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Three rapidly moving developments are shaking busi-nesses across the world: cloud technology, mobile adoption and data proliferation. Core construction In-

dustry have been dramatically impacted by the three. Con-struction projects, always a little tradition bound, is suddenly witnessing brisk change. The enhanced availability of rich data and mobile adoption is ensuring that the data reaches end users quickly and can be acted upon instantly. Enabling this is scalable and standardized infrastructure on cloud. Cloud is reshaping the way technology is used. It has made computing power, storage, back up, development platforms, testing environments and the ability to run a variety of ap-plications available with cost savings and increased options for users.

Everything from Business Support Systems to Opera-tions Support Systems is migrating to cloud. Businesses are

making saving through higher efficiencies, by lowering IT staff requirements and circumventing capital investments in IT. It’s time Enterprise resource planning (ERP) to migrate to cloud to make itself more user friendly and efficient tool.

Enterprise resource planning (ERP) systems are the backbone of many organizations, helping them manage their accounting, procurement processes, projects, and more throughout the enterprise. For large Construction Projects, ERP systems have often meant large, costly, and time-con-suming deployments that might require significant hardware or infrastructure investments. The advent of cloud comput-ing and software-as-a-service (SaaS) deployments are at the forefront of a change in the way businesses think about ERP. Moving ERP to the cloud allows businesses to simplify their technology requirements and more quickly see a return on their investment. According to a 2013 survey by McGladrey,

Ankita Adhikary

An Cloud Based ERP: The Next Step in Construction Project Management

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54 percent of respondents say changing or upgrading their existing applications is their most time-consuming ERP task. With cloud-based ERP deployments, however, businesses see lower support costs and no maintenance or upgrades for the IT staff to perform. Cloud-based ERP suites are ma-ture offerings that now have many of the same features and functionality as their on-premise counterparts. In addition, the cloud deployment model easily enables the integration of other key technologies like mobility, decision support sys-tems, and collaboration and social systems.

in their attempt to mitigate. A typical project launches at the departmental level. Thus, the solutions to plan and execute the project are typically decided on and deployed at that lev-el. Project management and monitoring is often undertaken separately, in different parts of the organization. This results in disparate solutions that often take the form of a myriad of ERP and custom solutions and it often brings with it a lack of integration among the projects collectively and individually within the enterprise as a whole.

In fact, adopting a cloud-based ERP suite today means neither IT nor the business needs to settle for applications or infrastructure that are deficient or lacking in features. Mod-ern ERP cloud-based applications have a consumer-like user experience, embedded collaborative capabilities, and in-context analytics to support real-time decision making, a critical activity in today’s fast-paced business environment. A modern cloud platform is agile, reliable, and secure, so that organizations can confidently pursue growth opportunities.

A modern ERP cloud solution simplifies, standardizes, and automates business processes helping organizations take full advantage of growth opportunities. A modern ERP cloud also enables a workforce to collaborate, analyze, and work on the move, accelerating performance and attracting great talent. Finally, a modern ERP cloud reduces costs and makes smarter use of scarce IT resources in a construction company.

Powering Project-Driven Businesses

Project management has historically been a complicat-ed and convoluted process. The ad hoc, often one-off nature of projects and proposed solutions create numerous pain points that organizations within enterprises often exacerbate

While senior management and project leads may lament this lack of integration, those in the trenches typically remain loyal to the applications with which they are familiar. Often this means Microsoft Excel on a user basis and Microsoft Proj-ects on a departmental basis, with both sometimes finding themselves rolled up into an enterprise-wide ERP system.

Project Portfolio Management (PPM) through Cloud based ERP

To truly understand the advantages of Project Portfolio Management (PPM) Cloud brings to project management it’s important to understand the foundation of PPM. PPM enables corporate and business users to organize a series of projects into portfolios and provide reports based on the various project objectives, costs, resources, risks, and other pertinent associations. PPM software enables users, usually management or executives within an enterprise, to review the portfolio that will assist in making key financial and busi-ness decisions for projects.

PPM has six core components, each of which is found in any project undertaken:

- Project costing - Project contracts and billing - Performance reporting - Resource management - Project management - Collaborative planning

All of these components flow into the two other com-ponents of ERP, procurement and financial management, putting PPM in an interesting position in the ERP landscape. Projects, by their very nature, are rarely stand-alone. While a good PPM software package will centralize resource and project tracking, there is no getting away from financial ac-countability or resource optimization, both of which are im-portant to managing the project and helping the enterprise

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achieve its goals. PPM solutions play a dual role, directly and immediately benefitting the project at hand and benefitting the whole enterprise indirectly and in the long term.

There are different ways to serve project management, which has processes that sit within CRM, HCM, project management itself, and accounting/financials software, and then applying business intelligence across the entire pro-cess. Some ERP apps have extensions to cater to the needs of businesses that are oriented around projects; other ERP apps take a project-based focus; and other solutions may be a mix of best-of-breed capabilities from CRM, HCM, project management, and accounting.

The Example of Oracle based PPM Module is mentioned as below:

PPM Cloud is designed to meet, with ease, the many challenges that senior executives, project organizations, and project leaders face. PPM Cloud offers an integrated yet modular project management suite designed to automate, streamline, and control project management processes end-to-end without expensive hardware and system man-agement overhead costs. Nine solutions focusing on project financial management and project execution make up the PPM Cloud solution. Because PPM Cloud is so modular in nature, enterprises can choose the products to deploy and add more products when they are ready. Billing and con-tracts is also popular for customers that bill for project-based work.

The following solutions make up Oracle PPM Cloud:

1. Project Performance Reporting 2. Project Costing 3. Project Control 4. Project Contracts 5. Project Billing 6. Project Management 7. Resource Management 8. Task Management 9. TAP for PPM

Project Performance Reporting: Uses a multidimen-sional reporting model to give project stakeholders answers to critical business questions to enable them to take action in real time.

Project Costing: Provides a highly automated and streamlined project costing process, allowing project-cen-tric organizations to capture and account for project costs and commitments across other Fusion applications and third-party integrations for standardized cost collection pro-cesses.

Project Control: Manages the planning, budgeting, pro-gressing, and forecasting aspects of a project from a user perspective using an intuitive interface to provide simplified viewing and control to better oversee critical activity.

Project Contracts: Delivers a common contract frame-work that allows users to manage the customer contract terms and conditions for products and services, independent of how the project is executed.

Project Billing: Works with Project Contracts to ensure compliance with the customer contract when billing and recognizing revenue for a customer project.

Project Management: Provides easy to use collaborative planning and essential scheduling capabilities for the project manager and team members.

Resource Management: Optimizes the allocation and utilization of resources to ensure best-fit candidates from a global repository are assigned for every project.

Task Management: Offers real-time, in-context collabo-ration to enable team members to work socially and move the project along easily.

Tap for PPM: Delivers a complete view of projects on any iOS device so users can understand the health of the project and performance while on the road or even at the client’s site.

Running through all of these solutions are the central tenets of Simplify, Accelerate, Collaborate, and Control. The result is cloud services that are more sophisticated around the financial planning and control of the project and that free up resources to be better utilized in other ways.

From the modern user experience through power user settings, becoming confident and proficient with the cloud service is a straightforward process. This is also achieved through Oracle PPM Cloud’s integration with popular desk-top tools, including Microsoft Project, which enables users to work in a comfortable environment. And, this support helps the transition from reliance on fragmented project tools to a complete project portfolio management cloud solution.

Oracle PPM Cloud also delivers greater insight and makes analysis simpler. Analytics drive key decisions in many enterprises. While analytics have the potential to lead to better decision making, they are unable to do so unless the right people have the right data. The Project Performance Reporting Cloud delivers insights via graphs and embedded analytics with over 150 pre-seeded data points in a rich set of subject areas. It is also able to work across multiple projects and historic indicators to determine what is going on. Rather than looking at custom reports, project managers and oth-ers can look at a standard set of key performance indicators (KPIs). Having this critical information at one’s fingertips is vital to the project’s success.

Tight integration and customized analytics mean less time and resources are spent tweaking and searching for data. More time can be spent analyzing the data and taking action. This insight into what might have gone wrong, “gives critical information to project managers for historical data,” making it easier to get the information needed to make deci-sions.

Since projects are inherently social, Oracle facilitated the meshing of project information among team members, making it easier for them to collaborate internally or exter-nally. The Team Connect feature offers an integrated, easily configured project space for all project members where any team member can upload documents or files for everyone working on a project to see and contribute. Users find func-tionality such as activity streams, forums, blogs, presence, document management, calendaring, and polling in Team Connect that make it comprehensive enough to meet all of their collaboration needs. For users who want the ability to work any time, Oracle offers “Oracle Tap for Oracle PPM Cloud,” a Fusion application that can be accessed from any iOS device.

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Integration, analytics, and collaboration are valuable, but they have great potential for the wrong information to end up in the wrong hands or for users to overstep their roles. Hence, controls are critical. Oracle PPM Cloud offers nu-merous built-in controls. Role-based usage and access are built into the services. This enables internal and external us-ers to have direct access to exactly what they need. In addi-tion, for critical areas across the PPM suite, such as con-tracts, role-based analytics are also available. Project team members are given access to the financial information on an as-needed basis. Controls also prevent missteps, particu-larly where expenses come into play. The standardized cost policies, for example, enforce validation at the point of entry. If costs are not validated upfront, the transaction will not go through. This way, “if it isn’t right, it doesn’t come in”.

PPM is a critical component of any enterprise involved with projects. PPM is hardly a stand-alone service though. Many of the components in PPM such as project costs, capi-tal assets, budgets, billing, and revenue flow into financial management. Oracle PPM Cloud brings these complex and critical integrations together seamlessly and transparently for the organization and the user. The benefits of this cohe-sive offering are truly felt when the big picture is examined.

Advantages of ERP on Cloud

businesses that want applications with modern functionality, but without the overhead of IT infrastructure, maintenance, and upgrades. In fact, 84 percent of CFOs surveyed in the 2013 Gartner Financial Executives International CFO Tech-nology Study believe that half of their transactions will be delivered through SaaS over the next four years (up from 53 percent in 2012).

As they consider how their technology strategy supports business objectives, forward-thinking businesses are now exploring and adopting cloud-based ERP applications. In addition to saving costs on infrastructure and maintenance, these businesses are reaping the benefits of modern, ma-ture ERP applications with the ability to easily integrate with new or existing business processes. ERP on Cloud delivers best-in-class functionality with integrated analytics, mobile accessibility, and collaboration built into its services to cre-ate a powerful, integrated suite of modern business applica-tions. The applications in the Cloud based ERP are designed with users in mind. Dashboards put the information users need at their fingertips when they need it; there’s no search-ing for action items. Users can work with their existing tools and applications; there’s no need to learn an entirely new process to get work done. The role-based design of ERP on Cloud means users focus only on the tasks and information they need to get their work done.

“ERP in cloud is poised to play a major role in Construc-tion Industry. As organizations attempt to increase flexibility, enhance customization, lower costs and drive the integra-tion of emerging technologies, cloud will become central to success.”

Reference

- https://www.oracle.com/webfolder/s/delivery_production/docs/FY14h1/doc3/Quinstreet-OracleERPCloud-eBook.pdf

- http://www.wipro.com/documents/ERP-on-Cloud-the-winds-of-change.pdf

- http://www.iaeng.org/publication/WCE2011/WCE2011_pp681-684.pdf

- http://www.fronde.com/assets/PDF/wp-abrdn-saas-and-cloud-erp-observations-060713.pdf

- http://www.emkor.com/upload/whitepaper1.pdf- http://resources.idgenterprise.com/original/AST-0111292_ERP_

US_EN_WP_IDCERPInTheCloud.pdf- http://airccse.org/journal/ijccsa/papers/3313ijccsa01.pdf w

- Enhanced flexibility (modular implementation)- Enhanced customization (new ERP solutions are highly

configurable)- Lowered cost of ERP implementation (due to lowered

infrastructure requirements)- Lower cost of ownership (pay-as-you-go model)- Better integration with emerging technologies (mobile,

data, analytics)- Lowered cost of IT talent (no on-premise installations to

manage)

The developments indicate that ERP on cloud is not a matter of when manufacturing will become part of the trend, but what it will choose to deliver via cloud first before it em-braces ERP on cloud completely.

Conclusion

Cloud-services are growing in popularity among leading

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An Experimental Study on Pile Spacing Effects under Lateral Loading in SandMahdy Khari, Khairul Anuar Kassim, & Azlan AdnanDepartment of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia

Superstructures are supported by pile foundations so that it had its origin in prehistoric time. These founda-tions may be subjected to significant horizontal loads

such as dynamic and static loadings. Two criteria shall be controlled to satisfy of functioning such structures: (1) their deflection which must be within the permissible limit and (2) safety of pile against ultimate failure. The behavior of the pile group and the single pile is usually different ow-ing to the impacts of the pile-to-pile interaction (so called shadowing effects). In addition, soil-pile coupling behavior is important when the load transfer occurs [1]. Evaluation of the pile group behavior and the soil-pile interaction has developed by several investigators in experimental and ana-lytical modeling [2–4].

Existing methods of the analytical modeling can be classified into numerical approaches, Beam on Nonlinear Winkler Foundation method (BNWF), and simplified formu-lations [5]. Although most of these approaches are attended on evaluation of the stiffness of the soil-pile system, they are less focused on the bending moment and the lateral re-sistance of the group.

It is worth noting that the estimations of ultimate lateral resistance and lateral subgrade modulus within a pile group are known as they are the key parameters in the soil-pile interaction phenomenon. Several theoretical methods have been developed to determine these parameters in cohesion-less soils. However, the predictions of these approaches are often different. On the other hand, the laterally loaded pile group behavior has received a little attention. Moreover, the experimental data on the determination of active pile length and bending moment are inadequate. Therefore, it is neces-sary to increase the experimental data for the response of the pile group under lateral loads.

This paper presents the results of a series of experimen-tal investigations carried out on single and grouped piles sub-jected to the monotonic lateral loads in Johor Bahru sand in the southern portion of Malaysia. Emphasis was focused on group efficiency and load-deflection behavior owing to the influence of relative density, size group, and pile spacing.

Brief Review

As mentioned in the foregoing section, the shadowing phenomenon affects the pile behavior within the group un-der the lateral loading [6]. Although many researchers have studied the ultimate lateral resistance and deflection of the pile group to a lateral loading, they are complex due to the

interaction between the surrounding soil and the pile [7].In 1962, Prakash carried out the pile group behavior un-

der the lateral loading using aluminum pipes (od = 12.7 mm; =pile diameter) inthemediumsand.Basedon these tests, it

was stated that the sum of pile capacities was more than that within the group when the spacing center-to-center of piles was less than3 and 8 in the direction perpendicu-lar and the direction to load, respectively. Meyerhof et al. [8] conducted tests in homogeneous sand on pile groups and rigid single pile under central inclined loads. The bored piles were tested by Franke [9] in the experimental tests. The results showed that the displacement of a group was more than a single pile in the same loading when the piles spacing was less than 6 . Patra and Pise [10] studied the ultimate lateral resistance on six types of configurations of pile group with different embedment length-to-diameter ratios equal to 12 and 38. Their results were compared with the results of analytical methods. Based on their report, it can be stated that the isolation spacing is six times of pile diameter for l/ = 12.

Kim and his workers [11] investigated lateral load tests on aluminum single pile (driven and drilled) in dry sand. In ad-dition, they considered the head conditions of the piles.

The lateral loads of the preinstalled were less than those of the driven piles. Zhang et al. [12] proposed the ulti-mate lateral resistance in cohesionless soils. They collected the experimental data done by other researchers on rigid piles and a simple method was developed by them to pre-dict the ultimate lateral resistance (involving of side shear resistance and frontal soil resistance) to piles considering the shape factor. Another method was developed by Prakash and Kumar [13]. In this method, loaddisplacement relation-ship was predicted by means of considering soil nonlinearity using subgrade reaction. Erdal and Laman [14] purposed the behavior of short pile subjected to lateral loads in a two-layer sand deposit.The pilemodeled had an embedded length-to-diameter ratio of 4 and fabricated from steel for all the tests. Based on their results, it can be stated that the lateral load capacity of short rigid piles in the dense sand was 5 times that in loose sand.

Experimental Setup

The schematic diagram of the test setup is shown in Figure 1. The model tests were performed in a rectangular soil tank with dimensions of 900mm in length, 700mm in width, and 65mm in height. To consider the boundary condi-

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tions, the size of the soil tank was extended up to 8–12 (=pile diameter) and 3-4 in the direction and perpendicu-lar to the lateral loading, respectively [15]. In additional, to minimize the influence of box boundaries, the soil thickness was kept below the pile tip at least 6 .

in this research to reconstruct the dry sandy soil samples using the dry pluviation method. The newlyMobile Pluvia-tor developed was consisting mainly of a soil bin (hopper), the diffuser system (the three sieves), sand collector, and a fixing device to set up these components so as the whole of the system was carried by a moveable steel frame. The interchangeable circular wood plates (shutter plates) were installed in the bottom of the sand hopper. The four patterns of the shutter plates were formed in a manner of the distri-bution differently of the holes for the sake of control of the rate of the soil discharge. While the apparatus was movable, the different factors were examined to obtain a wide range of the relative density. The falling height and the rate of pouring had the opposite effects on the relativedensity. Based on the results obtained, the two patterns selected consisted of 11 holes (diameter = 18 mm) and 16 holes (diameter = 10 mm) distributed evenly in the shutter to achieve the dense and the loose sand samples with relative density of 75% and 30%, respectively. The falling height was kept constant a 700mm from the surface of the model ground which was more than the critical height so that to obtain terminal velocity. The raining was stopped when the sand rained in the soil tank was 30mm thicker than required and then the extra soils were removed.

Figure 1: Side view of experimental setup.

The model piles with an open end and hollow circular section were fabricated from aluminum alloy tubes ( = 69.8 GPa) of 15.88mm out diameter, 1mm wall thickness and an embedded depth of 500 mm. It is worth noting that, for the pile properties and the selected soil, pile behaves as flexible pile.

Three plates made of steel were used as pile cap for different spacing. To satisfy fixed head conditions, the piles were passed through exiting holes in the cap and then screwed to angle profiles (length = 50 mm) welded on these holes. Lateral loads were applied to the model piles using a 650N capacity electric motor through a pulley supported by a loading platform with flexible wire attached to the cap. The horizontal deflection of the pile group was measured by means of two Linear Variable Differential Transducers (LVDT) to the angle profiles of the two corner piles. The ro-tation of cap was determined from axial displacement mea-sured by other two LVDTs fixed on front and behind of the cap in load direction. A load cell was placed between the flexible wire and electric motor to monitor the total loads ap-plied to the pile cap.

Soil Properties and Sample Preparation

The tests were conducted in dried sand (in the labora-tory temperature) from Johor Bahru sand. The sampled sand was classified as SP, according to the Unified Soil Clas-sification System (USCS).The medium diameter (D50) and uniformity coefficient (Cu) of sand were 0.532 and 0.17mm, respectively, and particle sizes in a range of 0.075–0.97mm with the gradation are shown in Figure 2. Based on a stan-dard density test, minimum and maximum unit weights of sand were 13.74 kN/m3 and 16.38 kN/m3.

To reconstruct the sand samples, several methods have been developed by investigators such as vibration, tamping, and pluviation [16]. The prepared samples using the pluvia-tion and tamping technique often result in a specimen of ho-mogenous and nonuniform density, respectively. Based on this defect, the newly designed Mobile Pluviator was utilized

Figure 2: Gradation curve of the Johor Bahru sand.

Test Procedure

Different configurations of pile groups in different spac-ing are shown in Figure 3. The center-to-center spacings of the piles were 6 and 3 , and embedment ratio of 32 was tested. Spacing ratio (SR = S2 /S1 where S2 and S1 are the piles spacing in perpendicular and direction of lateral load applied, resp.) was equal to 0.5,1 and 2. In addition, several tests were conducted on single pile. The piles (fixed with the cap) were first located in the center of soil tank and then were kept in a vertical statue using a supporting frame. After placing the model pile, the Mobile Pluviator apparatus was installed over soil box. To monitor uniformity and the relative density during the samples preparation, three small boxes cylinder shaped of 455 cm3 were placed on the surface of sample prior to sand spreading. The surface of the model ground was leveled when the required height was achieved. At least 24 hours elapsed before applying any test on the pile group. The data measured from LVDTs and load cell were stored on a computer data acquisition system.

Experimental Results and Discussion

A series of 45 tests were performed on piles to investigate

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the influences of soil density and different pile configurations on the ultimate lateral resistance and pile group efficiency. The pile groups were loaded in an incremental manner. The nonlinear load versus lateral displacement and vertical set-tlement of the pile cap could be adequately defined.

tion and group behavior for the 3 x 3 pile group with a square arrangement are shown in Figures 5 and 6. For a particu-lar value of lateral movement, the magnitude of lateral load decreased when the piles’ spacing decreased in dense and loose sand. At the deflection of 0.1 , the lateral load of the pile group was about 2.90 times higher than that of the single pile in the case of 6-diameter, 1.85 times higher for s/ = 3.

Figure 7 illustrates the influences of piles number in group on the value of the deflection against the lateral load. Compared to Figure 5, it is observed that when the piles’ spacing was the same, the magnitude of lateral load how-ever was higher for larger groups. comparing between Fig-ures 6 and 7, the load-deflection curves were almost similar. This may be due to the area of ground pressure in front of the pile group. This indicates that, although the number of piles contributes to the value of lateral resistance, the piles’ spacing is the most significant factor.

Figure 3: Pile group configurations and pile spacing ratio (| is thelateral load-ing direction).

The soil density effects on single pile against the average pile deflection are presented in Figure 4. From the figure it is seen that the load-deflection curves were nonlinear and a similar trend was observed in loose and dense conditions. Vertical displacements were negligible compared to horizon-tal deflections and it is in agreement with previous studies which stated that soil-pile interaction could be determined separately under lateral and vertical loads. The differences of the lateral deflection increased when the relative density increased from 30% to 75% under the same moment of load. Therefore, a higher relative density will provide a stiffer re-sistance for pile subjected to lateral loading. This is owing to the increasing of shear strength of sand as it becomes denser. In other words, pile behavior subjected to lateral loads depends on the interaction between the surrounding soil and pile material.

The influence of the piles’ spacing on the lateral deflec-

Figure 4: Lateral load versus deflection diagram for single pile(H = horizontal; V= vertical).

Figures 8 and 9 illustrate the behavior of the load deflec-tion of pile group in both series and parallel arrangements were investigated for a three-piles group in the spacing of the center-to-center piles of 3 and 6 . From these Figures, it can be seen that the piles’ deflection in parallel arrangement was less than that in series arrangement under a given lat-eral load. The higher lateral load capacities in parallel ar-rangement was governed by the increased passive pressure zone existed in front of the pile group. A similar comparison was made for different relative densities of soil, which shows that a similar phenomenon occurred.

Figure 5: Lateral load versus lateral deflection (3 x 3 pile group; s/d = 6).

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It should be noted as shown in Figures 8 and 9 that the effect of the stressed zone around piles for series arrayed piles was less than that for parallel arrangement. However, both stress zones may be dependent on the dimensions and the elastic modulus of the piles. Since the piles were assumed flexible, the failure of the surrounding soil will be earlier as compared to the piles.

the relative density affects ultimate resistance because of passive pressure zone existed in front of the pile group. The piles’ spacing in the perpendicular direction to load applied may affect the ultimate resistance load due to the stressed zone in front of the pile group. The ultimate lateral resis-tance of single piles was 84.013 and 44.5 (N) for loose and dense sand, respectively. With note to the ultimate load in group and single pile, the effects of the shadowing phenom-enon can be observed so as the increasing of the pile spac-ing causes the same in group and individual. The ultimate lateral load in single pile was about 25% of the ultimate load for 3 x 3 pile group (T3433) while this percentage for T3032/3 was about 47%. In fact, with increasing of the pile spacing from 3 to 6 , the value of ultimate lateral load about 0.53% is increased.

Group Efficiency

Variation of the pile group resistance at a given deflection is expressed by group efficiency ( ) and is calculated as follows:

(1)where LG and LS are ultimate lateral capacity of pile

group and single pile, respectively. n1 is number of rows in a pile group; n2 is number of columns in a pile group.

Wakai et al. [19] performed the laboratory tests on a 3 x 3 pile group with free and fixed head conditions (s = 2.5 ).

Figure 6: Lateral load versus lateral deflection (3 x 3 pile group; s/d = 3).

Ultimate Lateral Resistance

The ultimate lateral resistance in the different arrange-ments of pile groups was estimated by the load-deflection curves. The soil resistance to piles under lateral load may be involving of the side friction and the frontal normal reac-tion [17]. However, these two reactions are dependent on shape factor taking in account nonuniform distribution of earth pressure in front of pile and lateral shear drag. There are several methods to estimate the ultimate lateral resis-tance such as double tangent and log-log method. In this study, the ultimate lateral resistance was taken as the load corresponding to the reference deflection of 0.2 on the load-deflection curves [18]. The results obtained exhibit that the increasing rate of deflection was reached at about 0.2–0.35 . Figure 10 shows the influence of the piles’ spacing versus ultimate lateral resistance. The ultimate lateral load was constant with an increase from 3 to 6 in parallel ar-rangement of piles for group 1 x 3 in dense sand. However, the increasing can be observed more than that in series ar-rangement of piles. From the figure, It is worth noting that

Figure 7: Lateral load versus lateral deflection (2 x 2 pile group; s/d = 6).

Figure 8: Lateral load versus lateral deflection for three-pile group in series layout; (a) Dr =75%and (b) Dr = 30%.

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The group efficiency was estimated 0.45–0.70 at the deflection of 0.1 . However, the group efficiency obtained based on the ultimate lateral loading can be higher than that at a given deflection. Kim and Yoon [20] carried out the static loading tests on the different pile arrangements. They calculated the group efficiency when the deflection was reached 0.1 . In 3 x 3 pile group, the coefficient was 0.4–0.7 and 0.5–1.04 for the medium dense and the medium sand, respectively.

Gandhi and Selvam [21] stated that, at the 10mm dis-placement, pile behavior is crossed through elastic to plastic range. They considered this deflection to estimate the group efficiency. Based on their results, the efficiency increases with an increase in the s/ ratio and this raising can be due to the increasing of the overlapping zones.

Patra and Pise [10] and Oteo [22] carried out a series of tests on different configurations of pile groups under the lat-eral loading. Oteo reported model tests on 3 x 3 piles group in medium send. Patra and Pise reported the groups’ effi-ciencies for 2 x 1, 3 x 1, 2 x 2, and 3 x 2 for pile spacing from 3 to 6 . As Figure 11 shows, the experimental results in this study for 3 x 1 pile group at 3 and 6 were about 50% less than those the reported by Patra and Pise. However, the measured group efficiencies were in good agreement with those of Oteo.

The variation group efficiency against of spacing ratio (SR = S2 /S1, where S2 and S1 are the piles spacing in perpen-

Figure 9: Lateral load versus lateral deflection for three-pile group in Parallel layout; (a) Dr =75%and (b) Dr = 30%.

Figure 10: Ultimate lateral load versus pile spacing; (a) Dr =75%and (b) Dr = 30%.

Figure 11: Comparison of group efficiencies.

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dicular and direction of lateral load applied, resp.) is pre-sented in Figure 12 for 3 x 3 piles group in the different rela-tive densities. It can be stated that the group efficiency was decreased about 0.35 and 0.25 in the loose and the dense sand where S1 = S2. However, this value was increased for

. The group efficiencies were the same for the S2 /S1 ratio almost equal to 0.5 and 2. The group efficiency was higher in relative density by 30%. For a 3 x 3 piles group, the observed efficiency was about 0.23–0.28% and 0.32–.41% for Dr=75% and Dr=30%, respectively. As Figure 13 shows, the group efficiency was decreased with an increase in the number of piles arrayed in group. This decreasing with an increase of the number of piles in pile spacing of 6 and 3

was almost the same. However, the group efficiency was

about 0.68–0.84% for S = 6 and 0.35–0.68% for S = 3 . Pise and Patra [10] carried out a series of the tests for 3 x 3, 3 x 1, 2 x 2, and 2 x 1 piles groups. The efficiencies obtained were 0.752–1.0 and 0.9–1.2 for the 3 x 2 and the 3 x 3- piles group, respectively. These efficiencies were higher (about 42%–78%) than those obtained in this study for s/ = 3 and 6.

Conclusions

The behavior of single pile and grouped is believed to be understood, especially for soils where the subgrade modu-lus is independent of time. Based on this demand, a series of tests were carried out on pile group under lateral static loading in sandy soils. A new method of the reconstruction of sand samples was developed for large area of samples. Based on the results of present experiment, the following conclusions are drawn.

(1) Load-deflection curves were estimated with scaling factors to determine the ultimate lateral resistance of group. The qualitative and quantitative effects of the relative density of the sand have been carried out. The ultimate lateral load was increased 53% in increasing of s/ from 3 to 6.

(2) The subgrade modulus decreased with increasing deflec-tion. Width and pile stiffness were two important factors effective on this decreasing.

(3) Vertical deflection of pile group can be neglected with comparison to horizontal deflection under the lateral loading.

(4) The increase of the number of piles in-group decreased group efficiency owing to the increased overlapping zones and active wedges.

(5) A ratio of s/ more than 6 was large enough to elimi-nate the pile-to-pile interaction and the group effects. It may be more in the loose sand.

(6) Flexible piles of series arrayed were more resistant than those parallel arrayed to lateral loadings.

Acknowledgments

The research was undertaken with support from re-search university Grant (no. Q.J130000.2513.03H63) under the University Teknologi Malaysia (UTM). The first author would like to thank the Ministry of Education (MOE) and the Research Management Center for the financial supports during this study.

References

[1] M. Khari, A. K. Kassim, and A. Adnan, “Kinematic bending moment of piles under seismic motions,” Asian Journal of Earth Sciences. In press.

[2] M.H. ElNaggar, M. A. Shayanfar, M. Kimiaei, and A. A.Aghakouchak, “Simplified BNWF model for nonlinear seismic response analysis of offshore piles with nonlinear input ground motion analysis,” Cana-dian Geotechnical Journal, vol. 42, no. 2, pp. 365–380, 2005.

[3] M. Khari, A. K. Kassim, and A. Adnan, “Dynamic soil-pile interac-tion under earthquake events,” in Proceedings of the AICCE/GIZ’12, Park Royal Penang Resort, Penang, Malaysia, 2012.

[4] M. Khari, A. K. Kassim, and A. Adnan, “Effects of soil model on site response analyses,” Asian Journal of Scientific Research. In press.

[5] M. Khari, A. K. Kassim, and A. Adnan, “Development of p-y curves of laterally loaded piles in cohesionless soil,” Scientific World Journal. In press.

Figure 12: Group efficiency versus spacing ratio of piles; (a)Dr =75% and(b) Dr = 30%.

Figure 13: Group efficiency VS number of piles; (a) Dr =75% and(b) Dr = 30%.

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[6] D. Brown, C. Morrison, and L. Reese, “Lateral load behavior of a pile group in sand,” Geotechnical and Geological Engineering, vol. 114, pp. 1261–1276, 1988.

[7] M. Khari, A. K. Kassim, and A. Adnan, “The effects of soil-pile inter-action on seismic parameters of superstructure,” in Proceedings of the 2nd International Conference on Geotechnique, Construction Materials and Environment (GEOMAT ’12), pp. 479–484, Kuala Lum-pur, Malaysia, November 2012.

[8] G. G. Meyerhof, A. S. Yalcin, and S. K. Mathur, “Ultimate pile capac-ity for eccentric inclined load,” Journal of Geotechnical Engineer-ing, vol. 109, no. 3, pp. 408–423, 1983.

[9] E. Franke, “Group action between vertical piles under horizontal loads,” in Deep Foundations on Bored and Auger Piles, W. F. V. Impe, Ed., Balkema, Rotterdam,The Netherlands, 1988.

[10] N. R. Patra and P. J. Pise, “Ultimate lateral resistance of pile groups in sand,” Journal of Geotechnical and Geoenvironmental Engineer-ing, vol. 127, no. 6, pp. 481–487, 2001.

[11] B. T. Kim, N.-K. Kim, W. J. Lee, and Y. S. Kim, “Experimental load-transfer curves of laterally loaded piles in Nak-Dong River sand,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 130, no. 4, pp. 416–425, 2004.

[12] J. Zhang, R. D. Andrus, and C. H. Juang, “Normalized shear modu-lus and material damping ratio relationships,” Journal of Geotech-nical and Geoenvironmental Engineering, vol. 131, no. 4, pp. 453–464, 2005.

[13] S. Prakash and S. Kumar, “Nonlinear lateral pile deflection pre-diction in sands,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 122, no. 2, pp. 130–138, 1996.

[14] U. Erdal and M. Laman, “Lateral resistance of a short rigid pile in a two-layer cohesionless soil,” Acta Geotechnica Slovenia, vol. 2, pp. 19–43, 2011.

[15] S. Narasimha Rao, V. G. S. T. Ramakrishna, and M. Babu Rao, “In-fluence of rigidity on laterally loaded pile groups in marine clay,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 124, no. 6, pp. 542–549, 1998.

[16] M. Khari, A. K. Kassim, and A. Adnan, “Snad sample preparation using mobilepluviator,” The Arabian Journal for Science and Engi-neering. In press.

[17] T. D. Smith, “Pile horizontal soil modulus values,” Journal of Geo-technical Engineering, vol. 113, no. 9, pp. 1040–1044, 1987.

[18] B. Broms, “Lateral resistance of piles in cohesive soils,” Soil Me-chanics and Foundations Division, vol. 90, pp. 27–63, 1964.

[19] A.Wakai, S.Gose, and K.Ugai, “3-D elasto-plastic finite element analy-ses of pile foundations subjected to lateral loading,” Soils and Founda-tions, vol. 39, no. 1, pp. 97–111, 1999.

[20] B. T. Kim and G. L. Yoon, “Laboratory modeling of laterally loaded pile groups in sand,” Civil Engineering, vol. 15, pp. 65– 75, 2011.

[21] S. R. Gandhi and S. Selvam, “Group effect on driven piles under lateral load,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 123, no. 8, pp. 702–709, 1997.

[22] C. S. Oteo, “Displacements of vertical pile group subjected to lateral loads,” in Proceedings of the 5th European Conference on Soil Mechanics and Foundation Engineering, pp. 397–405, Madrid, Spain, 1972. w

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The Thermal Integrity Profiler (TIP) uses the tempera-ture generated by curing cement (hydration energy) to assess the quality of cast in place concrete foundations

(i.e. drilled shafts or piles). Whereas other methods of integ-rity testing have limits in assessing the full cross-section or length, TIP measurements evaluate the concrete quality from all portions of the cross-section along the entire length. The durability of drilled piles relies heavily on the thickness and quality of the concrete cover around the steel reinforcing cage. Until recently, this concrete cover went largely untest-ed as non-destructive test methods could not test this region or were severely limited in the detection capability. Further, the concrete cover contributes significantly to the moment of inertia resisting bending moments (at least on the side in compression) and is imperative to proper rebar bond/de-velopment length. TIP is capable of detecting the presence

(or absence) of intact concrete both inside and outside the reinforcing cage, thus providing a 100% scan of the pile. The method was developed in the mid 1990s at the University of South Florida, Tampa, and has been used commercially since 2007. The test measures the internal temperature of the pile, which is elevated by the cementitious materials present, and which react exothermically during hydration. The temperature rise from hydration energy has historically been considered an undesirable side effect that has been well studied in an effort to combat thermal-induced crack-ing. As high strength concrete has been used more often, the associated higher cement content has caused higher internal temperature. As an example of this effect, Figure 1 shows the modeled core temperature versus time relation-ship for three, 1.8 m diameter piles constructed with 18.6 MPa, 31.0 MPa and 62.0 MPa concrete with cement contents

Thermal Integrity Profiling for Quality Monitoring of Pile FoundationSonjoy Deb, B.Tech, Civil Associate Editor

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of 255, 356 and 510 kg per m3 (430, 600 and 860 lbs per cubic yard - PCY)of concrete, respectively. No flyash or slag was used in these example mixes.

of the reinforcing cage. The individual temperature readings will indicate any cage eccentricity, but the average tempera-ture will still allow for the determination of necks and bulges within the pile. Note that the gradient for the various pile siz-es is similar at the location of the cage. This is dependent on the time of testing and mix design, but is affected very little by pile diameter. In this way, the local radius of the pile is indicated by increases or decreases in temperature whereby the radius (or cover) is equally and oppositely higher or lower than that on the opposite side of the pile when the cage is eccentric. As the gradient is independent of pile size, bulges or necks in the pile are similarly detected as Increases or decreases in the average temperature, respectively. The magnitude of a bulge (or neck) is computed using the same gradient that identifies cage offset. Remember, when the av-erage temperature stays constant, the pile diameter stays constant; changes in the average temperature are the easi-est way to identify section changes.

Field Testing

Two approaches can be used to perform TIP: 1 use of a single thermal probe that is lowered into stan-

dard 38 mm ID steel or plastic access tubes affixed to the reinforcing cage, like CSL, or

2 by installing into the cage multiple, a full length Thermal Wire either in lieu of or in conjunction with each access tube. The plurality of access tubes or Thermal Wires has most often been the same as CSL testing where one tube or Thermal Wire is used for every 305 mm of pile diameter. For larger piles, fewer tubes or Thermal Wires have been shown to be similarly effective.Probe Option: When using access tubes, TIP is performed

by lowering a thermal probe equipped with radially oriented infrared sensors that record the internal wall temperature of the tubes in four orthogonal directions. The measured tem-peratures and depth of the probe are monitored and record-ed with a miniature computerized data acquisition system that plots the real-time progress for the operator to observe (Figure 3a,3b). One thermal profile is required from each tube, but often a second profile is obtained for data verifica-tion. The rate of descent is generally maintained at or below 0.15 m/sec making the test duration around 7 minutes per

Figure 1: The effect of cement content on core temperature of a 6 ft (1.8 m) dia. shaft

The presence of flyash or slag in the mix design can drastically change the time to peak temperature (Refer Fig-ure 1) up to 50 or 60 hours. Retarders further delay the time to peak temperature. Thermal Integrity Profiling is intended to be performed near the peak temperature (after hydration has completed), but can be conducted several days afterward depending on pile size and mix design. When considering the 31.0 MPa pile mix (Refer Figure 1), 600 PCY or 356 kg/m3 ), elevated pile temperatures above 52ºC persist for 5 or 6 days. As a rule of thumb, TIP can be performed up to D days after concreting (where D is the pile diameter in feet) and as early as 8 to 12 hours after concreting (depending upon pile diameter and concrete mix), thus expediting the continuation of construction. The internal temperature distribution within the pile is bell shaped as shown in Figure 2. Larger diameter piles develop the highest core temperatures but vary little as the pile size exceeds 1.8 m.

Figure 2: Knowledge of the normal temperature distribution is used to identify both cage alignment and local shaft radius (and cover)

Thermal Integrity Profiling measures the temperature at the radial location of the reinforcing cage where the gradient is highest. As a result, the measured temperature is highly sensitive to the cage alignment and subtle offsets are easily detected; in this case, a change of 1.9ºC equates to 25 mm of cage offset. Therefore, when the cage is off center, mea-suring temperature at opposite sides of the cage are equally affected; one is hotter and the other is cooler. The average of both represents the temperature at the average location

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Data Analysis

In general, two levels of analysis can be employed with-out using advanced numerical modeling.

- The first level makes observations of the raw thermal profiles, which with site experience, may provide enough insight into pile acceptance.

- The second level of analysis superimposes construction logs and concrete placement information to both con-firm first level observations and to convert temperature measurement into pile shape (radius, cage alignment and concrete cover).

Field measurements alone highlight glaring irregulari-ties since the average temperature profile shows the gen-eral shaft shape. This level of review reveals cage alignment irregularities, casing location, locations of over-pour bulges or necking, and can easily alert the user or an owner of areas of concern. Superposition of construction and concreting logs can calibrate the average diameter with average tem-perature, particularly when multiple concrete trucks per pile are used. The highest level of analysis uses thermal model-ing to simulate the shaft, the surrounding soil, the climatic history and energy generated from the concrete mix design. Results from simulations can define the best testing time for probe data acquisition (data for embedded wires is evaluated at time of peak temperature), or match the field measure-ments to a probable concrete shape. These models define the slope of the temperature to radius relationship near the edge of shaft where the cage is located. Finally, the mea-sured temperatures when converted to radius can be used to provide a 3- D rendering of the as-built shaft as well as 2-D slices of the shaft cross section at any depths of interest and vertical slices through any radial orientation.

Various TIP analyzing software’s are available in the market. A standard result produced by a software is shown in Figure 5.

Conclusion

Unlike above-ground concrete structures, drilled piles rely on effective post construction evaluation via nonde-structive testing methods. Thermal Integrity Profiling utilizes the heat generated by curing cement (hydration energy) to evaluate the integrity of cast in place concrete foundations such as drilled shafts, bored piles, augered cast-in-place,

Figure 3: Thermal probe system used to perform thermal integrity profiles

Figure 4 : Thermal wire system (Digital thermal sensor on cable)

30.5 m of tube length (2 scans per tube). TIP testing does not require water in the access tubes as testing is performed relatively quickly after concrete placement and the method is insensitive to debonding, allowing for the use of less costly PVC tubes; a cost savings to the project. If water has been introduced during construction for other integrity tests, it is removed, stored and returned after testing. Use of the same warm water prevents thermal shock to the tubes.

Thermal Wire Option

TIP can also be performed using an unmanned option where Thermal Wires are tied into the cage with discrete temperature sensors along its length. Each wire is connect-ed to a dedicated data collection box secured somewhere near the top of the pile. In this approach, data is continuously collected at user defined intervals (e.g., every 15 minutes) until the boxes are retrieved (Figure 4). This is convenient for scheduling testing personnel; no knowledge of the time to peak temperature is required. Rather, multiple tests are performed automatically and the optimal time of testing is selected from the library of recorded profiles. When used in conjunction with the probe option, preselected piles can be periodically instrumented with Thermal Wires that both per-form TIP tests and verify predictions of the temperature/time relationship. Piles not pre-selected can be spot checked with the thermal probe when unforeseen mishaps occur. An additional Thermal Wire can be installed in the pile with a known offset, typically 51 mm, from the reinforcing cage and the thermal gradient can be measured directly.

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continuous flight auger piles and drilled displacement piles. The technology may also be used to evaluate the shape of jet grouting columns and diaphragm or slurry walls, or other concrete structures. The expected concrete temperature at any point within the foundation is dependent on the shaft diameter, concrete mix design, time of measurement and distance to the center of the shaft. Regions that are colder than expected are indicative of necks or inclusions - a cross-sectional area smaller than intended for the shaft. Regions that are warmer than usual indicate bulges - an excess of concrete in a particular location. Temperature measure-ments may therefore be used, along with concreting logs, to estimate the actual shape of the shaft. It is also possible to identify misalignments of the shaft reinforcing cage and estimate the concrete cover along the entire length of the shaft. TIP has the potential to challenge or replace the pre-vailing methods of assessing quality of cast in place concrete foundations such as drilled shafts because TIP evaluates the entire foundation element and provides earlier results.

Other current methods of integrity testing have limitations. Cross Hole Sonic Logging (CSL) can only evaluate the con-crete inside the cage, leaving the concrete cover unexplored. Gamma-Gamma testing assesses only a limited zone near the access tubes. Pulse Echo testing has restriction on length and cannot evaluate the shaft below the first major cross section change. TIP scans the entire shaft for concrete anomalies, both length-wise without maximum length limi-tations and through the entire cross-section including the concrete cover outside the reinforcing cage. It also shows if the foundation reinforcement is properly aligned, something other test methods cannot do. Lastly, current test methods can only be performed after the concrete of the foundation has cured, a process that takes several days. This some-times results in construction delays since construction can-not proceed until foundations are approved. TIP, on the other hand, can yield results as early as 12 to 24 hours (depending on shaft diameter).

Reference

- http://www.pile.com/pdi/products/tip/tipwhitepaper.pdf- http://www.palanalys.se/brochures/tip.pdf- http://www.cif.org/awards/2013/04_-_Thermal_Integrity_Profiler.

pdf- http://www.dot.state.fl.us/research-center/Completed_Proj/Sum-

mary_SMO/FDOT_BD544_20_rpt.pdf- http://www.loadtest.co.uk/services/TIP%20datasheet.pdf- http://louisianacivilengineeringconference.org/yahoo_site_ad-

min/assets/docs/Non_Destructive_Thermal_Integrity_Test-ing.272132311.pdf

- http://www.pile.com/reference/DeepFoundationsMagazine/DFI_MAY_JUN2012_pg51-54.pdf w

Figure 5 : TIP Reporting Software

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Analysis of Multi Storey Building with Precast Load Bearing Walls

J.D. Chaitanya Kumar1, Lute Venkat2 1PG Scholar, Department of Civil Engineering, GVP College of Engineering (A), Visakhapatnam2Associate Professor, Department of Civil Engineering, GVP College of Engineering (A), Visakhapatnam

Abstract: Pre-cast construction is gaining significance in gen-eral and urban areas in particular. It is gaining more popularity with the rapid urban infrastructure growth. In this context G+11 storey residential building with precast reinforced concrete load bearing walls has been attempted for analysis. The struc-tural system consists of load bearing walls and one-way slabs for gravity and lateral loads have been taken for analysis using ETABS. Various wall forces, displacements and moments have been worked out for different load combinations. Data base is presented for the worst load combination. This work is limited to the analysis of structural elements only not the connection details.

Now a day, there is an increase in housing requirement with increased population and urbanization. Building sector has gained increasing prominence. However, the fact that the suitable lands for building construction. Precast load bearing walls provide an economical solution when compared to the conventional column beam in fill wall system for the advantage of speed of construction and elimination of wet trades. In multi-storey buildings, lateral loads that arise as a result of winds and earthquakes are often resisted by a system of shear walls acting as vertical cantilevers. Such walls are usually perforated by vertical bands of openings which are required for doors and windows to form a system of shear walls.

Mazen (2013) has stressed that the small openings in the shear wall will yield minor effect on the load capacity of shear walls, cracking pattern and maximum drift. In case of small openings, the shear walls behave as coupled shear walls. Thakkar (2012) has concluded that the design of shear wall is a complex procedure, especially if the cross section of the shear wall is not regular in shape. The design of shear walls takes horizontal forces into account by shear and bending. The design of shear in the walls can be managed by computing the shear stress distribution over the cross section and reinforcing appropriately. Potty (2008) has concluded that the difference in the deflection of shear wall modeled by beam element and the shell element is only 1.6 mm for the ten storey building.

Habibullah (2007) has worked on physical object based analysis and design modeling of shear wall system using ETABS. It has been concluded that grouping of the area objects into piers is a very powerful mechanism to automatically obtain design moments and shear across a wall section from a finite element analysis. Dar (2007) had stressed that the large open-ings are generally achieved by use of large transfer beams to

collect loading from the upper shear walls and then distribute them to the widely spaced columns that support the transfer girders.

Wdowicki and Wdowicki (1993) have stressed calculating stress and displacements in three-dimensional shear wall structure with uniform properties throughout the height. The analysis is carried out on the basis of the continuous connec-tion method. The system allows for considering lateral and gravity loads, arbitrary located in the plan and arbitrary distrib-uted along the height.

Benjamin (1968) worked on variability analysis of shear wall structure where both rigidity and the strength of shear walls are highly variable. Bozdogan et, al. (2010) carried out vibration analysis of asymmetric shear wall structures using the transfer matrix method. He concluded that the governing differential equations of equivalent bending-warping torsion beam are formulated using the continuum approach. Xiaolei et, al. (2008) worked on numerical analysis of cyclic loading test of shear walls based on openSEES. Carpinteri et, al. (2012) carried out lateral load effects on tall shear wall structures of different heights. The accuracy of the results is investigated by a comparison with finite elements solutions, in which the brac-ings are modeled as three-dimensional structures by means of shell elements. Biswas et, al. (1977) carried out three dimen-sional analysis of shear wall multi storey building. He studied the importance of torsion in multi storey building having asym-metric layout of shear walls. Greeshma et. al., (2011) carried out the analysis of flanged shear walls using ANSYS concrete model. He has studied the possibilities of modeling reinforce-ment detailing of reinforced concrete models in practical use. Fahjan et, al. (2010) studied nonlinear analysis method for re-inforced concrete buildings with shear walls. The different ap-proaches for linear and non linear modeling of shear walls in structural analyses of buildings are studied and applied to RCC buildings with shear walls.

In this present study, G+11 storey precast load bearing wall structure is taken for analysis. The modeling and analy-sis has been done in using ETABS. The parametric study has been done to observe the effect of axial compression load, out of plane moments, tensile force, shear force, storey drift, lat-eral load and storey shear on shear walls. Finally data base is prepared for various storey levels. Although the connection details in the precast construction plays vital role but presently the details of connections not included in the present paper.

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Hence the emphasis on the analysis of load bearing wall struc-ture.

Modeling of Shear Wall Structure

In this present study Ground +11 storey shear wall build-ing is considered for one acre of site with 350 units. Around 400sqft of carpet area per unit is taken with 300 units per floor. The constriction Technology is total precast solution with load bearing RCC shear walls and slabs. The modeling is done in ETABS as follows. 1. The structure is divided into distinct shell element. The

shell element combines membrane and plate bending be-havior, as shown in fig.1. It has six degrees of freedoms in each corner point. It is a simple quadrilateral shell element which has size of 24 x 24 stiffness matrix.

ments, shear forces and normal forces across a wall section. Appropriate meshing and labeling is the key to proper model-ing and design. Loads are only transferred to the wall at the corner points of the area objects that make up the wall. Gener-ally the membrane or shell type element should be used to model walls. Here the shell type is used for modeling the wall element. There are three types of deformation that a single shell element can experience axial deformation, shear defor-mation and bending deformation as shown in Fig.3

Figure 1: Shell element

Material name Concrete

Type of material Isotropic

Mass Per Unit Volume

2.5 kN/m3

Modulus of elasticity 32 kN/mm2

Poisson’s ratio 0.2

Concrete strength 30 MPa

Section name Wall

Wall thickness 150 mm

Table 1: Material and element property for wall element

2. Grid lines are made for the x, y and z coordinates and the wall is drawn from scratch.

3. Boundary conditions are assigned to the nodes wherever it is required. Boundary conditions are assigned at the bot-tom of the wall i.e., at ground level where restraints should be against all movements to imitate the behavior of shear wall.

4. The material properties are defined such as mass, weight, modulus of elasticity, Poisson’s ratio, strength character-istics etc. The material properties used in the models are shown in Table.1

5. The geometric properties of the elements are dimensions for the wall section.

6. Elements are assigned to element type, as shown in Table.27. Loads are assigned to the joints as they will be applied in

the real structure.8. The model should be ready to be analyzed forces, stresses

and displacements.In ETABS single walls are modeled as a pier/spandrel sys-

tem, that is, the wall is divided into vertical piers and horizontal spandrels. This is a powerful mechanism to obtain design mo-

Fig.2 A Typical Floor plan of structure under consider

a) Axial Deformation b) Shear Deformation c) Bending DeformationFig.3 Deformation of a shell element

Wall pier forces are output at the top and bottom of wall pier elements and wall spandrel forces are output at the left and right ends of wall spandrel element, see Fig.4

Fig.4 Pier and Spandrel forces in ETABS

At the upper level of this model, pier P1 is defined to ex-tend all the way across the wall above the openings. Pier P2 makes up the wall pier to the left of the top window. P3 occurs between the windows. Spandrel labels are assigned to vertical area objects (walls) in similar fashion to pier labels. The pier

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and spandrel labels must be assigned to wall element before performing analysis.

The lateral load analysis that is seismic and wind analysis requires certain parameters to be assigned in ETABS. These parameters are listed in table.2

Results and Discussion

Shear wall structure having G+11 storey is analysed for garvity and latral loads. The effect of axial force, out of plane moments, lateral loads, shear force, storey drift, storey shear and tensile force are observed for different stories. The analy-sis is carried out using ETABS and data base is prepared for different storey levels as follows:

1. Effect of axial force on shear wall:

The load bearing wall structure mostly caries axial com-

Seismic coefficientsAS PER IS: 1893-2000

Wind CoefficientsAS PER IS: 875-1987

Seismic Zone Factor 0.1 Wind speed (Vb) 50m/s

Soil Type III Terrain Category I

Importance Factor (I) 1 Structure Class B

Response Reduction (R) 3 Risk Coefficient k1

factor 1

Topography k3 factor 1

Windward coefficient 0.8

Leeward coefficient 0.5

Table: 2 Seismic and Wind parameters

STOREY WALL LOCATION AXIAL COMPRASSION LOAD (KN) OUT OF PLANE MOMENTS (KN-M)

12Top 15.358 20.010

Bottom 57.277 21.573

11Top 91.473 -37.385

Bottom 131.874 34.478

10Top 170.653 -42.314

Bottom 209.962 45.532

09Top 253.931 -46.156

Bottom 291.969 57.054

08Top 340.620 -47.442

Bottom 377.376 68.345

07Top 430.030 -46.705

Bottom 465.494 79.316

06Top 521.423 -46.841

Bottom 555.598 89.867

05Top 614.088 -55.166

Bottom 646.985 100.005

04Top 707.363 -63.545

Bottom 739.008 109.844

03Top 800.846 -71.943

Bottom 831.300 120.038

02Top 894.543 -80.360

Bottom 924.026 132.461

01Top 994.804 -89.367

Bottom 1026.764 142.603

Table: 3 Axial force and out of plane moments for different storey levels

STOREY MAXIMUM TEN-SILE FORCE (kN)

MAXIMUM SHEAR FORCE (kN)

STOREY DRIFT(mm)

Lateral loadIn (kN)

STOREY SHEAR (kN)

12 -16156.865 -907.77 0.199 736.67 -608.25

11 -35756.738 -2012.3 0.199 734.36 -598.27

10 -51933.454 -2925.14 0.201 730.37 -1337.36

09 -65018.616 -3664.54 0.2 604.65 -1946.62

08 -75343.36 -4248.75 0.197 494.90 -2436.02

07 -83237.752 -4696.04 0.189 387.14 -2855.50

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pression force and transfer on to the foundation. The entire vertical load of all the stories is carried by ground floor load bearing wall. In order to design that wall it is quite essential to understand the variation of axial force in the walls. This force in the shear wall is from worst load combination of gravity and lateral loads. For the worst load combination, the axial force in the wall is plotted on y-axis against at each storey level. From Fig.5, it is observed that maximum axial force in storey one is 1026.764 kN. The difference in maximum axial force between storey 11 and 12 is 7.26%. It indicates that the variation in maxi-mum axial force with storey level is linear for worst load com-bination.

06 -89030.468 -5024.66 0.177 293.35 -3125.17

05 -93048.654 -5252.87 0.16 217.52 -3334.92

04 -95617.871 -5398.93 0.138 151.66 -3504.71

03 -97062.088 -5481.08 0.11 97.78 -3604.59

02 -97703.854 -5517.6 0.077 55.88 -3634.55

01 -97864.264 -5526.73 0.036 25.94 -3674.50

Table: 4 Shear force and displacements for different storey levels

Fig.5 Axial force on shear wall

2. Effect of out-of-plane moments on shear walls

Load bearing RCC walls are slender compression ele-ments subjected to in and out-of-plane bending. For the worst load combination, out-of- plane moments in the wall is plotted on y-axis against at each storey level. it is concluded from Fig.6 that the maximum out-of- plane moments in walls of storey one is 142.603kN-m. The difference in maximum out of plane moment between storey 11 and 12 is 9.04% .It indicates that the variation in maximum out of plane moment with storey level is linear for worst load combination.

Fig.6 Out of plane moments on shear walls

3. Effect of storey lateral load on shear wall :

Most lateral loads are live loads whose main component is horizontal force acting on the structure. The intensity of these loads depends upon the building’s geographic location, height and shape. For the worst load combination lateral load in the wall is plotted against each storey level. From Fig.8, it is ob-served that maximum lateral load in storey 12 is 736.67 kN. The difference in maximum lateral loads between storey 11 and 12 is 0.54%. It is observed form fig.7 that this is non-linear variation of lateral load.

Fig.7 Lateral loads on shear walls

4. Effect of shear force on shear wall:

Shearing forces are unaligned forces pushing one part of a body in one direction, and another part the body in the op-posite direction. For the worst load combination shear force in the wall is plotted against at each storey level. From the Fig.8, it is observed that maximum lateral load in storey one is 5526.73 kN. The difference in maximum lateral loads between storey 11 and 12 is 19.98%. It indicates that the variation in maximum shear force with storey level is non-linear for worst load com-bination.

Fig.8 Shear force on shear walls

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5. Effect of storey drift on shear wall:

One of the major shortcomings high-rise structures is its increasing lateral displacements arising from lateral forces. For the worst load combination storey drift in the wall is plotted on y-axis against at each storey level. From the Fig.9, it is ob-served that maximum storey drift in between storey 12 is 0.199 mm. It indicates that the variation in maximum storey drift with storey level is non linear for worst load combination.

Fig.9 Storey drifts on shear walls

6. Effect of Storey shear on shear wall :

For the worst load combination storey shear in the wall is plotted on y-axis against at each storey level. From the Fig.10, it is observed that maximum storey shear in storey one is 608.25kN. It indicates that the variation in maximum storey shear with storey level is non linear for worst load combination.

Fig. 10 Storey shear on shear walls

7. Effect of tensile force on shear wall :

The tensile force is the maximum stress that a structure can withstand while being stretched or pulled before failing or breaking. Tensile strength is the opposite of compressive strength and the values can be quite different. For the worst load combination tensile force in the wall is plotted against at each storey level. From the Fig.11, it is observed that maximum tensile force in storey one is 97864.264 kN. The difference in maximum tensile force between storey 11 and 12 is 20.02% .It indicates that the variation in maximum tensile force with sto-rey level is non-linear for worst load combination

Summary and Conclusion

In this present work ETABS is used to analysis the shear wall structure of G+11 considering the gravity and lateral loads. The following conclusion is drawn from present work.

1. The variation of axial force with stories is linear. The differ-

ence in maximum axial force between storey 11 and 12 is 7.26 %.

2. The variation of out-of-plane moment with stories is linear. The difference in maximum out-of-plane moment storey 11 and 12 is 9.04 %.

3. The variation of lateral loads with stories is non-linear. The difference in maximum lateral loads between storey 11 and 12 is 0.54 %

4. The variation shear force with stories is non-linear. The dif-ference in maximum shear force between storey 11 and 12 is 19.98 %.

5. Variation of storey drift with storey is non-linear. The maxi-mum storey drift in storey 12 is 0.199 mm.

6. Variation of storey shear with storey is non-linear. The maximum storey shear in storey one is 608.25kN.

7. The variation of tensile force with stories is non-linear and the difference in maximum tensile force between storey 11 and 12 is 20.02 %

References

1. Wdowicki, J. and Wdowicka, E. (1993) “System of programs for analysis of three-dimensional shear wall structures” The structural design of tall buildings, Vol.2, pp 295-305.

2. Benjamin, J.R. (1968) “variability analysis of shear wall structures” Earth-quake Engineering Research vol2, pp B3-45.

3. Musmar, M.A. (2013) “Analysis of shear wall openings using solid65 ele-ment” Jordan journal of civil engineering, vol 7, no.2.

4. Thakkar, B.K. (2012) “Analysis of shear walls under compression and bend-ing” Current trends in technology and science vol: 1, Issue: 2.

5. Hauksdottir, B. (2007) “Analysis of a reinforced shear wall” M.Sc Thesis, DTU6. Bozdogan, K.B. and Ozturk, D. (2010) “Vibration analysis of asymmetric

shear wall structures using the transfer matrix method” Iranian journal of science & technology, transaction, Vol.34, No.B1, PP1-14.

7. Xiaolei, H., Xuewei, C., Cheang, J., Guiniu,M. and Peifeng, W. (2008) “Nu-merical analysis of cyclic loading test of shear wall based on openSEES” World conference on earthquake engineering.

8. Carpinteri ,A., Corrado ,M., Lacidogna, G. and Cammarano, S. “Lateral load effect on tall shear wall structure of different height” Structural engineering and mechanics, vol. 41, No.3 PP 313-337.

9. Biswas, J.K. (1974), “Three dimensional analysis of shear wall multi storey building” Opendissertations and theses.

10. Greeshma, S., Jaya, K.P and SheejaA, L. (2011) “Analysis of flanged shear wall using ANSYS concrete model” International journal of civil and struc-tural engineering vol.2, No.2.

11. Fahjan, Y.M., Kubin, J. and Tan, M.T., (2010) “Nonlinear analysis method for reinforced concrete buildings with shear walls” ECEE 14.

12. Habibullah, A., S.E (2007) “Physical object based analysis and design model-ing of shear wall system using ETABS” computers & structures

13. Dar, O.J. (2007) “Analysis and design of shear wall-transfer beam structure” boring pengeshan status thesis.

14. Potty, N.S., Thanoon,W.A., Hamzah, H.H. and Hamadelnil, A.M.M. (2008) “Practical modeling aspects for analysis of shear wall using finite element method” International conference on construction and exhibition w.

Fig.11 Tensile forces on shear walls

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Influence of Steel Fibres, Used in Conjunction With Unconfined Rebar Configurations, on the Structural Performance of Precast ElementsGary P. Robinson*, Alessandro Palmeri1 and Simon A. Austin1 *Centre for Innovative and Collaborative Engineering (CICE), Loughborough University, Sir Frank Gibb Building, Loughborough, UK1Dep. Civil and Building Eng., Loughborough University, Sir Frank Gibb Building, Loughborough, UK

A joint experimental and computational research program has been carried out to demonstrate the potential ben-efits of using Steel Fibre Reinforcement (SFR) within

the design and manufacture of two key structural elements, namely slender walls and thin lintels with dapped ends, often adopted within the pre-cast concrete industry. The investiga-tions specifically focus on the advantages of utilising SFR in conjunction with traditional bar reinforcement in an unconfined layout. This configuration allows cost savings in regards to pre-cast manufacture and enjoys good performance in terms of du-rability and fire resistance, though its use is currently limited by the brittle mode of failure. The paper sets out to prove that the inclusion of SFR within the concrete matrix is capable of induc-ing a more ductile response in the structural members under consideration, therefore potentially making it possible to justify the adoption of such unconfined layouts in the design practice.

Historical testing and research studies [1],[2] [3] have dem-onstrated that the adoption of single, centrally placed or mini-mum reinforcement configurations in RC wall elements, which are subjected to an eccentric axial load, results in a sudden and brittle failure mechanism. In addition, research undertaken to date [1] has also shown the ‘flexural cracking’ response of the slender RC wall elements to be critical in determining the re-sulting buckling behaviour and ultimate failure load of the panel. This is opposed to the more conventional assumption that the element’s capacity and response can be found by consideration of the component’s ultimate flexural capacity. This method however, has been shown to only be suitable for sections using a double layer of confined longitudinal reinforcement, where the longitudinal reinforcement ratio of this section ( = As lt ) is greater than 1% [3], where s A l is the cross-sectional area of reinforcement per unit length of the panel and t is the thickness of the panel. The term flexural cracking is used here to describe the situation where the concrete section at the critical location cracks in flexure (and the resulting concentrated loss of stiff-ness, combined with the lack of influential tension steel) con-trols the resulting structural behaviour and ultimate stability of the panel much more than would occur with doubly reinforced panels, where =As/ lt 1% [4]. Hence, the axial capacity of the RC wall element becomes dependent on the element’s flexural stiffness up to and post cracking. Consequently, appropriate ac-count now needs to be taken of the contribution of the concrete

acting within both the tension and compression stress block as part of the design of the element. Further, this flexural cracking response has been shown to control the response and capac-ity of centrally reinforced panel elements adopting unconfined rebar configurations, up to a steel ratio of =As/ lt 3%[5].

Thus the controlling failure mechanism of the identified RC wall elements will, in part, be influenced by the formation and subsequent progression of flexural cracks in the concrete at the panel’s critical section. It follows therefore that if, as argued, the initiation and behaviour of such cracks in the concrete sec-tion can be considered to be significant when determining the structural response of such panels, the incorporation of steel fibre reinforcement should therefore be seen to substantially influence the resulting behaviour and ultimate capacity of the panel elements under consideration. This is because the use of SFR concrete mixes has been shown to bring about a number of improvements in the mechanical performance of concrete, relating to aspects such as: a delay in micro-crack propagation to a macroscopic scale, the hindrance of macroscopic crack development and an improved structural ductility [6]. Aimed at demonstrating, as well as better understanding and designing for this predicted influence, the paper summarises the results of experimental and computational analyses for the relevant panel types and SFR concrete mixes.

From the literature reviewed as part of this investigation, few resources or research studies appear to currently exist, which aid in the design of slender panel elements, using a com-bination of both SFR and the traditional longitudinal reinforce-ment configurations proposed. Aimed at improving this current situation, the paper proposes and evaluates the possible use of a computational procedure, in which ‘lumped plasticity’ is used to predict the behaviour and buckling capacity of the resulting structural members. The method has previously been shown to provide a good correlation for slender precast panel elements, albeit for test samples adopting only a traditional unconfined reinforcement configuration and a standard (C40/50 grade) concrete mix design [3]. It is believed however, that if this design method is suitably modified to account for the SFRC material behaviour, the proposed technique could also be used to derive a design capacity for the panel elements adopting the hybrid of reinforcement types considered. The method utilises a non-lin-ear fibre hinge at the known critical cross section of the panel, in

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order to simulate the buckling response of the slender walls.The second aspect of the paper considers pre-cast lintels,

supported on end projections that have been reduced in height. Such ‘dapped end’ or ‘halving joint’ details are common in pre-cast construction because they beneficially lead to a reduction in the construction depth required. The experimental investiga-tion undertaken therefore aims to increase the understanding of the shear behaviour and capacity of these resulting discon-tinuity shear or ‘D-regions’, for situations in which: a centrally placed, unconfined and welded reinforcement mesh is to be used in conjunction with varying percentages of additional steel fibre content. Additionally, the structural testing undertaken will also aid in the development and verification of an analytical Strut-and-Tie Model (STM), capable of accounting for the use of such a non-traditional reinforcement strategy.

2 Current Limitations of Existing Design Methods in Relation to Unconfined and Steel Fibre Reinforcing Strategies

2.1 Design of Eccentrically Loaded Precast RC Panels

Both the major national codes of structural design practice reviewed (ACI-318 [7], EC2 [8]) currently devote specific sec-tions to the design and detailing of simply supported RC wall panels, subjected to an eccentric axial load. Each of the speci-fied design standards allows for the design of such elements through the adoption of one of two possible design methods. The first of these alternatives involves the use of simplified de-sign equations that have been empirically (or semiempirically) derived from a limited amount of experimental data [9]. These expressions however, allow no account to be taken in regards to either the quantity or the distribution of longitudinal rein-forcement. Also, the simple design equations do not currently allow for or enable the modification of the concrete material model, required in this instance to account for, and potentially take advantage of, the modified concrete behaviour due to the presence of the steel fibres within the concrete mix. In addition, the existing empirical design equations do not currently allow for design situations in which the eccentric load application is required to fall beyond the ‘kern point’ of the section. That is, the largest off-set at which a load can be applied to a section without it developing tensile stresses. One such load case is however, investigated as part of this study in order to assess the ability of, and therefore the potential for using the proposed hybrid reinforcement configurations to resist a larger, non-standard value of load eccentricity.

One potential alternative design method however, currently available within each of the regulatory guides considered [7-8], is the consideration of the wall component as a column of an ‘equivalent’ structural width. This method, prima facie, appears to potentially offer a suitable design method, for the hybrid panels under consideration. This is because, it would enable the engineer to account for the necessary modification to the concrete material model, as well as being able to include for the longitudinal reinforcement quantity and its distribution. By using this method, one could also allow for a load applied at the larger eccentricity. However, the use of this method requires the buckling failure load of the panel element to be dependent upon, and thus determined through consideration of, the flex-ural capacity of the component’s cross section [3]. As defined within section 1, this is not true for the minimally and centrally reinforced panels that are the focus of this study. Therefore, neither of the existing design procedures currently available,

appear suitable for the design of panels reinforced through a combination of minimum, centrally placed and unconfined lon-gitudinal re-bar, with secondary reinforcement also provided by using a quantity of SFR.

2.2 Strut and Tie Design for D-Regions

The strut-and-tie analytical model is an extension of the Ritter-Mörsch truss analogy, with particular application to the shear design of discontinuity regions (D-Regions) in cracked reinforced elements [10]. The model assumes that structural loads are carried through a set of compressive stress fields and interconnected tensile ties. Previous studies ([11],[12]) have demonstrated that the use of steel fibre reinforcement, in con-junction with traditional longitudinal reinforcement, significant-ly improves the capacity of the D-regions considered within the precast structural elements. However, the past investigations do not consider the validity of adopting an STM in their design. Hence, of particular interest as part of this study is; how a tradi-tional STM analytical model should be modified or augmented to suitably account for the behaviour and failures observed, when adopting the hybrid reinforcement proposed, within the critical structural regions?

Another important consideration in adopting the STM methodology, as part of the development of an acceptable de-sign for the proposed precast lintel elements, is that due to the lower-bound nature of the method, a number of potential (or compliant) models are possible. However, a poorly selected and detailed strut-and-tie model may potentially result in se-vere damage and cracking to the element, even under service loading [13]. Because of this, the experimental investigation and validation of any potential STM analytical model is therefore considered as an essential component in the development of a design procedure for the precast dapped end beams.

3 Experimental Investigation

3.1 Test Samples and Experimental Arrangements

Eight 450mm wide, 100mm thick and 3000mm tall panel el-ements were cast adopting C40/50grade concrete mix (500kg/m3 CEMI, 840kg/m3 Gravel<20mm, 900kg/m3 Sand<4mm, 0.8% Superplasticizer, w/c=0.36, Flow=650-700mm). Four of the samples were reinforced solely using a single, centrally placed layer of mesh reinforcement to form the unconfined reinforce-ment configuration illustrated in Figure 1(d). The four additional panels tested adopted an identical reinforcement configuration

Figure 1: Experimental arrangement (a); Test Rig Elevation (b); Test Rig Sec-tion (c); Pin Joint Loading (d); Reinforcement Cross Section

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to that illustrated although, in these cases, an additional steel fibre content (1% by volume) was also incorporated within the specified mix design. In this way, the potential for any improved performance through the use of such a hybrid reinforcing strat-egy will be quantified, relative to the conventionally reinforced panels. The double hooked end type fibres used were: 50mm long, 0.75mm in diameter, had an aspect ratio of 67mm and a tensile strength greater than 1100N/mm2.

The eight panel elements were then axially tested, using the experimental setup illustrated within Figures 1(a) and (b). The testing rig used for the experiments was capable of apply-ing a load of 4000kN, with the loading beam designed in order to ensure the transmission of a uniformly distributed load across the top of each panel at eccentricities of 17mm(t /6) and 33mm(t /3) . The smaller of the adopted eccentricities was chosen to re-flect the maximum load off-set allowed for within the major in-ternational design regulations (t/ 6) investigated [7-8]. This limit on load eccentricity is commonly referred to as the ‘kern point’ and has been widely adopted as part of a number of experimen-tal studies into the axial capacity of one-way spanning panel elements [2-5]. Additionally, a load case involving a larger ec-centricity (t /3) has also been incorporated as part of this study, in order to investigate whether the use of SFRC in conjunction with un-confined longitudinal reinforcing steel could potentially offer an engineer the opportunity to justify the use of such panel elements for resisting such a demanding loading condition

The top and bottom hinged support conditions were each simulated by placing a 25mm high strength steel rod on a 50mm thick bearing plate (Figure 1(c)). Displacement trans-ducers were utilised at the locations illustrated within Figure 1(b) in order to record out-of-plane displacements at the centre and top of the panel, as well as providing a means of determin-ing the rotation at the top of the wall. Strain readings were also taken utilising a digital portal gauge at the known critical sec-tion (i.e. the mid-span of the RC wall element). This allowed the strains induced at this section to be recorded as the axial load was incrementally increased.

As part of the secondary focus of the experimental study, four precast lintel elements were additionally cast and tested to failure. The geometry of the specimens tested and the weld mesh reinforcement layout adopted are illustrated within Fig-ure 2. Because the objective of the experimental program is to study the behaviour of the D-Region of the precast lintel com-ponent, a member length of 1415mm was adopted so as to ensure that the region controlling the element’s capacity was that under investigation. All reinforcing bars used in the manu-facture of the samples were 16mm in diameter, with a cover of 25mm maintained throughout. The bars were MIG welded, with all anchorage forces and requirements appropriate to the resulting welds calculated in line with the relevant EC2 provi-sions [8].

The testing of the beam samples in shear was undertaken using the experimental setup detailed within Figure 3, with a loading rate of 1 kN s adopted. Bearing plates with sizes of 100x100x12.5mm were used at both the support and loading positions in order to suitably spread the applied load and thus ensure the appropriate strut propagation within the sample. Digital strain gauges were used to collect data in regards to the strains at the surface of the sample continuously during test-ing. The positioning of the gauges was designed so as to collect results both for the tensile region at the re-entrant corner and over the primary compression strut that will form the dap. The rosette pattern adopted allowed the angle of principal stress in the half-joint detail to be calculated and recorded through-out the loading of the specimen. Consequentially this will allow the collected data, through the application of Mohr’s circle, to be used to validate the geometry of the adopted Strut-and-Tie model (STM). The digital strain gauges used were 60mm in length, with Figure 3 identifying the end locations of this instru-mentation.

Figure 2: Welded Mesh Reinforcement Configuration

3.2 Experimental Findings

Table 1 summarises the experimental failure loads ob-served for each of the panel elements tested. In addition Figure 4(d) details the measured relationship between the applied load and the deflection of the panel at its critical section, up until buckling failure occurred. It should be noted that the loads have been normalised (in order to allow an effective comparison of panel performance), according to the expression:

(1)

Where N is the axial load applied to the panel at the set ec-centricity (kN ), c f is the average measured concrete cylinder strength for the samples (N mm2 ), with L and t the width and thickness of the concrete wall elements respectively (mm).

As can be seen, the inclusion of the 1% volume fraction of steel fibre reinforcement in addition to the unconfined rein-forcement mesh traditionally adopted, leads to an increase in both axial load and deformation capacity of the panel. Both ef-fects appear to be more significant within the panels, to which the load was applied at an increased eccentricity. An average increase of 12% in normalised buckling capacity was seen for panels loaded at an eccentricity of 33mm (t/3), with the lat-eral deflection prior to failure increasing from a minimum of 10.5mm in the traditionally reinforced panel to a maximum of 20.55mm for a panel adopting the hybrid reinforcement op-tion considered. This increased lateral deflection could also be clearly observed for the SFRC panel elements, with a distinct bowing evident prior to the failure of the wall (Figure 4(c)). For panels loaded at an eccentricity of 17mm (t /6), a lesser average

Figure 3: Lintel Testing Schematic and Demec Rosette Detail (box)

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increase in normalised buckling capacity was recorded (9.8%). Lateral deflections of 17.51mm and 19.61mm were measured for panels SFR1 and SFR2 respectively compared to the mini-mum value of 11.02mm observed for Panel RC1.

The most significant difference in the behaviour of the two panel types investigated however, was perhaps associated with the buckling failure typologies observed for the hybrid and tra-ditionally reinforced elements. In the instances where a cen-trally placed, unconfined reinforcement layout was solely ad-opted the observed failure was of a sudden, brittle and explosive nature Figure 4(a). In contrast for the cases when a 1% volume fraction of the double hooked end steel fibres was incorporated, a much more acceptable (from a structural design perspective) ductile failure resulted.

Similarly, Table 2 details the failure capacities recorded for each of the six dapped-end lintel samples fabricated. For the control samples (RCL1 and RCL2) first cracking was seen to occur at the re-entrant corner, quickly followed by flexural cracking at the mid-span. As the loading was increased how-ever, the mid-span flexural cracking was seen to propagate at a rate greater than that which was observed at the re-entrant corners. It was then observed that both the samples exhibited a significant propagation of tensile cracking along the diago-nal compressive strut. This cracking next propagated upwards towards and subsequently along the beam’s top face. The pro-gression of this cracking was then observed to cause the brittle shear failure captured within Figure 5(a), with the concrete ma-terial forming the dap of the lintel, spalling away post failure to expose the welded mesh reinforcement. Interestingly, it was also observed that plastic hinges had formed within the longi-tudinal steel of the mesh, adjacent to the welded vertical bars. This perhaps indicates the potential failure mechanism for the sample.

Similar cracking patterns and propagation sequences were also then observed for the samples cast using a combination of a welded mesh and an additional content of steel fibre rein-forcement (samples SFRL 1-2). The first crack again occurred at the sample’s re-entrant corner and this was again followed by more extensive flexural cracking at the mid-span. However, a noticeably slower and less extensive crack propagation was observed for all samples adopting a percentage content of steel fibres relative those using the more traditional mix. This pro-vides evidence therefore that the content of steel fibres within the mix were acting as expected to provide a means of crack control. In addition to slowing crack formation the fibres also significantly reduced the level of the resulting spalling observed at failure (Figure 5(b)). Also worthy of note was that the extent of

flexural cracking away from the daps appeared to significantly multiply as the fibre content in the samples was increased.

Figure 5(c) illustrates the load deflection behaviour re-corded for each of the beam elements tested. Normalisation of loading values was undertaken in order to enable a compari-son between each of the samples in relation to how efficiently the steel weight incorporated is being used within each of the designs considered, as well as to allow for the variations in con-crete strength seen for the samples cast. The values were cor-rected according to the expression:

(2)

As would be expected, the plots of load displacement rela-tionship for the beam elements tested (Figure 5(c)) show that all samples had a similar elastic range. However, both samples incorporating the 1% volume of steel fibre content exhibited a much greater ductility, with the maximum deflection at the point of failure almost double that of the non-fibre samples. Such a response is indicative of the successful application of steel fi-bre reinforcement causing a more plastic/ductile response under loading and controlling the cracking, which would have otherwise resulted in failure. An average increase of 32.1% in

Figure 4: Eccentrically loaded panels (a): Brittle failure of traditional RC pan-els (b); SFRC panel section failure (c); Increased lateral deflection of SFRC panel prior to failure (d); Experimental load-deflection curves for panels with varying eccentric load and use of SFR

Figure 5: Brittle failure of traditional RC lintel (a); SFRC Lintel Failure (b); Ex-perimental load-deflection curves for traditional and hybrid lintel samples (c)

ElementRef

f2 (N/ mm2) e(mm)

Nu(kN)

Test Comp

RCW1 37.28 17 597 531

RCW2 37.28 17 572 531

RCW3 38.48 33 336 302

RCW4 38.48 33 322 302

SFRW1 40.21 17 713 623

SFRW2 40.21 17 689 623

SFRW3 41.11 33 407 345

SFRW4 41.11 33 394 345Table 1: Panel buckling capacities

Element Ref f2 (N/ mm2)Nu(kN)

Test Comp

RCL1 61.28 190 194

RCL2 32.96 100 124

SFRL1 42.16 175 158

SFRL2 32.96 140 124

Table 2: Lintel shear capacities

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normalised shear capacity was also measured for the SFRC halving joints.

4 Proposed Design Methods for Precast Elements Adopting Hybrid Steel Fibre and Unconfined Reinforcement Configu-rations

4.1 Lumped Plasticity

Lumped plasticity idealisation is a widely adopted compu-tational model, particularly utilised in earthquake engineering and robustness assessment, in order to determine the ultimate performance of a structural system by increasing step by step the load multiplier until failure (push-over or pushdown analy-sis). It has been demonstrated within previous studies [1] [3] that it is possible to consider, as part of a computational as-sessment, the entire inelasticity of an RC panel element to be concentrated at the critical section for the span, with this ‘lumped plasticity’ modelled through the use of a non-linear hinge (Figure 6(a)).

Such a computational model is effective for the cases con-sidered as part of this study, because the location of the maxi-mum moment (and thus the critical section) is known for the simply supported elements. In this representation the compo-nent’s cross section is subdivided into a number of elements or fibres, to which the appropriate material properties are then assigned (Figure 6(b)). In this way, the non-linear moment-cur-vature relationships and limits of the fibre hinge can then be determined for a range of axial loads (assuming plane cross sections). As such, the arrangement illustrated can therefore be used in order to provide an effective representation of sys-tem non-linearity, and consequentially, of buckling capacity.

Importantly, because the proposed computational method allows the designer to modify for the relevant concrete mate-rial model, it can therefore facilitate the incorporation within the analysis of other concrete types, such as the fibre reinforced mix adopted as part of this study. Therefore the Mander [14] model adopted for the unconfined concrete material within the tradi-tional RC panels was replaced by the material model suggested by Al-Taan and Ezzadeen [15] (Figure 6(c)) for fibre reinforced concretes adopting a 1% fibre volume fraction. Additionally however, in order to correctly quantify the rotational capacity of a concrete member, the length of the resulting plastic hinge (Lp) that will be formed during loading and subsequent failure must also be accounted for. Accordingly, the hinge lengths were computed for both panel types from the expression proposed by Panagiotakos and Fardis [16] for unconfined RC panels and column elements subjected to monotonic loading:

(3)

where Ls=H/2 is the shear span of the member, db=t/2 (for the panels considered as part of this study) is the effective depth of the reinforcement and fy is the yield strength of that reinforce-ment. As can be seen from Table 2, the resulting computational predictions for both the traditionally reinforced panels and those adopting the hybrid reinforcing strategy show a good cor-relation with the actual experimental capacities seen. This re-lationship is also illustrated within Figure 6(d) which shows the least-squares best fit to slope 1 = 0.833 and 2 = 0.846, for the RC and SFRC hybrid panel types respectively, to be acceptably close to the = /4 ideal. The poorer correlation seen within the panels where the secondary fibre reinforcement was incorpo-rated is likely due to the fact that a degree of calibration in rela-tion to the length of fibre hinge is required. However, a greater number of data points would be required in order to inform how Eq 3 should be modified to account for the use of SFRC.

4.2 Design Using Strut and Tie

To aid in the development of the proposed analytical strut-and-tie model for the beam elements considered, an elastic analysis was first undertaken in order to analyse the stress flows occurring, a method strongly advocated within existing literature [10]. A 2D finite element (FE) analysis was carried out, using shell elements due to the size of the section (100mm) in relation to the size of the shells considered. These stress flows were then used in the development of a relevant STM. Addition-ally the outputs of the FE model were used to verify the angle of the stresses against those obtained by experimental measure-ment, with the angle used for the analytical STM (59o) found to lie between the maximum measured angle of principal stress (52o) and that predicted through linear computational analysis (66o). The lower bound model developed is illustrated within Figure 7 (a) and compares well to those proposed within lit-erature [9] for concrete elements with a similar geometry and reinforcement provision. The precedent cited however, consid-ered the response of confined concrete without a steel fibre content.

A key assumption made when arriving at the most ap-propriate analytical STM, was regarding the width of the criti-cal compressive strut formed. Although the bearing plate was sized to spread loads across the full width of the beam it was assumed that the effective width was that confined by the welded mesh configuration (Figure 2). Therefore the width of concrete considered was that within the centreline of the rein-forcement bars, as this was felt to best represent the ‘pinching’ or confining point. The design model was then used to calcu-late the capacity of the section, with the theoretical predictions summarised as part of Table 2. Because the experimental work conducted identified that crushing of the primary compressive strut, positioned at the support bearing plate, resulted in ele-ment failure it could therefore be considered to be critical. It follows then that the size of this strut and thus the capacity of the section is then dictated by both the angle of the strut formed and the width of the bearing plate used. The remaining struts were still subsequently assessed for adequacy however, along with checks also required to ensure the tensile capacity of the reinforcement provided would not be exceeded within any of the associated ties.

Interestingly, and as can be seen from Table 2, the proposed

Figure 6: Lumped plasticity computational panel representation (a); Fibre hinge at critical panel section (b); Unconfined [14] and SFRC [15] material models (c); Comparison of theoretical and experimental panel capacities (d)

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STM overestimates the strength of the two samples adopting the welded mesh reinforcement without any additional steel fibre content by (2-24%). This is perhaps to be expected given the brittle nature of unconfined concrete and the sudden and explosive failure observed in the testing of the element. This finding perhaps indicates that unconfined concrete elements should not be designed using STM models without a further safety factor being applied to the current strut capacity equa-tion given within EC2 [8]:

(4)

where RD,max is the allowable axial stress within the com-pressive strut, fc is the concrete cylinder strength th and =1-(fc/250) is a reduction factor applied for cracked compres-sion zones within the Eurocodes. In contrast however, the STM model for samples where a 1% content of SFR, by volume was incorporated, tends to underestimate the capacity of the ele-ment by an average of 12%. This suggests that the use of stan-dard STM design is valid for situations in which un-confined reinforcement configurations are adopted and perhaps even indicates that a beneficial factor of safety could be applied to the strut capacity expression (Eq 4) for such design cases. How-ever, a much larger degree of testing would be required before any such conclusions or design recommendations could be provided. A potential need for such further investigation and the establishment of more appropriate correction factors is well illustrated by the comparison of actual lintel capacities to the ideal least squares correlation illustrated in Figure 7(b).

5 Conclusions and Recommendations for Future Work

The paper demonstrates that the incorporation of Steel Fi-bre Reinforcement (SFR) has significant effects on the structur-al performance of both eccentrically loaded panels and shear discontinuity regions for precast elements adopting unconfined configurations for the traditional bar reinforcement. The paper also shows the effectiveness of design methods that could en-able an engineer to justify the use of such hybrid reinforcing strategies in practice.

As far as the slender wall elements are concerned, the in-troduction of SFR was seen to increase both axial capacity and structural ductility for load eccentricities of e=t/6 and e=t/3, with a more significant improvement in the latter case. Moreover, an improved (and more acceptable) failure mechanism was observed, when compared to the sudden, brittle failure seen in the control samples. Lumped plasticity idealisation and fibre-hinge elements were shown to provide a good correlation with the experimental data relating to the singly and centrally rein-forced panels adopting both traditional and SFR concrete mix

alternatives. However, the computational method was found to be less effective in presence of steel fibres as secondary rein-forcement, suggesting that further testing is required in order to calibrate the length of the fibre hinge.

As far as the lintels with dapped ends are concerned, it has been similarly shown that the introduction of SFR leads to in-creased capacity and ductility. This is believed to be because the fibres act to control cracking at the re-entrant corner, in-ducing a greater degree of flexural action prior to failure. The investigations conducted have also developed and validated a suitable Strut-and-Tie Model (STM) for the design of halving joint details where an unconfined steel reinforcement layout is adopted, which however tends to overestimates the actual ca-pacity. The findings also suggest that a modification (or safety) factor should be applied to the strut element to account for the brittle nature of the unconfined concrete without SFR. In con-trast however, when a 1% volume of double-end hook SFR were introduced in the mix, the use of the STM design method could be justified, with the experimental values also indicating that a beneficial modification factor could be warranted. Also in this case, further testing would be required in order to adequately demonstrate and quantify what the value of such a beneficial factor should be.

References

[1] G.P. Robinson, A. Palmeri and S.A. Austin, Tension Softening Effects on the Buckling Behaviour of Slender Concrete Wall Panels, Proc. of ISEC-6 Modern Methods and Advances in Structural Engineering and Construction, Zurich (2011)

[2] J.H. Doh, S. Fragomeni, Evaluation of Experimental Work on Concrete Walls in One-Way and Two-Way Action, Aus. J. Struct. Eng, 6(1), 103-115 (2005)

[3] G.P.Robinson, A.Palmeri, S.A.Austin Design Methodologies for One Way Spanning Eccentrically Loaded Minimally or Centrally Reinforced Pre-Cast RC Panels, J. of Engineering Structures, Currently Under Review (2011)

[4] K.M. Kripanarayanan, Interesting Aspects of the Empirical Wall Design Equation, ACI Stuct. J, 204-207 (1977)

[5] S.U. Pillai, C.V. Parthasarathy, Ultimate Strength and Design of Con-crete Walls, J. of Bld. and Env, Vol 12, 25-29 (1977)

[6] H.H. Abrishami, D. Mitchell, Influence of Steel Fibers on Tension Stiff-ening, ACI Struct J, 769- 776 (1997)

[7] American Concrete Institute, ACI 318-05 Building Code Requirements for Structural Concrete, Farmington Hills (2005)

[8] Comité Européen de Normalisation, EN 1992-1-1 Eurocode 2 Design of Concrete Structures Part 1-1 General Rules for Building, Brussels (2004)

[9] J.K. Wight, J.G. MacGregor, Reinforced Concrete Mechanics and De-sign, 5th Edition, Pearson Education International, San Jose (2009)

[10] J. Schlaich, K. Schafer, M. Jennewith, Towards a Consistent Design of Structural Concrete, PCI Journal, 32(2), 74-150 (1987)

[11] D.R. Sahoo, S.H. Chao, Use of Steel Fiber Reinforced Cocncrete for En-hanced Performance of Deep Beams with Large Openings, Proc. of ASCE 2010 Structures Congress, 1981-1989, Orlando (2010)

[12] Z. Fu, Use of Fibres and Headed Bars in Dapped End Beams, Masters Thesis, McGill University, Montreal (2004)

[13] D. Kuchma, S. Yindeesuk, T. Nagle, J. Hart Experimental Validation of Strut-and-Tie Method for Complex Regions, ACI Structural J., 105(5), 578-589 (2008)

[14] J.B.Mander, M.J.N Prestly, Park R Theoretical Stress-Strain Model of Confined Concrete, J. of Structural Engineering, 114(8), 1804-1826 (1988)

[15] S.A. Al-Taan, N.A. Ezzadeen Flexural Analysis of Reinforced Fibrous Concrete Members Using the Finite Element Method, J. of Computers and Structures, 56(6), 1065-1072 (1995)

[16] T.B.Panagiotakos, M.N. Fardis, Deformations of Reinforced Concrete Members at Yielding and Ultimate, ACI Struct. J., 98(2), 135-148 (2001) w

Figure 7: Proposed STM for the design of lintel members (a); Comparison of theoretical and experimental lintel capacities (b)

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Practical and Economical Design Aspects of Precast Concrete Large Panel Building Structures

Bob van Gils (Director)WBK Engineering Services Pvt. Ltd. and Van Boxsel Engineering Pvt. Ltd.

Abstract: Precast concrete shear walled structures, also called large panel systems, are a good solution for multistoried resi-dential and commercial buildings. This paper describes the practical and economical aspects of designing and construct-ing these kinds of structures.

The large panel systems are made of large precast walls and slabs that are connected to each other in vertical and hori-zontal direction. The precast wall panels should be load bear-ing members and shall be capable of carrying the vertical and lateral loads. The wall panels can be connected to each other in various ways and together with the floor diaphragm they will form box type structures (figures 1 to 4). The external precast wall panels shall be a finished product and no cement plaster shall be required.

The precast concrete structures with load bearing wall panels have several advantages compared to RCC frame structures.

- No brickwork infill walls are required- Superior quality and durability of the high grade concrete

panels- No plastering required on the precast panels

- Saves time and manpower- The thin precast wall structure increases the carpet area- Better health and safety standards for laborers during

constructionAs quality and speed of construction are becoming more

important for builders the precast large panel system could prove to be a viable solution. But in every building project the following aspects are important.

- Architecture- Structure- MEP Services- Manufacturing- Erection

The importance of these aspects shall be briefly explained in this paper.

Architectural design aspects

Full advantage of precast concrete construction can be achieved when the buildings shall been designed for high con-struction speed and maximum repetition. The architect should be considering the following points in his design:

- Integration of architecture, services and structure has to be achieved.Figure 1. Large precast panel construction

Figure 2. Large precast panel construction

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- Prepare design with simple and symmetrical layouts and elevations

- Avoid many offsets and re entrant corners in the building plans

- Achieve standardization and repetition in the precast ele-ments

- Use modular grids with multiple spacing of 1200mm for standard slabs

- In case of standard precast slab sizes the modular design can have a big impact on the costing

- Minimize joints and plan location of joints- In façade minimize horizontal or low sloped elements that

can collect dirt- Keep precast elements as large as possible- Design should not be a conversion of cast in-situ structure- Not everything has to be made in precast- Explore the unique capabilities that can be achieved with

precast concrete- When using prestressed floor slabs prepare building lay-

outs with larger floor spans- Avoid last minute design changes when precast produc-

tion has started

Modular design

Modular design is important when the proposed precast system is utilizing standardized precast production methods with less flexibility. For example when using standard slab sizes the modular design is guided by the standard size of the precast slabs. Positioning and alignment of other precast ele-ments like walls, columns and beams has to be planned as per the modular system. It may prove cost effective to avoid offsets and align the load bearing structural elements. Modular de-sign principles can be strictly followed but give less freedom to the architect. In case other production methods are used with more custom components the architect shall have more freedom in his design.

In any kind of precast building the position of lifts, stair-cases and shafts are critical and have to be properly planned

where they do not complicate the layout of the load bearing walls and precast floor slabs.

Architectural features and finishes

The exterior of the large panel buildings will be formed by the load bearing precast wall panels. The concrete surface of the panels will be exposed and can have factory made finish-ing like sandblasting, polishing, exposed aggregate finish and form line finishing. Various color finishes of the precast panels are possible by changing the type of cement, type of aggre-gates and by using pigments in the concrete mixture (figure 7). Precast wall panels can also be provided with false joints to achieve a better architectural design patterns (see figure 8).

Figure 3. Large precast exterior wall panels

Figure 4. Large precast interior wall panels

Figure 5. Modular design pos-sibilities

Figure 6. Mass production of standard slabs

Figure 7. Exposed aggregate finish Figure 8. False joints in precast panel

Cantilevered balconies can be made by providing canti-levered brackets on the precast cross walls and resting the balconies on these brackets. The precast balconies can also be made cantilevered with protruding top reinforcement con-necting to the rcc topping of the floor slab. Cantilevered sun-shades are a common feature in Indian building projects. The precast walls are generally made on flat steel moulds and it is not possible to make the sunshade as one part with the wall. Sunshades can be prefabricated and connected in a later stage to the precast walls.

Flexibility in layouts

Flexibility in the layout of precast concrete building projects can be achieved by creating larger floor spans with larger open spaces. Especially in office buildings this concept will provide a lot of advantages to the end user (see figures 9 and 10). The non load bearing partition walls can be made as light weight blocks, dry walls or other suitable light weight systems.

Figure 9. Flexibility with large open spaces

Figure 10. Flexibility with large open spaces

Structural design aspects

India being an earthquake prone country the seismic re-sistant requirements are the most important criteria of the structural design. Looking at the requirements we can draw the conclusion that the basic earthquake resistant design rules are suitable for precast concrete buildings. Generally the following design rules should be followed:

- Simple and symmetrical building layout

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- Uniform distribution of mass and structural stiffness over the height of the building.

- Avoid torsion- Achieve ductile behavior of the structure- Avoid progressive collapse of the structure

Simple, symmetrical and uniform buildings are normally easy to optimize and very much suitable for precast concrete construction.

Precast floor slab systems

Basically three different floor slab systems can be consid-ered for multistoried precast buildings.

1. Prestressed precast hollow core slabs2. Precast half slabs with lattice girder reinforcement3. Precast solid slabs

Prestressed precast hollow core slabs

These are prestressed floor slabs with longitudinal voids (figure 11). The presence of the voids results in material sav-ings and weight savings. With hollow core slabs large one way spans can be achieved and no temporary propping is required. Hollow core slabs only have longitudinal prestressing rein-forcement and no other reinforcement. Due to manufacturing methods it is not possible to make slabs with anchored tie bars, protruding stirrups or embedded welded plates. Diaphragm action is achieved through special joint design. Especially in high seismic zones an rcc topping has to be added to join the slabs and achieve proper diaphragm action (figure 12).

phragm action and prevention of progressive for multisto-ried buildings.

- In case of rcc topping the top surface of the hollow core has to be roughened.

- It is difficult to place MEP services within the hollow core slabs, services have to be placed either below or above the slab.

- Connection of the hollow core slabs to the shear walls has to be properly designed and detailed for transfer of vertical and lateral loadings.

- Pay attention to fixation of hollow core units in between load bearing walls. Provide extra top reinforcement at this location.

Figure 11. Hollow core slabs Figure 12. Hollow core with RCC top-ping

Structural design aspects for using hollow core floor slabs in multistoried residential buildings are as follows:

- Standard width of the slabs is 1200mm.- Some hollow core suppliers are also providing slabs of

2400mm.- For large scale projects with many repetitions it could be

useful to have a combination of 1200mm and 2400mm slabs.

- Floor slab layout has to be designed on a multiple grid of 1200mm and 2400mm.

- Slabs can be cut in longitudinal direction if required to achieve different size of slab. However longitudinal cutting should be avoided as much as possible.

- Minimum slab thickness can be 100mm but most manu-facturers offer minimum thickness of 120mm or 150mm.

- No propping required to support the hollow core slabs during the construction phase.

- Camber and deflection should be checked in design and detailing.

- Minimum 60mm rcc topping is recommended for dia-

Figure 13. Example of different hollow core slab sizes

Precast half slabs with lattice girder reinforcement

These are composite slabs made of precast concrete planks of 50mm thick with an rcc topping. The bottom rein-forcement is placed within the precast planks and the top reinforcement is placed within the rcc topping. Basically the composite slab behaves the same as an rcc one way slab or two way slabs. The precast planks serve as the shuttering and have to be supported during casting and curing of the concrete. It is a very flexible system where size of planks can be easily adjusted and MEP services can be placed in the rcc topping.

Figure 14. Example of 2400mm wide hollow core slab

Figure 17. Precast half slab with lattice girders

Figure 18. Precast half slab with lattice girders

Profile h (mm) b (mm)Weight

(joints filled kN/m2

Joint filling l/m2(*)

HC-200 200 1196 2,60 7,0

HC-265 265 1196 3,80 10,0

HC-320 320 1196 4,10 12,0

HC-400 400 1196 4,65 17,0

Figure 15 Figure 16

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Structural design aspects of precast planks with lattice girders:

- Standard width is generally 2,4m or 3,0m, but can also be customized to room sizes.

- Flexible system, any type of slab size can be made.- Bottom reinforcement is placed in the precast plank.- Top reinforcement is placed inside the rcc topping.- Minimum thickness is generally 50mm precast plank with

100mm rcc topping.- Propping of the slabs during casting and curing of concrete

is required.- MEP services can be placed inside the rcc topping.

- All reinforcement is placed inside the slab.- Connection to the precast shear walls can be by protruding

reinforcement like u-bars, reinforcement couplers or site welded steel plate connections.

- In case of proper detailing then rcc topping can be avoid-ed.

- All provisions for MEP can be placed inside the slabs.

Lateral load resisting system

The structural behavior of precast concrete large panel buildings with shear walls is different than rcc frame struc-tures. The shear walls are to be considered as cantilevering from the foundation (see figures 28 and 29).

The precast floor units have to be properly joined together to act as a floor diaphragm that transfers the lateral loads to the shear walls. The connections between the floor diaphragm and the shear walls have to be properly detailed. The shear walls will transfer the lateral loads to the foundation by acting as cantilevered walls.

Figure 19. Example of lattice girder reinforcement

Figure 20. Complicated slab layout Figure 21. Top reinforcement

Figure 22. Various precast slab shapes

Figure 23. Casting of concrete topping

Precast solid slabs

Precast solid slabs without topping (Figures 24 to 27) can come in different systems. The slabs are made as tradition-ally reinforced solid slabs that are generally supported by load bearing walls at all sides. If the connections with the walls are properly detailed and executed then rcc topping can be avoid-ed.

The connections between the solid slabs and the walls can be made by protruding reinforcement like u-bars or by site welded steel plates.

The solid slabs can be made on stationary steel tables and during erection can be supported by props or by erection angles. All the services can be embedded inside the solid slabs during the production process.

Design aspects of solid precast slabs:

- Size of the slabs can be customized.- Flexible system but weight of the slabs has to be checked

as they are heavier than hollow core or precast planks.

Figure 24. Solid slab with welded plates

Figure 25. Erection of solid slab with welded plates

Figure 26. Solid slab with protruding reinforcement

Figure 27. Solid slab with stitching reinforcement

Figure 28. Forces acting on shear wall Figure 29. Forces acting within shear wall

Connections

The wall panel connections can be classified into horizontal joints and vertical joints. The horizontal joints have to transfer vertical loads as well as lateral loads. The vertical joints can be open and not transferring any loads or they can be connected to transfer shear loads.

In many countries the horizontal joints between precast wall panels are made with grouted corrugated ducts. The pre-cast wall panels are lowered into position over the vertical rein-forcement bars which are protruding from the below element (see figure 30). The ducts and the horizontal joint are fully filled

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with non shrink high strength grout with at least 10MPa higher strength as the precast concrete. In the plastic hinge regions the ducts can be provided over the full height of the precast wall and the reinforcement bars can be lapped inside the duct. Another option is to use the splice sleeve type 2 connection according to ACI 318 (see figure 31). It can also be decided to design the bottom stories, where yielding will occur, in cast in-situ concrete.

be replaced by coupler bars (see figures 34 and 35). It is advised to use these connections only for internal shear walls as the vertical joint has to be finished with plastering at both sides and this requires a lot of extra work.

Figure 30. Connection through grouted corrugated ducts

Figure 31. Splice sleeve connections

Filling of joints

Filling of horizontal joints with non shrink high strength ce-ment based mortar or grout can be done in several ways:

1. Placing the precast wall in thixotropic mortar bed (see fig-ure 32)

2. Fill the joint with mortar by hand placement3. Pump thixotropic grout in the joint (see figure 33)4. Fill the joint with flowable grout5. Injection of flowable grout

Because of high temperatures in India and because clean filling has to be achieved it is advised to follow the third method and fill the joints by pumping thixotropic grout in the joints. Fill-ing of the corrugated ducts is generally done by pouring flow-able grout from the top or by injection/pumping from the bot-tom of the duct.

Vertical joints can either be structural joints which have to transfer shear forces or non-structural joints which don’t have to transfer any forces. In case fully monolithic behavior has to be achieved the best option is to use a protruding reinforce-ment connection in combination with drop-in stirrups. To ease the manufacturing process the protruding reinforcement can

Figure 32. Placing wall in mortar bed

Figure 33. Pumping grout in horizontal joint

MEP Services

In precast concrete building projects it is important that the MEP services consultants and the MEP vendors are part of the design team. Services like air-conditioning, electrical and plumbing have to be an integrated part of the precast design. For example wall panels can be provided with electricity con-duits, electricity boxes and openings for ducts (see figures 36 and 37).

Figure 34. Vertical connection detail Figure 35. Vertical connections between internal walls

Figure 36. Electricity in precast wall panel

Hollow core slabs can be provided with electricity boxes and block outs. Placing MEP services within the hollow core slabs is not possible. Services have to be place above or below the floor, special hangers can be used (see figure 38). Precast planks with rcc topping can be provided with electricity boxes and block outs. Furthermore small conduits, ducts and plumb-ing pipes can be embedded in the rcc topping (see figures 39 and 40)

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Precast manufacturing aspects

Basically there are two different types of precast plants which are the temporarily site based precast plant (casting yard) and the permanent precast plant. Furthermore we can differentiate between precast plants with ordinary reinforced precast concrete elements and plants where prestressed concrete elements are manufactured. In case prestressed concrete elements have to be produced the system usually requires long line beds on which the concrete elements will be formed either with casting machines or with shuttering sys-

tems. The prestressing steel has to be anchored in abutments which are heavy concrete foundations.

Some possibilities for precast factory equipment are:

- Stationary flat bed moulds- Tilting tables- Battery moulds- Central shifter system with pallets- Side shifter system with pallets- Carrousel system with pallets- Hollow core plant- Other customized solutions

Industrialized precast building systems are consisting of standard prefab elements made in standard moulds with minimum customization and suitable for mass production (see figure 41).

Customized prefabrication systems are methods where the precast elements are made according to a standard con-cept but with flexibility to customize according to the require-ments of the project. These customized systems require more flexibility in the shuttering and moulding.

The design and project team has to understand the capa-bilities and limitations of the precast manufacturing unit and following aspects have to be considered while designing.

- Type of factory? Conventional, semi automated or fully au-tomated precast plant?

- Ordinary reinforced precast concrete elements and/or prestressed concrete elements?

- Horizontal tables or vertical battery moulds (see figures 42 and 43)

- Wooden side shuttering or steel side shuttering?- Custom made wooden or steel moulds for special ele-

ments (see figure 44)- Minimum and maximum size and weight of the precast el-

Figure 38. Typical load hangers for hollow core slabs

Figure 39. MEP services in topping of plank floor

Figure 40. Plumbing pipes in topping of plank floor

Figure 41. Industrialized building system components

Figure 42. Circulating pallet system (flat moulds)

Figure 37. Several openings in precast wall panel

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ements- Production tolerances- Standard embedded parts like anchors, lifting eyes, rein-

forcement etc.- Minimum variation in embedded parts (see figure 45)- Avoiding penetrations through the mould- Stripping methods of the precast elements en conse-

quences for design- Shape of block outs and rebates - Chamfer the edges of wall panels to reduce edge damage

and to mask differences in alignment between panels at the joints.

- Detailing of the reinforcement cages- Curing and finishing methods

Figure 43. Battery mould (vertical moulds)

Figure 44. Custom made wooden mouldExecution / erection aspects

Erection of the precast structure will be done by building cranes which can be placed at a fixed location like tower cranes or by mobile cranes which can move around the building (see figure 46). In case of mobile cranes or crawler cranes there should be enough space to maneuver comfortably around the building. The speed of the crane often determines the speed of erection, especially in case of high rise structures where it takes more time to lift the elements. Another important aspect of the erection sequence is the casting of the rcc topping on hollow core or plank floors and should be well planned.

Design aspects:

- Transportation and temporary storage of the precast ele-ments

- Crane position and lifting capacities- Lifting speed and speed of erection- Lifting systems and safety aspects- Space for movement of mobile cranes or crawler cranes

(see figure 29)- Easy access to connections- Clean construction process- Tolerances- Easy and fast erection- Erection sequence- Grouting methods for joints- Casting of rcc topping (see figure 47)- Position of props and supporting structures- Sealing methods for joints w

Figure 45. Standard reinforcement couplers

Figure 46. Space for crawler crane

Figure 47. Casting rcc topping on precast slabs

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Hollowcore Manufacturing and Factory Design

Stephen Carr C Eng MI Mech ESpiroll Precast Services Ltd.

Abstract:This document is designed to provide guidance and outline the main considerations in the initial planning of a Hol-lowcore plant. The information presented is based on 40 years of Spiroll experience. To summarise the key points: -

- The Plant should be planned with both the short term and long term capacity targets for hollowcore manufac-ture based on beds numbers and length. The normal bed length can vary from 60 to 150 metres - 120 metres is the most common as it provides good flexibility and fits well in to the daily production cycle.

- The aim of any plant is to achieve maximum efficiency by filling every bed every day. To achieve this, adequate time must be allowed for curing the concrete, cutting the slabs to length, lifting the slabs and cleaning and preparing the beds for casting again. Of particular importance is the cur-ing time as the strength of the concrete must be adequate to hold the bond when the wires are cut. Before releas-ing the tension in the free strand to transfer the strain energy into the concrete, the concrete must have enough strength.

- The factory layout must also take account of finished prod-uct handling and storage, concrete batching and distribu-tion, and maintenance and service requirements.

- An economic approach to the investment is to have staged investment. The factory layout should then include provi-sion for immediate and future production levels so that the

production can grow to meet market demand without dis-rupting the ongoing production.

- A low cost start-up can also be considered with minimal plant. This can be achieved with mobile plant, initially han-dling of the concrete and the product can be with a five tonne forklift. This system can be replaced by a gantry cranes, overhead cranes or travel lifts in the future.

- This system using extruder casting machines with mobile plant can be extended to a mobile hollowcore production plant. In this way the plant is sited at the construction proj-ect site and moved when the project is complete.

Hollowcore Slabs

In terms of selling hollowcore slabs into your local mar-ket, the slab is a versatile precast element that can be utilised in a wide range of applications and thus expand the available markets.

Few building materials available today offer the economy, flexibility and reliability of precast prestressed concrete. The advantages of hollowcore slabs are significant for the following reasons: -

Durability: Hollowcore slabs provide long-term perfor-mance in extremely harsh conditions that could destroy lesser materials. It is extremely resilient to deterioration from the weather and the dense concrete and high cover to the steel allows design for high fire ratings.

Speed: Factory production of hollowcore allows the pro-

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ducer to have full control over all the variables, which affect the durability, strength and appearance of the slab. The high quality and excellent finishes of the slabs reduce site work to an absolute minimum.

Flexibility: Hollowcore slabs used for floors have good sof-fits which allow for direct application of ceiling finishes. They can also be used for wall panels. Speed and economy make them a good solution to which can add decorative finishes by using a thin layer of different aggregates and colours on the soffits and tops of the slabs.

Economical: Hollowcore slabs themselves are up to 30% lighter than the equivalent in-situ floor. With the prestress and the low self weight, longer spans can be achieved for the same loads or greater loads for the same depths. The build-ing foundations can be lighter as they are required to support less weight. Alternatively the number of supporting columns and beams can be significantly reduced. Hollowcore therefore gives the opportunity for longer spans, greater loads and less foundation costs.

Features of High Freq. Vibration

- Fire resistance (2-4 hours fire rating) depending on design- High density product- No strand slippage- Low cement content- Greater span/depth characteristics- Consistent camber- Greater span load characteristic- Eliminates costly propping during installation- All weather construction- Immediate working surface- Custom made to length and detail- Excellent sound barrier (due to hollows)- Carpet direct top surface- Speedy erection, reducing interim financing- Maintenance free- Economical long line Production- Unlimited design possibilities, compatible with almost all

building materials- Flexibility in design and application

In summary there are a variety of uses for hollowcore with applications for floors, roofs and wall panels being the most common. Also some of the more innovative producers have found use for hollowcore in such projects as parking decks, bridge deck (permanent forms), basement walls, retaining walls, pedestrian bridges and parapet walls (air displace-ment).

Tests and Approvals

Hollowcore slabs have world-wide recognition and ac-ceptance as a building element. Many tests have been done for different purposes and in different countries. The design of hollowcore is covered in the British Standards, the EC by Euro Codes and in the USA by the Precast Concrete Institute (PCI).

Many tests on hollowcore have been carried out initiated by some of the early tests, for example

- Report on Structural Test on Spiroll Extruded Hollow Core

Slabs, Report K68-05 Stockholm, Sweden, August 1968. - Report on Test to Demonstrate the Adequacy of Floor or

Roof Assemblies using Spiroll Panels (By: S.B. Barnes and Associates).

- In addition to the published design codes mentioned above some more recent publications included.

The Extrusion Process

The most common casting system for casting hollowcore is the extruder. It was the World’s first machine for producing hollowcore slabs that did not require a separate driving force to move the machine along the production bed. The same effort that feeds the concrete mix through the machine and forms it into the final precast slab also provides the motivation to drive the extruder along the bed. This natural process propels the extruder along the production bed and allows the compacted concrete to reach the required density.

With high frequency (HF) vibration in the Spiroll machines, the intense vibration and pressure within the machines, means the concrete mix is effectively ‘plasticised’ during the short time that it is passed through the extruder. This results in dense concrete with little air retention and moulds the concrete to form the required section.

The formed slab then reverts to its ‘dry’ state and reaches a density high enough to stand on the slab immediately after the extrusion process. After a period of natural or accelerated cur-ing, the slabs are then cut to length, stripped from the casting bed and transported to the storage area. Concrete strength of a minimum of 35N/mm2 is required to hold the bond between the concrete and the strands.

The casting beds are prepared by cleaning and the applica-tion of a release agent. The high tensile steel strands are pulled down the length of the bed and stressed. The extruder hopper is filled with concrete and the machine moves along the bed, pushed by the pressure generated by the compacted concrete. The casting takes one and half to two hours depending on the length of the bed. The daily routine is established depending on the number of beds to be cast and the shift operated.

Extruders are by far the simplest hollowcore machine on the market in terms of their design and ease of use. Once the machine has been commissioned and set-up to suit the local material, it simply requires the required mix to be put in the hopper and the machine to be started. Some adjustment of the mix may be necessary to achieve the desired quality and curing times and once set, one man is required to operate the ma-chine. Maintenance is extremely easy. Wear components are designed for extended life. The simplicity, reliability, low main-tenance, low labour costs and high strength of the finished product make the machine extremely popular and have stood the test of time with many reputable customers.

Extruders are capable of producing hollow core slabs from depths of 150mm-470mm with widths from 600mm-1800mm.

Factory Design

Scope of Plant Layout

The layout of a new plant should be considered with a view to the future requirement for increased numbers of beds. This leads to reviewing the product handling and the distribution of the concrete. Consideration is required of the maintenance fa-

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cility, the drainage, access, wiring of the beds, stressing of the beds and storage of the finished product.

For a low cost start up, the plant would be designed with a production facility with two (2) 120 meters long Production Beds and 1.2 metres wide. This will provide an approximate output of 65,000m² of slabs per annum based on an average of two hundred fifty (250) working days per year.

Provision would be made for future expansion by the addi-tion of two (2) to four (4) identical beds in the future. The basic system would include one (1) extruder; one (1) saw, stressing equipment and lifting equipment.

By locating the mixer in the middle of the factory the dis-tribution of the concrete and the lifting and handling of the finished product can be completed with two overhead cranes. This minimizes the travel time for the concrete and allows the second crane to continue with other activities. If concrete is to be distributed to more than one bay then a batching plant at the end of the factory is usually necessary. Concrete distribution can then be aided by using an overhead travelling bucket and transfer crane.

With both systems, overhead cranes are used to strip the product. Also the opportunity exists to extend the crane longi-tudinal travel beyond the production buildings. This enables it to be used for transfer of product to the yard and some for yard functions in the future.

The batching plant should have the provision for handling of two (2) or three (3) aggregates and silo storage of cement. Batch size should match the machine usage of concrete to en-sure continuous operation during casting.

Transport of the concrete delivery skip/buckets to the ex-truder is accomplished by forklift truck(s), overhead cranes or other suitable methods.

Stressing Abutments and Production Bed foundations are to be designed as per details provided by your consultant and Soil Investigation Report provided by the customers.

Civil Work

- Foundations for Batch Plant, electrical and mechanical distribution centre and cement silos.

- Roofed, insulated structure to cover production area( Al-though this depends on local climate).

- Fully enclosed areas for parts storage and maintenance.

Production Beds

The bed length is dictated by a number of factors. These factors are plant capacity, available space, concrete distribu-tion time, batch size, bed production time, flexibility of product depth and strand patterns, bed utilisation and bed end wastage.

Shorter Production Beds give quicker production cycle time. They are more flexible for scheduling of multiple ma-chine sizes, but are not so productive. We would normally rec-ommend a bed length of 120 metres if space is available. In practise bed lengths vary between 60m to 150m.

Bed construction techniques vary, but generally heavier construction gives more dimensional stability and longer life.

To reduce heat loss insulation should be installed under the beds. Hot water pipe for heating is installed above the insu-lation. The bed can be filled with concrete, before turning onto insulation to improve bed stability and reduce transmission of vibration.

Abutments

The capacity of the abutment design should take account of future requirements for deeper hollowcore slabs as a small additional cost at the installation stage will save significant costs later (The stressing load for a 500mm deep unit could be as high as 400 tonnes). When preparing the ground works for the abutments provision should be made for future beds to minimise costs.

Fixed Steel Posts

The simplest and cheapest method is to have fixed steel posts at both ends of the production bed. With this method the strands are tensioned individually using a hydraulic pump unit and stressing jack.

These posts can be in line with the stressing load or a de-flected strand system can be used with the post below the bed level. This later system allows strand patterns to be readily changed, facilitates drainage and is a safer system.

If ‘shock-detensioning’is employed there is a potential for

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cracking and damaging to the slabs. Extra care needs to be taken when cutting the strand. Preferable is the slow release of tension using a hydraulic detensioning system.

Hydraulic Detensioning System

To avoid the problems associated with shock detension-ing, Spiroll has developed a simple and cost effective hydraulic detensioning system.

The design of the abutment is based on two posts, which are cast on site into the concrete at an angle; this allows the highest point of the assembly to be below the level of the beds. The Stress is transferred to the posts by a yoke, which fits over the posts and is locked off, to allow the hydraulic detensioning assembly to be fitted and removed.

Multi-stressing System

The most effective system for stressing and release of tension is hydraulic multi-stressing. This method allows all the strands to be both stressed and detensioning at the same time. Multi-stressing significantly reduces the time it takes to stress and eliminates the possibility of bond slip or damage to the slabs caused by shock detensioning. However this is the more expensive option and not recommended for start ups.

Drainage

Control of the water used during sawing, and maintaining a uniform level of water on the bed ahead of the casting machine can be achieved more easily if the beds or the channels be-tween beds are installed with a fall of approximately 3 to 4mm per metre of bed run, over the length of the beds.

Production Processes

Batching/Mixing

Concrete usage is approximately 1m3 every 6mins per ma-chine. For concrete distribution to match a batch size of 1m3 is preferable. To run two machines together the minimum batching capacity would therefore be 20 to 30m3/h (note: if the pan size is reduced then the capacity needs to be increased).

Needs only 20-70 litres of water but must be accurate to 1litre. Admix needs to be able to mix with water before going into mix.

Concrete Mix Design

The Spiroll system uses an extremely dry concrete mix, typically a water/cement ratio of approximately of 0.30 The mix design will depend on the availability of local cement and ag-gregates and can be easily fined tuned to suit local conditions. To reduce curing times and to allow ‘double casting’ within a 24-hour period the cement proportions can be increased. A survey of customers suggests that the proportion of course to fine aggregates does vary to suit local conditions. Admix is normally not required but can be added to improve flow and workability with angular aggregates or assist to reduce curing times.

Material Recommendations

Course Aggregates: 10mm/14mm Aggregates (Max. Size 16mm for mechanical clearance). Irregular shape is recom-mended. Extremes of very rounded or extremely angular re-spective are prone to sagging and lower speeds or are difficult to compact.

Sand: Clean Zone 2 or equivalent.Cement: Cement can be normal Portland cement or high

early strength cement as they contribute to workability and benefit to rapid curing.

Water: This could range from 23 to 70 litres per cubic me-ter of mix depending on the moisture content and/or degree of absorption of the aggregates.

Admix: Admixtures may be useful for workability or set control, but are not normally required.

Concrete Distribution

While the Extruder is the heart of the Hollowcore Plant, ad-ditional equipment is essential to perform other tasks. Most important is the transportation of the concrete mix from the batching plant to the Extruder.

Delivery of the concrete must match the requirements of the Extruder so that it does not run out of mix and slow down production. Several methods of concrete delivery can be used such as overhead cranes, fork lifts or automated Concrete Dis-tribution Systems (CDS).

To maintain continuity of supply of concrete to a Spiroll Ex-truder producing (as an example) a 200mm deep slab would require 1m³ every 6/7 minutes.

The Extruder can be stopped between loads but it is pref-erable to maintain the continuity of the casting once the line has been started. The permissible standing time before the machine has to be lifted clear of the curing concrete would be established by trials but would normally be between 5 and 10 minutes.

When delivering concrete the transfer between skips should be kept to a minimum to avoid segregation. The skip should be bottom opening with a wide mouth (1m²) to avoid trapping and segregation of the stone from the fines.

Using an average extrusion speed of 1.2m/min and a Bed length of 120 metres, the casting time per bed would be around 100 minutes. Transfer for lifting of the Extruder, setting-up, cleaning time etc. would add approximately 15 minutes.

Consideration should be given to the systems available to distribute the concrete as follows.

Method Benefits Disadvantages

Forklift Low initial cost Readily available Floor space required

Portal CraneLow cost, reliable, flexible, no building

required

Reduced floor space Danger of Legs

Overhead Gantry Crane

Low cost, reliable, flexible, clear of floor

space, faster than Portal Crane

Part of building Cost of structural supports

CDS SystemAutomation, more

than one bay, speed, low labour content

High cost of investment,

maintenance. Poor reliability

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Curing

The curing process is the longest part of the production cycle. As such it is critical to the overall production cycle time. This means that all efforts to reduce this process will most af-fect the whole length of the production cycle. By having a con-crete mix with a low water content the curing time is greatly reduced. The application of heat into the cast slabs through pipes under the bed initiates and accelerates the curing of the concrete. The Production Beds can be heated by either hot oil steam or hot water. Of these hot water is the cheapest to install and maintain and is by far the most popular as it is reliable, cost effective and manageable. Inlet temperatures of 60 – 80°c should be maintained with enough flow to maintain outlet tem-peratures at around 25 – 35°c. To ensure good early strengths, the beds should be hot when casting and the heat applied dur-ing the casting to maintain a concrete temperature of 60 de-gree centigrade. To trap the moisture and for efficient use of heat the product should be covered at the earliest opportunity after casting. Plastic sheet can be used but for efficiency par-ticularly in colder climates the concrete should be covered with a good quality insulated sheet.

Cutting The Slabs

The estimated time for a cut is 1½ to 2 minutes. With mov-ing and positioning this gives a cycle time of 4 to 5 minutes per cut. The blades are diamond tipped and require water during the cutting process. Water can supplied to the saw using a hose Cable Reeler or directly with a trailing hose. The later is not an efficient system.

Lifting (Stripping) Clamps

Special Lifting Clamps are utilised to lift the product off the beds either by crane or forklift. The product can be transferred from the bed to the Stock Yard by crane, boggie trailers, forklift, purpose made lifters or directly onto trailer.

Care is required to match the logistics of handling the fin-ished product with the production cycle to ensure the beds are stripped at the optimum rate.

Transporting Slabs

Options for transfer of product are:-

- Forklift Truck Front Loader- Forklift Truck Side Loader- Stacker Lifters- Overhead Crane- Direct onto road trailers- Low trailer system- Bogie Trolleys

The production rate will call for movement of: approxi-mately 80 square metres per hour (or approximately 8 to 10 pieces per hour assuming average lengths 6 to 8 metres).

Preparing the Beds

Once the hollowcore slabs have been cut to length and lifted away from the production beds, the beds then need to be cleaned and oiled. The prestressing strands are then pulled the full length of the bed from the strand dispensers, threaded through the abutments and the anchors fitted prior to stress-ing.The stages of preparation are: -

- Clean the Casting Bed- Clean the Bed Rails- Push Debris off the Bed- Spray the Bed Oil/Release Agent- Pull the Prestressing Wires/Strands- Stress the Wires/Strand

These activities can be done by hand. Equipment is also available to speed up the processes and reduce the labour costs.

Quality Control Equipment

Efficient Hollowcore Production requires good quality con-trol systems to ensure the consistent quality of the aggregates, the concrete, curing conditions, good bond and dimensional accuracy of the finished product.

To achieve this, the normal aggregate testing and cube testing equipment is required. Consistent concrete is achieved with batching calibration procedures. Preparation of the cubes with heavy vibration to match the extruder is necessary and extra cubes should be made to check the “transfer strength of the concrete is required in addition to the 28 day strengths.” Stressing and detensioning procedures require to be estab-lished with correct calibration.

Conclusion

The manufacture of hollowcore is not difficult. Low cost start up units can be designed with the potential to increase the capacity to match future demand.

The degree of automation depends upon the capacity re-quired and the local cost of labour to ensure good pay back periods.

Start up factories can be run with a low level of automation; this will reduce the capital expenditure and increase the reli-ability of the plant.

A high standard of product can be guaranteed by using the correct procedures and equipment.

High Frequency Vibration Extruders as made by SPIROLL produce the strongest and most consistent product. w

Method Benefits Disadvantages

90 o Cross Cut Low initial cost Faster cutting times

Need a secondary cutting station for long and angle

cuts

Long Rip Cuts

Cut slabs longitudinally when still wet, which is

faster

Poorer Finish

Multi Angle Cuts any angle and long cuts on the bed

Heavy and more expensive saw

Secondary Cutting Station

Frees up production bed faster. Cheap method of cutting

angles

Two stage cutting

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Precast Concrete Codal Provisions – Comparison of Various Codes

Prasad. C.A. M.Tech, M(ASCE), FIE, MSEI, CE(I)Managing Partner, PS Engineering ConsultantsSecretary, Pre Engineered Structures Society of India

Design Considerations

The precast structure should be analyzed as a monolithic one and the joints in them designed to take the forces of an equivalent discrete system. Resistance to horizontal loading shall be provided by having appropriate moment and shear resisting joints or placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise. No account is to be taken of rotational stiffness, if any, of the floor-wall joint in case of precast bearing wall build-ings. The individual components shall be designed, taking into consideration the appropriate end conditions and loads at vari-ous stages of construction. The components of the structure shall be designed for loads in accordance with Part 6 ‘Struc-tural Design, Section 1 Loads, Forces and Effects’. In addition members shall be designed for handling, erection and impact loads that might be expected during handling and erection

Robustness:

Ronan Apartments

- Watershed event for progressive collapse- 22 story precast panel construction supported by cast in place

concrete structure including parking garage- Gas explosion occurred in 18th story apartment- Wall panel blew out, causing loss of support of panels on 19-

22nd flrs- Debris of upper floors caused each floor below to successively

collapse

Schematic of the Ronan Point Collaps, Modified After Dragosavie (15)

- March 2, 1973- While concrete was being placed on the 24th floor and shoring

removel was occurring on the 22nd floor a collapse occurred for the full height of the tower

- Impact of debris also caused horizontal progressive collapse of entire parking garage under construction adjacent to the tower

- 14 workers killed, 34 injured

Skyline Plaza

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Codal Provisons

There are, in general, three alternative approaches to de-signing structures to reduce their susceptibility to dispropor-tionate collapse… as adopted by Major International Codes of Practice:

- Redundancy or alternate load paths (Bridging Elements etc.,)

- Local resistance (Ductility)- Interconnection or continuity (Ties etc.,)

British Standards (BS) and New UK Regulations provide explicit rules for Robustness. Hence these are discussed in detail in this presentation

Provisions for Robustness in British Standards initiated first in 1974

2.2.2.2 Robustness

Structures should be planned and designed so that they are not unreasonably susceptible to the effects of accidents.

In particular, situations should be avoided where damage to small areas of a structure or failure of single elements may lead to collapse of major parts of the structure.

Provisions in British / Indian Standards

Unreasonable susceptibility to the effects of accidents may generally be prevented if the following precautions are taken.

a) All buildings are capable of safely resisting the notional horizontal design ultimate load applied at each floor or roof level simultaneously equal to 1.5 % of the characteris-tic dead weight of the structure between mid-height of the storey below and above or the roof surface at each floor or roof level simultaneously. Unreasonable susceptibility to the effects of accidents

may generally be prevented if the following precautions are taken.

a) All buildings are capable of safely resisting the notional horizontal design ultimate load applied at each floor or roof level simultaneously equal to 1.5 % of the characteris-tic dead weight of the structure between mid-height of the

- Tower was reinforced concrete flat plate construction- Study of failure indicated premature removel of 22nd floor

slab shoring lead to punching shear failure of the slab around one or more columns at the 23rd floor

- The weight of debris caused the failure of all the lower floors for the full height

storey below and above or the roof surface at each floor or roof level simultaneously.

b) All buildings are provided with effective horizontal ties1) Around the periphery;2) Internally;3) To columns and walls.

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c) The layout of building is checked to identify any key ele-ments the failure of which would cause the collapse of more than a limited portion close to the element in ques-tion. Where such elements are identified and the layout cannot be revised to avoid them, the design should take their importance into account. Recommendations for the design of key elements are given in 2.6 of BS 8110-2:1985

Provisions in British Standards

d) Buildings are detailed so that any vertical load-bearing element other than a key element can be removed without causing the collapse of more than a limited portion close to the element in question. This is generally achieved by the provision of vertical ties in accordance with 3.12.3 in addition to satisfying a), b) and c) above. There may, however, be cases where it is in-appropriate or impossible to provide effective vertical ties in all or some of the vertical load-bearing elements. Where this oc-curs, each such element should be considered to be removed in turn and elements normally supported by the element in question designed to “bridge” the gap in accordance with the provisions of 2.6 of BS 8110-2:1985

Provisions in Standards

Safeguarding against vehicular impact

Where vertical elements are particularly at risk from ve-hicle impact, consideration should be given to the provision of additional protection, such as bollards, earth banks or other devices.

Flow chart of design procedure( cl 3.1.4.6, BS 8110)Figure 3.1 summarizes the design procedure envisaged by

the code for ensuring robustness.Flow chart for Design Procedure

3.12.3.7 Vertical ties

Each column and each wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of resisting a tensile force equal to the maximum design ultimate dead and imposed load received by the column or wall from any one storey

3.1.4.1 General check of structural integrity

A careful check should be made and appropriate action taken to ensure that there is no inherent weakness of structur-al layout and that adequate means exist to transmit the dead, imposed and wind loads safely from the highest supported level to the foundations.

Figure 5.4 Concept of horizontal and vertical ties

Figure 3.1 Flow chart of design Procedure

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Provisions in British Standards

Key Elements

2.6.2 Key elements

2.6.2.1 Design of key elements (where required in build-ings of five or more storeys). Whether incorporated as the only reasonable means available to ensuring a structure’s integrity in normal use or capability of surviving accidents, key elements should be designed, constructed and protected as necessary to prevent removal by accident.

2.6.2.2 Loads on key elements. Appropriate design loads should be chosen having regard to the importance of the key element and the likely consequences of its failure, but in all cases an element and its connections should be capable of withstanding a design ultimate load of 34 kN/m2, to which no

Provisions in StandardsTies Design Requirements

Type of tie Reinforcement required

Amount Disposition etc.

Vertical (only required for buildings exceeding four storeys high)

Minima provided to comply with require-ments for reinforced concrete walls and columns will suffice.CP110 only: For plain concrete walls where p<0.2% and for precast struc-tures see provisions in Clause 5.1.2.4

Horizontal (required in buildings of any height)

Peripheral To resist T = F1kNAt each floor and roof level.In peripheral wall or within 1200 mm of edge of building.

InternalIf (gk +qk)l < 37.5 kN/m, to resistT = F1kN/m widthIf (gk +qk)l > 37.5 kN/m, to resistT = 0.0267 (gk + qk)l F, kN/m width

At each floor level.Either spread evenly through slab or grouped at beams, walls etc.In walls (Within 500 mm of top or bot-tom of floor slab), floor slab or beams.

External column and wall

To resist the greater of either

(i) values for walls are perme-tre of horizontal lenth(ii) 0.03 x total ultimate vertical load for which member has been designed, at floor level considered.

At each floor level, to anchor column or wall to floor structure.Reinforcement required may be partlyor wholly provided by extending that used for peripheral or internal ties.Corner columns should be tied in both directions to resist forces T specified.

Number of Storeys no 1 2 3 4 5 6 7 8 9 10 or more

Tie-force coefficient Ft 24 28 32 36 40 44 48 52 56 60

Provisions in British / Indian Standards

Ties Anchorage Details

Provisions in British Standards

Ties Anchorage Details

Figure 3.1 Flow chart of design procedure

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partial safety factor should be applied, from any direction. A horizontal member, or part of a horizontal member that pro-vides lateral support vital to the stability of a vertical key ele-ment, should also be considered a key element. For the pur-poses of 2.6.2, the area to which these loads are applied will be the projected area of the member (i.e. the area of the face presented to the loads).

Provisions in British / Indian Standards

Key Elements contd.,

2.6.2.3 Key elements supporting attached building compo-nents. Key elements supporting attached building components should also be capable of supporting the reactions from any attached building components also assumed to be subject to a design ultimate loading of 34 kN/m2. The reaction should be the maximum that might reasonably be transmitted hav-ing regard to the strength of the attached component and the strength of its connection.

NBC 2005

A Key element is such that its failure would cause the col-lapse of more than a limited area close to it, and the area may taken as equal to 70 m2 or 15% of the area of the storey which-ever is lesser.

Provisions in British Standards

Bridging Elements

2.6.3 Design of bridging elements (where required in build-ings of five or more storeys)

2.6.3.1 General. At each storey in turn, each vertical load-bearing element, other than a key element, is considered lost in turn. (The design should be such that collapse of a signifi-cant part of the structure does not result.) If catenary action is assumed, allowance should be made for the horizontal reac-tions necessary for equilibrium.

2.6.3.2 Walls

2.6.3.2.1 Length considered lost. The length of wall con-sidered to be a single load-bearing element should be taken as the length between adjacent lateral supports or between a lateral support and a free edge (see 2.6.3.2.2).

2.6.3.2.2 Lateral support. For the purposes of this sub-clause, a lateral support may be considered to occur at:a) a stiffened section of the wall (not exceeding 1.0 m in

length) capable of resisting a horizontal force(in kN per metre height of the wall) of 1.5 Ft; or

b) a partition of mass not less than 100 kg/m2 at right angles to the wall and so tied to it as to be able to resist a horizon-tal force (in kN per metre height of wall) of 0.5 Ft; Where Ft is the lesser of (20 + 4 n0) or 60, where n0 is the number of storeys in the structure.

New Regulations in UK

Class 3 Buildings

All Class 2B Regulations apply; in addition:For Class 3 buildingd – A systematic risk assessment of

the building should be undertaken taking into account all the normal hazards that may reasonably be foreseen, together

with any abnormal hazards.Critical situations for design should be selected that reflect

the conditions that can reasonably be foreseen as possible during the life of the building. The structural form and concept and any protective measures should then be chosen and the detailed design of the structure and its elements undertaken in accordance with the recommendations given in the Codes and Standards give in paragraph 5.2.

New Regulations in UKClass 3 Buildings: Risk Assessment Procedure

1. Identify hazards (see Section 6.4) to form the basis of a risk register. This is an absolute minimum for Class 3 build-ings, to demonstrate that the possible hazards have at least been thought about by the designer.

2. Determine or estimate the severity of the consequences of each hazard.

3. Assess the likelihood of each hazard occurring.4. Estimate the risk of each hazard. The risk is usually ex-

pressed as a function of the severity and the likelihood for each hazard.

5. Evaluate which hazards have unacceptable levels of risk.6. Propose risk mitigation measures for any unacceptable

risks.The hierarchy of risk control is a) to prevent the hazard

from occuring, b) to reduce the probability of the hazard occur-ring, and c) to reduce the severity of the consequences. Further guidance is provided in Section 6.5.

New Regulations in UK

Class 3 Buildings: Risk Assessment Procedure:Sources of further guidance

BS 7974: 2001 (19)

This code of practice provides a framework for develop-ing a rational method for designing buildings using fire safety engineering. However, there are several aspects that could be applied more generally to Class 3 Buildings, particularly the Qualitative Design Review (QDR).

ISO 2394:1998 (20)

This International Standard specifies general principles for the verification of the reliability of structures subjected to known or foreseeable types of action. Section 8 provides guid-ance on the principles of probability-based design and Annex B provides examples of permanent, variable and accidental ac-tions. The information contained within this standard is simi-lar to that contained in EN 1990 Eurocode: Basis of Structural Design (21).

New Regulations in UK

Class 3 Buildings: Risk Assessment Procedure:Sources of further guidance

pr EN 1991-7-7 (5)

This document contains a great deal of helpful information and guidance that can be applied to Class 3 Buildings. Annex B provides guidance on risk assessment methods, acceptance criteria and mitigation measures. Section 3 includes guidance

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on identifying accidental actions. Sections 4 (Impact) and 5 (In-ternal Explosions) provide guidance on the size of loads that accidental actions might cause. This is likely to be a key source of guidance of engineers designing Class 3 buildings.

SCI publication P244(18)This publication provides guidance on the protection of

commercial buildings and personnel from the effect of explo-sions caused by the detonation of high explosives. It is aimed at engineers and architects who are involved in buildings designs where this type of protection is required. Particularly useful topics that are covered are; calculation of blast loads, struc-tural design approach and non-structural enhancements.

American Standards Approaches

ACI 318

RCC:

Specifies Structural Integrity rules such as continuation of reinforcement etc., but mention neither specific Tie force nor check to limit damage due to removal of single element.

Precast Concrete:

Specifies Tie force no check to limit damage due to re-moval of single element.

Approaches of other American Standards such as ASCE 7-02, GSA etc., are too not very explicit.

General Services Administration (GSA) is an independent agency of US GovtGSA’s Facilities Standards for the Public Buildings Service (PBS)

Provisions in Australian Standards

Flat Slabs as an example

In order to prevent a progressive collapse in flat slabs, the Australian code (AS3600) has mentioned that there should be bottom steel at the slab column connection. As per observa-tion the top bars are ineffective during a punching shear failure event. The bottom bars begin to take the force in the form of a catenary

As per NBC 2005

Bearing for Precast Units

Precast units shall have a bearing at least of 100 mm on masonry supports and of 75 mm at least on steel or concrete. Steel angle shelf bearings shall have a 100 mm horizontal leg to allow for a 50 mm bearing exclusive of fixing clearance.

Figure 2. Lack of continuous reinforcement across the beam-to-columnconnection can lead to progressive collapse (Reference 4)

Figure 4. Structural integrity requirements for top bars of perimeter beams (7.13.2.2(a))

Figure 5. Structural integrity requirements for bottom bars of perimeter beams [7.13.2.2 (b)]

Figure 6. Structural integrity requirements for stirrups in perimeter beams (7.13.2.3)

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When deciding to what extent, if any, the bearing width may be reduced in special circumstances, factors, such as, loading, span, height of wall and provision of continuity, shall be taken into consideration

7 Joints

7.2 The following are the requirements of a structural joint:

a) It shall be capable of being designed to transfer the im-posed load and moments with a known margin of safety;

b) It shall occur at logical locations in the structure and at points which may be most readily analysed and easily rein-forced;

c) It shall accept the loads without marked displacement or rotation and avoid high local stresses;

d) It shall accommodate tolerances in elements;

e) It shall require little temporary support, permit adjustment and demand only a few distinct operation to make;

f) It shall permit effective inspection and rectification; g) It shall be reliable in service with other parts of the buildingh) It shall enable the structure to absorb sufficient energy

during earthquakes so as to avoid sudden failure of the structure. Precast structures may have continuous or hinged con-

nections subject to providing sufficient rigidity to withstand horizontal loading. When only compressive forces are to be taken, hinged joints may be adopted. In case of prefabricated concrete elements, load is transmitted via the concrete. When both compressive force and bending moment are to be taken rigid or welded joints may be adopted; the shearing force is usually small in the column and can be taken up by the friction resistance of the joint. Here load transmission is accomplished by steel inserted parts together with concrete.

When considering thermal shrinkage and heat effects, provision of freedom of movement or introduction of restraint may be considered Joining techniques/materials normally employed are Welding of cleats or projecting steel, Overlapping reinforcement, loops and linking steel grouted

by concrete, Reinforced concrete ties all round a slab, Prestressing,

Epoxy grouting, Bolts and nuts connection, or a combina-tion of the above, and any other method proven by test and any other method proven by test

8 Tests for Components/structures

8.2 Testing on Individual Components

The component should be loaded for one hour at its full

Approaches for design against disproportionate collapse adopted in selected codes and stan-

dardsRedundancy Local Resistance Inter-connec-

tionThreat-dependent

analysis

ASCE 7-02 -

ACI 318-02 -

GSA…PBS, 2000 -

GSA…PBS, 2003 -

GSA PC Guidelines -

ASCE: American Society of Civil Engineers | ACI: American Concrete Institute

General Services Administration (GSA) is an independent agency of US GovtGSA’s Facilities Standards for the Public Buildings Service (PBS)

Would use of these codes and stan-dards in their design have improved

the performance of Ronan Point, Murrah and WTC ?

Redundancy Local Resistance

Inter- connection

Threat-dependent

analysis

Ronan Point

Murrah Building

WTC 1& 2

ASCE 7-02 - ? N N

ACI 318-02 - Y ? N

GSA…PBS, 2000 - ? N N

GSA…PBS, 2003 - N Y N

GSA PC Guidelines - N N N

Provisions in Australian Standards

Flat Slabs as an example

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span with a total load (including its own self weight) of 1.25 times the sum of the dead and imposed loads used in design. At the end of this time it should not show any sign of weak-ness, faulty construction or excessive deflection. Its recovery one hour after the removal of the test load, should not be less than 75 percent of the maximum deflection recorded during the test. If prestressed, it should not show any visible cracks up to working load and should have a recovery of not less than 85 percent in 1 h.

8.3 Load Testing of Structure or Part of Structure

Loading test on a completed structure should be made if required by the specification or if there is a reasonable doubt as to the adequacy of the strength of the structure.

8.3.1 In such tests the structure should be subjected to full dead load of the structures plus an imposed load equal to 1.25 times the specified imposed load used in design, for a period of 24 h and then the imposed load shall be removed. During the tests, vertical struts equal in strength to take the whole load should be placed in position leaving a gap under the member.

NOTE — Dead load includes self weight of the structural members plus weight of finishes and walls or partitions, if any, as considered in the design.

8.3.1.1 If within 24 h of the removal of the load, a reinforced concrete structure does not show a recovery of at least 75 per-cent of the maximum deflection shown during the 24 h under load, test loading should be repeated after a lapse of 72 h. If the recovery is less than 80 percent in second test, the structure shall be deemed to be unacceptable.

8.3.1.2 If within 24 h of the removal of the load, prestressed concrete structure does not show a recovery of at least 85 per-cent of the maximum deflection shown during the 24 h under load, the test loading should be repeated. The structure should be considered to have failed, if the recovery after the second test is not at least 85 percent of the maximum deflection shown during the second test.

8.3.1.3 If the maximum deflection in mm, shown during 24 h under load is less than 40 l2 /D , where l is the effective span in m; and D, the overall depth of the section in mm, it is not necessary for the recovery to be measured and the recovery provisions of 8.3.1.1 and 8.3.1.2 shall not apply.

9 Manufacture, Storage, Transport and Erection of Precast Elements

9.1 Manufacture of Precast Concrete Elements

9.1.1 A judicious location of precasting yard with concret-ing, initial curing (required for demoulding), storage facilities, suitable transporting and erection equipments and availability of raw materials are the crucial factors which should be care-fully planned and provided for effective and economic use of precast concrete components in constructions.

9.1.2 Manufacture

The manufacture of the components can be done in a fac-tory for the commercial production established at the focal point based on the market potential or in a site precasting yard set up at or near the site of work.

9.1.2.1 Factory prefabrication

Factory prefabrication is resorted to in a factory for the commercial production for the manufacture of standardized components on a long-term basis. It is a capital intensive pro-duction where work is done throughout the year preferably un-der a closed shed to avoid effects of seasonal variations. High level of mechanization can always be introduced in this system where the work can be organized in a factory-like manner with the help of a constant team of workmen.

9.1.2.2 Site prefabrication

Prefabricated components produced at site or near the site of work as possible.

This system is normally adopted for a specific job order for a limited period. Under this category there are two types that is semi-mechanized and fully-mechanized.

9.1.2.2.1 Semi-mechanized

The work is normally carried out in open space with locally available labour force. The equipment machinery used may be minor in nature and moulds are of mobile or stationary in na-ture.

9.1.2.2.2 Fully-mechanized

The work will be carried out under shed with skilled labour. The equipments used will be similar to one of factory produc-tion. This type of precast yards will be set up for the production of precast components of high quality, high rate of production.

Though there is definite economy with respect to cost of transportation, this system suffers from basic drawback of its non-suitability to any high degree of mechanization and no elaborate arrangements for quality control. Normal benefits of continuity of work is not available in this system of construc-tion.

9.1.5 The various stages of precasting can be classified as in Table 2 on the basis of the equipments required for the various stages. This permits mechanization and rationalization of work in the various stages. In the precasting, stages 6 and 7 given in Table 2 form the main process in the manufacture of precast concrete elements. For these precasting stages there are many technological processes to suit the concrete product under consideration which have been proved rational, economical and time saving. The technological line or process is the theoretical solution for the method of planning the work involved by using machine complexes. Figure 5 illustrates dia-gramatically the various stages involved in a plant process.

d) Better working conditions for the people on the job; and e) To minimize the effect of weather on the manufacturing

schedule.

9.2 Preparation and Storage of Materials

Storage of materials is of considerable importance in the precasting industry, as a mistake in planning in this aspect can greatly influence the economics of production. From ex-perience in construction, it is clear that there will be very high percentages of loss of materials as well as poor quality due to improper storage and transport. So, in a precast factory where everything is produced with special emphasis on quality, prop-er storage and preservation of building materials, especially

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cement, coarse and fine aggregates, is of prime importance. Storage of materials shall be done in accordance with Part 7 ‘Constructional Practices and Safety

9.3 Moulds

9.3.1 Moulds for the manufacture of precast elements may be of steel, timber, concrete and plastic or a combina-tion thereof. For the design of moulds for the various ele-ments, special importance should be given to easy demoulding and assembly of the various parts. At the same time rigidity, strength and water tightness of the mould, taking into consid-eration forces due to pouring of green concrete and vibrating, are also important.

9.3.2 Tolerances

The moulds have to be designed in such a way to take into consideration the tolerances given as follows:

9.3.3 Slopes of the Mould Walls

For easy demoulding of the elements from the mould with fixed sides, the required slopes have to be maintained. Other-wise there is a possibility of the elements getting stuck up with the mould at the time of demoulding.

9.4 Accelerated Hardening

In most of the precasting factories, it is economical to use faster curing methods or artificial curing methods, which in turn will allow the elements to be demoulded much ear-lier permitting early re-use of the forms. Any of the following methods may be adopted:

i) Length

± 5 mm 1, 7

± 5 mm or ± 0.1 percent whichever is 2, 3, 8

greater

± 0.1 percent subject to maximum 4

of + 5 mm

-10

± 2 mm for length below and up to 5

500 mm

± 5 mm for length over 500 mm 5

± 10 mm 6, 9,10

Thickness/Cross-sectional dimen-sions

± 3 mm 1

± 3 mm or 0.1 percent whichever is 2, 8

greater

± 2 mm up to 300 mm wide 4

± 3 mm greater than 300 mm wide

± 2 mm 3, 7

± 4 mm 6, 9, 10

iii) Straightness/Bow

± 5 mm or 1/750 of length whichever 2, 4, 8

is greater

± 3 mm 1, 5

± 2 mm 7

iv) Squareness

When considering the squareness of the corner, the longer of two adjacent sides being checked shall be taken as

the base line.

The shorter side shall not vary in 2, 5, 8

length from the perpendicular by

more than 5 mm

The shorter side shall not vary in 1, 7

length from the perpendicular by

more than 3 mm

The shorter side shall not be out of 4

+2

square line for more than −5 mm

v) Twist

Any corner shall not be more than

the tolerance given below from the

plane containing the other three

corners:

± 5 mm (Up to 600 mm in width and 2, 8

up to 6 m in length)

± 10 mm (Over 600 mm in width and

for any length)

± 1/1 500 of dimension of ± 5 mm 4

whichever is less

± 3 mm 1

± 1 mm

vi) Flatness

The maximum deviation from 1.5 m

straight edge placed in any position

on a nominal plane surface shall not

exceed

± 5 mm 2, 8

± 3 mm 4

± 2 mm 1, 7

± 4 or maximum of 0.1 percent 5

length

NOTES — Key for product reference

1 Channel unit 2 Ribbed slab unit/hollow slab 3 Waffle unit 4 Large panel prefabrication 5 Cellular concrete floor/roof slabs 6 Prefabricated brick panel 7 Precast planks 8 Ribbed/plain wall panel 9 Column10 Step unit

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a) By Heating the Aggregates and Water Before Mixing the Concrete

b) Steam Curingc) Steam Injection During Mixing of Concrete d) Heated Air Method e) Hot Water Methodf) Electrical Method

9.4.1 After the accelerated hardening of the above prod-ucts by any of the above accepted methods, the elements shall be cured further by normal curing methods to attain full final strength.

9.5 Curing

9.5.1 The curing of the prefabricated elements can be ef-fected by the normal methods of curing by sprinkling water and keeping the elements moist. This can also be done in the case of smaller elements by immersing them in a specially made water tanks.

9.5.2 Steam Curing

9.5.2.1 The steam curing of concrete products shall take place under tarpaulin in tents, under hoods, under chambers, in tunnels or in special autoclaves. The steam shall have a uni-form quality throughout the length of the member. The pre-cast elements shall be so stacked, with sufficient clearance between each other and the bounding enclosure, so as to allow proper circulation of steam.

9.6 Stacking During Transport and Storage

Every precaution shall be taken against overstress or damage, by the provision of suitable packings at agreed points of support.

9.6.1 The following points shall be kept in view during stacking:

a) Care should be taken to ensure that the flat elements are stacked with right side up. For identification, top surfaces should be clearly marked.

b) Stacking should be done on a hard and suitable ground to avoid any sinking of support when elements are stacked.

c) In case of horizontal stacking, packing materials shall be at specified locations and shall be exactly one over the other to avoid cantilever stress in panels.

d) Components — should be packed in a uniform way to avoid any undue projection of elements in the stack which nor-mally is a source of accident.

9.7 Handling Arrangements

9.7.1 Lifting and handling positions shall be clearly de-fined particularly where these sections are critical. Where nec-essary special facilities, such as bolt holes or projecting loops, shall be provided in the units and full instructions supplied for handling.

For precast prestressed concrete members, the residual prestress at the age of particular operation of handling and erection shall be considered in conjunction with any stresses caused by the handling or erection of member. The compres-sive stress thus computed shall not exceed 50 percent of the cube strength of the concrete at the time of handling and erec-

tion. Tensile stresses up to a limit of 50 percent above those specified in Part 6 ‘Structural Design, Section 5 Concrete’ shall be permissible

9.9 Transport

Transport of precast elements inside the factory and to the site of erection is of considerable importance not only from the point of view of economy but also from the point of view of design and efficient management. Transport of precast ele-ments must be carried out with extreme care to avoid any jerk and distress in elements and handled as far as possible in the same orientation as it is to be placed in final position.

9.10 Erection

In the ‘erection of precast elements’, all the following items of work are meant to be included:

a) Slinging of the precast elementb) Tying up of erection ropes connecting to the erection

hooksc) Cleaning of the elements and the site of erectiond) Cleaning of the steel inserts before incorporation in the

joints, lifting up of the elements, setting them down into the correct envisaged position

e) Adjustment to get the stipulated level, line and plumbf) Welding of cleatsg) Changing of the erection tacklesh) Putting up and removing of the necessary scaffolding or

supportsj) Welding of the inserts, laying of reinforcements in joints

and grouting the jointsk) Finishing the joints to bring the whole work to a workman-

like finished product. w

Author’s BioMr. C.A Prasad, is an Engineering graduate (B.Tech, civil

Engineering), from Jawaharlal Nehru Technological Univer-sity Hyderabad in the year 1982, and Post graduate in Engi-neering Structures (M.Tech) from Regional Engineering Col-lege, Kakatiya University, Warangal in the year 1989. He has 30 years of experience to his credit in the various fields of civil engineering, viz., Construction, Design, Quantity Surveying, and Project management of works.

He has worked in the Middle East for ten years and worked in the international firms like Balfour Beatty, WS At-kins, and Engineers Office. His Design works include the Burj Al Arab tower, the building in the waters of Ocean, Jumeirah Beach works, Millennium Grand Stand, Ware Houses and towers in Dubai and Doha, etc.,

He is a well-known personality in the field of precast works and is encouraging and promoting Precast structures in India, by advising and assisting the developers in the set-up of Precast factories and delivering precast consultancy services to them.

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SFRC: Practical Considerations andCommercial Feasibility

Abstract: During the last three decades SFRC was considered a new technology for Construction Industry. However this tech-nology has found high acceptance among today’s Construction industry. Currently, steel fibers are used mainly in Industrial flooring, Tunneling and Pavements etc.

Construction Time and durability are the main factors among the various advantages which help SFRC to command its superiority over other methods.

In our country lot has been written or published about SFRC, but we are not using this technology as it is being used in other countries there is a definite and detail approach on how to design Fiber concrete and achieve a homogeneous dispersion of Steel fibres. Steel fibre geometry and grading of concrete play a very important in role in practicalities of SFRC.

Following article talks about various aspects of Steel Fi-bre reinforced Concrete Viz. Design Methods, Design of SFRC Floor based on lose berg’s yield line Model Selection Criteria, Mix Design and other practical considerations and commercial feasibility.

Definition

Steel fibre reinforced concrete is defined as a concrete, containing discontinuous discrete steel fibres. Steel fibres are incorporated in Concrete to improve its Crack resistance, Duc-tility, Energy absorption and impact resistance characteristics. Properly designed and dosed SFRC can reduce or even contain cracking, a common cause for concern in plain concrete.

Scope

Concrete composition, admixtures, placing and curing play another evident role but here focus will be on design Principals and Methods a sample design of SFRC Industrial floors using Drapro and selection Criteria of Steel Fibre.

Design Methods

SFRC necessarily behaves very different as that of plain concrete. The performance of SFRC varies when compared

in post crack stage. Conventional methods do not necessar-ily consider post crack behaviour of concrete. Design method based on Lose bergs yield line model considers post crack strength of concrete in a right manner hence it is till date the best method to design SFRC as Shown in Table 1 and Picture 1

CONCRETE: SFRC

Sr Design Methods Applicability Why Test results Limitation/Economy

1Elastic – Elastic (Westerguardard or FEM )

Applicable but not suitable

Post crack behaviour and sys-tem properties are not taken in

to account

Far from reality (Actual Test

results)

Rather very safe Hence not economical

2 Elastic –Plastic

Applicable and closer to more accurate Plas-

tic- Plastic

Post crack behaviour Proper-ties are taken into account to

some extent related toFlexural Strength

Closer to Reality

Fibres do not increase flex-ural Strength of the section

within the section but increase load bearing capac-

ity of the system

3 Plastic- Plastic Applicable and Suitable

Considers Ductility of steel fibre reinforced concrete and both material as well as system

properties in account

Closer to actual results

Generally economical as compared with Plain or Rebar

reinforced concrete

Table No 1 Comparison of various design methods

Picture 1 contains a comparison of real scale test results and the results of back-calculation according to the different design approaches. It demonstrates the importance of taking the right design approach for elastically supported steel fiber reinforced concrete slabs. As a simple guideline, the results of elastic-elastic calculation can never be more economic than those of a plastic-plastic calculation providing same material properties and level of safety. The elastic-plastic approach is in the range of plastic-plastic approach.

Design of an Industrial Floor

Industrial floors are generally subjected to Loads such as point load, UDL and Wheel Load. In Interest of explaining load effects certain loads and sub base values are assumed to ar-rive at Flexural Stress and corresponding dosage. Other as-sumptions such as Temperature, Joint distance, loading factor can be made available on request.

- Input –loads- Point Loads

Picture 1: comparison of real scale test results and results of back-calculation

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Above figure (Picture 2) illustrates Point loads arising from Rack loads, Stacking Area, Lines Etc.

We need to design a floor which is efficient of taking these loads at various locations such as joint of panels, centre of pan-els etc.

Anticipated Location of Load

Input Sub base

Sub base plays an important role in Floor. Generally fol-lowing sub base (Picture 6) is seen in industries. To analyze the effect of sub base on floor design, it is necessary to arrive at equivalent E modulus or CBR value of the sub base.

If there are more than 2 layer of sub-base defined the equivalent E-modulus of the ground is calculated using the formula below

Picture 2: Point Loads

Picture 3: Various location of loads

Wheel Loads

Wheel loads are loads coming form Moving Equipments like Fork Lift , The diagram gives details of Loads arising out of a 6 ton Capacity Fork Lift having a tire pressure of 1.5 N/mm ^2 ( Picture 4)

UDL

Above Figure (Picture 5) illustrates UDL of 5 Ton /M ^2

Picture 4: Wheel loads

ResultAs it is not known beforehand which yield will occur first,

we have to consider all possible load combinations. After con-sidering various load combinations and locations following maximum moments (Table 2) are foreseen.

Assumptions / Design Criteria

Steel Fibres

Selection Criteria

The most important aspects controlling the performance of steel fibres in concrete are as follows

- Tensile Strength on the wire( > 1225 Mpa)- Aspect ratio- Geometrical shape

Higher aspect ratio (Picture 8) always gives better perfor-mance of the SFRC with respect to flexural strength, impact resistance, toughness, ductility, crack resistance etc.

Unfortunately, the higher the aspect ratio and volume con-centration of the fibre, the more difficult the concrete becomes to mix, convey and Pour. Thus there are practical limits to the

Ultimate Limit State Serviceability Limit State

Bending moments (kNm)

Loads (m+m’)max 5.67 kNm (m+m’)max 4.02 kNm

Shrinkage Ms 1.41 kNm

Temperature MT> 1.84 kNm

Settlement Mw 0.00 kNm

Floor thickness 120 mm

Required SF concrete flexural stress 1.09 N/mm2 0.86 N/mm2

Table No 2 Result

Picture 5: UDL

CONCRETE: SFRC

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Ultimate Limit State: for a dosage of 15 kg/m3 Dramix RC 80/60-BN.

Serviceability Limit State: for a dosage of 15 kg/m3 Dramix RC 80/60-BN

bres present no difficulty in mixing. They are added as an extra aggregate and require no special equipment to be added to the mix, whether dry mix or wet mix. The hooked ends improve the bond and anchorage of the Dramix steel fibres in the concrete/shotcrete and increase the reinforcing efficiency and ductility. Hooked ends are proved to be best as compared to any other shape of fibres. Bekaert has done extensive research on same copies of which can be made available on request.

Fibre Dosage

This is one of the most important elements in SFRC. As discussed earlier fibre performance clearly depends upon pa-rameters like tensile strength, Aspect Ratio, Anchorage. The dosage of fibres for a certain performance varies as per type of fibre used .This can be established by making a proper design followed by field test. Following table gives comparison of vari-ous types of fibres in terms of dosage.

Comparison with Alternatives

A conventional pavement with 200 mm Thk with single Mesh can be replaced by a 120 mm Thk (SFRC) pavement with following combinations.

Although unit cost of lower aspect ratio (45) fibre is less, due to high dosage ( 31.5 ) Kg) per M ̂ 3 cost of SFRC becomes very high as compared to that of SFRC with lower dosage ( 15 kg ) of High Aspect ratio ( 80 ) Fibres.

Ultimate Limit State Serviceability Limit State

Concrete design stress 1.45 N/mm2 2.18 N/mm2

Dramix®

Type RC 80/60-BN Type RC 80/60-BN

Dosage 15 kg/m3 Dosage 15 kg/m3

1.14 N/mm2 1.39 N/mm2

SF Ductility (%) 41.08 50.00

E k value : 3000.00 N/mm2

Concrete compressive strength, f ck :

C20/25

For ultimate limit state, the gov-erning load case is :Four wheels in a rectangle - Saw Cut

5.67 kNm

For serviceability limit state, the governing load case is :Four wheels in a rectangle - Saw Cut

7.28 kNm

Temperature differential between top and Bottom of the slab

28 °C

Coefficient of friction (µ) between slab and sub base :

0.50

Dramix ® Solution

Floor thickness : 120 mm

Dosage : 15 kg/m3

Fibre type : RC 80/60-BN

Re,3 value : 41.08 %

Equivalent flexural strength (Ffct,eq,150) :

1.52 N/mm2

Max joint spacing : 4000 mm * 4000 mm

Table No 4 Governing case & proposals

amount of single fibres, which can be added to SFRC, with the amount varying with the different geometrical characteristics of the several fibre types. Loose steel fibres with a high 1/d as-pect ratio, which is essential for good reinforcement, are diffi-cult to add to the concrete and to spread evenly in the mixture.

BEKAERT has glued (Picture 9) the loose fibres together with water-soluble glue into bundles of 30-50 fibres to facilitate handling of the Dramix steel fibres. The individual Dramix steel fibres have the necessary high 1/d aspect ratio, but as they are glued together in compact bundles, they have approximately the same size as the other aggregates. Glued Dramix steel fi-

Picture 7: Dramix® steel fibres

Picture 8: Aspect Ratios Picture 9: Glued Dramix® steel fibres

Fibre Type Type Len-gth

Diam-eter

Aspect Ratio( L/D)

Dosage perM ^ 3 *

mm mm Length/Diameter

Kg

RL 45/50 Loose 50 1.05 48 31.5

RC 65/60 Glued 60 0.9 67 20

RC 80/60 BN Glued 60 0.75 80 15

Table No 5 comparisons of various types of fibres

CONCRETE: SFRC

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Practical considerations

Steel fibre reinforced concrete is better concrete as com-pared to RCC in certain applications. To make this technology practically possible it is very much necessary to give impor-tance to fibre geometry, Concrete consistency, gradation Etc. What we want is concrete with right mix and Homogeneous dispersion of steel fibres (As below)

tion. What fibres want is concrete with enough paste around the aggregates.

Case I –Practical Project at Coimbatore Given factsMix Design

Picture 10: Fresh Concrete Picture 11: X-ray image of SFRC

Fibre Geometry

Length of the fibre should be more than sum total two Ag-gregate sizes (Picture 12). At the same time fibre length should not exceed 2/3rd of the inner dia of the conveying system (Pic-ture 13).

Here first factor is related to interlocking of two aggregates whereas second factor is related to workability of concrete through the pumping system.

Picture 12: (Minimum length of fibre)

Picture 13: (Maximum length of fibre)

Mix Design for Hansen/Shapporji Project As on 11.2.8 Reference PSG COLLAGE REPORT P/SM/T &CON/LN1309/2007/34D DATED 22.01.08

Description

Grade of Concrete M30

Required Slump 40-80

Type Of Cement OPC 43 GRADE

Grading of Sand Zone II

Maximum Size of Coarse aggregate 20

Specific Gravity

Cement 3.15

Sand 2.67

Coarse Aggregate 2.69

60 to 40 ratio of 20 and 12.5 Dia Aggregate

Bulk Density KG/M ^3

Cement 1440

Sand 1570

20 MM Coarse Aggregate 1542

12.5 MM Coarse Aggregate 1565

Water Absorption ( %)

Sand

Coarse Aggregate 0.41

Target Mean Strength ( N/MM ^2) 38.25 Mpa

Standard Deviation = 5.0 Mpa

Water Cement Ratio 0.4

Water content per m ^3 of concrete ( kg) 144

Sand as percenatge of total aggregate by Absolute volume

35

Entrapped Air as % of Volume of Concrete 2

Cement Content per M ^3 of concrete (kg) 360

Sand per M ^3 of Conccrete (KG) 674.4

Coarse Aggregate per m ^3 of Concrete (KG)

1261.9

(20 MM AND 12.5 mm In ratio of 60.40)

Admixture ( kg) 1.44

Mix Proportion by Weight

C, S ,CA ( 20 MM) ,CA(12.5MM) ,W 1:1.873,2.103,1.402. ,0.4

C= CEMENT, S = SAND, CA COARSE AGREGATE, W = WATER

Quantities of Materials( KG) Per M ^3 OF CONCRETE

Cement 360

Sand 674.4

Coarse Aggregate ( 20 MM) 757.14

Coarse Aggregate ( 12.5 MM) 504.6

Water 144

In order to have more networking of fibres it is suggested to have fibres with highest available L/D Ratio or least available diameter which finally gives more fibres per kilo (Picture 13)

Concrete Consistency and Gradation In addition to selection of appropriate fibres it is very much

necessary to have consistent concrete with continues grada-

Picture 14: Network of fibres Picture 15: Sieve curves

CONCRETE: SFRC

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Steel Fibres

Type 1

Length : 60 MMDiameter : 0.9 MMFormation : Glued Anchorage : Hooked End (Dramix)Tensile Strength : > 1000 N/MM ̂ 2Dosage : 30 KG/ M^3

Type 2

Length : 60 MMDiameter : 0.75 MMFormation : Glued Anchorage : Hooked End (Dramix)Tensile Strength : > 1000 N/MM ̂ 2Dosage : 20 KG/ M^3

In order to create more paste in existing formulae of con-crete following suggestions were made to job site.1. Depending on availability pl. add either of following (30-50

Kg per M ̂ 3, Fine sand <= .125mm, Fly Ash, 3. GGBS)2. Start from W/C Ratio of 0.5 and take trials up to 0.463. Increase cement content to 380-400 KG ( Trail and error)4. Increase slump to minimum 80 and maximum 120 ( Trial

and Error)It was difficult to get fine sand of required fineness so it

was decided to increase 20-40 KG of existing fine grade sand (ZONE II).

Six Samples of various combinations were checked for fi-bre dispersion as follows.

Case II – Commercial as Per Annexure I

Conclusion

Although proper design and economics is important for the project it is very much necessary to engineer the concrete to suit the selected fibre geometry. Concrete consistency and gradation should be different for every mix and should depend on the type of fibre as suggested by manufacturer.

Steel fibre reinforced Industrial floors can be designed us-ing Lose berg’s Yield line model. At www.bekaert.com/building one can register to get a free design of Steel fibre Industrial floors based on the inputs provided.

Steel fibres being an essential part of this design should be selected very carefully as discussed in the paper. More em-phasis should be given on total cost impact than per unit cost as mentioned in the Annexure II

Admixture 1.44

Confirmatory Test Result

7 days Compressive Strength 33.7

Expected 28 Days Compressive Strength 50.5

Workability

Slump 62

Required fiber content

Actual as per Sieve Test

Variation in % Slump

Grams Grams % MM

1060 974 8.11% Collapse

1060 1041 1.79% 80

1060 891 15.94% 80

706 729 -3.26% 130

706 570 19.26% 130

706 635 10.06% 170

Average 8.65%

Table No 7 Results of washout test

No balls were observed during the mix W/C Ratio maintained was 0.48/0.49Further improvements at the time of actual project can be

as follows.

1. Make fine sand available and reduce cement content2. Reduce water cement ratio to 0.463. Maintain slump in the range of 80-1204. If possible increase mixer speed to 18 RPM

References

1. Gerhard Vitt Design –Presentation at Malenovice approach for Dramix In-dustrial floors

2. Beckett D, Humphreys J The Thames Polytechnic , Dart ford : Compara-tive tests on Plain , Fabric Reinforced and Steel Fibre reinforced Concrete Ground Slabs ,

3. Lose berg A : Design Methods for structurally Reinforced Concrete Pave-ments , Sweden, 1961

4. Thooft H : Dramix Steel Fibre Industrial floor Design in accordance with the Concrete Society TR34

5. Practical guide to the installation of Dramix Steel fibre concrete floors.6. Ganesh P. Chaudhari , Design of SFRC Industrial floor Indian Concrete In-

stitute , Seminar on Flooring and Foundations7. Ganesh P. Chaudhari, Design of Durable SFRC Industrial Floor, Interna-

tional conference of “Sustainable Concrete Construction “ACI, 8-10 Febru-ary, Rantagiri, India. w

Parameter Acceptance Criteria Significance Remark

Tensile Strength

Rm nom = 1225

N/MM^2

Higher tensile strength , Better

performance

Anchorage Hooked end Better Anchorage

Hooked end gives Better anchorage as compared

with other forms of anchorage such as Flat or

corrugated

Length ( MM) 60

Length of Fibre should at least

cover three major aggregates

Diameter ( MM) 0.75

Lesser the diameter , more number of

fibres per kg

More fibre gives more Length , More surface

Area/Volume , Better Cor-rosion Resistance

Aspect Ratio ( L/D) 80

Higher aspect ratio leads to better

performance

Length Per KG 280 Meter

More length per KG gives optimum

results

Formation Glued fibreGlued fibre ensures

better dispersion and no fibre balling

Tolerance ± 7.5 AvgCloser Tolerance leads to designed

performance

No Of Fibres Per KG 4600

More fibres more network , More

ductility

Standards

CE-label system 1 according

EN 14889-1

Table No 8 Annexure II Selection Criteria for Steel fibres

CONCRETE: SFRC