Residual Flexural Behaviour of Reinforced Self-Compacting Concrete Beams After Fire Exposure: Effects of Fine-to-Coarse Aggregate Ratio, Heating Rate, and Boundary Restraint

Self-compacting concrete (SCC) is widely used in reinforced concrete structures, yet its residual structural behaviour after fire exposure remains insufficiently understood, particularly under the combined effects of mixture composition, heating rate, and boundary restraint. This study experimentally investigated reinforced SCC beams incorporating fine-to-coarse aggregate (F/C) ratios of 0.90 (SCC-L), 1.00 (SCC-R), and 1.10 (SCC-H). A total of 90 beams were tested at ambient temperature and after exposure to 300 and 500 °C using heating rates of 20 and 30 °C/min under simply supported and restrained-end conditions. Residual ultrasonic pulse velocity (UPV) measurements and four-point flexural tests were conducted. The ambient simply supported reference beams had ultimate loads of 11.2, 11.8, and 10.7 kN and first crack loads of 4.95, 5.20, and 4.50 kN for SCC-L, SCC-R, and SCC-H, respectively. Exposure temperature was the dominant factor governing deterioration, while higher heating rates accelerated thermal damage. At 500 °C and 30 °C/min, simply supported beams retained 66.95–78.50% of their ambient ultimate loads, whereas first crack loads decreased to 36.54–60.00% of their corresponding ambient- temperature values. SCC-H consistently exhibited the highest retention of residual mechanical and structural properties. End restraint increased the residual ultimate load by up to 21.52% and first crack load by up to 89.47% relative to simply supported beams with the same mixture, exposure temperature, and heating rate. The developed empirical model showed a good fit to the experimental ultimate flexural loads (R2 = 0.9637) within the investigated parameter ranges. The results demonstrate that mixture composition and boundary restraint significantly influence post-fire performance, while UPV provides a reliable indicator of residual structural condition.