Effect of post-fire curing on compressive strength of ultra-high performance concrete and mortar

Abstract

• Compressive strength of both UHPC and mortar increases at 300 ℃ but decreases at higher temperature. • Post-fire water curing is more effective on recover the compressive strength of the UHPC mix and Mortar than the post-fire air curing. • Strength recovery of the Mortar is greater than that of the UHPC. • Precipitation of portlandite heals the coarsened microstructure and recovers compressive strength. This paper investigates effects of post-fire curing on strength recovery and changes in microstructure of ultra-high performance concrete (UHPC) and mortar. Specimens were heated to elevated temperatures 300, 600, and 900 °C with a heating rate of 1 °C/min. After cooling, the specimens were subjected to post-fire curing in either air or in water for 56 days. Compressive strength, X-ray diffraction (XRD), mercury intrusion porosimeter (MIP), and field emission scanning electron microscope (FESEM) tests were conducted to examine the changes at both macro and micro scale and to reveal deterioration mechanisms due to fire effect and healing mechanisms due to post-fire curing. The test results showed that compressive strength of both UHPC and mortar increased slightly after 300 °C exposure but decreased significantly at higher temperature due to the coarsening of microstructure. Post-fire curing in air showed very limited strength recovery since new curing products did not heal thermal cracks larger than 1 μm. On the other hand, curing in water led to substantial strength recovery of the mortar mix due to formation of portlandite which compactly filled the microcracks in fire-damaged samples. However, the loose ettringite phase formed in the UHPC mix did not contribute to regaining the initial compressive strength.