## 3 Durability Coefficients of Constructed Reinforced Concrete  
### Concrete Cover Thickness and Compressive Strength  
The service life of reinforced concrete was determined by concrete cover thickness and its properties, and the margins for the cover thicknesses suggested by DuraCrete were 20 mm, 14 mm, and 8 mm, which represent high, normal, and low repair cost, respectively. The concrete compressive \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Component**} & \multirow{2}{*}{**Cement**} & \multirow{2}{*}{**Fly Ash**} & \multicolumn{4}{c}{**Aggregate (mm)**} & \multirow{2}{*}{**Sand**} & \multirow{2}{*}{**Water**} & \multirow{2}{*}{**Super**} & \multicolumn{2}{c}{**Air**} \\  & & & & **5–10** & & & **10–20** & & **20–31.5** & \\ \hline Land section & 315 & 135 & 207 & 414 & 414 & 721 & 149 & 4 & 2.7 \\ Marine section & 336 & 144 & 208 & 416 & 416 & 709 & 144 & 5.28 & 2.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mix proportions of concrete used in the land and marine sections (kg/m\({}^{3}\)). Figure 1: Reinforced concrete pier of the Jiaozhou Bay sea-crossing railway bridge (marine section). strength was measured by a rebound apparatus (manufactured by Runjie Corporation), and also the cover thickness of the pier structures was tested by a cover thickness meter (manufactured by Meiyu Corporation), as shown in Figure 2. The actual concrete cover of the reinforced concrete piers in Jiaozhou Bay sea-crossing railway bridge is shown in Figure 3. The measured concrete cover thickness of the piers in the marine and land sections followed a normal distribution. The mean cover thicknesses of the concrete piers in the marine and land sections were 52 mm and 36 mm, respectively. The corresponding standard deviations were 5.21 mm and 3.18 mm, respectively. The maximum difference between the measured and designed cover thicknesses of the concrete piers was less than 15 mm. Therefore, to ensure that the concrete is made more durable, it is very important to control the cover thickness in situ. After curing in situ, the compressive strength and rebound strength of the casting concrete specimens (100 mm \(\times\) 100 mm \(\times\) 100 mm) used in the marine and land sections were measured by a compression-testing machine and rebound hammer, respectively. As shown in Figure 4, the linear relationships between the compressive strength and the average rebound strength of the concrete  
Figure 3: Concrete cover thickness of piers in the marine section and the land section. (**a**) Frequency (marine section); (**b**) Probability (marine section); (**c**) Frequency (land section); (**d**) Probability (land section). Figure 2: Rebound strength and cover thickness tests at piers. (**a**) Rebound strength; (**b**) Cover thickness. specimens were obtained. In order to evaluate the actual compression strength of the large volume of pier concrete (exceeding 300 m\({}^{3}\)), the rebound strengths of the pier concrete in the marine and land sections were measured by a rebound hammer, and then the compressive strengths were calculated using the regressed linear relationships (Figure 4) based on the measured rebound strengths, which are shown in Figure 5. The compressive strengths of the concrete in the marine and land sections also follow a normal distribution. Further, the mean compressive strengths of concrete in the marine and land sections are 56 MPa and 46 MPa, respectively. The corresponding standard deviations are 2.45 MPa and 2.67 MPa, respectively. The designed compressive strengths of the concrete in the marine and land sections are 50 MPa and 40 MPa, respectively. Hence, the actual mean compressive strength of the concrete is higher than the designed strength. However, the minimum measured compressive strength is less than the designed strength, by about 7.2%. Figure 4: Linear relationships between compressive strength and average rebound strength for concrete in the marine and land sections. (**a**) Marine section; (**b**) Land section.