Study On Design Method Of Secondary Composite Mix Proportion And Mechanical Properties With Different Humidity Of Recycled Aggregate Concrete

Recycled aggregate concrete in China produces a large amount of construction waste every year due to the demolition of old buildings.If the construction waste can be treated and prepared into coarse aggregate,on the one hand,it can realize the recycling of construction waste,on the other hand,it can reduce carbon emissions and protect the environment.Compared with natural aggregate(NA),recycled aggregate(RA)has small apparent density,high porosity and crushing value,which will lead to the reduction of mechanical properties of recycled aggregate concrete.At the same time,natural disasters such as rainstorm and flood often make urban buildings in short-term water environment,structures such as dams,piers,swimming pools,reservoirs and basements are in long-term water environment.The increase of concrete humidity will lead to the increase of its elastic modulus and the decrease of compressive and splitting tensile strength.At present,there are few studies on the mechanical properties of recycled aggregate concrete with different humidity,especially the strength calculation method and constitutive relationship of recycled aggregate concrete with different humidity.In order to explore the strength variation law,calculation method and constitutive relationship of recycled aggregate concrete with different humidity.This paper takes recycled aggregate concrete as the research object,adopts the combination of experimental research and theoretical analysis as the research means,studies the mix proportion design method and the mechanical properties with different humidity of recycled aggregate concrete on the basis of considering water/cement ratio,coarse aggregate types,volume content of coarse aggregate and modification method.On the basis of previous research,this paper mainly carries out the following research work:(1)Study on mix proportion design method of recycled aggregate concreteThe effects of water/cement ratio,sand/cement ratio,coarse aggregate types and volume content of coarse aggregate on the mechanical properties of concrete were studied through experiments,the optimal range of sand/cement ratio and volume content of coarse aggregate were found.Concrete is regarded as a two-phase composite material composed of coarse aggregate and cement mortar,cement mortar is regarded as a two-phase material composed of fine aggregate and cement paste.The design method of secondary composite mix proportion based on sand/cement ratio,volume content of coarse aggregate and additional water consumption was established.The analysis results show that the mix proportion design method proposed in this paper has a clear process and is not affected by coarse aggregate types and apparent density.It optimizes the proportional relationship between each component,simplifies the mix proportion design process,expands the application scope,which provides a new idea for recycled aggregate concrete mix proportion design method.(2)Study on humidity and pore structure of recycled aggregate concrete in different environmentsThrough experiments,the moisture content,effective water absorption and humidity of cement mortar and recycled aggregate concrete test block were studied in different environments,the water immersion humidity of different types of concrete and the variation law of its effective water absorption with time were analyzed.The relationship between pore structure and humidity of recycled aggregate concrete was studied from the micro point of view.(3)Study on experiment of mechanical properties of recycled aggregate concrete in different humidityThe compressive and splitting tensile strength of different types of concrete were studied through experiments,the effects of water/cement ratio,coarse aggregate types,modification method,age and different humidity on the compressive and splitting tensile strength of concrete were analyzed.The stress state of concrete under compression and tension was analyzed by using the principle of effective stress.Based on the principle of stereology,the calculation method of cement mortar wrapping thickness of coarse aggregate with 99.7%reliability was proposed.The calculation formula of optimal coarse aggregate distance was deduced.Based on the factors of water/cement ratio,coarse aggregate types,volume content of coarse aggregate and steel fiber,simplified calculation models of compressive and splitting tensile strength of concrete with different humidity were proposed.The analysis results show that the calculation formula of cement mortar wrapping thickness of coarse aggregate and the simplified calculation models of compressive and splitting tensile strength have high prediction accuracy,which provides theoretical support for the calculation of compression and splitting tension of recycled aggregate concrete in different humidity.(4)Study on the uniaxial monotonic loading stress-strain relationship of recycled aggregate concrete in different humidityThe effects of water/cement ratio,coarse aggregate types,volume content of coarse and fine aggregate and steel fiber on the uniaxial compressive stress-strain relationship of recycled aggregate concrete with different humidity were studied.Based on micromechanics and surface energy theory,the influence of coarse aggregate types and different humidity on concrete energy dissipation was analyzed,the mechanism of the influence of coarse aggregate types and different humidity on concrete strength was explained from the perspective of energy.The calculation method of energy dissipation of concrete under uniaxial compression was proposed,the quantitative calculation was carried out according to the stress-strain curve of concrete under uniaxial compression.Based on the water/cement ratio,coarse aggregate types and steel fiber,the Weibull-Lognormal statistical distribution constitutive model with different humidity was established.The analysis results show that the established constitutive model is in good agreement with the test curve in the rising section,peak point and falling section,which provides a theoretical mathematical model for the stress-strain relationship of recycled aggregate concrete in different humidity.(5)Study on the uniaxial cyclic loading stress-strain relationship of recycled aggregate concrete in different humidityThe effects of water/cement ratio,coarse aggregate types,volume content of coarse and fine aggregate and steel fiber on the cyclic loading stress-strain relationship of recycled aggregate concrete with different humidity were studied.According to the characteristic points of uniaxial cyclic loading stress-strain curve of concrete,the plastic strain,stress degradation and stiffness degradation of recycled aggregate concrete were analyzed.The elastic-plastic envelope model,unloading curve and reloading curve calculation model based on unloading elastic modulus were established.The analysis results show that the whole curve model is in good agreement with envelope,unloading curve and reloading curve,which provides a new method for the constitutive model of concrete under cyclic compression.