Time-varying Models Of The Heat Source Function And Thermal Conductivity Function Of Concrete Under Coupled Multi-factor Effects

The cracking of large volume concrete elements,a major element type in modern bridge construction,has traditionally been one of the greater concerns of construction personnel.In order to guide construction and reduce the occurrence of temperature cracks,simulations can be carried out to develop specific temperature control measures for each stage of construction.However,the thermal parameters used for the analysis of the temperature and stress fields in the finite element analysis of mass concrete,the values are usually obtained by means of both experimental and empirical calculations.The experiments are costly,time consuming and rarely used,and the empirical calculations are subject to certain errors,and these parameters are influenced by temperature,time and space,loading and construction conditions during construction,which often leads to distortions in the parameters used,and to unpredictable errors in the results of temperature control in the process of carrying out temperature control of mass concrete,so it is because of this situation that the The operators concerned are not able to regulate the temperature correctly,which leads to the existence of cracks in some of the structures produced by mass concrete.In order to solve this problem,a time-varying model of the thermal conductivity under multi-factor conditions is developed based on simple experimental modifications of the heat source function based on existing research results.It is applied to the finite element simulation of mass concrete in order to improve the accuracy of the finite element model and thus reduce the risk of cracking in mass concrete.The main research contents and results of this paper are as follows.(1)The thermal conductivity isothermal calorimeter test method(accurate and stable test but more costly and the test time cannot be too long),the semi-adiabatic method using a home-made indoor test device(low cost,less accurate and can be easily used in construction sites)and the chemical reaction to calculate the heat generated by the hydration process of cement(low cost but can only be used to calculate the total heat generated).A modified four-segment time-varying model of the heat source function according to the rate of hydration reaction is obtained and applied to subsequent finite element simulations to reduce the errors arising from the inaccuracy of the heat source function in the current bulk concrete hydration simulation.(2)Based on the TPS transient planar heat source method,the thermal conductivity curve of the cement hydration process under the influence of different factors was tested using a thermal conductivity tester.sand content > external admixture.According to the experimental results,the thermal conductivity of concrete increases linearly with the age before final setting,and decreases non-linearly with the age after final setting.The time-varying model of thermal conductivity under the coupling effect of multiple factors in a double-folded form was then developed.The model is then applied to the subsequent finite element simulations to reduce the errors arising from the current empirical value of the thermal conductivity of bulk concrete.(3)For the simulation of the exothermic process of hydration of mass concrete,the heat source function model and the time-varying model of thermal conductivity are substituted into the finite element simulation and compared with the results of the finite element model with conventional concrete thermodynamic parameters and the measured results.