DOI: 10.14457/tu.the.2017.991
title: Behavior of PET FRP-confined plain and reinforced concrete under compression

description: An efficient way of improving the seismic capacity of structurally deficient RC columns is retrofitting with some suitable materials. In recent years, the use of externally bonded fiber-reinforced polymer (FRP) sheets has been widely increased. An FRP jacket with a large rupture strain (LRS), for example, polyethylene terephthalate (PET) FRP, is generally more beneficial as it leads to high ductile behavior with the same degree of strength enhancement. The compressive response of PET FRP-confined concrete has been studied in circular columns, however, for non-circular columns it needs to be studied in detail. This study is aimed at developing an understanding of the compressive behavior of PET FRP-confined concrete in square and rectangular concrete cross-sections. Two experimental studies were performed at the Sirindhorn International Institute of Technology, Thammasat University. In first study, a total of 54 PET FRP-confined plain concrete circular, square and rectangular small scaled specimens were tested in uni-axial compression. The parameters considered were the number of FRP layers, corner radii and cross-section shape. In second study, the combined effect of internal steel reinforcement and externally wrapped PET FRP was investigated in detail by testing a total of 32 small square and rectangular RC column specimens. Articulated in seven parts, the dissertation presents the findings of above two experimental studies in detail.The experimental results indicated that, because of low elastic modulus, significant strength softening was observed in plain concrete square and rectangular cross-sections compared to the circular one. Unlike other commonly used FRPs (carbon FRP, glass FRP and aramid FRP) with low rupture strain and high elastic modulus, PET FRP confinement efficiently controlled the softening response by redistribution of internal damage of concrete core and displayed a hardening response until failure. PET FRP-confinement also resulted in significantly improved ultimate conditions of specimens with sharp corners and/or cross-sectional aspect ratio of 2. The lateral strains and confining stress non-uniformity were found to be significantly influenced by the cross-section shape, corner radius and number of FRP layers. In addition, the experimental results also showed that the confined concrete exhibited high values of dilation characteristics (Poisson’s ratio, dilation rate and volumetric strains), however, because of LRS, PET FRP gradually stabilized the post-peak response. Buckling of longitudinal steel bars in reinforced concrete specimens with longer unsupported length was a more noticeable event than in specimens with shorter unsupported length. However, with an increase in PET FRP confinement, the effect of bar buckling length became less significant.                      Apart from these experimental results, performance of existing strength and strain models developed for other confining materials and different cross-sections was also evaluated in determining the ultimate axial stresses and strains of PET FRP-confined plain concrete. Considering their inefficiency, a new stress-strain model was proposed which provided a good agreement with the experimental results.