【正文】 e 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。 (b) presence of mobile groundwater。 (c) presence of carbonate generally in coarse and/or fine concrete aggregates。 (d) low temperatures. Secondary factors (a) type and quantity of cement used。 (b) quality of concrete mix and paction。 (c) changes to ground chemistry and water regime resulting from construction。 (d) type, depth and geometry of buried concrete. Instructions were issued to all the Maintenance Agents in England to mence an investigation. This was divided into two parts, initially based on a risk based desk study, and followed up where necessary with site investigation work. The aim of the desk study was to determine the coexistent presence of the primary factors, and particularly whether carbonate bearing aggregates had been used in buried concrete, and whether structural backfill or adjacent ground included sulfate/sulfides. The assessment was also related to structural significance and work needs, such as planned maintenance work. Of particular interest were structures such as those exhibiting signs of distress above ground, such as settlement or other ground movements or unexplained cracking. This desk study required the critical examination of structures records and soil survey information. In some cases there was insufficient information available, and limited confirmatory material testing was required. As part of the desktop assessment, structures were prioritised on the basis of risk. Follow up investigations were then conducted on a limited number of the higher priority structures in each area (if necessary), and with judgement made on the ease of access to buried concrete, so as to minimise the amount of temporary works and traffic management required. The aim of the investigation was to determine whether there were any visible signs of thaumasite attack present. If there was, then detailed sampling of the backfill/soils, and concrete coring was undertaken, backed up by laboratory testing. It was important to confirm for the highest priority structures that record information was correct. If all the critical factors were present, both positive and negative results were important. The absence of thaumasite, where carbonate in the aggregates and sulfates/sulfides in the soils was present, was just as important as if thaumasite was found. Significant results of the investigation were passed back to BRE to assist with their future research. The results of this investigation were very much as expected, and confirmed the findings of the Expert Group Report, and particularly the perceived areas of greatest risk. Apart from the Gloucestershire/Avon area where further cases emerged, the only significant positive result was from County Durham, and is reported later in this paper. However the investigation was invaluable, as it also confirmed that no thaumasite was present in the lower risk areas. Some words of caution about areas of risk are required, in particular about the presence of carbonates in aggregates. Aggregates can and often were transported substantial distances from their quarry source to their eventual