| The phenomenon of resource waste caused by the massive use of concrete is continuing to occur,and how to properly dispose of concrete waste has become a social issue.However,compared with natural aggregate concrete,the durability of concrete containing recycled coarse aggregate is poor.Among them,the problem of concrete carbonation has always been one of the main reasons affecting the durability of concrete,and the current experimental research on the durability of recycled concrete is mostly under stress-free state,and the actual serviced concrete often bears the combined effect of stress and environment.Based on this,this paper uses the self-designed axial tension and axial compression long-term loading device to study the carbonation performance of recycled concrete with different coarse aggregate replacement rate under long-term loading,in order to study the carbonation problem of recycled concrete in service under different stress states,and to provide some guidance for the durability of recycled concrete structure.In this paper,using C30 as the benchmark concrete,using 0%,30%,50%,and 70%recycled coarse aggregates to replace natural aggregates to configure recycled concrete specimens.The cubic compressive strength of four groups of concretes with different contents is measured.Carry out the rapid carbonation test of recycled concrete with four mix proportions under the ultimate load of 0%,20%,40%and 60%,and explore the influence of recycled coarse aggregate replacement rate and different stresses and stress ratios on the anti-carbonation performance of concrete.On this basis,a prediction model of the carbonation depth of recycled concrete under different axial stress states is established;at the same time,the scanning electron microscope test is carried out on the recycled concrete test block to observe and analyze the difference of the internal micro morphology of the recycled concrete test block before and after carbonation,and explain the material from the micro level.The performance of the macro test.The test results show that:(1)The compressive strength of concrete decreases with the increase of the replacement rate of recycled coarse aggregate.The compressive strength of natural aggregate concrete is the highest,and the strength of recycled concrete with a 70%replacement rate is the lowest.(2)The carbonation depth of recycled coarse aggregate concrete is greater than that of natural aggregate concrete,and the greater the replacement rate,the deeper the carbonation depth.(3)Stress will change the microscopic pore structure of concrete,and then affect the carbonation resistance of recycled concrete.Among them,the axial compressive stress inhibits the progress of the carbonation reaction;the axial tensile stress accelerates the progress of the carbonation reaction.Both the axial tensile stress influence coefficient and the axial compressive stress influence coefficient have a binary primary relationship with the stress level and the carbonation age;(4)Observed by scanning electron microscope,it is found that recycled concrete has a large number of weak parts-the interface transition zone,which is regenerated coarsely.The greater the amount of aggregate,the greater the number of defects and pores in the concrete micro structure,which will reduce the mechanical properties and carbonation resistance of the concrete;the calcium carbonate generated by the carbonation reaction can effectively fill the pores,improve the concrete micro structure,and reduce the porosity of the concrete structure.,And then delay the carbonation reaction rate;(5)Calculate the influence formula of recycled coarse aggregate content,axial compressive stress,axial tensile stress on the concrete carbonation rate,and establish a predictive model of recycled concrete carbonation depth considering the stress effect. |
