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Not fatal, but flawed

Having seemed to be under control, Having seemed to be under control, alkali silica reactivity is re-emerging as a alkali silica reactivity is re-emerging as a problem problem

Alkali silica reactivity (ASR) was headline news between 1976 and 1987. It was dubbed 'concrete cancer' and photographs of cracked concrete facades weeping a slimy gel caused something akin to panic among engineers responsible for major infrastructure. Over this time more than 100 cases of ASR were reported - but almost all turned out to be manageable.

Aesthetic qualities may have suffered, but the structures remained serviceable.

No new cases of UK ASR have been reported since 1987.

This is generally put down to the energetic research programme during the previous decade and the amendment of concrete specifications that followed.

These effectively banned high alkali cements in combination with highly reactive aggregates.

Coupled with an increasing tendency to specify concrete with part of the Portland cement content replaced by significant fractions of pulverised fuel ash (PFA) or ground granulated blast furnace slag (GGBFS), these changes gradually laid the spectre of ASR to rest.

So, for many years, cement manufacturers have supplied their main customers - the ready mix and precast concrete industries - with the information they need to design low risk concrete mixes. This is basically what is termed the declared mean alkali content - the average of individual tests on representative samples of 25 consecutive days' production from each works, plus one standard deviation of the results.

This margin is added to take care of the natural and inevitable variation between mixes and within batches.

A mix designed with this information in accordance with the most authoritative guidance documents and specifications is believed to have a negligible risk of developing significant or damaging ASR.

However, even before the shock news that readymix concrete suppliers in south west England had inadvertently been delivering concrete with higher alkali contents than specified, some voices were beginning to warn that the risk of ASR was still significant. If it does return, either in those structures made with the suspect Lafarge Westbury cement or older structures built before the 1980s revolution, there will be new challenges for the concrete industry to face - mostly in convincing anxious infrastructure operators not to overreact.

It is worth emphasising that in the UK so far no structure has ever collapsed as a result of ASR and very few have been demolished. Even the Montrose Bridge in Scotland, which was cut up and lifted away last year (NCE 28 October 2004), was still structurally safe, even though it was first diagnosed as suffering from well developed ASR in 1987. And the crossing was 70 years old when it became uneconomic to maintain any longer.

Three factors are generally accepted as being necessary for ASR to occur in any given concrete mix:

. Enough alkalis in the concrete . A critical amount of reactive amorphous (non-crystalline) silica in the aggregate . A plentiful supply of water.

If all these factors are present, then the alkaline salts of potassium and sodium dissolved in the pore water begin a slow reaction with the available silica.

An alkali silicate gel forms, calcium ions from the hydrated cement are taken up, and the gel becomes hydrophilic. It swells, cracks the concrete matrix, allows more water to penetrate, and swells even more.

The reaction can continue for years if the alkali/silica ratio is suitable and water continues to be available.

The problem for those who may now have the responsibility of evaluating suspect structures in the south west is that not every expert agrees on what level of alkalis or reactive silica represents significant risk.

According to ASR expert Dr Jonathan Wood of Structural Studies & Design, the alkali levels derived by the Hawkins Committee and first published in Concrete Society Technical Report 30 in 1983 'reduced the safety margin for variability of cement alkali content in concrete' (NCE Letters 3 February).

According to Wood, this was down to 'cement industry pressure' on the committee.

Inevitably, others disagree.

'The TR30 limits are fairly conservative and well below the level that causes a problem, ' says British Ready Mixed Concrete Association technical director Tom Harrison. 'All the available evidence supports the Hawkins Committee conclusions.' There are also differences over the way any suspect concrete should be evaluated.

The traditional way is a petrographic examination of cores cut from the area of concern, perhaps over a period of decades. The problem is, as Wood points out, 'most UK concretes contain a few particles with ASR gel, but this is well below the threshold at which any adverse consequences arise.

'Assessment using the 1992 Institution of Structural Engineers guidance enables these cases to be put into perspective.' Most would prefer a relatively early assurance of the scale of the problem they could be facing, preferably before any ASR appears. One option is a modified version of a test for aggregate reactivity in BS 812, known as the concrete prism method.

Using the actual aggregate source in the suspect structure and making prisms with a range of cement alkali levels representing the probable worst case scenario will give a reliable prognosis in just one year, the method's proponents maintain.

Whatever the method adopted, the question of remedial options will be high on the agenda.

Extra waterproofing may be one realistic option in some cases, or, where the structure is well reinforced and the safety margins high, a programme of regular inspections may be all that is needed.

Anyone confronted with a diagnosis of ASR would do well to remember the immortal words of Corporal Jones. Panic is the wrong reaction. In the long term, ASR is more like acne than cancer - nobody wants it, it can look disgusting, but fatalities are rare.

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