Development of hybrid recycled aggregate concrete for sustainable high-performance structural applications: Experimental evaluation and constitutive modelling

The use of recycled aggregate concrete in the structure is restricted because of the poor quality of recycled coarse aggregate, which ultimately leads to the decrease in mechanical properties, durability, and reliability of the structure. In this work, a hybrid recycled aggregate concrete (HRAC) is developed using 30% ground granulated blast furnace slag as a substitute for cement, 35% copper slag as a partial replacement for fine aggregate, and a hybrid fibre system comprising 1.0% hooked-end steel fibre and 0.5% polypropylene fibre to address the performance limitations of recycled aggregate concrete. The fresh properties, mechanical performance, durability, reinforced concrete beam behaviour and microstructural characteristics were examined by experimental tests on M30 and M40 concrete with recycled coarse aggregate from 0 to 100%.Experimental investigations have been performed on M30 and M40 concrete with RCA content (0–100%) to investigate their fresh properties, mechanical performance, durability, reinforced concrete beam behaviour and micro structural characteristics through scanning electron microscopy and X-ray diffraction. Other analytical tools used to develop a detailed performance evaluation framework include statistical analysis, regression modelling, constitutive stress–strain modelling, and embodied carbon assessment. As there was a progressive increase in recycled aggregate replacement, the strength, stiffness, and durability decreased due to increased porosity, decreased ITZs, and pre-existing microcracks. These properties were significantly recovered in the proposed hybrid system with matrix densification, re-formation of hydration-product development, refinement of ITZ, and multi-scale crack-bridging mechanisms. The hybrid mixtures produced improved compressive, tensile, flexural, durability, and structural performance compared with the corresponding RCA mixtures. However, fibre incorporation increased the absolute embodied carbon of the hybrid mixtures, partly offsetting the reductions achieved through GGBS and RCA. The stress–strain response was well captured by the constitutive model and good correlation was obtained between the compressive strength and other engineering properties. The results indicate that HRAC mixtures with 25–50% RCA provide a balanced combination of mechanical performance, durability, and environmental performance.