Experimental study of concrete-filled CHS stub columns with inner FRP tubes

An experimental study into the axial compressive behaviour of concrete-filled circular hollow section (CHS) steel columns with internal fibre reinforced polymer (FRP) tubes is presented in this paper. A total of 17 concrete filled steel tubular (CFST) columns were tested, 15 with an inner FRP tube and 2 with no inner tube. Complementary material tests and tests on 15 FRP-confined concrete (FCC) columns were also carried out. The varied test parameters included the concrete strength, the ratio of the diameter of the steel tube to that of the FRP tube, the diameter to wall thickness ratio of the inner FRP tube and the type (influencing principally the rupture strain) of the FRP. It was found that the presence of the inner FRP tube led to considerably improved axial compressive behaviour due to the greater levels of confinement afforded to the 'doubly-confined' inner concrete core; the load-bearing capacity was increased by between about 10% and 50% and the ductility was also enhanced. Greater benefits arose with (1) increasing diameter of the inner FRP tube due to the increased portion of the cross-section that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. The load-deflection responses of all tested specimens were reported, revealing that failure was generally gradual with no sharp loss in load-bearing capacity, implying that the embedment of the inner FRP tube within the concrete enables it to continue to provide a reasonable degree of confinement even after the initiation of fibre rupture; this is different to the sudden loss of confinement typically observed in FRP externally jacketed concrete columns.