## 3 Test Results and Discussions
### Effect of Chloride Environment  
The tensile test results of TRM coupons were analyzed based on ACI 549.4R-13 [33]. The idealized stress-strain curve includes the initial segment as the uncracked stage, and the second segment as the cracked stage, which is distinguished by the significant stiffness change (transition point), as shown in Figure 11. The initial linear segment of the curve corresponds to the TRM uncracked linear behavior, and it is presented by the cracking tensile strength \((f_{c})\), cracking tensile strain \((\varepsilon_{c})\), and tensile modulus \((E_{1})\) of elasticity of the uncracked coupons. In the first stage, the elastic modulus is governed by the mechanical characteristics of the mortar and carbon fiber textile based on the perfect bond at the interfaces. The second linear segment, which corresponds to the TRM cracked linear behavior, is presented by the ultimate tensile strength \((f_{u})\), ultimate tensile strain \((\varepsilon_{u})\), and tensile modulus \((E_{2})\) of elasticity of the uncracked coupons. In this study, tensile stress \((\sigma)\) was defined as the applied load (N) divided by the cross-section area \((A_{s})\) of the TRM coupons (80 mm \(\times\) 30 mm) [39]:  
The uncracked tensile modulus of elasticity was calculated for the first linear segment as:  
\[E_{1}=\Delta_{\sigma}/\Delta_{\varepsilon}, \tag{1}\]  
where \(\Delta_{\sigma}\) is the difference in tensile stress at the point where 60% of the cracking strength (MPa) is applied and \(\Delta_{\varepsilon}\) is the corresponding difference in tensile strain (see point A in Figure 11). The cracked tensile modulus of elasticity was calculated for the second segment [33] as:  
\[E_{2}=\Delta f/\Delta\varepsilon=(0.9f_{u}-0.6f_{u})/(\varepsilon_{0.9f_{u}}- \varepsilon_{0.6f_{u}}), \tag{2}\]  
where \(\varepsilon_{0.9f_{u}}\) and \(\varepsilon_{0.6f_{u}}\) are the strains at a tensile stress level, equal to \(0.9f_{u}\) and \(0.6f_{u}\), respectively (see points B and C in Figure 11). This is due to the fact that the mechanical characteristics of carbon fiber mesh mainly govern the elastic modulus in the second stage. Figure 11: Idealized stress–strain curve of the TRM coupons [33]. Figure 10: Failure mode from the uniaxial tensile test of TRM coupons. The first test series, which included the coupons CE, CE_E, and CE_ES, were prepared to investigate the effect of chloride environment on the tensile behavior of TRM coupons. The coupons CE mean the TRM coupons without surface treatment, subjected to a 3.5 wt.% chloride environment. The coupons CE_E and CE_ES mean the TRM coupons with epoxy impregnation, and with sand coating after epoxy impregnation, respectively. Figure 12 shows the stress-strain relationship of the first test series with different surface treatment methods in the chloride environment. In the uncracked stage, the stress-strain relationship of the TRM coupons showed a linear elastic curve with a high slope by the combined working of both the mortar matrix and carbon fiber textile. After that, at around 0.75 MPa of tensile stress, the first cracks appeared in the middle of the TRM coupons, then more cracks appeared near the transition zone, and these cracks continued to widen, until failure (Figure 9a). Figure 13 compares the average values of initial stiffness, ultimate strain, cracking tensile strength, and peak tensile strength between TRM coupons with different surface treatments. There was no significant difference in most of the tensile behavioral characteristics, regardless of the surface treatment (Figure 13a and Table 2). However, the ultimate strains of the surface-treated TRM coupons decreased, in particular, the CE_E coupons showed a decrease of 13.09% compared to that of the CE coupons. The chloride environment might negatively affect the interfacial bonding once the chloride solution penetrates the interface made of epoxy and coating with aluminum oxide powder.