Study On Mixing Approach Of Recycled Aggregate Concrete

The application of recycled aggregate concrete (RAC) has environmental benefits, social benefits and economic benefits, which has profound significance to the sustainable development. Research and utilization of RAC are flourishing all over the word. However, there is not enough attention and research to mixing approach of RAC. The traditional mixing approach takes the macroscopic homogeneity of fresh concrete as the evaluation criterion for valuing equipment performance, which ignores the microscopic homogeneity. As a result, the effects and returns of improving mechanical property by means of changing the traditional mixing technology are not realized. For solving these problems, a secondary mixing technology is put forwar and studied.Firstly, the author in this paper analyzed the similarities and differences in interfacial transition zone between normal concrete and recycled concrete, find out that interfacial transition zone is the weakest link among recycled concrete matrixes. The research status of the mixing approach of RAC at home and abroad are surveyed, and the author points out that the traditional mixing technology is not perfect enough to enhance the cement’s dispersity and hydration degree, it can not improve the microscopic homogeneity of RAC either, then shows the reasons for using secondary mixing thchnology. The effect mechanisms of the secondary mixing technology on RAC establishment are studied:From the differences between recycled aggregate and natural aggregate, change the material inventory and feeding sequence, and make the surface of recycled aggregate and natural aggregate are wrapped in cement slurry of low water cement ratio, and fill in the pores of recycled aggregate efficiently, and make the interfacial transition zone dense. This not only decrease the stress concentration of surface between aggregate and slurry, but also make Ca (OH)2crystal lack enough space to growth in interfacial trasition zone. At the same time, it also makes the C-S-H gel be formed fully at the surface of aggregate, and enhance the adsorption effect with aggregate to improve the bond strength between aggregate and slurry.Therefore, the author thinks that the secondary mixing technology can enhance the microstructure of RAC and integrally enhance performance of recycled concrete.Based on theory analysis and the literature research, six kinds of secondary mixing processes are tested, based on mix proportion design method of free water cement ratio of RAC, mix proportion of RAC with recycled coarse aggregate replacement rate of25%,50%,75%,100%were determined. Under the same mix proportion and conditions, RAC were made to achieve their parameter matches by1-6#six kinds of the secondary mixing technology with strength, work performance and durable performance as evaluation index, and compared with the traditional mixing process. The experimental results show that:4#and6#secondary mixing approach are the better process,and5#secondary mixing approach is the optimal process. Under the same mix proportion,5#secondary mixing approach made the compressive strength at3days, splitting tensile strength at3days, the compressive strength at28days and splitting tensile strength at28days of RAC with recycled coarse aggregate replacement rate of25%,50%,75%,100%enhanced generally by10%-20%、-10%-10%、10%-25%、10%-40%respectively, so strength was improved obviously. Slump dropped by an average of about10mm, so there is no obvious improvement for liquidity.The chloride ion diffusion coefficient at28days dropped by5%-15%generally, so improvement of durability is good. What’s more, with the recycled coarse aggregate replacement rate drop, the improvement of strength tends to be better, liquidity tends to be worse, the drop of chloride ion diffusion coefficient tends to be bigger. In conclusion, compared with tranditional mixing approach,5#secondary mixing approach can enhance the comprehensive performance of high flowing RAC efficiently.