## 3 Test Results and Discussion
### Concrete Core
The maximum width of cracks in the outer concrete core at the spring-line was 2.2 mm via field observation, as plotted in Figure 17.  
Figure 16: Phenomena of test pipe after wire breakage. After the operation of wire breakage, the subsequent load stage (load stage III) decreased the internal water pressure to the artesian pressure (0.2 MPa). The strains in the core concrete showed a decline with the water pressure decreasing, apart from the measuring point #1 in the outer concrete core. The occurrence of cracks and the dynamic changes of the internal water pressure led to the redistribution of the stress in the test pipe. As a result, the strain in #1, which was at the spring-line around the broken point in the outer concrete core and in the area where cracks were densely distributed, showed a slight rise in this load stage. On the basis of field records, the micro-cracks in the concrete core and mortar coating displayed a slight closure. However, the maximum width of cracks in the outer concrete core at the spring-line was still 1.2 mm. The fourth load stage was the tensioning operation of the strands. Some fluctuation appeared on the strain curves of the core concrete, due to the stepwise tensioning and some inevitable artificial disturbance. In general, the overall changing tendency of the strains in the core concrete was evident. The strains in the core concrete all showed a drastic drop during the process of tensioning. The maximum width of cracks in the outer concrete core at the spring-line was reduced from the former 1.2 mm to 0.1 mm (Figure 17). Most visible cracks show a closure property and are eventually difficult to find. The final load stage (load stage V) involved increasing the water pressure again until the pressure reached the design value, aiming to verify the strengthening effect of the external strands. The comparison of the strain in each section of the core concrete before and after the tensioning operation was given in Figure 18. The strain of each measuring point when the water pressure was again up to 0.6 MPa was far lower than the strain before strengthening. Although the strain at each measuring point showed a slight growth with the continuing increase in water pressure till load step 24, the strain level was still much lower than the level when load step 10 finished. The maximum width of the cracks in the outer concrete core at spring-line almost remained invariant at approximately 0.1 mm as the pressure increased. This phenomenon indicates that the prestressed strands can not only constrain the crack propagation, but also actively restore the prestress loss due to the wires breakage. In addition, the concrete core was restored to be compressive again. The water tightness of test pipe was still in a good situation according to the field observation. Figure 17: The change of maximum width of cracks in the outer concrete core. The strains of the core concrete were all no more than \(207\times 10^{-6}\), which means that the cracks did not pass through the areas where the measuring points are attached when they propagate. Although the strain gauges did not capture the initiation and propagation of cracks, the values of the strain gauges are still able to reflect the actual overall condition of the test pipe.