Mesoscopic Numerical Simulation And Experimental Study On The Chloride Diffusivity Of Recycled Aggregate Concrete

Using of waste concrete as aggregate of new concrete (recycled concrete) can be realized the recycle of the nonrenewable resources—natural sand, stone, etc. It has attracted widely attention due to its distinguished environmental benefits, economic advantage and social returns. Current research on recycled aggregate concrete (RAC) suggests that It can meet the basic requirements of the concrete that building structures asked, such as, workability of flesh concrete and physic mechanics property of the harden concrete. Relatively low durability of RAC, however, often limit the extensive application to structural occasions in coast, de-icing salt and other harsh environments.Firstly, in this paper, the chloride diffusivity of recycled aggregate concrete was systematically investigated by experiment. Secondly, concrete including recycled concrete, was analyzed as a composite consisting of coarse aggregate, mortar and interfacial transition zone (ITZ). Because permeability and mechanical properties of each phase are not the same, their distribution in concrete will affect the permeability of concrete. The effect of mortar, aggregate content and the distribution of ITZ in concrete were studied by experiment and numerical simulation. The main research results are as follows.(1) There are several defects in the ASTM C1202accelerated chloride. A multi-channels chloride ion migration apparatus and an automatic data collection system based on TM7709were developed in this study to improve it. Some improvements are also put forward to NEL chloride ion migration apparatus.(2) By the improved test apparatus and NEL test, the chloride diffusivity of recycled aggregate concrete (RAC) was investigated. The experiment result shows that there is excellent linear relationship between charge passed and chloride diffusion coefficient from ACMT, so we can evaluate the chloride diffusivity of concrete through charge passed from specimen. It also shows that the chloride diffusion coefficients have excellent linear relationship from ACMT and NEL methods, so we can save time and labor by using of NEL method.(3) The experiment result shows that the chloride diffusion coefficients are large with the increment of the replacement rate of recycled coarse aggregate. The replacement rate of recycled coarse aggregate being not surpass25%, has a little influence on permeability of concrete, but if the replacement rate is100%, the chloride diffusion coefficient of recycled concrete is about twice as that of natural concrete. It is also found that, the water-binder ratio is higher; the chloride diffusion coefficient of concrete is larger, irrespective of binder and aggregate type.(4) An addition of activated admixture to the recycled aggregate concrete can improve property of interfacial transition zone and pore structure, concrete is compacter than ordinary concrete, and thus it can enhance the resistance to the chloride ion diffusion of the concrete. In this paper, the partial replacement with activated admixture such as fly ash, slag and silica fume is dramatically effective in decrease in the chloride diffusion coefficient. The usage of25%silica fume as partial cement substitution in the recycled aggregate concrete, the chloride diffusions of them are approximate to those of natural coarse concrete. So the partial replacement with activated admixture is effective way to enhance the resistance to the chloride ion diffusion of the recycled aggregate concrete.(5) The effect of aggregate content and the distribution of ITZ in concrete were investigated in this paper. Shaped aggregates were put into the mortar in order that the chloride diffusion coefficient of each component was obtained by using the improved accelerated chloride migration test. The result show that, the diffusion coefficient of ITZ of RAC is about7.7times as that of natural aggregate concrete, the activated admixture, fly ash, can sharply decrease the chloride diffusion coefficient of ITZ in the recycled aggregate concrete. In order to investigate the effect of coarse aggregate content on the chloride diffusion coefficient of concrete, three series concrete specimen with different coarse aggregate volume fractions were tested by ACMT. The result indicate that:to natural aggregate concrete, the aggregate content is higher, the concrete chloride diffusion coefficient is smaller and there is same phenomenon to RAC concrete with fly ash, but to RCA concrete, the aggregate content is higher, the concrete chloride diffusion coefficient is larger.(6) The effect of mesoscopic structure to the chloride diffusivity of recycled aggregate concrete was investigated by numerical simulation. Firstly based on physical engineer (PhysX.net), the mesoscopic structure models of concrete were established. The program of the model were developed where the3-D ellipsoid and spatial convex polyhedral aggregates can be random distributed and automatic generated. Based on the3-D aggregate distribution model, the chloride diffusion numerical model was established which contain mortar, aggregate and ITZ. The concrete models of chloride penetration with different coarse aggregate volume fractions, different type of binder and different distribution of ITZ were simulated utilizing the finite element method. The results are consistent with the experimental results.