## 2 Experimental Determination of FRP-Concrete Natural Bonding by the Push-Out Test
### Experimental Observations of the Push-Out Test  
There was one common failure mode for all four specimens, namely the shear fracture of the steel bolts (Figure 2). This failure mode has also been found in other push-out tests [43; 44] when the strength of the steel bolts is not high. The ultimate load (\(P_{u}\)) and ultimate slip (\(s_{u}\)) of each specimen can be seen in Table 1. Since it is difficult to evaluate the performance by \(P_{u}\) because the bolt number (\(n\)) of each specimen varies, the load per bolt (\(P_{ub}\), computed as \(P_{u}\)/\(n\)) was calculated and listed in Table 1. The average \(P_{ub}\) of specimens S-1,2 was 25.32 kN while the average \(P_{ub}\) of specimens S-3,4 was 28.13 kN, which was 11.1% higher than that of S-1,2. Therefore, it can be concluded that using SIP forms can slightly improve the ultimate shear capacity. Figure 3 shows the load-slip curves for all of the specimens. There were three distinct stages in specimens S-3.4: the non-slip stage, the debonding stage, and the bearing stage. Apparently, there were two different load-slip phases in specimens S-3.4 when compared with specimens S-1,2. In the non-slip stage, the load increased while the slip remained close to zero, which was caused by the initial natural bonding between the FRP and concrete. In the debonding stage, the load decreased dramatically while the slip increased suddenly, which was caused by the release of the natural bonding strength. After the debonding phase, both the load and slip increased gradually until the bolt shank fracture of the \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Longitudinal** & **Longitudinal** & **Longitudinal** & **Longitudinal** & \\  & **Tensile** & **Tensile** & **Compressive** & **Compressive** & **Shear Strength** \\  & **Strength** & **Modulus** & **Strength** & **Modulus** & \\ \hline FRP girder & 420 MPa & 25 GPa & 350 MPa & 13 GPa & 11.8 MPa \\ SIP forms & 390 MPa & 22 GPa & 350 MPa & 12 GPa & 13.5 MPa \\ \hline \end{tabular}
\end{table} 
Table 2: Material properties of the FRP profiles. Figure 1: Illustration of a push-out test specimen (unit in mm): (**a**) Half cross section of specimens S-1,2; (**b**) Half cross section of specimens S-3,4; (**c**) Half illustration of vertical cross section of S-2; (**d**) Half illustration of vertical cross section of S-4 (note that ‘Sym’ denotes the ‘symmetric line’); (**e**) Schematic graph of push-out test of specimen S-2; and (**f**) Cross section of the FRP segment. bolts, which was called the bearing stage and the load was only carried by the steel bolts. The load-slip curves of specimens of S-3/4 are depicted in Figure 3. The characteristic loads and slips of the above three stages can be found in Table 3.  
In Table 3, it can be seen that the non-slip stage shear capacity of the SIP-concrete block for specimens S-3 and S-4 was 184 kN and 200 kN, respectively. The load drop from the end of the non-slip stage to the end of the debonding stage for specimens S-3 and S-4 was 83 kN (from 184 to 101) and 86 kN (from 200 to 114), respectively. Therefore, the average value of the shear force carried by the natural bonding was 84.5 kN with a small variation. Assuming that the load drop was only caused by the release of the natural bonding and the shear stress distributed uniformly along the SIP surface, the interfacial shear strength, \(\tau_{bond}\), can be obtained by dividing the load drop by the surface area of the SIP form (2 \(\times\) 300 mm \(\times\) 500 mm), which gave an average value of 0.27 MPa. It should be noted that in the test in [42], the shear stress was not distributed uniformly, to be specific, it was higher in the middle and lower at both ends, so the \(\tau_{bond}\) underestimated the bond shear strength. Figure 3: The load-slip curves for all of the specimens. Figure 2: Typical failure mode for the push-out test specimens.