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Alkali-silica reaction
What is alkali-silica reaction (ASR) and how can I recognise it?
Answer
Alkali-silica reaction (ASR) is the most common form of alkali-aggregate reaction. It occurs when the alkaline pore fluid and siliceous minerals in some aggregates react to form a calcium alkali silicate gel. This gel absorbs water, producing a volume expansion which can disrupt the concrete.
The introduction of specifications to limit alkali content and reactive aggregates in concrete has meant that no confirmed incidence of ASR has been noted in the UK since 1987.
The main external evidence for damage to concrete due to alkali silica reaction is cracking. In unrestrained concrete the cracks have a characteristic random distribution often referred to as ‘map cracking’ where there is a network of fine cracks bounded by a few larger cracks. However the only reliable evidence for diagnosing ASR as the cause of damage is by microscopic examination of the interior of concrete to identify positively the presence of the gel, of aggregate particles which have reacted and the presence of internal cracking, characteristic of that induced by ASR.
Acknowledgement:
The Concrete Society
Other references:BRE Information Paper IP1/02 Minimising the risk of alkali-silica reacttion: alternative methods
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TR30 Alkali-silica reaction
- minimizing the risk of damage to concrete
