Alkali Silica Reaction - External and Internal causes

1. EXTERNAL CAUSES
   • Alkalinity in the water to which the concrete is exposed may cause problems if reactive aggregate was used in the concrete. The water may acquire its alkalinity from soil through which it passes.
   • Deicing solutions may provide alkalis that could cause ASR if reactive aggregate was used in the concrete.
   • Sea water and water from industrial processes may also introduce alkali to concrete.
2. INTERNAL CAUSES
   • Alkali in the cement and reactive aggregate in the concrete in sufficient concentrations will cause ASR.

   Both of these elements must be present for the reaction to occur unless an E-glass is used in the mixture  The following is a list of reactive aggregate. Aggregates listed here have the potential to react by may be stable in some situations.

   argillite	dacite	porphyries	tuffs
   slate	siltstone	opal	obsidian
   cristobalite	tridymite	chalcedony	andesites
   rhyolites	greywacke	phyllite	schist
   gneisses	gneissic granite	sandstone	latites
   artificial glass	volcanic glass	flint	shale
   cryptocrystalline and strained or metamorphic quartz	chert	some dolomitic limestones	granite

   If the unstable component is the coarse aggregate, a higher alkali content is required to cause problems because there are fewer reaction sites (Hobbs, 1988).  If the fine aggregate portion is unstable, greater expansion will be experienced. Most concrete cracked by ASR contains reactive sand. (Hobbs, 1988) Reactive aggregate has a random, disorderly, amorphous microstructure while stable aggregate has a well-organized crystalline structure.

   • Using mix water with high alkalinity will increase the concrete’s alkali content. This is prevalent at ready-mix concrete plants where recycled wash water is used to make concrete.
   • Pozzolans with a high silica concentration should be avoided. However, they cause fewer problems than a high-alkali cement because they produce a less expansive gel.
   • The use of glass fibers or bars (E-glass) in the concrete will cause ASR problems if a sufficient alkali concentration exists.
   • Internal humidity may cause ASR if the concrete is not permitted to dry properly.