Design of a Concrete Shear Device and Investigation of the Shear Performance of New-to-Old Concrete Interfaces

Shear strength, which indicates the interfacial bond performance between new and old concrete, is critical in the field of structural reinforcement and rehabilitation. However, the absence of standardized testing equipment has hindered the accurate quantification of this parameter. To address this gap, a dedicated shear-loading apparatus was designed in this study, and finite element modeling was conducted to simulate the shear performance of concrete with different interface roughness. The results show that failure consistently occurs at the interface and that roughness has significant influence on shear capacity. In order to reveal the relationship between shear strength and surface roughness, shear experiments were conducted on new-old concrete using the device we designed. The surface of old concrete was treated by water-jetting, electric hammering, grooving, or grout seal strip to create different profiles, the roughness was quantified by 3D scanning and Fourier transform analysis, and fresh concrete was then cast atop the processed surfaces to form composite specimens. The results show that the correlation between shear strength (τ) and Fourier transform roughness (FTR) can be described with the equation τ (MPa) = 0.546FTR² + 1.832FTR - 0.447.