## 3 Test Results and Discussion  
### Results Revised  
Hoop compressive strains were produced in the core concrete owning to the contribution of the prestress applied by prestressing wires. Therefore, the hoop tensile strain caused by internal water pressure in each component of the PCCP will be balanced by the compressive prestrain. In other words, only when the tensile strain is larger than the compressive prestrain will the real tensile strain be measured. Therefore, the real strain state of the core concrete and prestressing wires should be revised by the following equation:  
\[\varepsilon_{r}=\varepsilon_{i}+\varepsilon_{m} \tag{1}\]  
Figure 12: Arrangement of the measuring points for each strand wire.  
Figure 13: Data acquisition system.  
where \(\epsilon_{r}\) is the real strain of the component, \(\epsilon_{i}\) is the initial prestrain caused by prestress, \(\epsilon_{m}\) is the strain measured by strain gauges. Moreover, positive values represent the tensile strain and negative values represent the compressive strain.  
According to ANSI/AWWA C304-2014 [15], the state of stress with a single layer of prestressing is calculated by the following equations.  
The initial prestress in the core concrete:  
\[f_{ic}=\frac{A_{s}f_{sg}}{A_{c}+n_{i}A_{s}+n_{i}^{\prime}A_{y}} \tag{2}\]  
The initial prestress in the prestressing wire:  
\[f_{is}=-f_{sg}+n_{i}f_{ic} \tag{3}\]  
The final prestress in the concrete:  
\[f_{cr}=\frac{f_{ic}\big{(}A_{c}+n_{r}A_{s}+n_{r}^{\prime}A_{y}\big{)}-\big{(}A _{s}E_{s}+A_{y}E_{y}\big{)}\text{s}-A_{s}Rf_{sg}}{A_{c}+\big{(}n_{r}A_{s}+n_{r} ^{\prime}A_{y}\big{)}(1+\varphi)} \tag{4}\]  
The final prestress in the prestressing wire:  
\[f_{sr}=f_{is}+\ Rf_{sg}+\frac{A_{c}\big{(}f_{ic}\varphi n_{r}+E_{s}s\big{)}- RA_{s}f_{sg}n_{r}(1+\varphi)}{A_{c}+\big{(}n_{r}A_{s}+n_{r}^{\prime}A_{y} \big{)}(1+\varphi)} \tag{5}\]  
The final prestrain in the concrete:  
\[\epsilon_{cr}=\frac{f_{cr}}{E_{c}} \tag{6}\]  
The final prestrain in the prestressing wire:  
\[\epsilon_{sr}=\frac{f_{sr}}{E_{s}} \tag{7}\]  
where  
\(A_{s}\)--total area of prestressing wire (mm\({}^{2}\)/m),  
\(A_{c}\)--core concrete area (including steel-cylinder area, mm\({}^{2}\)/m),  
\(A_{y}\)--steel cylinder area (mm\({}^{2}\)/m),  
\(f_{sg}\)--gross wrapping stress in prestressing wire (MPa),  
\(n_{i}\), \(n_{r}\)--modular ratio of prestressing wire to core concrete at wrapping and at maturity,  
\(n_{i}^{\prime}\), \(n_{r}^{\prime}\)--modular ratio of steel cylinder to core concrete at wrapping and at maturity,  
\(E_{c}\), \(E_{s}\), \(E_{y}\)--design modulus of elasticity of core concrete, prestressing wires and steel cylinder,  
s--design shrinkage strain for a buried pipe,  
\(\varphi\)--design creep factor for a buried pipe,  
\(R\)--wire-relaxation factor for a single layer of prestressing.  
The compressive prestrain in the core concrete caused by prestress is \(-227\times\)10\({}^{-6}\). The tensile prestrain of prestressing wires is \(4492\times 10^{-6}\). It should be noted that the data recorded were all revised and the strains given in this paper are the real strains of each component.  
To assess the state of the PCCP, the onset of micro-cracking in the concrete core is when the strain of core concrete [15] reaches \(1.5\epsilon_{t}^{\prime}=1.5\times\frac{0.52\sqrt{f_{c,b}}}{E_{c}}=207\times 1 0^{-6}\), where \(\epsilon_{t}^{\prime}\) and \(f_{c\cup b}\) represents the elastic strain when the stress of the core concrete reaches the design value of tensile strength, and the standard compressive strength of core concrete in N/mm\({}^{2}\), respectively. Moreover, the standard compressive strength of core concrete, \(f_{\text{c,b}\times}\), is equal to 1.25. f\({}_{c}^{\prime}\), in which f\({}_{c}^{\prime}\) stands for the design 28-dayscompressive strength of core concrete in American Society for Testing Materials(ASTM) [19]. Once the strain reaches \(11\epsilon_{t}^{\prime}=1522\times 10^{-6}\), visible cracks are likely to appear.