Alkali-aggregate reactions, strengthening or total collaps? The different effects of AAR on concrete structures

Alkali-aggregate reactions (AAR) can have many different effects, ranging from nothing happening to total cracking of the structure. The reason for this is related to the fact that chemistry and mechanics interfere with each other in the reactions. So, factors such as the concrete composition, the production procedure, the design, and the climatic conditions, and in repair situations, the repair conditions, influence the development of the reactions.An overview of the many consequences and the influencing factors is necessary, when a certain structure suspected of being influenced by AAR is going to be analysed. In the present paper a proposal for the time development is given. The time development of the harmful reactions consists of an initiation period, a propagation period, and a rest period. - For a given concrete in a given environment, the length of the initiation period depends upon the square of the dimension of the structural member in the direction of diffusion. - The time rate in the propagation period is determined by the rate of the chemical processes and of the creep and fracture properties of the concrete.Decisions about repair ought to be made on the basis of a plot of the time development. As a case study a road bridge is analysed according to the principles presented. Keywords: Alkali-aggregate reactions, alkali-silica reactions, bridges, concrete structures, dams, repair decision, time development.