Multiscale characterization of bond performance of UHPC-NC with different interface agents

The interface bonding properties of bi-material structures are influenced by a variety of factors, including surface roughness, presence of interface agents, moisture content of surface, etc., and they are critical factors in determining the overall performance and durability of structures. Interface between dissimilar materials exhibits heterogeneity and discontinuity. In this study, four types of interface agents (i.e., epoxy resin, nano-epoxy resin, high-strength mortar, and fiber-reinforced mortar) are proposed to investigate the multiscale effect and bonding mechanism of interface heterogeneity and continuity on the bond performance between hybrid recycled tire steel fiber reinforced concrete or ultra high performance concrete (UHPC) and normal strength concrete (NC). The multiscale analyses include the macroscopic (i.e., surface morphology, tensile bond strength, and shear bond strength), mesoscopic (i.e., fiber distribution, size of interface transition zone, and pore structure), and microscopic (i.e., micro-morphology and hydration product characteristics) characterization. The findings indicate that improving the interface heterogeneity and continuity enhances the bond performance of UHPC-NC interface. Compared with the reference group without the interface agent, the samples reach the highest tensile and shear bond strength, which are 71.46 % and 150 % higher than those of the reference ones, respectively, when the chemical interface agent with similar components and properties between the UHPC-NC layers is deployed. The present study reveals that the improvement in interface bond performance is attributed to the formation of hydration products and the densification of interface transition zone, governed by the chemical bonding, fiber reinforcement, and physical adsorption effects. Furthermore, the relationship between the size of interface transition zone and interface bond strength is established. Based on the theory of stress wave propagation, the influencing mechanism of interface heterogeneity and continuity on the failure mode and bond performance of UHPC-NC composites is elucidated. It concludes that altering the interface continuity between the UHPC-NC layers is the key to control the interface bond strength and its failure modes.