Experimental Research On The Thermal Conductivity Of Concrete Under Constant Low And Negative Temperature Conditions

With the implementation of the "Belt and Road" cooperation initiative,the frozen soil engineering in the cold area is also developing in an orderly manner.However,during the development process,many new problems and challenges have been encountered,especially in the frozen soil area of the Qinghai-Tibet Plateau in my country.In order to ensure the normal operation of lines in frozen soil areas,bridges with bored piles as the main foundation are often used to cross sections with poor geological conditions.Taking into account the engineering problems encountered during the construction of the pile foundation,when applying bored piles in frozen soil areas,it is necessary to ensure not only the thermal stability,but also the critical strength of frost resistance.Thermal conductivity is a key parameter that affects the thermal stability of pile foundations in frozen soil areas.Therefore,studying the properties of thermal conductivity of pile foundations in frozen soil areas is of great significance for optimizing engineering design and ensuring engineering safety.This paper takes the low and negative temperature environment in the permafrost regions of the Qinghai-Tibet Plateau as the research background,combines experimental research,model prediction and other research methods,and uses a combination of macro and micro research methods to analyze concrete that is in an unsaturated state at different temperatures and different ages.Conduct research on thermal conductivity.The main research content and results of the thesis are as follows:(1)Using the heat flow method thermal conductivity meter,based on the steady-state method,the thermal conductivity of early-age concrete under the influence of different factors is obtained.According to the experimental results,the influence of age,curing temperature,and water-binder ratio on the thermal conductivity of concrete is analyzed,It is found that the thermal conductivity of concrete shows a nonlinear downward trend with the increase of age.In view of this,a composite exponential time-varying model of the thermal conductivity of concrete at early age is established.(2)In order to explore the time-varying model of thermal conductivity of early-age concrete under the influence of multiple factors,considering the multiple effects of curing temperature and water-binder ratio on thermal conductivity,a time-varying model of thermal conductivity under the coupling effect of multiple factors was established.This model can accurately describe the relationship between the thermal conductivity of concrete,age,curing temperature,and water-binder ratio.(3)Through the study of the relationship between the thermal conductivity and compressive strength of concrete at an early age under the influence of different curing temperatures and different water-binder ratios,it is found that under the influence of a single factor,the thermal conductivity of concrete is negatively correlated with the compressive strength,but the curing is considered comprehensively.Under the influence of temperature and water-binder ratio,the thermal conductivity of concrete has little correlation with the compressive strength.In view of this,a coupling model of the early-age concrete thermal conductivity,compressive strength,curing temperature,and water-binder ratio is established.(4)Using nuclear magnetic resonance instrument to test the porosity and pore size distribution of the early-age concrete at different curing temperatures at different ages,and use the drying method to study the change process of the early-age concrete moisture content.According to the test results,Based on the gray correlation theory,the correlation between the thermal conductivity of concrete and porosity,the proportion of different pore diameters and the moisture content is calculated,and it is revealed that the dynamic change of the thermal conductivity of concrete at early age is mainly affected by moisture content,porosity and harmful pores.(5)Based on the 28-day-old concrete specimens,the immersion method was used to study the influence of the single factor of moisture content and porosity on the thermal conductivity of concrete.It was found that when the moisture content was constant,the thermal conductivity and porosity of concrete became exponential.Decrease trend;when the porosity is constant,the thermal conductivity of concrete is in the state of low water content,and the two satisfy a linear relationship.Considering the coupling effect of moisture content and porosity on the thermal conductivity of concrete,a multivariate nonlinear model of concrete thermal conductivity,moisture content and porosity is established.This model has a good overall prediction effect on thermal conductivity data under different moisture content and porosity.