| The thermal conductivity of cement-based materials is an important parameter to characterize its thermal properties,and also a key thermodynamic index for the analysis of temperature field and temperature stress field of mass concrete.Therefore,the accurate determination of its thermal conductivity is crucial to the analysis of heat conduction of concrete structures and the prediction and control of temperature cracks.For this reason,this paper conducts a comprehensive study on the thermal conductivity test and model of cementbased materials under the condition of periodic temperature variation,in order to provide a calculation basis for the temperature field simulation calculation of mass concrete in cold region,as follows:(1)The projection pursuit regression(PPR)no-hypothesis modeling technique was used to analyze the weight degree of the factors affecting the thermal conductivity of cement-based materials.The results showed that temperature and saturation were the two factors with the greatest influence,and the saturation was closely related to the water-binder ratio.Therefore,temperature,saturation and water-cement ratio were selected as the main influencing factors in the test design.(2)Under the same conditions,by combining the high frequency resistivity test and the low frequency thermal conductivity test of cement mortar,the variation rule and correlation of the two under the condition of periodic temperature change were analyzed,determine the reliability of the model by using an accuracy test method that conforms to the qualified rate of the model and test samples,based on this,the change rule of thermal conductivity of cement mortar under periodic temperature change is analyzed.The results show that the thermal conductivity and resistivity of cement mortar are negatively correlated with the temperature under the condition of periodic temperature variation.There is a good exponential relationship between the thermal conductivity and the resistivity of cement mortar,and the model can describe the evolution law of the thermal conductivity of cement mortar under the condition of periodic temperature variation,which proves the feasibility of using the resistivity high-frequency test to obtain the experimental data of the thermal conductivity of cement mortar.(3)Based on the mesoscopic composite material theory and the calculation model of thermal conductivity of two-phase composite material,the calculation model of thermal conductivity of cement-based material under the condition of periodic temperature variation is established.The ice crystal growth process in the pores of cement mortar under the condition of periodic temperature change was analyzed by using the ice model of resistivity method.Secondly,the characteristics of pore structure change under the condition of periodic temperature change were obtained by means of the microscopic test of mercury injection instrument,and the evolution mechanism of thermal conductivity of cement mortar was expounded from the microscopic perspective.The results show that the calculation model of thermal conductivity of cement-based materials under the condition of periodic temperature variation has high accuracy and reliability,water-cement ratio and pore structure(porosity,pore size and distribution)have important effects on the freezing amount of cement mortar under the condition of periodic temperature variation,and the change of pore structure and the induced growth of pore ice crystal are the essential reasons for the thermal conductivity evolution of cement mortar. |
