Alkali-silica reaction - overview

Barrage damaged due to an alkali-silica reactionBarrage damaged due to an alkali-silica reaction

In an alkali-silica reaction (ASR) alkali-reactive siliceous constituents of the aggregates react with the alkali and hydroxide ions (K+, Na+ and OH-) in the pore solution of the concrete to swellable alkali silica gel. Under adverse conditions, the volume of the alkali silicate gel may gradually expand to such an extent as to cause localised swelling pressure. This can damage the concrete microstructure and lead to cracking, efflorescence and spalling. Experts speak of a "damaging ASR", while colloquially the term "concrete cancer" is used. The durability of the concrete may be affected, although the stability of the components is not normally at risk. It usually takes 5 to 10 years or more for such damage to appear. The requirements for a damaging ASR in concrete (concrete cancer) are:

  • a sufficient supply of water
  • a sufficiently high alkali hydroxide concentration in the pore solution of the concrete
  • alkali-reactive aggregates


Spalling on the surface of a concrete test specimenSpalling 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). 

In the case of concrete pavements according to ZTV Beton-StB, load classes Bk100 to Bk1.8 pursuant to the guidelines for the standardisation of road surfacing (e.g. motorways) subject to high dynamic loads and external supply of alkali in the form of de-icing salts, the requirements are defined in TL Beton-StB 07 and the General Circulars on Road Construction (ARS) No. 04/2013 issued by the Federal Ministry for Transport, Building and Urban Development (BMVBS). 

In recent years ASR performance test methods have been developed to investigate whether a damaging ASR can be excluded for a specific concrete composition. The advantage of this approach is that the interaction of the individual concrete constituents is evaluated instead of individual constituents. When new concrete pavements are nowadays built, ASR damage can be more reliably prevented by investigating the potential of the concrete composition for ASR damage by carrying out ASR performance tests prior to installation. 

According to ARS No. 04/2013 the suitability of coarse aggregates for concrete belonging to the moisture class WS can be verified by means of “WS basic testing”. Such testing firstly involves determining the alkali-reactivity of all fractions of the excavation site to be used in concrete road surfaces with the help of the accelerated mortar bar test (reference or alternative method) in accordance with Part 3 of the Alkali Guidelines. This is carried out on three samples to take account of natural deviations. These results are used to determine the mineralogical/petrographical characteristics and to perform testing using the “WS concrete test” for selected fractions. Depending on the concrete composition used, a distinction is made between “WS basic testing” for concrete with a maximum particle size of 8 mm for the fraction to be used (top layer concrete (0/8)) and for concrete with a maximum particle size > 8 mm (top layer concrete (D>8) and bottom layer concrete). 

Where a concrete passes ASR performance testing or where an aggregate passes WS basic testing, the concrete or the aggregate can then be assessed by means of a “WS confirmation test” at short notice and may be used for the construction of road surfaces if there is sufficient conformity with initial values. To assess the level of conformity, accelerated mortar bar tests and mineralogical/petrographical investigations are carried out on current aggregate samples. Suitability can be confirmed either by third-party inspection on a regular basis or in a test carried out once prior to the start of casting the concrete in a building project. In the case of concretes the other concrete constituents should also undergo testing. 

ASR-damage to a concrete pavement may lead to dangers arising from the formation of cracks and eruptions
In a damaging ASR, the volume of alkali-silica gel is increased so much that the concrete cracks.
Even buildings can be affected by a damaging alkali-silica reaction.
400x magnification of an air pore in concrete, which is filled with alkali-silica gel.
Accumulation of alkali-silica gel in a pore next to a pebble grain.
Preparing a test piece in VDZ's concrete laboratory for testing the resistance to alkali-silica reaction of concrete.


Dr. Wibke Hermerschmidt

Dr Wibke Hermerschmidt

+49-211-45 78-374

+49-211-45 78-219


Ingmar Borchers

+49-211-45 78-368

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Dr. Christoph Müller

Dr Christoph Müller

+49-211-45 78-351

+49-211-45 78-296

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Concrete and mortar: testing of fresh and hardened concrete – tests regarding alkali-silica reaction – testing aggregates and admixtures