ASR is a damaging mechanism in concrete that causes cracking and deterioration of the concrete. ASR happens when concrete is made using certain aggregates that are reactive with alkali. These aggregates are composed of silica and so ASR stands for Alkali-Silica Reaction (ASR).

ASR causes cracking and deterioration of the concrete. (Image courtesy of the US Department of Transportation)

The alkali part of the reaction comes primarily from the cement in the concrete mix. ASR occurs at the interface between the cement paste and the surface of aggregate pieces forming a layer of what is called silica-gel. This silica-gel will readily absorb water and when it does, it swells in size. The resulting expansion pressure is enough to fracture the concrete and the result is concrete that is covered in interconnected cracks that look like a map of London. The process can take as little as a few years to develop serious cracks.

Cracks are not good for concrete – especially since most concrete contains steel reinforcement, which will corrode when exposed to water and salts entering through cracks. Corrosion creates its own expansion and this only further disrupts the concrete and accelerates its deterioration.

There are three essential elements needed for ASR to occur: alkali, reactive aggregates and water. Efforts to prevent or mitigate ASR have nearly always focused on measures to reduce or eliminate the first two. Special low-alkali cements have been used. Partially replacing cement with pozzolins such as fly-ash and slag have also been demonstrated to help. Of course, avoiding the use of reactive aggregates would definitely prevent ASR. But these various measures are not always available or economical.

The idea of eliminating water has been largely ignored. This is likely because water is generally accepted to be ever present. While normal concrete is porous and will absorb water, there are multiple ways to reduce or prevent the penetration of water into the concrete.

The incorporation of Krystol Internal Membrane (KIM) to a well-proportioned mixture can aid in the prevention of water into concrete and protection against the harmful expansion forces of ASR. When it comes to creating durable and sustainable structures, it’s important to consider the benefits of keeping water out of concrete.