Stress–strain relationship analysis of recycled aggregate concrete based on orthogonal analysis

The use of recycled coarse aggregate (RCA) in natural aggregate concrete is a sustainable solution to the problem of solid waste worldwide. The effects of RCA gradation, RCA content, fly ash content and water–cement ratio (w/c) on mechanical properties of RCA are studied in this paper. The stress–strain curves of each specimen under 28 days axial loads were obtained by experiments. Through the stress–strain curves of each specimen, the effects of the above four factors on the maximum strain, ultimate strain, elastic modulus and toughness index of RAC were analyzed, and the optimal selection of the four factors were determined. Mathematical models of strength and four factors were established by fitting the experimental data. Finally, the RAC’s hydration failure mechanism with different RCA contents was studied by scanning electron microscopy. The test results show that: influence degree of the four factors on the compressive strength of RAC is: w/c > RCA content > RCA gradation > fly ash content. Based on a comprehensive analysis of compressive strength, peak strain, ultimate strain, elastic modulus and toughness index, it is recommended that RCA gradation is 5–20 and 5.0–31.5, RCA content is 50–70% and fly ash content is 20%.