Alkali-silica Reaction (ASR) causing irreversible deterioration of concretes occurs when silica in reactive aggregates chemically reacts with alkaline components of Portland cement forming ASR gel.   Sodium (Na) and potassium (K) in the cement react with the reactive silica to create the gel.  When the internal relative humidity of concrete is high enough, the gel absorbs water and swells.

Over time, this expanding ASR gel exerts internal pressure that can lead to cracking of the concrete. Cracks allow ingress of potentially deleterious materials like water, sulfates and chlorides to the interior of the concrete, which in turn can lead to durability issues such as freeze/thaw damage, sulfate attack or steel corrosion.

ASR expansion can be reduced to acceptable levels by use of Type F fly ash and by use of lithium nitrate additive in accordance with the manufacturer’s recommendations.  [::More...]

Alkali-Carbonate Reaction (ACR) is similar to ASR in that the alkaline environment of concrete attacks the aggregate that includes reactive particles.  In ACR, the alkalines react with dolomite limestone, replacing it with less stable and expansive products.  This reaction usually occurs early and structures may show cracking within 5 years after construction.

Over time, the ACR products create a "rim" around the aggregate, weakening the bond and creating micro cracks and voids.  Cracks allow ingress of potentially deleterious materials like water, sulfates and chlorides to the interior of the concrete, which in turn can lead to durability issues such as freeze/thaw damage, sulfate attack or steel corrosion.

ACR damage can be prevented by the use of non-reactive aggregates, reducing available hydroxides, controlling moisture and temperature and minimizing porosity. 

Once ACR begins, it will continue until the reactants (that is, dolomite and hydrated lime) are exhausted. Because of this, structural damage caused by ACR can only be repaired by complete replacement of affected members

ACR is relatively rare because aggregates susceptible to this reaction are usually unsuitable for use in concrete for other reasons, such as strength potential.