The impact of concrete and post-tensioning cable degradation on strain measurements of concrete containment buildings

Concrete containment buildings (CCBs) are important safety structures in nuclear power plants; however, degradation may occur in CCBs as they age. For post-tensioned CCBs, prestressing losses could occur and may affect the CCBs' performance under accident conditions. CANDU CCBs contain cement-grouted post-tensioning (P-T) cables. The grouting of P-T cables prevents direct monitoring of prestressing losses by traditional lift-off testing. Instrumented monitoring has been recommended as an indirect approach by some guidelines for integrity evaluation of CCBs with grouted prestressing systems. As part of the investigation on the relationship between instrumentation data and the integrity of CCBs, sensitivity analyses have been performed using finite element models to develop an understanding of the sensitivity of strain changes to degradation factors that contribute to prestressing losses, such as creep and shrinkage of the concrete, stress relaxation, and deterioration of prestressing systems. Strain measurements from a CANDU CCB were analysed to assess the measurement noise, which was compared with the predicted strain changes due to degradation to evaluate whether the degradation of concrete and prestressing systems can be captured by strain instrumentation. The analysis reveals that the strain changes due to degradation, except the creep and shrinkage during the early years of CCBs, were comparable with the level of noise observed in the measured strain data. Degradation mechanisms related to prestressing losses have conflicting effects on strain changes and are difficult to assess individually. Therefore, it could be difficult to detect the prestressing losses and the effect of individual degradation issues using strain instrumentation. (author)