Study On Performance Degradation Of Recycled Concrete With Different Water-cement Ratios Under Coupling Of Freeze-thaw And Sulfate

With the continuous development of cities,the disposal of construction waste becomes a problem,and the use of recycled concrete arises at the historic moment.Northeast China,Northwest China,North China,and parts of Central China and East China have been damaged by freezing and thawing to varying degrees.The range of acid rain in North China has gradually expanded and increased in acidity,which has caused serious damage to concrete structures.Therefore,this paper studies the performance degradation of recycled concrete with different water-cement ratios under the combined action of freeze-thaw cycle and sulfate attack,and mainly completes the following work:(1)The freeze-thaw test method is carried out by the"quick-freezing method",and the freeze-thaw experiment is carried out on the recycled concrete with a replacement rate of 30%,a sodium sulfate concentration of 5%,and a water-cement ratio of 0.35,0.45,and 0.55,respectively,and Macroscopic mechanical performance test and microscopic experimental sampling were carried out on specimens under different freeze-thaw cycles.(2)Macroscopically analyze the deterioration rules and causes of various mechanical properties through apparent morphology,quality,relative dynamic elastic modulus and compressive strength.The process of quality loss,compressive strength,and relative dynamic elastic modulus performance of recycled concrete with different water-cement ratios decreased in three stages;as the freeze-thaw cycle progressed,erosion products continued to accumulate,internal defects continued to expand,and various indicators declined sharply Among them,the water-cement ratio of 0.55 specimens is the most obvious.(3)Microscopically observe the internal corrosion products and structural changes through SEM experiments to verify the cause of macroscopic performance degradation.The size of the water-cement ratio and the performance of the recycled coarse aggregate will affect the crack development of the interface transition area,thereby affecting its macro-mechanical properties;the larger the water-cement ratio,the weaker the interface transition area,so the water-cement ratio should be fully considered for the recycled concrete The effect of frost resistance and macro mechanical properties.(4)Microscopically analyze the product type and content changes by XRD experiment,and further analyze the deterioration of recycled concrete and sulfate corrosion mechanism.During the freeze-thaw cycle and sulfate erosion,the calcium hydroxide content gradually decreases,and the erosion products ettringite and gypsum are sequentially precipitated,and their content increases rapidly in the late freeze-thaw period;the amount of calcium hydroxide content seriously affects the freeze resistance of recycled concrete and anti-erosion ability,so effectively reducing calcium hydroxide in recycled concrete is of great significance to improve the frost resistance and anti-sulfate corrosion ability of recycled concrete.(5)Based on the damage mechanics theory,establish a damage model on the water-cement ratio and the number of freeze-thaw cycles,and perform fitting verification and life prediction.Under the coupling effect,the durability of recycled concrete with water-cement ratio of 0.35 can reach the best;it is recommended that the water-cement ratio of recycled concrete should not be greater than 0.45 in engineering applications.