混凝土方面畢業(yè)論文外文翻譯--回顧英國公路管理處理硫酸鹽硅灰石膏侵害的經(jīng)驗

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混凝土方面畢業(yè)論文外文翻譯--回顧英國公路管理處理硫酸鹽硅灰石膏侵害的經(jīng)驗_第1頁

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1、　　A review of the experience of thaumasite sulfate attack by the　　UK Highways Agency　　Neil Loudon　　Highways Agency, SSR CE SDM Division, Heron House

2、, 49-53 Goldington Road, Bedford MK40 3LL, UK　　Abstract　　The paper summarises and reviews the UK Highways Agency experience of thaumasite sulfate attack.</p

3、>　　Thaumasite sulfate attack was found in February 1998 in a number of bridge foundations and buried columns on the M5Motorway in Gloucestershire. The paper will highlight the investigation of these structu

4、res, and assess the implications of the resultsof the extensive testing undertaken at these sites, for other structures.　　Subsequent to the discovery of thaumasite in Gloucestershire the Highways Age

5、ncy participated in the Thaumasite Expert　　Group set up by the Minister for Construction. The report produced by the group influenced the development of the Agencies own　　guidanc

6、e. Investigations were undertaken nationally using risk based criteria, to determine whether the occurrence of thaumasite　　was a local phenomenon, or of more widespread concern. Concurrently guidance

7、 was also issued for managing and minimising therisks of thaumasite in new construction, and this culminated in the development of a new specification for buried concrete. There arealso implications for ground investigat

8、ions, and these will be discussed.　　The paper will also highlight the case study of the A1 Mally Gill Bridge in County Durham, where a markedly different set of　　circumstances ga

9、ve rise to thaumasite sulfate attack in a foundation.　　The Highways Agency has been involved with the development of improved guidance, as a result of the Expert Group Report, to

10、　　ensure that that there is a consistency of approach across the construction industry. The paper will acknowledge that there are still　　many unknowns, particularly the mechanism governing the occurr

11、ence of thaumasite and the speed of thaumasite production.　　A particular aspect of concern to the Highways Agency is the ongoing management of structures that are known to be or may be<

12、;p>　　affected by thaumasite.　　What are the methods for investigation and testing and are there suitable methods for remote detection?　　Do we need to introduce new inspection r

13、egimes?　　What are the requirements for the repair of thaumasite affected structures?　　Mention will be made of some of the research in this area, particularly the trial of repaire

14、d concrete at Moreton Valence.　　The paper will summarise the lessons learnt so far from the thaumasite experience, the implications for the future management of　　structures in po

16、cy　　1. Background　　In early 1998, in the course of other maintenance and　　bridge strengthening works, unusual concrete deterioration　　wa

17、s found to have occurred in the foundations　　and buried columns on a couple of bridges on the M5　　Motorway in Gloucestershire. Subsequent investigations　　pro

18、ved that the observed defects were the result of　　thaumasite sulfate attack, and the Highways Agency　　were faced with a number of immediate questions.　　How s

19、erious was the attack?　　Were there structural implications?　　How many bridges were affected?　　Was it a local problem, or national?　　What

20、 were the costs?　　How do we undertake repairs or other remedial action?　　Were there any faults in the original design and construction?　　Were there implicati

21、ons for new construction?　　What were the implications beyond the Highways　　Agency interests?　　Do we need to carry out any research?　　095

23、1051–1058　　www.elsevier.com/locate/cemconcomp　　This paper covers the responses to some of these　　questions, and summarises the current situation, and</p&g

24、t;　　identifies other issues for the future.　　2. Tredington-Ashchurch Bridge　　Thaumasite sulfate attack was first observed in early　　1998 on a High

25、ways Agency structure in the substructure　　of the Tredington-Ashchurch Bridge, an overbridge　　carrying a local road over the M5 Motorway situated　　between Ju

26、nctions 9 and 10 in Gloucestershire. At about　　the same time similar, but less severe defects, were also　　found in Grove Lane Bridge foundations, also on the

27、　　M5, but further south, between Junctions 12 and 13.　　Tredington-Ashchurch is a four span overbridge with　　a reinforced concrete deck. Supporting columns are

28、;　　sited in the central reserve and verges, and extend through　　placed fill for about 5 m to reinforced concrete spread　　footings. The bridge was built about 30 years ago, to the

29、　　applicable contemporary Ministry of Transport standards　　and specifications. Regular structural inspections　　and testing of the concrete elements above gro

30、und have　　not indicated any observable signs of distress to the　　bridge in the intervening period.　　Planned strengthening work to the columns of the

31、;　　bridge necessitated excavation of some of the backfill　　surrounding the concrete supports, and in the course of　　this work site staff observed some unusual deterior

32、ation　　in the exposed concrete. This indicated that some of the　　concrete surfaces had turned to a _mushy’ consistency,　　and of _warty’ appearance with evide

33、nce of expansion of　　the residual material. This was unexpected to the say the　　least, and potentially extremely serious. Diagnosis was　　first made by our Ma

34、intenance Agents, Gloucestershire　　County Council/Halcrow and later confirmed by the　　Building Research Establishment (BRE) as thaumasite　　sulfate attack, af

35、ter initial concrete samples had been　　analysed.　　Further excavation of the backfill ensued to completely　　expose the buried columns and t

36、he top surface of　　the foundations, entailing extensive temporary works to　　provide the necessary support. A large programme of　　investigation was initiated,

37、 including extensive concrete　　sampling, and soil and groundwater testing, backed up　　by a thorough laboratory testing regime, and analytical　　work to correl

38、ate soil conditions with concrete defects.　　The objective was to determine the precise causes of the　　deterioration, and to see if parameters could be found

39、　that would assist future identification of affected structures　　and their investigation.　　Other papers at the conference will deal with the　　Tredington-Ashc

40、hurch case study in much more detail,　　however suffice to say that it transpired that a number of　　critical factors in combination had occurred (listed　　belo

41、w), which were significant in terms of the deterioration　　that occurred, and particularly in relation to the　　severity of the thaumasite sulfate attack:　　(a)

42、 use of limestone aggregates in the concrete;　　(b) use of sulfate and sulfide bearing materials for backfilling　　around the foundations and buried columns　　(

43、the soil excavated to construct the foundations contained　　iron pyrites (sulfides) which started to oxidise　　after exposure to the atmosphere, and added to the<p&g

44、t;　　reservoir of available sulfates);　　(c) groundwater movements;　　(d) physical disposition of the structure (deep foundations　　and slender concrete elements

45、);　　(e) construction regime, particularly the excavation of a　　large hole through undisturbed ground which was　　subsequently backfilled with the excavated ma

46、terials　　and created a _sump’ around the buried columns;　　(f) leaking motorway drainage;　　(g) relatively cold conditions.　　These factors

47、 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 groun

48、ds, 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 c

49、entral core of concrete remaining. Assessment　　of the structure showed that the road could be safely　　reopened whilst the investigations continued.　　Based on

50、 the information available at the time, and　　the need to undertake the planned strengthening work　　on the columns against vehicular impact, it was decided　　t

51、hat the columns should be removed. This decision was　　reached after considerable deliberation, and assessment　　of costed options, but was influenced by the lack of<

52、;p>　　available information on how to repair structural elements　　affected by thaumasite. There was also uncertainty　　as to whether the concrete deterioration had&l

53、t;p>　　_stabilised’ or was continuing. In the event the most effective　　strategy was the complete removal of the bridge　　columns. A system of temporary propping was introduced&

54、lt;/p>　　to allow the columns to be cut up and removed,　　and a new foundation and supports cast on top of the　　existing foundation.　　Alongside t

55、he continuing site investigation work, records　　of the structure were inspected, and also available　　geotechnical information. It was evident that the design

56、　　of 30 years ago had been undertaken in accordance with　　the standards and specifications of the time, based on　　the contemporary soil survey. This had duly taken account</p&

57、gt;　　of the prevailing soil conditions in regard to the　　occurrence of sulfates. However at that time there was　　no requirement to consider sulfides, and so in compari

58、son　　to the rules developed in 1999 in response to the　　discovery of thaumasite, may have underestimated the　　influence of sulfates by 2–3 classes (based on

59、BRE Di-　　1052 N. Loudon / Cement & Concrete Composites 25 (2003) 1051–1058　　gest 363 [1]). Clearly this was very significant, but no　　fault could be atta

60、ched to the design or construction　　process of 30 years ago.　　Whilst work was taking place at Tredington-　　Ashchurch Bridge, a further maintenance contract a

61、t　　Grove Lane Bridge further south on the M5 Motorway,　　revealed similar signs of thaumasite deterioration in　　buried concrete, although evidently not as sev

62、ere or as　　widespread. In this case after initial investigation, conventional　　concrete repairs were conducted, after removal　　of the contaminated concrete.&

63、lt;/p>　　3. DETR Expert Group　　When the concrete deterioration was first discovered　　in the motorway bridges, it was immediately apparent　　that

64、the thaumasite attack was very serious, and there　　were potential implications for all new construction as　　well as existing buildings and civil engineering structures.

65、　　The Highways Agency briefed the Department of　　the Environment Transport and the Regions (DETR)　　and Government Ministers. It also transpired that BRE,&l

66、t;p>　　through Dr. Norah Crammond and other colleagues,　　had been investigating thaumasite related concrete deterioration　　for nearly 10 years, though the results of this</p

67、>　　research had not been published, and the current edition　　of BRE Digest 363 [1], made only passing reference.　　A decision was taken at Ministerial level to set u

68、p a　　National Expert Group under the chairmanship of　　Professor Les Clarke of University of Birmingham, with　　a remit to investigate thaumasite and report ba

69、ck within　　six months. This was a very tight constraint, but it was　　decided that it was essential to provide authoritative　　guidance at the earliest opportu

70、nity to allay public and　　industry concerns. The Expert Group was derived from　　a wide cross section of industry and the author was also　　seconded to represe

71、nt the Highways Agency.　　After considerable discussion and deliberation a report　　entitled “The Thaumasite Form of Sulfate Attack:　　Risks, Diagnosis, Remedia

72、l Works and Guidance on　　New Construction“ [2] was produced and published in　　January 1999. As such, it was a significant achievement　　to publish a very deta

73、iled report in such a short timescale,　　and represented a collaborative and very constructive　　approach by all the participants, although it　　was apparent th

74、at there were differing views on many　　aspects, and especially the detailed and complex chemistry　　involved in thaumasite. It was also apparent that　　many of

75、 the construction industry representatives, had　　understandable concerns about the commercial implications　　for their own sector of the market.　　The report d

76、ealt with the nature and risk of thaumasite　　and sulfate attack, inspection and testing requirements,　　structural assessment, remedial works, and a specification for new works, a

77、nd it is particularly with this last section I was involved. However the report as a whole provided authoritative guidance and recommendations for both new works, and for clients and property owners managing structures a

78、nd buildings.　　The report itself set out _guidance’ in areas where only limited research and case studies had been undertaken,but _recommendations’ where more reliable and well documented information

79、 was available. It was acknowledged that not all the answers were known. In particular there was much debate surrounding the mechanisms for occurrence of thaumasite, and the timescales over which it occurred, and in this

80、 area further research was essential. Consequently some caution had to be exercised in the detai　　One aspect of major importance, particularly as regards new construction, was the need to strike the

81、correct balance between technical issues and commercial interests. To do this the Report had to deal authoritatively with the technical concerns without unnecessarily placing restrictions on the use of the constituent ma

82、terials to produce concrete. At the time it was published　　the Report acknowledged that the information was of an interim nature and would need to be updated as more evidence concerning thaumasite be

83、come available. It also sought to _minimise’ the risk of future occurrence,rather than the introduction of draconian measures to completely eliminate the risks.　　Key facets to the Report were the ide

84、ntification of a　　number of primary and secondary factors governing the　　occurrence of sulfate (these are referred to later in the　　paper) and the classifica

85、tion of thaumasite in affected　　structures:　　(a) _TSA’ for thaumasite sulfate attack where there is　　significant damage to the matrix of the concrete or</

86、p>　　(b) _TF’ for thaumasite formation where thaumasite is　　present in pre-existing voids in the concrete, but　　there is no deterioration evident (a latent stage in&

87、lt;/p>　　development of TSA).　　The Report also contained practical information on　　the requirements and methods for identification and testing of concrete and soils

88、in existing structures, to determine if thaumasite is present. Particularly important was the strong message to clients and designers of new structures to undertake a thorough soil survey as part of the design process. T

89、his has always been undertaken on road building projects and major civil engineering contracts, but less so on smaller works, and in　　the domestic property market.　　Chapter 9 of

90、the Report provided detailed specification　　requirements for new construction to minimise the risk of thaumasite sulfate attack, based on the approach already adopted in BRE Digest 363 [1], namely by

91、 classifying the sulfate conditions in the soil adjacent to the designed concrete, and by setting a series of restrictions and options for the concrete mixes for each classification. However this approach was extended to

92、 adopt a “package” of measures to deal with all ground　　classifications. It sought to leave as many options open　　to designers, and particularly to the contracting and concrete s

93、upply industries, and all such options were deemed to be equally effective. There was also a desire to keep all guidance and recommendations as simple as possible, and user friendly.　　The Report reco

94、mmendations consisted of detailed materials requirements for concrete mixes and this was allied to a risk based strategy based on different structural　　performance levels, depending on the required&l

95、t;/p>　　service life and usage of the structure. There was an　　overarching assumption that the soil/ground water　　classification had been correctly assessed, and the

96、 recommendations　　in chapter 6 of the Report had been　　followed, and so consequently the concrete requirements　　were appropriate to the existing and anticipa

97、ted future　　ground conditions. The recommended concrete options　　followed on from this correct sulfate classification of the　　soils, with due allowance for a

98、ny sulfides present. Other　　related guidance involved consideration of the construction　　operations and the need, where appropriate, to provide additional drainage around structu

99、res, to avoid creating sumps, and where possible not to use reworked sulfate/sulfide bearing backfills.　　The concrete materials requirements embodied various　　controls on cement

100、content, and free water/cement ratios, but adopted a new classification of aggregates by defining different carbonate ranges depending on the amount of carbonate present in the fine and coarse aggregate fractions. In ter

101、ms of the structural performance 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

102、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

103、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/sensi

104、ti　　where structures had short anticipated service lives.　　Since there were a number of areas where the dearth　　of research data prevented authoritative reco

105、mmendations for materials, it was agreed that it would be prudent to adopt a _multi-layered’ protection approach, whereby a number of additional protective measures would be instigated, in addition to the material requir

106、ements, 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 low

107、er 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 la

108、rge, 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 als

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