Study On The Variation Of Thermal Conductivity Of Mass Concrete At Early Age

As the social economy continues to develop,mass concrete has been widely used.Temperature fluctuations in mass concrete can cause cracks and affect its own stress situation,with temperature stress in some cases higher than the stress caused by other loads.Therefore,a scientific and reasonable analysis of temperature stress and the thermal conductivity of concrete is necessary.Previous studies have treated thermal conductivity as a constant value,but in fact,it changes due to the temperature of mass concrete,especially in the early age stage where temperature varies greatly.This dissertation investigates the change of thermal conductivity of mass concrete at the early age stage by testing the thermal conductivity of concrete at 10℃,40℃ and 70℃.The effects of water-cement ratio,curing age,compressive strength,porosity,and water content on thermal conductivity are also studied.The results show that(1)Thermal conductivity is negatively correlated with age,temperature,and water-cement ratio,with a linear relationship to temperature and water-cement ratio and a non-linear relationship to age.A composite exponential prediction model of thermal conductivity and age is established.(2)A nonlinear regression analysis is used to establish a prediction model between thermal conductivity and age,temperature,and water-cement ratio.(3)The compressive strength is linearly and negatively correlated with thermal conductivity,while the multi-factor correlation analysis shows it has the least effect on thermal conductivity.A prediction model between thermal conductivity and age,temperature,water-cement ratio and compressive strength is established.(4)NMR method is employed to obtain T2 profiles,porosity,and pore size distribution of concrete with different water-cement ratios at each age.The correlation between thermal conductivity and porosity is found to be the largest,followed by harmful pores.(5)Immersion method is utilized to study the magnitude of thermal conductivity of concrete with different porosity and water content.It is shown that the relationship between thermal conductivity and water content is linear positive correlation and that the relationship between thermal conductivity and porosity is nonlinear negative correlation with a constant water content.The expressions of the relationships between thermal conductivity and porosity and water content are fitted,and a prediction model of thermal conductivity under the influence of coupling porosity and water content is established by multivariate nonlinear regression.