Abstract
The study reported herein aims at investigating the behavior of medium-scale circular reinforced concrete columns wrapped with fiber reinforced polymer (FRP) sheets under concentric and eccentric axial loads. The experimental program was devised to assess the effects of loading conditions, absence/presence of an FRP jacket as well as the FRP wrapping system. To achieve the study objectives, four column groups were tested under axial compression at 0, 25, 50 and 65 mm loading eccentricities corresponding to eccentricity-to-diameter ratios of 0, 0.13, 0.26 and 0.34, respectively. Specimens in a fifth group were tested in pure bending simulating axial compression at infinite loading eccentricities. Three column subcategories were tested under each of the 5 loading eccentricities: unwrapped; wrapped with one ply of hoop FRP sheets; and wrapped with two FRP plies with fibers oriented at 0 and 90° to the longitudinal column axis thereby providing externally-bonded longitudinal reinforcement and hoop confinement, respectively. Tests confirmed that FRP confinement enhances the axial–flexural column resistance even at large eccentricities that exceed the balanced state of unconfined columns. Although axial column resistance decreased with increasing bending moments, relative enhancements (25–35%) in axial resistance provided by FRP confinement were found to be more significant under eccentric loading than in pure compression. Compared to hoop FRP-confined columns, using additional longitudinal sheets resulted in minor (7–9%) but stable enhancements in axial resistance that were unaffected by the increase in loading eccentricity. The FRP hoop wraps had a minor effect on the flexural resistance of specimens tested in pure bending but managed to double their resistance when combined with the externally-bonded longitudinal FRP sheets. Finally, three stress–strain models of FRP-confined concrete were used in conventional section analysis to assess the axial–flexural interaction in the FRP-jacketed columns. Strength predictions made using the stress–strain model proposed in ACI 440.2R-17 design guidelines did not agree with the test results of the eccentrically-loaded columns and underestimated the moment resistance at a given axial force even when considering higher confinement ratios than those permitted by the guidelines. Strength predictions made using eccentricity-dependent stress–strain models showed better results especially when accounting for the increase in ultimate axial strains under eccentric loading.
Highlights
External wrapping using fiber reinforced polymer (FRP) sheets has become a well-established technique that is typically used to retrofit some of the commonly encountered inadequacies of reinforced concrete (RC) columnsAl‐Nimry and Al‐Rabadi Int J Concr Struct Mater (2019) 13:53 which exhibit their largest strength along the fiber direction, are subject to the type of applied loading.Being aware that columns in real structures are expected to experience flexural loading as a result of applied end moments or at least accidental loading eccentricities, researchers have addressed the effects of different loading combinations
Hoop FRP confinement provided by the C wrapping system resulted in a minor increase of about 8% in flexural resistance accompanied with an increase in lateral deformation ability of about 41% compared to the unwrapped control specimen leading to an increase of about 58% in ductility
5 Conclusions Based on the test results of 25 medium-scale circular RC columns under axial–flexural interaction and the numerical analysis conducted in this study, the following main conclusions are drawn: 1. Reductions in axial resistance with the increase in load eccentricity were found to be less pronounced in FRP-wrapped columns than in unwrapped columns
Summary
External wrapping using fiber reinforced polymer (FRP) sheets has become a well-established technique that is typically used to retrofit some of the commonly encountered inadequacies of reinforced concrete (RC) columns. Bisby and Ranger (2010), Al-Nimry and Neqresh (2019), Chaallal and Shahawy (2000) and Pham et al (2013) have used stress–strain models derived from concentric loading tests of FRP-confined concrete to develop axial force–bending moment (P–M) interaction diagrams and have reached contradicting conclusions. Chaallal and Shahawy (2000) showed that the experimental moment capacity under a certain axial force is smaller than the theoretical value calculated using a stress–strain model derived from concentrically-loaded tests whereas Bisby and Ranger (2010) and Al-Nimry and Neqresh (2019) arrived at an opposing conclusion, i.e. the theoretical P– M interaction diagrams provide conservative predictions of the actual response. Test results are used to construct P–M interaction diagrams for the FRPwrapped columns
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