Required research for predicting AAR expansion: the Ichikawa theory revisited

Various unresolved issues surrounding alkali–aggregate reaction (AAR) expansion are revisited in this article, employing the Ichikawa theory as a framework. According to the Ichikawa theory, alkali–silica gel (ASG) generated by the alkali–silica reaction (ASR) within the aggregate interacts with the surrounding cement paste to form calcium silicate hydrate (C-S-H). This reaction allows ions and water to pass through while blocking the movement of ASG, thereby generating internal expansion pressure and causing cracking within the aggregate, ultimately leading to AAR expansion. The theory offers explanations for various phenomena associated with AAR expansion, including the discrepancies observed between expansion behaviour in accelerated tests and exposure environments, the reduced expansion rate under excessive acceleration, the pessimum effects related to temperature, alkali content, curing age and aggregate composition, as well as the contribution of alkali release from the aggregate to long-term expansion. The study concludes with the need to develop a model that balances the ASR rate of the aggregate and the reaction rate of C-S-H formation from ASG, which encapsulates the ASG.