Autogenous shrinkage and early cracking of massive foundation slabs

The risk of early cracking due to temperature gradients in the early period of concrete hardening of massive monolithic reinforced concrete structures determines the relevance of research into the possibilities of regulating temperature-shrinkage stresses. An important task is to improve the algorithm for calculating temperature-shrinkage stresses due to temperature gradients and autogenous shrinkage, taking into account the kinetics of heat dissipation of concrete, heat exchange conditions, and ambient temperature. Numerous factors influencing the formation of temperature-shrinkage stresses determine the relevance of the use of numerical methods for modeling temperature fields and stresses, which requires dependencies of changes in heat generation and temperature gradients over time, temperature deformations and autogenous shrinkage deformations, tensile strength of concrete, elastic modulus, creep coefficient. Purpose of the study: obtaining dependencies describing changes in the kinetics of heat dissipation and strength, taking into account the properties of cements and the presence of additives; methodology for taking into account autogenous shrinkage deformations when calculating stress values; modeling the stress-strain state of a massive monolithic structure in the early period of hardening and comparison of calculated and experimental values, taking into account the influence of autogenous shrinkage of concrete on the stress level. Materials and methods: modeling temperature fields and stresses from temperature differences and autogenous shrinkage depending on the class and kinetics of heat dissipation and hardening of concrete; experimental studies of temperature fields and stresses in the early period of hardening. Results: the equations describing the kinetics of heat dissipation, strength, and autogenous shrinkage of concrete up to 5 days old depending on concrete hardening rate are proposed. It is shown that failure to take autogenous shrinkage into account can lead to an overestimation of the tensile stress level, depending on the concrete class and the autogenous shrinkage value by up to 30 %. An algorithm is proposed for calculating the kinetics of the stress level in the early period of hardening of massive monolithic structures, taking into account the deformations of autogenous shrinkage, the class, and hardening rate of concrete. The expediency of limiting the temperature difference “center-top” depending on the required reliability in the range from 20...23 °C to 26...28 °C is substantiated.