Mechanical effects of alkali silica reaction in concrete studied by SEM-image analysis

The occurrence of alkali-silica and alkali-silicate reactions causes damage in concrete. Even though the reaction has been known for some time, the progress of reaction in affected structures is difficult to predict. This research programme aims to study the relationship between the progress of the reaction and the mechanical properties of the concrete in order to support better prognosis of the effect of ASR on affected structure. The basic principal of the research programme is to characterise the chemical, microstructural and mechanical state of the concrete, the degree of expansion in the initial and final states and how the rate of change of the chemical reaction is related to the changes in mechanical properties. In practice this is difficult to do because: In the field the induction and reaction periods are very long necessitating accelerated testing in the laboratory The amount of reactive material in the aggregate is usually small, and hence difficult to measure The relation between the degree of reaction, expansion and the change of mechanical properties is not known. The microstructural characteristics of some Swiss aggregates have been studied and quantified using microscopic techniques. Mortar and concretes samples were made with the aggregates and subjected to ASR. Image analysis of SEM-BSE obtained from polished samples was used to quantify the reaction degree. In this project, a series of mechanical tests, in parallel to the chemical reaction were undertaken in order to determine the effect of ASR on the mechanical properties of concrete. The principal objective of this research is the development of a tool relating laboratory results to real structures. Important aspects of the research were: A multidisciplinary approach to chemical, microstructural and mechanical characterisation to increase the chances of finding a combination of techniques which can be used to assess the degree of reaction Use of modelling approach to link between microstructural changes and mechanical properties, which can in turn, be used to extrapolate the evolution of properties in the long term The results show a strong relationship between the observed reactivity and the degrees of expansion in the concretes and mortars. The presented micromechanical modeling is able to correctly reflect the laboratory results and sheds further light on the mechanism of ASR pertinent to field structures.

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