Long-term Behavior Of Recycled Aggregate Concrete Filled Steel Tubuluar Stub Columns

Reusing waste concrete as a source of aggregate in producing the recycledaggregate concrete (RAC) can alleviate the burden of landfilling while conservingthe natural resources. The concept of putting RAC into the steel tube makes itpossible to expand the use of RAC to structures. Such member will increase theultimate capacity of RAC while its shrinkage and creep decreases due to theconfinement effects and the sealed state provided by the steel tube. As a new type ofstructural member, limited attention has been devoted to the long-term behavior ofrecycled aggregate concrete filled steel tube (RACFST). This thesis intends toprovide the experimental and numerical analysis on the time-dependent responses ofRACFST stub columns under axial compressive loading.As part of this work, existing concrete models proposed particularly for RACwas benchmarked against51available long-term tests to evaluate the reliability ofthose models in predicting the long-term response of RAC. Based on the analysisresults, the important factors which influence time effects of such member havebeen specified to propose a modified model with the capability of considering theproperties of the original concrete. The new proposed model has an acceptableaccuracy in predicting the long-term response of RAC.The long-term experiments were conducted to investigate the creep behaviourof RAC sealed inside steel tube. For this purpose,12RACFST short columns withdifferent recycled coarse aggregate replacement rate were subjected to the samelevel of sustained axial loads applied at different concrete ages. The modified modelproposed by this thesis for predicting the creep effects of RAC was validated usingthe test results. Parametric analysis was then performed using this model toinvestigate the time effect of the RAC core on static response of RACFST members.At the completion of the long-term tests, specimens were tested to failure toevaluate the time effect on their ultimate responses.The time effect on the creep buckling behaviour of RACFST columns wasevaluated by the theoretical calculation and the recommendation for the designprogram has also been provided. In addition to this, an extensive parametric studywas performed to investigate how creep deformation influences the stability of such members. Considered parameters include the slenderness ratio, the ratio of the steelarea over the concrete area, the concrete age at first loading, the duration of thesustained load, the concrete strength, the recycled coarse aggregate replacementratio and the steel yield strength. Based on the analysis, the simplified formulationfor the bearing capacity of RACFST members was proposed taking the time effectinto account.