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 (σn, 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 σn, exhibiting varying degrees of enhancement compared to those of PC. The values of the 3D roughness parameter C gradually increase while Rs and the joint roughness coefficient (JRC) decrease with increasing σn. The evolution of shear strength components is affected clearly by σn. The calculation results of shear damage constitutive model match well with the experimental results.