Effectiveness of CFRP Confinement and Compressive Strength of Square Concrete Columns

Existing strength models for square concrete columns confined with fiber-reinforced polymer (FRP) laminates are almost all proposed on the basis of test data from relatively small-scale specimens. The possibility of extrapolating the models to represent the behavior of real-scale square columns still needs further studies. In particular, the quantitative evaluation of size effect is still a subject that requires systematic research. To address this knowledge gap, this paper reports the results of testing on 42 relatively large-scale square concrete columns (300x300x600 mm) under axial compression. The testing demonstrated that the carbon FRP (CFRP) wrapping strain around the perimeter exhibits a certain regularity, even under very high axial load. The strain efficiency factor at corner from the large-scale columns is lower than those from small specimens, and its value shows an increasing trend as the corner radius ratio increases. Based on the available data, confinement effectiveness provided by FRP wraps was quantified as a comprehensive function considering the influence of lateral confinement level, corner radius ratio, and size effect. A database of FRP-confined square columns was assembled and used to assess the predictability of typical existing models. The results show that the existing models may significantly overestimate the strength of real-size structural columns wrapped by FRP. A modified strength model for FRP-confined square columns is also presented and is shown to have good correlation with experimental results.