Study On The Effect Of Cooling Method On The Mechanical Properties Of High Strength PVA Fiber Concrete After High Temperature

High Strength Concrete（HSC）has the advantages of high strength,low permeability,good durability and small deformation,etc.In engineering practice,it can give full play to these advantages to meet the development needs of large span,large space and super high-rise buildings,and can reduce the cross-section of members,save raw materials,thus reducing project costs and meeting the strategic requirements of energy conservation,emission reduction and sustainable development.The strategic requirements of energy saving and sustainable development.However,the engineering structure will face the threat of fire in the process of use,so it is necessary to carry out relevant research on the high temperature performance of HSC.Meanwhile,in order to study the influence of fiber on the improved performance of HSC,this paper selects polyvinyl alcohol fiber（PVA fiber）to be added to HSC in the form of single blending to explore the prospect of engineering application of PVA fiber HSC.This paper mainly explores the effect of cooling mode on the mechanical properties of HSC with different heating temperatures and different fiber doping.The main research contents and results are as follows.（1）High temperature tests（20℃,200℃,400℃,600℃,800℃）were conducted on HSC with different fiber doping,and the changes of specimen appearance morphology and mass loss rate with temperature were analyzed after the specimens were cooled down by different cooling methods.The results show that:with the increase of heating temperature,the surface color of the specimen changes from dark greenish gray to grayish white,and the number of cracks on the surface of the specimen increases gradually;the change of mass loss rate is not obvious before 400℃,and increases significantly after 400℃;the mass loss rate of the specimen under natural cooling method is greater than that of water spray cooling and water immersion cooling.（2）The experimental study on the mechanical properties of HSC with different cooling methods,different heating temperatures and different fiber doping was carried out to analyze the changes of dynamic elastic modulus,compressive strength and flexural strength of the specimens.The results showed that the dynamic elastic modulus and mechanical strength decreased gradually with the increase of heating temperature;when the heating temperature was the same,the dynamic elastic modulus and mechanical strength showed a trend of increasing and then decreasing with the increase of PVA fiber admixture;the compressive strength was the largest at 0.2%of PVA fiber admixture;the flexural strength was the best at0.3%.The dynamic modulus of elasticity and mechanical strength of concrete with different fiber admixtures at different temperatures are basically greater than those with water immersion cooling and water spray cooling after natural cooling.（3）The microscopic morphology of PVA fiber HSC after high temperature was analyzed by SEM test and XRD test to study the effect of different heating temperatures on the microscopic morphology of plain HSC and HSC mixed with 0.2%PVA fiber.The results show that:below 200℃,the overall structure is basically intact,after 200℃,the fiber starts to melt,with the increase of heat temperature,Ca（OH）₂ keeps dehydrating and decomposing,the gel structure is destroyed,the pores and cracks increase,and the structure becomes more loose;at low temperature,the main components of concrete are Ca（OH）₂ and C-S-H gel,with the increase of temperature,Ca（OH）₂and other hydration products gradually decompose,resulting in the gradual loss of concrete strength.At 800°C,Ca CO₃ decomposes into Ca O,which is manifested in macroscopic terms as a reduction in the compressive strength of concrete.