Post-Heating Performance of Concrete Containing Grading-Controlled Recycled Refractory Brick Fine Aggregate

Recycled refractory brick aggregate (RRBA) offers a potential means of reducing natural-sand consumption while improving the post-heating performance of concrete. However, comparisons among previous studies are frequently affected by differences in aggregate grading. In this study, a grading-controlled design was therefore adopted by blending two RRBA fractions to closely match the particle-size distribution of natural sand. Nine concretes with 0–60% fine-aggregate replacement were characterised at 28 days. All mixtures exceeded the target mean compressive strength of 50 MPa, while mean flexural strengths showed limited variation without a systematic trend with RRBA content. Separate specimens were heated at an age of 90 days to 100, 150, 200, 400, 600 and 800 °C. Mass loss and macroscopic damage were assessed after heating and cooling, while compressive strength, water-accessible porosity and saturated electrical resistivity were determined following 28 days of moist reconditioning. Thermal deterioration was generally less pronounced at 100–200 °C and intensified from 400 °C onwards. At 600 °C, the reference concrete retained only 25.4% of its unheated compressive strength, compared with 62.2% for RRBA-10. At 800 °C, the reference exhibited negligible compressive strength, whereas RRBA-15 achieved the highest observed mean post-thermal compressive strength, at 24.38 MPa (41.8% of its unheated strength). At 600 °C, RRBA-05 limited the relative porosity increase to approximately 10%, compared with 54.9% for the reference. Electrical resistivity losses for RRBA-05 to RRBA-15 ranged from 42.2% to 45.8%, compared with 75.8% for the reference mixture. A weighted geometric index combining compressive-strength, porosity and resistivity conservation identified 5–10% RRBA as a favourable interval at 600 °C, with RRBA-05 and RRBA-10 also occupying the first two positions under all eight alternative weighting schemes examined. The favourable replacement level therefore depended on exposure temperature and the performance criterion considered.