Concrete Deterioration (Hansard, 1 April 1998)

§ Baroness Hayman

During recent strengthening operations on two of the older overbridges on the M.5 in Gloucestershire, an unexpected deterioration was noted in some of the concrete columns below ground level. Consultations with the Building Research Establishment (BRE) identified this as the thaumasite form of sulphate attack. Foundations on a further three bridges were examined and were found to be exhibiting similar deterioration.

The Highways Agency's view is that this does not pose a threat to the structural stability of the bridges, nor is there any risk to the safety of road users. The agency is drawing up a programme to identify which 45WA bridges might be subject to this form of problem and to investigate those bridges.

BRE has been conducting research into this form of sulphate reaction since 1990 for the Department of the Environment, Transport and the Regions, but, until the cases on the M.5 were discovered last week, only three such incidents of damage to concrete foundations had previously been identified in the UK. It appears that compliance with standard design guidance on protection against sulphate attack does not guarantee protection of concrete against the thaumasite form of reaction. At present, BRE is undertaking both laboratory and long-term field studies into this phenomenon. These studies have led it to the conclusion that this form of sulphate reaction can only occur when there is a combination of:

(a) A source of sulphates, as commonly found in clay soils;
(b) Very wet, cold conditions; and
(c) A source of calcium carbonate in the concrete (limestone aggregate in the cases investigated).

The expert independent advice received from the BRE is that, while it believes that its initial findings indicate the need for wider study, it is strongly of the view that any damage to buildings would be gradual and there is no danger to people in buildings.

In the light of the problems discovered on the M.5, we have today appointed Professor Leslie Clark to chair an expert group to study the phenomenon further and to report to me as soon as possible. The group will develop guidance and advice on any implications for existing buildings and structures and on the design and specification of new construction.