Essay on Alkali Aggregate Reaction

602 Words 3 Pages
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 [e.g. crushed greywacke type sandstone] Moisture
Structures most at risk are bridges, hydraulic structures, exposed frames [e.g. open multistorey car parks] and foundations. In relation to total construction volume, the incidence of
…show more content…
At a time when recycling of materials has become a priority it should be noted that this report recommends that recycled aggregates (including crushed concrete and other demolition wastes) should be regarded as highly reactive. This is a precautionary measure pending further knowledge about their long-term behaviour.
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 an Information Paper, IP 1/02 Minimising the risk of alkali-silica reaction: alternative methods, has now been published.
3.2 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

Related Documents