## 5 Calculation of Flexural Load Capacity  
According to the Chinese specification (GB50010-2010) [32], when the damage of reinforced concrete beam occurs, the bending moment of the section is provided by the  
Figure 11: The performance indexes vary with duration of service. upper compressed concrete together with the lower tensile reinforcement. The formula for calculating the cross-sectional bending moment (\(M\)) is as follows. \[M=\alpha_{1}f_{\rm cu}^{*}bx(h_{o}-\frac{x}{2})+f_{\rm y}^{\prime}A_{\rm s}^{ \prime}(h_{\rm o}-a_{\rm s}^{\prime}) \tag{2}\]  
\[x=\frac{f_{\rm y}A_{\rm s}-f_{\rm y}^{\prime}A_{\rm s}^{\prime}}{\alpha_{1}f_{ \rm cu}b} \tag{3}\]  
Note: \(\alpha_{1}\) represents the discount factor of high-strength concrete (\(\alpha_{1}\) is taken as 1 when the concrete strength grade does not exceed C50, \(\alpha_{1}\) is taken as 0.94 when the concrete strength grade is C80 and the interval is determined by linear interpolation, \(\alpha_{1}=1\) in this paper). \(f_{\rm cu}\)* represents the cube compressive strength of concrete under a conventional atmospheric environment. \(b\) represents the section width (\(b\) = 150 mm in this paper). \(h_{\rm o}\) represents the effective height (\(h_{\rm o}\) = 175 mm in this paper). \(x\) represents the height of compressed concrete. \(f_{\rm y}^{{}^{\prime}}\) represents the compressive yield strength of compressed steel. \(A_{\rm s}^{{}^{\prime}}\) represents the total cross-sectional area of compressed steel. \(a_{\rm s}^{{}^{\prime}}\) represents the distance between the compressive reinforcement and the upper edge of the section (\(a_{\rm s}^{{}^{\prime}}\) = 25 mm in this paper). \(f_{\rm y}\) represents the tensile yield strength of the tensile reinforcement. \(A_{\rm s}\) represents the total cross-sectional area of the tensile reinforcement. The specimen in this paper is a four-point bending beam, so its bending load capacity (\(P\)) is calculated as  
\[P=2 aM \tag{4}\]  
Note: \(a\) represents the shear span length (\(a\) = 350 mm in this paper). In this paper, it was found that the changes in the load-bearing capacity of reinforced concrete beams after tidal environment exposure were mainly due to changes in material strength. The reinforcement in this study was not eroded in the tidal environment of 270 d due to the protection of the protective layer. Therefore, this paper only considers the effect of the degradation of concrete strength on the flexural bearing capacity of the specimen beams in the tidal environment. The concrete strength correction factor (\(Q\)) is introduced, which is obtained by fitting the data of this paper with those of the literature [33; 34] (as shown in Figure 12). Its calculation equation is as follows. \[Q=1-5.2\times 10^{-4}\cdot t-9.47\times 10^{-7}\cdot t^{2} \tag{5}\]  
Note: \(t\) represents the duration of service, its unit is days (d). Therefore, the bending capacity of reinforced concrete beams in a tidal environment is calculated by the following formula. \[P=2a\Big{(}\alpha_{1}Qf_{\rm cu}^{*}bx(h_{o}-\frac{x}{2})+f_{\rm y}^{\prime}A _{\rm s}^{\prime}(h_{\rm o}-a_{\rm s}^{\prime})\Big{)} \tag{6}\]  
Figure 12: Fitting formula for concrete strength correction factor (\(Q\)). The theoretically calculated values of the flexural bearing capacity of reinforced concrete beams in a tidal environment are summarized in Table 3. It can be seen from the test that the specimen beam does not reach its ultimate load or breaking load when the reinforcement reaches near its yield strength, but makes a significant turn in the rigidity of the specimen (the point is defined as the yield point of the specimen in this paper). Therefore, the yield point strength of the specimen should be selected for comparison with the theoretical calculated value. As can be seen from Table 3, except for the SSCB-R11 specimen, the ratio of the calculated value to the test value of the remaining specimens is between 0.81 and 0.95, which indicates a conservative prediction.