A rational approach toward strain efficiency factor of fiber-reinforced polymer-wrapped concrete columns

This paper aims to provide a rational approach to the concept of circumferential strain efficiency factor kₑ of circular concrete columns wrapped with fiber-reinforced polymer (FRP) jackets. The variable kₑ reflects the reduction in the FRP hoop rupture strain observed in experiments, relative to the true rupture strain of the FRP material obtained from standard coupon tests or manufacturer datasheets. Currently, the ACI 440.2R-08 design guide uses a constant empirical kₑ factor of 0.55 for any column, regardless of geometric or mechanical properties. A rational approach is proposed to calculate kₑ. It employs a simplified Tsai-Wu interaction failure envelope of FRP laminates and acknowledges the biaxial state of stresses in the jacket. A large database of 454 circular concrete specimens wrapped with unidirectional FRP fabrics has been compiled from literature and subjected to rigorous statistical evaluations. While the proposed model performed quite similarly to the current ACI 440.2R-08 in that it did not lead to significant improvement in prediction, it has the advantage of offering analytical rational bases to the observed phenomenon. It also distinguishes between columns of different geometric and mechanical properties, leading to a variable kₑ of 0.05 to 0.98 with a 0.65 average, which is consistent with experimental observations and more logical than the constant value of 0.55.