The effect of recycled fibers on shear properties of concrete at different ages and damage constitutive model based on 3-dimensional morphology analysis.

To study the shear properties and damage constitutive model of plain concrete (PC) and nylon fiber recycled from waste reinforced concrete (RNFRC), the direct shear test under different normal stresses (σ n , from 1 to 5 MPa) with a continuous horizontal rate of 0.5 mm/min is conducted on the cube specimens at different ages. The 3D roughness parameters of shear surface and the evolution of shear strength components are studied. Results show that the peak shear strength of RNFRC at 3, 7 and 28 d ranges from 6.65 to 12.09, 8.07–13.94 and 8.93–15.27 MPa under different σ n , exhibiting varying degrees of enhancement compared to those of PC. The values of the 3D roughness parameter C gradually increase while R s and the joint roughness coefficient (JRC) decrease with increasing σ n. The evolution of shear strength components is affected clearly by σ n. The calculation results of shear damage constitutive model match well with the experimental results. • The feasibility of using recycled nylon fibers to enhance the shear properties of concrete is discussed. • The roughness parameters of shear surface are analyzed by quantifying the 3D morphology characteristics. • The evolution shear strength components and their contribution to the peak under different test conditions are studied. • A new peak shear strength criterion based on the 3D morphology parameters and the damage constitutive model are proposed. [ABSTRACT FROM AUTHOR]