The problem concerning the construction process of a three to five metre thick concrete wall built in contact with the upstream face of an old masonry gravity dam has been analyzed. This structural modification is required to augment the height of the dam crowning and improve, according to Italian Technical Standards [1], the structural safety of the barrage during the new operational conditions of the reservoir characterized by an increase in the maximum water level.

The wall is realized with layers of about three metre height: during their placement, as a combination of the initial high heat generation, long dissipation path and the small thermal conductivity of the concrete castings, the inner temperature of the wall rise to values considerably greater than the one in the masonry, thus generating high gradients normal to the interface. Since the wall is fixed to the existing structure by means of horizontal steel bars anchored to its upstream face, the differential dilatation resulting from the temperature distribution induces tensile stress in the masonry. Furthermore the concrete elastic modulus significantly increases during this process and, when the internal temperature progressively decreases, restrained contractions produce the rise of internal tractions in the concrete.

These tensile stresses could generate a cracking system near the upstream face of the new composite structure: beside deteriorating mechanical properties of the materials (Neville [3]), this would cause water penetration from the reservoir during the exercise conditions and following pore pressure which will make the static condition of the structure worse (Manenti [2]).

Correct evaluation of the internal state of stress resulting from the described phenomena requires a sequential analysis (multi-physics) which provides:

1. time-spatial evolution of the temperature field due to generation and dissipation of the concrete hydration heat during the construction phase;
2. stress induced by self weight and thermal stains.

Thus a non-stationary thermal analysis has been carried out in order to simulate the hydration process in the concrete blocks; thermal results have been employed as body loads in the subsequent non-linear elastic analysis in which time-dependent evolution of the concrete elastic modulus E has been considered.

Simulation of the wall construction has been obtained by means of the progressive elements re-activation (element birth and death) in the model, thus reproducing the placement of the concrete layers.

In order to test both the finite element code capabilities and the influence of the model parameters, two one-dimensional problems have been investigated: they reproduce in a schematic form the placement of the layers and the hydration heat generation. In this way it has been possible to check birth and death features coupled with the transient non-linear analysis: in fact the existence of a reference solution allows the comparison with numerical results, providing information on their accuracy.

Subsequently a two dimensional plane strain model of the composite structure including part of the foundation rock has been developed. The main results achieved with thermo-elastic sequential analysis are shown.

1

D.M. LL. PP. 24/3/82, "Norme tecniche per la progettazione e la costruzione delle dighe di sbarramento", (in Italian).

2

S. Manenti, U. Ravaglioli, F. Bontempi, "The heightening of an old masonry gravity dam", proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing, Rome 30 August-2 September (2005), Civil-Comp Press. doi:10.4203/ccp.81.5

3

A.M. Neville, "Properties of Concrete", Pitman Paperbacks (1968).

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