## 4 Factors Affecting the UHPC-NC Interface Adhesion
### Interface Roughness  
Interface roughness is an important factor affecting the UHPC-NC interfacial adhesion [28; 57; 58]. The concrete surface of UHPC-NC composite members must be treated to improve the original interface roughness. Current common treatment methods include high-pressure water jet roughening, grooving, brushing, roughening, drilling, sandblasting, and planting reinforcement [28]. Some interface treatment methods are shown in Figure 11. \begin{table}
\begin{tabular}{c c c} \hline
**References** & **Calculation Formula** & **Calculated Parameters** \\ \hline [PERSON] [49] & \(\tau_{u}=(-0.012R^{2}+0.138R+0.037)f_{cu}^{0.55}\) & \begin{tabular}{c} \(R\): roughness; \(0\leq R\leq 4\); fct: ordinary concrete cube; \\ compressive strength: 35 MPa \(\leq\)\(f_{cu}\leq 60\) MPa. \\ \end{tabular} \\ \hline [PERSON] [27] & \(\tau_{u}=(\sigma_{S1}+\sigma_{S2})(a\cdot e^{x/b}+c)\) & \begin{tabular}{c} \(\sigma_{S1}\) and \(\sigma_{S2}\): compressive strengths of UHPC and NC; \(x\): \\ roughness coefficient; \(a\) to \(c\): relevant parameters. \\ \end{tabular} \\ \hline [PERSON] [53] & \(V_{i}=V_{k}+V_{sm}+V_{coh}\) & \
\begin{tabular}{c} \(V_{k}\): shear-bearing capacity of the UHPC key teeth; \(V_{sm}\): friction resistance of the flat joint; \(V_{coh}\): cohesive shear resistance of the adhesive layer at the joint interface. \\ \end{tabular} \\ \hline Li [56] & \(V_{WJ}=\eta(0.989+0.5076\lambda_{J})A\sqrt{f_{c}}\) &
\begin{tabular}{c} \(f_{c}\): axial compressive strength of concrete; \(\lambda_{J}\): characteristic parameter of steel fiber; \(\eta\): direct shear strength ratio (i.e., ratio of the peak direct shear strength of the flat joint \\ specimen to the peak direct shear strength of the same type \\ of monolithic casting specimen). \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: UHPC–NC interface shear strength calculation formulas. Figure 11: Different surface treatments for NC substrates [9]. The interface roughness is affected by different interface treatment methods. The relationship between the roughness and bonding strength is determined by different tests. [PERSON] (2011) demonstrated that the interface bonding strength between UHPC and NC increases with the increasing surface roughness of the NC substrate. The highest interface bonding strength was obtained at a surface roughness depth of 4 mm. Water spraying as an interface treatment can considerably improve the shear strength of the construction joints ([PERSON] et al., 2016). [PERSON] (2011) investigated the UHPC-HSC structural interface bonding strength under different roughness conditions using a direct tensile test. The bonding strength at the UHPC-HSC structure interface was reported to increase with an increasing roughness. Moreover, the specimen developed cracks in the HSC layer, and the interface bonding strength was higher than those obtained by [PERSON] (2011) and [PERSON] (2011) using pull-off tests. [PERSON] (2011) performed slant shear and split tensile tests to examine the effect of different roughness values on the UHPC-NC structure interface bonding strength. In the slant shear test, the bonding strength of the specimens with interfaces with low and high roughness values increased by 26% and 56%, respectively, compared to those of the smooth interface components. Both shear and slant shear tests can provide better evaluation of the effect of interface roughness; however, the sensitivity of the core pull-off and composite beam tests was lower than that of the former. [PERSON] (2011) and [PERSON] (2013) performed the UHPC-NC core pull-off and bending tests. Regardless of the treatment of the NC substrate (untreated or sandblasted), the specimen substrate was always damaged. [PERSON] (2011) used five interface treatment methods on the specimen. The high-pressure water jet was found to be the most effective method for improving the interface bonding strength.