Repair Effects and Acoustic Emission Technique–Based Fracture Evaluation for Predamaged Concrete Columns Confined with Fiber-Reinforced Polymers

During the last two decades, the use of fiber-reinforced polymer (FRP) in the retrofitting of concrete structures has been widely studied. However, there is limited research on the effect of initial concrete damage on the retrofitting effect of FRP on concrete columns, especially when the initial damage is severe. For FRP-confined concrete columns, the fracture process inside the concrete is covered by FRP and is not visible to inspectors during the service period. This paper investigates these problems through an experimental study of 28 $100\times 100\times 300\mathrm{mm}$ concrete columns using axial compression tests and acoustic emission (AE) fracture monitoring. Prior to FRP wrapping, three different damage levels were assigned to the concrete columns. The stress-strain results showed that the predamage levels had a small effect on the ultimate strength of the FRP-confined predamaged concrete columns, even when the predamage was severe, whereas the axial/lateral strains were more affected by the predamage levels. AE parameter analysis showed that at the early loading stage, AE signals generated from FRP-confined predamaged concrete columns contained less energy than those from the FRP-confined intact concrete columns. The fracture processes of unconfined and FRP-confined concrete columns were also studied through concrete failure modes, $b$-value and improved $b$-value ($Ib$-value) analyses, and fractal analysis. The following conclusions were drawn from the analyses: (1) during compression of the FRP-confined concrete columns, the concrete failure modes shifted from cracking type to cracking-crushing mixed type; (2) the fracture process of the confined concrete columns could be divided into three stages, which are different from those for unconfined concrete columns and reinforced concrete beams; (3) the fractal dimension of the crack network had a relationship with the $b$-value and decreased from a value between 3 and 2 to a final value of 2 during the fracture process for unconfined and FRP-confined concrete columns.