## 3 Results
### Response of Optical Fibers to Simulated Alkaline Pore Water  
The impact of the highly alkaline environment of the concrete on the optical glass fibers is summarized in Table 2 and Figure 5. The failure strain tests of the polyimide coated fiber had to be stopped after 14 days. This is due to the detachment of the polyimide coating from the glass fiber (Figure 5b). In addition, for the test on the polyimide coated fiber after been exposed to alkaline 5% NaOH solution, only three fiber samples could be measured, since all other fiber samples were already broken during mounting to the tensile testing machine. In case of the optical glass fiber with carbon coating, two different fabrication batches were evaluated. Fiber samples of the carbon coated optical glass fiber from the first fabrication batch were already consumed after 28 days of exposure to alkaline pore water. Therefore, a second batch of this fiber was ordered to at least conduct the resistant experiments to alkaline pore water over a period of one year. As can be seen in Table 2, the carbon coated optical glass fiber from the first fabrication batch did not show a clear trend towards a lower failure strain for increasing alkaline attack. However, in the case of the optical fibers with carbon \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Fiber-Type** & **10 turns [dB]** & **20 turns [dB]** & **30 turns [dB]** & **40 turns [dB]** \\ \hline
**Corning CC** & \(-\)0.06 & \(-\)0.11 & \(-\)0.2 & \(-\)0.28 \\
**OFS PI** & 0 & 0 & 0 & 0 \\
**OFS C/PI** & 0 & 0 & \(-\)0.1 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of the micro- and macro-bending test of the optical glass fibers with different protective coatings. CC: ClearCurve; PI: Polyimide; C/PI: Carbon/Polyimide. Figure 4: Fiber optic Mach-Zehnder (MZ) interferometer to characterize the sensor response from the FCS under test. coating from the second fabrication batch, a continuous degradation of the failure strain for increasing alkaline attack was observed. It is assumed that the defects in the coating from the second fabrication batch of the carbon coated optical glass fiber had allowed simulated pore water to attack the silica glass matrix of the fiber cladding. It can also be seen in Table 2 that the optical glass fiber with an acrylate coating shows continuous mechanical degradation with increasing alkaline attack. Figure 5 shows different fiber coatings after being subjected to simulated alkaline attack. As can be clearly seen in Figure 5b, the polyimide coating is detached from the fiber after only 14 days of alkaline attack. No defects on the fiber coating could be detected for the optical glass fibers with acrylate (Figure 5a) and carbon (Figure 5c) coatings. The results obtained are consistent with [24; 25], where acrylate and carbon coated fibers showed resistance against the highly alkaline environment. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Acrylate** & **Polyimide** & **Carbon/Polyimide** & **Carbon/Polyimide** \\  & & & **(Batch \#1)** & **(Batch \#2)** \\ \hline \multirow{2}{*}{**Before exposure**} & 1.27 GPa & 3.29 GPa & 2.29 GPa & 2.10 GPa \\  & N = 5 & N = 5 & N = 5 & N = 5 \\ \hline \multirow{5}{*}{**After exposure**} & 0.90 GPa & 0.59 GPa & 2.20 GPa & 1.94 GPa \\  & (14 days) & (14 days) & (14 days) & (14 days) \\ \cline{1-1}  & N = 5 & N = 3 & N = 5 & N = 5 \\ \cline{1-1}  & 0.83 GPa & & 2.23 GPa & 0.91 GPa \\ \cline{1-1}  & (3 months) & & (28 days) & (30 days) \\ \cline{1-1}  & N = 5 & N = 5 & N = 5 \\ \cline{1-1} \cline{2-5}  & & & & 0.34 GPa \\ \cline{1-1}  & & & (1 year) \\ \cline{1-1}  & & & & N = 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Change of the mean failure strain of the optical glass fibers with different fiber coating for different exposure times to highly alkaline 5% NaOH solution. N specifies the number of samples per test.