Mechanism of damage for the alkali–silica reaction

A novel mechanism for the damage induced by alkali–silica reaction (ASR) is proposed. Two reaction steps are taken into account in the mechanism: the Q3 tetrahedrons formation by breaking up siloxane bonds and the dissolution of these Q3 tetrahedrons. We demonstrate that the formation of Q3 tetrahedrons in the aggregate prevails over dissolution during the swelling step. The formation of Q3 tetrahedrons causes a swelling and a micro-cracking of the aggregate: we observe a significant increase of the aggregate pore volume. A model based on a volume balance between the aggregate expansion and the swelling of mortar bars is proposed. This model enables us to measure an amplification factor of the aggregate swelling. This amplification factor is high (about 3) and related to the stiffness of the low porosity cement paste and to the cracking propagation process.