Effective strain of FRP for confined circular concrete columns

There is a consensus in the literature that the full material strength of fiber-reinforced polymer (FRP) cannot be utilized in FRP confined circular concrete columns and an efficiency factor is applicable to calculating the effective confinement pressure. However, large discrepancies of the efficiency factor have been reported, with values of the factor ranging from 0.274 to 1.133. An extensive experimental and analytical study was conducted in this work to reveal the causes of these discrepancies. The results indicated that the curvature and multi-axis stress condition of FRP jackets do not significantly reduce the efficiency factor, which contradicts the general understanding in the literature. Smaller factor values are generally caused by differences between the strain measurement systems used for the FRP jacket and the flat coupon, and differences in workmanship when they are made. These differences can be overcome by normalizing FRP fabrication methods and strain measurement systems. With the normalization scheme proposed here, a consistent efficiency factor of about 0.9 can be achieved for circular columns.