## 4 Factors Affecting the UHPC-NC Interface Adhesion
### Interfacial Adhesive Agents  
Interfacial adhesive agents are broadly classified into cement-based composite and polymer materials [29]. The hydrating products of cement-based composites fill the pores at the interface, forming a compact transitional layer and improving the mechanical properties of the layer interface. Polymer penetration effectively inhibits crack expansion and increases the bonding force of the interface layer by permeating through the NC microcracks [28, 64, 73].  
In [65], the effects of the absence of an interfacial adhesive agent and the use of cement paste, silica ash paste, cement swelling slurry, and butadiene emulsion on the interfacial bonding strength of UHPC-NC was compared using split tests. Except for butadiene emulsion, using interfacial agents improved the interfacial bonding strength. Typically, an interface agent is required for precast UHPC-NC composite components. [PERSON] [57] used structural adhesives and grouting materials of cement as the interface agents of prefabricated UHPC-NC components. The superior effect of the structural adhesives over cement-based grouting materials was reported. Therefore, structural adhesives are good interfacial agents.  
The interfacial agent ratio influences the effect of interfacial bonding. [PERSON] [74] conducted an experimental study and reported the different degrees of improvement of the interfacial bonding strength with the incorporation of different interfacial agents. The bonding split tensile strengths of UHPC-NC gradually increased as the interfacial agent incorporation ratio increased. [PERSON] [75] investigated the effect of the interfacial agent on the interfacial bonding strength of cast UHPC-NC and precast UHPC-NC specimens. During the slant shear, four-point bend, and split tensile tests, the interfacial bonding strength of the precast specimens using epoxy resin increased by 12.20%, 26.10%, and 60%, respectively, compared with the cast samples (Figure 14).  
[PERSON] [76] compared the interface bonding strengths of precast and cast UHPC-NC components with multiple interface agents using pull-off tests. Phenyl  
Figure 14: Effects of the epoxy-based adhesive agent on the interfacial bonding strength [26].  
Figure 13: Load–displacement curves of two test blocks [59]: (**a**) UHPC–UHPC; (**b**) UHPC–NC.  
was demonstrated as the best interfacial agent, followed by modified fly ash, silica fume paste, expanded cement paste, and cement paste. Moreover, the structural adhesives considerably improved the interfacial shear bonding strength compared to cement grouting materials [74]. Under an established shear connector, the direct shear strength of the specimen bonded by a grouting material was lower than that required by the regulations issued by the American Concrete Institute (ACI). However, the direct shear strength of the specimen bonded by a structural adhesive can satisfy the ACI requirements.  
Through drilling core and bending tests, [PERSON] [35] discovered the specimen failure at the NC substrate with UHPC as the interfacial agent. [PERSON] [40] designed two UHPC samples with different interfacial roughness values to investigate its effect as the interfacial agent between UHPC and NC. Compared to the specimens without interfacial agents (S and R), the interface bonding strength of the specimens with interfacial agents (SB and RB) decreased by 14% and 50%, respectively (Figure 15). Thus, the enhancement effect using UHPC as the material interfacial agent with different interface roughness conditions should be investigated.