Microstructural Changes in Concrete: Postfire Scenario

The thermal and mechanical properties of coarse aggregates, cement mortar, and cement concrete specimens of varying mix proportions were studied after exposure to a range of high temperatures. For the tests, 70.6-mm mortar cubes, concrete cubes of 100 and 150 mm, and concrete cylinders of 100 and 150 mm diameters and 200 and 300 mm heights, respectively, were cast. The water to binder ratio was varied from 0.35 to 0.5. These specimens were exposed to a wide range of temperatures varying from room temperature ($24°\mathrm{C}\pm 2°\mathrm{C}$) to 1,000°C and tested to understand the degradation in strength and spalling behavior of normal-strength concrete (NSC) and high-strength concrete (HSC). The crack initiation and propagation was studied using micrographs obtained from scanning electron microscopy (SEM). Changes in the porosity of coarse aggregates and concrete at elevated temperatures is quantified by incorporating digital image processing on the SEM micrographs. The influence of high temperature on the physical and chemical properties of concrete and concrete constituents at the macroscale and microscale are discussed. Simple empirical relationships to compute residual compressive strength of NSC and HSC at elevated temperatures are proposed. It is concluded from the results that the degradation in the concrete strength at high temperatures is primarily a result of nonuniform thermal expansion of concrete and its individual constituents leading to porosity changes in the cement–sand matrix, coarse aggregates, and interface between the two.