Simulation And Life Prediction Of Chloride Transporting In Recycled Aggregate Concrete In Chloride Environment

Recycled aggregate concrete（RAC）is a type of concrete which using recycled aggregate to replace natural aggregate partially or completely.Extensive use of recycled concrete in the field of civil engineering can not only recycle construction waste and reduce environmental pollution,but also alleviate the current situation of increasing demand for natural aggregate in the construction industry,which is in line with the national concept of green and sustainable development.However,compared with ordinary concrete,the mechanical and durability properties of recycled concrete are both poor.In particular,recycled concrete serving in coastal areas is more susceptible to chloride erosion,and descreasing its durability.In order to comprehensively understand the transmission of chloride ions in recycled concrete,transmission mechanism of chloride ions in recycled concrete was used to establish the transmission model of chloride ions under different service states.Simulation studies of chloride ion transport in non-destructive recycled concrete,loading recycled concrete and cracked recycled concrete are systematically carried out at the mesoscopic level.At the same time,the influence of relevant parameters on the resistance to chloride ion corrosion of recycled concrete was analyzed.On this basis,the durability life of recycled concrete servicing in the marine environment is predicted.The main research contents and results are as follows:（1）Simulation of chloride ion transport in recycled concrete.Considering the random distribution of recycled aggregate in concrete,a five-phase composite recycled concrete model including new mortar,new mortar-old mortar interface transition zone（NITZ）,old mortar,old mortar-natural aggregate interface transition zone（OITZ）and natural aggregate was established at the mesoscopic level using Matlab software.Based on Fick’s second law,the simulation of chloride ion transport in recycled concrete under saturated state is carried out,and then compared with the existing test results.Finally the influence of relevant parameters on chloride ion transport in recycled concrete is analyzed.On this basis,a multiphase composite recycled concrete model considering surface strengthening of recycled aggregates at the mesoscopic level was established,and simulations of chloride ion transport inside recycled concrete were carried out.The model was validated using relevant experimental data.The results show that the simulation results are in good agreement with the experiment results.The transport rate of chloride ions in recycled concrete increases with the increase of the replacement rate of recycled aggregate,the diffusion coefficient of the old mortar,the thickness of ITZ and the diffusion coefficient of the nano-coating layer.When the replacement rate of recycled aggregate increased from 0%to 50%and 100%,the corrosion depth of chloride ions at the critical concentration increased by 18.18%and 68.18%,respectively,and the chloride ion concentration at the steel bar location increased by 40.68%and 117.97%,respectively.（2）Simulation of chloride ion transport in loading recycled concrete.Considering the influence of load on concrete porosity and the change of concrete porosity on chloride ion transmission rate,the relationship between load and concrete volumetric strain,volumetric strain and concrete porosity,and porosity and chloride ion diffusion coefficient was analyzed respectively,thus the correlation equation between loading and chloride ion transport rate is derived.On this basis,a mesoscopic transport model of the five phase composite recycled concrete was established.And the parameter sensitivity analysis was carried out.The results show that the simulation results are in good agreement with the existing domestic experimental data.Under the action of low horizontal tensile stress,the chloride ion corrosion resistance of recycled concrete decreases with the increase of tensile stress;under the action of low horizontal compressive stress,the chloride ion corrosion resistance of recycled concrete increases with the increase of compressive stress.However,the chloride ion corrosion resistance of recycled concrete under both stresses has a small change.Increasing the initial porosity of recycled concrete,the replacement rate of recycled aggregate and the thickness of ITZ can accelerate the transmission rate of chloride ions in recycled concrete.When the initial porosity increases from 0.1 to 0.2 and 0.3,the erosion depth increases by 214.53%and 413.91%,respectively.（3）Simulation of chloride ion transport in cracked recycled concrete.Based on the chloride ion transport mechanism in cracked recycled concrete,a chloride ion transport model in cracked recycled concrete related to parameters such as crack width and spacing was established.The results show that the numerical simulation results are in good agreement with the existing experimental data of the research group.The larger the crack width,the lower the tortuosity and the smaller the spacing,the worse the chloride ion erosion resistance of recycled concrete.The closer the distance to the crack,the smaller the effect of increasing the replacement rate of recycled aggregate,the adhesion rate of old mortar and the thickness of ITZ on the transmission rate of chloride ions in recycled concrete.When the adhesion rate of old mortar was 10%,20%,30%and 40%,the chloride ion concentration at crack increased by 0.64%,0.52%,0.42%and 0.36%than the non-crack position respectively.（4）Durability life prediction of recycled concrete in chloride environment.Taking steel depassivation as the durability limit state of recycled concrete in chloride environment,based on Monte Carlo method and reliability theory,a prediction model for the durability life of recycled concrete in chloride environment is established,and the calculation is carried out according to statistical parameters.The results show that reducing the replacement rate of recycled aggregate,load level,old mortar adhesion rate and ITZ thickness can prolong the durability life of recycled concrete.When the recycled aggregate replacement rate is 0%,30%,50%and 100%,the durability life of recycled concrete is 56a,49a,46a and 46a respectively.The use of nano-Si O₂to strengthen recycled aggregates can significantly increase the durability life of recycled concrete,and 0.4%content of nano-Si O₂can increase the durability life of recycled concrete most.