Thermal stresses in concrete structures at early ages

Abstract: This thesis deals with the problem of formulating constitutive models for the analysis of early-age thermal stresses in concrete due to hydration. The main features of the work are constitutive modelling of early-age concrete behaviour experimental studies to check the models and to determine material parameters application of the models to some design and construction situations. The proposed model for thermal stress analysis in young con- crete is based on a rate-type viscoelastic law which is supplemented with a strain-softening model for high stress level nonlinearity in tension and by an adjustment for low- stress nonlinearity in compression. In order to demonstrate the potential of the model it has been applied to some typical cases in practical engineering. Thus, the risks of thermal cracking are computed for A - a section poured between inflexible supports B - a differential temperature field across a wall section C - a wall cast on a concrete slab D - a box tunnel element E - a concrete cover layer applied on a bridge pier. From the computations it is concluded that the magnitude of thermal stresses do not only depend on temperature variations but to a critical extent also on the mechanical behaviour of the hardening concrete as well as on the nature and degree of the restraint. The thermal stresses are in most cases not proportional to the temperature field within the structures as defined by maximum temperature rise, maximum temperature differences etc. It is evident from this study that, in some early-age applications, an analysis solely based on the temperature field may not even reveal whether a part of the concrete structure is in tension or compression.