3.2　Load-Strain Response

Figure 4 shows the typical load-axial strain curves considering the effect of replacement ratio.It can be found that the load-strain curves of the specimens confined by the GFRP tubes presented a monotonically ascending bi-linear shape where there were no load drops and the loads kept increasing until the GFRP tubes ruptured[5].The second ascending portion of the bi-linear curve is defined as the hardening range.For DSTCs and DTCCs,the ultimate load capacity decreased with an increase in the replacement ratio of the recycled coarse aggregates and the thickness of the GFRP tubes.However,the ultimate axial strain shows the opposite trend.