Research On Performance Of Ultra High Toughness Ccmentitious Composite And Composite Structures Under Extreme Temperatures

Ultra high toughness cementitious composite(UHTCC)is a kind of high performance fiber-reinforced cementitious composite material,which possesses the strain-hardening property and crack-controlling ability.It is particularly promising to use UHTCC in building structures.Building structures may encounter extreme temperature conditions during their life cycle,such as building fire(extremely high temperature)and liquefied natural gas(LNG)leakage(extremely low temperature).The change of the temperature will result in the change of the properties of concrete materials.Therefore,extreme temperature conditions may have a great impact on the safety,applicability and durability of building structures,resulting in incalculable casualties and economic losses.In order to explore the safety of UHTCC composite structures in the whole life span,it is necessary to investigate the elevated temperature performance and low temperature performance of UHTCC and UHTCC composite structures.The relevant researches were conducted in this paper supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China with Grant No.2012B AJ13B04 and the National Natural Science Foundation of China with Grant No.51622811.The main research contents are as follows:1.The influence of elevated temperatures on the thermal properties,mechanic properties and microstructures of UHTCC was investigated.It was found that the thermal conductivity of UHTCC increases slightly with the increase of temperature when it is kept for 50 min at elevated temperatures,and generally remains at about 0.5 W/(m·K);the flexural strength of UHTCC decreases with the increase of temperature when it is kept for 60 min at elevated temperatures;the residual compressive strength of UHTCC increases slightly when the temperature is not more than 200?,whereas it gradually decreases when the temperature is higher than 200?;the spalling can be effectively avoided in UHTCC matrix,whereas elevated temperatures may cause PVA fiber melting,steel fiber corrosion and matrix degradation.2.A method was proposed by modifying the specific heat of UHTCC to simulate the influence of moisture migration on the temperature field distribution accurately.Accurate temperature field distributions of UHTCC structures in fire condition were calculated using this method.3.A nonlinear constitutive model for UHTCC under uniaxial compression at elevated temperatures was proposed which can reasonably capture the compression performance of UHTCC at elevated temperatures.The constitutive model can be used in finite element(FE)analysis to obtain the fire performance of UHTCC structures.4.The fire performance of steel-reinforced UHTCC(SRU)columns was investigated.By FE analysis,the fire performance of SRU column was investigated,including temperature field distribution,structural response at elevated temperature,etc.The model was validated by comparing with the test results.The results showed that only slight spalling occurs to SRU columns;the temperature in the cross section of SRU column is lower than that of SRC column under the same test condition,which means that UHTCC can better protect the steel embedded in the specimen in fire condition;under the same column load ratio(n),the fire resistance of SRU column is higher than that of SRC column.According to the standard design method,the fire design method of SRU columns was proposed which can be used to guide the engineering practice5.The fire performance of steel-reinforced UHTCC(SRU)beams was investigated.The temperature field distributions and structural response of SRU beams at elevated temperature were analysed by FE method.Compared with the experimental results,the correctness of the FE analysis model was verified.According to the standard design method,the fire resistance design method of SRU beams was given,which can provide reference for structural designers6.The fire performance of steel-reinforced UHTCC(SRU)joints was investigated.The FE analysis method was used to study the temperature field distribution of the beam and column cross section.The temperature difference between the cross section in the joint zone and the non-joint zone was mainly analyzed.It was found that the cross-section temperature in the joint zone is lower than that in the non-joint zone,and the closer the distance is to the fire-exposed surface,the more obvious the temperature difference is.The influence of the thickness of the UHTCC protective layer on the fire resistance of SRU structures was analysed by FE method.The results showed that increasing the thickness of the UHTCC protective layer can delay the temperature rising inside the specimen in fire condition,thereby achieving the purpose of improving the structural fire performance7.The influence of low temperature on the mechanic performance of UHTCC was investigated,with the temperature ranging from-160? to ambient temperature.The experimental results showed that the brittleness of UHTCC specimens with single PVA fiber increases as the temperature gradually decreases,and the addition of steel fibers can improve the toughness of UHTCC at low temperatures;the uniaxial compressive strength and the elastic modulus of UHTCC increase approximately linearly with decreasing temperature,and the values at-160?are more than twice of the value at ambient temperature.A nonlinear constitutive model for UHTCC under uniaxial compression at low temperatures was proposed which can be used in finite element analysis to obtain the low temperature performance of UHTCC structures.