Alkali Silica Reaction In Concrete [ASR]
D. K. Doran
1. Introduction
ASR is one example of a more general problem known as Alkali Aggregate Reaction [AAR]. ASR is a chemical process in which alkalis, usually predominantly from the cement, combine with certain types of silica in the aggregate when moisture is present. This reaction produces an alkali-silica gel that can absorb water and expand to cause cracking and disruption of the concrete. For damaging reaction to take place the following need to be present in sufficient quantities.
- High alkali cement
- Reactive aggregate [eg crushed greywacke type sandstone]
- Moisture
Structures most at risk are bridges, hydraulic structures, exposed frames [eg open multistorey car parks] and foundations. In relation to total construction volume, the incidence of significant damage due to ASR in the UK is small. ASR was discovered in the USA in the 1940s; no case was found in the UK until 1971 [in a concrete dam in Jersey] although recent publicity has revealed that the Montrose Bridge in Scotland [now demolished because of ASR] was about 75 years old. The problem is known to exist in at least 35 countries.
Affected concrete often exhibits map cracking of the surface, known colloquially as Isle of Man cracking after that island`s three legged emblem. In heavily loaded sections the cracking will probably follow the line of the main reinforcement.
An excellent overview of the phenomenon can be found in BRE Digest 330.
2. Minimising the risk
Since the early 1980s recommendations have been produced and refined to the stage where ASR is unlikely if these are adopted. Principal methods include.
- Controlling the level of available alkalis in the concrete mix.
- Using pfa and/or ggbs as a partial cement replacement
Full details of these recommendations can be found in Concrete Society report TR No: 30 [Third Edition] often known as the Hawkins report.
Further work is going on to investigate the use of silica fume, metakaolin and lithium salts to achieve the same objective. Research has indicated that air entrainment is not significantly beneficial in inhibiting the problem. This work is being supervised by a BRE Working Party and a progress report is expected shortly.
At a time when recyling of materials has assumed a high priority specifying engineers should be aware that CS Report 30 [Third Edition] recommends that recycled aggregate derived from demolition waste be classified as highly reactive. This is suggested as a precautionary measure pending further knowledge about their long term performance.
3. Diagnosis
If ASR is suspected, then an investigation should follow. The extent of this investigation should be related to the criticality of the structure and the perceived intensity of the reaction. Full details of a diagnosis programme can be found in BCA report The diagnosis of alkali-silica reaction.
In essence this report recommends the following:
- A desk study to ascertain the source and analyses of materials used.
- Critical examination of site to determine if causes other than ASR are present
- Sampling eg by coring in selected areas
- Thin section petrography and other analysis of samples to determine the cause of cracking
- Report and recommendation on whether remedial treatment is required
4. Structural Aspects
If ASR has been diagnosed, then remedial treatment may be required. This may take one of the following forms:
- Regular monitoring of the structure to check that deterioration does not reach dangerous proportions. This requires experienced engineering judgement
- Detailed check on the structural details to establish criticality; in particular a critical examination of the robustness of the reinforcement detailing
- Measures to reduce the amount of water available to the structure.
- Limited strengthening of the structure
- Partial or full demolition followed by re-building
Further details of these possible actions are available from the IStructE report Structural effects of alkali-silica reaction.
5. Bibliography
The literature on this subject is considerable; in excess of 2000 papers are available. ASR is the topic of bi-annual international conferences; the proceedings of these conferences are available from the Library of the Institution. In the UK some of the most relevant documents are:
British Cement Association, The diagnosis of alkali-silica reaction. 1988
Concrete Society, Technical Report No. 30 Third Edition, Alkali-silica reaction: minimising the risk of damage to concrete. 1999
Institution of Structural Engineers, Structural effects of alkali-silica reaction. Technical guidance on the appraisal of existing structures. July 1992
Building Research Establishment,Digest 330 Pts 1,2,3 & 4, Alkali-silica reaction in concrete. 1997
Hobbs Dr D W, Alkali-silica reaction in concrete. T. Telford 1988
West G, Alkali-silica reaction in concrete roads and bridges. T.Telford 1996
Swamy R N, Alkali reaction in concrete. Blackie. 1992
This feature was compiled by David Doran [F]: Chairman of IStructE Task Groups; Member of Hawkins Committee & Member of BRE Working Party.
30 May 2001 [revised 25 Nov 2004] |
