Influence of releasable alkalis from aggregates on a deleterious alkali-silica reaction in concrete

The European Committee for Standardisation CEN/TC 154 had planned to replace the essential characteristic "alkali-silica reactivity" in the European harmonized product standard EN 12620 "Aggregates for concrete" by the "releasable alkali content". The requirement "durability against alkalis" of Mandate M/125 was thus to be fulfilled. In the meantime, this plan has been discarded again and the "releasable alkali content" should not appear as an essential characteristic in the future aggregate standard.

Technically, the releasable alkali content of aggregates is nevertheless of interest. For example, there is evidence that certain minerals in aggregates release alkalis and that these can increase the effective alkali content of the pore solution of concrete. This would promote a deleterious alkali-silica reaction (ASR). The alkali contribution could explain why some structures are ASR-damaged even though they were built with a low alkali cement (Na₂O_equ ≤ 0.60%) or with less than 3 kg/m³ Na₂O_equ (from the cement).

If the releasable alkali contents measured in the laboratory are transferred to a concrete with 1850 kg/m³ aggregates, then aggregates could contribute up to 12.7 kg Na₂O_equ/m³ in the worst case (Fig. 2).  However, this is only an indicative laboratory result.

The actual alkali release from aggregates under practical conditions cannot currently be predicted with the laboratory test results, as the relationship is as yet unknown. There is currently hardly any knowledge about the actual release under practical conditions. This question will therefore be investigated in the research project. Furthermore, the relationship between the releasable alkali contents from aggregates in laboratory tests and possible effects on the ASR in concrete will be investigated. Since there is no experience to date on alkali release from German aggregates, the relevance of alkali release for a spectrum of different aggregate types from German deposits will be examined.