A multi-scale study on UHPC-NC bonding: Effects of NC substrate moisture and bonding agents

In ultra-high-performance concrete (UHPC) and normal concrete (NC) composite structures, the performance of the UHPC-NC interfacial transition zone (ITZ) plays a decisive role in the mechanical behavior and durability of the overall structure. To improve this weak area, this study systematically investigates the effects of bonding agent types and NC substrate moisture conditions on interfacial bonding performance. Through multi-scale experimental techniques such as oblique shear tests, splitting tensile tests, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD), the macro-mechanical properties and microstructural characteristics of the interface under different influencing factors were comprehensively evaluated. Furthermore, by integrating macro- and micro-analyses, the multi-scale bonding mechanisms of the interface under varying moisture and bonding agent conditions were revealed.The results indicate that silica fume, as a bonding agent, exhibits the highest shear and splitting tensile strengths under dry, moist and saturated conditions, with values reaching 48.91 MPa and 4.01 MPa, respectively, under the optimal moist condition. Although bisphenol-A epoxy resin (E-51) slightly improves the splitting tensile strength, it significantly reduces the interfacial shear strength. The interfacial failure mode shows good consistency with the bonding strength. The moisture state of the NC substrate significantly influences interfacial performance, with the moist condition being optimal, as it promotes sufficient hydration and enhances the density of the ITZ. Additionally, micro-test results demonstrate that silica fume effectively refines the pore structure of the interfacial transition zone, promotes the hydration reaction, and consumes Ca(OH)₂. Moist treatment optimizes the distribution of hydration products and the pore structure. Although E51 reduces porosity to some extent, it forms a barrier layer at the interface that restricts the hydration process, resulting in interfacial bonding that primarily relies on chemical bonding, which is insufficient for achieving high bonding strength. The micro-analysis provides a comprehensive explanation for the macro-mechanical performance.