Time-Dependent Behaviour Of Steel-Coarse/Fine Recycled Aggregate Concrete Continuous Composite Slabs

Steel-recycled aggregate concrete(RAC)composite slab is a kind of structural member with higher construction efficiency,less requirement for the mechanical properties of concrete,and extensive usage of concrete,which is an effective structural solution for applying RAC safely and efficiently.Studies showed a non-uniform shrinkage with larger top strain and smaller bottom strain exists in the concrete of composite slabs due to the sealed condition at the bottom of the slabs,which significantly increase the long-term deflection of the slabs.The application of the coarse recycled aggregate(CRA)has been found to further exacerbate the influence of the non-uniform shrinkage.Considering that the mechanism of the influence of fine recycled aggregate(FRA)is similar to CRA,the long-term properties of composite slabs with FRA is also considered to be significantly influenced.However,no investigation of the application of FRA in composite slabs has been reported.Meanwhile,the elastic modulus,tensile strength,and shrinkage are the essential mechanical properties of RAC that influence the long-term behavior of composite slabs.However,the study on the above properties is not adequate yet.For example,no investigation was performed on the possible interactive influence of CRA and FRA on the mechanical properties of RAC,no research focuses on the influence of CRA or FRA on the development trend of the shrinkage,and no autogenous shrinkage test is reported on concrete with the inclusion of FRA.Therefore,this thesis will focus on the investigation of the mechanical properties of RAC with FRA and the long-term behavior of composite slabs with RAC.(1)Experiment with the parameters of the replacement ratios of CRA and FRA(r CRA and r FRA)and the water absorption ratios of FRA is conducted to study the influence of FRA on the basic mechanical properties of RAC with different r CRA,analyze the mechanism of the influence of the FRA,and quantify the interactive influence of CRA and FRA.Based on the three-phase-compositematerial theory,considering the influence of the old cement paste and the old ITZ,adopting the volume of CRA and FRA as main parameters,prediction models for the prediction of the elastic modulus and the splitting tensile strength of RAC were established and validated using collected test data.These two models are suitable for RAC with any r CRA and/or r FRA ratios.(2)Considering the influence of r CRA and r FRA,and the water absorption ratios of FRA,an 500-day experiment is conducted to investigate the different influence of FRA on the shrinkage strain at different r CRA ratios,and the influence of FRA on the development trend with time for the concrete shrinkage;Taking the water absorption ratios and the volume content of CRA and FRA as main parameters,a new RAC drying shrinkage model with higher accuracy and universality was established and validated using collected test data.(3)Taking r CRA,r FRA,and the water absorption ratios of FRA as the main parameters,a 360-day autogenous shrinkage test is conducted to investigate the difference in the influence of FRA at different r CRA ratios.Accounting for the interactive influence of the CRA and FRA,the influence of the old cement paste on the stiffness of aggregates,and the self-curing effect,a unified autogenous shrinkage model was theoretically derived for RAC with any replacement ratio of FRA and/or CRA based on the two-phase-composite-material theory.The reliability of the model was validated using available test data.(4)Based on previous study,the internal relative humidity and shrinkage strains of concrete specimens with the dimension of 510 mm×600 mm are monitored to investigate the influence of FRA on the distribution of relative humidity and shrinkage strains of concrete specimens.Considered parameters includes the r CRA and r FRA ratios,the depths of concrete,and the surface moisture conditions.Based on the tested distribution of the relative humidity and shrinkage of double-surface-exposed specimens,the expression for the relationship between relative humidity loss and the shrinkage strain was established.Based on the measured distribution of the relative humidity of single-surface-exposed specimens,a model for the distribution of the shrinkage of single-surface-exposed RAC specimens was established by cross-sectional analysis and regression analysis.The proposed model was validated using the test data.(5)Long-term tests were carried out on two-span continuous composite slabs with total length of 6300 mm to investigate the influence of recycled aggregate and the non-linear shrinkage on the long-term properties of continuous composite slabs.In addition to the deflections and strains,particular attention was paid on quantifying the influence of recycled aggregates and non-linear shrinkage on the development of the crack width at the negative moment region.Considering the influence of non-linear shrinkage,concrete crack,and creep effect,an FEM model was established for the prediction of the long-term properties of two-span continuous composite slabs,and the model was validated using available test data.