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Strength development of concrete containing coal fly ash under different curing temperature

conditions Mohammed A. Elsageer1, Steve G. Millard1 and Stephanie J. Barnett1 1University of Liverpool, Department of Engineering, Brodie Tower, Brownlow Street, Liverpool, UK, L69 3GQ KEYWORDS: Temperature, Compressive strength, Fly ash Abstract Concrete containing fly ash (FA) has been investigated in order to determine the effect of temperature curing conditions on the early age strength. Trial concrete using Portland cement only and concrete containing FA with cement replacement levels of 15%, 30% and 45% and varying water/binder ratios have been investigated under standard 20 °C curing. For a given water/binder ratio, the 3, 7 and 28 day strength was observed to be lower when using fly ash to replace cement. A concrete with target mean strength of 70 N/mm2 was then designed using Portland cement and FA concrete, with different water/binder ratios required to achieve 28day target mean strength of 70 N/mm2. The strength development of Portland cement and FA concrete with the target mean strength of 70 N/mm2 at 28 days has been investigated under isothermal (10 °C, 30 °C, 40 °C, 50 °C) curing regimes and compared to the strength development using standard curing conditions. At 10 °C and 20 °C, the strength development of FA concrete with target 28-day strength of 70 N/mm2 was found to be equivalent to that of Portland cement concrete. At an elevated curing temperature all concrete samples were observed to gain strength more rapidly than at 20 °C and had higher 32-day strength with increasing levels of FA. However, the longer term strength is detrimentally affected by the higher curing temperatures, with Portland cement concrete being more detrimentally affected than FA concrete.

2009 World of Coal Ash (WOCA) Conference - May 4-7, 2009 in Lexington, KY, USAhttp://www.flyash.info/

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Materials Throughout this study, single batches of Portland cement (PC) and fly ash were used. PC was provided by Castle Cement Ltd and FA was provided by Hargreaves Coal Combustion Products Ltd. Chemical analyses of these materials are given in Table 1. The fly ash used was low-calcium fly ash. The coarse aggregate was crushed granite graded 20-5 mm. The fine aggregate used was 0-4 mm irregular to round sand, which had 66% passing through a 600 µm sieve. All aggregates were oven dried before use and allowance was made for water absorption when calculating batch weights for mixing. The superplasticiser used was polycarboxylate polymer (Fosroc Structuro 11180). Table 1 Chemical analyses of materials Component Portland cement

% (by mass) Fly ash

% (by mass) CaO 63.4 1 - 5 SiO2 20.6 45 - 51 Al2O3 5.5 27 - 32 Fe2O3 2.5 7 - 11 SO3 2.8 0.3 - 1.3 MgO 2.6 1 - 4 K2O 0.7 1 - 5 Na2O 0.2 0.8 - 1.7 Loss on ignition 1.8

Mix design methods The normal BRE method 8 was used to design concrete with w/b ratio of 0.4 and above to obtain normal strength concrete. The ratio free water/ (cement+k*fly ash) as defined by the method was used to design FA concrete. The cementing efficiency factor, k was proposed by Smith 9 to give the same strength as PC concrete of similar workability (the workability required was 60-180 mm). In this study, k was taken as 0.3. For w/b ratio of 0.4 and below, where the BRE method produced concrete mix proportions with very high cement content, the modified maximum density theory method (MMDT) 10 was used. The fine:coarse aggregate ratio giving minimum void volume was determined in the same manner as used in the usual maximum density method. In the modified method, the fine:coarse aggregate ratio was set slightly lower than that required to give the minimum void content and enough binder and water was added to fill the void volume and give a slight excess of binder and water (defined as a percentage overfill). A polycarboxylate polymer based high range water reducing admixture (Fosroc Structuro 11180) was used to maintain workability in these mixtures. A total of 51 trial concrete mixes were produced to determine the strength development under standard curing conditions of concrete with PC and also with different levels of 15%, 30% and 45% FA (as a percentage of the total binder content).

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A mix volume of 0.012 m3 was prepared for each concrete according to BS1881-125:1986 11. After the concrete was mixed, the concrete was cast into steel 100 mm cube moulds and then compacted on a vibrating table. The specimens were covered with damp hessian and plastic sheeting. After 24 hours, the specimens were demoulded and cured under water at 20 °C. The compressive strength of three replicate specimens was tested at ages of 3, 7 and 28 days. To investigate the strength development of concrete cured under 10 °C, 30 °C, 40 °C, 50 °C and standard (20 °C) isothermal curing conditions, mix proportions for concrete with a target mean strength of 70 N/mm2 at 28 days were obtained from the results from the above work. The mix proportions for PC concrete and concrete with 15%, 30% and 45% cement replaced by FA are shown in Table 2. These mix proportions were obtained using the Modified Maximum Density Method. The mixing was done according to BS1881-125:1986 11 in a 0.1 m3 capacity pan mixer. Table 2 Mixture proportions for grade 70 concretes % Fly ash

Cement (kg/m3)

Fly ash (kg/m3)

Granite (kg/m3)

Sand (kg/m3)

Free water (kg/m3)

SPA (%)

Free W/B

0 316 0 1426 612 145 0.20 0.46 15 284 50 1426 612 136 0.25 0.41 30 243 104 1426 612 123 0.31 0.36 45 202 165 1426 612 110 0.37 0.30

The concrete was cast into 100 mm cubes for compressive strength testing. These cubes were cured under 10 °C, 30 °C, 40 °C, 50 °C and standard curing conditions and were tested at ages of 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 and 256 days. Strength development of the trial concrete mixes under standard (20 °C) curing conditions The strength development under standard curing conditions of the trial mixes is shown in Figure 2; the 3 and 28 day strengths are shown as function of water/ binder ratio. In all mixes, the strength development depends on the water / binder ratio. The 3 and 28 days strength were highly dependent on the level of FA in the mix. The strength using the same water/binder ratio was lower for the concretes containing higher levels of FA.

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Figure 2. Trial mixes at 20°C – 3 and 28-day compressive strength vs. water / binder

Strength development under standard (20 °C), 10 °C, 30 °C, 40 °C and 50 °C curing conditions The strength development under standard (20 °C) curing condition for PC and FA concrete is shown in Figure 3. At the standard 20 °C curing temperature, the early strength of this particular FA concrete was not significantly affected by the standard curing condition as indicated by earlier studies 3, 4, which reported a lower strength gain of FA concrete at early ages. From an age of 32 days, the strength of FA concretes continued to develop and was higher as the level of FA increased. This finding is consistent with that of the earlier studies 3, 4. The strength development of PC and FA concrete at 10 °C, 30 °C, 40 °C and 50 °C isothermal curing temperatures is shown in Figure 4. At an early age, the strength development of PC and FA concretes at higher curing temperatures is greater than at lower curing temperatures. This is attributed to an increase in the hydration reaction rate. However at a later age, the strength achieved at higher curing temperatures was reduced. The later age strength of Portland cement concrete was much more detrimentally affected by higher curing temperatures than that of fly ash

water / binder

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.900

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50

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70

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110PC 15% FA30% FA45% FABRE Mix Design

a) 28-day strength

b) 3-day strength

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concrete. This is the so-called "crossover effect", where concrete cured at higher temperatures initially has higher strength but later has lower strength than concrete cured at lower temperatures. It is believed to be due to the reaction products not having time to become uniformly distributed within the pores of the hardening paste. In addition, shells made up of low permeability hydration products build up around the cement grains. The non-uniform distribution of hydration products leads to larger pores that reduce strength 12. As shown in Figure 5, at curing temperatures of 10 °C and 30 °C, the strength development of FA concretes is more or less equivalent to that of PC concrete up to the age of 32 days. From this age onwards the strength of the FA concretes continues to develop due to the pozzolanic reaction. The FA reacts slowly with the lime produced by reaction of the cement to produce cementitious hydrates, providing additional strength gain for up to three to ten years 13, 14. At curing temperatures of 40 °C and 50 °C, the FA concretes have strength equivalent to PC concrete at early ages. After 4 days at 50 °C, the 30% FA and 45% FA concretes achieved 86% and 97% respectively of their 32-day strength at 20 °C, whereas the equivalent figure for PC concrete was 73%. At later ages, the strength is greater as the level of FA increases, since the Portland cement concrete is more detrimentally affected by the high curing temperatures.

Figure 3. Strength development of PC and FA concrete, under standard (20 °C) curing condition

Age (Days)

0.5 1 2 4 8 16 32 64 128

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Conclusions

• The strength development of this particular FA concrete was observed to be similar to that of an equivalent Portland cement concrete at standard curing temperature (20 °C) up to 32 days. From this age onwards the strength continues to develop and is higher as the level of FA increases.

• At 40 °C and 50 °C, the strength development of FA concretes is similar to that of an equivalent Portland cement concrete at early ages. At later ages the strength development is dependent on the level of FA and is higher as the level of FA increases.

• At 10 °C, FA and Portland cement concretes gain strength more slowly than at 20 °C and the strength of FA concrete is approximately equivalent to that of Portland cement concrete.

• The crossover effect is observed earlier as the level of FA decreases and the curing temperature increases. 

• This work indicates that FA concrete could be used in projects when early age strength is required without having a detrimental effect on the early or later age strength development. Its early age strength was found to be equivalent to that of Portland cement concrete. The later age strength in a structural element, where temperatures are likely to exceed standard 20 °C curing temperatures, may be significantly higher than the target mean strength of the concrete when FA is used. In contrast to slag cement15, which is detrimentally affected by cold temperatures, FA concrete showed the same strength development at 10 °C as Portland cement concrete and could potentially be used at significant levels even in colder conditions without causing delays to construction schedules. This applies for this particular FA and that it may be different for FA from another source. The effect of temperature on this particular FA seems to be the same as that for PC at early ages– irrespective of whether the temperature is higher than or lower than normal curing temperature.

Acknowledgements The principal author would like to express gratefulness for the PhD scholarship sponsored by Libyan government and Dr Marios Soutsos for his advice throughout the research. The authors would also like to thank Mr Paul Farrell for his assistance with the laboratory work.

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References [1] Balendran, R.V., and Pang, H. W., Strength development deformation properties and mix design of Pulverised fuel ash concrete. Structural Survey, 1995, Vol. 13, No.1, pp.7-11. [2] United Kingdom Quality Ash Association. Pulverised Fuel Ash (PFA) for concrete, Technical Datasheet 1, http://www.ukqaa.org.uk/Datasheets_PDF/Datasheet_1-0_Nov_2006.pdf. [3] Sear, L.K.A, The Properties and Use of Coal Fly Ash, text book, 2001. [4] VARGAS José A, A designer's view of fly ash concrete: A summary of benefits and precautions, Concrete international, 2007, Vol. 29, No.2, pp. 43-46 [5] Dhir, R.K, “Pulverised-fuel ash", in Swamy, R.N. (Eds), Concrete Technology and Design, Cement Replacement Materials, Surrey University Press, London, 1986, Vol. 33, pp.197-255. [6] Soutsos, M.N, Barnett, S.J., Bungey, J.H. and Millard, S.G., fast track construction with high strength concrete mixes containing ground granulated blast furnace slag 7th ACI international symposium on the utilization of high strength/high performance concrete, ACI SP-228,Vol.1. pp. 255-270. [7] Centre for advanced cement based materials www.acbm.northwestern.edu [8] Building research establishment. Design of normal concrete mixes. 2nd edition [9] Smith I A. The design of fly-ash concretes. Journal Institution of Civil Engineers, 1967, Vol.36. pp.769-790. [10] Soutsos M.N, Mix design, workability, adiabatic temperature and strength development of high strength concrete. PhD thesis, University college of London, 1992 [11] British Standard Institution. Testing concrete. Methods for mixing and sampling fresh concrete in the laboratory. BS1881-125:1986. [12] Malhotra, V. M., Nicholas J. Carino, Handbook on Nondestructive Testing of Concrete, text book, 2003. [13] Hansen, T.C., “Long-term strength of high fly ash concretes", Cement and Concrete Research, 1990, Vol. 20, pp.193-6. [14] Thomas, M.D.A., "An investigation of conventional ordinary Portland cement and PFA concretes in 10 year old concrete bridges", Proceedings of the Institution of Civil Engineers Part 1, 1989, Vol. 86, pp.1111-28.

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[15] Barnett, S.J., Soutsos, M.N., Millard, S.G., and Bungey, J.H., Strength development of mortars containing ground granulated blast-furnace slag: effect of curing temperature and determination of apparent activation energies, Cement and Concrete Research 36 (3) (2006), pp. 434-440. [16] Barnett, S., Soutsos, M., Bungey, J., and Millard. S., the effect of ground granulated blast furnace slag on the strength development and adiabatic temperature rise of concrete mixes, proceeding global construction: ultimate concrete opportunities, Dundee, event 1: cement combinations for durable concrete, July 2005, pp.165-172. [17] Mani, A. C., Tam, C., and Lee, S. L., Influence of high early temperatures on properties of PFA Concrete. Cement and concrete composites 1990, Vol. 12, pp. 109-115. [18] Ganesh Babu, K., Siva Nageswara Rao, G., early strength behaviour of fly ash concretes. Cement and concrete research, 1994, Vol. 24, No.2, pp. 277-284. [19] Eren, O., Strength development of concretes with ordinary Portland cement, slag or fly ash cured at different temperatures. Materials and structures, November 2002, Vol. 35, pp. 536-540.