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Experts cast doubt on Thaumasite report

CONCRETE EXPERTS have questioned the main conclusion of the Government's official report into Thaumasite Sulphate Attack published this week.

The report was produced after an eight month investigation by an expert group appointed by construction minister Nick Raynsford. It was prompted by the discovery of TSA on bridges on the M5 in Gloucestershire (NCE 2 April 1998).

It concludes that the risk of TSA - which converts concrete to the white, mushy mineral thaumasite - on buildings and structures is 'small and poses little serious cause for concern'. But materials specialists claimed this conclusion was based on too little research, and that more widespread TSA damage to structures could not be ruled out.

'Not many structures have actually been investigated so I think it remains to be seen whether this is as rare as the expert group suggests,' said director of materials consultancy STATS Dr Ian Simms. Similar views were expressed by other experts contacted by NCE.

Sims said that without further investigations the report was likely to be inconclusive. He added that STATS had received 'a lot of inquiries' about TSA from worried local authorities, and he now expected to proceed with structural investigations.

But presenting the results to the House of Commons on Monday, Raynsford sought to minimise concern over the phen- omenon. He claimed TSA could only occur in buried Portland cement concretes where there is a high sulphate presence, low temperatures, a high water presence and a source of carbonate.

'The probability of all these factors being present simultaneously is low for most existing buildings and structures and the number potentially at risk of the occurrence in the UK is considered small,' he assured the House.

This conclusion came despite confirmation from expert group chairman and president of the Institution of Structural Engineers Professor Leslie Clark that the findings of the 180 page report were based on limited research. Only 14 structures on the M5 in Gloucestershire and Avon were considered, together with three houses in the Cotswolds. Other high-risk areas in the Midlands, Thames Valley and Lincolnshire have so far been ignored, including over 70 potentially at-risk M5 bridges.

'We had a very broad remit and in the time available we were not in a position to go out and look at new sites,' he said.

But Raynsford dismissed suggestions that the conclusions of the report were ill founded.

'Engineers will find a very detailed analysis of the geographical area where TSA is likely to be a particular problem in the report, and that is the area where concentrated research is taking place,' he said. But so far no details have been discussed over when this research will be carried out or who will do it. Raynsford said only that the expert group will review the report's conclusions in a year.

Highways Agency senior technical advisor Neil Loudon said it would now carry out 'five or six excavations' in each of its areas likely to be affected by TSA as part of the Agency's bridge maintenance programme. The overall cost of repairs due to TSA is expected to be 'under £1M a year,' he said.

(More recommendations page 4)

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