General Problem Known As Alkali Aggregate Reaction example essay topic

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 multi storey 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 1940's; 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 1980's 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 pf a and / or gobs 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, meta kaolin 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.