Design method for concrete columns strengthened with prestressed CFRP sheets

To improve upon the bearing capacities of short circular concrete columns strengthened with carbon fiber reinforced polymer (CFRP) sheets, this paper conducted experimental and numerical investigations to study the influencing factors on the compressive strength of CFRP-strengthened concrete columns, including the preloading of columns, the prestressing applied to CFRP sheets and the number of layers of CFRP sheets. The test columns consisted of an un-strengthened control column in addition to 10 preloaded and non-preloaded columns strengthened in compression with one or two CFRP layers at prestress levels of either 300 MPa or 600 MPa. These columns were tested under an axial load. The failure modes, stress changes, bearing capacities and load-longitudinal displacement behaviors of the tested specimens were recorded and discussed. Furthermore, the finite element models (FEM) of 10 strengthened columns were developed and validated by comparing the load-longitudinal displacement curves obtained through finite element analysis (FEA) with those retrieved experimentally. A parametric study was subsequently performed using the validated FEM to investigate the effects of preloading, prestressing and the number of CFRP layers. The parametric study demonstrated that the strain lag caused by preloading was negatively related to the compressive strength of the CFRP-strengthened columns. In addition, prestressing could reduce the negative effects of preloading and could improve on the utilization of CFRP tensile strength. Furthermore, the yield load of CFRP-strengthened concrete columns was improved correlative with an increment of the number of layers and the prestress levels of the CFRP sheets. Conversely, whereas the ultimate load only improved with an increment of the number of layers of CFRP sheets, it had no obvious relationship with the prestress levels of the CFRP sheets and was instead related to their ultimate tensile strength. Based on the test data, the bearing capacity formulas for preloaded and non-preloaded short circular concrete columns strengthened with prestressed CFRP sheets were proposed. Subsequently, the presented formulas were confirmed to be reasonably accurate by comparing the bearing capacities calculated numerically with those measured experimentally.