Control Temp in Mass Concrete

CONTROLLING TEMPERATURES IN MASS CONCRETE

Saeed Ahmad*, University Engineering & Technology Taxila, Pakistan Safdar Iqbal, University Engineering & Technology Taxila, Pakistan

Imran A Bukhari, University Engineering & Technology Taxila, Pakistan

34th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 - 18 August 2009, Singapore

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34th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 18 August 2009, Singapore

CONTROLLING TEMPERATURES IN MASS CONCRETE

Saeed Ahmad*, University Engineering & Technology Taxila, Pakistan Safdar Iqbal, University Engineering & Technology Taxila, Pakistan

Imran A Bukhari, University Engineering & Technology Taxila, Pakistan

Abstract

Concrete generates heat as the cementitious material hydrate and for thin sections, heat dissipates almost as quickly as is generated. In case of massive structures, temperature increases due to generation of more heat of hydration therefore management of concrete temperatures is necessary to prevent damages, minimize delays and meet specifications. This study was carried out at Dubai International Airport(DIA) project during expansion phase2. During the study different Trial Mixes were used and their suitability was ascertained through temperature monitoring data. Three Trial Mixes of ratio 1 : 1.40 : 2.86 were made with constant workability and varied percentage of Cement and GGBFS as follows: One Heat Block Cube of plain concrete (3x3x3 m) and one Scaled Model of dimensions 3x3x2.8 m, having actual Rebar of Raft were made for each trial Mix. Temperature monitoring was carried out for both blocks. In addition, some more experimental work was carried out to study the effects of cement and slag combinations on temperature as well as on strength. This work was studied under two cases: Case-1: Effect of increasing the quantity of cementitious contents (cement and slag) on temperature

and strength of mass concrete. Case-2: Effect of GGBFS on temperature and strength of mass concrete.

Results indicated that increasing the quantity of cementitious contents results in increase of the concrete temperature as well as strength. It was also concluded that GGBFS is a useful material to control the temperature in mass concrete although it reduces the 28 days cylinders strength of concrete.

INTRODUCTION

Mass concrete is extensively being used in the construction industry all over the world. A lot of research work has been done on behavior of mass concrete all over the world however there is still a need of further evaluation especially in hot climates. According to ACI 116R,1 any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume changes, to minimize cracking is categorized as mass concrete. Traditionally mass concrete has been associated with dams and other extremely large placements but this is no longer the case. Use of concrete with high cement contents for durability and rapid strength gain demands to treat increasing number of placements as a Mass Concrete. A successful completion of mass concrete pour demands special attention to the logistical and technical considerations like Concrete supply, Casting sequence, Cold joints, Plastic settlement and Heat of Hydration. One of the major benefits of mass concreting is cost effective as it reduces stop ends and speed up construction. Moreover the elimination of potential cracks at construction joints results in a monolithic unit. Continuous casting also takes advantage of the ability of ready-mixed concrete companies to supply concrete at high delivery rates. All concretes generate heat as the cementitious materials hydrate and an exothermic reaction occurs. Most of this heat generation occurs in the first few days after placement. For thin items such as

as pavements, thin slabs and walls, heat dissipates almost as quickly as it is generated. However for thicker concrete sections (mass concrete), heat dissipates more slowly than it is generated and the temperature of the mass concrete increases. Significant Tensile stresses in mass concrete are developed from volume changes associated with Heat of Hydration. If proper measures are not taken, cracking due to thermal behavior may occur in mass concrete which may cause:

1. Loss of structural integrity and monolithic action, 2. Excessive seepage 3. Shortening of the service life of the structure, 4. Esthetically objectionable 5. Worst effect can occur in Dams and other Hydraulic Structures where cracks are more

objectionable. There are various factors which effect Heat of Hydration like cement content, cement type, size of

concrete pour, type of formwork, concrete temperature, thermal coefficeient of aggregates and ambient temperature. Generally higher the cement content, the more will be heat of hydration. To predict concrete temperature, a simplest method (described in a PCA document.3) can be used according to which every 45 kg of cement increase the temperature of concrete 70 C. Using this method, the maximum concrete temperature of a concrete element that contains 365 kg/m3 of concrete (as in case of Raft at a Case Study project) and is cast at 300C, is approximately 860C which is above the safe limit (650C). There are some other methods like Fitz Gibbon method, Schmidts method etc and software has also been developed for more accuracy and precision which can be used for 1, 2, and 3 dimensional analyses to predict maximum concrete temperatures and temperature differences. Similarly there are various methods of controlling temperatures in mass concrete like use of low-heat Materials (replacement of cement with Pozzolanic Materials or Slag), pre-cooling of concrete ingredients, cooling arrangements during pouring, post-cooling of concrete, concrete surface insulation and using aggregates with low thermal expansion-e.g Limestone.

This research work was carried out to evaluate the behavior of mass concrete with respect to temperature effects and as a case study, Phase II of Dubai International Airport was selected.

Overview of Dubai International Airport Expansion project- Phase II:The major structural details about the project are given in Table 1. Terminal Building: The Terminal Building is a totally underground Structure comprising of 6 Levels, and accommodates passengers processes, Air Line and governmental service offices, Food Courts, Duty Free Areas and all special airport systems, baggage handling systems (BHS), flight/baggage information display (FIDS/BIDS). The Roof of the Terminal Building forms a part of the air side Apron. The Terminal Building is connected to the public Levels of Concourse 2. It is also connected to Concourse 3 building via an Automated People Mover (APM). Car Park: The Car Park is totally underground Structure on 3 Levels and connected to Terminal 3 Arrival and Departure levels . Capacity is about 3000 cars. The LRT related facilities are located above the Car Park. This consists of two elliptical structures. The one closer to the Terminal is the Ticketing Hall and the other is the actual station / platform. CONCOURSE 2: The Concourse 2, which is directly linked to Terminal 3, is dedicated to the Emirates Airlines passengers. (26 contact gates of which five for A380 aircraft stands). This Concourse accommodates ground operation and immigration services, Gates /Holding Lounges, Duty Free Area and electromechanical equipments. The Hotel levels are also situated in C2. These consist of 5-star (46 keys) and 4-star (207 keys) rooms along with a business centre and a health club. CONCOURSE 3: Concourse 3 (C3) is an extension and continued development of Concourses 1 (C1) and 2 (C2). C3 is partially an underground structure comprising the APM stations (both arriving and departing) and extends above ground under a metal shell to accommodate 20 aircraft stands, out of which 18 are for A-380. Similar to C2, the lower and upper levels of C3 are connected by means of a special vertical transportation system (sky train) that acts as a focal point feature in a central atrium.

EXPERIMENTAL PROGRAM

Three trial mixes of concrete were prepared with varying ratios of cement and GGBS to study various properties. Following requirements for mass concrete were specified:

1. Use of OPC Type-1 with max heat of Hydration 325 Kj/kg, when tested as per ASTM C186. 2. Use of Ground Granular Blast Furnace Slag(GGBS) which complies with ASTM C989,Grade

100 or 120. 3. Max size of coarse aggregates was 20 mm

4. Water Cement ratio of 0.4 5. High Range water-Reducing Admixture (Super plasticizer) complying with ASTM C494

To be in line with the specification for massive cast in place concrete at Dubai International Airport, following design criteria for the temperature control was established.

1. Concrete Temperature at time of placement should be maintained below 300 C. 2. Concrete Temperature at core of the structural elements shall not exceed 650 C. 3. Temperature Differential within the concrete body shall not exceed 250 C.

Tests were carried out on different materials to confirm their compliance with the specified requirements for Trial Mix Design. For the temperature monitoring, three Heat Blocks of 3m*3m*3 m each was casted. In addition, Casting of prototype section of raft 3m*3m*2.8 m with actual reinforcement was also done for each of the concrete trial mix. Two probes were installed at center of the mass concrete i.e. Center Middle-1 and Center Middle -2 positions to find the accurate value of core temperature and temperature was noted using a data logger as shown in Figure 1.

In addition to the trial mixes, 4xconcrete blocks with 500mmx500mmx500mm were cast at Dubai International Airport Site Concourse 3. Each block was cast using different concrete mix designs. Due to smaller size of the block, only one transducer/ probe was placed at the center of the block and activated just after the completion of pouring of each block. Temperature monitoring was recorded for minimum fourteen days of each block.

RESULTS AND DISCUSSION

Results of Trial Mix-1 indicate that both probes at central location have very consistent results, with a core temperature of 67 degree Celsius while specification allow only 65 degree Celsius. A steep rise observed (especially in core temperature) during the first three days after concrete pouring. Trial mix was rejected due to high core temperature and ingredients of mix were revised. The details are given in Table 2 and temperature monitoring data for the heat block and prototype raft are given in Figures 2 and 3 respectively.

Results of Trial Mix-2 show that both probes at central location have very consistent results, with a core temperature of 63 degree Celsius which is in acceptable limits. Again a steep rise was observed during the first three days after concrete pouring. However, concrete temperature at poring time, always kept less than 30 degree Celsius which is as per the specification requirement. Although core temperature noted (63 degree) is meeting the specified requirement which is 65 degree maximum, however keeping in view the worst weather conditions in Gulf region, high importance of the project ie Dubai International Air port Airport Project and expected improper handling of concrete at site, it was decided that Peak temperature to be further reduced. Since cement contents were already reduced to 30% , another suitable option was selected for the next Trial and that was use of slag (GGBFS) with Reducing Blaine (fineness). The details are given in Table 2 and temperature monitoring data for the heat block and prototype raft are given in Figures 4 and 5 respectively.

In Trial Mix-3, again only two probed were installed at center of the mass concrete i.e. Center Middle-1 and Center Middle-2 positions to find the accurate value of core temperature with higher degree of certainty. Probes at other points were not installed to save time and cost. Results indicate that both probes at central location have very consistent results, with a core temperature of 60 degree Celsius which is within safe acceptable limits. As the Trial Mix no-3 meets the core temperature requirement it was decided that this mix design should be further studied in actual site condition. Hence first Raft with actual size (28m*28m*2.80 m) was poured with Mix Design no-3 and results were recorded. The details are given in Table 2 and temperature monitoring data for the heat block and prototype raft are given in Figures 6 and 7 respectively.

Eight probes were installed to monitor the temperature in the massive structure of an actual size raft. The core temperature peak was observed 58 degree Celsius (Well below specified limit of 65 degree Celsius) as recorded by probes (Center Middle -1 & 2). Core temperature reached at its peak, about 68 hours after completion of casting of Raft and remained at peak around 62 hours (until 5.5 days from completion of casting). Temperature within the mass of raft (excluding reading of corner top and corner bottom) remained within the specified limit of 25 degree Celsius. Core to surface temperature differentials average was less than 10 degree Celsius for the first four days and then gradually increased. A max differential of 21 degree was recorded at approx 08 days after casting. The temperature differentials between the mass of raft and extreme outer corners peaked at 31 degree. However this was not replicated within the body of raft or between core and surfaces and no detrimental effects (e.g. cracking) were observed on removal of insulation. The Temperature Monitoring Data of the raft is shown in Figure 8.

Four concrete blocks (500mmx500mmx500mm) of different concrete mix designated as Mix Design A, B, C and D were cast and temperature was monitored using one transducer/ probe placed at the center of each block for minimum fourteen days. Cement content of the concrete mixes was kept 115, 122,125 and 390 kg/m3. GGBS was also used in varying quantities in three concrete mixes A,B and C whereas it was not used in mix D. The details of the mix designs are given in Table 3.

The compressive strength of the mix D (with no cement replacement) was found higher than the other mix designs as shown in Figure 9. Increase in the peak temperature was also observed with the increase in quantities of cementitious materials as shown in Figure 10.

CONCLUSIONS

1. Replacement of cement with GGBFS is an effective remedial measure to control the HOH in mass concrete at design stage and upto 70% replacement of cement can be made.

2. Rate of exothermic reaction in mass concrete can be controlled by using GGBFS with reduced Blaine(fineness)

3. For temperature monitoring in mass concrete, center point was found critical and gives maximum temperature.

4. Peak temperature in mass concrete can be achieved in first 3 to 4 days of pouring of concrete. 5. Increasing the cementitious material results in increased peak temperature and compressive

strength. 6. GGBFS slows down the strength gaining process of concrete. 7. Test results indicate that mix design no-3 fulfils the specified requirements and hence

recommended for Mass Concrete in raft for any project especially in Gulf region where weather conditions are very hot and humid.

REFERENCES

[1] ACI Committee 116, Cement and Concrete Terminology (ACI 116R-00). [2] ACI Committee 207, Mass Concrete (ACI 207.1R-96). [3] ACI Committee 207, Effect of Restraint, Volume Change and Reinforcement on Cracking of Mass Concrete (ACI 207.2R-95). [4] Portland Cement Association, Design and Control of Concrete Mixtures 13th Edition, Stokie, III., 1988, pp212. [5] The CIRIA Guide to concrete construction in the Gulf Region, Special Publication 31. [6] Springenschmd, R.editor, Prevention of thermal cracking in concrete at early ages London: Spon, 1988(RILEM Report 15). [7] ACI 207.4R-93, Cooling and insulating systems for mass concrete Farmington Hills, Michigan: American Concrete Institute, 1998, p207.4R-7. [8] Campbell-Allen, D and Thorne, C.P The thermal conductivity of concrete, Magazine of Concrete Research, vol.15, No.43. March1963,pp 39-48. [9] Gibbon, G.J and Ballim,Y. Laboratory test procedures to predict the thermal behavior of concrete Journal of SAICE, Vol 38, No 3,1996, pp21-23. [10] Koenders, E.A.B. and Van Breugel, K. Numerical and experimental adiabatic hydration cure determination, Proceedings of the International RILEM Symposium: Thermal Cracking in Concrete at early ages, Munich, 1994, pp 3-10. [11] Taylor, H.F.W Cement Chemistry, 2nd Edition, London: Thomas Telford, 1997, pp212.

KEY ELEMENTS C2-T3 & CP C3

Piles 8700 Nos. 4560 Nos.

Excavation 10 million m3 4 million m3

Diaphragm Walls 57,600 m2 35,000 m2

Concrete 2.4 million m3 1.2 million m3

Reinforcement 450,000 tons 220,000 tons

Structural Steel 29,000 tons Approx. 16,000 tons

Table 1: Major Civil Structural Statistics

Weight Kg/m3 Materials Specified limits

Trial Mix 1 Trial Mix 2 Trial Mix 3

Cement (Kg/m3) ASTM C 150 -Type -1 135 (35%) 110 (30%) 110 (30 %)

GGBS Replacement (Kg/m3) ASTM C 989 -Grade 100/120 245 (65%) 255 (70%) 255 (70%)

Blaine Fineness of GGBS 420 420 380

Total Cementations Content Min = 350 kg/m3 380 365 365

Max Water Cement Ratio Max 0.4 0.36 0.37 0.37

Fine Aggregate:(kg/m3) 585 510 510

Course Agg :(kg/m3)

- 20 mm lime stone 690 690 690

- 10 mm lime stone 345 355 355

Specific Gravity Min=2.6 2.79 2.79 2.79

Admixture (Lit/m3) 6 7 7

Fiber reinforcement 0.6 0.6 0.6

Core Temperature Max=65 C

Table 2: Trial Mix Designs for mass concrete

Figure 1: Thermocouple affixed to steel reinforcement

Pour Temperature ProfileHeat Block-1 (Trial mix-1)

0

10

20

30

40

50

60

70

80

0 25 50 75 100 125 150 175 200 225 250 275 300

Time in Hours

Tem

pera

ture

(C)

cen-top cen-middle-1cen-middle-2 cen-bottomcor-top cor-bottomedge-top edge-middle

Figure 2: Temperature Monitoring Data (Trial Mix 1- Heat Block)

Figure 3: Temperature Monitoring Data (Trial Mix 1- Prototype raft)


Pour Temperature Profile of Prototype Raft (Trial mix-1)

0

10

20

30

40

50

60

70

80

0 25 50 75 100 125 150 175 200 225 250 275 300Time in Hours

Tempe

ratu

re (C

)


0

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20

30

40

50

60

70

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425

Tem

per

atu

re (C

)

Time in Hours

Pour Temperature Profile)2-(Trial MixHeat Block

cen-top cen-middle-1 cen-middle-2cen-bottom cor-top cor-bottomedge-top edge-middle

0

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20

30

40

50

60

70

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425

Tem

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ture

(C)

Time in Hours

Pour Temperature Profile)2-(Trial MixPrototype Raft





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70

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425

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ture

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Time in Hours

Pour Temperature Profile)3-(Trial MixHeat Block


0

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50

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70

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425

Temp

eratur

e (C)

Time in Hours

Pour Temperature ProfilePrototype Raft (Trial Mix-3)

cen-top cen-middle-1cen-middle-2 cen-bottom#REF! cor-topcor-bottom edge-topedge-middle

0

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70

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425

Tem

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(C)

Time in Hours

Pour Temperature ProfileActual Raft -Trial Mix-3

cen-top cen-middle-1cen-middle-2 cen-bottomcor-top cor-bottom

Figure 8: Temperature Monitoring Data (Trial Mix 3- Actual raft)

Mix Design-Weight Kg/m3 Materials

Specified limits A(Block 1) B(Block 2) C(Block 3) D(Block 4)

Cement (Kg/m3) ASTM C 150

-Type -1 115 122 125 390

GGBS Replacement (Kg/m3) ASTM C 989

-Grade 100 245 258 265 0

Blaine ( Fineness) of GGBS) 380 380 380 0

Total Cementitous Content kg/m3 Min = 350 360 380 390 390

Max Water Cement Ratio Max 0.4 0.37 0.37 0.37 0.36

Fine Aggregate:(Kg/m3)

0-5mm crushed sand 744 662 655 675

Dune sand 227 198 195 202

Microsilica(Kg/m3) 7 7 12 12

Course Aggregate:

- 20 mm lime stone 687 721 720 730

- 10 mm lime stone 318 329 325 333

Admixture Reobuild 858 (Lit/m3) 6 5.5 5.5 5.5

Table 3: Mix Design

Figure 9: Compressive Strength gain Figure 10: Peak temperature and Cementatious with time Material

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Control Temp in Mass Concrete