Effects of cement dosage and cooling regimes on the compressive strength of concrete after post-fire-curing from 800 °C

Post-fire-curing is an effective method to recover fire-damaged concrete. In order to investigate the effects of cement dosage and cooling regimes on the recovery of fire-damaged concrete, three kinds of concrete specimens, with different cement dosages of 412, 392, 372 kg/m3, were prepared and tested. After cooling either in a furnace or ambient environment to the room temperature, these specimens were first soaked in water for 24 h and then subjected to 29 days' post-fire-curing. Based on temperature history, residual compressive strength and cracks in the cross sections were tested and analyzed. And scanning electronic microscope (SEM) was used to help the morphology analysis. A new parameter named heat accumulation factor was proposed to estimate the strength loss and the recovery potency of the fire-damaged concrete. It was known from the experiments that the difference of relative residual compressive strength of specimens with three cement dosages is less than 3 percentage points. After post-fire-curing, the relative residual compressive strength of the specimens cooled in the air recovered from 77% to 83%, while that of the specimens cooled in the furnace recovered from 65% to 87%. The longer exposure under high-temperature due to the cooling in the furnace was responsible for the lower relative residual compressive strength and higher recovered strength. This is because the high temperature can promote dehydration processes, accompanied with more rehydration products with a denser microstructure. The crack lengths were observed much longer in the specimens cooled in the ambient environment due to the higher temperature gradient during the cooling process. The higher temperature and temperature gradient near the surface can result in more intensive cracks.