Cracks due to alkali-silica reaction
Alkali-silica reaction occurs when the alkaline pour fluid in the cement paste reacts with the minerals in some aggregates. The resulting gel takes up pour solution water and expands, which can disrupt the concrete.
In the early stages, in unrestrained concrete, surface cracks form in a random manner (similar to those caused by freeze-thaw) often referred to as ‘map’ cracking. When the expansive forces are restrained, for example by reinforcement or by applied loads in a column, the cracks tend to run parallel to the direction of restraint, i.e. parallel to the reinforcement or vertically in a column. The cracks may exude gel which shows up as white deposits on the surface.
The earliest time at which cracking due to alkali-silica reaction has been observed and identified in structures in the UK has been about five years after casting, but it could be earlier for some aggregates. With other aggregates it may not appear for 40 years.