Alkali release from aggregates (ARA) in concrete is a hot topic on the international concrete research scene. ARA is part of the puzzle of understanding the mechanisms of the deleterious alkali silica reactions (ASR) in concrete.

First, what is ASR?

Simply put, concrete is a very alkaline material with a pH of 13+, which is in the range of the pH of drain cleaner. If there is sufficient moisture present (e.g. rain exposed concrete or concrete dams), specific siliceous minerals in the aggregates (=rocks), used to produce the concrete, might start to dissolve in this high pH. After that, ASR products (ASR-gel) form and cause the concrete to expand. These expansions lead to cracking and structural failure of reinforced concrete structures. In Norway, we have a lot of ASR reactive aggregates, and ASR typically affects dams and bridges. ASR is a slow degradation mechanism. The first signs of ASR are often visible after 10-15 years. Though as it mainly affects structures with design lives up to 100 years, it does post an enormous cost on our society. As it is difficult or nearly impossible to repair ASR affected concrete structures, often structures simply are replaced way before their design service life. The only way to avoid ASR is through the concrete mix design, meaning using safe combination of aggregates (potentially reactive) and cement. This is also what regulations focus on.

Now, what is ARA?

Generally, the cement is the main contributor to the high pH of concrete. Thus, in the Norwegian regulations we use the alkali content of the cement to calculate the alkali load (and thereby indirectly the pH) of the concrete. The regulations describe a maximum alkali content of the concrete, and thereby the amount of cement that can be used. Though, aggregates themselves might potentially also contribute with alkali, but this is currently not included in the regulations. In 2021, a new method is published to determine the potential release of alkali from aggregates (ARA). This method is called (RILEM AAR-8) and was developed by a team of international researchers within the RILEM TC 258-AAA (2014-2019) lead by Børge J. Wigum (HeidelbergCement Northern Europe) assisted by Jan Lindgård (SINTEF). Whether the value obtained with this method reflects the condition in real concrete is unknown. There is an urgent need to critically discuss the results from this method and to verify whether they at all reflect reality. Børge J. Wigum is part of the European standardization committee for concrete aggregates. In 2020, he witnessed that the AAR-8 method or a related method was almost adopted as a new European standard. This could lead to a problematic situation where either many aggregate sources are excluded from the market without founded reason, or unrealistic demands are put on the alkali content of the cement to compensate for alkali coming from the aggregates. The verification of ARA is therefore a very important topic for cement producers as well as aggregate and concrete producers.

The project leader for the ARA project is Børge J. Wigum from HeidelbergCement Northern Europe (HCNE). Terje Rønning also from (HCNE) is central in the ARA project. The research project partners are SINTEF with Jan Lindgård and Tobias Danner and the Concrete Group at NTNU, with Mahsa Bagheri (postdoc), Klaartje De Weerdt and Mette Geiker.