Alkali-silica reaction - overview
In an alkali-silica reaction (ASR), alkali-sensitive constituents in the aggregates react with the alkalis (potassium and sodium hydroxide) in the pore solution of the concrete to form an alkali silicate gel having water absorption tendency. Under adverse conditions, the volume of the alkali silicate gel may gradually expand to such an extent as to cause swelling pressure locally. This may in turn damage the concrete microstructure and lead to reticular crack formation, efflorescence and pop-outs visible on the surface. Experts might be talking about a “damaging ASR”, whereas the term used in the media is “concrete cancer”.
It usually takes 5 to 10 years or more for the damage to appear. The durability of the concrete may be affected, whereas the stability of these structural elements is usually not imperiled.
Requirements for a damaging ASR in concrete are:
- a sufficient supply of water,
- a sufficiently high alkali hydroxide concentration in the pore solution,
- a critical amount of reactive mineral species.
In Germany, the “alkali guidelines” issued by the German Committee for Structural Concrete (DAfStb) regulate the measures intended to prevent a damaging ASR. In order to determine measures for the prevention, the concrete’s environmental conditions by means of the moisture class and alkali-reactivity-class of the aggregates have to be specified. Each aggregate applied for concrete in accordance with EN 206-1/DIN 1045-2 has to be classified into an alkali-reactivity-class according to DIN EN 12620 in Germany. In case of certain combinations of moisture class, alkali-reactivity-class and, if any, cement content of the concrete, preventive measures have to be taken. The alkali-guidelines comprise test methods as well as criteria for the classification of aggregates into one of the alkali-reactivity-classes and preventive concrete technology based measures.
Substantially, a damaging ASR can be prevented by two measures:
- replacement of the alkali-reactive aggregate
- reduction of the alkali content in the pore solution of the concrete.
The latter measure can be implemented by using special cements with low effective alkali content. In Germany, these cements, which are also called NA-cements, are standardised in DIN 1164-10. They comprise Portland cements, Portland-slag cements and blastfurnace cements. Basically, the use of pozzolana as main cement constituent or concrete addition also serves this purpose. No regulations are yet applicable in this regard in Germany.
In many cases also alkali-reactive aggregates in concrete can be used for constructions in the field of building and structural engineering without having taken particular measures. However, that depends on the concrete composition (cement content) and the environmental conditions (moisture class).
In case of concrete road pavements in accordance with ZTV Beton-StB, classes SV and I to III (e. g. motorways) due to high dynamic load and external alkali supply by de-icing salts, the requirements are defined in the TL Beton-StB 07 and road construction circulars issued by the Federal Ministry for Transport, Building and Urban Affairs (BMVBS).
In the last years so-called ASR-performance test methods were developed. These test methods consist of testing whether a damaging ASR can be ruled out for a certain concrete composition. This approach holds the advantage that the interaction of the individual concrete constituents is evaluated instead of individual constituents being assessed. Today, when new concrete road pavements are being built, ASR-damages can be prevented much better by investigating the ASR-damaging potential of concrete compositions prior to use by means of ASR-performance tests.