Impact of high temperature on PFA concrete

After being subjected to high temperatures, the residual properties of pulverized fly ash (PFA) concrete have been investigated. Both mechanical and durability properties of concrete were tested on concretes made with different water to binder ratios and PFA contents. Microscopic techniques were then employed and the pore structure and microhardness values of hardened cement paste (hcp) were determined. The results of rapid chloride diffusion tests revealed that concrete durability deterioration commences after exposure to temperatures which are lower than those at which compressive strength deterioration commences. The rise in compressive strength, which occurs after exposure to 250°C, may be largely due to the hardening of cement paste caused by drying and the further hydration of cementitious materials. The simultaneous loss of durability, however, can be explained by a weakened transition zone between hcp and aggregate, and by the concurrent coarsening of the hcp pore structure. When PFA is included, an improvement of fire resistance as characterized by the residual compressive strength was observed, and this relative improvement over non-PFA concrete was the most pronounced for maximum exposure temperatures of 450°C and 650°C.