## 2 Experimental Determination of FRP-Concrete Natural Bonding by the Push-Out Test  
### Material Properties and Setup of the Push-Out Test  
To estimate the bond strength between the FRP profile and concrete, a push-out test of four FRP-concrete connection specimens (see Figure 1 and Table 1) was conducted. As a comparison, specimens S-1,2 were equipped with steel bolts as the interfacial connection and specimens S-3\(A\) were equipped with the SIP form and steel bolts together. The geometric dimensions of the concrete blocks were 150 mm thick, 300 mm width, and 500 mm high (Figure 1e). The thickness of the flanges and webs of the FRP profiles was 10 mm. At the corner of the flange-web junctions, the radius was 10 mm. The SIP form was a plate with a thickness of 10 mm, a width of 300 mm, and 500 mm height (Figure 1b,d,e). All of the parameters can be seen in Table 1 and Figure 1.  
The FRP I-profiles and SIP forms were produced by pultrusion manufacturing. Unidirectional E-glass fibers were used as the main reinforcement while fiber mats were used in the outer layers to create smooth surfaces. Unidirectional fibers were oriented at zero degree to the longitudinal direction and the glass fiber continuous strand mat was randomly oriented. Unsaturated polyester resin was used as the matrix, and additives made of CaCO\({}_{3}\) powders, weighted about 20% of the matrix, were added to lower the cost according to the manufacturer's design. The fiber content was 55% in volume according to the data provided by the manufacturer. The fiber layups and fractions of the additives in the matrix varied across the FRP I-profiles and SIP forms, so the material properties were not the same. The material properties of the FRP are reported in Table 2, according to the data provided by the manufacturer. The compressive strength of concrete was tested using twelve \(150\times 150\times 150\) mm\({}^{3}\) cubic coupons cured at the same conditions as the push-out test specimens. Tests showed that the concrete had an average compressive strength of 35.38 MPa, with a coefficient of variation (CoV, also known as the relative standard deviation) of 0.207. The embedded length of bolts, (from the top of the steel bolt to the surface of the FRP) in the concrete block was 100 mm for all specimens. One day before casting the concrete, the surface of the SIP and FRP were cleaned using alcohol. The diameter of the bolt shank and the predrilled holes was 10 mm and 10.5 mm, respectively. The steel bolts were fixed into the holes by two washers and two nuts, with a predefined torque of 50 kN\(\cdot\)m. The torque was applied and controlled by a torque trench. Steel washers, with an outer diameter of 32 mm, an inner diameter of 12.5 mm, and a thickness of 1 mm were used to disperse the stress caused by prestressing the steel bolts. There was a thin layer of bond adhesives between the SIP form and FRP girders. The bond adhesive used was a commercially available product in China that consists of a 1:1 ratio of epoxy resin to curing agent in mass. According to the manufacturer's data, the tensile strength of the adhesive is larger than 14 MPa. Reinforcing bars were made of ribbed steel with a yield strength of 235 MPa. The vertical bars had a diameter of 10 mm and the transversal bars had a diameter of 6 mm. Six LVDTs were positioned at opposite sides of the concrete block and FRP webs, capturing the average slip between the FRP girder segment and concrete. The load was applied to the push-out specimens via a calibrated 5000 kN capacity universal testing machine.