Alkali-Silica Reaction in Concrete Caused by Densified Silica Fume Lumps: A Case Study

Petrographic and scanning electron microscopy (SEM) studies of a lightweight aggregate concrete exhibiting severe cracking indicate that a relatively large, nonuniformly distributed and densified formation (lumps) of silica fume (SF) may have substantially contributed to the mechanism of concrete deterioration. The lumps of SF (from 100 to 800 μm in size) were observed to react in a manner similar to reactive aggregates with cement alkalies to form silica gel. Such an alkali-silica reaction (ASR) is often associated with an expansion that leads to cracking of concrete. The poor dispersion of SF also increases the effective water to cement-plus- pozzolan ratio of portions of the concrete, possibly contributing to drying shrinkage cracks. The early drying shrinkage cracks may have led to increased water penetration and subsequent ASR.