Risk of damaging alkali-silica reaction
For ASR to occur in its damaging form, sufficient alkalinity of the pore water solution (hydroxyl ion concentration) must be present to react with certain forms of silica in the aggregate to form a gel. The gel absorbs pore water fluid, swells and exerts a pressure causing cracking.
Damage can only occur if there is;
- sufficient alkalinity, expressed as equivalent Sodium Oxide (Na2Oeq)
- critical amount of reactive silica in the aggregate
- sufficient moisture.
The majority of hydroxyl ions derive from the Portland cement. The cement manufactures publish ‘declared mean alkali contents’. These values are used by BS 8500, Concrete Society TR 30 and BRE Digest 330 in conjunction with other sources of alkali, aggregate type, additions etc., to minimise the potential risk of damaging ASR occurring.
UK specifications are currently based on a 3.5 kg/m3 Na2Oeq, using declared mean values, below which there is negligible risk of damaging ASR in a concrete containing ‘normal reactivity aggregate’. For high alkali cements (declared mean >0.75 %Na2Oeq) this level is reduced to 3.0 kg/m3 Na2Oeq. However, this value is considered conservative, the risk gradually increasing when the total actual alkali content of the concrete exceeds 5.0 kg/m3. Additionally, it has been reported that the inclusion of at least 8% silica fume increases this threshold by a further 0.5 kg/m3 Na2Oeq.
It may be considered that where actual values of alkali and the source of the concrete constituent materials is known, an assessment of an existing concrete for the potential risk of damaging ASR could be carried out against an agreed higher threshold alkali content, so as not to condemn sound concrete.