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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
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]. 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.
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. 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 e.g. 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
3.3 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
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