Sustainable Utilization of Demolition Waste as Recycled Coarse Aggregate in Concrete: Strength Enhancement Using Silica Fume and Steel Fibres

Abstract

Construction and demolition waste (CDW) is one of the major solid-waste streams generated by rapid urbanization and infrastructure renewal. Recycled coarse aggregate (RCA) obtained from CDW can reduce the consumption of natural aggregates and support circular construction; however, its higher porosity, adhered mortar, and weaker interfacial transition zone may influence concrete performance. This study experimentally investigates the use of RCA as a partial replacement for natural coarse aggregate in M20, M30, and M40 grade concretes. RCA was incorporated at 0, 10, 20, 30, 40, 50, and 100% replacement levels, and mechanical properties were evaluated at 7, 28, and 90 days. Based on the first-stage results, 30% RCA was identified as the optimum replacement level. At 28 days, the 30% RCA mixes improved compressive strength by 9.24%, 7.60%, and 8.79% for M20, M30, and M40 grades, respectively, compared with corresponding control concretes. Split tensile strength increased by 25.71%, 24.41%, and 23.08%, while flexural strength increased by 9.24%, 7.60%, and 8.77%, respectively. Further modification of the optimized RCA mixes using silica fume at 5, 10, and 15% showed that 15% silica fume produced the highest strength gain, with 28-day compressive strengths of 35.00, 51.50, and 58.00 MPa for M20, M30, and M40 grades. Steel fibres further enhanced post-cracking resistance and flexural performance, with the C3S15F1.5 mix reaching 60.25 MPa compressive strength and 14.50 MPa flexural strength at 28 days. The results confirm that CDW-derived RCA can be used effectively up to 30% replacement, and its performance can be significantly improved by silica fume and steel fibre incorporation.