CONTRACTORS WILL bear the main responsibility for producing thaumasite- resistant concrete under new Highways Agency guidance, it emerged this week.
In interim advice notes, the Agency says a 'designated outline' approach to the problem of thaumasite sulphate attack will be favoured. This will require the Agency and its consultants to supply information only on design constraints and ground conditions, and require the contractor to suggest preventative measures, including mix designs.
Initially thought to be a rare, non-threatening form of sulphate attack, serious TSA was first discovered early last year on bridges on the M5 in Gloucestershire (NCE 2 April 1998). Key factors triggering the attack have been provisionally identified as the use of limestone aggregates, the presence of high levels of sulphates in the backfill around the foundations and a wet, cold environment.
Agency senior technical advisor Neil Loudon said the new guidance was largely based on the recent Thaumasite Expert Group report (NCE 4 March 1999). 'But,' he added, 'we need the flexibility for contractors to select the most appropriate recommendations from the report'.
Concrete industry insiders believe most contractors will opt for a combination of 'low-carbonate' aggregates and blended cements, rather than take a risk on unproven protective coatings.
Loudon also revealed that of the 27 structures in Gloucestershire identified as being at high risk, 19 had fully developed TSA. Two more had preliminary thaumasite formation.
All the affected structures were founded on Lower Lias clay, which contains sulphides. These can oxidise when disturbed, increasing long term sulphate levels well above those measured initially.