Multi-scale investigation on the interfacial behavior of between NC and UHPC in flexural members considering influence of the interface morphology

To study the interfacial behavior between NC (normal concrete) and UHPC (ultra-high performance concrete) in composite flexural members, a multi-scale numerical simulation framework was proposed, and then verified by three-point bending tests conducted on RC (reinforced concrete)-UHPC composite girders, considering different interface morphologies. The numerical model of a RC-UHPC girder based on this method can take into account the influence of the material mesoscale components as well as interfacial mesoscale morphology on the mechanical behavior. The results indicate that the damage and failure at the NC-UHPC interface of a flexural element is controlled by the horizontal shear. The interface roughness has positive effects on both the interfacial performance and the mechanical performance of a composite girder. When the root mean square roughness increases from 0.2 mm to 4.5 mm, the interface shear strength grows from 2.04 MPa to 5.20 MPa by 2.55 times, and the peak load applied to the girder grows from 197.9 kN to 239.3 kN by 1.21 times. The relationship between the resistance of a composite girder and the interfacial shear strength/ interface roughness could be described by the exponential function/ linear function.