STUDY ON THE BEHAVIOR OF RECYCLED AGGREGATE CONCRETE UNDER HIGH TEMPERATURE EXPOSURE AND RECOVERY THROUGH RE-CURING

Abstract

The effect of high-temperature damage on Recycled Aggregate Concrete (RAC) remains understudied. Strength reduction due to high-temperature exposure is caused by a thermal expansion mismatch between the aggregate and the cement paste. This mismatch generates thermal stresses in the Interfacial Transition Zone (ITZ), which significantly impacts the performance of RAC, particularly during the re-hardening phase after heat exposure. This study investigated the effectiveness of re-hardening as a method to restore the compressive strength of RAC that has been exposed to high temperatures. Recycled Aggregate Concrete (RCA) was sourced from concrete waste with original concrete compressive strengths of 25 MPa and 40 MPa at replacement levels of 0%, 25%, 50%, 75%, and 100%. Mixes with water-cement ratios of 0.3, 0.45, and 0.5 were cured for 28 days before being heated to 550°C and 800°C, respectively. Compressive strength tests showed that as the RCA content increased, the concrete experienced a greater decrease in compressive strength at 800°C, with conventional concrete exhibiting the sharpest decrease. Seven days of water curing restored 44–87% of the compressive strength of concrete with RCA 0–100%, w/c 0.45, and 0.5, but w/c 0.3 required a longer time due to its low permeability. More cracks and pores facilitate hydration recovery in normal concrete, while high-strength concrete (w/c 0.3) limits water penetration. For RCA 25 and 40, water reinfiltration after exposure to high temperatures can restore pre-fire strength and halt further degradation, significantly improving post-fire performance.