The Generation And Extension Of Concrete Crack And Its Thermodynamic Mechanism

Concrete is an important engineering material in the field of engineering construction.As a large amount of hydration heat is generated when concrete is formed,the release of hydration heat to the environment during hardening will cause the generation and extension of concrete crack due to shrinkage deformation,which not only affects its normal service life,but also brings potential security risks.Therefore,how to effectively prevent the generation and extension of concrete crack has always been a key technical problem in the field of engineering construction.In recent decades,people have conducted a lot of researches on the mechanism of the generation and extension of concrete crack by adopting the methods of experiment and numerical simulation.It is found that the factors,which affect the generation and extension of crack,are very complex,such as the variety and dosage of cement,water-cement ratio,water content,sand percentage,the variety and dosage of admixture,aggregate variety,and the temperature,humidity,wind speed of environment and so on.Although these results can provide some technical guidance for controlling the generation and extension of concrete crack,they have not revealed the common mechanism of these factors.From the perspective of thermodynamics,based on the principle of cement hydration and the first law of thermodynamics,firstly,this paper analyzed the heat of cement hydration,and at the same time also analyzed the influencing factors of cement hydration heat.Because the heat generated by hydration of cement is very high,most of the heat will be released from the concrete to the external environment.As the basic thermodynamic process,the release process of cement hydration heat will inevitably interact with the generation and extension of concrete crack.Secondly,because the release of cement hydration heat includes the conduction heat of concrete material itself and the latent heat of vaporization taken away by evaporation of water in concrete,the factors that affect intensity of the two heat release will inevitably affect the generation and extension of concrete crack.As an evaluation index of concrete heat conduction capacity,thermal conductivity affects the intensity of heat release in a thermally conductive manner between the concrete and environment.Therefore,the calculation methods and influencing factors of thermal conductivity of concrete were analyzed.Subsequently,by analyzing the heat carrying process in theform of latent heat of vaporization,it is found that the amount of heat taken away by latent heat of vaporization is related to the rate of water evaporation.Finally,in order to reveal the interaction mechanism between the generation and extension of concrete crack and the heat transfer process between concrete and the environment,it based on the generalized heat engine model.It is found that in the isolated system composed of the concrete and environment,the heat flow rate released by the high temperature heat source of the concrete is partly transmitted to the low temperature heat source where the environment is located.The thermodynamic process is a spontaneous heat transfer process.The other part is converted into elastic-plastic deformation work in the concrete.The thermodynamic process is a non-spontaneous generalized work process,which contains the thermodynamic coupling mechanism that the non-spontaneous generalized work process must be driven by the spontaneous heat transfer process.Therefore,the generation and extension of concrete crack is essentially caused by the release of cement hydration heat.In addition,by establishing the relationship between thermal energy and elastic-plastic deformation work,it is found that the degree of concrete deformation is not only related to the temperature difference between concrete and the environment,but also related to the intensity of heat release between concrete and the environment.The intensity of heat release is mainly composed of the intensity of heat release in a thermally conductive manner and the strength of phase change heat transfer.Therefore,although the current influencing factors for the study of generation and extension of concrete crack are intricate and complex.They can be summarized into three aspects that include the temperature difference between concrete and environment,the intensity of heat release between concrete and environment in a thermally conductive manner,and the intensity of phase change heat transfer between the concrete and environment.Therefore,it is found that the thermodynamic nature of the factors is to influence the generation and extension of concrete crack by affecting the temperature difference between concrete and environment,the intensity of heat release in a thermally conductive manner,or the intensity of phase change heat transfer,which provides a unified thermodynamic description for the influencing factors of concrete crack generation and extension.The smaller the temperature difference,the intensity of heat release in a thermally conductive manner or the intensity of phase change heat transfer between concrete and environment,the smaller the degree of concrete deformation,and the more difficultthe generation and extension of concrete crack.Accordingly,the factors that can reduce the temperature difference,the intensity of heat release in a thermally conductive manner or the intensity of phase change heat transfer can effectively prevent the generation and extension of concrete crack.Based on this conclusion,we will find more measures that effectively control the generation and extension of concrete crack from these aspects.