Numerical Simulation On The Thermodynamic Performance Of Early-age Concrete By Mesoscale Method

Non-loading factors in hydrating concrete structures present a significant risk of cracking,which has a major impact on the concrete durability.The temperature stress can be produced by the boundary constraints and varying temperature caused by the hydration heat,then the cracks occur when the temperature stress is above than concrete strength.Therefore,it is an important basis for the evaluation of concrete anti-crack performance to carry out the study on the distribution and time-varying rules of temperature and strength for early-age concrete.The lattice network model,in which concrete is regarded as a three-phase composite material consisting of coarse aggregate,mortar and interfacial transition zone(ITZ)between the aggregate and the mortar matrix,is developed to analyze heat transport process in concrete by assuming that mortar is the main heat source during the hydration process.A numerical simulation study on the distribution and time-varying rules of temperature field is carried out by means of the mesoscale lattice network model,in which the “thermal-chemical” coupling mechanism has been taken into account.At the same time,the analysis has been performed on the influence of aggregate precooling,embedded cold water pipe and heat dissipation boundary on the temperature field.Based on this,the numerical simulation research about the strength development of early-age concrete considering temperature and curing age was conducted,and the influence of temperature control measures and heat dissipation boundary on the strength field wad analyzed.The main contents and conclusions in this paper can be briefly summarized as follows:(1)The lattice network model is developed in terms of the mesoscale structure of concrete to analyze heat transport process,and a heat conduction equation considering the "thermal-chemical" coupling is established,which can be solved by the Galerkin Method of Weighted Residuals and Finite Difference Method.The capabilities of the lattice network model are shown through the agreement between the numerical analysis of adiabatic temperature rise and experimental results.At the same time,the numerical analysis of the influence of aggregate content on adiabatic temperature rise is carried out,and the results show that the higher the aggregate content of concrete,the lower the adiabatic temperature rise.This proves that it is necessary to carry out the research on the heat conduction of early-age concrete in terms of the mesoscopic level,and it is feasible to solve the heat conduction problem of early-age concrete by the lattice network model.(2)A numerical simulation study,which is aimed at the influence of precooling aggregate,embedded cold water pipe and heat dissipation boundary on the temperature field,is carried out.The results show that precooling aggregate can rapidly reduce the overall temperature of concrete,which is not effected by the aggregate particle size in the condition of equal amounts of aggregate.The cooling effect of embedded cold water pipe is better than the precooling aggregate,because it can take away heat persistently.The temperature distribution of early-age concrete varies with the heat dissipation boundary,in general,the temperature of concrete rises rapidly in the early stage of hydration and descends to ambient temperature gradually in the later hydration period,and the distance to the heat dissipation boundary is closer,the peak of temperature is higher.The internal temperature of concrete will change periodically under the influence of the periodic heat dissipation boundary.Compared with the third-type boundary condition,the amplitude of temperature variation is lager and the frequency of temperature variation is loser under the first-type boundary condition.We can know that the influence on the temperature distribution of the first-type boundary condition is stronger than that of the third-type boundary condition.(3)A numerical simulation study on strength field of early-age concrete considering temperature and curing period is carried out.The capabilities of the lattice network model,applied to the prediction of strength field,are shown through the agreement between the numerical analysis and experimental results.At the same time,we also carry out a numerical simulation study,which is aimed at the influence of precooling aggregate,embedded cold water pipe and heat dissipation boundary on the strength field.The results indicate that the development of concrete strength is association with temperature,and the strength of early-age concrete would develop faster with a higher temperature.Precooling aggregate and embedded cold water pipe can also hinder the development of concrete strength,but the influence of the embedded cold water pipe on the development of strength is more significant.Overall,the concrete strength develops rapidly in the initial stage and gradually stabilizes in the later stage.The heat dissipation boundary can influence the development of concrete strength.Compared with the third-type boundary condition,the first-type boundary condition has a more significant influence on the development and distribution of the strength of concrete.