Børge Johannes Wigum

Alkali–silica reactivity of some common rock types. A global petrographic atlas

The correct identification of potentially alkali–silica reactive aggregates is important for the prevention of alkali–silica reaction (ASR) in concrete. Although a number of standards for assessment of concrete aggregate by petrography are available, distinction of potentially deleterious from innocuous rock types can be problematic. The application of geological nomenclature alone is insufficient, as the geological history and hence mineralogical texture of a given rock type may strongly influence its performance in concrete. One of the goals of RILEM TC 219-ACS is to develop a worldwide photo atlas as a guide for petrographers in the identification of the mineral compositions and textures that are characteristic of alkali-reactive rocks. The atlas is based on micrographs of rock types recognized as potentially deleterious by field performance and/or laboratory expansion testing. It is intended to serve as an independent reference work and aims to unify rock terminology so as to improve the petrographic characterization of aggregates. The mission of RILEM, the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (Réunion Internationale des Laboratoires et Experts des Matériaux), is to advance scientific knowledge in the field of construction. Its Technical Committee TC 219-ACS focuses on alkali reactions and their effects on concrete.

Grain Size Analysis of Quartz in Potentially Alkali-Reactive Aggregates for Concrete: A Comparison Between Image Analysis and Point-Counting

This article describes a new image analysis method for grain size analysis of quartz in potentially alkali-reactive aggregates for concrete. The accuracy and speed of this new method are compared against the accuracy and speed of the traditional point-counting method. Quantitative petrographic examination of potentially alkali-reactive aggregates, based on grain size analysis by point-counting method, has shown that a relationship does exist between the total grain boundary of quartz and expansion results from accelerated mortar bar testing (Wigum, BJ (1995): Examination of microstructural features of Norwegian cataclastic rocks and their use for predicting alkali-reactivity in concrete. Engineering Geology (40): 195–214; Wigum, BJ, Hagelia, P, Haugen, M, and Broekmans, MATM (2000): Alkali aggregate reactivity of Norwegian aggregates assessed by quantitative petrography. In: Berube, MA, Fournier, B, Durand, B (editors): Proceedings of the 11th International Conference on Alkali-Aggregate Reaction in Concrete, Quebec: 533–542). In this article, the authors performed quantitative petrographic examination of potentially alkali-reactive aggregates, based on grain size analysis by image analysis, of a few selected samples. The results were compared against the results obtained by the traditional point-counting method for the same samples. This approach revealed that image analysis is more time efficient than point counting without compromising the accuracy of the results.