Study On The Compressive Properties Of Ultra-High-Performance Concrete After High Temperature

With the development of Ultra-High-Performance Concrete(UHPC)in terms of ultra-high strength,ultra-high toughness,high durability,low cost,green ecology and sustainability,it has gradually been applied to high-rise building structures,nuclear In engineering,anti-blast structure,subway and bridge,UHPC has become a research hotspot in the field of civil engineering.However,most researches focus on the preparation technology of UHPC and the mechanical properties at room temperature.There are few studies on the high temperature fire resistance of UHPC,and the high temperature burst problem has not been effectively solved.Therefore,based on the existing research,this paper has carried out systematic research on the preparation of UHPC and its compression performance after high temperature.The main results are as follows:(1)Through the compressive strength test of 42 sets of UHPC cementitious material cube specimens after high temperature,the influence of silica fume and fly ash content on the fire resistance of UHPC cementitious material was studied.The test results show that: with the increase of the content of silica fume,the bursting degree of the specimens deepens and the strength loss is serious;with the increase of the content of fly ash,the specimens not only did not burst after high temperature,but also no obvious cracks were found,and residual the intensity is increasing.Fly ash improved the fire resistance of UHPC cementitious materials better than silica fume.The optimum content of silica fume and fly ash was 5% and 15%,respectively.(2)Through the different combinations of standard curing,steam curing at 90℃and dry heat curing at 200℃,the influence of the curing system on the compressive strength of the hardened slurry of UHPC cementitious material was studied.The test results show that compared with standard curing,the combined curing system of 7 days90℃ steam curing,21 days standard curing and 24 hours 200℃ dry heat curing was the best,which could increase the strength of the hardened slurry of UHPC cementing material by 68.8%.(3)Through the compressive strength test of 21 sets of UHPC cube specimens after high temperature,the influence of high temperature on the mass loss rate and compressive strength of UHPC specimens was studied.The test results showed that with the increase of temperature,the mass loss in the two temperature ranges of200℃～500℃ and 600℃～800℃ is the most serious;the compressive strength of UHPC specimens with 3% steel fiber increases with temperature.The increase shows a trend of first increase and then decrease,and the strength reaches its peak at 200°C.Through statistical analysis of the test results,a UHPC cube compressive strength calculation model considering the effects of high temperature in fire is established.(4)Through the uniaxial compression test of 12 sets of UHPC prism specimens after high temperature,the influence of high fire temperature on the axial compression characteristic parameters of UHPC specimens was studied.The test results show that with the increase of temperature,the peak stress and elastic modulus of UHPC all show a trend of first increasing and then decreasing,reaching the peak at 200℃;while Poisson’s ratio shows a trend of decreasing first and then increasing.Through the statistical analysis of the test results,the calculation models of peak stress and elastic modulus considering the effects of high temperature of fire are established respectively.(5)Through the analysis of the UHPC compressive stress-strain curve,the influence of high fire temperature on the characteristics of the curve is studied.The results show that,compared with the normal temperature environment,the slope of the stress-strain curve increases and the peak point moves forward after a high temperature of 200℃.In the range of 200℃～800℃,as the temperature increases,the stress-strain curve gradually tends to flatten and the peak point shifts back,and the area enclosed by the curve gradually decreases;after experiencing a high temperature of 1000℃,most of the hydration products Decomposition,the steel fiber loses the ability to transfer stress,UHPC becomes brittle,and the peak point of the stress-strain curve moves forward again.Finally,through theoretical derivation,a UHPC axial compression constitutive model considering the effects of fire high temperature is established.(6)Through XRD phase analysis and SEM scanning electron microscope observation on the hardened slurry of UHPC cementing material after undergoing different temperatures,the degradation mechanism of UHPC mechanical properties after high temperature is revealed from the microscopic level.The results show that:compared with normal temperature environment,high temperature can promote the hydration process of UHPC cementing material.As the temperature increases,the CS-H gel composition inside the matrix increases first and then decreases.400℃ is the dividing point;the bonding interface between the steel fiber and the matrix begins to weaken after 400℃.