Research On Mechanical Properties And Impact Resistance Of Nano-SiO₂ And PVA Fiber Reinforced Recycled Concrete Under Freeze-thaw

The application of recycled concrete provides a new way to recycle waste resources and reduce environmental pollution,which has significant economic and social benefits.However,compared with natural aggregate,recycled aggregate has the characteristics of large porosity,high water absorption,and high crushability index,which makes the performance of RAC and ordinary concrete have certain differences,and these differences are more obvious in low temperature environments.The research on RAC mainly focuses on short-term mechanical properties such as compression,tensile,and flexural properties at room temperature.Research on the mechanical properties of RAC after freeze-thaw cycles is still insufficient.Therefore,how to improve the mechanical properties of RAC after freezing and thawing is the current research hotspot.Existing studies have shown that the compounding of polyvinyl alcohol(PVA)fiber and nano-SiO2(NS)into RAC can inhibit the development of cracks while increasing the density of RAC,so that the concrete has good crack resistance and toughness properties.In addition,the RAC structure is inevitably subjected to impact loads during service,which puts higher requirements for the strength and stability of the RAC structure.However,the current design theory and experimental research on nano-material fiber composite reinforced concrete mainly focus on the performance under static load at room temperature,and mechanical research on impact load after freeze-thaw cycles is still insufficient.In this paper,through experiments and theoretical analysis,the effects of different PVA and NS content on the compressive,flexural,split tensile and impact resistance of the RAC under different freeze-thaw ages are studied.Moreover,a freeze-thaw damage model is proposed on the basis of the research on the frost resistance of NS-PVA reinforced recycled concrete,in order to provide a reference and theoretical basis for the later research work of composite reinforced recycled concrete and its promotion and application in actual projects.The research content and conclusions are as following:(1)The freeze-thaw cycle test was carried out on different content of NS-PVA reinforced recycled concrete,and the macro-destructive morphology and influence law of the compressive,split tensile,and flexural strength of the concrete before and after freeze-thaw were analyzed.The experimental results show that with the increase of PVA fiber and NS content,the compressive strength,splitting tensile strength and flexural strength of NS-PVA reinforced recycled concrete show a trend of first increasing and then decreasing,but they are all higher than those without mixing PVA fiber and NS specimens;with the increase in the number of freeze-thaw cycles,the compressive,split tensile and flexural strength of NS-PVA fiber-reinforced recycled concrete show a downward trend.When the content of PVA fiber is0.075% and the content of NS is 1%,the performance of the specimen is improved most obviously after freezing and thawing.(2)According to the concrete drop hammer test method recommended by the American Concrete Institute ACI544,Using a self-designed drop hammer impact test device,the impact resistance test of NS-PVA reinforced recycled concrete under different freeze-thaw ages was carried out,and the impact of different freeze-thaw ages,NS and PVA fibers content on the impact resistance of RAC was discussed.The test results show that the impact resistance of RAC gradually decreases with the increase of freeze-thaw age;the incorporation of PVA fiber and NS significantly improves the impact resistance of RAC.And compared to mixing PVA fiber alone,the impact resistance of the test piece is improved more obviously by the composite mixing of PVA fiber and NS.By comparing the specimens with different dosages under different freeze-thaw ages,when the dosage of PVA is 0.075% and the dosage of NS is1%,the impact resistance of the specimens is improved most significantly.(3)Based on the above test results and theoretical analysis,the influence of the content of PVA fiber and NS on the durability of freezing resistance is studied,the mechanism of freeze-thaw damage of concrete is analyzed,and the freeze-thaw damage model is established based on the loss rate of compressive strength,which is a good response Evolution of freeze-thaw damage of NS-PVA fiber reinforced recycled concrete.