Influence of granitic aggregates from northeast Brazil on the alkali-aggregate reaction

Abstract

The alkali-aggregate reaction (AAR) in concrete structures is a problem that has concerned engineers and researchers for decades. This reaction occurs when silicates in the aggregates react with the alkalis, forming an expanded gel that can cause cracks in the concrete and reduce its lifespan. The aim of this study was to characterize three coarse granitic aggregates employed in concrete production in northeastern Brazil, correlating petrographic analysis with the kinetics of silica dissolution and the evolution of expansions in mortar bars, assisted by SEM/EDS, XRD, and EDX. The presence of grains showing recrystallization into individual microcrystalline quartz subgrains was associated with faster dissolution of silica and greater expansion in mortar bars. Aggregates showing substantial deformation, such as stretched grains of quartz with strong undulatory extinction, experienced slower dissolution, with reaction and expansion occurring over longer periods that could not be detected using accelerated tests with mortar bars.

Relationship between Degree of Deformation in Quartz and Silica Dissolution for the Development of Alkali-Silica Reaction in Concrete

“…However, in most samples the recrystallized grains occur only at the edges of the original grains, as Figure 3d shows. Neto et al [13] also observed a presence of recrystallized sub-grains in deformed quartz.…”

“…On the other hand, many studies have shown that microcrystalline quartz grains are more reactive and promote greater silica dissolution than grains with intense undulatory extinction [12,13,14]. However, the continuous increase in the displacements of the crystal leads to a clustering of the networks, known as sub-grain walls which, with the boost in tensions, can break the original grain and thus form new recovered grains, that is, grains which do not have displacements.…”

Study of the alkali-silica reaction rate of Spanish aggregates. Proposal of a classification based in accelerated mortar bars tests and petrographic parameters

“…G. Neto et al (19) have carried out several studies trying to link the potential reactivity of aggregates with the dissolution of the silica of different aggregates (20,21). He classified them into two groups, according to the reaction kinetics.…”