Testing and analysis of basalt FRP-confined damaged concrete cylinders under axial compression loading

Fiber-reinforced polymer (FRP) composites has been widely used in the retrofitting of earthquake-damaged concrete structures. In literature, the studies on FRP-confined intact concrete columns have been well documented. However, research on FRP-confined damaged concrete columns is relatively rare. Aiming to solve this problem, the confinement performance of basalt FRP (BFRP) on pre-damaged concrete columns was investigated in the present study. In total, twenty-six concrete cylinders were constructed and tested under axial compression. The effects of pre-damage levels and number of BFRP wrapping layers on the compressive performance of the concrete cylinders were investigated. Eighteen plain concrete cylinders were preloaded at three different stress levels (i.e., up to f′co, 0.9f′co and 0.8f′co on the descending branch of the axial compressive stress-strain curve of plain concrete, f′co was compressive strength of plain concrete and was 37.7 MPa here) to represent the moderate, moderately-severe, and severe damage levels, respectively. The thin surface layer of the damaged plain concrete specimens was removed and the damaged concrete specimens were repaired using early strength cement mortar. Then, the cylinders were wrapped with BFRP sheets and retested under axial compression. The experimental results showed that compared with the plain concrete, the ultimate compression strength and axial strain capacities of the pre-damaged concrete were improved by 10-104% and 186-590% respectively after the BFRP confinement. The ultimate compressive strength and the initial elastic modulus of the BFRP-confined concrete tended to reduce with an increase in pre-damage level. Based on the test results, a modified strength expression and a strain expression were developed to predict the compressive behavior of the BFRP-confined concrete (undamaged). Based on the modified strength and strain expressions, a modified stress-strain expression was further proposed for BFRP-confined damaged concrete. This research concludes that BFRP is an effective external confining material for damaged concrete column members after earthquakes.