Thaumasite investigations to M4 overbridges

Abstract

Abstract In March 1998, thaumasite sulfate attack (TSA) was discovered affecting several buried raking supports of the Golden Valley Interchange at Junction 11 of the M5 Motorway in Gloucestershire. Following this discovery and the subsequent issue of the Thaumasite Expert Group Report in January 1999, the Highways Agency required all Network Agencys to investigate the likelihood and extent of any such deterioration. The investigations were to be targeted to all suspect structures, i.e. those with some or all of the details and conditions thought necessary for the initiation of TSA. The thaumasite investigation undertaken by Parsons Brinckerhoff Infrastructure as agents for the Highways Agency Area 5 Network commenced with a desk study to determine the structures most at risk from the initiation of the TSA. Of the 675 bridge structures within this network, the total with an identifiable risk priority of 1 was found to be 50; these having some form of buried foundations in reworked sub-soils affected by either natural ground water sulfates or contaminated road run-off. Of these 50 structures, 18 were initially selected for Phase 3 detailed site investigations. Following the identification of those structures which would most likely have TSA, site investigations were programmed and undertaken in October 1999. Five structures were initially investigated, with concrete cores being removed from selected locations of the buried post-tensioned ground beams. These core samples were subjected to petrographic, scanning electron microscopy and X-ray diffraction examinations following discussions with experts from the Building Research Establishment. This paper discusses the methodology adopted in the preparation of the desk study, the Special Inspection and the means by which the samples were extracted, tested and examined. This paper focuses on the ‘Engineers methods’ for the identification of thaumasite and the results of the investigations are presented.