【正文】 formance level the philosophy adopted was one based on banding of structures into high, normal and low performance, representing a range from long service life structures to low performance for short service life structures, and structures with massive buried concrete foundations or those with slender or critical buried elements. This recognized that thaumasite attack in reality was assumed to be a relatively slow process, and may be insignificant for structures with only a short design life. More restrictive measures would be required when dealing with longer service lives for structures and critical/sensitive buried elements, and conversely less severe requirements imposed where structures had short anticipated service lives. Since there were a number of areas where the dearth of research data prevented authoritative remendations for materials, it was agreed that it would be prudent to adopt a _multilayered‘ protection approach, whereby a number of additional protective measures would be instigated, in addition to the material requirements, in appropriate situations. They would represent an important first line of defence and consisted of the adoption of surface protection and drainage requirements, as well as the choice of lower carbonate range aggregates in certain circumstances. Overall the Expert Group Report provided a number of important messages. The number of structures potentially at risk was not thought to be large, and the structural consequences not generally serious. The deterioration would generally provide early warning signs above ground where significant thaumasite sulfate attack was occurring below ground. There was also not thought to be any significant problem for domestic properties. The Report, if followed would minimise the risk of thaumasite sulfate attack, but also pointed out that not all the answers were known and that further research was needed. It also acknowledged that there was a need to update various documents and in particular BS 5328 [3] and BRE Digest 363 [1]. 4. Highways Agency guidance for new construction In parallel with the work of the Thaumasite Expert Group the Highways Agency set down its own advice for new construction, based on the Expert Group requirements. Interim Advice Note 25 [4] was issued, which laid down requirements to be adopted for highway structures, to minimise the risk of thaumasite sulfate attack in new construction. In particular it assumed that in most cases highway structures would be deemed to be high performance level, as befitting the required 120 year service life. The Advice Note also provided some more specific guidance on drainage around structural foundations and the type of protective coatings appropriate for application to buried concrete in highway structures. 5. Highways Agency strategy for existing structures However as well as dealing with new construction, the Highways Agency also needed to address the issues of the occurrence of thaumasite in existing structures and particularly whether it was confined to the existing area of Gloucestershire or more widespread. A decision was taken to embark on a national investigation. The Expert Group had identified a number of primary and secondary factors that must coexist or would influence the occurrence or severity of the attack. Primary factors (a) presence of sulfates and/or sulfides in the ground。 (d) physical disposition of the structure (deep foundations and slender concrete elements)。 Highways Agency 1. Background In early 1998, in the course of other maintenance and bridge strengthening works, unusual concrete deterioration was found to have occurred in the foundations and buried columns on a couple of bridges on the M5 Motorway in Gloucestershire. Subsequent investigations proved that the observed defects were the result of thaumasite sulfate attack, and the Highways Agency were faced with a number of immediate questions. How serious was the attack? Were there structural implications? How many bridges were affected? Was it a local problem, or national? What were the costs? How do we undertake repairs or other remedial action? Were there any faults in the original design and construction? Were there implications for new construction? What were the implications beyond the Highways Agency interests? Do we need to carry out any research? 09589465/$ see front matter Crown Copyright _ 2021 Published by Elsevier Ltd. All rights reserved. doi:(03)00146X Cement amp。 Structures。 (c) groundwater movements。 (g) relatively cold conditions. These factors are illustrated in Fig. 1. When the deterioration was first observed, a decision was taken to close the local road carried by the bridge, on safety grounds, until such time as the extent and nature of the problem could be determined. Subsequently the defects were found to be surface effects, with a sound central core of concrete remaining. Assessment of the structure showed that the road could be safely reopened whilst the investigations continued. Based on the information available at the time, and the need to undertake the planned strengthening work on the columns against vehicular impact, it was decided that the columns should be removed. This decision was reached after considerable deliberation, and assessment of costed options, but was influenced by the lack of available information on how to repair structural elements affected by thaumasite. There was also uncertainty as to whether the concrete deterioration had _stabilised‘ or was continuing. In the event the most effective strategy was the plete removal of the bridge columns. A system of temporary propping was introduced to allow the columns to be cut up and removed, and a new foundation and supports cast on top of the existing foundation. Alongside