Mechanical and stress-strain behavior of hybrid fiber-reinforced metakaolin-based recycled aggregate concrete

This study investigates the effects of fiber on the mechanical performance and stress-strain characteristics of metakaolin-based recycled aggregate concrete (RAC). Ten experimental groups were designed, comprising single-fiber systems (steel fiber (SF), polyvinyl alcohol fiber (PF), and basalt fiber (BF)) and hybrid fiber systems combining SF-PF or SF-BF at blending ratios of 8:2, 7:3, and 5:5. Mechanical property evaluations included compressive strength, splitting tensile strength, elastic modulus, and full-range stress-strain curve analysis under uniaxial compression. Some constants are proposed based on test results to predict the compressive strength, splitting tensile strength, and elastic modulus of RAC. Furthermore, nonlinear regression analysis was performed to characterize the ascending and descending branches of stress-strain curves by the existing constitutive equations. The results indicate that the inclusion of fibers can effectively improve the mechanical properties of RAC. Based on some existing equations for compressive stress-strain curves of concrete, the parameters for the calculation of fiber-reinforced metakaolin-based RAC stress-strain curves were determined. Progressive substitution of SF with PF or BF in hybrid systems inversely correlates with strength and elastic modulus, while positively influencing peak strain development. A bilinear constitutive model derived from Al-Hassani's equation demonstrates superior fitting accuracy for hybrid fiber-reinforced metakaolin-based RAC. These results provide critical insights into the design of sustainable fiber-reinforced concrete with tunable mechanical properties through metakaolin activation and recycled aggregate utilization.