sustainable concrete for the 35 storey karle zenith towers in bangalore

SUSTAINABLE CONCRETE FOR THE 35 STOREY KARLE ZENITH TOWERS IN BANGALORE

S A Reddi1, Gurumurthy D M2

1. Value Engineering Consultant, India

2. QA Engineer Karle Group, India

ABSTRACT. Three High rise residential towers are under construction near Hebbal in Bangalore. The developer was keen on application of Value Engineering for the Project. As part of the Value Engineering proposal, the first author had proposed substantial saving in cement and an economical concrete mix with cementitious material content limited to the minimum specified in IS 456 -2000. Accordingly a series of trial mixes were conducted by the contractor under the advice of the authors. Use of minimum cementitious material for M40 concrete mix was specified in the tender as 350kg/m3 of concrete. A series of trial mixes were conducted at the project site to determine a choice of mix satisfying workability of concrete strength and conformity to IS 456-2000. Conventionally, M40 mix is being realized by various agencies in India mainly with cement content of about 420kg/cm3 of concrete. Psychologically the concerned execution agencies as well as independent test laboratories in Bangalore (and the rest of India) were not tuned to the idea of reducing cementitious material content. As such a large number of trials had to be conducted to convince all concern about the practicability of using 350kg cementitious material (cement + fly ash/GGBS). About 150 Trials were conducted before choice of the mix. Refinements in trial mixes included calibration of batching plant, temperature control curing tank, moisture probe for sand, laboratory pan mixer etc. These were introduced progressively, accounting for the increased number of trial mixes. The chosen mix was validated by a reputed recognized third party laboratory before implementation in actual concreting. The paper describes in detailed the process of arriving at the most economical mix. The results of work tests using the chosen mix over 28000m3 till November 2012 are also analyzed and found to be in conformity with the requirements of IS 456. Keywords: Durability, Sustainability, Cement, Material selection, lab maintenance

S A Reddi is a consultant at Value Engineering, Bangalore, India. He has been actively involved in the formulation of Codes and Specifications as a member of the committees of Bureau of Indian Standards, Indian Roads Congress, fib, and International Association of Bridge and Structural Engineering, Zurich. Gurumurthy D M is with QA Engineer Karle Group, India

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INTRODUCTION Concrete mix proportioning is being done using a number of published methods. The author has found trial mix method yielding satisfactory results. In the case of Zenith Towers, the tender document has included requirement of mix proportioning for use of minimum cementitious material content of 350kg/m3 of concrete. As per IS 456, for Bangalore environment, the minimum specified cementitious material content is 320kg/m3 concrete (severe environment). The successful contractor shall proportion the mix accordingly. Based on trial mixes, any variation in cementitious material content in the approved mix is payable in addition. After a number of trial mixes, it was possible to get an approved mix with the specified cementitious material content. The paper details the procedure followed and improvements practiced which enabled a mix with minimum cementitious material content. The approved mix contained 260kg cement plus 90kg fly ash or 175kg cement plus 175kg ground granulated blast furnace slag (GGBS). Due to uncertainty in obtaining regular supply of GGBS, the developer opted for use of fly ash. More than 32000m3 of M-40 concrete has successfully been carried out for raft, beams, columns, retaining walls, slabs etc. till December 2012.

GRADE OF CONCRETE The design consultant had earlier specified different grades of concrete (M25, 30 and 40) for slabs, beams and columns. They were persuaded to adopt M40 grade for all components to simplify operations, assured that cementitious material content will be minimum specified in the code. The developer was assured that cement consumption will be equivalent to that of the lowest grade of concrete (M20) permitted in IS 456:2000. Thus only one grade of concrete (M40) is being used for the project. This decision confirms to Cl. 10.2 Batching, of IS 456: “To avoid confusion and error in batching, consideration should be given to using the smallest practical number of different concrete mixes on any site or in any one plant”. Number of advantages arises out of single grade of concrete at a project site: • Saving in cost and time of proportioning different mixes • Problem of pouring concrete at interface between columns, beams and slabs • No confusion in batch plant settings • Error-free batch plant operation

CONCRETE MIX PROPORTIONING IS 10262 – Indian Standard Concrete Mix Proportioning - Guidelines (First Revision) was published in 2009. In the foreword: “The objective of proportioning concrete mixes is to arrive at the most economical and practical combinations of different ingredients to produce concrete that will satisfy the performance requirements under specified conditions of use. An integral part of concrete mix proportioning is the preparation of trial mixes and effect adjustments to such trials to strike a balance between the requirements of placement, that is, workability and strength, concomitantly satisfying the durability requirements”. The title of the Standard has been modified as “Concrete Mix Proportioning- Guidelines" from the earlier title “Recommended Guidelines for concrete mix design”.

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The concrete mix design was carried out by trial mix method, within the framework of IS 10262 –2009, Indian standard concrete mix proportioning – Guidelines ( First revision 2009). Annexure B of the code gives illustrative example of mix proportioning of concrete using fly ash as part replacement of OPC. The example coincidently was for M40.

WORKABILITY OF CONCRETE As all concrete was proposed to be pumped, target workability was assumed as 75-100mm, as per Cl 7.1 of IS 456 in the mix proportioning trials. Initially, there was resistance from the constructor, but due to continuing efforts, pumping concrete has stabilized at slump of 80-100mm at the delivery point. Subsequent trial mixes were successfully realized with 100mm slump. Normal tendency at Indian sites is to have higher slump, with mistaken notion of easy pumpability; actually higher slump leads to segregation, separation of coarse aggregate etc. Field personnel prefer higher slump for ease of placement; in fact, such higher slumps in pumped concrete may have detrimental results including segregation, bleeding, blocking of pipeline etc. Hours are lost to clear blockages; in some cases the concrete sets and the pipe is lost! (Figure 1). Other factors contributing to need for higher slump included truck mixers and pipelines not cleaned after each pour, lack of coordination between batch plant, truck mixer movement and concrete pumping. These were investigated and corrective action taken during quality site visits.

Figure 1 Concrete Pump Pipe Chocked Figure 2 Temperature control curing tank By persistent efforts and training, it was possible to successfully pump concrete with a slump of 80-100mm for subsequent pours. INCREMENTAL IMPROVEMENTS IN QUALITY ASSURANCE Minimize cementitious materials content within the limits of IS 456: 2000 (350kg/m3 concrete for M40); cement content 175 kg/m3 with GGBS and 250-260kg with fly ash). Use manufactured sand in the context of nationwide difficulties in getting river sand. Fix thermostat in the curing tank to maintain water temperature at about 27°C (Figure 2).


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Fit the sand bin with moisture probe (Most of the batch plants in India do not have one) (Figures 3a and b).

Figure 3a Moisture Probe in sand bin Figure 3b Typical Moisture Probe

Daily monitor the weighing tolerances in concrete component by checking the printout. Calculate cumulative monthly standard deviations; any increase triggers warning for corrective action. Every consignment of constituent materials, including cement, is independently tested, compared with BIS requirements. Any change in source of cement, even with same brand, is implemented only after tests Regular Sieve analysis of coarse and fine aggregates. Form removal time based on tests of cubes site cured, not in curing tank.

VALIDATION OF SELECTED MIX The choice of final mix was validated by independent tests at recognized laboratory. The component materials were sent to recognized laboratory along with the accepted proportion of mix. The Recognized laboratory prepared cubes, cured them under standard conditions as per IS and tested the same for 28 days strength and approved the mix. Strength achieved at 28 days was 51.5MPa. Subsequent work cube test results for the same mix varied from 44 to 48 MPa. Actual slump range during works is 80-100mm, with a tolerance of 25mm. Thus the mix used satisfies all the parameters given in IS 456. In the present case the cement content is 260kg, fly ash is 90kg. Proportioning of the concrete mix has been done by trial mix method, considered most reliable. Trial mixes were conducted with varying proportions of natural and manufactured


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sand. Trial mixes have been conducted for cube strength as well as slump of concrete for every trial mix. The following mix was finally approved for use, after extensive trials lasting months. Trials were started as the first activity after establishing the site. The approved mix was ready for use by the time the site was ready for the first pour.

Table 1 Details of the approved mix used

APPROVED MIX

Cement 260kg Fly Ash 90kg Water 154L CA 20mm 684kg 12.5mm 293kg Natural Sand 448kg Crushed San 448kg Admixture 0.60%* * By weight of cementitious material

NEED FOR USING MINERAL ADMIXTURES

Mineral admixtures are used in the concrete mix primarily to reduce the heat of hydration of the fresh concrete and increase the workability of concrete at reduced water content. They also assist in reduction in cost of concrete. IS 456 permits fly ash as per IS 3812 up to 35% or ground granulated blast furnace slag (GGBS) up to 70% or the combination of fly ash and GGBS. As per IS 456 -2000, the definition of cement content includes cement & mineral admixtures. Fly ash shall confirm to IS 3812 and GGBS to BS or EN standards.

PHYSICAL REQUIREMENTS OF FLY ASH AS PER IS 3812 Fineness: Specific surface of 320 m2/kg (minimum) by Blaine’s permeability method. Particles Retained on 45 micron IS sieve (wet sieving): 34% Max Lime Reactivity: Average compressive strength, 4.5 N/mm2 Compressive Strength at 28 days (minimum): Not less than 80% of OPC mortar cubes. These requirements were verified by regular tests in third party laboratory in Bangalore. It would have been preferable to use processed fly ash. However, the only reliable fly ash processing unit is in Nashik, Maharashtra. Logistics rendered the use of Nashik processed fly ash impractical. Fly ash was sourced from JSW from Bellary and Raichur Thermal Power Station. As per the relevant IS Code, the fly ash supplier is required to provide test certificate for each consignment. Thermal power stations are the source of fly ash; they consider fly ash as waste, not interested in supplying test certificates. Fly ash is generally collected by third parties and supplied to users. Thus no test certificates are normally available to the end user in advance. The developer or contractor has arranged for regular tests to verify the quality of fly ash supplied.


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SEPARATE SILOS FOR CEMENT, FLY ASH At any instant of time, there is possibility change over from one brand of cement to another approved brand. Fly ash source also varied. Flexibility in the use of different brands of cement, as well as fly ash from different sources demanded installation of three silos at the batch plant.

SITE LABORATORY

The contractor had established a concrete testing laboratory at site with equipment for following tests, as per contract requirement: • Sieve analysis of aggregates • Flakiness and elongation of coarse aggregates. • Bulk density of aggregates • Specific gravity of aggregates. • Water absorption of aggregates • Wet sieve analysis of river sand and M sand • Cement test • Cube test • Pan mixer • Temperature control for curing tank • Compressive strength testing machine • Fly ash sieve analysis

BENCHMARK FOR SLUMP AND TIME TAKEN FOR ENFORCEMENT

Slump achieved ranged from initial 150mm to 140mm after 60 minutes. Strength achieved at 28 days was 51.5MPa. The approved trial mix by independent laboratory was carried out as per the mix proportion approved by author. The works cube test result for the same mix varied from 44 to 48 MPa. Actual slump during works ranged between 110mm and 125mm. As per IS 456 for pumped concrete, suggested slump range is 75-100mm, with a tolerance of 25mm. Thus the mix used satisfies all the parameters given in IS 456. The contractor started the pours with a slump of around 150mm. Due to persistent efforts of the Value Engineering consultant over a period of months, the slump at pour point was brought down to about 80-120mm, in line with BIS limits in IS 456.

CHOICE OF CHEMICAL ADMIXTURES Trial mixes were conducted using different brands of chemical admixtures - BASF (Glenium B233) ,SIKA- viscocrete, Fosroc- Aura mix, Rheobuild, DON- Supa flow, Grace – adva 950. Based on observed performances, and contractor preference, BASF Glenium B233 PC based


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admixture was selected for trial mixes and eventually for works. Tests were conducted at third party laboratory for compatibility between each brand of chosen cement and the chemical admixture before commencement of trial mixes.

MIXER FOR TRIAL MIXES Tender envisaged laboratory pan mixer for trial mixers which was procured by the contractor after some delay. In the meanwhile, trial mixes were conducted at the contractors regular batch plant at nearby site. After the arrival of laboratory pan mixer at site, all further trial mixes were conducted using the same.

TEMPERATURE CONTROLLED CURING TANK FOR CUBES As Per IS 516: Method of Test for Strength of Concrete, temperature of water in the curing tank shall be maintained at 27°C with a tolerance of plus or minus 2°C. This requirement is rarely followed at Indian sites. In view of intention to use minimum cement, the contractor had installed temperature control system in the curing tank. This has contributed in maintaining uniformity of cube strength results. Mixes were initially proportioned for a standard deviation of 3MPa. After installation of temperature control in the curing tank, the Standard Deviation of works cube strength has come down to about 2MPa. Indicating excellent control.

CONCRETE TECHNOLOGIST PROVIDED BY THE CONTRACTOR Planning for quality concrete begins at the tender stage. Tender documents specified that a full time concrete technologist shall be provided at site by the successful tenderer, highlighted during pre-bid meetings. The contractor has posted a competent engineer at site for the purpose. All trial mixes were conducted by him, witnessed by the developer’s quality engineer and by the Value Engineering consultant during weekly site visits.

BATCHING PLANT SOFTWARE The concrete batch plant at site employs computer aided control; assist in fast, accurate measurement of input ingredients, tie together various parts and accessories for coordinated and safe operation. Concrete performance depends on accurate water measurement, moisture probe used to measure water in sand while being weighed, automatically compensate the water target. Initially the batch plant was received from another project without moisture probe. Due to persistence of the employer, after some delay, a reliable moisture probe was imported and fitted at the inlet of the sand bin. Automated moisture measurement in sand ensured water reduction in each batch to compensate for moisture present in sand in each batch. This also ensured slump control. Why do we need moisture sensors in the sand bins? Because mix proportioning assumes aggregates, cement and water are present in the correct proportions, according to the dry material weights. If sand contains 10% moisture, when you weigh out 1000kg, only 900kg is


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sand; the rest is water. You can allow for this by estimating or measuring the moisture of the sand by conventional method and modify the amount that you weigh in proportion. If your sand moisture decreases by 2% without being noticed, however, the batching system will weigh out 2% more sand than you require and will add appreciably less water than needed, making a dry batch. Moisture content varies through the day. Traditional methods of using frying pan to assess moisture once or twice daily is not good enough. Moisture probe is unique; it can continuously adjust the load water as the material flows over the probe, a method that ensures consistent slump predictability in high specification concrete and concrete product manufacturing. Batch plants should be fitted with moisture probes at the sand feed point. Probe is connected to the computer, which will continuously adjust the mix water accordingly. The aggregate moisture sensor ensures that the batch is proportioned according to the dry weight mix design.

BATCH PLANT CALIBRATION All materials are weighed with high quality load cells for accuracy and ease of calibration. Weighing accuracy of the aggregates is within +/-2% and cement/water to within +/-1% of the batch size. This has been ensured by frequent calibrations. Permitted weighing tolerance applies to each batch of concrete, not averaged out per truck mixer as is being commonly practiced in India. This improvement was made possible by persistent efforts of the employer’s engineers over a period of time. The batch plant supplier had to be called to site. After initial reluctance, the controls were put in place.

TRIAL MIXES FOR M40 GRADE CONCRETE More than 100 trial mixes were conducted at the site laboratory for various purposes such as the selection of materials, selection of concrete mix, suitability of cement, selection of cement brand. Trials with aggregates from different quarries were also undertaken. Trial started with first quarry from –Hosur, mix was finalized with Sonappanahalli quarry – Zone I & II. Trials with five brands of cement were also conducted. The selected mix was intended to be used in all components of the building: raft, retaining wall, shear wall, column, beam & slab.

STANDARD DEVIATION Initially standard deviation was assumed as three based on past experience; used for calculation of target mean strength for proportioning the mix for trials. This is based on the author’s prior experience; IS 456 Table 8 gives a value of five. The assumption was validated by analyzing cube test results of trial mixes initially and work test results cumulatively at monthly intervals. The last cumulative standard deviation has been reported as 1.9 in December 2012. This is considered indicating excellent quality control at site.


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THIRD PARTY TEST REPORT ON CONCRETE MIX PROPORTIONING BY CIVIL AIDS BANGALORE DATED 4 MAY 2012 Two trial mixes were validated in the third party laboratory as per the employers requirement in Bangalore with cement content of 260kg and 245kg, fly ash of 90kg and 105kg to make a total of 350kg cementitious material, water 154 litre, admixture 0.7% and other components as per approved mix proportions. Their observations:

Table 2 Compressive strength and slump of two trial mixes

SLUMP Initial 150mm 170mm

After 30min 140mm 165mm After 60min 140mm 110mm

COMPRESSIVE STRENGTH 3 day 25.9MPa 24.4 mPa 7 day 37.6MPa 34 mPa 28 day 51.5MPa 53 mPa

Special Remarks By the third party laboratory: "Trial mix was carried out as per the mix proportions Suggested in letter dated 3.4.2012 and using materials as supplied by CCCL. The concrete making materials such as cement, fly ash, fine & coarse aggregates, admixture and water should conform to relevant Indian Standards. Correction for water absorption in crushed sand aggregates and coarse aggregates to be applied at site. Fine aggregate of natural river sand & crushed stone sand shall be used in the ratio of 50:50. Coarse aggregate of 20mm and 12.5mm shall be used in the ratio of 70:30 repective1y." "Performance of admixture shall be checked and verified periodically at site." "Trial mix proportions considered above shall be modified in the field using the proposed materials and finalized by observing the actual performance of the concrete mix. Suitable modifications may be necessary to suit field conditions in the light of experience as for the particular materials used in each case."

INDUSTRY PERCEPTION ON THE PROPOSED MIX The developers and contractors in Bangalore and the rest of India are used to much higher cement contents for M40 mix, in the range of 400 to 450kg per m3 of concrete. To the best of the authors’ knowledge this is the first time in India that such low cementitious material content is used successfully. Even the proof testing laboratory specialists were surprised!


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PRODUCTION OF CONCRETE: QUALITY ASSURANCE MEASURES Quality assurance in construction relates to proper design, use of adequate materials and components to be supplied the producers, proper workmanship in execution of the works by the contractor.

OTHER QUALITY ASSURANCE MEASURES Modern concrete batch plants employ computer aided control; assist in fast, accurate measurement of input ingredients, tie together various parts and accessories for coordinated and safe operation. Concrete performance depends on accurate water measurement, use moisture probes to measure water in sand while being weighed, and automatically compensate the water target.

TRUCK MIXERS Six cubic meter capacity truck mixers were deployed for the project. Except for some major pours, four m3 units would have been more appropriate from quality assurance point of view, but rarely employed in India. Mixer drivers were prohibited from adding water en route. Truck mixers were made to come under the plant with their drums in discharge mode to ensure that there is no water in the drum prior to loading concrete. Occasionally there was no coordination between the batch plant production, number of truck mixers employed, and the deployment of concrete pump. This has resulted in delay between mixing and placing concrete beyond the permitted gap as per the code. Attempts are made to correct the situation.

WEEKLY QA VISITS During the first months after commencement of concreting, the authors conducted weekly QA visits to project sites, verified quality of concrete, reinforcement, formwork, false work etc. and enforced compliance until stabilized. Site visits were preceded by review of compliance of the previous weeks observations and followed by preparing minutes of the current visit.

Figure 4 (a) Raft top surface (b) Slab underside finish (c) Column finish


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Figure 5 Front Elevations of Three Towers

Credits

Developer: Karle Group, Bangalore

In House QA: Mr Gurumurthy DM, Quality Manager

Design Consultant: BURO Singapore & CRN Chennai

Value Engineering Consultant: SA Reddi, Fellow, Indian National Academy of Eng.


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