Behavior Of Recycled Fine Aggregate Concrete Filled Circular Steel Tubular Stub Columns Under Axial Compression

The reuse of recycled aggregates in real application can be beneficial to the development of economy,society,resources and environmental protection.Due to the significant effects of recycled fine aggregates(fine-RA)on the mechanical properties and durability of concrete,the use of recycled fine aggregates in structures has been strictly limited.Concrete filled steel tubular(CFST)columns are gaining increasing usage in practice owing to their inherent advantages,including high load capacity,anti-seismic performance and the ease of construction.Encasing recycled concrete into steel tube can reduce the negative effects induced by recycled aggregates and therefore promote the structural application for recycled aggregates.At present,there are limited researches on the mechanical properties of recycled fine aggregate concrete filled steel tubular stub columns(RFACFST)under axial loading,and the parameters considered in these researches are not enough.Besides,there is no investigation on the stress-strain relationship for the steel tube and concrete core of RFACFSTs with the consideration of confinement effects.In this context,this paper reports a further experimental investigation to reveal the effect of different parameters on the mechanical behaviors of recycled aggregate concrete filled steel tubes under axial compressive loading.The study parameters include recycled coarse aggregate(coarse-RA)substitution level,fine-RA substitution level,steel ratio,concrete strength and source of recycled concrete.The main work is presented as follows:(1)Three CFSTs and 51 RACFSTs were tested under axial compression.Based on the test results,the effects of coarse-RA and fine-RA replacement ratio,steel ratio,concrete strength and source of recycled aggregate on the mechanical properties of specimens were discussed.The confinement effect between steel tube and concrete core has been also investigated based on experimental results.The investigation results show that the recycled fine aggregate does not considerably influence the mechanical properties of RACFSTs,since the maximum difference is 13% for the mechanical properties between RACFSTs with different parameters.(2)The solid finite element model of RACFSTs under axial compressive loads has been established by ABAQUS.The predicted results by FE models were compared with the available experimental data to evaluate the adequacy of finite element model in predicting the mechanical behaviors of RACFSTs.Parameter study has been carried out to investigate the effects of coarse-RA and fine-RA replacement ratio,concrete strength and steel ratio on the mechanical properties of specimens subjected to axial loads.The ability of design codes(i.e.EC4,AISC,AIJ and GB 50936)to predict the sectional capacity of RACFST columns was evaluated based on benchmarking analysis.(3)Based on the test results,the stress-strain relationship of steel and concrete core for CFST has been modified to account for the influence of coarse-RA and fine-RA replacement ratio.This newly proposed model was benchmarked against the test and FEA results to verify its accuracy in predicting the static resp onse of RACFST subjected to axial loading.