## 3 Results  
### Monitoring Results  
#### 3.1.1 General Measure Points  
After about 230 h of measurements, the data were processed with temperature compensating and then exchanged to strain [19]. The results from measuring point No. 1 (MP 1) to measuring point No. 4 (MP 4) are plotted in Figure 7. The vertical direction strain is shown by the red line while the circumferential direction strain is shown by the green line. In addition, pressure time table is a dashed \begin{table}
\begin{tabular}{c c} \hline \hline
**Load Step** & **Time (unit: h)** \\ \hline Gravity & 20 \\ Prestress & 20 \\ Inner pressure & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Load step history. Figure 6: The FEM model. (**a**) Concrete; (**b**) Tendon and rebar; (**c**) Steel liner. line in every picture. As can be seen from Figure 7, the height of pressure time table is set the same which indicates the pressure variation trend. The pressure value 0 MPa and 0.483 MPa is marked beside the pressure time table. From Figure 7a-d, it is clear that the strain in two directions had the same trend compared with pressure variation. Using FBG to monitor PCCV is feasible. Take the results of MP 1 as an example. The strain maintained almost the same at 0 \(\mu\varepsilon\) during the first 12 h, then the strain of cylindrical wall increased along with the pressure increase. At the time of pressure 0.483 MPa, the circumferential direction strain reached the maximum 200 \(\mu\varepsilon\) whereas the vertical direction strain was about 40 \(\mu\varepsilon\). This meant circumferential direction expansion was larger than the vertical direction expansion. After the highest pressure, the strain decreased along with the pressure decrease. However, there were many differences from MP 1 to MP 4 which might be caused by different location, concrete material property and so on. There was a phenomenon that the measuring strain from 121 h to 148 h was 10 \(\mu\varepsilon\) lower than the strain from 73 h to 98 h (see Figure 7a-d). However, the pressure was the same value, 0.42 MPa, during the two periods. Meanwhile, the temperature changed regularly on the order of 5 \({}^{\circ}\)C during the test. If the thermal expansion coefficient of concrete is \(1\times 10^{-5}\), 5 \({}^{\circ}\)C would cause 50 \(\mu\varepsilon\) every day, which is visible in the vertical direction strain (see Figure 7a). The strain discrepancy between 73 and 98 h and 121 and 148 h may be caused by the temperature fluctuation. However, the temperature between these two periods seemed not very large, so there may be another reason explaining this phenomenon. The reason for this phenomenon might be that the FBG was fixed on the concrete wall by glue instead of a bolt. Although a bolted connection might be a better choice, a bolt was not used in order to protect the concrete surface. Obviously, glue creep might occur during the ILRT. As a consequence, the creep led to a tiny strain on each FBG. During the last 30 h (200 h to 230 h), as shown in Figure 7a,d, the vertical direction strain decreased with value \(-30\)\(\mu\varepsilon\) and \(-100\)\(\mu\varepsilon\). Considering that the vertical direction strain changed about 50 \(\mu\varepsilon\) at highest pressure, this decrease was very huge. The main cause of this phenomenon was concrete material thermal contraction. As mentioned before, \(\alpha_{h}\) was not considered in Equation (1) for  
Figure 7: The monitoring results in vertical direction and circumferential direction. (**a**) Measuring point MP 1; (**b**) MP 2; (**c**) MP 3; (**d**) MP 4.  
simplification. Therefore, temperature compensation FBG could not compensate the thermal expansion of concrete. Fortunately, the thermal expansion or contraction strain of concrete material was relatively small when the temperature changed regularly. In site, an abrupt temperature drop of about 11 \({}^{\circ}\)C happened at between 200 h and 230 h (see Figure 8). This phenomenon caused an abrupt strain decrease as shown in Figure 8a.