| With the continuous advancement of China’s western development and northeast revitalization strategy,the construction of infrastructure such as railways,highways and construction projects in the plateau high altitude and high latitude frozen soil area was in the ascendant.Various engineering foundation structures were built in the frozen soil foundation,the bored pile foundation was a commonly used form of foundation structure,the interaction between cast-in-place pile and frozen soil foundation constitutes the bearing system of pile foundation.The installation of cast-in-place piles has caused disturbance to the frozen soil,at the same time,the frozen soil environment has a very adverse impact on the early age concrete strength development and pore structure formation of cast-in-place pile body,leads to the reduction of pile strength and insufficient bearing capacity of cast-in-place pile,which finally endangers the structural safety.The research on the time-varying temperature field between the bored pile and the frozen soil environment was the basis for analyzing the strength,durability and life prediction of the concrete of the bored pile,it has important theoretical significance and engineering application value.The hydration heat and thermal conductivity of concrete are the key parameters affecting the time-varying temperature field of bored pile.The influencing factors of hydration heat and thermal conductivity of concrete were studied.The influence mechanism of water-binder ratio,curing temperature and other factors on hydration heat and thermal conductivity of concrete was expounded.The time-varying models for calculating the hydration heat and thermal conductivity of concrete were established respectively,it provides theoretical support for the reasonable selection of heat generation function and concrete thermal conductivity in the study of the time-varying temperature field of bored pile concrete in frozen earth area.Based on thermodynamics and mathematical physics,a time-varying temperature field model of bored pile concrete in frozen soil envirment was established.The laboratory test of temperature field of artificial frozen soil bored pile was carried out.Considering the time-varying properties of hydration heat and thermal conductivity of bored pile,the finite element model of artificial frozen soil pile was established.The applicability of the parameter assumptions of the finite element model and the theoretical calculation model of the temperature field were verified.Based on the above work,the following research results were obtained:(1)The direct method(test method for semi-adiabatic temperature rise)was used to test the hydration heat of the cement paste,so as to simulate the heat generation of the cement after the cast-in-place concrete was poured,and the influences of water-binder ratio,molding temperature and fly ash content on hydration heat of cement paste were analyzed.The results show that at the age of 1d,as the water-binder ratio increases,the heat of hydration of the cement paste gradually decreases,but the high water-binder ratio increases the cumulative hydration heat of the cement paste at the age of 28 d.The increase in water-binder ratio delayed the acceleration period of cement hydration,and the peak hydration rate also decreased.At the beginning of cement hydration,the molding temperature of cement paste has little effect on the hydration heat,but higher molding temperature brings a higher final hydration heat.With the increase of fly ash content,the hydration heat of cement paste gradually decreases,the peak of cement hydration rate gradually decreases,and the age when the cement hydration rate reaches the peak was also delayed accordingly.Based on the empirical formula for calculating the hydration heat of cement,a time-varying model for calculating the hydration heat considering the influence of the concrete molding temperature and the fly ash content was established.(2)The hydration heat of cement paste under constant temperature curing condition was tested by the solution calorimetry,so as to simulate the heat generation process of cement hydration after the temperature of bored cast-in-place concrete stabilized.The effects of water-binder ratio and constant curing temperature on the heat of hydration of cement were analyzed.The results show that under the constant temperature curing at 20 ℃,the hydration heat of cement paste gradually decreases with the increase of water-binder ratio at 1d age,after the 1d age,as the water-binder ratio increases,the hydration heat of the cement paste gradually increases.Under constant temperature curing at 5 °C and 0 °C,the hydration heat of cement paste gradually increases with the water-binder ratio at each age.Under constant temperature curing at-5 °C,the hydration heat of cement paste gradually decreases with the increase of water-binder ratio at each age.The cumulative hydration heat of cement paste decreases with the decrease of curing temperature,and increases with the age.The time-varying models of hydration heat under constant temperature curing were established by calculation model based on empirical formula,calculation model based on equivalent phase and calculation model based on cement hydration dynamics.(3)The thermal conductivity of unsaturated concrete at different ages under constant temperature curing at 20 °C,5 °C and-5 °C was measured by steady-state method,and the influence of water-binder ratio,initial gas content,fly ash content and curing temperature on the thermal conductivity of concrete was analyzed.The results show that the water-binder ratio,initial air content,fly ash content,curing age and curing were all in inverse proportion to the thermal conductivity of concrete.The correlation between each influencing factor and the thermal conductivity of concrete was confirmed,that was curing temperature>initial air content>curing age>water-binder ratio>fly ash content.A time-varying model of concrete thermal conductivity was established by using multiple regression method.(4)Based on the physical model of bored pile concrete in frozen soil environment and the basic theory of heat transfer,a time-varying temperature field model of bored pile concrete in frozen soil environment was established.The time-varying temperature field model of bored cast-in-place concrete in frozen soil environment was solved by using the separation variable method.From the point of view of physics and thermodynamics,the method of "thermal impulse method" was proposed to solve the inhomogeneous differential equation in the temperature field model.Based on the bassel function,the basic calculation parameters of time-varying temperature field model of bored pile concrete in in frozen soil environment were determined.The time-varying temperature field model of bored pile concrete in frozen soil environment was established to provide theoretical support for the quantitative reverse design of raw materials and mix ratio of bored pile concrete.(5)The temperature field tests of in-door cast-in-situ concrete pile and artificial frozen soil model experiment were carried out.Considering the time-varying characteristics of hydration heat and thermal conductivity of concrete of the cast-in-place pile in in frozen soil environment,the parameters of concrete hydration heat and thermal conductivity expressed by piecewise function were assumed,and the finite element model of artificial frozen soil filling pile was established.The laboratory test data,finite element analysis data and theoretical model calculation data of the temperature measuring point at the pile center were compared.The results show that the finite element calculation model could accurately reflect the temperature change process of concrete in the cast-in-place pile within the reasonable error range,and the rationality of applying the heat generation function and thermal conductivity coefficient of concrete in the form of piecewise function in the finite element model was verified.The variation trend of temperature at the measured point at the pile center calculated by time-varying temperature field theory model was basically consistent with the variation trend of measured data of the artificially frozen soil cast-in-place pile.The calculated value of the time-varying temperature field model established in this paper has little deviation from the actual measured value of the artificial frozen soil cast-in-place pile concrete temperature field in the existing literature.Under appropriate boundary conditions,the theoretical model can be used to quantitatively analyze the time-varying temperature field of bored pile concrete in frozen soil environment. |
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