Research On Action Mechanism Of Recycled Aggregate On Concrete Structure Durability

For the requirement of the sustainable development and construction of civil engineering, as a green building material, recycled concrete feasible to civil engineering, particularly to structural engineering, becomes an inevitable trend. Therefore, inspected from a durability standpoint, carbonation performance, chloride ion penetration performance and steel bar corrosion performance of recycled concrete are tested in this paper. And the steel corrosion mechanism is studied based on the microstructure of recycled aggregate and recycled concrete.Firstly, this paper established the microstructure of recycled aggregate and recycled concrete in order to determine the micro factors that influence the macro structure properties,which are water-cement-ratio and recycled aggregate mix proportion. Carbonation performance, chloride ion penetration performance and steel bar corrosion performance of recycled concrete are studied by changing water-cement ratio and recycled aggregate content to obtain necessary experimental data for reinforcement corrosion mechanism of recycled concrete. Targeting at the situation of almost not considering recycled aggregate content and few carbonation performance and steel bar corrosion study on recycled concrete, compressive strength and carbonation model of recycled concrete are suggested considering recycled aggregate content as main segment parameter. Based on the former research, this paper emphasizes on the effect factors and mechanism of reinforcement recycled concrete by establishing penetration model and pore-fracture dual-percolation model. The main research contents and achievement are summarized as follows:Considering a "recycled aggregate effect layer" as recycled aggregate performance characterization, the micro-structure model of recycled aggregate is developed. The layer volume fraction is calculated and analyzed to establish a functional relationship between recycled aggregate water absorption ratio and the effect layer volume fraction. Then, a three-phase micro-structure model of recycled concrete is developed, which suggested that the recycled aggregate, the new interfacial transition zone and cement mortar shoule be considered in the model. Through the micro-structure and macro-performance correlation dependence analysis, the relationship that is K=C (W/C, M) between macro-performance and micro-structure of recycled concrete is established, as a theoretical basis to expand the durability of recycled concrete structures.According to the theoretical basis of K*=C (W/C, M),15groups of recycled concrete specimens are disposed to test28d compressive strength and3d,7d,14d,28d,56d carbonation depth, which water-cement ratio is0.45,0.55,0.65and recycled aggregate content is0,30%,50%,70%,100%, And the influence of water-cement ratio and recycled aggregate content on compressive strength and carbonation performance is discussed. The results show that water-cement ratio has same effect on recycled concrete performance as ordinary concrete, that is compressive strength and carbonation depth increase with the increasement of water-cement ratio. But when water-cement ratio changes from0.45to0.65, the growth rate of recycled concrete carbonation depth is not as significant as normal concrete impacted by its high water absorption. On the other hand, recycled aggregate content has obvious affect on compressive strength and carbonation performance. The degree of influence depends on contrast of two conflicting factors:one is beneficial effect due to high water absorption; another is harmful effect caused by damage structure, which is related to water-cement ratio. Taking into account the interactive effect of water cement ratio and recycled content, compressive strength model and carbonation model are suggested on the condition of50%recycled aggregate content as a staging point. The reference values of the models agree well with the experimental data by error analysis.Around the reinforcement corrosion mechanism of recycled concrete,6h electric flux of15groups of recycled concrete specimens is tested, and their chloride ion permeability coefficients are calculated. Then the influence of water-cement ratio and recycled aggregate content on recycled concrete chloride ion permeability is analyzed. The results indicate that recycled concrete chloride ion permeability is medium in different proportions, which is not poorer than ordinary concrete. With the increase of water-cement ratio,6h electric flux and chloride ion permeability coefficient of recycled concrete increase, which has the same rule with ordinary concrete. And recycled aggregate content has some non-monotonic affect on recycled concrete chloride ion permeability that is related to water-cement ratio. A dual-phase composite of recycled concrete permeability model is proposed based on these. According to the model, high and low permeability phase transforms each other when a change of water-cement ratio and recycled aggregate content occurs, and the permeability coefficient of recycled concrete can be calculated by GEM theory.According to carbonization and chloride ion penetration test results, experiment on properties of recycled concrete reinforcement corrosion is carried on, and rust start time and reinforcing bar corrosion rate of15groups of recycled concrete are measured. It is shown that rust start time is impacted by water-cement ratio and recycled aggregate content in recycled concrete. When recycled aggregate content is less than50%, the effect of water-cement ratio in recycled concrete is similar to that of ordinary concrete, but the effect becomes different if recycled aggregate content is higher than50%. Besides these, with the increase of recycled aggregate quantity, reinforcement corrosion start time is postponed. Reinforcement corrosion rate increases with the increase of water-cement ratio, and the influence of recycled aggregate content on steel corrosion rate is related to water-cement ratio, depending on the two conflicting sides of recycled aggregate.Combined with dual-phase composite of recycled concrete permeability model and percolation theory, this paper presents Pore-fracture dual-percolation model of recycled concrete. Based on it, the important conclusions, that key percolation phenomenon occurs when water-cement ratio increases to0.55, while block percolation phenomenon happens when recycled aggregate content increases to52.4%, is drawn by calculating percolation threshold and verified by former test data. So steel bar corrosion mechanism of recycled concrete is clarified on condition that water-cement ratio and recycled aggregate content interactive change. And a physical model of recycled concrete reinforcement corrosion is established. On the other hand, some ideas for experience model of recycled concrete reinforcement corrosion are demonstrated in order to provide theoretical foundation for further recycled concrete steel bar corrosion research.