Compression behavior of expansive concrete-encased-steel filled square CFRP tubes

The concrete-encased-steel filled FRP tube (CSFFT) is a composite that enables taking advantage of combined effects of material properties of FRP, concrete and steel. Such composites outperform the conventional concrete or steel structures, by integrating outer FRP wrapping as lateral confinement and chemical barrier to concrete core and encased steel section, as well as the load-carrying capacity of concrete and steel section. While the CSFFT composites exhibit a great potential for broader applications, the issues associated with concrete shrinkages and stress hysteresis experienced in FRP confinement as observed by previous studies could weaken their performance at some cases. Therefore, to overcome these drawbacks, this study was to investigate active confinement on the CSFFT composites using expansive concrete-generated prestress. A total number of 24 concrete-encased-steel filled square CFRP tube (CSFSCT) specimens were casted and tested under axial compression. The variables studied included the number of CFRP layers (one, two and three layers), and the area of the section steel (type A and B). Test results showed that the CSFSCT failed by the rupture of the CFRP in the hoop direction. For test specimens with the same sectional area of steel and CFRP layer, the normalized ultimate load capacity of the prestressed specimen was 1.03-1.13 times that of the non-prestressed specimen, which indicated that the pre-stress effect made the tube specimens had higher strength enhancement ratio. As the number of CFRP layer increased, the ultimate load capacity and ultimate axial strain increased, and the slope of the second linear segment of the load-strain curve increased. Moreover, it was revealed that the area of the steel was effective in increasing the P-u/P-o and strain reduction factor of tube specimens. The models of ultimate stress and axial strain of confined concrete were formulated to account for both FRP-induced passive confinement and expansive concrete-generated active confinement and were validated by the experimental data.