Behavior of square and rectangular ultra high-strength concrete-filled FRP tubes under axial compression

This paper presents results of an experimental program undertaken to investigate the behavior of square and rectangular ultra high-strength concrete (UHSC)-filled fiber reinforced polymer (FRP) tubes (UHSCFFTs) under axial compression. The effects of the amount of confinement, cross-sectional aspect ratio and corner radius were investigated experimentally through the tests of 24 concrete-filled FRP tubes (CFFTs) that were manufactured using unidirectional carbon fiber sheets and UHSC with 108 MPa average compressive strength. As the first experimental investigation on the axial compressive behavior of square and rectangular UHSCFFTs, the results of the study reported in this paper allows a number of significant conclusions to be drawn. Of primary importance, test results indicate that sufficiently confined square and rectangular UHSCFFTs can exhibit highly ductile behavior. The results also indicate that confinement effectiveness of FRP tubes increases with an increase in corner radius and as sectional aspect ratio approaches unity. It is found that UHSCFFTs having tubes of low confinement effectiveness may experience significant strength loss along the initial portions of the second branches on their stress-strain curves. Furthermore, it is observed that the behavior of UHSCFFTs at this region differs from their normal-strength concrete counterparts and is more sensitive to the effectiveness of confining tube. The second half of the paper presents the performance assessment of the existing FRP-confined concrete models in predicting the ultimate conditions of the HSC and UHSCFFTs. The results of this assessment demonstrate that the existing models provide unconservative estimates for specimens with higher concrete strengths. To address this, a new model that was developed on the basis of a comprehensive experimental test database and is applicable to both NSC and HSC of strengths up to 120 MPa is proposed. The model comparisons demonstrate that the proposed model provides significantly improved predictions of the ultimate conditions of FRP-confined HSC compared to the existing models. (C) 2013 Elsevier Ltd. All rights reserved.