## 4 Factors Affecting the UHPC-NC Interface Adhesion
### Interfacial Water Content  
The effect of the interfacial water content on the bonding properties of the UHPC-NC interface bonding zone primarily occurs when water exists on the NC surface. This slows down water migration in UHPC and promotes the production of hydration products and the reduction of porosity in the interfacial transition zone [28; 77; 78; 79]. Microscopic tests have been performed to investigate the effects of different interfacial water contents on the hydration products and porosity of the interfacial transition area [80; 81]. As shown in Figure 16, Group SSD (interfacial pre-wetting) had a higher proportion of hydration products, which improved the compactness of the interfacial transition area.  
[PERSON] [28], [PERSON] [29], and [PERSON] [80] demonstrated that a pre-wetted NC surface affects the interface bonding properties of new and old materials, and a wetted NC substrate can improve the interface bonding strength of composite members. The bonding property of the UHPC-NC interface is considerably affected by the wetting degree of the NC base and is positively correlated with the interface shear strength. The references [81; 82; 83; 84] conducted an axial tensile test on specimens after treating them with SSD, ASW, and ASD. It was found that increasing the wetting degree of the NC substrate under natural curing conditions improved the crack load and load capacity of the UHPC-NC specimens under the axial tensile test. Meanwhile, [PERSON] [82] and [PERSON] [80] believed that interfacial pre-wetting treatment does not improve the bonding properties of the UHPC-NC interface because the water from the UHPC flows to the NC substrate during the slant shear test, increasing the water-to-binder ratio. According to [PERSON] [85], SSD has a negative impact because of the increase in the porosity and water-cement ratio of the interface, which are related to the fluidity of the UHPC and material properties of NC.  
Figure 15: Average bond strength for each group of tests.  
[PERSON] [80] investigated the effect of different interfacial moisture contents on the interface bonding strength between UHPC-NC and NC-NC. Under SSD conditions, the interface is conducive to the interface bonding strength improvement. [PERSON] [83] discovered that wetting surface treatment effectively inhibits interfacial failure, reduces the randomness of the failure mode, and improves the overall mechanical properties of specimens compared to the drying treatment of the NC surface. Figure 17 illustrates the influence of the interfacial moisture content on the tensile bonding strength.The interfacial bonding strength of UHPC-NC increased by 34%, similar to that obtained by [PERSON] [84].  
Several researchers noted that pre-wetting has no considerable effect on the interfacial bonding properties. By performing three-point bending and single-plane shear tests, [PERSON] [82] demonstrated that the shear strength of the interfacial bonding of UHPC-NC on a wet surface is higher than that on a dry surface. However, the slant shear test results were contradictory, consistent with the conclusions of [PERSON] [80]. Moreover, [PERSON] [85] investigated the effect of different water contents on the interfacial bonding characteristics and microstructures of three NC samples (i.e., different porosities and water absorption characteristics). Wetting the NC surface did not improve the interface bonding strength but increased the water-cement ratio and porosity of the interfacial transitional zone. Therefore, the interfacial bonding characteristics are affected by the interfacial moisture content, material properties of NC, and fluidity of UHPC.  
Figure 16: Overall porosity and hydrated and unhydrated distributions on the grout side of the interface for the control and SSD specimens [80].  
Figure 17: Effect of the substrate’s moisture condition on the tensile bonding strength [17].