Review of the key parameters of the international calculation guidelines for non-circular RC columns confined with CFRP composites. Comparison with medium and large-scale tests

Abstract

Several international guides offer recommendations for the calculation and design of reinforcements with fibre reinforced polymer (FRP) composites. To strengthen axially-loaded columns, the guides adopt design stress–strain models based on extensive research into small-scale specimens of plain concrete confined with FRP. However, through this type of testing, it is difficult to accurately predict the behaviour of existing full-scale reinforced concrete (RC) columns confined with FRP composites.This paper presents a review of the formulation proposed by four international guides. The study compiles the available test results on medium and large-scale rectangular RC columns confined with carbon FRP composites subjected to axial compression concentric load.The predictions of the axial strength and the ultimate strain of the FRP-confined concrete are compared to the experimental results. The minimum confinement ratio (CR) recommended in the guides to guarantee sufficient confinement level is also analysed, alongside the estimation of the effective strain in the CFRP jacket at failure. The results obtained have proven uneven.The best predictions of the concrete strength improvement have been obtained with the ACI and fib guides. In general, to comply with the minimum CR proposed by the guides fails to prevent a softening stress–strain response, frequent in the rectangular tests analysed. The more restrictive CR criteria (from the CNR and fib guide) have shown themselves to be more suitable.The ultimate strain of the confined concrete is underestimated by the guides and the errors obtained are much higher than in the prediction of the strength enhancement ratio.Despite the widespread dispersion of the results, the mean value of strain efficiency factor obtained in the tests is similar to the value of 0.55 recommended by ACI. This parameter is not easy to determine for rectangular columns, but its estimation is essential for the calculation.