Study On The Heat Transfer Characteristics Of Early-age Concrete Under The Environment Of Drastic Daily Temperature Changes

As China’s hydropower construction center gradually moves westward,the global climate environment is changing drastically,coupled with the rapid changes in daily temperature in the northwest region.Compared with mature concrete,Due to the low strength of early age concrete,thermal parameters such as thermal conductivity coefficient are greatly affected by the change of ambient temperature.When the temperature difference exists in different parts of concrete,the temperature stress will be generated,which will lead to concrete cracking.The research shows that the thermal parameters of concrete are affected by many factors such as mix ratio,environmental temperature and humidity,and will change with the age.Therefore,to explore the heat conduction characteristics of early age concrete under the environment of drastic change of daily temperature has great theoretical research value and practical significance on how to effectively reduce the impact of external environment temperature on dam concrete during construction period and reduce the temperature stress.In this paper,the heat conduction characteristics of early age concrete under the environment of drastic change of daily temperature are studied by means of combining macroscopic test with microscopic test and comparing laboratory test with numerical simulation.The main conclusions are as follows:(1)In order to explore the effect of curing temperature and age on the thermal conductivity of early age concrete,the experiment of heat conduction characteristics of early age concrete under the environment of severe change of daily temperature was carried out by using constant temperature and humidity curing box.The results show that the thermal conductivity of early age concrete increases with the increase of age,and the change of specific heat capacity with age is small,which leads to the change trend of thermal conductivity is basically consistent with that of thermal conductivity.The test data in the curing environment with drastic change of daily temperature is less than that in the standard curing environment,and there is a certain lag.At the same time,the thermal parameters of early age concrete in dry condition are all smaller than those in real condition.(2)In order to explore the influence of the microstructure evolution of early age concrete on the macroscopic heat conduction performance,a scanning electron microscope(SEM)was used to carry out the microscopic experimental study of early age concrete under the environment of drastic changes in daily temperature.The results show that the porosity and pore size of early concrete decrease with the increase of age.Under the condition of drastic changes in daily temperature,the concrete specimens have low or even negative curing temperature,which hinders the progress of cement hydration reaction.Part of the hydrated cement particles has produced substances that hinder further hydration due to the surface hydration.The mineral components involved in hydration are reduced,and the internal pore structure of the concrete cannot be effectively filled,resulting in the porosity and pore size distribution of the concrete specimens under standard curing conditions.Low temperature will delay the hydration reaction of cement,which makes the development of early age concrete under the condition of drastic daily temperature change slower than that under the standard curing condition.In addition,through the second-order nonlinear regression analysis of porosity,pore size distribution and thermal conductivity of early-age concrete under different curing conditions,it is found that thermal conductivity has obvious positive correlation with the porosity and pore size distribution of concrete.(3)In order to explore the influence of curing temperature on the internal heat conduction of early age concrete,the finite element software COMSOL Multiphysics was used for numerical simulation.In the numerical simulation process,the difference between the thermal parameters of early age concrete in the environment of drastic daily temperature change and that in the standard curing environment was considered,and the influence on the internal temperature field of early age concrete was considered.The results of numerical simulation show that the internal temperature field of early age concrete are more close to the experimental results under the environment of drastic changes in daily temperature.