Equivalent Load Method in Calculating Thermal Stresses during the Construction of Massive Monolithic Concrete Structures

Construction of massive monolithic structures requires assessing the risk of early cracking at the stage of concrete hardening. Most finite element method (FEM) programs do not allow taking into account the change in the mechanical properties of concrete over time in thermal stresses calculation for hardening monolithic structures. The purpose of this article is to eliminate this shortcoming.

A new method is proposed that allows the use of standard finite element software in the task of calculating thermal stresses in hardening concrete without any modifications. It was shown that, instead of a structure with a time-varying modulus of elasticity, it is possible to calculate a structure with a constant modulus of elasticity by replacing the actual temperature load with an equivalent one.

The developed method was tested using the example of the foundation slab. The discrepancy between the calculation results according to our method and the results for a time-varying modulus of elasticity of concrete is insignificant. There is also a satisfactory agreement (discrepancy is less than 5%) with the experimental data up to 6 days, when the risk of early crack formation is highest.

A number of works suggest expressing temperature stresses through the temperature difference between the center and the surface of the structure. It was shown that instead of the actual temperature difference, an equivalent temperature difference should be used.

The proposed method allows to perform calculations of thermal stresses in hardening concrete using standard FEM software. To obtain true stresses that take into account changes in the elastic modulus over time, it is sufficient to apply an equivalent temperature load to a structure with a constant elastic modulus.