Thaumasite (Hansard, 25 January 1999)

§ Mr. Raynsford

I am today publishing the Report of the Expert Group which has under the chairmanship of Professor Leslie Clark, President of the Institution of Structural Engineers, been studying the thaumasite form of sulfate attack on concrete.

I announced the appointment of Professor Clark to chair an expert group on 1 April 1998, Official Report, columns 532–33, following the discovery by the 46W Highways Agency during bridge strengthening operations on the M5 in Gloucestershire of unexpected deterioration in some of the concrete columns below ground level. This was identified by the Building Research Establishment as the thaumasite form of sulfate attack.

I asked the Expert Group to consider the available research and other evidence on thausmasite and to report giving advice and guidance on the implications for existing buildings and structures and on the specification and design of new construction. The Group was also asked to recommend any longer term programme of investigations and research which it considered necessary.

The Expert Group reported to me at the beginning of December and I have since discussed the conclusions and recommendations with Professor Clark. I am very grateful to him and all his colleagues on the Expert Group for producing such a thorough and well-considered Report in a technically complex area in such a short period of time. It provides a great deal of re-assurance about the extent of the problem.

The main findings of the Group are that: the thaumasite form of sulfate attack will only occur in buried Portland cement based concretes when all the primary risk factors are present simultaneously and developed to a significant degree. These primary risk factors are:

the presence of sulfates and/or sulphides in the ground;
the presence of mobile groundwater;
the presence of carbonate, generally in the concrete aggregates; and
low temperatures

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; the structural consequences of thausmasite attack are rarely of serious concern in respect of public safety, as most buildings would exhibit warning signs of distress well before there was any danger of collapse; it is not necessary to specifically inspect domestic properties for occurrence of thaumasite attack in foundations or above ground. Data from the National House Building Council indicate that the incidence of sulfate attack in domestic properties is very small in relation to other building defects. Thaumasite can be addressed as part of general repair and maintenance of houses.

The Report recommends a range of options open to specifiers and designers to minimise the risk of occurrence of the thaumasite form of sulfate attack in new construction.

Although the Group's findings are re-assuring, the Report stresses the need for the construction industry not to be complacent. It recommends that the industry, with appropriate support from Government, instigates a structured programme of research aimed at understanding the outstanding issues.

My Department is already taking steps to put in hand a programme of research in collaboration with the industry to take forward the Report's specific recommendations.

The BRE will be amending Digest 363, which gives advice to designers, to reflect the Report's recommendations. We have also asked the BRE to continue to monitor occurrences of thaumasite and the conditions under which these take place.

47W

The Highways Agency is currently reviewing its interim guidance on thaumasite and will be issuing updated guidance shortly. It is completing its programme of investigations and repairs in Gloucestershire and Avon and is undertaking a phased limited programme of continued investigations in association with its planned bridge inspection programme.

I have also asked Professor Clark to review the Report's conclusions in a year's time with help from the experts at BRE and, in the light of the findings of further research and monitoring now being commissioned, to consider whether there may be a need to reconvene the Group.