Dynamic And Static Mechanical Properties Of Fiber-Nano SiO₂ Modified Recycled Aggregate Concrete

Under the background of carbon neutrality,the main problems faced by engineering construction are high energy consumption,high resource development and high pollution emissions.The emergence of recycled aggregate concrete is in line with the development requirements of the times,and is an effective way to construction solid waste resources.The efficient use of recycled aggregates and the widening of the application of recycled aggregate concrete have become the key.Nano-SiO₂solution was used to pre-soak recycled aggregates,composite steel-PVA fiber was used to prepare recycled aggregate concrete,and the dynamic and static mechanical properties of fiber-nano SiO₂ modificed recycled aggregate concrete were studied.The test mainly includes three stages,namely the selection of the reference mix ratio,the static orthogonal test and factor analysis,and the SHPB impact test.The main research contents and conclusions are as follows:（1）Optimize the aggregates gradation,determine the physical indicators of coarse aggregates and fine aggregates,and determine the proportion of each component in the benchmark mix through the analysis of the compressive strength and slump of the benchmark group specimens.The results show that the addition of steel fibers can improve the strength and plasticity of recycled aggregate concrete,and the superplasticizer can improve the fluidity of recycled aggregate concrete.（2）The compressive strength and splitting tensile strength of fiber-nano SiO₂modificed recycled aggregate concrete were measured by orthogonal test,and the optimal ratio of each factor was obtained by combining extreme value R,horizontal value K analysis and comprehensive balance method.On this basis,the static performance comparison test was carried out.The results show that the factors affecting the compressive strength are the content of steel fibers,the replacement percentage of recycled coarse aggregates,the concentration of nano-SiO₂ and the content of PVA fibers,and the factors affecting the splitting strength are the content of steel fibers and the concentration of nano-SiO₂,PVA fiber content,recycled coarse aggregates replacement percentage.Through the comprehensive index evaluation of the orthogonal test,the comprehensive optimal factors for the static strength of the recycled aggregate concrete block are as follows:the nano-SiO₂ concentration of 2.0%,the steel fiber content of 2.0%,and the PVA fiber content of 0.2%.（3）Comparative tests show that with the increase of the replacement percentage of recycled coarse aggregate,the static strength of the specimen decreased.The addition of nano SiO₂ and steel fiber improves the static strength of recycled aggregate concrete.The addition of 2.0%nano-SiO₂ increases the compressive strength of the specimen by16%and the split tensile strength by 12%.For the 2.0%nano-SiO₂ modified recycled aggregate concrete specimens mixed with fibers alone,the strength of the specimens improved by the single addition of 2.0%steel fibers is greater than that of 0.2%PVA fibers.Compared with the 2.0%nano-SiO₂ modified sample,the compressive strength of the 2.0%steel fiber sample was increased by a maximum of 37%,and the split tensile strength was increased by a maximum of 44%.The mixed specimen of 2.0%steel fiber-0.2%PVA fiber is not conducive to the improvement of strength and reduces the mixing performance of recycled aggregate concrete.In the hybrid fiber specimen group,the 1.0%steel fiber-0.2%PVA fiber hybrid specimen showed better static strength.（4）According to the test results of quasi-static and SHPB impact test,observe the failure mode of each group of specimens and define the representative strain rate under different air pressures,and analyze the quasi-static compressive strength and impact stress-strain curve characteristics of the specimens.The results show that the quasi-static specimens undergo shear failure,the cracks expand from the top down,and the quasi-static compressive strength of each group of specimens is similar to the static compressive strength.As the impact speed increases,the damage of the specimen increases.The hybrid fiber specimen under high strain rate has good integrity.With the increase of the strain rate,the stress and strain values of the specimen increase,and the peak stress shows a significant increase trend.After the steel-PVA fiber was added,the deformation capacity of the specimen was improved.（5）Combined with the characteristics of impact stress-strain curve,dynamic mechanical indicators such as impact strength,peak strain,initial elastic modulus and post-peak secant modulus,DIF value,and specific energy absorption of the specimen are analyzed.The results show that with the increase of strain rate,the impact strength,peak strain,DIF value and total energy consumption of the specimen show an increasing trend,and the impact strength shows a strong linear growth relationship.At high strain rate,the initial elastic modulus and post-peak secant modulus increases;the addition of nano-SiO₂ and steel fibers is beneficial to improve the impact strength and modulus of the specimen,while the addition of steel-PVA hybrid fibers improves the deformation and energy dissipation capacity of the specimen more significantly.