Dynamic Mechanical Properties Of PVA-nano-SiO₂ Concrete After Fatigue And Freeze-thaw Performance Study

Concrete structures are often subjected to damage from fatigue loads and freeze-thaw cycles,which reduce the durability of concrete structures.At present,scholars focus on the effect of adding fibers and nanomaterials on the durability of concrete.This paper will study the nano SiO₂of concrete mixed with PVA fiber（Polyvinyl alcohol fiber）and the influence of the fatigue resistance and freeze resistance.This paper takes 243 concrete prism specimens as the research object.PVA fiber concrete（P）,nano-SiO₂concrete（S）and PVA-nano-SiO₂concrete（SP）are subjected to fatigue loading and dynamic loading tests after freeze-thaw cycles,respectively.Based on the changes of mechanical properties such as mass loss rate,relative dynamic elastic modulus,and strain rate on the mechanical properties of concrete are analyzed.The improvement of fatigue resistance and frost resistance of concrete by adding PVA fiber and nano-SiO₂is studied.The microscopic morphology of concrete after freeze-thaw damage is observed by scanning electron microscope,and the deterioration mechanism of freeze-thaw damage is analyzed.Based on the analysis of the test data,the freeze-thaw damage model of SP concrete（PVA-nano-SiO₂concrete）is established and numerical simulation is carried out.The main results are as follows:（1）The uniaxial dynamic compression tests of PVA fiber concrete,nano-SiO₂concrete and PVA-nano-SiO₂concrete after fatigue loading are carried out.The effects of fatigue times and strain rates on the strength and deformation properties of concrete are studied.The results show that under the same strain rate,with the increase of fatigue times,the compressive strength of the three groups of concrete specimens decreases continuously,with a maximum decrease of 16.61%.The peak strain increases with the number of fatigue.Under the same fatigue times,the compressive strength of the three groups of specimens increases with the increase of the strain rate.The peak strain decreases with increasing strain rate.From the perspective of compressive strength,the fatigue resistance of the three groups of concrete is analyzed,and it can be concluded that the SP group>S group>P group.（2）The uniaxial dynamic compression test of PVA fiber concrete,nano-SiO₂concrete and PVA-nano-SiO₂concrete after freeze-thaw cycles is carried out.The effects of freeze-thaw cycles and strain rates on the strength and deformation properties of concrete are investigated.Combined with the microscopic test,the freeze-thaw damage and deterioration mechanism of concrete is analyzed.The results show that under the same strain rate,the compressive strength of the three groups of specimens decreases with the increase of freeze-thaw times,and the maximum decrease is 37.13%.With the increase of freezing-thawing times,the peak strains of the three groups of concrete specimens also increase gradually.Under the same freezing-thawing damage degree,the compressive strength of the three groups of concrete specimens increases with the increase of strain rate,and the peak strain decreases with the increase of strain rate.From the perspective of compressive strength,the frost resistance of the specimens in group S is weaker than that in groups P and SP.Compared with the plain concrete group,the peak strain of the three groups of specimens mixed with PVA fiber and nano-SiO₂increased significantly,and the ductility is better.（3）The numerical simulation study of uniaxial dynamic compression of PVA-nano-SiO₂concrete after freeze-thaw cycles is carried out.By analyzing the test data,the freeze-thaw damage model of PVA-nano-SiO₂concrete is established.The data obtained from the numerical simulation are compared with the experimental data.The results show that the compressive strength and peak strain of SP concrete obtained by numerical simulation have little error with the experimental results.The maximum error of compressive strength is 0.81%,and the maximum error of peak strain is 1.82%.The obtained stress-strain curve is basically consistent with the experimental results.The proposed freeze-thaw damage model of PVA-nano-SiO₂concrete can more accurately reflect the mechanical properties of SP concrete after freeze-thaw damage.