## 3 Test Results and Discussion
### Concrete Core  
Figure 15 presents corresponding strains in the inner core concrete of Section 1 (Figure 15a), Section 2 (Figure 15b), the outer core concrete of Section 1 (Figure 15c), Section 2 (Figure 15d) and Section 3 (Figure 15e). A growth in the internal water pressure linearly increased the strains in the inner core concrete and the outer core concrete during the initial load stage (load stage I). The strains at crown, invert and spring-line orientations of core concrete, no matter inner or outer core concrete, were almost equal. Meanwhile, the strains all show negative values owning to the contribution of the prestressing wires, which means that the concrete core was still under compression. There was no crack initiation in the concrete core surface in this load stage. The strain in the core concrete changed non-linearly as the percentage of broken wires increased during load stage II. A rapid rising in the strains of each section, except for Section 3 in the outer concrete core, appeared when the percentage reached a certain degree due to the abrupt prestress loss. The strain in the inner core concrete at the crown of each section showed a greater magnitude of change than the invert and spring-line as the percentage of broken wires increased. Taking Section 1  
Figure 15: The strain curves of core concrete: (**a**) Section 1 of inner concrete core; (**b**) Section 2 of inner concrete core; (**c**) Section 1 of outer concrete core; (**d**) Section 2 of outer concrete core; (**e**) Section 3 of outer concrete core. for example, the strain at crown turned to be a tensile strain at load step 9 while strains at invert and spring-lines were still compressive. This phenomenon reveals that breaking of the wires has the most influence on the crown than the invert and spring-line of inner concrete core. The strains at measuring point #1 and #3 of each section show a slight distinction although they were both located at the spring-line in the inner concrete core. The strain at #1 showed a sudden increase while the strain at #3 suddenly decreased to some extent during the operation of wires breakage, as depicted in Figure 15a,b. The abrupt rupture of prestressing wires can induce instantaneous stress relief around the broken points (between #1 and #4, anticlockwise, near #1) and stress localization away from the broken points (between #2 and #3, clockwise, near #3). Consequently, the strains at the spring-line of each section in the inner core concrete are not completely identical. Different from the phenomenon in inner core concrete, the strain at spring-line was larger than the crown in outer core concrete, as illustrated in Figure 15c-e. The operation of wires breakage greatly impacts on the strain at the spring-line near the broken points in the outer concrete core. In addition, as exhibited in Figure 15e, Section 3 in the outer concrete core showed a slight variation in strain, suggesting that breaking wires manually performed no remarkable influence on the area away from wires breakage. Visible cracks on the surface of the mortar coating first occurred at the pipe spring-line when the percentage of broken wires was up to 20.18% through field observation. An increase in the percentage of broken wires accelerated the crack propagation in the mortar coating. Cracks were soon found in outer concrete core at the pipe spring-line which was closer to the broken point (as shown in Figure 16a). They propagated rapidly along the longitudinal direction as the percentage of broken wires increased and induced several branches gradually. New cracks occurred simultaneously when the former cracks extended, which formed a crack band together. The appearance of these cracks intensified the damage of the pipe structure and made the pipe unable to sustain any tensile stress. Interfacial debonding between the mortar coating and outer concrete core was discovered at the spring-line (Figure 16b). Meanwhile, a loud sound was heard, which may be induced by the cracking of concrete core.