Study On Frost Resistance Durability Of Modified Reclaimed Coarse Aggregate Concrete

Recycling of recycled concrete is of great significance to the construction of green ecological environment,and how to improve the anti-freezing performance of recycled concrete is one of the hot topics in current research.Most winter areas in our country are cold,therefore,research on the anti-freezing durability of recycled concrete is very necessary.In this paper,the recycled aggregate is modified and strengthened,fly ash is added,the preparation process is changed,the cracks of the recycled aggregate are blocked and the compactness of the recycled concrete is enhanced,so as to strengthen the mechanical properties of the recycled concrete and improve the frost resistance durability.On the basis of the test,the freeze-thaw cycle and uniaxial compression process of concrete are simulated by finite element software,and the test data and finite element analysis results are compared and analyzed.Specific research contents are as follows:(1)The regenerated aggregate was modified and strengthened with different concentration of sodium silicate solution and soaking time.After soaking the reclaimed aggregate in sodium silicate solution,the apparent density and bulk density increased,while the water absorption rate and crushing value index decreased.The study showed that the comprehensive strengthening effect was better when soaking time was 1 hour and the concentration was 5%.(2)Regenerated concrete specimens were made by changing the regenerated aggregate substitution rate,fly ash content,water-binder ratio and preparation technology.Based on freeze-thaw cycle test,ultrasonic wave velocity,mass and uniaxial compressive strength of reclaimed concrete specimens were tested.With the increase of the number of freeze-thaw cycles,the quality,ultrasonic wave velocity and compressive strength of recycled concrete all decrease.The freeze-thaw cycle degrades the internal structure of recycled concrete and reduces the frost resistance durability.(3)The influence of different factors on the freeze-resistance of recycled concrete and the freeze-resistant durability life were obtained by analyzing the test data.The orthogonal test results show that when fly ash content is increased,the recycled concrete with low substitution rate and low water-binder ratio has excellent compressive performance.Fly ash content of 20%,water-binder ratio of 0.43,the replacement rate of reclaimed aggregate of 50%,and the preparation process using two-stage mixing is the best combination of different factors.The limit freeze-resistance times of recycled concrete in different experimental groups were predicted by damage attenuation model,and the longest freezeresistant durability life was 40.1 years and the shortest freeze-thaw life was 33.7 years under natural freeze-thaw conditions in central China.(4)ABAQUS simulation software was used to analyze the whole process of freezethaw cycle and uniaxial compression of recycled concrete,and the temperature changes in the heat transfer process of freeze-thaw cycle test were simulated.The results show that the recycled concrete with low substitution rate has low temperature stress and better frost resistance.The variation range of the external temperature of the concrete specimen is larger than that of the internal temperature,and the temperature stress on the external surface of the concrete specimen is larger and the failure occurs first in the freeze-thaw process.The results of uniaxial compression simulation show that the recycled concrete with high replacement rate has higher compressive strength loss rate and poor frost resistance durability after freeze-thaw cycle,and its compression failure mode is "X" type.