Study On Water And Chloride Transport Properties In Recycled Aggregate Concrete Under Axial Compressive Loading

The recycling of waste concrete can not only effectively alleviate the resource crisis brought by excessive exploitation,but also eliminate the environmental issues caused by construction and demolition waste（C&DW）.The performance of recycled coarse aggregate（RCA）is inferior to that of natural coarse aggregate（NCA）due to the damage accumulation caused in the service environment and during the process of RCA production.In addition,the residual mortar attached to the surface of aggregate as well as the formed mutliple interface transition zone（ITZ）has a significant influence onthe durability of recycled aggregate concrete（RAC）.The coupled action of loading and corrosive ion(Cl^-,SO₄^2-and CO₃^2-,etc.)erosion is usually one of the main reasons causing the durability degradation of RAC structures serving in harsh environment.External loadings can result in the damage and cracking of concrete,and even promote the initiation and propagation of internal microcracks,providing the transport channels for water and aggressive agents and accelerating the deterioration of material performance.Therefore,it is of great significance for perfecting the durability design theory of RAC structures and the prediction of service life to study the transport properties of water and chloride in RAC subjected to loadings.Based on this,the investigation on the transport properties of water and chloride in RAC under axial compressive loading was conducted.The influence of stress level and replacement ratio of RCA on the transport process of water and chloride in RAC was analyzed.According to damage mechanics and unsaturated flow theory,the constitutive model of water and chloride transport in RAC subjected to compressive loading was established.Considering the characteristics of multiple ITZs within RAC,a mesoscale model of RAC composed of five-phase composite materials was developed,and the numerical simulation of water and chloride transport in RAC under loading was conducted.The specific contents of the research work and main conclusions are summarized as follows:（1）The experimental investigation on water and chloride transport in RAC after repeated axial compressive loading was carried out.The influence mechanisms of stress level and replacement ratio of RCA on the water and chloride transport properties were analyzed.The results show that both the cumulative mass of capillary absorbed water and chloride content within RAC increase as the stress level and RCA content increase.The damage model recommended by the Chinese codes of GB50010-2010 was adopted to establish the quantitative relationship among the damage variable,the replacement ratio of RCA and the index of transport property（sortivity and chloride diffusion coefficient）.The numerical method for calculating the distribution of water and chloride content within unsaturated damaged RAC was proposed,and the accuracy of the proposed model was verified by comparing with the experimental results.（2）An improved equipment for real-timely monitoring capillary water absorption by concrete subjected to sustained compressive loading was designed.The experiments on capillary absorption and chloride penetration into RAC under sustained loading were conducted.The influence of the sustained stress levels and the replacement ratio of RCA on the behavior of capillary absorption and chloride penetration into RAC was analyzed.Based on the obtained testing results,the fitted relationship curves between sustained stress level or replacement ratio of RCA and mass transport coefficient were respectively obtained.The results indicate that the sorptivity and chloride diffusion coefficient of RAC increase sharply when the content of RCA exceeds 50%.The chloride diffusion coefficient of RAC with 100%RCA is 2.78 times as much as that of NAC without loading.In addition,the chloride penetration into RAC is more sensitive to sustained compressive loading than that of NAC due to the higher porosity and more initial defect within RCAs.（3）The Monte Carlo approach was employed to develop a two-dimensional model of RAC with random distribution of circle coarse aggregates.Considering the characteristics of multiple ITZs within RAC,a five-phase mesoscale model of RAC composed of new mortar,new ITZ,old mortar,old ITZ and original aggregate was developed.Based on the theory of Mazars’damage model,the constitutive models of water and chloride transport in RAC considering the damage caused by sustained loading were established.The calculation method of water and chloride content within RAC was proposed.Meanwhile,in order to investigate the detailed process as well as Mesoscale simulation of the process of water and chloride transport in the modeled RAC with sustained loading damage was realized and validated by comparing with the experimental results.The studies indicate that the proposed FEM model of sustained loaded-damaged RAC can accurately describe the damage evolution,and simulate the process of water and chloride transport in RAC under sustained compressive loading.The proposed approach can provide favorable support for the investigation on the durability degradation mechanism of RAC under the coupled action of loading and environmental conditions.