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Industry ignored warnings on thaumasite threat

WARNINGS THAT thaumasite sulphate attack in structural concrete would become a widespread problem were ignored when first discovered 25 years ago, a leading cement technologist claimed this week.

The revelation came as the Department of the Environment, Transport and the Regions admitted that attacks could spread beyond the 50 bridges on the Gloucestershire section of the M5 originally targeted (NCE 2 April).

Professor John Bensted - an ex-senior scientist for BP - claims to be the first person to prove the chemical reaction leading to thaumasite sulphate attack while working for Blue Circle in 1974. But he insisted his warnings at the time to the cement and concrete industry 'fell on deaf ears'.

'It was just plain ignorance really. People don't listen and then problems arise. The industry should learn lessons from that,' he said.

And it also emerged this week that the threat of thaumasite sulphate attack in concrete was highlighted and ignored in the late 1980s. Independent materials technologist Philip Owens claimed his warning to a British Standards Institution committee on cement specifications in 1988 was 'met with derision'.

But a spokesman for the DETR denied these claims. 'The reaction can be produced in the laboratory fairly easily and this has been known for some years,' he said. 'But in all that time only four cases had been documented worldwide prior to the findings on the M5.'

Construction Minister Nick Raynsford announced to the House of Commons last Wednesday that an expert group, headed by Professor Les Clark of Birmingham University (see Profile), is to be formed to investigate the problem.

A DETR spokesman said that the expert group would consider the whole of the UK but it seemed unlikely that thaumasite sulphate attack would occur in Scotland and Wales, as the necessary environmental conditions were not present.

But Bensted predicted that more cases of thaumasite sulphate attack - which can occur even in concrete made with sulphate-resisting Portland cement - will soon come to light. He warned that increased sulphate levels in cement may mean that a source of sulphate in the ground is not necessarily needed for the reaction to take place. This would make it possible for the phenomenon to occur in any location.

'Personally speaking, I think thaumasite sulphate attack is going to affect above ground structures as well as sub-soil structures,' he said.

The expert group has been given six months to report back to DETR. It will be expected to produce guidelines on the identification of thaumasite sulphate attack, appraisal of damage, design and prepare any specification changes needed.

Investigations will initially target the higher risk areas where clay soils combine with the use of limestone aggregate in concrete. Other ingredients vital for the reaction, the DETR confirmed, are wet conditions and temperatures below 15C.

Highways Agency director of civil engineering Alan Pickett stressed that as attack rates found so far are relatively slow, it is not thought to threaten public safety. But investigations could take several years, and materials testing firms are understood to have been told to prepare for a major increase in workload.

Bill Price, a senior associate at materials testing consultant Sandberg claimed that thaumasite could already be present in concrete piles and basement walls. 'I think the fact it hasn't been noticed widely is because it is below ground. We tend to assume that below ground structures will last forever,' he said.

Matthew Jones

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