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Cotswold study of thaumasite

FEARS THAT thaumasite sulphate attack in concrete could also hit thousands of house foundations have prompted the government to commission a major insitu research project, it emerged this week.

The research will take place at a site in the Cotswold village of Shipston- on-Stour, where cases of thaumasite in house foundations were discovered as long ago as 1990 (NCE 23 April). The tests will monitor 176 concrete specimens over periods of three and10 years.

Contractors are expected to finish placing samples by the end of the week. Some 43 mix types will be used with four different aggregate and 10 different binders. A combination of pre-cast and cast insitu specimens will be placed in two 11m long by 2.5m deep trenches.

But independent thaumasite experts this week raised doubts over the validity of the tests. They claimed that thaumasite is most likely to occur when concrete is placed during cold weather, and that placing the samples during the current hot weather would affect the results.

'Though it is disputed by the Building Research Establishment, the initial conditions are important. If the concrete is put into cold ground during cold weather then thaumasite will form much quicker,' said one expert.

But the research is a clear indication of the Government's increasingly fears over the possible scale of the problem. The Highways Agency revealed on Monday that has now discovered 10 more cases of thaumasite on bridges on the M5 in Gloucestershire. A further 27 structures are understood to have been identified as vulnerable to thaumasite attack on Gloucestershire county roads.

Agency senior technical advisor Neil Loudon, who is a member of the Government's expert group on thaumasite, said it had identified 93 structures at risk in the Gloucestershire area, of which 20 will now be investigated. Cases excavated so far all showed signs of thaumasite attack.

The Agency also this week issued interim guidance to its designers prohibiting the use of limestone aggregates in foundations in areas thought to be at risk. And it warned its 34 maintenance agents to be on the look-out for thaumasite attack throughout the country.

A desk study will be carried out to identify areas most at risk from thaumasite attack - those with sulphate bearing clays where limestone aggregates are currently used (see map). High risk structures with poor drainage systems or slender foundations will then be identified for physical investigation.

Loudon said that the guidance would remain in place until more considered advice can be given by the Government's expert group. 'We do regard it as a serious problem but a relatively rare problem and we have taken the line that we need to be safe.'

Matthew Jones

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