Alkali Reactivity of Aggregates and Concretes

Alkali-reactive aggregates in concrete may cause expansion due to a damaging alkali-silica reaction (ASR), resulting in serious cracking. The BAW's construction materials laboratory conducts special laboratory tests to prevent such damages from ocurring in new construction or repair measures or to determine the residual reactivity of constructions already damaged.

If the concrete contains materials that are prone to a damaging ASR, i.e. materials with a higher alkali content and alkali-reactive rocks, waterway structures are significantly more likely to exhibit this reaction due to the ever-present moisture (solid structural elements, exposure to water) than e.g. engineering structures in building construction. Even minor previous damages, e.g. resulting from cracks caused by restraints due to hydration heat, can trigger a damaging ASR in the long term, even if the aggregates in question show only low reactivity (picture 1).

Therefore, to prevent any such damages, additional suitability tests following the alkali guideline issued by the German Committe for Reinforced Concrete (DAfStb) are performed whenever a material is suspected of being alkali-reactive (e.g. when using specific aggregates that cannot be clearly assessed according to the relevant regulations). For these tests, specimens are manufactured, using the planned concrete formulations, and kept for 9 months in a fog chamber at a temperature of 40°C (picture 2). During that time, data is gathered, not only on obvious damages (formation of cracks, gel exudation) but also on expansions (picture 3) and changes in the structure (relativ elastic modulus).

If concrete structures already exhibit damages with the typical ASR-related features, core samples are obtained from the damaged structures and exposed to a fog chamber (40°C) to accelerate the reaction; thus it is possible to identify the cause and degree of the damage as well as the residual potential expansion of the concrete. In addition to the above-mentioned evaluation procedures, any potential losses in strength are immediately rated by determining the residual strength after exposure to the fog chamber. These examinations are necessary for assessing bearing capacity and serviceability as well as possible repair measures, if required.