Aging approach to water effect on alkali-silica reaction degradation of structures

The maintenance of a considerable number of concrete structures the durability and serviceability of which are endangered by the alkali-silica reaction (ASR) requires a quantitative structural evaluation and prediction, in time and space, of the structural effects of ASR expansion. Considerable research effort has been dedicated to the mechanical modeling of ASR-induced concrete swelling. However, its predominant factor, the internal humidity, has not yet been clearly understood. Recent laboratory tests on the influence of humidity on ASR expansion allow us to better understand the swelling mechanisms. The aim of this paper is to discuss the, role of water at both reaction and material level, to propose a comprehensive mechanical model for the material swelling with a hydrochemomechanical approach, and finally, to study structural effects of ASR by means of the new model. The proposed model adopts a two-stage mechanism for the swelling kinetics, consisting of the formation of an amorphous gel for which a characteristic time of reaction is identified and of the combination of important quantities of water by the gel. Because the combination of water shows an aging effect a second reaction with a characteristic time of aging is introduced. Furthermore, the initial phase of the material swelling is explained by the filling process of internal pores by the swollen gel. The model is verified by using experimental data. At a structural level, a characteristic ASR water diffusion length is proposed to evaluate the,concrete surface delamination depth. Its range is calculated by means of an one-dimensional analysis of ASR swelling activated by water diffusion. As a case study a reactive retaining wall is analyzed with the new model integrated into a finite element scheme. The material degradation process and the structural responses are illustrated and discussed.