Microstructural examination of concrete exposed to elevated temperature by using plane polarized transmitted light method

In this study, the influence of elevated temperature on the residual compressive strength and ultrasonic wave velocity of the concrete specimens has been investigated. In addition, the effect of elevated temperature on the aggregate–cement paste interface and aggregate microstructure of the concrete specimens was also has been examined by plane polarized transmitted light (PPTL) method. Investigation was carried out by using one concrete mixture which was produced by normal Portland cement (PC), quartzitic natural sand and crushed basaltic coarse aggregate. The water–cement (w/c) ratio used in the mixtures was 0.50. The produced concrete specimens were exposed to 200, 400, 600, 800 and 1000 °C. Test results indicated that the residual compressive strength and ultrasonic wave velocity values of the concrete specimens decreased depending on increase in elevated temperature. In addition PPTL examinations showed that, increasing temperature caused weakening of the adherence of aggregate and cement matrix. Film layers and discontinuities observed in the aggregate–cement matrix depend on increasing temperature. Fast cooling (FC) method resulted in strength losses when compared to slow cooling (SC) method.