Research on size effect of Z-shaped shear interface at the interface between new-old concrete

Ignoring the size effect of shear strength at the interface between new-old concrete in the design of structural reinforcement and retrofitting projects will significantly overestimate the structural load-bearing capacity, leading to safety hazards. Based on the shear tests of Z-shaped new-old concrete interface specimens with three groups of 11 different interface treatment methods and different lateral restraint conditions, as well as one monolithic Z-shaped concrete specimen, the shear-slip curves of each specimen were measured. The experiments showed that the shear strength of the unconstrained new-old concrete interface was 0.25 to 0.50 times that of the monolithic specimen, and the section shear strength of the specimens without prestressed screw restraints increased by an average of 1.55 times. The cracking stress, crack propagation stress, and ultimate stress of each group of specimens exhibited a significant size effect, decreasing as the height of the shear plane increased. A calculation formula for the shear bearing capacity of the interface was proposed, and an expression for calculating the coefficient of shear strength size effect at the interface was fitted. Finally, the accuracy of the calculation formula of the interfacial shear capacity is verified by the simulation of the equivalent substitution model established by ANSYS.