Time-dependent Behaviour Of Composite Steel-recycled Aggregate Concrete Slabs Considering Non-uniform Shrinkage Effects

Composite steel-recycled aggregate concrete (RAC) slabs show their advantages due to the fast construction speed and high degree industrialization.Recent research highlighted that the flexural and longitudinal shear capacities of RAC composite slabs are similar to the ones exhibited by natural aggregate concrete(NAC) members and current design approaches are still applicable for RAC composite slabs. In addition to the ultimate limit state behaviours, further studies on the serviceability limit state behaviours of RAC composite slabs are still required,and this is because that a shrinkage gradient develops through the composite slab thickness due to the inability of the concrete to dry from their underside. To date no research has been undertaken on the non-uniform shrinkage models for RAC composite slabs and current design procedures for NAC composite slabs do not consider the combined effects of concrete cracking, creep and non-uniform shrinkage. Besides, the mechanical properties of RAC are influenced by various parameters, and the high variation in RAC mechanical properties is another key factor that limits the use of RCA in structural members. For instance, the parent concrete quality and service ages may influence RAC elastic modulus and shrinkage behaviours, however, these influences are not considered in current prediction models, which affect the accuracy of these models. In this context, experimental investigations and theoretical study were conducted in this thesis, aiming to investigate the long-term behaviour of composite slabs prepared with RCA. The main work is presented as follows.(1) 189 cubes and 135 prisms were tested to investigate the influence of RAC mixing methods, RCA service ages and RCA replacement ratios on the mechanical behaviours of RAC. Test contents included compressive strength, elastic modulus,and shrinkage deformation over time. Based on the experimental results, a model was proposed to account for the influence of RCA replacement ratios on the compressive strength development of RAC. An equation to predict RAC elastic modulus was developed accounting for the influence of residual mortar content,based on two-phase composite material theory. Suggestions were provided for the structural classifications of RCA.(2) 40 autogenous shrinkage and 40 drying shrinkage prisms were tested to investigate the influence of parent concrete quality on the shrinkage properties of RAC. The design parameters included the RCA service ages, the water-to-cement ratios of parent concrete and the water-to-cement ratios of RAC. Based on the experimental investigations, both autogenous and drying shrinkage models for RAC were proposed. Particularly, the parameters of residual mortar content and water absorption were included to the RAC autogenous shrinkage model to account for the effects of RCA rigidity reduction and the “internal curing effects",respectively;while the drying shrinkage model for RAC was proposed accounting for the combined effects of residual mortar content, parent concrete strength and the different influences of RCA on the drying shrinkage behaviour of RAC with different strengths.(3) 17 shrinkage slab samples with plane size of 510 mmx600 mm were tested and the design parameters included RCA replacement ratios, slab depths and the relative humidity boundary of the slabs. The shrinkage deformation and the relative humidity distributions through the thickness of slab samples were monitored over time. Based on the experimental investigations, the nonlinear free shrinkage profiles through the thickness of slabs with their soffit sealed were proposed. Based on the equilibrium of internal force, compatibility condition and plane cross-section assumption, the linear shrinkage profiles were further derived using cross-sectional analysis by including the proposed autogenous and drying shrinkage models. As part of this chapter, a numerical model capable of incorporating the effects of non-uniform shrinkage inside composite slabs was proposed by means of cross-sectional analysis and the long-term concrete deformations were modelled by age-adjusted effective modulus method, based on which the influence of the non-uniform shrinkage on the long-term behaviour of RAC composite slabs was evaluated.(4) 7 full-scale composite steel-concrete slabs with a span of 3300 mm were tested, and the influence of RCA and non-uniform shrinkage on the time-dependent deflections was experimentally investigated. Based on MATLAB, a finite element model was developed using Newton-Rapson method to account for the time-dependent behaviour of RAC composite slabs by including the combined effects of concrete cracking, creep and non-uniform shrinkage, and the long-term concrete deformations were modelled by step-by-step methods. A systematically parametric study was then carried out to evaluate the influences of various parameters on the long-term behaviour of RAC composite slabs. The parameters included RCA replacement ratios, residual mortar content, parent concrete strength and RAC strength, etc. Following the formula form used to calculate the effective stiffness of reinforcement concrete slabs accounting for the influence of concrete cracking, a design approach to be used for routine design of RAC composite slabs under simply-supported conditions was proposed not only to consider the influence of concrete cracking caused by external loading and concrete creep, but also to account for the non-uniform shrinkage and its influence on concrete cracking.