Influence of concrete age on stress–strain behavior of FRP-confined normal- and high-strength concrete

Abstract The potential applications of fiber reinforced polymer (FRP) composites as concrete confinement in retrofitting existing concrete columns and in the construction of new high-performance composite columns have received significant research attention. In practical applications, the ages of concrete in retrofitted columns are significantly different from those of newly constructed columns. Without a full understanding on the influence of concrete age on their compressive behaviors, the validity of existing experimental findings, which are based the age of concrete at the time of testing, remains ambiguous when the design application lapses in time. This paper presents the results of an experimental study on the influence of concrete age on the compressive behavior of FRP-confined normal-strength (NSC) and high-strength concrete (HSC). The first part of the paper presents the results of 18 FRP-confined and 18 unconfined concrete specimens tested at 7 and 28 days. To extend the investigation with specimens with concrete ages up to 900 days, existing test results of FRP-confined concrete was assembled from the review of the literature. Based on observations from both short- and long-term influences of concrete age on compressive behavior of FRP-confined concrete, a number of important findings were drawn and are presented in the second part of the paper. It was observed that, at a same level of FRP confinement and unconfined concrete strength, the stress–strain behavior of FRP-confined concrete changes with concrete age. This difference is particularly pronounced at the transition zone of the stress–strain curves. It is found that, in the short-term, the ultimate condition of FRP-confined concrete is not significantly affected by the age of concrete. However, in the long-term, slight decreases in the compressive strength and the ultimate axial strain are observed with an increase in concrete age.