Experimental Study On Frost Resistance And Durability Of Air-Entrained Recycled Concrete Pavement

After being crushed,sieved,washed,and classified,waste concrete can be reused as recycled coarse aggregate to partly or completely replace natural coarse aggregate.Then it can be mixed with cement,water and natural aggregate to prepare recycled concrete.The recycling of waste concrete has obvious environmental,economic and social benefits.The application of recycled concrete in road construction is one of the directions in the development of recycled concrete.This paper conducted research mainly into the failure mechanism of freeze-thaw and service life in freeze-thaw environment,focusing on analyzing the pavement freezing resistance and durability of air entrained recycled concretes when the recycled coarse aggregate’s substitution rates were at 0%,25%,50%,75%,100% respectively.The key contents and achievements of the research are as follows:(1)Research was carried out on the influence patterns of different recycled aggregate substitution rates and air entraining agents on the mechanical properties and basic pavement performances of recycled aggregate concrete.The study revealed that,when the substitution rate of recycled aggregate was lower than 75%,it could reach the level two and below level two concrete pavement performance indexes,and when the substitution rate of recycled aggregate was 100%,it could meet the requirements of level three and level four concrete pavement.(2)Research was conducted on the changes in the physical and mechanical performance indexes after air entraining agents and recycled coarse aggregates were employed in the freezing and thawing test of recycled concretes.Analysis was conducted to find out the changes in mass loss rate,relative dynamic modulus of elasticity,concrete scaling per unit and flexural strength after the freezing and thawing of recycled concrete.The result indicated that,when the substitution rates of recycled coarse aggregate were the same,the adding of air entraining agent could enhance the freezing resistance and durability of recycled concrete;the freezing resistance and durability of natural aggregate concrete outperformed those of recycled concrete,and the recycled concrete with 50% of recycled coarse aggregates turned out to have the freezing resistance and durability closest to those of natural aggregate concrete.(3)NMR(nuclear magnetic resonance)test was adopted to analyze from the micro-scale the influence of the adding of air entraining agent and the substitution rate of recycled coarse aggregate on the evolving properties of concrete freezing and thawing.The adding of air entraining agent increased the porosity of recycled concrete and changed the pore structure of concrete,thus significantly improving the work-ability and freezing resistance of recycled concrete;compared with natural concrete,recycled concrete has higher porosity and higher ratio of large pores to fractures.In addition,when the substitution rates of recycled coarse aggregate were 25% and 50% respectively,the pore structures of recycled concrete were more similar,and when the substitution rate of recycled coarse aggregate was above 75%,a notable increase in the ratio of large pores to fractures appeared in recycled concrete.(4)In combination with the experiment data,the freezing and thawing damage models were built using elastic modulus and rupture strength respectively as damage variables.In combination with the definition of freezing resistance index DF,the conversion between relative dynamic modulus of elasticity and rupture strength loss rate was established for the pavement performance of recycled concrete.Through the model,the freezing resistance and durability life of the air-entrained recycled concrete in Xi’an were forecast,and the rupture strength loss of air entrained recycled concrete within its service life was also forecast.With the substitution rates of aggregate at 0%,25%,50%,75%,and 100%,the lives of air entrained recycled concretes in Xi’an turned out to be 41,36,36,34,and 32 respectively,and the rupture strength losses within the service life were 41.62%,47.07%,46.28%,50.67%,57.2% respectively.