## 3 Test Results
### Tensile Properties  
Table 2 gives the average values with standard deviations of the tensile strength (\(f\)) and elastic modulus (\(E\)) of the specimens at specific ageing time. The average values of the tensile strength retention and elastic modulus retention are plotted against ageing time in Figures 6 and 7, respectively. It can be seen that the variation of error values was small overall, which can be considered that the results of the tensile test were accurate and reasonable. It can be found from Figure 6, in the first 14 days, the tensile strength decreased rapidly by 20%-40%. Afterward, the tensile strength showed a slower decline with the increase of ageing time. After an ageing of 180 days, the tensile strength retention of specimens B\({}_{1}\), B\({}_{2}\), and B\({}_{4}\) in deionized water and alkaline solution were 34.0%, 37.7%, 40.6% and 20.1%, 26.3%, 33.1%, respectively. It can be seen that the thicker specimens had higher  
Figure 4: Degradation mechanism of epoxy resin in 60 \({}^{\circ}\)C deionized water or alkaline solution. (**a**) breaking down of epoxy molecular chains (**b**) ageing damage of epoxy resin in macroscopy. Figure 5: SEM images of (**a**) unaged specimen surface; Aged surfaces of the specimens soaked (**b**) deionized water and (**c**) alkaline solution at 60 \({}^{\circ}\)C after 180 ageing days. tensile strength retention than the thinner specimens. Figure 7 shows elastic modulus retention in deionized water and alkaline solution. The change trends of elastic modulus retention were similar to those of tensile strength retention. However, the degradation rate of elastic modulus is far less than that of tensile strength. After the ageing of 180 days, the maximum decrease of elastic modulus retention of all specimens was around 5%-20%. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Specimen Thickness**} & \multirow{2}{*}{**Ageing Days**} & \multicolumn{2}{c}{**Deionized Water**} & \multicolumn{2}{c}{**Alkaline Solution**} \\ \cline{3-6}  & & \(f\) & \(E\) & \(f\) & \(E\) \\  & & **(MPa)** & **(GPa)** & **(MPa)** & **(GPa)** \\ \hline \multirow{6}{*}{1 mm} & 0 & \(1763\pm 41\) & \(81.8\pm 0.8\) & \(1763\pm 41\) & \(81.8\pm 0.8\) \\  & 7 & \(1345\pm 34\) & \(79.1\pm  
According to the SEM images of tensile failure surfaces of unaged and aged specimens shown in Figure 8, the degradation of tensile properties of BFRP laminates mainly comes from two parts. One is the degradation of epoxy resin and the corresponding interfacial debonding between the basalt fiber and the epoxy resin. Figure 8a exhibits the tensile failure surface of unaged specimen, where the basalt fiber was perfectly bonded to epoxy resin. It means that the basalt fiber and the epoxy resin play a synergistic role in the process of stress transmission during tension for unaged specimens. At the beginning of ageing, as shown in Figure 8b, the interfacial bond between basalt fiber and epoxy resin was gradually weakened due to the hydrolysis reaction of epoxy resin, leading to partial interfacial debonding. As the increase of ageing time, the hydrolysis of epoxy resin would be more severe as shown in Figure 8c (180 days). The other is the ageing of basalt fiber. SEM images of basalt fiber before and after ageing were shown in Figure 8d-f, respectively. It can be seen that the basalt fiber was also deteriorated due to the chemical reactions. Basalt fibers are composed of SiO\({}_{2}\) with tetrahedral network structure, in which silicon and oxygen account for 27% and 44% of the total elements, respectively [47]. When basalt fiber was exposed to the solution, H\({}_{2}\)O and OH\({}^{-}\) played a significant role in the deterioration of basalt fiber. The chemical reaction formula of basalt fiber is shown as Equation (3). OH\({}^{-}\) attacks the molecular chains of SiO\({}_{2}\), causing a part of SiO\({}_{2}\) chains to relax. After that, the produced SiO\({}^{-}\) further react with H\({}_{2}\)O to produce OH\({}^{-}\) to intensify the reaction of Equation (3) [47; 48], as follows in Equation (4).