## 3 Test Results  
### Polarisation Curve Test Results  
Figure 4a-d shows that, from 0 to 90 days, the corrosion potential moved in a positive direction, from \(-0.75\) to \(-0.55\) V. From 90 to 180 days, the corrosion potential shifted from \(-0.55\) to \(-0.53\) V. From 180 to 270 days, the corrosion potential changed from \(-0.53\) to \(-0.56\) V. From 270 to 360 days, the corrosion potential changed from \(-0.56\) to \(-0.60\) V. From 360 to 450 days, the corrosion potential changed from \(-0.60\) to \(-0.52\) V. Finally, from 450 to 2160 days, the corrosion location moved in both the positive and the negative directions. The corrosion potential of group B also moved during the whole immersion period. It can be seen that the corrosion potential of groups A and B moved in both positive and negative directions during the whole immersion period. These positive and negative movements of the corrosion potential reflect the difficulty in corroding (coated) steel bars. The positive movement indicates that corrosion is more difficult, while the negative movement indicates that corrosion is easier [31; 32; 33]. Figure 5 shows that the anodic polarisation curve is steep and that the cathodic polarisation curve is gentle during part of the immersion cycle, but the anodic polarisation curve becomes gentle, and the cathodic polarisation curve becomes steep as the immersion progresses. The anode polarisation curve is slightly flat, indicating that the Tafel slope is somewhat small, while the cathode polarisation curve is steep, indicating that the cathode Tafel slope is somewhat large. That is, there is slight resistance to anodic dissolution, indicating that the coated steel bar is in an easily corrodible state. A slightly steep anodic polarisation curve indicates a slightly larger Tafel slope, while a gentle cathodic polarisation curve indicates a slightly smaller Tafel slope. That is, there is slightly greater resistance to anodic dissolution, indicating that the coated steel bar is in a state in which corrosion is difficult. \begin{table}
\begin{tabular}{c c c c c} \hline \(i_{\text{corr}}\) (\(\mu\)A\(\cdot\)cm\({}^{-2}\)) & \(i_{\text{corr}}\) \textless{} 0.1\(\quad\) & \(0.1\leq i_{\text{corr}}\) \textless{} 0.5\(\quad\) & \(0.5\leq i_{\text{corr}}\) \textless{} 1\(\quad\) & \(i_{\text{corr}}\) \textgreater{} 1\(\quad\) \\ \hline \
\begin{tabular}{c} rusting \\ condition \\ \end{tabular} & no corrosion & low corrosion &
\begin{tabular}{c} medium \\ corrosion \\ \end{tabular} & serious corrosion \\ \hline \end{tabular}
\end{table} 
Table 4: Corresponding relationship between corrosion current density and corrosive degree of rebar. Figure 3: Fourier transform infrared spectrometer. Figure 4: Polarisation curves for group A specimens. Figure 5: _Cont._  
For group A, the corrosion mechanism is as follows [34]:  
\[\mathrm{Fe^{2+}+2 Cl^{-}+4H_{2}O\to FeCl_{2}\cdot 4H_{2}O} \tag{1}\]  
\[\mathrm{FeCl_{2}\cdot 4H_{2}O\to Fe(OH)_{2}+2 Cl^{-}+2H_{2}O} \tag{2}\]  
When oxygen is sufficient, Fe(OH)2 is oxidised to Fe(OH)3, and Fe(OH)3 is dehydrated to form Fe2O3 (red rust):  
\[4 Fe(OH)_{2}+O_{2}+2H_{2}O\to 4 Fe(OH)_{3} \tag{3}\]  
\[2 Fe(OH)_{3}\to Fe_{2}O_{3}+3H_{2}O \tag{4}\]  
In contrast, under the condition of insufficient oxygen, Fe(OH)2 is oxidised insufficiently to form Fe3O4 (black rust):  
\[6 Fe(OH)_{2}+O_{2}\to 2 Fe_{3}O_{4}+6H_{2}O \tag{5}\]  
Therefore, the continuous accumulation of steel corrosion and the continuous penetration of chloride ions into the corrosion layer further led to the positive and negative movement of corrosion potential and the changes in the slopes of the anode and cathode polarisation curves [35; 36]. For group B specimens, the protection provided by asphalt to reinforcements is mainly a barrier function; that is, the asphalt prevented direct contact between Cl\({}^{-}\) ions and reinforcements and prevented them from accelerating the corrosion of reinforcement.