Reliability assessment of axially loaded uPVC tube confined reinforced concrete columns

Un-plasticized polyvinyl chloride (uPVC) tubes are increasingly used as stay-in-place formwork and protection for reinforced concrete columns from chemical attacks, peeling of concrete cover and corrosion of steel in a harsh environment. The provision of uPVC lateral confinement to the concrete improves the strength, ductility and energy absorption of the columns. However, since there is no available design guideline for uPVC tube confined concrete columns, the contribution of uPVC confinement to the column resistance has not been considered during the design stage. In this study, the strength reduction factor 4) provided by ACI 318-14 for a reinforced concrete column with circular tie reinforcement is modified to account for uPVC confinement for application in design. The subset simulation method was applied to assess the effect of uPVC confinement on the reliability of uPVC confined concrete columns. Based on the reliability study, a new strength reduction factor was proposed for uPVC confined reinforced concrete columns. Additionally, the effect of column variables on the reliability index was also investigated. The result shows that the additional uncertainties associated with the uPVC material properties and the peak strength model have an effect on the strength reduction factor. Therefore, the strength reduction factor needs to be reduced to achieve the same reliability level of the unconfined column. However, for the same design load used, the reliability index and strength reduction factor increased from 4.03 to 4.53 and 0.65 to 0.75, respectively as the 2t/D ratio increases from 0 to 0.04. Therefore, a strength reduction factor (phi) of 0.75 can be used instead of 0.65 to design a reinforced concrete column according to ACI 318-14 if the uPVC tube is going to be used as stay-in-place formwork; otherwise, the new proposed model can be used to calculate the strength reduction factor and design a uPVC confined concrete column according to ACI 318-14.