Spalling on the surface of a concrete test specimen
In Germany, the "alkali guidelines" issued by the German Committee for Structural Concrete (DAfStb) specifies measures designed to prevent the occurrence of any damaging ASR. To determine preventative measures, it is necessary to characterise the ambient conditions of the concrete by means of the moisture class and the alkali-reactivity potential of the aggregate. For this purpose, every aggregate according to DIN EN 12620 to be used in Germany for concrete in accordance with EN 206/DIN 1045-2 needs to be categorised by an alkali reactivity-class. Preventative measures have to be taken in the case of certain combinations of moisture class, alkali reactivity-class and the cement content (if any) of the concrete. The alkali guidelines include test methods as well as criteria for the categorisation of aggregates into one of the alkali reactivity-classes, in addition to preventative measures based on concrete technology.
A damaging ASR can basically be prevented in two ways:
- replacement of the alkali-reactive aggregate
- reduction of the alkali content in the pore solution of the concrete.
The second measure can be achieved by using special cements with low effective alkali content. In Germany, these cements, which are also referred to as “NA cements”, are standardised in DIN 1164-10. They include Portland cements, Portland-slag cements and blastfurnace cements. The use of pozzolana as a main constituent of cement or an additive to concrete is in principle also suitable for this purpose. No regulations in this regard are yet applicable in Germany.
In many cases alkali-reactive aggregates in concrete can also be used for structures in the building and structural engineering sector without any need for special measures. This, however, depends on the composition of the concrete (cement content) and the environment (moisture class).