Effect of Fly-ash and Silica Fume on the Properties of Concrete Exposed to High Temperature

Abstract

The development of high temperatures in concrete could cause a number of effects that have been shown to be determining to long-term concrete performance. High temperatures in concrete increase the rate of hydration, thermal stresses, the tendency for drying shrinkage cracking, permeability, and decrease of long-term concrete strengths and durability as a result of cracking. The objective of this study is to investigate the effects of using fly-ash and silica fume on the strength and absorption of cement concrete and pozzolanic cement concrete in hot condition. The cement concrete and pozzolanic cement concrete specimens containing fly-ash or silica fume were first cured in water for a periods of 1, 3, 7, 28, 90 and 180 days then exposed to high temperature 45°C ( maximum summer temperature in Egypt and Arabian Gulf countries) in oven for 2,8,12 and 24 hours. The effect of fly-ash and silica fume contents on the absorption capacity of concrete specimens was evaluated by measuring 48-hours absorption and volume of permeable voids. The specimens were also tested for compressive strength and moisture content. The results show an increase in the compressive strength of cement concrete and pozzolanic cement concrete with increasing both curing and exposing temperature time. The absorption of theses concretes decreased with time of curing and increased with exposing temperature time The absorption of both fly-ash and silica fume cement concrete specimens decreased significantly at early stages (3-7days) of curing. While the absorption of cement concrete specimens were effected significantly by exposing temperature period, no significant change in the absorption of pozzolanic cement concretes due to increasing exposing temperature period was observed, especially after 7 days of curing. The volume of permeable voids also decreased with curing. The highest reduction in the absorption and volume of voids was observed in the fly-ash concrete specimens.