Study On Dynamic And Static Mechanical Properties And Micro Mechanism Of Recycled Concrete Fine Aggregate

Recycled concrete aggregate is a kind of recycled material obtained from the demolition of construction waste after crushing and screening.Its annual output is huge,and the existing stockpile stock accounts for one-third of China’s solid waste stockpile stock.RCA has great potential to replace natural aggregate for recycling,but compared with natural gravel aggregate,RCA has weak strength and high compressibility,so it is urgent to study the reinforcement technology of recycled concrete aggregate.Some studies have speculated that the special physical and mechanical properties of RCA are mainly due to a layer of cement mortar coating on the particle surface.However,the existing research is lack of comparative research on the properties of RCA after stripping.In this paper,the mechanical properties of dilute aggregate removal and regeneration of hydrochloric acid film are explored from the macro scale and the micro scale.The main research results are as follows:(1)Influence mechanism of film peeling on micro contact mechanical properties of RCA particlesTaking RCAs from Nanjing and Sydney as test materials,single particle compression test and inter particle cyclic loading test were carried out on aggregate particles with particle size of 1-2mm by using self-developed high-precision micro soil mechanics loading instrument.The strength and stiffness characteristics of RCA particles,as well as the effects of particle color,type and coating on strength and stiffness are revealed from the micro level;With the help of SEM,SEM and EDS,the surface structure and element composition of aggregate particles are analyzed.Combined with the micromechanical test results,the composition of RCA particles and the influence mechanism of cement coating on the contact mechanics of recycled aggregate are clarified.Basalt is the main geological material of RCA.In addition,some gray particles in Nanjing are pure cement polymers and red particles are bricks.(2)Macroscopic dynamic characteristics and influencing factors of recycled concrete aggregate under cyclic loadThe unit cyclic loading test of RCA is carried out by dynamic triaxial tester.The factors such as confining pressure,cyclic deviator stress,gradation and whether there is film coating are changed.The dynamic parameters such as permanent axial strain,rebound strain,rebound modulus and damping ratio of RCA and the influence of different variables on the variation law of these parameters are quantitatively analyzed.It is found that the resilient modulus increases with the increase of confining pressure and cyclic deviatoric stress;The effect of water content on the modulus of resilience is small,while the effect of pore ratio on the modulus of resilience is greater;Under multiple cyclic loading,the damping ratio first increases and then decreases with the increase of strain;After pickling,the resilient modulus of the sample decreased by 2.3%～26.5%.Combined with research(1),the dynamic characteristics of recycled aggregate are revealed from micro and macro scales.(3)Macroscopic static properties and critical state mechanical properties of recycled concrete aggregate and its influencing factorsThe triaxial shear test of RCA is carried out by static triaxial tester.The factors such as confining pressure,gradation and whether there is film coating are changed.The static parameters such as stress-strain relationship,shear strength,critical state strength and stiffness of RCA are quantitatively analyzed,as well as the influence of different variables on the variation law of these parameters.It is found that strain softening occurs under low confining pressure and strain hardening occurs under high confining pressure;After pickling,the shear strength of RCA decreases by 7%～8%.By drawing the critical state line(CSL)of soil,it is concluded that the critical state strength of soil after pickling decreases and the stiffness increases.(4)In the framework of critical state soil mechanics,a relationship model between state parameters and modulus of resilience is proposedThrough the investigation of the literature,it is found that the void ratio e is not considered in the model for predicting the modulus of resilience MR in the literature.According to the research(2),the void ratio e has an important impact on the modulus of resilience Mr.therefore,this paper creatively adds the void ratio e to the original model and modifies the model;Combined with research(3),the state parameters are calculated under the framework of critical state soil mechanics ψ Combined with the modulus of resilience Mr,the relationship model between state parameters and modulus of resilience is established.Finally,the elastic modulus prediction model before and after pickling is unified with the critical state,and the model is simplified.After a series of experimental studies,this paper reveals the macro dynamic and static mechanical properties and micro contact mechanical properties of RCA and their influencing factors and variation rules from the macro and micro point of view,creatively discusses the modulus of resilience in the critical state,and puts forward the relationship model between state parameters and modulus of resilience.