Research On Mixture Ratio Design And Physical And Mechanical Properties Of Recycled Concrete

Renewable resource utilization of construction waste is an important aspect of achieving sustainability, developing the circular economy and building a conservation-minded society for construction industry. The cement and concrete of annual production and consumption in China nearly accounts for half of the world, at the same time, with the development of urbanization in our country, construction waste emissions increased year by year, so the research of recycled concrete is of great significance for saving resource, protecting environment and realizing the sustainable development of construction industry. At present, there is more research on the compressive properties of recycled concrete. But because of the limitation of the technical and economical, there is less research on its tensile and flexural performance. The purpose of this paper is to research the mix proportion design of recycled concrete and its physical and mechanical performance, and provide some references for the study of high strength and high performance recycled concrete.Recycled coarse aggregate is gained by breaking discard waste concrete and clay brick, some physical properties of the recycled coarse aggregate and natural stone are determined such as particle size gradation, apparent density, water absorption and crushing index. Besides, the physical properties of the recycled aggregates are compared with those of the natural stone, and the reasons causing these differences are analyzed.First of all, in order to study the compressive behavior of recycled concrete at different strength grade, under the condition that without prewetting of recycled coarse aggregate and adding the high efficiency water reducing agent, this thesis respectively designs concrete specimen of strength grade C30, C40, C50, which needs to design 6 kinds of recycled coarse aggregate replacement ratio for each strength grade, namely, 0, 20%, 40%, 60%, 80% and 100%. The ordinary concrete without adding fly ash is taken as the benchmark concrete. At the same time, in order to improve the flow properties of concrete, high quality of fly ash is used to replace partial cement, and the replace rate of fly ash is 20% in this study. The results show that the concrete specimen strength of strength grade of C30 first decreases and then increases; for the concrete specimens of strength grade of C40, when the recycled aggregate replacement ratio is in the 0 ~ 40% and 40% ~ 40% range, the compression strength of concrete first increases and then decreases; for strength grade of C50, the specimen strength that recycled coarse aggregate content is under 40% improves with increasing replacement ratio, but the specimen strength that recycled coarse aggregate content is more than 40% reduces with increasing replacement ratio. Therefore, considering the workability, strength, and economic factors of concrete, this study will use the recycled aggregate replacement ratio of 60% to carry out subsequent experiment.Secondly, in order to study the tensile performance of recycled concrete at the different strength grade, under the condition that with the recycled aggregate replacement ratio of 60% and adding the high efficiency water reducing agent, this paper designs three recycled concrete specimens of the strength grade of C30, C40, C50. Besides, their splitting tensile strength and cubic compressive strength are made a experimental research, which the ordinary concrete without adding fly ash is taken as the benchmark concrete. Experimental results show that at the same strength grade, recycled concrete can achieve and even exceed that of the splitting tensile strength of ordinary concrete; the ratio of splitting tensile strength and cubic compressive strength is close between the recycled concrete and normal concrete; with increasing the strength grade of concrete, the ratio of splitting tensile strength and cubic compressive strength gradually decreases.Finally, in order to study the flexural performance of recycled concrete at the different strength grade, under the condition that with the recycled aggregate replacement ratio of 60% and adding the high efficiency water reducing agent, this paper also designs three recycled concrete specimens of the strength grade of C30, C40, C50. Besides, their flexural strength and cubic compressive strength are made an experimental research, which the ordinary concrete without adding fly ash is taken as the benchmark concrete. The results show that at the same strength grade, recycled concrete can achieve and even exceed that of the flexural strength of ordinary concrete; the ratio of flexural strength and cubic compressive strength is close between the recycled concrete and normal concrete; with increasing the strength grade of concrete, the ratio of flexural strength and cubic compressive strength gradually decreases.