The Effect of Simulated Large Pour Curing Conditions on the Temperature Rise and Strength Growth of PFA Containing Concr
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THE EFFECT OF SIMULATED LARGE POUR CURING CONDITIONS ON THE TEMPERATURE RISE AND STRENGTH GROWTH OF PFA CONTAINING CONCRETE M.J. COOLE and A.M. HARRISSON Blue Circle Industries PLC, Group Research, 305 London Road, Greenhithe, Kent, United Kingdom Received 20 October,
1986; refereed
ABSTRACT When concrete is poured in large volumes, it is necessary to be able to predict the temperature rise which may occur inside the mass because of the effect this may have on the ultimate properties of the hardened concrete. It is known that the elevated temperatures generated may have a detrimental effect on final strengths and that if the difference in temperature between 0 the centre and the surroundings exceeds 20-25 C, cracking may occur. In order to study these effects, a calorimetric controlled apparatus has been designed that is able to simulate the temperature rise profile occurring within any size of concrete pour. The apparatus is also used to control a curing bath thus enabling the compressive strength of match cured concrete to be determined. Results have been obtained for both temperature rise and strength growth at the centre of simulated 0.8, 1.5 and 3 m deep pours, using plain Portland and Portland PFA cement concrete. These show that in the larger sized pours the strength of concrete from a Portland PFA cement blend grows, after 2-3 days, at a greater rate than that of pure Portland cement, while giving lower temperature rises. Comparative strengths at 28 days are 48 Nmm- 2 for the PFA cement concrete and 38 Nmm- 2 for the pure Portland cement concrete. The influence of temperature on the reactivity of the PFA under these conditions has been studied using a dilute acid dissolution method. The hydrates formed and the progress of the pozzolanic reaction within the actual concretes has been monitored using scanning electron microscopy. INTRODUCTION The hydration of cement is an exothermic reaction which, in the case of large pours, can cause a considerable temperature rise in the concrete placed. When concrete is poured in a large quantity, it is necessary to be able to predict the temperature rise which may occur inside the mass because of the effect this may have on the ultimate properties of the hardened concrete. It is known that elevated temperatures may have a detrimental effect on the strength growth pattern of the concrete and that if the difference in temperature between the centre and surroundings of a pour exceeds 20 to 250 C cracking may occur [1,2]. Whether cracking occurs depends both on the magnitude of the temperature induced stress and the capacity of the concrete to accommodate the strain. The need to be able to predict these temperature rises is greater with composite cements, containing PFA or blast furnace slag, where the use of these materials introduces an additional uncertainty as to the effect of increased temperature on the development of strength with time. In the past, various methods have been employed to try to ascertain these effects, such as the adiabatic calorimeter and the vacuum
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