Experimental Study On Mechanical Properties Of Mixed Recycled Aggregate Concrete At Multiple Ages

With the growth of waste concrete and the increasing prominence of environmental problems,research and utilization of recycled concrete has gradually shown its importance.At present,the research of recycled concrete technology is still in the initial stage,so it is necessary to carry out a deeper research on recycled concrete.It is well-known that coarse aggregate acts as a framework in concrete and the strength of the framework determines the strength of concrete.In order to improve the strength of skeleton in recycled concrete,a new concept was provided to improve the composite of coarse aggregate for RAC by mixing large particle natural aggregate with small particle recycled aggregate,in which the particle size of recycled aggregate is made less than that of mother aggregate in waste concrete.Meanwhile,in this new RAC,only the fine recycled aggregate is used as the fine aggregate.To explore the developments of compressive strength,axial compressive strength,rupture strength and splitting tensile strength of this new RAC,640 groups of cubic specimens were tested at different curing ages from 3 days to 360 days.The main research contents include:(1)Experimental study on material properties of mixed recycled aggregate.The mass ratio of natural aggregate with large particle size and overall coarse aggregate is 0% ~ 70%.The apparent density,bulk density,particle gradation,crushing index and water absorption rate of mixed recycled aggregate are studied.The results show that with the increase of natural aggregate content,the apparent density and bulk density of mixed recycled aggregate increase continuously.The water absorption and crushing indexes decrease continuously,and the gradation of the four groups of mixed recycled aggregate conforms to the requirements of the national standard.(2)Experimental study on mechanical properties of new recycled concrete.To measure the compressive strength,axial compressive strength,rupture strength and splitting tensile strength in this paper,the water/cement ratio was taken as 0.60,0.49,0.41 and 0.36 to represent four strength grades of RAC.Based on test results,the strength development was represented as the curves of compressive strength,axial compressive strength,rupture strength and splitting tensile strength with curing age and the regularities are analyzed based on the hardening mechanisms of RAC.It can be concluded that the amount of large particle natural aggregate had a reasonable range with different strength grade of RAC having better strength development,which could be lower than 30% when water/cement ratio was 0.60 and 0.49,and should be no less than 50% when water/cement ratio was 0.41 and 0.36.(3)Prediction of mechanical properties and strength of new recycled concrete.The prediction formulas of cubic compressive strength,axial compressive strength,flexural strength and splitting tensile strength of RAC at any curing age are analyzed and put forward.