Thermal stress evaluation and safety aspects associated with massive concrete constructions joined to existing structures: The case of a masonry gravity dam

This study has been carried out with the aim to investigate, by means of the Finite Element analysis, thermal stresses induced by hydration heat generated during the construction phase of a massive concrete structure. An useful numerical tool has been developed in order to analyze the construction process of a concrete wall three-five meters tick built in contact with the upstream face of an old masonry gravity dam. This structural modification is needed to augment the height of the dam crowning and improve structural safety of the barrage during the new operational conditions of the reservoir characterized by an increase of the maximum water level and seasonal storage. The wall is built with layers about three meters height: during their placement, as a combination of initial high heat generation, long dissipation path and small thermal conductivity of the concrete, the inner temperature of the wall rise to values considerably greater than the one into the masonry, thus generating high gradients normal to the wall-dam interface. Furthermore during the hydration process the elastic modulus of concrete rises significantly producing an increase of the internal constraints. Being the two structures fixed each other, they becomes coupled for both thermal and elastic events: differential dilatation and constrained deformations induce tensions of traction that, when exceed the tensile stress, induce a cracking system. Beside deteriorating mechanical properties of the materials, this would cause water penetration from the reservoir during the exercise conditions and following pore pressure which worsen the static condition of the structure. Correct evaluation of the elastic internal stresses due to described phenomena requires to perform a sequential transient non-linear analysis providing time-spatial evolution of the temperature field due to the hydration heat and stresses induced by thermal stains. Preliminary studies have been carried out by means of a one dimensional model including the main features of the problem in order to test the Finite Element code capabilities and the influence of parameters.