Quantifying influence of interfacial morphology in NC-UHPC bonding via mesoscale phase-field modeling and bi-shear testing

This study investigates the influence of interfacial morphology on the bonding behavior and fracture mechanisms between normal concrete (NC) and ultra-high performance concrete (UHPC) through bi-shear experiments and mesoscale phase-field analyses. The surface texture of NC substrate, roughened by toothed plates with pitches ranging from 3.5 mm to 7.0 mm, was calibrated and characterized as the superposition of three components, namely the primary profile, waviness profile, and roughness profile. The primary and roughness profiles follow Gaussian distributions, whereas the waviness profile is determined by the dimension of tooth pitch. Representative interfacial profiles were reconstructed from these characteristics. The reconstructed profiles were then incorporated into the phase-field models to define the NC-UHPC interface geometry. Experimental results reveal an approximately linear pre-peak load-relative slip response followed by an abrupt loss of capacity. As the tooth pitch increases from 3.5 to 7.0 mm, the average ultimate load decreases from 191.49 kN to 86.84 kN, with the corresponding nominal interfacial shear strength decreasing from 4.08 MPa to 1.90 MPa. These reductions are attributed to weakened mechanical interlocking. The mesoscale phase-field model, incorporating reconstructed morphologies, successfully replicates experimental failure modes and load-slip relationships, providing insight into the brittle failure mechanisms. Interfacial debonding, identified as the predominant failure mode, initiates at the interfacial zone between NC and UHPC near the loaded top surface, and propagates primarily along the interface. Small tooth pitches amplify stress concentrations at the tooth tips, leading to crack deviations from the interfacial zone into the NC substrate. This shifts the failure mode from predominant interfacial debonding toward mixed interfacial-substrate failure. These findings provide a basis for optimizing surface roughening techniques of NC-UHPC composites.