Study On Fatigue Performance Of Recycled Concrete Modified With Nano Calcium Carbonate

With the acceleration of urban construction and urbanization,the demolition of old houses,road renovation,bridge reconstruction and other acts will generate huge amount of waste concrete.In order to realize the recycling of waste concrete,this paper partially replaces natural coarse aggregate with recycled coarse aggregate made from waste concrete,and conducts research work on the mechanical properties and fatigue characteristics of concrete by nano-modification and recycled coarse aggregate replacement rate,aiming to provide theoretical basis for scientific evaluation of long-term stability of nano-modified recycled concrete under fatigue loading environment.The main research works are as follows:(1)The recycled coarse aggregate was determined as Class Ⅱ by physical test,and the mix design was improved by slump test.The results show that the higher the replacement rate of recycled coarse aggregate Q,the lower the concrete strength;the strength and toughness of recycled concrete are obviously enhanced by nano calcium carbonate,and the optimum content of nano calcium carbonate is 1%.Based on the analysis of microstructure by central effect,nano calcium carbonate promotes hydration reaction,plays a role of nucleation barrier and bridging.(2)With static loading and dynamic cyclic loading,the influence law of nanomodification and recycled coarse aggregate substitution rate on the resistance fatigue characteristics was compared and analyzed,and the performance law of fatigue residual strength under different cycle times was explored.The results show that:recycled coarse aggregate incorporation has a significant negative impact on fatigue characteristics,and recycled concrete is more sensitive to fatigue cycles;nano can effectively improve concrete integrity and increase concrete resistance to deformation.(3)Based on the statistical probability distribution model,the fatigue life was analyzed,and the two-parameter Weibull distribution probability model was proved to be more reliable.Based on the Weibull distribution model,the double logarithmic S-N curves and corresponding fatigue life equations were derived for the four grouping conditions,and the fatigue characteristics of the four groupings were investigated in depth;the double logarithmic P-S-N curves and fatigue equations were established considering the relative fatigue life under certain survival probability,and the relationship between the fatigue times of the four groupings of concrete was further investigated by comparing the static strength and the conditional fatigue strength:W2Q50<W2Q30<W2Q50N1<W2.(4)The results show that the fatigue strain evolution is in accordance with the three-stage strain curve development law,and the agreement is low for the second stage fatigue strain of recycled concrete.A new equation is proposed to describe the second stage strain curve of recycled concrete through the study of fatigue strain,and the agreement is high.The deformation volume was introduced and the relationship between deformation volume and cycle ratio function was established.