High-Strength Recycled Concrete: Hybrid Material Evaluation and Predictive Modeling

The demand for sustainable construction has intensified the search for structurally viable recycled concrete aggregate (RCA) applications, historically limited by high water absorption, low density, and a weak dual interfacial transition zone (ITZ). This study experimentally evaluates mix designs using supplementary cementitious materials (SCMs) and fibres against the ACI 363R high-strength threshold (55 MPa) at a low water-to-cementitious (w/c) ratio of 0.32. Seventeen mixes were evaluated across three RCA replacement levels (0%, 34%, 67% by weight) containing fly ash (FA: 15%, 25%), silica fume (SF: 7%, 15%), polypropylene fibres (300, 600 g/m3), and steel chips (3.75 kg/m3). One-way analysis of variance (ANOVA) and Welch pairwise t-tests confirmed a statistically significant effect of RCA replacement level on 28-day compressive strength (F(2,6) = 382.3, p < 0.001); at 67% RCA, no mix reached 55 MPa (maximum 46.45 MPa). At 34% RCA, 25% FA achieved a 28-day strength of 59.00 MPa, not significantly different from the natural-aggregate control (p = 0.356). SF15% combined with steel chips yielded the highest flexural (9.90 MPa) and splitting tensile (5.51 MPa) strengths, an improvement over the plain 34% RCA baseline. Based on indirect durability indicators, SF7% showed the most favourable ultrasonic pulse velocity (UPV = 4.59 km/s) and lowest water absorption among the 34% RCA mixes. All mixes maintained placement viability (slump > 85 mm) at 1.3% superplasticizer. A multiple linear regression (MLR) model calibrated on the 17-mix dataset predicted 28-day compressive strength (R2 = 0.874, RMSE = 2.03 MPa): f’c (MPa) = 61.35 − 0.265·RCA (%) + 0.169·FA (%) + 0.119·SF (%), in which the RCA and FA terms are statistically significant but the SF term is not. These findings identify 34% RCA as the highest replacement level tested at which the ACI 363R threshold was met, and provide criterion-specific, statistically grounded design guidance for structural RAC within the tested range.