Long-term Behavior Of Circular Recycled Aggregate Concrete-filled Steel Tubular Columns With Different Water To Cement Ratios Of Source Concrete

As a vertical load-bearing member, recycled aggregate conrete filled steel tubular(RACFST) columns have a promising future to be used in building structures. This is because filling recycled aggregate concrete(RAC) into steel tubes can improve the mechaincal properties of this kind of material. Available experiments have shown that the axial compressive loading capacities, eccentrical compressive loading capacites and cyclic loading capacities of RACFST members are similar to those of conrete filled steel tubular(CFST) members, within the r ange of 10%. Unlike the observation in ultimate tests, the incorporation of recycled aggregates can significantly influnce the long-term behaviour of RACFST members. However, limited attentions have been devoted to this area. For example, most available cr eep models only considered the effects of residual motar contents on the long-term responses of RACFST members. This thesis intends to highlight how the properties of the residual mortar, which highly depends on the water-to-cement ratio of the source concrete, also influences the creep of circular RACFST members. The main work is presented as follows.(1) Long-term tests were conducted on 5 groups(2 columns in each group) of circular RACFST specimens. The influences of recycled coarse aggregate replacement ratio(0%, 50%, 100%), water-to-cement ratio of the source concrete(0.25, 0.45) and water-to-cement ratio(0.28, 0.45) were discussed based on the measured long-term deformations. The concrete age at first loading is 28 days and the initial concrete stress level is 0.3. The adequacy of using the model proposed by our research group to predict the long-term responses of the RACFST specimens was verified using the test results.(2) Based on the predicted model considering the effects of water-to-cement ratio of the source concrete, extensive parametric studies were carried out to numerically investigate the time effects on the long-term deformations of RACFST members. Considered parameters included recycled coarse aggregate replacement ratio, the ratio of the steel area over the concrete area, the concrete strength, concrete age at first loading, residual mortar content and water-to-cement ratio of the source concrete. Additionally, different algebraic methods(effective modulus method, mean stress method and age-adjusted effective modulus method) suitable for design calculations were applied to the long-term modelling of RACFST members and their accuracies were then investigated based on the results obtained using the step-by-step procedure.(3) Time effects on the static behavior of circular RACFST columns were investigated by means of the finite element method using ABAQUS. Extensive parametric studies were then carried out to investigate the time effects on axial compressive load capacities of circular RACFST columns. Considered parameters included recycled coarse aggregate replacement ratio, the concrete strength, slenderness ratio, water-to-cement ratio of the source concrete, the steel strength, the ratio of the steel area over the concrete area and the stress level in concrete core. Additonally, designing equations were proposed based on the finite element analysis results to predict the ultimate axial compressive loads of circular RACFST columns accounting for time effects.