Research On Creep Behaviour Of Coarse/Fine Recycled Aggregate Concrete Filled Circular Steel Tubes

Filling coarse and fine recycled aggregate(CRA and FRA)into the steel tube can alleviate the deteriorated mechanical properties of recycled aggregate concrete(RAC)with FRA,which was referred to as recycled aggregate concrete-filled steel tube(RACFST).Because of the improved mechanical properties,this kind of structural member has a promising future for the structure use as main bearing members in multi-level or even high-rise buildings.Because the complex confinement mechanisms for RACFST and the high scatter in the properties of RAC,the establishment of accurate and universal models for the mechanical properties of RACFST is a bottleneck problem in the use of such innovative RACFST structures.Up to date,several researchers have investigated the mechanical behaviours of RACFST with only CRA;while the research on the RACFST with both FRA and CRA is quite limited,especially for the long-term proformance.Therefore,this paper focused on the long-term behaviours of RACFST with both of FRA and CRA,investigated its effects on stability,revealed the related influencing mechanisms,and developed the accurate and universal creep models for RACFST.Meanwhile,this paper clarified the mechanisms for the effects of recycled aggregate(RA)on confinements,based on which the axial stress-strain relationship of RACFST was developed.The parametric study was then conducted.The main works were presented as follows:(1)Six(6)groups of sealed RAC specimens(totally 30 specimens)were prepared to investigate the basic creep of RAC,with the main parameters of FRA replacement ratio(r_F=0%,50%and 100%)and water-to-cement ratio(w/c=0.3 and0.6).With the help of the existing test data by our research group,this study focused on the influence of FRA on the basic creep of RAC with different w/c ratio and revealed the related mechanisms.Based on the two-phase-composite-material theory,the basic creep model of RAC was derived with the both consideration of compounding effect between FRA and CRA and the difference of FRA influence with different w/c.The model validation was then conducted against the test data with different loading age,concrete strength grade and replacement ratio of RA.Experimental results showed that the FRA influence on basic creep was different for the RACs with different w/c.(2)Experiments with the main parameters of CRA replacement ratio(r_C=0%,50%and 100%)and the FRA replacement ratio(r_F=0%,50%and 100%)were conducted to study creep of RAC under drying atmosphere.This study mainly focused the compounding effect between FRA and CRA as well as the RA influence on the creep development with time,revealing the related influencing mechanisms.Based on the two-phase-composite-material theory,the creep model was established considering the above effects.The model validation was then performed.Experimental investigation showed that the FRA and CRA had a pronounced compounding effect on drying creep and the creep development trend was significantly affected by the both inclusion of FRA and CRA.(3)Eighteen(18)groups of RAC prisms were prepared to investigate the CRA replacement ratio(r_C=0%,50%and 100%)and the FRA replacement ratio(r_F=0%,50%and 100%)on the compressive responses of RAC,focusing on the compounding effect between FRA and CRA,and revealing the influencing mechanisms of FRA and CRA.Based on the crack zone theory by Bazant and two-phase-composite-material theory,this study established a universal model to predict the stress-strain relationship of RAC with FRA,with the consideration of stiffness reduction of recycled aggregate(RA)and the deteriorated restraint to cracking provided by RA.The model validation was then performed based on the available test data.According to the test results,the FRA and CRA had a pronounced compounding effect on peak stress,peak strain and deformability.(4)With the parameters of CRA replacement ratio(r_C=0%and 100%),the FRA replacement ratio(r_F=0%and 100%)and concrete strength grade(C35 and C50),this study conducted experiments on the 6 groups of 18 RACFST specimens,mainly focusing on the FRA effect with different concrete strength grades and quantifying the FRA effect with different r_C.Based on the available cross-sectional analysis method and time effect analysis approach,the analysis method for the long-term deformation of RACFST was established.The parametric study was then performed.Investigation results showed that the FRA and CRA had a significant compounding effect on the long-term deformation of RACFST,and the FRA effects were different for RACFST with different concrete strengths.(5)With the deep analysis of the existing test data,the compounding effect between FRA and CRA on the compressive responses as well as the RA effect on the confinement effect of RACFST were studied.Based on this,the longitudinal stress-strain model of RACFST with FRA was developed.With the introduction of such stress-strain model of RACFST and creep model of RAC into ABAQUS,the finite-element model for the creep buckling behaviour RACFST with FRA was established.The parametric study was then performed to investigate the influence time effect on the stability of RACFST with different RA replacement ratio,sustained load level,slenderness ratio,strength grade of concrete core,loading age and steel ratio.Design recommendations were then presented according to the results of parametric study.Investigation results showed that within the limitation of slenderness ratio(λ≤80),the buckling load capacity of RACFST was decreased by up to 8.3%due to the prebuckling deformation induced by time effects.