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50 The Masterbuilder - August 2012 • www.masterbuilder.co.in

Ant Colony Optimization Module for Scheduling of Resource Constrained Construction Project

Project completion within a time limit is essential for successful project performance, regardless of the size and complexity of the project. Each delay

in the completion time constitutes a loss in revenue that can hardly be recovered later. The critical path method (CPM) has been widely used as a project management tool to improve schedule and project administration tasks supporting project managers to ensure project completion on time and on budget. CPM is developed based on the assumption that resources required by the activities are unlimited. In practice, some of resources are limited and is not available in required quantity. In most of the construction projects, planning without considering the limitations of the resources results in a non-credible schedule, since the start ability of activities is effected by resource availability.

A real time construction project is invariably composed of activities linked with constrains. The aim of the manager is to execute the activities in project with the aid of available resources within minimum duration. The expertise and skill of the manager help to optimize the resource constrained project. Present study aims to automate the involvement of

an expert manager in scheduling the resourced constrained construction projects. In this study, a module is prepared based on ant colony optimization (ACO) approach, a heuristic optimization method, which mimics the social behavior of real ants. The ACO module is integrated with commercially available software based on early start distribution, which can be used for optimized scheduling of resource constrained construction project. The duration of the project activities scheduled using CPM and ACO approaches are compared.

Need for the study

A four storied apartment project is scheduled using CPM method, early start distribution method, uniform distribution method and heuristic method. The total duration of the project scheduled based on various approaches is given in Table 1. The quantity of the river sand, a material resource in the project is constrained to 50 percent of the quantity required in the unconstrained condition. The material resource is constrained over a period of 15 percent of estimated duration of the project in the unlimited resource

Job Thomas1, Binsu C. Kovoor2, Jeesan Jose3, Krishnan C.E.4

1Corresponding Author, Reader in Civil Engineering, School of Engineering, 2Assistant Professor in Information technology, 3Graduate Student, 4Principal, Sree Gurukulam College of Engineering, Ernakulam, India *Cochin University of Science and Technology

The optimized scheduling of activities in a project is the difficult task for a manager. The scheduling of construction project activities under limited resources, such as crew sizes, limited equipment, finance and materials is complex. The direct methods used for solving project management problems are critical path method (CPM) and project evaluation and review technique (PERT). The effectiveness of direct methods in modeling the complex situation is limited. Optimization methods yield better approximation for the complex situation. However, optimization methods often require large computational and storage capacities for large construction projects. The details of scheduling of a resource constrained construction project using ant colony optimization method (ACO) are presented in this paper. An ACO module is developed in visual basic integrated with commercially available planning and scheduling software based on early start distribution. The module is customized to account for constrains on resources such as materials and finance. The results are compared with ordinary CPM schedules. The outcome of the study indicates that the solutions created by ant colony optimization (ACO) approach are efficient and satisfactory.

Project Management Research

www.masterbuilder.co.in • The Masterbuilder - August 2012 51

condition. The material resource is constrained during its peak period of demand.

of project is observed for ACO approach with five ants when compared to the corresponding results of single ant. The study indicated that the scheme of population of ants influences the results.

Thangavel [4] showed that an ant colony optimization algorithm is a good alternative to existing algorithms for hard combinatorial optimization problems like resource constrained project scheduling. Xiong and Kuang [5] developed a hybrid model combining ant colony optimization (ACO) with serial schedule generation scheme (SSGS) for the solution of resource leveling problems in the construction projects.

Liu and Wang [6] developed a flexible model for handling the optimization of scheduling problems in linear construction projects involving different objectives and resource argument tasks. The model is suggested for the scheduling of construction activities of high-rise building or bridge projects. Jianxing and Cang [7] demonstrated the use of ant colony optimization algorithm to solve the dynamic problem of resource scheduling in group project management.

Goncalves, Menes and Resende [8] used genetic algorithm for the scheduling of multi-resource constrained project. The chromosome representation of the problem is selected based on random keys. The schedules are constructed using heuristic approach that builds based on priorities, delay time and release dates. Rogalska and Bozejko[9] developed a hybrid algorithm for the analysis of time and cost relationships performed using time coupling method (TCM). Time couplings are internal interdependencies between construction process and sectors, which take into account the resource and technical constraints.

Drezet and Billaut [10] developed a solution to the resource constrained project-scheduling problem. The model is developed accounting for the limiting resource as labor-force and activity are required to complete within a given duration. In this approach, problem is formulated with linear programming and with tabu search algorithm for optimization. Wang and Liu [11] developed a profit optimization model based on constrained programming (CP) for resource constrained projects. The model accounts for limit on resource usage and cash flow and management requirements, namely, resource and credit limits and attempts to maximize the project profit.

Christodoulou, Ellinas and Aslani [12] proposed a mathematical model for the solution of resource constrained scheduling problems accounting for the project entropy. The entropy is a measure of the degree of disorder in a system and is an indicator of project progress. Hsie, Chang and Yang [13] proposed an optimization model to minimize

The analysis of the results given in Table 1 indicates that in resource constrained condition, the minimum project duration is achieved by heuristic approach. For resource constrained construction projects, the schedule prepared using early start distribution approach and uniform distribution approach is indicative and does not accounts for all the aspects considered in the heuristic approach. Hence, a need is identified to develop a module based on heuristic approach for the scheduling of resource constrained projects. In this study, ant colony optimization (ACO) algorithm, which resembles with the heuristic method, is integrated with the commercially available planning and scheduling software based on CPM.

Background information

Colorni, Dorigo and Maiezzo [1] developed ACO approach in 1991 based on the fact that real ants are able to find the shortest path between their nest and the source of food. This is done using pheromone trails, which ants deposit whenever they travel, as a form of indirect communication. Colorni, Dorigo and Maiezzo [1] designed artificial ants, which represents solutions and the collective intelligence of ants are transformed into useful optimization techniques that find application.

Dorigo and Blum [2] reviewed the convergence requirements of ACO, connections between ACO algorithms and cross entropy methods. The influence of search bias on the working of ACO algorithms is discussed in Dorigo and Blum [2]. Mohamed [3] developed ACO algorithms with single ant, and also with five ants. The model is used for scheduling resource constrained projects. In the five ants ACO algorithm, each ant finds a solution in all iterations and uses the best-found solution so far developed for the pheromone update. A significant reduction in the duration

Schedule approach

Resource condition Project duration

Project delay

CPM Method Unlimited resource condition 182 days 0 days

Early start distribution

(Primavera P6 version)

Constrained resource condition (Material resource 50%

constrained)

208 days 26 days

Uniform distribution

(Primavera P3 version)


constrained)

225 days 41 days

Heuristic approach


constrained)

203 days 21 days

Table 1. Comparison of project duration scheduled using different approaches



the project duration. An optimal set of production rates in limited resources atmosphere is used in the model. The model accounts for the lead-time, lead distances between operations and the limited availability of multiple resources. Kumar and Rao [14] applied data mining algorithms to explore the patterns in data generated by an ant colony algorithm. A rule based scheduling module which approximates the ant colony algorithm is developed.

Abdallah and Hassan [15] extended the use of ant colony optimization module in project evaluation and review technique (PERT). The ACO module is used for path minimization. The proposed PERT-ACO model imitates the natural ants, in which, the start node is equivalent to the ant’s nest and an end node equivalent to the food location. Thiagarasu and Devi [16] presented a multi-agent coordination for environment consisting of a standard operating procedure for resource constrained project scheduling. The coordination of overlapping of activities, task scheduling and the effective resource allocation is accounted for in the model.

The literature reveals that researchers have developed numerous methods and techniques to overcome the complex nature of resource constrained project-scheduling problem. In all these models, physically renewable resources, such as manpower and machineries have been considered as the constrained resource for the analysis. In construction projects, constraints on consumable materials and cash flow are important. The availability of construction materials, such as sand, cement and aggregates may be limited during the course of construction. In this study, a model is proposed to schedule the resource constrained construction projects. The constraint on finance and on consumable material, namely river sand is considered for the illustration of the performance of the proposed model. The proposed model is based on ant colony optimization algorithm that can be used as a module of CPM based scheduling software.

Ant colony optimization (ACO) approach

Figure 1. Finding of shortest path using pheromone based communication by ants

Ant colony optimization (ACO) is a nature inspired meta-heuristic approach used for solving combinatorial optimization problems. The inspiring source of ACO is the foraging behavior of real ants. When searching for food, ants explore the area surrounding their nest in a random manner. As soon as ants find a food source, part of the food is picked up and carried to the nest. During the return trip, the ant deposits a chemical pheromone on the ground. The behavior of real ant colonies is exploited in artificial ant colonies in order to solve discrete optimization problems.

In the subsequent stages, trail pheromone information is utilized to explore the sudden unexpected change in the terrain and is given in Figure1. A colony of ants traveling over a shortest linear path between nest A and food point E is shown in Figure 1(a). The presence of the pheromone trail helps the ants to recognize the path between nest A and food point E. The ant deposits a chemical pheromone on the ground during the travel and the concentration of pheromone increases in the subsequent trips. When an unsymmetrical object is present between the path A to E, some of the ants will move in the route A-B-F-G-H-D-E and others will move in the route A-B-C-D-E and is given in Figure 1(b). It is expected that the probability of selecting the path B-F-G-H-D-E and B-C-D-E from the point B is equal. For a given duration, the total number of ants travels over the shortest duration A-B-C-D-E will be more when compare to the corresponding number of ants in the route A-B-F-G-H-D. Consequently, the pheromone concentration over the path B-C-D will be greater than that over the path B-F-G-H-D. Thereafter, an ant reaching at the point B will move in a path B-C-D-E having higher concentration of pheromone. The optimized path A-B-C-D-E having shortest route length is established for the successful completion of the food transport and is given in Figure 1(c). The behavior of ant colonies inspired in developing ant colony optimization meta-heuristic algorithm, in which a set of artificial ants co-operate to find solutions to optimization problem by depositing pheromone trails throughout the search space.

Mathematical formulation of ACO module

In a combinatorial optimization problem, the constrain is the basic component (C) and is given by

C = c1, c2, ........cn (1)

The feasible solutions (F) of the optimization problem are given by

(2)

A constrain solution function z is defined over the solution domain (R) and is given by

(3)


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The objective is to find the optimum constrain solution (S*) and is given by

(4)

An ant constructs partial solutions for the problem in the process of iterations. The solution is updated in every iteration. At each step, each ant computes a set of feasible solutions. The probability for ant k to go to state i to state j , while in its tth iteration is given by

(5)

where is the trail level indicating the proficiency and ij is the attractiveness to make a particular movement

selection in the path. and are the impact factor of trail and attractiveness. If is small, shortest activities are favored. Small values of may result in choosing non-optimal paths. The tabuk is the tabu list of ant k. In each iteration t, trail is updated and is given by

and

(6)

where represents sum of the contributions of all ants that used to move to construct their solutions. is the pheromone evaporation rate. is the trails laid on edge and is given by

(7)

where R is the pheromone reward factor and the Lk is the fitness of ant k . Lk is computed by

(8)

where Zk is the total cost and Tk is the duration of tour. The attractiveness ij in Eq. (5) is computed by

(9)

where dij is the computed duration of the project under constrained condition.

A preliminary analysis is carried out for finding the suitable values of the ACO parameters. The results of the preliminary analysis of the various constrained construction projects

indicated that the refinement project duration is limited when the ACO parameters are assigned a value greater than the value given in Table 2. Hence, ACO parameters are assumed to take a value given in Table 2.

ACO parameter Magnitude

Number of ants (m) 20

Limiting number of iterations (n) 800

Impact factor for trails ( ) 0.5

Impact factor for attractiveness ( ) 0.9

Pheromone evaporation rate ( ) 0.4

Pheromone reward factor (R) 10

Table 2. Magnitudes for the ACO parameters used in the proposed model

Working of ACO module integrated to planning and scheduling software based on CPM

The project is scheduled and budgeted in commercially available software based on CPM. The reports are exported to the customized excel template, which is the input data file to the ACO module. The details such as activities, its duration and cost are saved in cash flow template of the ACO module. The details of constrains such as quantity and duration are recorded in resource template of the ACO module. The iterations simulating the ant movement in ACO module is started to identify the optimized solution of combinatorial problems. After performing the predefined number of iterations, the optimized duration of the activities and its cost are exported to the commercially available software based on CPM. The project is rescheduled accounting for the revised optimized duration of activities in the constrained environment.

A case study

A building construction project of total duration 545 days under limited resource condition is rescheduled using ACO module integrated with commercially available software based on critical path method (CPM). The project is rescheduled in unlimited resource condition using CPM based software. The cast flow is constrained to 80 percent of that required in unconstrained condition over period of 30 days during peak period of requirement in the project. The project is rescheduled using software based on CPM with early start distribution module and CPM with ACO module. The results are given in Table 3.

Results and discussions

The duration of the activities in the construction project is given in Table 3. The duration of the activities of the project incorporating constrain on cash flow is computed using critical path method (CPM) with ant colony optimization (ACO) approach and is given in Table 3. The total duration



of the resource constrained construction project when analyzed using CPM method is found to be 4 percent higher when compared to corresponding results under unconstrained condition. The total duration of the project when analyzed using ACO module is found to be 2.3 percent when compared to the unconstrained condition. The results indicated that ACO approach is effective method for reducing the duration of resource constrained construction projects. The results of the present study corroborates with the findings of Dorigo and Blum [2]. The results of the present study indicate that the deviation of duration is positive for most of the activities in the constrained construction project. Hence it may be concluded that ACO approach is effective in scheduling the resource constrained projects when compared to conventional CPM.

Parametric study

Influence of ACO parameters, namely, limiting number of iterations (n), impact factor for trails ( ) impact factor for attractiveness ( ) and pheromone evaporation rate ( ) on the duration of the project is examined. The variation of the duration of the project from the optimum solution is given in Figure 2.

Activities Scheduled duration of the activities in days

Unlimited Resource state in CPM

Constrained Resource state in CPM with

Deviation (%)

early start distribution ACO

(1) (2) (3) (4) (5)=[(3)-(4)] ÷(3) *100

Reinforcement work for beams & slab at floor 1 60 71 72 -1.41

Column concrete at floor 1 18 24 22 8.33

Beam slab concrete at floor 1 48 54 52 3.7

Reinforcement work for beams & slab at floor 2 60 72 70 2.78

Column concrete at floor 2 18 24 21 12.5


Reinforcement work for beams & slab at floor 3 60 71 66 7.04

Column concrete work at floor 3 18 24 25 -4.17

Solid block masonry for at G floor 32 44 46 -4.55

Lintel sunshade rack drop concrete at G floor 15 24 25 -4.17


P C C 1:4:8 for flooring at G floor 6 8 9 -12.5

Solid block masonry work at floor 1 32 44 41 6.82

M S grill work fixing for windows at G floor 16 18 16 11.11

Chasing walls & pipe laying at G floor 16 18 16 11.11

Chasing walls and electrical conduits at G floor 16 18 16 11.11

Total Project Duration 545 567 558 1.59

Table 3. Comparison of duration of activities of resource constrained project scheduled using CPM and ACO approach

The influence of the number of iterations (n) is given in Figure 2(a). The deviation is minimum when the magnitude of n is equal to 800. The influence of impact factor for trial ( ) is given in Figure 2(b). The deviation is minimum when the magnitude of is equal to 0.9. The influence of impact factor for attractiveness ( ) is given in Figure 2(c). The deviation is found to be minimum when the magnitude of takes a value of 0.5. The influence of pheromone evaporation rate ( ) is given in Figure 2(d). The deviation is found to be minimum when the magnitude of is equal to 0.8. The results given in Figure 2 indicated that the variation of total duration of the project from optimum solution is random and does not follow a trend. The parameters have no direct linear or nonlinear relationship with the performance of algorithm.

Conclusions

Based on the present study, the following conclusions are derived.

- The ant colony optimization (ACO) approach yields better solutions for scheduling of resource constrained projects when compared to the conventional approaches given in commercially software.



- The comparison of the schedules indicated that duration of the project computed based on ant colony optimization (ACO) method is less than the corresponding results of critical path method (CPM).

- Influence of the ACO parameters on total duration of the project is random. Hence trials are required to finalize the values for the ACO parameters

References

1. A. Colorni, M. Dorigi, V. Maniezzo, Distributed optimization by ant colonies, Proceedings of European Conference on Artificial Life, Elsevier Publishing, Amsterdam, 1991.

2. M. Dorigo, C. Blum, Ant colony optimization theory: A survey. J. of Theo. Com. Sc., 344 (2005) 243-278

3. A. Mohamed, Resource constrained project scheduling problem by ant colony optimization. NICMAR- J. of Const. Man., 3 (2005) 232-240.

4. K. Thangavel, Ant Colony Algorithms in Diverse Combinational Optimization Problems -A Survey, ASCE J. of Const. Man., 6 (2006) 78-89.

5. Y. Xiong, Y. Kuang, Ant colony optimization algorithm for resource leveling problem of construction project. Adv. of Con. Man. and Re. Es. 1 (2006).212-226.

6. S. Liu, C. Wang, Optimization model for resource assignment problems of linear construction projects. J. of Auto. in Con. 16 (2007) 460-473.

7. Y. Jianxing, Y. Cang, Study of resource scheduling in project group

management of offshore engineering based on ACO. J. of Sys. Eng. –Th. & Pra. 18 (2007) 204-215.

8. J. Goncalves, J. Menes, M. Resende, Genetic algorithm for the resource constrained multi-project scheduling problem. J. of Ope. Res. 189 ( 2008) 1171-1190.

9. M. Rogalska, W. Bozejko, Time/cost optimization using hybrid evolutionary algorithm in construction project scheduling. J. of Auto. in Con. 18 (2008) 24-31.

10. L. Drezet, J. Billaut, A project scheduling problem with labor constraints and time-dependent activities requirements. Int. J. of Pro. Eco. 112 (2008) 217-225.

11. C. Wang, S. Liu, Resource constrained construction project scheduling model for profit maximization considering cash flow. J. of Auto. in Con. 17 (2008) 966-974.

12. S. Christodoulou, G. Ellinas, P.Aslani, Entropy-based scheduling of resource- constrained construction projects. J. of Auto. in Con,.18 (2009) 919-928.

13. M. Hsie, C. Chang, T. Yang, C. Huang, Resource constrained scheduling for continuous repetitive projects with time- based production units. J. of Auto. in Con. 18 (2009) 942-949.

14. S. Kumar, C. Rao, Application of ant colony, genetic algorithm and data mining-based technologies for scheduling. J. of Rob. and Comp. Int. Man. 25 (2009) 901-908.

15. H. Abdalla, Hassan, Using Ant colony optimization algorithm for solving project management problems. J. of Exp. Sys. with App. 36 (2009)10004-10015.

16. V. Thiagarasu, T. Devi, Multi agent coordination in project scheduling: Priority rules based resource allocation. Int. J. of Rec. Tr. in Eng. 1 (2009) 314-326.

Figure 2. Influence of ACO parameters on the deviation of total duration of the project from optimum solution


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Critical Chloride Content in Reinforced Concrete

After it was recognised in the second half of the last century that chloride may induce steel corrosion in reinforced concrete structures, great research efforts

have been made in this regard: over the last fifty years, a considerable amount of papers has been published presenting values for critical threshold chloride content (CTL) in reinforced concrete. Considering marine exposure conditions and the extensive use of de-icing salts in many countries, chloride induced corrosion is one of the most

common causes of degradation of reinforced concrete structures. Both for the design of new structures and for condition assessment of existing structures, knowledge of reliable CTL values is important as the remaining service life is often considered as the time required to reach the chloride threshold value at the depth of the reinforcement. In probabilistic service life modelling, CTL has been identified to be one of the most decisive input parameters. Despite the multitude of studies undertaken, many aspects

Sonjoy Deb, B.Tech,’Civil’

Associate Editor

“The steel rebar inside reinforce concrete structures is susceptible to corrosion when permeation of chloride from deicing salts or seawater results in the chloride content at the surface of the steel exceeding a critical chloride threshold level (CTL). The CTL is an important influence on the service life of concrete structures exposed to chloride environments. The key factor on CTL was found to be a physical condition of the steel–concrete interface, in terms of entrapped air void content, which is more dominant in CTL rather than chloride binding, buffering capacity of cement matrix or binders.”

Corrosion in Concrete


of chloride induced reinforcement corrosion in concrete are still incompletely understood and no general agreement on a CTL value has been achieved. Results reported in the literature scatter over a large range. This is not only the result of different definitions, measuring techniques and testing conditions, but also owing to the stochastic nature and complexity of initiation of pitting corrosion. Thus, often conservative values are now a days used as critical chloride content: In European countries as well as in North America it has become common practice to limit the tolerable chloride content to or around 0.4% by weight of cement. In probabilistic modelling the critical chloride content is a stochastic variable as e.g. in the fib model code for service life design, where CTL is defined by a beta distribution with a lower boundary of 0.2% chloride by weight of cement and a mean value of 0.6% by weight of cement. Although there is a strong need for reliable CTL values, an accepted or standardized test method to measure critical chloride does at present not exist. The present review summarises the state of the art regarding critical chloride content in reinforced concrete. It is not only aimed at collecting CTL values reported in the literature, but also all the relevant details about experimental procedures are collated.

Marine exposure and the extensive use of de-icing salts in many countries are the most common causes of degradation of reinforced concrete structures

Concept of critical chloride content

Reinforcement corrosion in non-carbonated, alkaline concrete can only start once the chloride content at the steel surface has reached a certain threshold value. In the literature, this value is often referred to as critical chloride content or chloride threshold value. In the present work CTL is used. Two different ways of defining CTL are common: From a scientific point of view, the critical chloride content can be defined as the chloride content required for de-passivation of the steel (Definition 1),whereas from a practical engineering point of view CTL is usually the

chloride content associated with visible or “acceptable” deterioration of the reinforced concrete structure (Definition 2).

It has to be emphasized that the two definitions are related to different phenomena: the de-passivation-criterion in Definition 1 only considers the initiation stage, whereas in the case of Definition 2 with visible or acceptable deterioration as a criterion, also the propagation stage is included. As a result, the two definitions lead to different CTL values. Figure 1 illustrates this by combining Tuutti’s corrosion model with an assumed curve representing the chloride concentration at the steel reinforcement vs. time. The figure clearly shows that using the practical definition leads to higher CTL values. It is important to understand that this is only the result of a longer time passing until the chloride content is determined. The rate at which corrosion proceeds has a large influence on when this is done and thus greatly affects the chloride threshold value when applying this definition. Definition 1 is more precise, since it expresses the chloride content that is directly related to de-passivation. In Definition 2, the chloride content associated with an acceptable degree of corrosion has no theoretical background: the amount of chloride that is measured at that time has nothing to do with the degree of corrosion or the corrosion rate. Also the term “acceptable degree” is imprecise and thus definition 2 results in a larger scatter of CTL values. In the literature, these two definitions are often mixed up. Care has thus to be taken when comparing and evaluating results reported by different researchers.

Initiation Propagation

high corrosion rate

low corrosion rateacceptable

deterioration

Time

Time

2

2

2

1

1

Deg

ree

of

co

rro

sio

nC

hlo

rid

eco

ncen

tra

tio

n

at

rein

forc

em

en

t

Threshold according to scientific definition (depassivation)

Threshold according to practical definition

(visible or “acceptable” deterioration)

Figure 1: Definitions for chloride thresholds (based on Tuutti’s model)



Expression of CTL

- Free chloride content

The representation of CTL reflects the aggressive ion content and inhibitive properties of the cement matrix. Chloride ions which are removed from the pore solution as the result of an interaction with the solid matrix (bound chloride) are relatively immobile and may not be transported to the steel surface. This should in theory favour the use of the free chloride content (water soluble chloride) to represent the CTL. Results by Petters on show a wide range of the CTL values in terms of free chloride concentration, ranging from 0.28 to 1.8 M in mortar specimens with water/cement ratios between 0.3 and0.75. More recent works by Alonso et al. reported CTL values in terms of free chloride content by weight of cement, ranging from 0.3% to 2.0%.Early works suggested that only the free chloride contributes to the corrosion process and hence the free chloride content was regarded as the best expression of

Table: Chloride threshold level reported by various authors with varying conditions

Condition Threshold Values Detection method Reference

Total chloride (%, cem.) Free Chloride (%, cem.) [Cl–]/[OH–]

Pore solution 0.6 Half-cell potential [2]

0.3 Polarisation [33]

Specimen + internal C1” 8–63 Polarisation [34]

0.5–2.0 Macrocell current [1]

0.079–0.19 AC impedance [81]

0.32–1.9 Mass loss [14]

0.78–0.93 0.11–0.12 0.16–0.26 Half-cell potential [12]

0.45 (SRPC) 0.10 0.27

0.90 (15% PFA) 0.11 0.19

0.68 (30% PFA) 0.07 0.21

0.97 (30% GGBS) 0.03 0.23

0.35–1.00 0.14–0.22 C1”/OH” = 0.3 [48]

Speciment + external C1” 0.227 0.364 1.5 Polarisation [28]

0.5–1.5 Half-cell potential [11]

0.70 (OPC) Mass loss [15]

0.65 (15% PFA)

0.50 (30% PFA)

0.20 (50% PFA)

1.8–2.9 Polarisation [26]

0.5–1.4 Non mentioned [25]

0.6–1.4 Macrocell [50]

Structure 0.2–1.5 Mass loss [3]

Note: SRPC: sulphate resistant Portland cement, PFA: pulverised fly ash, GGBS: ground granulated blast furnace slag, OPC: ordinary Portland cement.

this. These proposals have been challenged by current thinking, when considering that (1) bound chlorides at the steel depth are released to form free chlorides when the pH drops due to depassivation, and (2) cement hydration products such as calcium hydroxide resist a fall in pH at a particular value of the pH. It should be noted that current guidelines and standards do not address the free chloride content in relation to corrosion risk, largely for the reasons mentioned above. The free chloride content is more often expressed as a function of hydroxyl ion concentration in the pore solution, or the mole ratio of chloride to hydroxylions.

- [Cl-]:[OH+]

This approach assumes that bound chlorides are not a risk to corrosion, and that the hydroxyl ion concentration reflects the inhibitor content of the environment by sustaining the high pH of the pore solution. In early works, the relation between free chloride and hydroxyl concentration was used to express the CTL in terms of the ratio of free chloride to


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hydroxyl concentration. This expression of the CTL is still currently used. A threshold ratio varying from 0.3 to 40.0, as given in Table 1, was reported.

- Total chloride

The representation of the CTL by the total chloride level is the most widely used approach, and is the approach adopted in standards. Table 2 gives the limit of the total chloride content of concrete from each standard. The representation of the CTL as the total chloride content as a percentage by weight of cement, is favored because it is relatively easy to determine and because it involves the corrosion risk of bound chloride and the inhibitive effect of cement hydration products. At the stage of corrosion initiation, the pH in the vicinity of the steel falls locally as a result of an electro chemical reaction with chloride and ferrous ions during pit nucleation. Corrosion is initiated in the form of pitting where the local pH falls below 10. The drop in pH releases at least 90% of the total surrounding chloride ions to participate in the corrosion process with access to oxygen and water as well as chloride accelerating the rate of corrosion. This suggests that the total chloride content is a more accurate indicator of corrosion risk and the inhibitive nature of cement may thus be better reflected by the total cement content rather than the pore solution pH. Hence, the total chloride content to cement weight is the better representation of the CTL because (1) the inhibitive properties of cement matrix are reflected by its cement content and (2) the total aggressive potential of chloride ions is represented.

- [Cl-]:[H+]

In a recent work, it was suggested that a more appropriate representation of the inhibitive and aggressive properties of concrete is provided, respectively, by its acid neutralization capacity (ANC) and acid soluble chloride content. The acid neutralization capacity has been used to quantify the buffering capacity of concrete. The content of acid needed to reduce the pH of concrete and cement paste suspended in water, up to a particular value, has been reported by Sergi and Glass. The acid (moles H+/kg binder) required to reduce the pH to 10 was determined as 18.9, 17.5, 15.4 and 14.5 mol/kg for OPC, sulfate resisting Portland

Table 2: Maximum chloride content values set by various ACI and BS documents

Type Maximum chloride content (%, cem.)

BS 8110 ACI 201 ACI 357 ACI 222

Prestressed concrete 0.10 0.06 0.08

Reinforced concrete exposed to chloride in service 0.20 0.10 0.10 0.20

Reinforced concrete that will be dry or protected from moisture in service 0.40

Other reinforced concrete 0.15

cement (SRPC), 30% pulverized fly ash (PFA) and 65% ground granulated blast furnace slag (GGBS), respectively. Thomas determined the CTL of OPC and 30% PFA content as 0.7% and 0.5% by weight of cement, respectively. Based on these data, the CTL for OPC and 30% PFA equate to the same mole ratio of 0.01[Cl-]:[H+]. A mole ratio of 0.01 also approximates to 0.65% and 0.5% chloride by weight of cement in SRPC and 65% GGBS concretes, respectively. The ratio of total chloride to ANC is probably the best representation of the CTL, since it considers all potentially important inhibitive (cement hydration products) and aggressive (total chloride) factors.

Influencing parameters

From an electrochemical point of view, it is the potential of the steel, Ecorr, relative to the pitting potential, Epit, that determines whether corrosion will start or not. The pitting potential depends on both environmental influences (chloride content) and on properties of the metal such as the degree of alloying (e.g. stainless steel). The open circuit potential of the passive steel, on the other hand, only depends on the environment (pH and oxygen content).Where as parts of the steel electrode are in contact with the concrete pore liquid, others might be covered with hydration products and thus to a certain extent be shielded from aggressive species in solution. The critical chloride content in concrete is thus not only a matter of pure electro chemistry, but also of physical protection of the steel electrode. Numerous parameters affect the value of CTL and many of them are interrelated:

- Steel–concrete interface

- Concentration of hydroxide ions in the pore solution (pH)

- Electrochemical potential of the steel

- Binder type

- Surface condition of the steel

- Moisture content of the concrete

- Oxygen availability at the steel surface

- w/b ratio

- Electrical resistivity of the concrete

- Degree of hydration



- Chemical composition of the steel- Temperature- Chloride source (mixed-in initially or penetrated into

hardened concrete)- Type of cation accompanying the chloride ion- Presence of other species, e.g. inhibiting substances.

It has been suggested to consider the condition of the steel–concrete interface as the most dominating influencing factor, together with the pH of the concrete pore solution and the steel potential.

The variety of factors involved indicates that the concept of critical chloride content faces some difficulties regarding a unique chloride threshold value applicable to a wide range of structures.

Raising threshold values

- Corrosion inhibitors

The advantage of using corrosion inhibitors to provide corrosion protection is that the inhibitor is well distributed throughout the concrete, which means that it protects all the steel. A corrosion inhibitor modifies the surface chemistry of steel to mitigate or prevent the corrosion process. While numerous corrosion inhibitors have been suggested, the detrimental effects of many of them in concrete limit their commercial use. Calcium nitrite has been widely used as a corrosion inhibitor in concrete since the middle of the 1970s, because of its inhibiting effect as well as its compatibility with concrete. It enhances the compressive strength at an early age, and accelerates the setting time within the range recommended by standards.

- Coating of reinforcing steel

Galvanisation

A galvanised (zinc) coating acts both as a sacrificial coating in protecting steel. It is reported in laboratory and field studies that galvanising increases the CTL. Treadaway et al. showed that galvanised steel in a concrete structure exposed to corrosive conditions delayed the initiation of corrosion and resulted in a CTL of 0.9% by weight of cement. The results of the monitoring of concrete structures in seaside environments over an 8–23 year period suggested a CTL of 0.64% when using galvanised steel, while untreated steel showed a CTL of 0.2%. Bautista and Gonzalez found that the corrosion rate for galvanised steels was much lower than that for bare steel; the corrosion rate for galvanised steel ranged from 0.2 to 1.2 lA/cm2, while for bare steel it ranged from 0.4 to 10 lA/cm2 after 12 months exposure to a chloride solution. The inhibiting effect of galvanizing appears to be enhanced in high-performance concrete, with the time to corrosion considerably delayed.

Barrier coating

The corrosion of reinforcement in concrete can be prevented by coating the steel with epoxy, which stops aggressive ions reaching the steel surface. Care is required in the handling, transporting, storing and placing of epoxy-coated steel since damage can impair its corrosion pro-

Hot dipped galvanized steel coil

tection performance. Erdogdu et al. showed that the corrosion rate of coated steel bars was below 0.01 mA/m2 after 25 months exposure, compared to 2–100 mA/m2 for uncoated steel bars after 5 months exposure. Al-Amoudiet al. showed the effect on CTL of epoxy-coated steel with various degrees of coating damage. With 1% damage to the coating, the CTL was about 2% by weight of cement, while at 2% damage, the CTL was below 0.4%.

Recent studies have shown that epoxy-coated steel can give good, long term performance even on severe exposure to chloride conditions and considering the effects of bond loss when properly coated and handled. Cement-based coatings rather than resin coatings have been suggested because they perform better due to the higher bond strength as well as corrosion protection.

Liquid Epoxy Coatings for steel



Conclusion

The present study revealed the following conclusions, with respect to CTL representation, influencing factors, and methods to raise the CTL.

- The CTL value depends on how it is expressed, such as the mole ratio of [Cl-]:[OH+],free chloride, or total chloride. The CTL has been expressed as free chloride or [Cl-]:[OH+] in many previous studies, as being very widely ranged. The free chloride content or [Cl-]:[OH+] has the disadvantage of poor accuracy and repeatability. It fails to consider the participation of bound chloride in sustained corrosion and the buffering capacity of the cement matrix. The representation most widely used for the CTL is total chloride content relative to the cement weight, as it takes into account the inhibiting effect of cement and the aggressive nature of chloride and is convenient. CTL values in total chloride content are within a relatively narrow range, compared to values expressed in free chloride or [Cl-]:[OH+].The mole ratio of total chloride to the acid neutralisation capacity of cement (expressed as the mole concentration of H+) has been proposed as a better method of capturing the inhibiting effect.

- The corrosion of steel is initiated at defects at the steel–concrete interface, commonly at entrapped air voids where there is an absence of cement hydration products. Hence, an increase in the air voids content at the interface leads to a greater probability of a lower CTL. The majority of previous studies of CTL have investigated the influence of binder type, in particular C3A content. This has not resulted in a more precise definition because bound chloride is freed when there is a local fall in pH. The influence of replacement materials, in particular pulverised fly ash and ground granulated blast furnace slag on the CTL is subject to debate. The CTL for concrete containing pozzolanic materials depends on whether the chlorides are introduced from an external environment or from the concrete constituents, and/or detecting on the method of corrosion initiation.

- Calcium-nitrite based corrosion inhibitors have been successfully applied to concrete structures for enhancing the resistance to chloride-induced corrosion. Calcium nitrite in general, remarkably raised the CTL, and thus a much longer time to corrosion is expected. However, nitriteions present in concrete allow external chlorides to more easily penetrate concrete, thereby off setting the effect of increased CTL in prolonging the service life of structures. Galvanisation and barrier coating have been used to protect the embedded steel in concrete from chloride or carbonation attack. However, their defect

at the steel–concrete interfaces (i.e. a reduction of bond), which may be attributed to hydrogen evolutionor smooth surface of coating, restricts the use in concrete structures.

Reference

- K. Tuutti, Corrosion of Steel in Concrete, Swedish Cement and Concrete Research Institute, 1982.

- D.A. Hausmann, Steel corrosion in concrete; How does it occur? Materials and Protection 6 (1967) 19–23.

- British Standard 8110: Part 1, Structural use of concrete – code of practice for design and construction, British Standards Institute, London UK, 1985.

- M. Thomas, Chloride thresholds in marine concrete, Cement and Concrete Research 26 (1996) 513–519.

- C. Arya, J.B. Newman, An assessment of four methods of determining the free chloride content of concrete, Materials and Structures 23 (1990) 319–330.

- K. Tuutti, Effect of cement type and different additions on service life, in: R.K. Dhir, M.R. Jones (Eds.),Concrete 2000, vol. 2, E& FN Spon, London UK, 1993, pp. 1285–1296.

- K. Pettersson, Chloride threshold value and corrosion rate in reinforcement concrete, in: R.K. Dhir, M.R.Jones (Eds.), Concrete 2000, vol. 1, E& FN Spon, London UK, 1993, pp. 461–471.

- C. Alonso, C. Andrade, M. Catellote, P. Castro, Chloride threshold values to depassivate reinforcing barsin a standardized OPC mortar, Cement and Concrete Research 30 (2000) 1047–1055.

- C. Alonso, M. Castellote, C. Andrade, Chloride threshold dependence of pitting potential of reinforcements, Electrochemica Act a 47 (2002) 3469–3481.

- B.B. Hope, J.A. Page, J.S. Poland, The determination of chloride content of concrete, Cement and Concrete Research 15 (1985) 863–870.

- G.K. Glass, B. Reddy, N.R. Buenfeld, The participation of bound chloride in passive film breakdown onsteel in concrete, Corrosion Science 42 (2000) 2013–2021.

- G.K. Glass, B. Reddy, N.R. Buenfeld, Corrosion inhibition in concrete arising from its acid neutralization capacity, Corrosion Science 42 (2000) 1587–1598.

- G.K. Glass, N.R. Buenfeld, The presentation of the chloride threshold level for corrosion of steel in concrete, Corrosion Science 39 (1997) 1001–1013.

- V.K. Gouda, Corrosion and corrosion inhibition of reinforcing steel; 1 – Immersion in alkaline solution, British Corrosion Journal 5 (1970) 198–203.

- T. Yonesawa, V. Ashworth, R.P.M. Procter, Pore solution composition and chloride effects on the corrosion of steel in concrete, Corrosion 44 (1988) 489–499.

- O.A. Kayyali, M.N. Haque, The Cl-/OH-ratio in chloride-contaminated concrete – a most important criterion, Magazine of Concrete Research 47 (1995) 235–242.

- A.K. Suryavanshi, J.D. Scantlebury, S.B. Lyon, Corrosion of reinforcement steel embedded in high water-cement ratio concrete contaminated with chloride, Cement and Concrete Composites 20 (1998) 263–281.


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- M. Castellote, C. Andrade, C. Alonso, Accelerated simultaneous determination of the chloride depassivation threshold and of the non-stationary diffusion coefficient values, Corrosion Science 44(2002) 2409–2424.

- C.L. Page, K.W.J. Treadaway, Aspects of the electrochemistry of steel in concrete, Nature 297 (1982) 109–115.

- J. Tritthart, Chloride binding: II. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding, Cement and Concrete Research 19 (1989) 683–691.

- G.K. Glass, N.M. Hassanein, N.R. Buenfeld, Neural network modelling of chloride binding, Magazine of Concrete Research 49 (1997) 323–335.

- ACI Committee 201, Guide to Durable Concrete, Manual of Concrete Practice, Part 1, American Concrete Institute, Detroit USA, 1994.

- ACI Committee 357, Guide for design and construction of fixed off-shore concrete structures, Manual of Concrete Practice, Part 4, American Concrete Institute, Detroit USA, 1994.

- ACI Committee 222, Corrosion of metals in concrete, Manual of Concrete Practice, Part 3, American Concrete Institute, Detroit USA, 1994.

- B. Reddy, Influence of the steel–concrete interface on the chloride threshold level, PhD Thesis, University of London, 2001.

- G. Sergi, G.K. Glass, A method of ranking the aggressive nature of chloride contaminated concrete, Corrosion Science 42 (2000) 2043–2049.

- R. Cigna, C. Andrade, U. Nürnberger, R. Polder, R. Weydert, E. Seitz (Eds.), COST 521:Final Report “Corrosion of Steel in Reinforced Concrete Structures”, Luxembourg, 2002.

- L. Bertolini, B. Elsener, P. Pedeferri, R. Polder, Corrosion of Steel in Concrete, WILEY VCH, 2004.

- R.J. Craig, L.E. Wood, Effectiveness of corrosion inhibitors and their influence on the physical properties of Portland cement mortars, Highway Research Record 328 (1970) 77–88.

- A.M. Rosenberg, J.M. Galdis, T.G. Kossivas, R.W. Previte, Corrosion inhibitor formulated with calciumnitrite for use in reinforced concrete, in: D.E. Tonini, S.W. Dean (Eds.), Chloride Corrosion of Steel in Concrete, ASTM STP 629, 1976, pp. 89–99.

- A.M. Rosenberg, J.M. Gaidis, Methods of determining corrosion susceptibility of steel in concrete, Transportation Research Record 692 (1978) 28.

- D. Chin, A calcium nitrite-based, non-corrosive, non-chloride accelerator, in: F.W. Gibson (Eds.),Corrosion, Concrete and Chloride, ACI SP 102, 1987, pp. 49–77.

- K.Y. Ann, H.S. Jung, H.S. Kim, S.S. Kim, H.Y. Moon, Effect of calcium nitrite-based corrosion inhibitorin preventing corrosion of embedded steel in concrete, Cement and Concrete Research 36 (2006) 520–525.

- A.U. Malik, I. Andijani, F. Al-Moaili, G. Ozair, Studies on the performance of migratory corrosion inhibitors in protection of rebar concrete in Gulf seawater environment, Cement and Concrete Composites26 (2004) 235–242.

- N.S. Berke, M.C. Hicks, Predicting long-term durability of steel reinforced concrete with calcium nitritecorrosion inhibitor, Cement and Concrete Composites 25 (2004) 439–449.

- P. Montes, T.W. Bremner, D.H. Lister, Influence of calcium nitrite inhibitor and crack width on corrosion of steel in high performance concrete subjected to a simulated marine environment, Cement and Concrete Composites 26 (2004) 243–253.

- B.B. Hope, A.K.C. Ip, Corrosion inhibitors for use in concrete, ACI Material Journal 86 (1989) 602–608.

- J.T. Lundquist, A.M. Rosenberg, J.M. Gaidis, A Corrosion Inhibitor Formulated with Calcium Nitrite for Chloride Containing Concrete Improved Electrochemical Test Procedure, The International Corrosion Forum, San Francisco USA, 1977.

- B. El-Jazairi, N.S. Berke, The use of calcium nitrite corrosion inhibitors in concrete, in: C.L. Page, P.B.Bamforth, J.W. Figg (Eds.), Corrosion of Reinforcement in Concrete Construction, Cambridge UK, 1990,pp. 571–587.

- A. Bentur, S. Diamond, N.S. Berke, Steel Corrosion in Concrete, first ed., E& FN SPON, 1997.

- D. Stark, Measurement techniques and evaluation of galvanized reinforcing steel in concrete structures in Bermuda, in: D.E. Tonini, J.M. Gaidis (Eds.), Corrosion of Reinforcing Steel in Concrete, ASTM STP 713,1978, pp. 132–141.

- A. Bautista, J.A. Gonza´lez, Analysis of the protective efficiency of galvanizing against corrosion of reinforcements embedded in chloride contaminated concrete, Cement and Concrete Research 26 (1996)215–224.

- N. Gowripalan, H.M. Mohamed, Chloride-ion induced corrosion of galvanized and ordinary steel reinforcement in high-performance concrete, Cement and Concrete Research 28 (1998) 1119–1131.

- L.H. Everett, T.W.J. Treadaway, The use of galvanized steel reinforcement in building, Building Research Station Current Paper CP3/70, Garston UK, 1970.

- O.A. Kayyali, S.R. Yeomans, Bond of ribbed galvanized reinforcing steel in concrete, Cement and Concrete Composites 22 (2000) 459–467.

- S. Erdogdu, T.W. Bremner, I.L. Kondratova, Accelerated testing of plain and epoxy-coated reinforcement in simulated seawater and chloride solutions, Cement and Concrete Research 31 (2001) 861–867.

- O.S.B. Al-Amoudi, M. Maslehuddin, M. Ibrahin, Long-term performance of fusion-bonded epoxy-coatedsteel bars in chloride-contaminated concrete, ACI Material Journal 101 (2004) 303–309.

- R.E. Weyer, W. Pyc, M.M. Sprinkel, Estimating the service life of epoxy-coated reinforcing steel, ACI Material Journal 95 (1998) 546–557.

- A.B. Darwin, J.D. Scantlebury, Retarding of corrosion processes on reinforcement bar in concrete with an FBE coating, Cement and Concrete Composites 24 (2002) 73–78.

- R. Vedalakshmi, K. Kumar, V. Raju, N.S. Rengaswamy, Effect of prior damage on the performance of cement based coatings on rebar: macro cell corrosion studies, Cement and Concrete Composites 22 (2000)417–421.

Photo Courtesy

www.epoxytec.blogspot.in, www.portstrategy.comwww.diytrade.com, www.stuartsteel.com


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An Approach towards High Performance Concrete

Long-term performance of structures has become vital to the economies of all nations. Concrete has been the major instrument for providing a stable and reliable infrastructure

since the days of the Greek and Roman civilization. At the turn of the 20th century, concrete compressive strength was in the range of 13.8 MPa, by the1960s it was in the range of 27.6 - 41.4 MPa. Deterioration, long term poor performance, and inadequate resistance to hostile environment, coupled with greater demands for more sophisticated architectural form, led to the accelerated research into the microstructure of cements and concretes and more elaborate codes and standards. As a result, new materials and composites have been developed and improved cements evolved. Today concrete structures with a compressive strength exceeding138 MPa are being built world over. In research laboratories, concrete strengths even as high as 800 MPa are being produced.

One major remarkable quality in the making of High per-formance concrete (HPC) is the virtual elimination of voids in the concrete matrix, which are mainly the cause of most of the ills that generate deterioration. ACI defines HPC as “Concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices”. Such concretes can be either normal strength or high strength. Normal strength concrete by ACI definition is a concrete that has a cylinder compressive strengths not exceeding 42 MPa. All other concretes are considered High Strength Concretes (HSC).HPCs with 140 MPa are currently being used in high rise structures in USA and Europe.

Important governing factors for HPCs are strength, long term durability, serviceability as determined by crack and deflection


Associate Editor

High Performance Concrete


control, as well as response to long term environmental effects. High performance concretes(HPC) are concretes with properties or attributes which satisfy the performance criteria. Generally, concretes with higher strengths and attributes superior to con-ventional concretes are desirable in the construction industry. HPC is defined in terms of Strength and Durability.

Therefore HPC can be considered as a logical development of cement concretes in which the ingredients are pro-portioned and selected to contribute efficiently to the various properties of cement concrete in fresh as well as in hardened states.

Salient Features of HPC[2]

- High Strength 42 – 100 MPa, Very High Strength 100 – 150 MPa, Ultra High Strength > 150 MPa.

- Water-binder ratio =0.25-0.35 ,therefore very little free water

- Reduced flocculation of cement grains - Wide range of grain sizes - Densified cement paste - No bleeding homogeneous mix - Less capillary porosity - Discontinuous pores - Stronger transition zone at the interface between cement

paste and aggregate - Low free lime content - Endogenous shrinkage - Powerful confinement of aggregates - Little micro-cracking until about 65-70% of characteristic

strength- Smooth fracture surface

Advantages of using HPC

The advantages of using high strength high performance concretes often balance the increase in material cost. The following are the major advantages that can be accomplished.[1]

- Reduction in member size, resulting in increase in plinth area / useable area and direct savings in the concrete volume saved.

- Reduction in the self-weight and super-imposed DL with the accompanying saving due to smaller foundations.

- Reduction in form-work area and cost with the accom-panying reduction in shoring and stripping time due to high early-age gain in strength.

- Construction of High –rise buildings with the accompanying savings in real- estate costs in congested areas.

- Longer spans and fewer beams for the same magnitude of loading.

- Reduced axial shortening of compression supporting members.

- Reduction in the number of supports and the supporting foundations due to the increase in spans.

- Reduction in the thickness of floor slabs and supporting beam sections- which are a major component of the weight and cost of the majority of structures.

- Superior long term service performance under static, dynamic and fatigue loading.

- Low creep and shrinkage.- Greater stiffness as a result of a higher modulus, Ec- Higher resistance to freezing and thawing, chemical

attacks, and significant improvement in long-term dura-bility and crack propagation.

- Reduced maintenance and repairs.- Smaller depreciation as a fixed cost.

Composition of HPC

The ingredients of HPCs are almost same as those of normal strength concretes (NSC). But, because of lower water cement ratio, presence of pozzolans and chemical admixtures etc., the HPCs usually have many features which distinguish them from NSCs. From practical con-siderations, in concrete constructions, apart from the final strength, the rate of development of strength

High performance concrete allows for longer spans with fewer beams High performance concrete requires lesser maintenance and repairs.



is also very important. The high performance concrete usually contains both pozzolanic and chemical admixtures. Hence, the rate of hydration of cement and the rate of strength development in HPC is quite different from that of normal strength concretes (NSC). The proportioning(or mix design) of normal strength concretes is based primarily on the w/c ratio ‘law’ first proposed by Abrams in 1918.For high strength concretes, however, all the components of the concrete mixture are pushed to their limits. Therefore, it is necessary to pay careful attention to all aspects of concrete production, i.e., selection of materials, mix design, handling and placing.[1][2]

In essence, the proportioning of HPC concrete mixtures consists of three interrelated steps:

- Selection of suitable ingredients - cement, supplementary cementitious materials (SCM), aggregates, water and chemical admixtures

- Determination of the relative quantities of these materials in order to produce, as economically as possible, a concrete that has the rheological properties, strength and durability,

- Careful quality control of every phase of the concrete making process.

Types of Supplementary Cementitious Materials

The most commonly used supplementing cementitious materials / mineral admixtures for achieving HPC are [1][2][3]

- Silica Fume - Fly Ash - GGBFS(Ground granulated blast furnace slag)- High Reactivity Metakaolin (HRM)

Mix proportions for HPC

Only a few formal mix design methods have been developed for HPC / HSC to date. Most commonly, purely empirical procedures based on trial mixtures are used. Therefore, it calls for extensive field trials for designing desired strength of concrete using various mix proportions of SCMs, admixtures and water / binder ratio.

Use of Super-plasticizers

Use of super-plasticizers becomes essential for designing mixtures to achieve HPC.A scan be seen, the water / binder ratio has an important bearing on achieving the strength parameters. In order to achieve dense concrete with reduced permeability, super-plasticizers of following types are in general use[1][3]

1. SNF – Sulphonated Napthalene based2. Melamine sulphonate based3. Lignosulphonate based4. Polycarboxylic type.

Of the above types, the latest and the most effective super-plasticizer is SNF based. ASTM also has recommended

use of this type for attaining the optimum benefits like good workability and minimum w/binder ratio. Around 2% by weight of cementitious materials is normally used for achieving required workability.

Chemical and physical propertiesof the SCMs and flow chart for typical design mix

The following table gives the chemical properties of the above SCMs. However, the values given here are only to appreciate the range and percentage of each of the elements contained in them.The cited values vary between products obtained from various sources for the same SCM. [1][2][3][6]

Curing of HPC -The most intricate part[1][3][5]

HPC has very low w / binder ratio and better particle distribution due to the use of mineral admixtures, which result in signifi-cantly less pore per unit volume of cementitious materials in the mixture than the NSC. Filling of the voids by hydration product in HPC is much faster than that of NSC as smaller pores needs less hydration products to fill. Therefore, moisture loss due to capillary action stops earlier in case of HPC compared to NSC under the same curing conditions. The moisture loss from HPC has been found predominant up to the first 24 hours. Owing to very low w / binder ration and use of super plasticizer, the early stage hydration rate of HPC is higher than NSC leaving less long term hydration potential. Curing duration after the initial moisture protection has been found to have little effect on long term chloride permeability of HPC containing micro-silica or fly-ash. All these indicate that the requirement of curing duration for HPC is less compared to NSC.

Duration of wet curing has significance on the shrinkage of HPC, which is not the case with NSC. Method of curing has similar effect on HPC both for creep and shrinkage of concrete, which are again influenced by the type and duration of curing.

Curing is the most intricate part of construction of the structures with HPC. For a given level of workability, HPC has

Chemicalcomposition

(%)

OPC (%) Fly Ash ( % )

GGBFS ( % )

Silica Fume(%)

SiO2 17.0-25.0 35.8 - 42.83 32.6 90.11

Al2O3 3.0-8.0 18.0 - 26.9 12.8 1.63

Fe2O3 0.5-6.0 6.5 - 8.2 1.3 1.98

MgO 0.1-4.0 3.5 - 4.1 7.2 0.78

SO3 1.3-3.0 2.2 - 3.5 0.03 -

Na2O+K2O 0.4-1.3 - - 1.97

P2O5 - - 0.05 1.18

CaO 60.0-67.0 18.8-19.8 41.0 -

Table 1: Percentage composition of chemical constituents in OPC and various SCMsComparison of Physical and Chemical Characteristics -- Silica Fume, Fly Ash and Cement


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lesser quantity of water compared to the conventional cement concrete, sometimes being lower than the minimum necessary for complete hydration and self-desiccation. Therefore, loss of moisture from the concrete at an early stage leads to detrimental effects on the soundness and long term properties of the concrete. Therefore, protection against moisture loss from fresh HPC is crucial for the development of strength, prevention of plastic shrinkage cracks as well as for durability.

Again, wet curing of HPC cannot be done at an early stage because this will increase the water-binder material ratio adjacent to the exposed surface causing deterioration of the concrete quality. In one of the studies, it was found that the

moisture loss from HPC is maximum during the first 24 hours after placement. Fresh concrete mix of HPC is more cohesive and bleeding is very less compared to that of NSC. Evaporation of bleed water takes place rapidly which makes HPC more prone to plastic shrinkage cracks. Critical time to start forming of plastic shrinkage cracks is around the initial setting time. Therefore, plastic shrinkage cracks can be very serious problem under curing condition of elevated temperature, low humidity and high winds, which accelerate the evaporation of water from fresh concrete. Therefore, to overcome this problem, curing process should start immediately after the placement of fresh HPC.

Wet curing, if applied immediately, after the placement of concrete to combat plastic shrinkage cracks, as in the case of NSC, would also have harmful effects on the quality of surface layer of the hardened concrete. In case, wet curing is applied before final setting of the concrete, the curing water will dilute the cement paste near surface thereby increasing w/c ratio. As a result, strength and impermeability properties of concrete will be seriously hampered. Therefore, HPC should be cured at an early stage without applying water directly on the exposed surface of fresh concrete. This calls for entire curing procedure for HPC to be divided into two stages.

Therefore, curing of HPC is generally done in two stages- initial curing and wet curing. Water is not used directly during the initial curing. Time of commencement of both stages of curing and their duration depends on the initial and final setting time of concrete. It is difficult to make a general specification for curing, applicable for all weather conditions as well as for all types of structural elements. Loss of moisture from fresh HPC depends on the ambient conditions, wind velocity, temperature and humidity and also exposed surface area to volume ratio(s/v).Structural geometry, reinforcement layout and construction methods have bearing on the initial curing procedure.

Curing Duration for HPC

The initial curing of HPC should be started immediately after the placement of fresh concrete and continued till the final setting of the concrete. A better proposition may be to extend it about an hour after the final setting time. The initial curing is followed by wet curing. Total curing duration of HPC is the sum of the initial curing duration and wet curing duration out of which the second part is the longer one.

Method of curing has similar effect on HPC both for creep and shrinkage of concrete, which is again influenced by the type and duration of curing. Overall, considering the above, a curing duration of 7 to 10 days seems to be necessary for HPC though curing duration of about 1-2 days could be sufficient from strength gaining.

Fresh Properties of High Strength Concrete

High-strength concrete can be made easier to place by substituting proportions of ultrafine particles for cement. In

Silica Fume Fly Ash Cement

SiO2 Content 85 - 97 35 - 48 20 - 25

Surface Area (m2/kg) 17,000 - 30,000 400 - 700 300 - 500

Pozzolanic Activity (with cement, % )

120 - 210 85 - 110 n/a

Pozzolanic Activity (with lime, psi)

1,200 - 1,660 800 - 1,000 Pozollanic activity Nil,

Grain Size is (15-20) micro

MtrGrain Size (0.1-0.12) micro

Mtr.(10-12) micro

Mtr

Table 2: Comparison of physical and chemical properties of SCMs with cement

Fig. 1 HPC design mix flow chart



the presence of super plasticizers, finer the micro filler, lower is the flow resistance and torque viscosity of the mixture. Up to 20% ground silica or limestone did not increase the super plasticizer requirement to achieve a constant workability, even though one of these fillers had a surface area as high as 10,000 m2/kg. Silica fume, however, while being the most effective filler from a rheological point of view, increased the super plasticizer demand at a constant workability. This may suggest that a high surface area is not the sole parameter influencing the super plasticizer demand of silica fume mixtures, and that silica fume may have a strong affinity for multi-layer adsorption of super plasticizer molecules. Micro fillers did not seem to reduce significantly the slump loss of fresh HSC, and were advantageous in some instances in maintaining better workability over time. Micro fillers were also successful in inhibiting the induced bleeding of fresh concrete. Therefore, it is possible to design triple-blended composite cements including different fillers to achieve improved rheological characteristics.

Mechanical Properties of High Strength Concrete[4]

The Pozzolanic material provides not only chemical strength affects but also physical packing effects. The following are the effects on the mechanical strengths.

Compressive Strength

The compressive stress curves may keep growing as the concrete ages. Compressive strength, in the beginning stage, is in inverse proportion to W/B due to the use of lower amount of cement and higher amount of pozzolanic materials. Therefore, the strength in the beginning stage is much influenced by the amount of cement in the sample. In other words, it is closely related to W/C and the ultimate strength is controlled by W/B. The strength of HPC with silica fumes is higher than HPC with slag to partially replace cement. However, one year later, the HPC strength of slag partially replacing cement is superior to samples with silica fumes to partially replacing cement, and it is superior to the control group strength as well. The reason might lie in the efficiency of the slag reaction which gets stronger as time passes or in the heterogeneous mix of silica fumes.

Splitting Tensile Strength

The splitting tension of HPC increases after aging. The splitting tension mentioned above is about 5% to 10% of the compressive strength and tends to decline as the strength increases. Splitting Tensile strength of control group is higher than ACI-318, because the HPC uses more pozzolanic materials which have not yet fully developed pozzolanic reactions at the age of 28 days splitting tension of HPC has a close relation with the square root of compressive strength, and its related equation may be expressed as follows:

Wherein (a, b) the mean values of the control group, the slag group, and the silica fumes group are (0.51, 0.03), (0.48, 0.08), and (0.49, 0.08) respectively.

Flexure Strength

Flexure strength is about 7.5% to 13% of the compressive strength. The ratio of flexure strength to compressive strength of the control group declines in response to the increment of compressive strength. The ratio of flexure strength to compressive strength gets higher in response to higher ratios of slag and silica fumes.[8][9]

Compressive Strength Efficiency of Cement

According to the strength development of various mixture proportions, every kilogram of cement in HPC offers about 0.24 to0.32MPa of strength. Normal high strength concrete, owing to more cement, has quicker strength development in the early stage. But, as far as compressive strength efficiency of cement is concerned, it is not so good as that of HPC. The reason is that HPC, based on the densified mix proportion algorithm, lowers water content and enhances the packing effect of aggregates, and the contribution of pozzolanic reactions becomes significant in developing better strength efficiency.[8][9]

Enhancing Ductility of High Strength Concrete members[1]

Ductility is an essential property in structures that have to respond to inelasticity in severe earthquakes, it is defined as a measure of the ability to undergo large deformations without failure (important in seismic zones). Experiments have shown that very high strength concrete are brittle as compared to normal strength concrete. The falling branch of the stress -strain diagram is much steeper and develops at a faster rate as the compressive strength is increased. For making use of high strength concrete in seismic zones special confining reinforcements are provided through use of spirals or rectangular ties to enclose the longitudinal reinforcement so as to form with the enclosed core a con-fined concrete area subjected to tri axial stress. Hence confinement is a technique to achieve high performance in high strength concrete (which is otherwise a brittle material) structural elements through the design of the required steel confining ties or spirals.

Timing and duration of curing is extremely important as it could alter and effect the results of the final product.



Durability of HPC[1]

Strength and Durability are the USPs of High Performance Concrete. The strength aspects have been addressed else where in the paper. In this section durability aspects are discussed.

High performance is characterized by the special attributes which are achieved both long term and short term wise. This is attained through use of non-conventional ingredients such as mineral admixtures (fly ash, blast furnace slag, silica fume), chemical admixtures (high range water reducing admixtures).

The durability of HPC can be achieved through the following parameters.

- Low Permeability to resist - Chemical attack (chlorides & Sulphates)- Reinforcement corrosion- Salt penetration- Freeze and thaw attack- Abrasion resistance (more relevant for bridge decks)- Fire Resistance- Resistance to alkali aggregate reaction- Resistance to Creep & Shrinkage

Economics of High Performance Concrete[1]

The high strength concrete being of higher quality and with inclusion of mineral admixtures (e.g. fly ash, GGBS, silica fumes etc.) and chemical admixtures (e.g. HRWR etc.) obviously costs more than normal strength concrete per unit quantity. But the increased per unit cost is offset by the reduction in the volume of concrete required to construct the structure (As characteristic compressive strength fck is increased for concrete & member size gets reduced). The most substantial savings in the use of High Strength Concrete comes from the reduction in nonmaterial costs associated with structures. For example in case of a bridge with I-Girders the use of high strength concrete reduces the number of girders, reduces the labour cost in the production of girders, reduces the transportation costs, reduces the erection cost, reduces the overhead expenses. Use of HPC ensures lesser cost of maintenance owing to enhanced durability and serviceability. In addition to the monetary savings, use of HPC substantially reduces construction time and thus the structures can be put into use much earlier than normal strength concrete structures. Therefore in totality the overall cost is reduced substantially with the inclusion of high strength concrete. Since cost is the major factor in the choice of material and the level of high strength concrete to be used for the facility to be constructed charts on cost-benefit data should be obtained from the reputed ready –mix suppliers in order to make a studied judgment.

Applications of HPC in India and some real time design mixes[3]

- M 60 grade HPC used for nuclear power plants at Kaiga

(Karnataka) executed by M/s L&T and RAPP (Rajasthan) executed by M/s HCC, both during the year 1998. Silica fume has been used to get the desired strength.

- M 75 grade HPC is presently being used in fly-overs being constructed by M/s AFCONS at PUNE. High strength is being achieved due to addition of Silica fume and flyash as supplementary binders. And because of this, it has been possible to design slender piers and post-tensioned segmental superstructure over longer spans.

- M 60 grade HPC is presently being used in the construction of Delhi Metro Rail.

- M 75 grade HPC has been used for the first time in India in 2002 in JJ Hospital Fly Over Project by Gammon India Ltd.

Conclusion

High Strength and durable concrete will dominate the new and rehabilitated infrastructure of the new millennium. Mixing procedures and equipment would have to be modified con-siderably using high speed mixers instead of truck mixing and robotic technology to place concrete in forms. Instrumentation of the infrastructure, both newly constructed and existing, will be a standard procedure to monitor per-formance. Engineering education will have to adapt to this highly sophisticated materials technology by the inclusion of adequate instruction to equip engineers with knowledge of the behaviour of these constituents. By adapting this technology safety would be enhanced and impending failure could be prevented. The inherent nature of major infrastructure projects will place increasing demands on the new millenniums engineer.

In short as stated by Mather[1]

“Concrete is International and as we have air to breath, water to drink, earth to grow plant in, it is the foundation of civilization.”

References

1. Nawy, Edward G, Fundamentals of High Performance Concrete.

2. Nevellie, A. M., Brookes, J. J., Concrete Technology.

3. Sai Prasad, P. V., and Jha, Kamlesh, High Performance Concrete, Project Work for Course No.624-Sr.Professional Course (Bridges & General)

4. Journal of the Chinese Institute of Engineers, Vol. 27, No. 7, pp. 1081-1085 (2004)

5. The Indian Concrete Journal,Vol-80, June 2006

6. Shetty, M. S., Concrete Technology.

7. Basu, P.C., and Mittal, Amit, High Performance Concrete for Indian Nuclear Power Plants, Transactions of the 15th International Conference on Structural Mechanics in Reactor Technology (SMiRT-15), Seoul, Korea, August 15-20, 1999.

8. ACI 318

9. ACI 363


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Innovative Construction Technology for Quality Construction of Rural Road

Rural development has become a matter of growing urgency for considerations of social justice, national integration, and economic upliftment and inclusive

growth. For rural development, the provision of rural road network is a key component to enable the rural people to have access to schools, health centers and markets. Rural roads serve as an entry point for poverty alleviation since lack of access is accepted universally as a fundamental factor in continuation of poverty. As India launched the era of planned development in 1951, she had a reasonably good railway system, a few ports and around 400,000 kms of serviceable road network. Accessibility to villages was poor as only about 20 percent of them had all-weather road links. The Government laid down a framework for accelerated growth through investments in irrigation, power, heavy industry and transport. Side by side, stress was laid on provision of social infrastructure (education and health) and integrated rural development including agriculture. Rural roads act as a facilitator to promote and sustain agricultural growth, improve basic health, provide access to schools and economic opportunities and thus holds the key to accelerated poverty reduction, achievements of Millennium Development Goals (MDG),

socio-economic transformation, national integration and breaking the isolation of village communities and holistic and inclusive rural development. A major thrust to the development of rural roads was accorded at the beginning of the Fifth Five Year Plan in 1974 when it was made a part of the Minimum Needs Programme. In 1996, this was merged with the Basic Minimum Services (BMS) programmes. The works of village tracks were also taken up under several employment creation and poverty alleviation programmes of the Central and State Governments.

There is growing empirical evidence that links transport investment to the improved well being of the poor. A study (Fan, Hazel and Throat, 1999) carried out by the International Food Policy Research Institute on linkages between government expenditure and poverty in rural India has revealed that an investment of ` 10 crore (at 2009-10 prices) in roads lifts 16,500 persons above the poverty line. States having low connectivity had higher poverty levels. Provision of good roads in rural areas also changes the characteristics of rural transport. With people tend to travel more, the ownership of vehicles increases. There is a shift from non-motorized vehicles to motorized ones and the cost and time of travel get reduced.

Dr. S.K. Chaudhary Assistant Engineer, Road Construction Department, Bihar

Rural Connectivity becomes a critical component in the socio-economic development of rural people by providing access to amenities like education, health, marketing etc. It has been established that investments in rural roads lifts rural people above the poverty line. The evidence also indicates that as the rural connectivity improves, the rural poverty levels come down. While building rural roads, the provisions based on the parameters that affect the sustainability are to be made, but at minimum cost. The conventional methods and specifications tend to recommend technology and materials, however difficult and distance away they may be, which normally result in higher cost of construction. Quality Construction of rural road has been a major challenge for engineering fraternity all over the world. This call for introduction of innovative and environment friendly approaches in rural roads building for achieving cost-effectiveness. Though such methods and technologies were tried world over, they could not become popular in India, due to procedural constraints and lack of awareness/exposure. In this paper author has made an attempt to present innovative, cost effective and environment friendly technology for quality construction of road. National scenario presents an interesting picture. Economic and environmental aspects were also discussed.

Road Engineering Rural Infrastructure


Presently about 740 million people of India live in rural area. Rural connectivity is being focused for the growth of economy, agricultural development and employment generation to rural people. India is having about 2.65 million km of road under rural road category out of total road network of 3.3 million km according to a statistic of Indian road network of National Highways Authority of India. Efforts are going on by central government and state government through different program like Pradhan Mantri Gram Sadak Yojana (PMGSY) to improve road access to rural people. Still about 40 percent of village people of the country are not connected by all weather roads. The rural connectivity is expected to have many positive impacts on economy, agricultural, employment and social services to rural masses.

India is distinguished for its geographical diversities with mountains, hills, rivers terrains, forest, wet lands, deserts and scattered habitations in remote areas. Also, there exists a wide range in the sub-grade soil types, rainfall, traffic pattern and availability of construction materials.These natural barriers create problems for developing a standard uniform technique to serve the requirements at all the sites. This requires adoption of different technologies based on site specific conditions.

For the construction of Rural Roads, Indian Roads Congress has brought out Rural Road Manual IRC SP:20-2002 for design and construction. The design is based on the CBR value of the soil sub-grade and the 10 years projected cumulative traffic with an assumed 6% traffic growth per year. Based on this concept, normally two layers of WBM with 75 mm thickness is laid over the granular sub-base with suitable material having minimum 15% CBR. However, there are situations in many states where the prescribed standards are not available at normal leads resulting in longer haulage and higher costs.

If the locally available materials, including marginal and industrial waste materials are utilized, it could be possible to reduce the cost of road construction. Several types of new materials are tried to establish the efficacy of new materials in road construction. However,the use of new materials and technologies is not becoming popular owing to certain procedural constraints as well as lack of awareness and therefore appropriate steps may have to be taken for popularizing the new technologies for building better rural roads with less cost. Adoption of such technique may also result in the conservation of natural resources, energy and environment.

Ground Improvement Techniques

One of the proven technologies for the use of local soil and marginal aggregates is stabilization. The stabilization can

be mechanical or chemical and several types of stabilizing agents have proved to be suitable under different conditions of soil and environment. The soil stabilization techniques include:

- Stabilization with lime.- Stabilization with cement.- Stabilization with a combination of lime and cement

Even though specifications for soil stabilization are included in both MoRT&H and MoRD book of specifications their adoption is not getting popular, due to problems associated in attaining homogeneity of soil-stabilizer mix in the field and achieving the desired results. The only constraint in the use of the above techniques lies on the procedures adopted in the field. It is possible to popularize the use of stabilization techniques through appropriate training and capacity building of the field engineers. Further, development of low end technology equipment, for use in the rural roads also facilitates wider use of these methods.

In addition to the above, several methods are being tried with the use of industrial waste by products in road building. The following are some of the important materials which have proved good.

- Fly Ash for the construction of the embankments and stabilization of sub-base and base-courses.

- Steel and copper slags for the construction of sub-base and base-courses.

- Marble dust in sub-grade and sub base.

Though the construction of different elements of the road with Fly Ash has been successfully implemented, the use of other materials is not so widely adopted except for inplant roads. However, construction technologies with the use of such materials can also be successfully adopted, if the field engineers are properly trained.

Studies were carried out on the use of waste materials like rice husk ash and lime sludge. These materials, if left un-used, may affect the surroundings and also create problem for their disposal. Use of those waste materials in road construction can alleviate the problem of their disposal to great extent. In India, studies were conducted at CRRI, IIT Roorkee and several other places for their use in stabilizing the soil. The results indicated that heir usage has great impact on the improvement of soil properties. The studies suggested that they are very useful for stabilizing clayey soils. The summary of the results indicate the following:

- Improve Atterberg limits to make soil suitable for road building.

- Increase the unconfined compressive strength of soil as well as CBR.



Innovations in Ground Improvement

Recently several environmental friendly enzymes have come into the market such Fujibeton,Terrazyme and Renolith etc. Use of these products indicates minimization, elimination of the use of aggregates and is referred to as Aggregate-Free Pavement Technology. Such materials can also be tried in the rural roads construction after proving their efficacy in the Indian conditions, through series of trial projects.

1. Fujibeton as a Soil Stabilizing Agent

The Fujibeton material, developed in Japan, is climatically stable material and suitable for stabilization of all types of soils. Basically, the product is an inorganic polymer that chemically binds with all compounds, where blended with ordinary Portland cement in 1 to 3% by weight of OPC. The blended mix is called ‘Fujibeton Mix’, which is used for stabilization of soil that improves the engineering properties of soil.

The design concept is based on the optimization of Fujibeton mix for stabilization based on unconfined compressive strength results determined on the given soil for different proportions of soil-Fujibeton mix and calculation of the thickness of the stabilization layer (Beton-Subbase) based on design CBR, wheel load and volume of traffic. The top layer of the pavement should be covered with 3 to 5 cm asphalt concrete.

The technology is advantageous not only for locations where aggregates are not available at economical rates but also for all types of soil conditions. With the use of new soil hardening agent, the material available at the construction site may be used as it is, eliminating the need for transporting of borrow soil from long distances, thus economizing and simplifying the work process. Fujibeton improves CBR of the sub-grade and does not create shrinkage cracks and is therefore highly effective for clayey/soils. With Fujibeton, a high dry density is obtained with only minor compaction. Therefore, small and simple equipments like tractor mounted equipment are sufficient. Also, this technology does not require skilled manpower for road construction. This tech-nology is efficient and economical for construction of embankment and sub-grade & sub-base course.

2. Terrazyme as a Soil Stabilizing Agent

Terrazyme is a natural, non-toxic; environmentally safe, bio-enzyme product that improves engineering qualities of soil reduces ruts and potholes resulting in more durable and longer lasting roads. The function of Terrazyme is to minimize absorbed water in the soil for maximum compaction, which decreases the swelling capacity of the soil particles and reduces permeability. The application of

Terrazyme enhances weather resistance and increases loadbearing capacity of soils especially in clayey/soils. This will provide cost effectiveness both in the initial construction cost and maintenance cost.

Advantages of Terrazyme Technology

- Considerable improvement in soil CBR.- Minimum loss of gravel due to erosion or abrasion

by the traffic preserving original transverse section of slopes.

- Impediment of widespread occurrence of dust from loose fine material on the road surface.

Terrazyme is used world wide in strengthening of layers of un-surfaced roads, in base layers and sub-base layers covered with asphalt material. Among the soil materials stabilized by Terrazyme are sandy clay, silty clay, sandy silt, plastic and non-plastic clay, sandy loam,fine loam, loam mixed with clay.

3. Soil Cement Renolith Stabilization Technique

Renolith is polymer based chemical, which is environmentally friendly and which facilitates the bonding of soil particles (a phenomenon which is known a micro-rubber bonds).Soil-cement with Renolith has a high modulus of elasticity and can disperse the wheel loads very effectively. It is a semi-rigid material. A noteworthy feature of this technology is that it require very little amount of aggregate, which is useful at places where the material haulage is more. The use of Renolith, when used in soil stabilization with cement, gives strong and durable base. This type of construction does not require surfacing for low volume roads, since the base course is stabilized. It is expected to give good performance with longevity and reduces maintenance costs in almost dust free environment. Limited research was carried out abroad, with soil cement Renolith Stabilization, but similar studies are yet to be carried out in India.

Alternate Technologies in Rural Roads Construction

There are several other techniques that can be adopted in conditions of low bearing capacity soils, marshy lands and location with drainage problems such as the use of geotextiles.Several types of geo-textiles including synthetic, jute coir etc. are proved to give good results and provide cost effectiveness for rural roads.

1. Use of Jute Geo-textile

Jute Geo-textile (JGT) is a kind of natural technical textile laid in or on soil to improve its engineering properties. It is made out of yarns obtained form the jute plant. Jute Geo Textiles have high moisture absorption, excellent drapability, high initial tensile strength, biodegradable and improved soil structure on degradation. The basic functions of JGT


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are separation, filtration, drainage and initial reinforcement. It is environment friendly. Jute Geotextiles can be more effective, eco-friendly and economical if used judiciously and jointly with other measures.

2. Flexible-Concrete Pavement Technology

IIT Kharagpur has developed a new technology for low cost cement concrete road construction, which has proved to be suitable in place of conventional CC roads for low volume traffic. Even though the initial cost of flexible-concrete road is high compared to cost of conventional flexible pavement, the life cycle cost with maintenance costs over a period of 10-20 years is less compared to the conventional one. The technology consists of placing a form work of plastic cells 150 x 150mm and 100mm deep over the prepared foundation of road and placing zero slump concrete in the cells and compacting with road roller/ plate compactor / earth rammer. On curing, a flexible-concrete pavement is obtained which will not wear even under iron tyred carts if aggregates of good quality are used.

3. Use of Waster Plastic Blended Bitumen

It is possible to improve the performance of bituminous mixed used in the surfacing course of roads. Studies reported in the used of re-cycled plastic, mainly polyethylene, in the manufacture of blended indicated reduced permanent deformation in the form of rutting and reduced low – temperature cracking of the pavement surfacing. Laboratory studies were carried out at the Centre for Transportation Engineering of Bangalore University, in which the plastic was used as an additive with heated bitumen n different proportions (ranging from zero to 12% by weight of bitumen) The results of the laboratory investigations indicated that, the addition of processed plastic of about 8.8% by weight of bitumen, helps in substantially improving the stability, strength, fatigue life and other desirable properties of bituminous

concrete mix, even under adverse water-logging conditions. The additions of 8.0% by weight of processed plastic for the preparation of modified bitumen results in a saving of 0.4% bitumen by weight of the mix or about 9.6% bitumen per cubic meter of BC mix.

4. Cold Mix Technology

Cold mix is a mixture of unheated aggregate and emulsion or cutback and filler. The main difference between cold mix and HMA is that aggregates and emulsion or cutbacks are mixed at ambient temperature (10°C-30°C) in case of cold mix and aggregates and binder are mixed at high temperature (138°C-160°C) in case of HMA. Dense graded cold mixtures have far lower permeability and good resistance to deformation. Open graded mixtures are storable and semi dense mixtures have good adhesion

and lower permeability.

Cold mix when used as paving mix can offer following advantages.

- It eliminates heating of aggregate and binder.

- It is environmental friendly and conserves energy. Cold mix pavement can provide energy savings of over 50% compared with hot mix. So it can be considered as green bituminous mix for rural road construction.

- It can be easily prepared using small set up on site. It can be produced manually for small scale job. Laying of HMA for rural road construction sometimes is not economical because setting up of a hot mix plant for small scale job increases the project cost.

- This paving mix is particularly suited for construction of roads in remote and isolated areas of a country where plant produced hot mix may have set before reaching site.

- Cold mix can be laid during wet or humid condition also.

- It is versatile also as a large number of grades of emulsion and cutbacks are available.

- It is economical and high production is possible with low investment

In India majority of road network is occupied by bituminous pavement only in which Hot Mix Asphalt (HMA) is used predominantly as a paving mix from many decades. However this bituminous mix is associated with some limitations. These include excessive emission of greenhouse gases (e.g. sulfur dioxide, nitrogen oxides, carbon monoxides and volatile organic compounds) from HMA plant, shut down of hot mix plant during rainy season and the laying of HMA is difficult in hilly areas and rural areas having long hauling distances, cost of putting up HMA plant is high and comparative budgets of small sections of rural road is very less, etc.

As, Indian rural road network is developing continuously, paving mix like cold mix asphalt or Warm Mix Asphalt (WMA) should be tried. This mix is started to lay on pavement to reduce the problems associated with HMA. Warm mix asphalt is a very new technology compared to cold mix asphalt.

Construction of rural road using conventional paving mix is sometimes not feasible in high rainfall area because it is difficult to produce and lay HMA. In case of high altitude or snow bound area, lower temperature of environment makes difficult to heat aggregate and binder at high temperature. In case of hilly roads, HMA is supplied from remote HMA plant; it is difficult to maintain mix temperature for long hauling distance. Cold mix can be produced on



site. Simple concrete mixture, motor pavers or specialized mixing plant can be used to produce cold mix on site. Cold mix can be lay down by hand for small scale job and compaction is carried out by vibrating roller. Hence Cold mix asphalt should be tried in India for construction of rural roads in hilly areas having high rainfall and difficult terrain.

National Scenario

1. Fujibeton as a Soil Stabilizing Agent: To evaluate the performance of this technology, using Fujibeton as soil stabilizer, small road stretch has been constructed within the campus of NCCBM’s in Ballabhgarh. With this study,it is revealed that because of faster setting and improved CBR of stabilized soil, the rural road can be opened to traffic within a day. Due to speedier construction practices, the Fujibeton stabilized rural roads will not only be economical but also prove to be effective under constraints of traffic diversion.

2. Terrazyme as a Soil Stabilizing Agent :Trial roads were built in India with Terrazyme stabilized road structure in the states of Kerala and Tamil Nadu. The soil used in these studies are mainly gravelly clay, silty clay,clayey sand, medium to fine sand-clay mixtures, silt and clay mix. It is proved that there is an increase in CBR value of more than 100% and relative compaction by more than 100%.Case study of the two roads built by PWD of Maharashtra revealed that the use of Terrazyme resulted in overall cost savings in the range of 18-26%.

3. Use of Jute Geo-textile: Based on the experiences of the use of Jute Geo Textiles, MoRD in collaboration with JMDC is implementing a pilot project in five states covering a length of about 48 Km under different soil and environmental conditions.

This project is taken up with different types of Jute Geo-textile and placement at different levels. The post construction performance monitoring is expected to give valuable data for arriving at standards and specifications of this technique which helps for wider application. The project is in progress and the results are expected shortly.

4. Flexible-Concrete Pavement Technology :A model rode has already been constructed in a village close to IIT Kharagpur using the technology “IITGP_ROAD”.Experimentation through pilot project for the “IITGP_ROAD” technology is being tried I the construction of the rural rods under PMGSY, so as to enable standardization and popularization of this cost effective solution.

5. Use of Waster Plastic Blended Bitumen:

- In Tamil Nadu, length of roads around 1000 m in various stretches were constructed using waste plastic as an additive in bituminous mix under the scheme “1000 km

Plastic Tar Road”, and found that, the performance of all the road stretches are satisfactory.

- The performance of the road stretches constructed using waster plastic in Karnataka is also found to be satisfactory.

The construction of roads using waste plastic in the above states is based on the guidelines developed by Bangalore University. CRRI and College of Engineering, Madurai. However, standard specifications are not available on the use of waste plastic in bituminous road construction.

5. Cold Mix Technology: North eastern states of India belong to hilly area and sometimes roads go through forest zone. Due to its topographical constraints and environmental rule and regulation, use of cold mix may be a promising mix under different site conditions. Field trials have been carried out by CRRI at some location in North eastern states of India. Cold mix is gaining considerable popularity in rural road construction.

Economic and Environmental Aspect

- The stabilization of soil with Enzyme based stabilizers like Fujibeton, Terrazyme and Renolith, can eliminate the need for the use of aggregate material in base course resulting in conservation of material. This results in reduction in the cost of construction. A typical analysis for saving of cost in terms of material, machinery and labour for two layers of WBM (75 mm each) and 3.75 m carriageway indicate a saving of about Rs. 5.0 lakhs with medium lead.

- Using Jute Geo-textile in the pilot project taken up under PMGSY, it is found that there is cost saving of about 12% in road construction.

- MoRD in collaboration with JMDC is implementing a pilot project in five States covering a length of about 48 Km under different soil and environmental conditions, the cost analysis of which is given below:

- Even though the initial cost of flexible-concrete road is high compared to cost of conventional flexible pavement, the life cycle cost with maintenance costs over a period of 10-20 years is less compared to the conventional one.The “IITGP_ROAD” technology need to be studied further because even through,the initial cost of Cement Concrete Pavement is at par with the conventional pavement, it is lower than the conventional flexible pavement if maintenance cost is also considered whose bitumen top is to be renewed every 5 years at a cost of over ` 5 lakhs.

- The additions of 8.0% by weight of processed plastic for the preparation of modified bitumen results in a



saving of 0.4% bitumen by weight of the mix or about 9.6% bitumen per cubic meter of BC mix.

- It Is not only the reduction of cost, but the real interesting part of this is the conservation of natural resources and energy along with preservation of the environment, which gives long way, if such aggregate free construction of rural roads are encouraged and popularized.

using conventional plant or by hand. So it can be laid as surface course or bituminous base course for rural road construction.

- Cold mix can be tried for paving mix in north east region of India.

- Large scale laboratory and field trials studies should be carried out to develop better understanding on the performance of cold mixes in rural road construction for different traffic, climate and terrain conditions.

- The use of new materials and technologies is not becoming popular in our country mainly due to lack of awareness. Failure to instill confidence in the field engineers by addressing their problems can be another reasons, the third being non-availability of suitable standard equipments.

References

- IRC:SP:20-2002. “Rural Roads Manual”, Indian Roads Congress.

- Dr Satish Chandra, Shiv Kumar & Rajesh Kumar Anand, “Soil Stabilization with Rice Husk Ash and line Sludge”, India Highways, Indian Roads Congress, vol33 No. 5, May 2005, pp.87-98.

- Raju, G.V.R.P., Chandrasekhar B. P., Kumar R.R.P. and Mariyanna G. “Strength Characteristics of Expansive Soils Stabilized with Lime and Rice Husk Ash”, Proceeding of the National Seminar on Road Transportation; Issues & Strategies, Patiala, India (1998), pp-20-29.

- Report on “Demonstration Project for Aggregate-Free Pavement Technology using Fujibeton for Rural Road Construction”, NCCBM, New Delhi, India.

- Gianni A.K. Modi, A.J., “Bio Enzymatic Soil Stabilizers fro Construction of Rural Roads”,International Seminar on Sustainable Development in Road Transport, New Delhi-India 8-10 November 2001.

- Report on “Demonstration Project using Soil-Cement – RENOLITH stabilization technique” by PWD Rajasthan, India.

- “Use of Jute Geo-textile as a separator-cum Reinforcement for Rural Roads under PMGSY on an Experimental basis” TECHFAB India, Mumbai.

- Teiborland L. Rynathiang, M. Mazumdar and B.B. Pandey, IIT Kharagpur, “Structural Behaviour of Cast n Situ Concrete Block Pavement”, Journal of Transportation Engineering,vol.131, Issue 9, pp.662-668 (September 2005).

- Punith, V.S. and Veeraraghavan, A., “Laboratory Fatigue Studies on Bituminous concrete Mixed Utilizing Waster Sherdded Plastic Modifier”, Proceedings of 21st ARRB Transport Research (ARRB) and 11th Road Engineering Association and Australia (REAAA) Conference,Caims, Australia, May 19-23, 2003

States Total length of Roads (Km)

(Cost in lakh) Savings in ` lakhConventional

DesignWith Jute

Geo Textile

Assam, Chattisgarh, MadhyaPradesh, Orissa, West Bengal

47.84 2022.95 1790.06 232.89

Conclusion

- Fujibeton improves CBR of the sub-grade and does not create shrinkage cracks and is therefore highly effective for clayey/soils.

- Terrazyme increases CBR of soil sub-grade by more than 100%. Impedes widespread occurrence of dust from loose fine material in the surface of the soil roadways and reduces cost of construction by 15-20%.

- The noteworthy feature of soil-Cement-Renolith Stabili-zation that it requires very little amount of aggregate, performs with increased life and reduced maintenance cost provide a good base for the field Engineers to experiment the construction of unsealed roads in rural areas and also in localities where aggregate are not available in normal leads.

- The Jute Geo-textile strengthens the soil sub-grade by preventing intermixing of sub-grade and sub-base by acting as a separation layer and further it prevents migration of fines of a sub-grade by acting as a filtration materials.

- The use of modified bitumen with the addition of processed waste plastic of about 8.0% by weight of bitumen helps in substantially improving the stability, strength, fatigue life and other desirable properties of bituminous concrete mix, resulting which improves the longevity and pavement performance with marginal saving in bitumen usage.

- Cold mix can be laid on low to medium volume road as a green paving mix. Mixture can be produced by


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An Insight into ETFE - History, Application & Future

“Ethylene Tetra Fluoro Ethylene (ETFE) - not the best of names; however, ETFE foil is fast becoming one of the most exciting materials in today’s design industry and has set the construction world alight with the potential it offers.”

Originally invented by DuPont as an insulation material for the aeronautics industry, ETFE was not initially considered as a mainstream building

material. Its principal use was as an upgrade for the polythene sheet commonly used for greenhouse poly-tunnels. The advantages of its extraordinary tear resistance, long life, and transparency to ultraviolet light offset the higher initial costs, and 20 years later, it is still working well. It wasn’t until the early 1980s, when German mechanical engineering student Stefan Lehnert investigated ETFE in his quest for new and exciting sail materials, that its use was reconsidered. Although discounted for Lehnert’s original purpose, he

saw its strength, high light transmission, and structural properties as advantages to the construction industry and started to develop the systems we see today. Over the past 20 years, Lehnert has increased awareness of the material and its uses, and it is rapidly bursting into the consciousness of architects and designers worldwide. Most recently, the Eden Projectin the UK (Refer Figure 1) and the Beijing Olympic Aquatics Centre, nicknamed the “Watercube,” have brought the material into public discussion. ETFE is increasingly being specified on a wide range of projects – from schools and offices, to government buildings and sports facilities. ETFE is under the architectural spotlight and intends to shine.

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Figure 1 – Eden Project Biomes, Cornwall, UK

The Principles of ETFE

ETFE foil is essentially a plastic polymer related to Teflon and is created by taking the polymer resin and extruding it into a thin film. It is largely used as a replacement for glazing, due to its high light transmission properties. Trans-parent windows are created either by inflating two or more layers of foil to form cushions or tensioning into a single-skin membrane.

Weighing approximately 1% the weight of glass, single-ply ETFE membranes and ETFE cushions are both extremely lightweight. This enables a reduction of structural framework and imposes significantly less dead load on the supporting structure (Figure 2). The reduced requirement for steelwork provides a large cost benefit for clients and is a key advantage when replacing glazing in old structures to meet current building codes (e.g., railway station roofs).

A major benefit of ETFE is its high translucency (Figure 3). Transmitting up to 95% of light. When high levels of light and UV transmission are not required, ETFE also has the ability to be printed or “fritted” with a range of patterns. This fritting can be used to reduce solar gain while retaining transparency; or alternatively, it can incorporatea white body tint to render the foil translucent. ETFE cushions can be lit internally with LED lighting to make them glow or may be projected onto externally like a giant cinema screen, creating dramatic results.

Unaffected by UV light, atmospheric pollution, and other forms of environmental weathering, ETFE foil is an extremely durable material. While no ETFE structure has been in place for longer than 25 years, extensive laboratory and field research have suggested that the material has a lifespan in excess of 40 years. ETFE scores well on the eco-friendly front as well. Being 100% recyclable and requiring minimal energy for transportation and installation means that it makes a significant contribution to green construction and sustainability. It has gained popularity mainly due to its daylight transmittance and the potential for energy savings. When

used as cladding ETFE sheets are usually assembled into cushions, which are inflated for structural reasons. ETFE cushions can provide thermal insulation with reduced initial costs and less structural supports as compared with a conventional glazed roof. The benefits of this material are extensive and have yet to be put to use in many areas.

Figure 2: Aluminum framing connects all panels together and carries the weight of the fabric cushions

Figure 3: In Wiltshire, England, the “SwindonDome” covers an atrium with an ETFE dome large enough to house the entire college and allows maximum light to be transmitted to the space below

Properties

Various types of ETFE films is shown in Figure 4 below.

Figure 4: Types of ETFE membrane



A comparison of physical properties of ETFE with Glass is shown in Figure 5.

Figure 5: Comparison of Properties of ETFE membrane and Glass

Thermal insulation properties of ETFE as against that of Glass is shown below (Refer Figure 6). It indicates that ETFE fils are as effective as glass from thermal insulation view point.

Figure 6: Thermal Insulation Properties of ETFE

The various physical properties of ETFE is shown in the below mentioned Figure 7.

Figure 7: Various Properties of ETFE Membrane

The Areas of Application of ETFE

Owing to its transparency property, ETFE found its use on projects such as botanical gardens, zoological gardens, swimming pools, and exhibitions spaces. However, ETFE is increasingly finding its place in more traditional buildings as roofing for courtyards, shopping malls, atria and stores. The ETFE material has been used on prominent architectural projects such as the Eden Centre and the Water Cube and it is currently considered for a number of high profile inter-national sports venues.

A Couple of Case Studies

Single-Ply ETFE on the Radclyffe School

The use of ETFE has been particularly popular in the construction of new schools. Hailed as environmentally friendly, architecturally aesthetic, and cost effective, it is not surprising that it has been included in both single-ply and cushion form. The covered street at Radclyffe School is a good example of the use of single-ply ETFE (Figures 8 and 9). The atrium area, which forms the intersection of five school buildings, needed to be covered for one simple reason: to provide an open but dry space for students and staff to gather, socialize, and learn. Without a requirement for insulation, with a need to keep costs down, and with a desire to maintain natural light, single-ply ETFE provided a good solution. A cable net accommodates the larger ETFE spans. The cables are inserted through pockets on the underside of the fabric. The intertwining of the lateral and longitudinal cable mesh helps the fabric resist snow loads and wind uplift. Additionally, a study was carried out on the support cable locations, which found that additional cables were needed in certain locations to avoid issues of ponding. The perimeter of the ETFE is fixed to the steelwork using aluminum and silicon rubber extrusions attached with stainless-steel fasteners. Single-ply ETFE has massive and somewhat untapped potential for creating interesting and dynamic structures in a range of settings and with a variety of effects. The installed structure at Radclyffe School is proof that it is possible to create an ETFE roof using the simplest of shapes, even with minimal curvature, but without losing any of the architectural impact.

Figure 8 – ETFE side panels join the roof and walls at the Radclyffe School

Figure 9 – ETFE roofing helps to create an inside/outside space, which is very popular for schools


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ETFE Cushion System on the NW Bus Interchange

At the new Westfield White City shopping de velopment in East London, it was important to the client to achieve eye-catching design as well as practicality. The North West Bus Interchange forms one of the main entrances to the shopping complex and is a valuable location for boosting general(Figures 10 and 11).The two-layer ETFE cushions form the main canopy and span approximately 60 my 18 m (197 ft x 59 ft), and the two layers are continually inflated using a high-techinflation system to create the bubble-like cushion form. The translucency of the membrane proves the feeling of a traditional bus shelter is a long way from this reality; however, the practicalities of weather protection are not lost (Figure 12).The double-skinned cushions include drainage to a central gutter and are supported by safety cables in case the power. supply fails during a storm (Figure 13). Each individual cushion was specifically designed in order to be easily removable for replacement, if necessary. The cushions also in clude wires fitted to the perimeter of all ETFE panels to deter perching birds. ETFE cushion structures such as the North West Bus Interchange are being designed more frequently as the principles of ETFE are becoming more widely understood. As ETFE becomes more mainstream, the demands made on design, inflation systems, and control will become more ambitious.

Figure 10 – NW Bus Interchange will be one of the busiest areas of the Westfield site and will see millions of visitors each year

Figure 11 – Over 60 m (almost 200 ft) long, the canopy shelters commuters from the rain

Figure 12 – Translucency of the ETFE gives travellers maximum view of their surroundings

Figure 13 – The cushions are supported by safety cables in case of power failure

Advantage of ETFE Film Structures

The major advantage of ETFE can be summarized in Figure 8.

Figure 8: Advantages of ETFE membrane structures

The Future

Much has happened very quickly in the development of ETFE. In 30 years, it has gone from creation to one of the industry’s most sought-after building materials. But there is plenty more advancement to come. The makings of ETFE as a long-term construction material will lie in the development of various high-tech coatings and methods


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of printing, which will modify not just the translucency, but also the thermal and acoustic properties of the fabric itself. By increasing the number of layers and by incorporating “nanogels,” it is possible to increase the thermal properties of ETFE foil. Its use in an internal setting has yet to be fully discovered, partly due to its current lack of acoustic absorption properties. The latter is a major selling point for foil for traditionally noisy areas such as indoor sports halls and swimming pools; the echoing noise now simply escapes through the roof. Still, when noise exclusion is required (e.g. external traffic noise and heavy rain and hail in airports), ETFE currently struggles. However, noise and rain suppression systems are now being incorporated into external structures with successful results, and there is much potential for this to be developed further to improve acoustics.

ArchitenLandrell, A renowned architect is running an active test program to develop IR reflective coatings that will allow multilayer ETFE systems to transmit visible light yet block (insulate) infrared transmission. Current systems have insulation levels similar to conventional glazing products, so the search is on for products that will dramatically improve on these values. All of these developments will move ETFE into a wider product arena.

Some of the Applications of ETFE Foil

Conclusion

What is clear is that the world is not short of architects, designers, and contractors who want to specify ETFE foil in their projects. Demand is high, and with demand comes increasingly adventurous design briefs, which constantly push the boundaries of what can be achieved.

ETFE is still in its infancy, but these are exciting times and there is much more potential to tap into. ETFE continues to open new horizons for architects and designers, and it is sure to remain in the architectural sphere for the foreseeable future.

Reference

- www.makmax.com- Robinson A.L. 2005. Structural Oportunities of ETFE (ethylene

tetra fluoro ethylene), Masters thesis at the Massachusetts Institute of Technology, USA

- Robinson-Gayle S., Kolokotroni M., Cripps A., Tannob S., 2001. ETFE foil cushions in roofs and atria, Journal of Construction and Building Materials 15, p 323-327

- Salz C., Schepers H., 2006. Arup’s ETFE Material note- www.konstruct-ag.comwww.photovoltaics.dupont.com- ETFE, The New Fabric Roof, Amy Wilson

Figure : Shenyang Yuanda, Entrance Gate (Liaoning, Shenyang, China)

Figure : Shenzhen Ocean Park, Indoor Water Park (Shenzhen, China)

Figure : Aoyama Great Brits 2005, Air Cabin (Tokyo, Japan)

Figure : Sahara Star Hotel, Mumbai (Mumbai, India)


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Example of Tension Fabric Structure Analysis

The principal material used for constructing tension fabric structures (TFS) is a coated woven fabric called architectural fabric. The fabric is made of woven fibres

(warp and weft) covered by a coating material, see Figure 1. Most of the technical woven fabrics are made of nylon, polyester, glass or aramid fibre nets covered by coating materials such as PVC, PTFE, or silicone. Additionally, it is possible to use architectural fabrics with some special features, e.g. high light transmission, surface coating with a self-cleaning photocatalyst or superhydrophobic material, etc.

The description of coated woven fabric deformation is the most important problem concerning materials modelling. Developing a realistic constitutive model to describe the material’s behaviour has been the main objective of research for the last decades. Great progress in the computational tools gives new perspectives to the evolution of constitutive models, however, the practical engineering applications of some models are limited due to difficulties with identification procedures (e.g. large number of parameters of a material). A brief characterization of the constitutive models which have been proposed for the last decades for material modeling of a coated woven fabric is given.

Stubbs and Fluss [1] have described a coated fabric element by a stable geometrically nonlinear space truss. The solution is determined by solving the system’s equations using the secant method. Experimental results are presented to demonstrate the predictive capability of the model. Stubbs and Thomas [2] have developed a nonlinear elastic constitutive model for coated fabrics. The model which accounts for the basic mechanisms of yarn rotation, yarn extension and coating extension is obtained by expressing the equations of equilibrium for a unit cell of the material. Argyris et al. [3, 4] have presented constitutive viscoelastic modelling including experimental testing procedures, and identification of rheological parameters for a PVC-coated fabric. Kato et al. [5] have proposed the formulation of continuum constitutive equations for fabric membranes. The formulation is based on a fabric lattice model where the structure of fabric membranes is replaced by an equivalent structure composed of truss bars representing yarns and coating material. Kuwazuru and Yoshikawa [6] have described a pseudo-continuum model. This model transforms deformations of a fabric into the axial tensile strain and transverse compressive strain, separately for warp and weft. Xue et al. [7] have presented a non-orthogonal constitutive model for characterizing woven composites. The relationships between the stresses and strains are obtained on the basis of a stress and strain analysis in orthogonal and nonorthogonal coordinates and rigid body rotation matrices. King et al. [8] have proposed a continuum model. The fabric structural configuration is related to macroscopic deformation through an energy minimization method, and is used to calculate the internal forces carried by the yarn families. Pargana et al. [9] have proposed a unit cell approach. The base fabric model consists of a series of nonlinear elastic and frictional elements and rigid links to represent the yarns. The fabric coating is modelled as an

Andrzej Ambroziak, PawelłKlosowskiDepartment of Structural Mechanics and Bridge Structures,Faculty of Civil and Environmental Engineering,Gdansk University of Technology, Poland.

The aim of this work is to examine two variants of non-linear strain-stress relations accepted for a description of the architectural fabric. A discussion on the fundamental equations of the dense net model used in the description of the coated woven fabric behaviour is presented. An analysis of tensile fabric structures subjected to dead load and initial pretension is described.

Figure 1. Visualisation of coated woven fabric

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isotropic plate. Galliot and Luchsinger [10] have proposed a simple model based on experimental observations of the yarn-parallel biaxial extension of a PVC-coated polyester fabric. A linear relationship is experimentally found between elastic modules and normalized load ratios. The material behaviour is assumed to be plane stress orthotropic for a particular load ratio, while the elastic properties can vary with the load ratio in order to represent the complex interaction between warp and fill yarns. Among these approaches, the dense net model used in this paper can also be mentioned.

Constitutive architectural fabric model

To describe the behaviour of a coated woven fabric the authors applied the dense net model (see [11, 12]). In this model, stresses T1 and T2 (stresses in warp and weft thread families, see Figure 2) depend on the strains in the same family only:

(1)

where F1(1) kN/m and F2(2) kN/m specify the tensile stiffness of the warp and

weft threads, 1 and 2 define the strains along thread families (along warp and

weft threads) and are defined by components of strains in the plane stress state:

(2)

The parameters, F1(1) and F2(2), are experimentally determined from uniaxial tensile tests in the warp and weft direction, respectively. In this approach it is possible to use different types of thread behaviour: nonlinear elastic, viscoplastic and viscoelastic. In the present paper two variants of the non-linear elastic approach are used to describe the behaviour of an architectural fabric. The identification process is presented in detail in papers [13]

Figure 2. Thread forces in dense net model

and [14]. On the other hand, in paper [15], the authors have proposed nonlinear viscoelastic behaviour to describe the fabric material.

The relation between components of the membrane forces in the plane stress state in the local coordinates (x, y) can be calculated from the equation ([11, 12]):

(3)

Following, it is possible to write:

where [D] defines the elasticity matrix and is expressed by the formula:

It should be noted that the angle _ (see Figure 2) between warp and weft thread families changes during the deformation and may be calculated from the following equation:

(6)

It should be noted that the initial angle = 0 must be specified. This angle is equal to 90o for most coated woven fabrics. This parameter is dependent on the kind of woven and initial stresses. Typical biaxial weave pattern styles for

0 =90o, woven fabrics, are shown in Figure 3.

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Figure 3. Typical biaxial weave pattern styles [16]

Structure Description

Geometrically non-linear calculations of a hyperbolic paraboloid TFS with edge ropes (see Figure 4), with non-linear strain-stress relations applied for a coated woven fabric, were performed. This structure of a doubly ruled surface, shaped like a saddle, is more and more often used nowadays, due to its structural properties merit, see, e.g. [17, 18]. The vertical roof surface coordinates of the initial configuration can be computed from the following equation:

(7)

where H1 is the height at the centre point, H2 is the height at the maximum height point, 2A is the diagonal horizontal span, X, Y , Z are the global coordinate system axes. Edge cables in the initial configuration are assumed as a second-order parabolic shape, see Figure 4.

Figure 4. Roof geometry

Value and unit

UTSwarp 75 [kN/m]

UTSweft 60 [kN/m]

weight 870 [g/m2]Table 1. Panama fabric properties

The geometrical data for the example are: A=20 m; H1 =2 m; H2 =4 m and B =2 m (see Figure 4). The coated woven Panama fabric, assumed for these calculations, is made as a polyester base fabric coated with PVC. The weight and ultimate tension strength values are given in Table 1. The warp and weft threads of the fabric have the global

coordinate system XY directions (warp – X and weft – Y direction) in plane. A steel edge rope, 12 mm in diameter, was taken. The initial ropes force of 50 kN were used.

The numerical analysis was performed in two variants of the non-linear elastic strain-stress relation:

- NLS P – the non-linear elastic of the piecewise stress-strain relation. The tensile stiffnesses of the warp and weft threads are taken directly from Table 2.

- NLS M – the non-linear elastic Murnaghan model relation. In this case, the tensile stiffnesses of the warp and weft threads are determined as derivations of potential energy:

(8)

The potential energy is accepted in the form [19]:

(9)

where: , µ are the Lame constants; l, m, n are the Murnaghan constants, and IE, IIE, IIIE are the invariants of the Lagrange-Green strain tensor. The mean values of the Murnaghan model coefficients are shown in Table 3.

[kN/m] [-]

WarpF1( 1)=904 1 (0÷0.0119)

F1( 1)=176 1 (0.0119÷0.093)

F1( 1)=471 1 (0.093÷0.180)

WeftF2( 2)=187 2 (0÷0.039)

F2( 2)=146 2 (0.039÷0.1495)

F2( 2)=340 2 (0.1495÷0.24)

Table 2. Non-linear elastic properties of coated fabric Panama [14]

[kN/m] µ [kN/m] i [kN/m] m [kN/m] n [kN/m]

Warp 188.9 −146.2 313.3 6366.0 634.4

Weft 48.2 −24.4 453.6 1781.2 −43.1

Pre-analysis of structure

One of the most important problems is to choose a proper value of the structure initial pretension forces. It is necessary to select a proper value of the initial pretension forces applied in the warp and weft directions, and also in cables. Wrong assumptions of pretension forces in the fabric can cause membrane wrinkling and also change the assumed shape of the structure, see e.g. [20].

The choice of force for this structure type is discussed in [12]. The initial pretension forces in the hyperbolic paraboloid tension structure must fulfil the following equation:

Table 3. Murnaghan coefficients for PVC-coated Panama fabric [13]

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(10)

where

T1,T2 are horizontal components of pretension forces in the warp and weft directions, Z(X,Y) are the vertical coordinates of the roof surface defined by Equation (7). Consequently, the relation between pretension forces can be rewritten as:

(11)

For the analysed structure we can assume that (shallow structure):

(12)

Therefore, the values of pretension forces for the geometric parameters used in the calculations can be determined as:

(13)

It is assumed that the value of T2 = 6.0 kN/m, therefore, T1 = 6.0 kN/m from Equation (13) is accepted for the analysed structure pretension forces in the initial configuration in the warp direction.

Due to the symmetry of the geometry and loadings, it is sufficient to analyse only a quarter of the roof with proper symmetry boundary conditions at X = 0 and Y = 0 coordinates. At the beginning of the numerical analysis, a convergence analysis of the FE mesh was performed. Basing on this calculations, the mesh of 12×12 elements (see Figures 5 and 6) was accepted. The length parameter in Figure 5 is taken first along the Y axis to the middle point of the roof. The length parameter is calculated as a polyline with two nodes: (0;0), (20;0).

Figure 5. Convergence analysis of FE mesh – vertical displacement along X axis

Numerical analysis

A self-made FEM code for a textile membrane analysis was used in the numerical calculations. An exact theoretical description, the FEM coding details and the application limits of this code are discussed in [21].

Comparisons of the vertical displacement in Figure 7 are made. In the next figures (see Figures 8 and 9) the stress isolines for the membrane roof subjected to the dead load and initial pretension forces Figure 6. FE mesh

are shown. The displacement profiles and membrane stress isolines obtained for the piecewise stress-strain and Murnaghan model relations are comparable.

Figure 7. Vertical displacement along X axis

Figure 8. Distribution of stresses (left T1 kN/m, right T2 kN/m) – NLS P

Conclusions and general remarks

A geometrically non-linear analysis of a hyperbolic paraboloid TFS with non-linear elastic physical equations for a dense net model was successfully carried out. Calculations for a typical polyester base fabric coated by PVC were preformed. Both the piecewise and Murnaghan

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model gave almost the same response in the calculated FE variants. The numerical results showed that the simplified piecewise model gave comparable results with a more advanced Murnaghan model.

The obtained results require revision for more complex roof shapes, analysis types and load kinds. The difference between material models is expected to grow when operational static loading or dynamic wind loads are applied. The authors are aware that many other material models exist, but their engineering applications are limited due to the difficulties with identification of a large number of material parameters.

Acknowledgements

The calculations presented in the paper were made at the Academic Computer Centre in Gdansk (TASK).

References

1 Stubbs N and Fluss H 1980 Applied Mathematical Modelling 4 (1) 51

2 Stubbs N and Thomas S 1984 Mechanics of Materials 3 (2) 157

3 Argyris J, Doltsinis J St and Silva V D 1991 Comput. Meth. in

Appl. Mech. and Engng 88 135

4 Argyris J, Doltsinis J St and Silva V D 1992 Comput. Meth. in Appl. Mech. and Engng 98 159

5 Kato S, Yoshino T and Minami H 1999 Engineering Structures 21 691

6 Kuwazuru O and Yoshikawa N 2002 The 5 th World Congress on Computational Mechanics, Vienna, Austria, pp. 1–10

7 Xue P, Peng X and Cao J 2003 Composities Part A 34 183

8 King M J, Jearanaisilawong P and Socrate S 2005 Int. J. Solids and Structures 42 (13) 3867

9 Pargana J B, Lloyd-Smith D and Izzuddin B A 2007 Engineering Structures 29 (7) 1323

10 Galliot C and Luchsinger R H 2009 Composite Structures 90 438

11 BranickiCzandKłosowskiP1983ArchivesofCivilEngineering29 189 (in Polish)

12 Branicki Cz 1969 Some Static Problems of Hanging Nets, PhD Thesis, Gdansk University of Technology, Poland (in Polish)

13 Ambroziak A 2006 TASK Quart. 10 (3) 253

14 Ambroziak A 2005 TASK Quart. 9 (2) 167

15 KłosowskiP,KomarWandWoznicaK2009ConstructionandBuilding Materials 23 (2) 1133

16 Bejan L and Poterasu V F 1999 Comput. Meth. in Appl. Mech. and Engng 179 53

17 Armijos S J 2008 Fabric Architecture: Creative Resources for Shade, Signage, and Shelter, W. W. Norton & Company, Inc.

18 Otto F 2001 Tensile Structures, MIT Press, New York

19 Jiyple A II 1970 TeorIIa yIIpyroctII, Nauka, Moscow, Russia (in Russian)

20 Stanuszek M 2003 Finite Elements in Analysis and Design 39 599

21 KłosowskiP1983StaticsandDynamicsofCable-MembraneHanging Roofs in Materially and Geometrically Non-linear Approach, PhD Thesis, Gdansk University of Technology, Poland (in Polish)

Figure 9. Distribution of stresses (left T1 kN/m, right T2 kN/m) – NLS M

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Pre-Engineered Metal Buildings

Buildings & houses are one of the oldest construction activities of human beings. The construction technology has advanced since the beginning from primitive

consturction technology to the present concept of modern house buildings. The present construction methodology for buildings calls for the best aesthetic look, high quality & fast construction, cost effective & innovative touch.

In developing countries massive house building construction is taking place in various parts. Since majority of population lives in towns and cities hence construction is more in the urban places. The requirement of housing is tremendous but there will always be a shortage of house availability as the present masonary construction technology cannot meet

the rising demand every year. Hence one has to think for alternative construction system like pre-engineered steel buildings. Pre Engineered Building (PEB) offers a solution to these problems with ease. In pre-engineered building concept the complete designing is done at the factory and the building components are brought to the site in knock down condition. These components are then fixed / jointed at the site and raised with the help of cranes. The pre-engineered building calls for very fast construction of buildings and with good aesthetic looks and quality construction. Pre-engineered Buildings can be used extensively for construction of industrial and residential buildings. The buildings can be multi storeyed (4-6 floors). These buildings are suitable to various environmental hazards.

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PEB concept has been very successful and well established in North America, Australia and is presently expanding in U.K and European countries. PEB construction is 30 to 40% faster than masonary construction. PEB buildings provides good insulation effect and would be highly suitable for a tropical countries. PEB is ideal for construction in remote & hilly areas. Refer Figure 1 for some photographs of Pre Engineered Steel Buildings.

Figure 1: Pre Engineered Buildings (Source Internet)

Technology of Pre Engineering Building with its Various Components

Pre-Engineered Steel Buildings use a combination of built-up sections, hot rolled sections and cold formed elements which provide the basic steel frame work with a choice of single skin sheeting with added insulation or insulated sandwich panels for roofing and wall cladding. The concept is designed to provide a complete building envelope system which is air tight, energy efficient, optimum in weight and cost and, above all, designed to fit user requirement like a well fitted glove.

These Pre-Engineered Steel Buildings can be fitted with different structural accessories including mezzanine floors, canopies, fascias, interior partitions, crane systems etc. The building is made water-tight by use of special mastic beads, filler strips and trims. This is a very versatile building system and can be finished internally to serve any required function and accessorized externally to achieve attractive and distinctive architectural styles. It is most suitable for any low-rise building and offers numerous benefits over conventional buildings.

Pre-engineered buildings are generally low rise buildings, however the maximum eave heights can go upto 25 to 30 metres. Low rise buildings are ideal for offices, houses, showrooms, shop fronts etc. The application of pre-

Metal Buildings


engineered concept to low rise buildings is very economical and speedy. Buildings can be constructed in less than half the normal time especially when complimented with other engineered sub-systems.

The most common and economical type of low-rise building is a building with ground floor and two intermediate floors plus roof. The roof of a low rise building may be flat or sloped. Intermediate floors of low rise buildings are made of mezzanine systems. Single storeyed houses for living take minimum time for construction and can be built in any type of geographic location like extreme cold hilly areas, high rain prone areas, plain land, extreme hot climatic zones etc.

There are basically nine major components in a pre-engineered building such as :

- Main framing or vertical columns- End wall framing- Purlins, girts and eave struts- Sheeting and insulation or prefab panels- Crane system- Mezzanine system- Bracing system- Paints and finishes- Miscellaneous services

Main Framing

Main framing basically includes the rigid steel frames of the building. The PEB rigid frame comprises of tapered columns and tapered rafters (the fabricated tapered sections are referred to as built-up members). The tapered sections are fabricated using the state of art technology wherein the flanges are welded to the web. Splice plates are welded to the ends of the tapered sections. The frame is erected by bolting the splice plates of connecting sections together. For normal housing the main framing columns are of ISMC category.

End wall framing

The endwall frame of a pre-engineered building may be designed as a main rigid frame (i.e. similar to the interior frame) or as a post and beam frame. The decision depends on the customer’s requirement (mainly as to whether he wants to go in for future expansion or not)and / or building’s requirements (is the endwall open for access). The post and beam end wall system of framing consists of columns (posts) with pinned ends, supporting horizontal beams known as endwall rafters. Girts are flush framed between posts to provide lateral stability and a neat appearance. Post and beam endwalls are assumed to be laterally stiff due to the diaphragm effect of the wall sheeting. The diaphragm action is proven to be sufficient enough to resist

the transverse wind force acting on the small tributary area of the end wall.

For single storeyed normal houses end wall framing is same as main framing.

Purlins, girts and eave struts

Purlins, girts and eave struts are also known as secondary cold-formed members. There is no welding involved in their preparation. They are prepared by just bending the steel coil giving it the desired shape (Z-shape for purlins and girts, and C-shape for eave struts).

Sheeting and insulation or prefab panels (Panels and insulation)

Single skin profile steel sheets are used as roof and wall sheeting, roof and wall liners, partition and soffit sheeting. The steel sheets are generally made from steel coils and aluminium coils. Minimum thickness of steel coils used is 0.5mm high tensile steel. The profiles depends upon the stiffness required, the governing loads (dead/live/wind) etc. The strength of the sheets depends on its profile, and the depth and number of ribs. The steel sheets are normally zincalume or galvanized profiled sheets permanently colour coated either plain or the sheets can be coated with special paints like PVF2, if required, for better anti-corrosion properties. These buildings can be properly insulated by providing fibrous insulation slabs / rolls of non- combustible Rockwool, Aluminium foil laminated, placed over a metal mesh bed created between the purlins, and then the roofing steel sheet fixed over it. The siding walls can also be insulated by providing a double skin profile steel sheet wall cladding having Rockwool Insulation slab sandwiched in between and held in position with the help of ‘Z’ spacers in between the two profile steel sheets. In similar pattern a double skin insulated roofing system can also be erected.

Crane system

Crane in industrial buildings are used to improve material handling productivity and to allow more efficient utilization of space by reducing or eliminating traffic due to forklifts etc. The crane runway beams are simply supported built-up sections with cap channels. Also, since it’s a built-up member, it can be tapered – saving the beam costs for large spans.

Mezzanine system

Generally, the mezzanine framing is connected to the main rigid frame columns for lateral, stability. Mezzanine beams and joists are analyzed and designed as simple span members. Standard mezzanine structure consists of built-

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up beams (that may be tapered for large spans or heavy loads) that support built-up, hot-rolled or cold-formed mezzanine joists which in-turn support a metal deck. A reinforced concrete slab is cast on the metal deck as a finished surface. The metal deck is not designed to carry the floor live loads, it is intended only to carry the reinforced concrete slab during pouring. The reinforced concrete slab must be designed to carry the floor loads. Interior mezzanine stub columns are hot rolled tube sections or built-up sections.

Sometimes, in place of concrete flooring, checkered plates or grating may be used. Sometimes, a structural framing system is mounted on top of the roof and is designed to support heavy roof accessories, such as HVAC units, water tanks and other miscellaneous roof equipment. These we call as roof platforms. Also, a narrow walkway, used primarily for maintenance crews to provide access to mechanical equipment supported on roof platforms, called as catwalk is provided at times. Catwalk are usually mounted alongside crane beams, suspended under rigid frame rafters or elevated above the top of the building roof.

Bracing system

Longitudinal cross bracing, used to provide lateral stability to the structure against wind, seismic or other forces, comprises of 7-strand twisted galvanized cables with an eye bolt and an adjusting nut at both ends, located near the outer flange of columns or rafters and attached at the web of the rigid frame. In buildings supporting cranes, crane longitudinal loads will be transferred to the foundation using smooth round bars or hot rolled angles in lieu of cables. Also, when the sidewall has to be open for access etc., portal bracing is provided. For narrow width buildings with low eave heights, the fixed base column can be designed in the minor axis direction to resist the lateral forces applied along the length of the building, thus saving the additional bracings. Bracings are usually provided in large roof area industrial sheds. This is not required for houses.

Paints and Finishes

Normally the primary and secondary steel are coated with one coat (35 microns) of redoxide paint without any special treatment to steel. However, if some special paint has to be applied to steel in order to give better anti-corrosion properties etc. then the steel members have to be shot-blasted and then coated with the special paints. Also, the other option is for going in for galvanized secondary steel and hot-dip galvanize the primary steel for better steel properties. For houses inside painting on walls & ceiling is to be provided.

Miscellaneous Services

a. Doors and Windows

Steel or aluminium framed doors and windows are fixed to the purlins either by welding or bolted to the flanges already fixed to the purlins. Proper flashings are applied wherever necessary.

b. False Ceiling

This is usually required for residential building or offices. A metal frame work is hung from the ceiling and false ceiling of rigid boards are either bolted or placed over the frame work.

c. Partition Walls

This is usually required for residential building or offices. Partition wall comprises of two rigid boards having insulation sandwiched in between and fixed to the steel columns and purlins. Alternatively prefab sandwich panels can also be fixed to the columns and purlins.

d. Flooring

Flooring is usually of conventional nature consisting of cement concrete.

Design Codes Referred

Design codes that govern the design procedures and calculations are as follows :-

- Frame members (hot rolled or built-up) are in accordance with AISC (American Institute of Steel Construction) Specifications for the design, fabrication and erection of structural steel.

- Light gauge cold-formed members are designed in accordance with AISI (American Iron and Steel Institute) Specification for the design of light gauge cold formed steel structural members.

- IS : 8750 - 1987 : Code of practice for design loads of buildings & structures.

- IS : 800 -1984: Code of practice for general construction in steel.

- IS : 801- 1975 : Code of practice for use of Cold formed light gauge Steel Structural Members in general building Construction

Benefits of Pre-engineered building

- Optimised design of steel reducing weight- Easy future expansion/modification- Voluminous space (up to 60M clear spans, 30 M eave

heights)- Weather proof

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- No fire hazards- International Quality Standards- Seismic & Wind pressure resistant- Quality design, manufacturing and erection- Quick delivery and Quick turn-key construction- Architectural versatility- Energy efficient roof and wall system using Rockwool &

PUF insulation- Water-tight roofs & wall coverings- Pre-painted and has low maintenance requirement- Easy integration of all construction materials - Erection of the building is fast- The building can be dismantled and relocated easily- Future extensions can be easily accommodated without

much hassle.

Applications

Applications of pre-engineered steel buildings include (but are not limited to) the following :

- Houses & Living Shelters- Factories- Warehouses- Sport Halls

Figure 2: Application of Steel structures in end-use segments (Research carried out by Deloitte)

Figure 3: Benefits and challenges of using structural steel in building segment (Research carried out by Deloitte)

- Aircraft Hangers- Supermarkets- Workshops- Distribution Centres- Commercial Showrooms- Restaurants- Office Buildings- Labor Camps- Petrol Pumps/Service Buildings- Schools- Community Centres- Railway Stations- Equipment housing/shelters- Telecommunication shelters- “Almost” any low-rise building

Figure 4: Demand from building segment (Research carried out by Deloitte)

However in a research carried by Deloitte for Indian market, Industry dominates the application of tell structures and buildings are the least (Refer Figure 2). Structural steel usage in building segment in India is presently driven by exception rather than practice. Penetration of structural steel in overall buildings segment is currently at <2%. However, this trend has seen a change over the past 2-3 years as developers are realizing the advantages of using structural steel in building segment. As seen below in Figure 3, the benefits of using structural steel in building segment out-weighs the challenges over the next 10 years.

Conclusion

All the developed countries have shifted to PEB structures as it is fast and easy to build at low cost. This can be easily seen from the Figure 4.

For increased use of PEB structures, s shift in mindset that concrete is the only default construction material in the buildings segment needs to be changed. There is a recent trend of using metal buildings as an option for affordable housing in various developing countries including India. These are typically small metal buildings, complete with civil foundations, electrical, plumbing and drainage solutions. To join the growth story as developed countries, the time has come to understand and start using PEB structures in all types of constructions.

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Condensation Phenomena in Metal Buildings

Condensation occurs at cold surface areas. Cold transfers through metal that is exposed to outside temperatures and forms on the warm air side. An

example of this is a water glass with ice(Refer Figure 1). The outside of the glass can become wet as the condensation forms on the warm air side. Visible condensation can be controlled by reducing the cold surface areas.

Water is the most common cause of rot and corrosion within buildings. Moisture inside a building can also con-tribute to the formation of mold. Any evidence of liquid

water within the building commonly is misunderstood by engineers and property owners as leakage or failure of roof covering materials. But condensation is frequently the cause. The principle thermodynamic properties responsible for condensation in buildings are identical for all building envelope materials. Moisture condensation is not unique to metal roofing systems but frequently is the cause of rotting of timbers or corrosion of metal decking, metal roof panels and metal fasteners. The control of condensation in steep slope metal roofing systems is important to maintain the effectiveness of the insulation and to protect the roof

MB Bureau Report

“Condensation in steel buildings is a drawback that is often mistaken for leakage from roofing failure. Excess condensation in metal buildings can lead to odors, mold and mildew growth which can further result into deterioration of building material. It can also decrease the effectiveness of insulation and encourage insect infestations.”

Metal Building Condensation


material from degradation. Increasing condensation in steel buildings can also lead to corrosion of metal roofing, metal decking and metal fasteners. Condensation problem in steel buildings can be solved by keeping the relative humidity in your building between 30 to 50 percent. With the help of a dehumidifier with a hygrometer, you can set a humidity level for your building. Increased air circulation and ventilation also help to solve the issue of condensation in a metal building.

Sources of Moisture

Moisture is given off in many different processes. Human beings give off a significant amount of water through respiration and perspiration. There are other common sources. Gas, oil-fired and propane space heaters give off a significant amount of moisture through the process of combustion. The moisture introduced through combustion is sometimes very difficult to detect because at the point of combustion, the gases are very hot and can hold large quantities of moisture. When this hot gas is mixed with cooler air, the temperature drops. As the temperature drops, the amount of moisture that the gas can hold will decrease and at some point away from the combustion device, the water will condense on anything that it comes in contact with that is below the dew point of that mixture. Flue gases should be vented outside to prevent this from happening. Even before a building is fully completed, there can be significant amounts of moisture introduced into the building. Excavated earth contains a significant amount of moisture. As the soil is exposed to surrounding air, the moisture will be given off. If the building is closed, this moisture will stay within the building. As soon as the temperature drops below the dew point of that air/water mixture, condensation will form. Fresh concrete is another source of large amounts of moisture. If the building is closed and concrete is poured, a way must be provided for moisture to be vented to the atmosphere. Ventilation should always be considered as a preventive measure during the construction schedule.

Effects of Condensation

Knowing the process by which condensation is formed, one can look at the effects of condensation on normal building materials. For the metal building industry, the most commonly used materials are steel sheathing and fiberglass insulation. Most of the metal in metal building is treated against corrosion and rust. Rust is a result of an interaction between the metal and salts, acids or alkalines. Another metal employed is aluminum which does not rust, but does oxidize. In both instances, the metal itself becomes weaker. In time, both materials will deteriorate and shorten life expectancy. While many surface treatments are applied

to ferrous and non-ferrous metals to prevent oxidation, the best protection is to eliminate a principal cause of oxidation--in other words, eliminating condensation. The insulating material most commonly used in metal buildings is fiberglass blanket. Water in its liquid form is a good conductor of heat. The presence of water vapor or condensed water in fiberglass insulations will increase its thermal conductivity because of the higher conductivity of water. However, glass fibers do not absorb water. Only surface wetting occurs. Once all the moisture is removed from the wet fiberglass surface, it will revert back to its original insulating value. The same cannot be said for other materials. Here again, an ounce of prevention is worth a pound of cure. Eliminating and preventing condensation in the fiberglass will help retain the insulating values of the fiberglass.

Condensation in Existing Buildings

Condensation in existing metal buildings happens because there is no insulation, not enough insulation, or the existing fiberglass or facing is old and torn, creating air leaks. Fortunately, there are ways to help prevent or reduce any of these problems. With high energy costs there is a growing trend to seal air leaks as well as add insulation to existing metal buildings. This is referred to as an insulation retrofit.

Retrofitting insulation between purlins or wall girts not only saves on energy bills, but it also addresses any existing condensation issues. Typically, structural members (roof purlins or wall girts) are 8” deep. It is recommended in retro fit jobs that the entire 8” cavity is filled with insulation to avoid creating an air space between the roof panel and insulation. This is important because air spaces can cause condensation to frequently form on the inside of the cold surface of the exterior panel.

In most cases, metal building insulation was installed upon construction between the roof or wall panels and steel roof purlins or steel wall girts. This type of install causes there to be a substantial heat loss at the area where the insulation blankets were compressed between the purlins and wall girts. The positive factors with this type of install is that the vapor barrier is continuous, and the compressed insulation gives some thermal break between the outside panels and structural members. This thermal break is important in an insulation retro fit to help avoid condensation in the inside where the structural members are exposed. Adding faced fiberglass blankets to fill the entire cavity, in addition to the existing thermal break will help prevent any further condensation from forming against the exterior panels or the exposed steel members.

Condensation issues are more difficult to address in cases where there is no insulation previously installed



between the outside panels and structural members. This is because there is no thermal break between structural members and the outside panels making it easy for cold to transfer from one metal to another, eventually making it into the building. Once the cavities between the purlins are filled with faced fiberglass and heat is introduced into the building, condensation or frost can still form at the bottom of the structural members. To help avoid this, a thermal break tape or strips of rigid board insulation can be installed at the bottom of the purlin or girt.

Refer Figure 2 and 3 for condensation process in steel buildings.

Condensation Control

In a typical residential roofing assemblyseveral components are recommended to minimize condensation in the attic space. They are:

- A vapor retarder located on the attic side of a ceiling board

- Insulation located above the air barrier on the warm side of the surface

- Vented air space above the ceiling

Figure 2: Condensation formed on underside of exterior panel shown above because the entire cavity was not filled with fiberglass insulation.

Figure 3: Cold transferred through the metal and formed condensation on the warm air side of the metal above.

Figure 4: Condensation in Building Structural Elements

- A moisture barrier or underlayment located between the roof covering materialand the roof deck

- A roof covering on top of the underlayment

Vapor Retarders

Vapor retarders are an important and effective component in a roofing system in areas where the mean daily temperature reaches 40o F or lower, and the indoor relativehumidity is 45% or greater. Vapor retarders physically block the flow of moisture-ladenair, which prevents it from contacting a cold surface. Vapor retarders are generally thin flexible membranes, with a perm rating of 0.50 or less. The perm rating is a measure ofthe resistance of a material to the passage of water vapor through it under specifiedtime rate, temperature, and humidity conditions. A rating higher than 0.50 perms is generally considered to have a diffusion too great to adequately serve as a vaporretarder. Perm ratings are established by standard ASTM E96.

Insulation

In commercial roofing applications, the insulation material may be loose-laid over the purlin or truss structure, with the panel attachment securing the insulation. A composite insulation board wood panel (wood panel laminated to a rigid insulation) is also acommon substrate used under architectural metal panel roof systems.Care must be taken not to compress and deform the insulation. A metal or woodbatten, counter-batten system can also be used to create ventilation space below themetal panels where needed.

Ventilation

Venting air and water vapor out of a building can be an effective way to minimizecondensation but ventilation alone does not cure the problem. Proper ventilationreplaces the warm moist air with air that contains less water vapor. Ventilation isrelatively easy to accomplish in conventional attic spaces. However it is morecomplicated when used in cathedral ceilings and with structural insulated panels. Positive ventilation and air flow is necessary to maintain consistent temperature between the vented area and the outside conditions. This condition will cause moistureto escape rather than to condense on cold surfaces. Ventilation is generally expressed by the number of times per hour the building air is replaced with outside air. This is referred to as air changes per hour. The number of air changes required per hour varies per application. For commercial building construction, general guidelines for whole building air exchanges, (i.e. in building types that do not have an attic space) are 3-5 air changes per hour for warehouses, 5-10 air changes per hour for light manufacturing facilities and 10-20 air changes per hour for heavy manufacturing.


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Underlayments or Moisture Barriers

On roof applications over solid decking, an underlayment (with slip sheet as needed) isrecommended as a secondary barrier against water penetration through a roof system. The underlayment should “breathe” wherever possible, allowing air to pass through while shedding moisture.

Moisture Balance

If a balance between wetting and drying is maintained, moisture will not accumulate and condensation related problems in roofing are unlikely. The extent and duration of wetting, storage and drying must be considered when assessing the risk of moisture damage. The concept of using materials with greater drying potential and storage capacity has begun to receive more attention.

A quick reference guide using this Moisture Balance approach is shown below:

Recommendations for New Buildings

Long term cost of ownership should be the governing element in determining a new building’s insulation needs. For any climate controlled building, we recommend a good quality vapor retarder facing as well as an insulation thickness that gives you the lowest cost of ownership. Local codes may govern the insulation minimums that are required. It is important that these codes are verified by the owner or contractor prior to making an insulation order. Metal building insulation offers several different options. Care should be taken to seal the vapor retarder facings and or provide a continuous covering such as polypropylene to help prevent the passage of vapor into the fiberglass blankets. A continuous vapor barrier will help prevent moisture from working its way into the fiberglass. It is also important to provide a thermal break at each roof purlin or wall girt to prevent heat or cold transfer. Thermal breaks are achieved by either laying insulation over the top of the girts and or purlins before the panels are screwed down; with ¼” Thermal Break Tape; or with ½” or 1” Thermal Blocks.

Conclusion

Careful design of roof systems can prevent or at least minimize condensation in building assemblies. The proper use of vapor retarders, insulation, ventilation, and moisture barriers can reduce the presence of moisture in a building and control the formation of condensation. Building codes and ASTM offer some guidance in the selection of these building components. However, it is always recommended that a moisture control or roof design professional be consulted for specific information on how to design metal roof systems to minimize condensation problems.

Reference

- http://www.designandbuildwithmetal.com/Columnists/Writers/danny_wirth_7_13_09.aspx

- http://www.rimainternational.org/wp-content/uploads/2011/03/condensation-faq.pdf

- http://sheds-support.leisurebuildings.com/threads/173-Condensation-on-metal-and-steel-sheds-and-buildings

- NRCA Metal Roofing Manual.

- MBMA Metal Roofing Systems Design Manual.

- ARMA Technical Bulletin – Ventilation and Moisture Control for Residential Roofing,1997.

- J.F. Straube, Moisture, Materials & Buildings, HPAC Engineering.

- J.F. Straube, Moisture in Buildings, ASHRAE Journal, January 2002.

- H. Hens and F. Vaes, Laboratory of Building Physics, KatholiekeUnversiteit,Leuven, Belgium, The Influence of Air Leakage on the Condensation Behavior ofLightweight Roofs, Air Infiltration Review, Volume 6, No. 1, November 1984.

Strategy Objective Measure

Control of moisture access

Eliminate air leakage Air barrier system

Restrict vapor diffusion Vapor retarder

Control moisture accumulation

Raise temperature of condensing surface

Insulation outside condensing surface

Allow harmless accumulation

High moisture storage capacity at condensing

surface

Removal of moisture Promote drying Vapor permeable layers

Capillary ventilation

Cavity ventilation

Remove condensation Drainage

Reduce moisture load Limit construction moisture

Initially dry materials

Condensation Control Materials

A benefit of metal roofing is that it does not absorb condensed water like some other roofing materials. Certain condensation-control fleece materials are available to absorb liquid condensate into capillary pores and release it back by evaporation or drainage at a later point when conditions favor evaporation. The water vapor then gets transported out of the system by convection and diffusion. These types of self-adhesive fleece materials are typically laminated to the underside of metal roof panels during manufacturing but before forming. The adhesive layer acts as a barrier to prevent the moisture from contacting the surface of the metal roof underside. In many cases, these materials can replace anti-condensation blankets and vapor retarders.


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Building Big: Civil Engineering Behind Skyscrapers

Planning a high-rise building would be in con-ceivable today without the help of experts and technical consultants. Extensive soil analyses are

required to determine the strength of the subsoil before deciding on the location for a high-rise building. In the majority of cases, cores are drilled into the load-bearing subsoil to obtain soil samples. The drilling profile of the geological strata making up the subsoil and laboratory analyses of the soil samples provide the basic data for the soil report which is in turn used as the basis for planning the supporting structures and choosing a suitable foundation structure with due regard for the

loads exerted by the high-rise building. The forces acting on the high-rise structure in the event of an earthquake must be taken into account when erecting high-rise buildings in areas prone to seismic activity. The same applies to wind loads and particularly to the dynamic effects of windstorm or earthquake loads. The additional vibration loads can result in overall loads of the same order of magnitude as the load exerted by the deadweight of the structure. The situation is particularly critical if the vibrations reach the resonant frequency of the building: in such a case, the vibrations can intensify until the entire building collapses.

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“Skyscrapers are gigantic projects demanding incredible logistics, management and strong nerves among all concerned in their planning and construction. As long ago as 1928, the American Colonel WilliamA. Starrett wrote that no peacetime activity bore greater resemblance to a military strategy than the construction of a skyscraper.”

High Rise Building Technology


Technologies Used

Many technologies are adopted in high-rise building construction to overcome problems. The following are most commonly used:-

Top-down construction method

In centre business area (CBA), the buildings general have a basement and connect other building and mass transit railway (MTR). During construction state, top-down and bottom-up method were adopted in construction. Top-down construction method become more popular because it shorter the construction period. Excavation will carry out without the need for strutting to support the excavation because the slabs act as the horizontal support. Therefore, it is another advantage for top-down construction method. The difficulties are the limited headroom for excavation. Therefore, a special machine maybe need during construction state.

Prestressed Concrete

As the height of building increase, the material and con-struction will be change to fit the actual need. For example, the reinforced concrete beam will change to prestressed reinforced concrete beam in order to decrease the depth of beam and/ or slab. Prestressing means the intentional creation of permanent internal forces and stress in a structure or assembly, for improving its behaviour and strength under service conditions. Since concrete is strong in compression and weak in tension, prestreeing the steel against the concrete would put the concrete under compressive stress that could be utilized to counterbalance tensile stresses produced by external loads.

Figure 1: Prestress tendon is post-tensioning slab, Condition of tendon before concreting

Pre-tensioning

High-tensile tendons are tensioned before the concrete casting. When the strength of concrete reaches the designed level, the wires are released to produce compressive stress. Suitable curing can accelerate the strength establishment process of concrete. For example, steam curing.

Post-tensioning

Prestress tendons are checked before concrete casting. The prestressing steel can be introduced after concrete has set by casting in duct-tubes at the appropriate positions that are extracted before the steel is inserted. The tendons are anchored at one end of concrete unit and stressed by jacking against the other end. The steel is subsequently grouted under pressure through holes at the ends of the unit to protect it from corrosion and to provide bond as an additional safeguard.

Advantages of Prestressed Concrete

- Better appearance and durability- More efficiently than RC- Allow for shrinkage creep at the design stage- Longer span- Greater rigidity under working loads than RC

Disadvantages of Prestressed Concrete

- Higher construction cost due to material and supervision- Maintenance is need after superstructure completed- Broken tendon may kill the worker during prestressing.

Figure 2: Concreting works for Pile Cap

Figure 3: Classification of Foundation



Transfer Structure

In order to have a large space for shopping mall, transfer structure will be adopted to transfer the loading to column. Pile caps also a transfer structure in Hong Kong. Pile Caps transfer the loading from column to pile (e.g. H-pile, bored pile, mini-pile, etc. Refer Figure 2 for a typical transfer structure construction.

Deep Foundation

Sometime, the soil condition is not suitable for high-rise building. A deep foundation (e.g. large diameter bored pile (LDBP), driven H-pile) will be developed to solve this problem.

Ground Investigation (G.I.)

Before foundation works, designer use the G.I. to found out the data for foundation design. Before actual foundation works (mini-pile, bored pile) commenced, second G.I. (pre-drill)is needed to check the design assumption After all foundation works (mini-pile, bored pile) completed, third G.I. (post-drill) is need to check the design. Interface coring and proof-drill are needed to check the quality.

Figure 4: Driven H-pile works in Progress Figure 5: Drilling rig is used in drilling works

Mini-pile

Sometime, the location is not suitable for large machine for foundation works. Other construction method will be produced, such as, mini-pile. Odex method is used in mini-pile construction works. Refer Figure 6 for the mini piling process.

Large Diameter Bored Pile (Refer Figure 7 to 12 for bored piling process)

Advantages of large diameter bored pile

a) length can readily be varied to suit the level of bearing stratum

b) Soil or rock removed during boringc) Less noise and vibration is produced compare with

driven H-pile

Disadvantages of large diameter bored pile

f) expensive compare with footingg) soil erosion may be occur if the intersection of soil and

rock is not horizontalh) Large machine is needed compare with mini-pile.

Figure 6: Mini-pile was used because of headroom limit

Figure 7: Steel casting is used in bored pile works

Figure 8: Equipment is used to ensure the drilling rig vertical Figure 9: Vibro hammer is used to insert the steel casting


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Figure 10: Machine used to construct the bell out Figure 11: Reinforcement fixing for bored pile in progress

Figure 12: Excavation works for pile cap and cut-off

Driven H-pile (Refer Figure 13 and 14 for H-Piling process)

The construction sequence of H-pile

1. set out the location of H-pile2. engineer check the setting out, dimension of pile and etc3. Driven the steel pile4. Check the verticality5. Connect the steel H-pile by welding6. Painting the pile in order to rust-proofing7. Final-set test is adopted to check the design assumption

Advantages of large diameter bored pile

a) length can readily be varied to suit the level of bearing stratum

b) Compare with bored pile, H-pile is cheaperc) No soil or rock were excavated; therefore, transportation

of debris is not needed.d) Settlement of adjacent is usually small compare with

large diameter bored pile.

Disadvantages of large diameter bored pile

a) No Soil or rock removed during drivingb) expensive compare with footingc) Large machine is need compare with mini-piled) Large noise and vibration is produced

e) Longer construction time is needed because only 3 hours per day for divining works

f) Adjacent buildings may be affected due to large vibration

Shoring

Shoring is the means to provide temporary support to structures that are in an unsafe condition till such time as they have been made more stable.

Sheet Piling and Slope stabilization (Geotechnical Works)

Normally in urban areas, the buildings are mainly surrounded by other building and slope. Therefore, a lot of technology was used to solve such problem. For example, sheet piling, grout column, grout curtain and pipe pile wall for excavation and lateral support. Soil nail and rock dowel for slope stabilization works.

Pre-cast facade

Pre-cast façade is very common in high-rise building. It is because government encourage the developer to use such construction method. Sometime, contractor will use precast concrete to reduce the amount of construction waste generated on construction sites, reduce adverse environmental impact on sites, enhance quality control of concreting work and reduce the amount of site labour. Refer Figure 19 and 20 for precast façade installation process.

Figure 13: H-piling works in progress Figure 14: Verticality was checked during H-piling works

Figure 15: Shoring was provided to support adjacent building



Advantage of pre-cast façade is below

- Better quality control: Better quality control is achieved because the precast façade was produced in a factory, where all procedure were closely monitoring

- Less debris produced: It is because all form formwork was the same. It can be recycled. However, in construction site, the formwork may be placed in a wrong position.

- Less noise produced: All the sequences were conduct in factory; therefore, less noise will be produced. For example, hammer hit the formwork.

- Faster construction period: The strength of the pre-cast façade can reach the design strength earlier because steam curing can be used.

Disadvantages of Pre-cast facade

- Damaged during transportation, lifting operation- It can be solve by careful design to ensure the pre-cest faced suitable for lifting up by tower crane.

- Water leakage- Water leakage between the construction joints is the major problem in high-rise buildings. However, it can be solved by the following:

Figure 16: Sheet piling was installing by vibrating hammer

Figure 17: Sheet piling were installed to protect the existing road in drainage improve works

- a careful design and- install a water-stop during concreting on site

Different Structural Types used in High-Rise Buildings Construction

Throughout time, there are many structural systems developed. It started for building the structure as a rigid frame to building as a long cantilever. Moment resistant frames can be effective options for buildings upto 20 to 30 stpries; tubular frames and trusses can reach a lot higher. Other systems have characteristics taken from both. There are many factors determining a structural type for a building, this include the general economic considerations, soil conditions, fabrication and erection considerations, mechanical systems considerations, fire rating considerations, community factors, legal factors and availability and cost of main structural materials.

Bearing Wall System

This is the traditional structural system in the erection of tall buildings. The vertical structural elements carry the loads directly to the foundations. The common building materials would be stone, brick and reinforced concrete. The height of the structure is limited by the strength of the bearing materials. Buildings in this type are not going to be too high because of the accumulated weight of the walls plus the other dead and live loads. Too high a structure will result in wall becoming so thick that lower floors can no longer function.

Bearing Walls with Core

In this system, one or more cores are added to the parallel alignment of the bearing walls in order to create a lateral load resistance in a direction perpendicular to the bearing walls. The core is formed by grouping two or four walls perpendicular to each other to create a closed geometry. Typical shape would be tube, round or square that is stiff and can resist torsion. The core is often placed in a central location for the convenience of distributing building services and for an increase in structural integrity. If placing the core off center, it can create additional torsion and rotation, which might require extra resistance mechanisms. This structural system allows a greater free floor area and this is a common system in Hong Kong for building 20 to 30 storeys reinforced concrete office building.

Self Supporting Boxes

In 1970s, this system was developed when the prefabrication of reinforced concrete structures was at the peak popularity. Prefabricated concrete floor are place on top of each other in a way that each later oriented perpendicular to the one directly below it.


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Figure 18: Pipe Pile Wall is used to support existing buildings in shaft excavation

Core with Cantilevers

In this system, floor slabs are cantilevered out from the solid core supports in the middle. The advantage will be the absence of interior columns and freeing the façade of the structure. The disadvantage is the additional thickness needed for larger cantilever.

Rigid Frame

Rigid frame system is developed by structural designers from the bearing wall system. However, it is not very efficient. Each member in the system must help in the transfer of lateral loads to the foundations through rigid connections. Buildings in this type are often very regular and not very tall. In order to reduce the lateral sway of the structure, a stiffening core would be added to the standard rigid frame.

Tube in Tube

The development of this system is very important in modern skyscraper of great height. The exterior and interior columns of the structure are placed so close together that they almost form a solid surface. The entire building acts as a huge hollow tube with a smaller tube in the middle. This resists a

great amount of torsional loading and the lateral loads are supported between the inner and outer tubes. A number of tallest buildings in Asian countries used this system.

Some Special construction methods

BMW Headquarters, Munich

The headquarters of BMW A.G. differs from conventional buildings to create an impressive corporate symbol in the form of a 100-m-high four-cylinder structure. The require-ments for appropriate office organization yielded a basicout line in the shape of a clover leaf. Stairways, elevators and sanitary areas are accommodated in the central core. In this way, all the offices can be reached by the shortest possible route. Trendsetting methods were also used for the construction work. A reinforced concrete version was chosen as the most economical solution. According to the design concept, the entire building with 18 office floors and a technical floor was to be suspended from a girder cross at the top of the roughly 100-m-high core via four central king posts. This is a modification of the outrigger truss. The entire load of the building is transmitted to the foundations via the core as the central element; it also absorbs all wind forces. A mighty girder cross with a projection of16 m is mounted at the top of the core. The four king posts are secured to this central girder cross, each king post comprising 105 threaded steel bars with a load-bearing capacity equal to a suspended weight of 4,600 Mp. Small Figure 19: Pre-cast facade installation works in progress

Figure 20: Pre-cast facade before installation



outer columns are additionally located between the floors. These outer columns are designed as compression columns above the technical floor (12thfloor) and as king posts below. Time and costs were the decisive reasons for choosing this innovative construction method. All 19 floors were successively produced at the foot of the shell and core; the first floors were even produced complete with facade and glazing during construction of the supporting cross. The finished floors were then connected to the supporting cross via the king posts and raised one floor at a time every week with the aid of hoisting gear so that another floor could be produced in the space vacated at the foot of the core and then connected to the floor above (lift-slab method). Completion of the facade, glazing, installation and interior finishing proceeded on the suspended floors, unimpeded by the structural works and lifting operations. In addition to reducing the construction time required, this method also eliminated the need for expensive tooling and assembly work.

LA Grande Arche, Paris

This building, which has already been mentioned, takes the form of a giant cube open on two sides with edge lengths of 110 m. It was completed at the end of 1989 on the 200th anniversary of the French Revolution and took 5 years to

build (Refer Figure 21).The building has a weight of more than 300,000 Mp and is mounted on neoprene bearings, the loads being transmitted 30 m into the subsoil via twelve concrete pillars. The cube’s main support is in the form of four prestressed upright reinforced concrete frames 21 m apart. They are complemented by horizontal members measuring roughly70 m at ground and roof level. Each of these members is 9 m high, the equivalent of a 3-storey building. Since the two vertical sides of the cube would be without roof-level transverse bracing during construction, the required stability for that phase of the work was produced by means of horizontal steel truss reinforcements. A total of 37 office floors are accommodated in the two 18-m-wide wings of the cube (each with an area of 42,000 m2).

Conclusion

High-rise buildings are still the essential form of building structure constructed extensively in urban are as, in particular, in the hearth of the commercial zones of metro-politan cities. On the other hand scarcity of land supply encourages the construction of high-rise buildings. For the construction of high-rise buildings, site planning including activity scheduling and site production layout has to be reviewed and re-plan from time to time in practice as site conditions and resources are dynamic and uncertain. Every building is special of its kind and has to be given special consideration. Because every building has its own surrounding condition which is different from the other. The techniques mentioned above are more general, it need to be applied with modifications to suit for every individual case.

Reference

- CHEW (2001), Construction Technology for Tall Buildings, Singapore University Press, Singapore

- R.C. Smith and C.K. Andres (1986), Principles and Practices of Heavy Construction, Prentice-Hall

- C.W. Griffin(1986), Manual of Low-Slope Roof Systems, 3rd Edition, McGrawHill, New York

- W. McElroy (1993), Roof Builder’s Handbook, PTR Prentice Hall, New Jersey

- C.K. Andres (1998), Principles and Practices of Heavy Construction, 5th Edition, Prentice Hall, New York

- D. T. Coates (1993), Roofs and Roofing Design and Specification Handbook, Whittles, UK

- S. Hardy (1997), Time-Saver Details for Roof Design, McGraw-Hill, New York

- H.W. Harrision (1998), Roofs and Roofing: Performance, Diagnosis, Maintenance, Repair and the Avoidance of Defects, Building Research Establishment, Watford, Hert

- W. Schuller (1990), The Vertical Building Structure, Van Nostrand Reinhold

- High-rise buildings in the course of history, Technology of high-rise buildings, Risk potential & Insurance by Munich Re Group.Figure 21: LA Grande Arche, Paris


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Use of Super Fibers in Sky Scrapers

Skyscrapers of the future in some sense are a kind of cosmic space elevators. Their height continues to grow and the design becomes more and more

original. In doing so these buildings won’t only be more seismically and winds resistant, but the most important thing – they will be absolutely safe for its inhabitants. One of the tools that allow achieving these effects are incorporating structural materials called super fibers.

Super Fiber

Super fiber shows extremely high strength compared to common fibers for clothes such as nylon or polyester and steel. It is mainly used as reinforcement for structural material these days, so that it is usually classified in high-tech industrial material other than fiber. The applications are aerospace, military, automobile, bicycle, sports, electronics, telecommunication, civil, construction and so on. Super fibers are classified according to raw material and can be divided into two groups of organic and inorganic matters. There are 5 types of super fiber and the famous super fibers are aramid fiber for armor jacket and carbon fiber in composite materials for golf shaft and airplane body. The inorganic carbon fiber and organic aramid fiber account for more than 90% of world production. The remainders are UHMWPE, Polyarylate and PBO which are organic. PBO is most recently developed and shows notable features but the production is not enough to be used widely. Polyarylate rates somewhat superior to aramid in chemical and abrasion resistance and the applications are almost the same. UHMWPE is made up of extremely long chains of polyethylene in the same direction and its molecular weight

numbers in usually between 2 and 6 million, which is the origin of the name. It is widely used for mooring lines, tug rope, etc. because it floats and does not absorb water. The ratio of strength to weight is the best among super fibers so that it was selected as material replacement for steel. And also it shows several merits of high shock resistance, high abrasion resistance, high anticorrosion and low friction but the melting point is lower than other super fibers.


Associate Editor

Classification Strength (GPa) Elasticity (GPa) Elongation (%) Density (g/cm3) Melting Point (0C)

Super fiber

Aramid 2.8 109 2.4~4.4 1.45 500~560

UHMWPE 3.5 110 3.5 0.97 150

Polyarylate 3.2 75 3.8 1.41 400

PBO 5.8 280 2.5~3.5 1.56 650

Carbon fiber 3.5 230 0.2~2.4 2.4~3.1 300~1500

OthersSteel 2.5~2.8 160~200 1.4 7.8 1150~1500

Polyester 1.1 15 25 1.38 260

Table 1: Comparison of properties between the super fibers

Figure 1: First prototype of ultra lightweight hoist rope using smart fibers

As can be seen from Table 1, the various properties of super fibers can be compared with various properties of steel and polyester fibers. The striking difference is in density, Super fibers comes with much lower density than steel fibers at considerably higher strength. Which gives an option to select it as a construction material in place of steel for controlling the weight of the skyscrapers. The process can allow for going into more high skyscrapers without adding

High Rise Building Super Fibers


any more weight on the founding ground. ”Additives like whisker-thin steel fibers are enhancing concrete’s strength and rigidity,” says John Fernandez, a professor at MIT’s interdisciplinary building-technology program. ”We’re also seeing research into smart fibers and carbon nanotubes that, when added to concrete, will increase compression strength beyond 200 MPa.” Fernandez’s work on smart fibers is also contributing to advances in so-called high-performance concrete, which is strong but optimized for other characteristics such as fire and blast resistance, vibration damping, and durability. A view of ultra lightweight hoist rope using smart fibers.

Some Proposed Projects with Super Fibers

Sky City 1000, Tokyo, Japan, Projected Height-3281 ft

The Takenake Corporation proposed Sky City 1000 back in 1989 to tackle Tokyo population-density problems. Tokyo-like congestion prompts a demand for green space and office space that vastly exceeds supply, and also introduces a host of environmental and social issues, from pollution to uncomfortably packed commuter trains. Takenake’s solution: Build up, way up, and place green spaces in the sky. “The feature of the proposal was making artificial land in the air,” said Masato Ujigawa, manager of the engineering department at Takenaka. To achieve this, Takenake will first start with a base that is 1300 feet per side, a footprint that equates to several city blocks (Burj Khalifa’s triangular footprint is just 300 feet or so). Then, in accordance with its name, Sky City 1000 will rise a full thousand meters (3281 feet), consisting of 14 levels stacked on top of one another. Each level will act as its own “town,” with a park-like plaza area in its center ringed by residences, schools and businesses. The structure would hold 10,000 homes and be used in some capacity by 130,000 people. Construction has not begun on Sky City 1000 since Japan’s population has begun shrinking as of 2005, Ujigawa says. Nevertheless, Ujigawa says that ideas originally espoused by the Sky City 1000 project have since been used in more conventional

construction. These include concrete reinforced with carbon fibers instead of iron to cut down on weight, and self-contained water-service systems in buildings that treat sewage and reclaim water.

Architect Peter Testa’s Carbon Tower, 40 story high

Figure 1: Sky City 1000, Tokyo, Japan

The project is a prototype 40-story skyscraper made entirely of composite materials, mostly carbon fiber. Such man-made composites, which also include better-known materials like fiberglass and Kevlar, are increasingly used in industry and for consumer goods—in everything from airplane fuselages to tennis rackets—because they are strong, lightweight, and easily molded into an almost endless variety of shapes. Although the materials seem well suited for architecture—in tension, carbon fiber is five times stronger than steel—their use in buildings has been rare. Testa, though, is convinced that composites will radically transform architecture during the next decade or two. His carbon skyscraper, which he likes to describe as a “woven building,” is designed to be not just less muscle-bound than the skyscrapers in which Americans work today but also more beautiful, environmentally friendly, and cheap to build.

The basic form of the carbon tower is not especially complex. Imagine, first of all, a cylindrical building 40 stories high. Then picture that cylinder strung together by 40 carbon-fiber strands, about 1 inch wide and nearly 650 feet long, that are arrayed in a helicoidal, or crosshatch, pattern. Filling in the structure between floors is an advanced glass substitute. A pair of ramps on the exterior of the building offers circulation and further stabilizes the structure. That, in simplified form, is the carbon tower. Perhaps the most striking thing about



it is that every major element in the building, including the floors and the exterior ramps, is made of some kind of composite material—there is no steel, concrete (apart from the foundations), or conventional glass. The important point is the structural use of continuous carbon strands, which are woven to form a structure that distributes its loads over its entire surface. (Most contemporary skyscrapers use steel or concrete, or both, in compression.) Taken together, the building’s innovations open up the potential for what Testa calls a new “organic minimalist aesthetic”—a building whose surface and structure are one and the same.

The 24 strands will be fixed into shape by something called a robotic pultrusion machine, which Testa envisions climbing up the structure like a spider and weaving the strands on the side of the tower as it’s built. “You just bring a bundle of fibers and some plastic to the site, and then you manufacture the building right there,” he says. “Each of the strands will have its own machine.” The method would offer obvious energy-saving advantages over traditional construction techniques, but there is another dramatic benefit as well. As Testa envisions it, this 40-story skyscraper won’t need any vertical structural elements: no columns between floors, no central core. Air circulation would be handled through a pair of continuous open cylinders, or “virtual ducts,” that run from the top of the building to the bottom and operate on the kind of displacement ventilation system that is already appearing in some European buildings. Three groups of elevators are distributed throughout the floor plan, instead of being crowded together at the core. The tower aims to reconfigure all three central elements of contemporary skyscraper design: structure, circulation, and heating-and-cooling systems. “It’s time people faced the fact that skyscrapers don’t work well from almost any vantage point,” Testa says. “They don’t work well from the standpoint of the organization of the workplace, urbanistically, or environmentally.”

Testa is adamant that his tower can be built within five to ten years. In his push toward real-world feasibility, he’s relying heavily on an association with his two institutional collaborators. Arup is a giant firm whose status in the design world is evident by the fact that it is working with four of the six teams of finalists on the redesign of Ground Zero. CTEK, which Testa describes as a “heterogeneous workshop,” is best known for building a custom futuristic Lexus for Steven Spielberg’s film Minority Report and the curving glass panels of Frank Gehry’s Condé Nast cafeteria, in New York.

At Arup, Testa’s carbon tower has caught the eye of structural engineer Markus Schulte. In addition to his work for clients including Richard Meier and David Childs, Schulte spends about one day a week, as he puts it, “trying to be a facilitator in making architectural dreams a reality.”

Schulte says he’s impressed with the prospects for the carbon tower so far. Tests run at Arup have suggested, among other findings, that the building would use about 50 percent less energy for heating and cooling than most skyscrapers of its size. And Schulte is impatient with any suggestion that this is a pie-in-the-sky project. “I don’t like the idea of it as a sort of experiment that Peter works on for a while and then puts on his shelf,” he says. “For me as an engineer that’s not good enough.” He is connected to the project, he stresses, primarily because he thinks it is buildable.

Even if the tower can’t be built, or if it evolves into some less radical form, it holds the potential to be an important step in twenty-first-century architecture. Testa, who for a while held a rather inflexible attitude about the tower’s development, has a new way of thinking after his experience in Cambridge. First of all, he’s realized that he may have to make changes in the design even if they’re not necessary structurally. “I’ve now come to feel, okay, we’ll change the design, we’ll build some redundancies in. We’ll put some stairs on the inside, even if I don’t think we need them, to help people’s psychological well-being. There’s no reason to be absolutist about it.”

Conclusion

A lot of things would have to happen for even a single example of the tower to be built in the next decade. The biggest hurdle is the cost of carbon fiber or other super fibers and its resin coating, which continues to be prohibitively high compared to steel or concrete. Then there’s the issue of building codes for skyscrapers, which are tightening around the country after the World Trade Center collapse. There’s also the question of whether using carbon fiber/super fibers at this scale will be safe for the people who use the building every day. (In the past materials that were heralded as breakthroughs—like asbestos—have sometimes turned out to be dangerous to our health.) And even if all of those barriers are somehow cleared, there is need to find a risk-taking first client. The question is how these materials will behave in the long run at this scale, and there one has every reason to worry. Even a force as seemingly innocent as sunshine can have a deleterious effect on some of the plastics that are used in composites and cause them to wear down over time.

Reference

- http://www.metropolismag.com/story/20030201/carbon-fiber-future

- http://www.popularmechanics.com/technology/engineering/architecture/4343115

- http://spectrum.ieee.org/energy/environment/how-to-build-a-milehigh-skyscraper

- http://www.iaarc.org/publications/fulltext/S25-5.pdf


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