Long-term Static Behavior Of Recycled Aggregate Concrete Filled Steel Tubes Under Different Stress Levels

The recycling and reuse of concrete wastes as recycled coarse aggregate(RCA) may reduce the usage of natural resources and it can also reduce the impacts on the environment. Compared with nature aggregate concrete, the recycled aggregate concrete(RAC) has lower strength and elastic modulus and higher shrinkage and creep. Encasing RAC into steel tube as recycled aggregate concrete filled steel tube(RACFST) can effectively improve the mechanical properties of the recycled concrete. The study has shown that the long-tem response of concrete filled steel tubular(CFST) is needed to be consdiered in the design. However, as a new structural member, limited attention has been paid to the creep behavior of RACFST columns. Therefore, this thesis intends to provide analytical and experimental investiagtions on the creep behavior of the RACFST columns.Existing recycled aggregate concrete creep prediction models were benchmarked against creep tests of recycled aggregate concrete filled steel tube(RACFST) to evaluate the reliability of these models in prediction of the long-term deformation of RACFSTs. An extensive parametric study was performed to evaluate the influence of creep effects on the static response of RACFSTs. Additionally, different algebraic methods were applied to the creep modelling of RACFSTs and their accuracy was investigated. Investigation shows that all the existing creep models provide similar prediction on the long-term deformation of RACFSTs with the maximum difference of 9.1%. All algebraic methods produce acceptable results, with the AAEM-B method yielding the smallest error of 4.0%.An accurate method was developed with the software ABAQUS to evaluate how the creep effects influence the static response of RACFST columns with different parameters. Investigation shows that time effects have considerable influence on the static responses of RACFST columns, leading to 38.5%-112.6% increase in the stress of the steel tubes, 8.2%-29.6% increase in the deflection of the columns, 3.1-8.7% decrease in the compressive capacity for columns under axial compressive loading. The time-dependent behavior of RACFSTs is mainly influenced by the variation on the replacement ratio of recycled coarse aggregate, load ratio, the ratio of steel area over concrete area, age at first loading and the slenderness ratio of the columns.The long-term experiments were carried out to investigate the creep behavior of RACFST members under high initial stress levels. 4 RACFST stub columns with different RCA replacement ratios were subjected to long-term axial compressive loading, while 4 companion RACFST stub columns were prepared for ultimate capacity tests. The test results show that nonlinear creep leads to the increment of 312% in the creep coefficient compared to the linear creep coefficient. The confinement effects provided by the steel tubes on the concrete core decreased during the long-term experiments due to nonlinear creep.