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

In the construction industry,the research on the utilization of waste materials has led to the development of recycled concrete technology,which can effectively solve the problem of waste concrete,continue the development of the construction industry,and promote the development of the construction industry to greening.The development of roads in permafrost and seasonal permafrost regions in China has led to the reserach of concrete antifreeze durability technology.However,the current research on the antifreeze durability of recycled concrete has reached different conclusions,the proper addition of air entrained agent can improve the frost resistance of recycled concrete.With the content of recycled coarse aggregate and the air entrained agent as the main factors,the content of recycled coarse aggregate is 0,25%,50%,75%and 100%,respectively,and air entrained recycled concrete is prepared for rapid freeze-thaw test.The test carry out for a total of 200 freeze-thaw cycles,and the quality,ultrasonic wave velocity,dynamic elastic modulus,and cubic compressive strength were tested every 25 freeze-thaw cycles.On this basis;the physico-mechanical damage characteristics of recycled concrete after freeze-thaw cycles were studied,and the freeze-thaw damage model of recycled concrete was dedced.The main conclusions were as follows:(1)During the freeze-thaw cycles,the quality of recycled concrete and air-entrained recycled concrete has a trend of increasing first and then decreasing.After 200 freeze-thaw cycles,the mass loss rate of recycled concrete and air-entrained recycled concrete with the replacement rate of 0%5 25%,50%are less than 2%,only the mass loss rate of 100%recycled concrete is slightly more than 5%,which exceeds the requirements of the specification.Therefore,the recycled concrete with 100%replacement rate does not meet the requirements of frost resistance durability.(2)Dynamic elastic modulus,ultrasonic wave,cubic compressive strength all show the same change trend,gradually become smaller with the freeze-thaw cycle progresses,and at 200 freeze-thaw times,the minimum value of three indexes meets the specification requirements.In the case of the same recycled coarse aggregate content and freeze-thaw cycle,the air-entrained recycled concrete has higher dynamic elastic modulus,ultrasonic wave and cubic compressive strength than the recycled concrete,which indicates the air entrainment can improve the the antifreeze performance of recycled concrete.(3)For the recycled concrete and air-entrained recycled concrete with different recycled coarse aggregate content,after 200 freeze-thaw cycles,the measurement of mass and dynamic elastic modulus can be expressed sequentially with recycled concrete as:RAC50>RACO>RAC25>RAC75>RAC 100,the measurement of the ultrasonic wave may be expressed sequentially with recycled concrete as:RAC0>RAC25>RAC50>RAC75>RAC 100,the order of cubic compressive strength may be expressed with recycled concrete as:RAC0>RAC50>RAC25>RAC75>RAC 100,comprehensive test results,recycled concrete with a replacement rate of 50%showed better frost resistance.(4)The stress-strain curves of recycled concrete and air-entrained recycled concrete with different proportion of recycled coarse aggregate have the same change trend,the slope of the stress-strain curve increases first and then decreases.When the recycled coarse aggregate is same,as the number of freeze-thaw cycles increases,the peak stress gradually decreases and the peak strain gradually increases.The effect of replacement rate of coarse aggregate and freeze-thaw times on the peak stress ratio of the two kinds of recycled concrete is very small,which fluctuates within the range of ±11%.However,it has a great influence on the peak strain.When the replacement rate exceeds 75%,the peak stress ratio changes approximately in parallel.(5)The mechanical model of freeze-thaw damage of recycled concrete and air-entrained recycled concrete with different recycled coarse aggregate content is derived.The damage variable D(n)of the recycled concrete gradually increases with the freeze-thaw cycle n,and the addition of the air entrained agent can reduce the value of the damage variable D(n).The freeze-thaw damage characteristics of concrete predicted by the freeze-thaw damage model of recycled concrete are in good agreement with the actual freeze-thaw damage.The regenerated concrete specimens with a replacement rate of 50%show a small damage variable value.