Direct shear test study on old and new concrete

The interface between new and old concrete is a common feature in prefabricated concrete structures and structural reinforcement. Its shear-slip performance directly impacts the stress performance and deformation capacity of members and structures. This paper aims to examine the influence of roughness density, roughness, roughness treatment, and rebar distribution on the shear performance of the interface by conducting direct shear tests on 14 Z-type specimens without dowel bars and 10 Z-type specimens with dowel bars. The tests reveal that specimens without dowel bars fail in shear brittle, with the roughness density of the interface having the most significant impact on shear strength, followed by roughness, and a minimal effect from the roughness treatment method. In contrast, specimens with dowel bars exhibit shear ductile damage. An increase in the number of inserted bars at a uniform reinforcement ratio leads to increased bond stiffness and shear strength, and a decrease in peak slip. The interface after rough chiseling exhibits the highest shear strength, followed by the interface after broaching, and the interface after grooving has the lowest shear strength. Increasing the roughness or designing dowel bars can enhance the strain coordination ability of the structure, which is beneficial for overall stress loading. Finally, finite element analysis was used to validate the reliability of the tests. Refined modeling of the grooved model without dowel bars revealed that peak height correlates best with shear strength, and that chiseling and other methods of improving the valley depth of the concrete interface at the base level are still superior roughness treatments.