Effect of mechanical connectors on interface shear strength between concrete substrates and UHPC: Experimental and numerical studies and proposed design equation

•The effect of mechanical connectors on the interface shear strength between UHPC and NSC was investigated.•Thirty-nine specimens were tested using the push-off test for different textures and reinforcement ratios.•The substrate surface roughness was evaluated using a non-contact 3D laser scanning method.•A proposed equation to estimate interface shear strength following AASHTO-LRFD approach is suggested.•A numerical model to simulate the interface between UHPC and concrete is presented. Composite concrete sections are used in new construction such as bridge deck overlay or in repairing and retrofitting existing concrete elements in buildings and bridges. One important property to achieve composite action between two concrete segments is “interface shear strength,” which should be sufficient enough to prevent any excessive sliding or slippage. There are sufficient experimental studies and code design equations related to interface shear strength between two concrete segments; however, there is a lack in similar studies and design equations between ultra-high performance concrete (UHPC) and concrete substrates, especially with the presence of mechanical connectors. Thirty-nine specimens were tested using a modified push-off test apparatus to evaluate the interface shear strength for a combination of different surface treatments and different reinforcement ratios for mechanical connectors. These treatments included smooth surface (as-cast) and rough surface with sandblasting. The reinforcement ratio ranged between 0 and 0.012 for each surface treatment. The experimental results showed that surface treatment of the interface and reinforcement ratio of the mechanical connectors significantly affects interface shear strength. The results from experimental testing were used to develop a design equation to estimate interface shear strength following AASHTO-LRFD approach in terms of cohesion (adhesion combined with aggregate interlock) and friction (clamping force). It was concluded that the current AASHTO-LRFD values for cohesion and friction coefficients between two concrete surfaces are considered conservative in the case of UHPC and concrete substrates. Besides, numerical modelling was conducted to calibrate interface model parameters with sufficient acceptance. The findings of the experimental and numerical studies and the proposed design equation can help engineers and researchers to estimate the interface shear strength between UHPC and concrete substrates