The Durability And Deterioration Mechanism Of Recycled Aggregate Concrete Subjected To Compound Erosion

Recycling waste concrete to prepare recycled aggregate concrete(RAC)can not only solve the problems of construction waste stacking and environmental pollution,but also make up for the shortage of natural aggregate resources.However,the research results of durability of RAC under the combined action of various environmental erosion factors are very little,which directly affect the application of RAC in practical engineering.Therefore,it is necessary to study it systematically,which is consistent with the basic national policy of sustainable development and the construction of a green conservation-oriented society.In this paper,the main environmental factors(freeze-thaw cycle,sulfate attack and carbon dioxide)affecting the durability of RA C structure were selected by using the method of laboratory test and theoretical analysis.The freeze-thaw cycle test,combined action of freeze-thaw cycle and sulfate attack test,combined action of freeze-thaw cycle and carbonization test were carried out.The microstructure characteristics of RAC before and after composite erosion were analyzed systematically by using scanning electron microscope(SEM),X-ray diffraction(XRD)and nuclear magnetic resonance(NMR)technology.On this basis,in order to improve the durability of RAC,the optimal replacement ratio of RAC aggregate(RCA)and optimal content of fly ash were studied.The main contents of this paper include:(1)According to the RAC specimens with different replacement ratio of RCA and different fly ash content,the mechanical properties and flow properties of RAC were studied by using the test method.The internal reasons for the change of macroscopic mechanical properties of RAC were discussed,and the difference mechanism between RAC and ordinary concrete in material properties was analyzed and revealed.(2)The frost resistance was studied on RAC with different replacement ratio of RCA and different fly ash content.The change law of mass loss,relative dynamic modulus of elasticity(RDME)and compressive strength of RAC under freeze-thaw cycle was analyzed.The influence mechanism of replacement ratio of RCA and fly ash content on frost resistance of RAC was revealed.The durability damage model was established considering the influence of RAC material parameters and freeze-thaw cycle.(3)According to the RAC specimens with different replacement ratio of RCA and different fly ash content,the durability test of RAC subjected to the combined action of freeze-thaw cycle and sulfate attack was designed and carried out.The change laws of mass loss,RDME and compressive strength of concrete were studied,and the damage mechanism of RAC was analyzed.The variation of sulfate transport performance of RAC under combined action of freeze-thaw cycle and sulfate attack was studied by ultraviolet spectrophotometer.The influence mechanism of sulfate solution on frost resistance of RAC was analyzed by introducing the coefficient of λ.A durability damage model considering the effects of RAC material parameters,combined action of freeze-thaw cycle and sulfate attack was established.(4)According to the RAC specimens with different replacement ratio of RCA and different fly ash content,the durability test of RAC under the combined action of freeze-thaw cycle and carbonization was designed and carried out.The carbonation depth,mass loss rate,RDME and compressive strength of RAC were studied.The relationship between carbonation rate and compressive strength of RAC under freeze-thaw environment was quantitatively revealed.The damage mechanism of RAC was discussed.The influence coefficient λC of composite damage of RAC was introduced,and the influence of carbonization on frost resistance of RAC was analyzed.The influence coefficient λF of composite damage of RAC was introduced,and the influence of freeze-thaw cycle on carbonation performance of concrete was analyzed.A durability damage model considering the effects of RAC material parameters,combined action of freeze-thaw cycle and carbonization was established.(5)By using NMR,considering the influence of RCA replacement ratio and fly ash content,the pore structure of RAC before and after composite erosion was analyzed.The evolution of porosity and pore distribution of RAC under composite erosion was explored.The pore micro-damage characteristics of RAC under composite erosion were revealed,which provided a new method and idea for the study of the durability of RAC under composite erosion.(6)By means of SEM,XRD and EDS,the development process of micro-structure cracks,the structure evolution of interface transition zone in concrete,the growth process of main erosion products,the structure form and characteristics of main erosion products were analyzed.The quantitative analysis of the erosion products were carried out and the damage and deterioration mechanism of RAC under composite erosion were revealed from micro-angle.The research work in this paper can make up for the defects of the mechanism explanation and model construction of the durability damage of RAC.It provides theoretical basis and technical support for the study of the durability degradation characteristics and service life prediction of RAC under the action of composite erosion,and has certain significance for perfecting the theoretical research system of the durability of RAC.