Alkali-Silica Reaction (ASR) in Concrete Petrography

Cause and Effect

In the alkali-silica reaction (ASR), alkalis in the pore solution react with soluble quartz (silicate) to form an expanding gel. This reaction requires the sufficient availability of alkalis, soluble quartz, and water, as it also occurs, like the hydration of cement, through dissolution processes.

Figure 1: Compaction pore in concrete slab with marginal AKR gel deposits and a crack system originating from the pore.

The availability of alkalis is usually regulated by the cement, where a known amount of alkalis is indicated in the form of the sodium equivalent (Naequ = Na2O + 0.658 * K2O). An exception is the direct and indirect exposure to de-icing salts in road and airport operations, as well as in parking garages and lots, where de-icing agents introduce additional alkalis into the pore solution of the concrete.

Another factor is the availability of easily soluble quartz, which can influence the susceptibility and rate of ASR. The crystalline structure of quartz, which varies significantly depending on the formation of the respective rocks, plays a crucial role. Magmatic crystalline quartz from granites is considered difficult to dissolve, whereas quartz from sedimentary rocks such as chert, sandstones, and greywackes, as well as silicate inclusions in carbonate rocks, exhibit significantly higher solubility.

In addition to the chemical and mineralogical prerequisites, the availability of water is essential for the formation of ASR. This depends largely on the density and structure of the concrete microstructure. Besides the type of cement used and the water-cement ratio, the proper placement (compaction, homogeneity, curing) of the concrete plays a critical role. Even small oversights during installation, such as inadequate curing, can make the microstructure more permeable due to a significant increase in capillary porosity. Subsequently, the use of additional de-icing agents can lead to a concrete-damaging ASR. Compaction errors can also result in non-ideal air pore systems or inhomogeneities, which promote water permeability and thus allow an alkali-silica reaction to occur.

When damage such as cracks and spalling occurs on concrete surfaces, it is always advisable to identify the actual damage mechanism, as this knowledge can save significant costs in repair and restoration planning, as well as in damage prevention for future projects. A clear diagnosis of the damage cause is only possible through microscopy (polarizing microscopy and/or scanning electron microscopy) of the concrete structure.

Figure 1 shows an example of ASR damage, where the combination of easily soluble quartz (chert and sandstone), de-icing agents, and insufficient curing of the concrete surface led to a damaging ASR with macroscopic network cracking.

...Concrete doesn't forget!

This fundamental principle is no news to the community of concrete petrographers, but its significance is vastly underestimated. By reconstructing the history of the concrete, from installation to current condition assessment, damage causes can be clearly identified or excluded. This provides infrastructure managers with better planning security, damage progression predictions, implementation of repair and maintenance measures, and knowledge for damage prevention.