FIFTY BRIDGES on the M5 motorway are under scrutiny following the identification of the first known cases in the UK of a rare type of sulphate attack on major concrete structures.
The Highways Agency confirmed on Monday that five bridges over the Gloucestershire section of the M5 are suffering from the rare form of the phenomenon known as thaumasite sulphate attack. This occurred despite the use of sulphate resisting cement in the structures (see below).
The Agency now fears that the problem could extend to many more late1960s structures in the area. A spokesman said: 'We need to pin this down with investigations. There could be upward of 40 or 50 other bridges affected if similar conditions apply.'
Engineers discovered the problem during strengthening work on the Tredington- Ashchurch overbridge near Tewkesbury. Excavations revealed that in places concrete cover to the bridge's columns had been eroded down to the reinforcement. The bridge has now been closed for further investigations and repairs.
Consultants working for the Highways Agency are still puzzled by the exact combination of conditions which led to the sulphate attack. Soil samples and concrete cores are being taken and investigations are likely to take several months to conclude.
A spokesman from the Department of the Environment Transport and the Regions claimed that prior to the M5 discoveries, only four other cases of thaumasite sulphate attack had been identified in the world; three in small scale foundations in the UK and one in Alaska.
The other four M5 bridges affected are at the A40 Golden Valley interchange at junction 11 and at Grove Lane near junction 13. An Agency spokesman said: 'Clearly because of the problems at Tredington we have sensibly looked at these other bridges.'
However, the Agency quashed fears over the safety of the bridges. 'There is no concern over the structural stability of the bridges in question,' stressed the spokesman.
The Building Research Establishment published its first reports into thaumasite sulphate attack only in 1996. Research was carried out on concretes and mortars containing different aggregates and cement types.
The results shook the BRE investigators. At low temperatures permanently wet concrete made with common limestone aggregates and sulphate resisting cement performed worse than expected. Only concrete containing ground granulated blast furnace slag had any real resistance to this form of attack.
Repairs to the M5 bridges are likely to involve breaking out the defective concrete and casting new concrete collars around the columns. The Agency said that it was still 'too early' to determine the full scale of the problem and cost of remedial works.