Experimental Program

The cross-sectional sizes of specimens are shown in Figure 1 where CC,CFST,CFFT,DSTC and DTCC stands for the concrete column,concrete filled steel tubular column,concrete-filled FRP tube,doubleskin tubular column,and the double-tube concrete column,respectively.The DSTCs and DTCCs are a promising type of FRPRAC-steel hybrid columns,and the DTCCs may be taken as a variation of DSTCs[2].Totally 34 column specimens were tested subjected to axial compression considering the effects of replacement ratio of RCAs and the thickness of GFRP tube.All specimens had a height of 400 mm.

Figure 1　Sizes and designations of specimens

The RCAs were taken from waste concrete after demolition.The replacement ratios of 0,50%and 100%,respented by R0,R50 and R100,were adopted.The compressive strength of R0,R50 and R100 on 28 days are respectively 45.6 MPa,41.6 MPa and 40 MPa.The ultimate strength,yield strength,elastic modulus,and Poisson's ratio of steel section were 504 MPa,315 MPa,205 GPa and 0.3,respectively.The hollow steel columns with a height of 400 mm were also tested under axial compression to obtain the compressive or buckling strength of steel.The solid GFRP tubes were fabricated in a filament-wound process with fiber winding angles of±80°to the longitudinal axis of the tube.Two types of GFRP tube,4-ply and 8-ply,were prepared,having a thickness of 1.96 mm and 3.69 mm,respectively.The axial strength of 4-ply and 8-ply is 77.7 MPa and 83.4 MPa,respectively.The compressive strength and elastic modulus of the 8-ply tubes were higher than those of the 4-ply tubes,but the Poisson's ratio was lower.The hoop strength of 4-ply(G4)and 8-ply(G8)is 40.3 MPa and 44.2 MPa,respectively.The 8-ply tubes also exhibited higher elastic modulus and lower Poisson's ratio,when compared with the 4-ply tubes.