Introduction

Due to the existence of interfacial transition zone(ITZ)and pre-cracks/damage in the parent concrete,recycled aggregate concrete(RAC)usually suffers from some drawbacks,such as reduced strength,inferior durability compared with the normal aggregate concrete(NAC).As a result,the use of RAC is mainly limited to non-structural applications to date.Using RAC in composite member is an easy and promising way to avoid the unfavourable effects(e.g.reduced strength and durability)of including recycled aggregates.Following this approach,the RAC was in combination with steel and fibre-reinforced polymer(FRP)in this study to form FRP-RAC-steel hybrid column so as to compensate/suppress the unfavourable effects of the said drawbacks.In such hybrid column,FRP tube was placed outsides and steel tube inside with the concrete filling inbetween.If the steel tube is filled with concrete,the hybrid column is termed as FRP-concrete steel double-tube concrete column(DTCC)and if there is no concrete filling in the steel tube,it is termed as FRP-concrete steel double-skin tubular column(DSTC).Existing studies already showed that if NAC was used,the said hybrid column possessed many good properties such as enhanced strength and deformation capacity.This is because the following favourable interactions exist in the said hybrid columns:(1)steel tube and FRP tube provide effective confinement to the concrete between the tubes;(2)the FRP confinement together with the confined concrete supresses the local outwards buckling of the steel tubes;(3)the FRP tube can provide protection to the concrete and steel tubes against harsh environment[1].Furthermore,both the steel tube and FRP tube can work as formwork for casting concrete which leads to ease in construction.In this study,the compressive behavior of both DTCCs and DTSCs with RAC(under axial compression loading)were tested.For comparison purpose,plain concrete columns(CC),concrete filled steel tubes(CFST)and concrete filled FRP tubes(CFFT)were also tested.The test results showed that beneficial interaction also existed in the difference components(FRP,steel and RAC)of the said hybrid column members.As a result,it should be possible to use RAC in the said hybrid columns.