Effects of high temperatures on PFA concrete

Abstract

The effects of high temperatures on pulverized fly ash (PFA) concrete were investigated by studying the residual mechanical properties (compressive strength and tensile strength) and the durability properties (resistance to chloride penetration). The observations were then explained by microscopic examinations: microhardness of hardened cement paste (hcp) in the interfacial transition zone (ITZ), porosity of cement pastes, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Parameters under investigation included PFA replacement level (0, 25% and 55% by weight of total binders), water to binder ratio (0.3 and 0.5), and curing regime (cured in water or air for 35 days to 180 days). The peak exposure temperatures ranged from 250 C to 800 C. The research results showed that the loss of compressive strength could only be observed after concretes were exposed to 450 C. The degradation of compressive strength became the predominant menace after concretes were exposed to temperatures higher than 650 C. It was found that, after they were exposed to 450oC, PFA concretes could basically maintain the compressive strengths at room temperature, and the best resu1ts were observed in high PFA content concretes. Reduction of tensile strength was negligible after concretes were exposed to 250 C, and the rate of reduction increased gradually with the rise in exposure temperature. When the PFA dosage was 25% by weight of total binder, the relative residual compressive strength and the relative residual tensile strength were improved by about 5% when the exposure temperatures were not higher than 650 C. When the PFA dosage was 55%, the improvement could be about 10%. However, severe deterioration of durability was observed after concretes were exposed to 250 C, which turned out to be the main concern for concrete at such an exposure temperature. The total charges passed in concretes increased almost linearly as the maximum exposure temperatures increased from 250 C to 650 C. Unfortunately, PFA concretes showed greater rate of degradation than OPC concretes with respect to resistance to chloride penetration. The worst results were observed in concretes made with a water to binder ratio of 0.3 and with a PFA dosage of 25%. The changes of the above-mentioned macro properties were then co-related with the microscopy investigation results and crack observations. It was proposed that the improvement of mechanical properties after exposure to 250 C could be attributed to the hardening of cement paste. The simultaneous loss of durability could be explained by the weakening of the transition zone between hcp and aggregates, concurrent with the coarsening of pore structure in hcp. For higher exposure temperatures, The detrimental changes such as the decomposition of the hydrates in hep and cracking played predominant roles. The beneficial effect of PFA on residual mechanical properties, which was evident at the exposure temperatures from 450 to 650 C, was confirmed by microscopic investigations. The cracking of PFA concrete, which could be greatly influenced by PFA contents and post-exposure treatments, was found to be an important damage factor controlling the residual properties of concrete after high temperature exposure. The better residual mechanical properties of PFA concretes could be well related to the reduction of crack when PFA was introduced into the concrete mixes. Lastly, different measures, included adding PFA, optimization of the grading of aggregates, modification of cooling method, and various post-cooling curing regimes, to reduce the damage in concrete after high temperature exposures were studied. Adding PFA and decent grading of aggregate were found to be effective methods. To relieve the damage, it was recommended that concrete be splashed with hot water when cooling to room temperature, followed by water curing. Three days of water curing might be enough to help the recovery of concrete in terms of crack healing.