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HERRADOR, M. F. et al. Prediction of mechanical behavior of an expansive 30-year old dam concrete. In: CONGRESSO INTERNACIONAL SOBRE O COMPORTAMENTO DE ESTRUTURAS DANIFICADAS, 2., 2000, Rio de Janeiro. Anais... Rio de Janeiro, 2000.
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Resumo

The analysis of concrete dams of over 20 years of age requires precise knowledge of their theological properties. Linear mathematical models generally proposed by most national codes prove to be of little use in this aspect. Further difficulties appear when concrete suffers the presence of expansive gels such as those produced by chemical reactions between cement alkalis and aggregate silica (alkali-aggregate reactions, AAR). Besides, when using a sophisticated model, real data are needed both to carry out calibration of the model and to verify the quality of its predictions. Whereas AAR expansion has been known for decades and delayed concrete behaviour phenomena have been thoroughly described, there are many unknowns yet to be clarified, such as those related to mechanical behaviour of old age concretes subject to very low application speed or sustained loads, of the kind induced by expansions, and which can produce non-linear creep strains; prediction of evolution of expansion with time; influence on expansion of stress state of concrete... The object of the present study is to propose a mathematical applied model for mechanical behaviour of concretes of over 20 years of age subject to sustained or very slowly increasing loads, including interaction with expansive phenomena. Nowadays, performance of tests in addition to those required by simple quality control has become usual to determine delayed concrete deformations and thus predict the life span of structures, particularly those for which a security analysis is specially interesting. The beginning of these test series coincides with the actual building of the structure, so the concrete object of study is obtained from the same sources and is treated in controlled transport, casting and curing conditions. When revisiting old aged structures, it is unfrequent to find results of this kind of tests, as the study of long time behaviour of the material was seldom or never considered due to the lack of calculation power needed to undertake structural problems by means of non-linear mathematical models. Thus, in the case of old age structures for which previous studies on materials are unavailable, the only remaining solution is to perform tests on the concrete as it is presently, with the subsequent experimental difficulties it implies. This paper describes an experimental which permits calibration and verifying of the mathematical model described, based on a posteriori mechanical testing of already built structures in real environmental conditions via core-drilled specimens, thus permitting evaluation of structural behaviour when no tests have been carried out at casting time. This procedure includes methods for specimen preparation, data acquisition and gauge protection in harsh environmental non-laboratory conditions.

Abstract

A series of creep and other mechanical tests are performed on core-drilled specimens from a 30 year old concrete arch dam. This dam presents expansion problems due to alkali-aggregate reactions (AAR) which show out in the form of upstream displacements and AAR expansion cracking. The main objective of these tests is the development of a material and structural model to predict further displacements of the dam, in order to undertake appropriate prevention measures. The model takes account of expansion-induced deformations, callibrated through experimental results, and introducing a relationship between AAR expansion and tensional state. The paper also describes an experimental procedure for a posteriori creep testing of already built structures in real environmental conditions via core-drilled specimens, thus permitting evaluation of structural behaviour when no tests have been carried out at casting time. This procedure includes methods for specimen preparation, data acquisition and gauge protection in harsh environmental non-laboratory conditions.
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