1、STDOBSI BS 7943-ENGL 1999 Lb24bb9 079LL93 267 BRITISH STANDARD Guide to The interpretation of petrographical examinations for alkali-silica reactivity ICs 91.100.15 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW BS 7943:1999 STD-BSI BS 7943-ENGL 1999 I Lb24669 0791194 LT3 II
2、BS 7943:1999 Committees responsible for this British Standard The preparation of this British Standard was entrusted by Technical Committee B502, Aggregates, to Subcommittee B/502/2, Aggregates for concrete including those for we in roads and other pavements, upon which the following bodies were rep
3、resented Association of Consulting Engineers British Geological Survey British Precast Concrete Federation Ltd. County Surveyors? Society Department of the Environment, Transport and the Regons Department of the Environment, Transport and the Regions (Building Research Department of the Environment,
4、 Transport and the Regions (Highways Agency) any Products Association Establishment) This British Standard, having been prepared under the direction of the Sector Committee for Building and Civil Engineering, was published under the authority of the Standards Committee and comes into effect on 15 Ju
5、ly 1999 O BSI 07-1999 Amendments issued since publication No. I Date I The following BSI references relate to the work on this standard Committee reference B/502 Draft for comment 9W101379 DC ISBN O 580 28273 2 STDmBSI BS 7743-ENGL 1979 m 3624bbq 0793395 03T W BS 7943:1999 Contents Page Committees r
6、esponsible hide front cover Foreword ii 1 Scope 1 2 Characteristics of UK aggregates 1 3 Classification of aggregates 2 quantitative petrographic examination of aggregates Annex B (informative) Gel-pat test for the detection of opaline silica Annex A (informative) Precision of the procedure for qual
7、itative and 7 7 Annex C (informative) Example of report form 14 Bibliography 15 Figure B.l- Qpicai pat specimen at the end of a test, showing several gel 11 Figure B.2 - Typica pat specimen at the end of a test, showing severai gel 12 Figure B.3 - Typical pat specimen at the end of a test, showing s
8、everal gel growths - An oblique view of the pat after drying in ambient conditions 13 3 growths - Specimen immersed in the storage solution growths - Specimen removed from the storage solution but kept wet Table 1 - Guidance on rock types Table 2 - Guidance on mineral types Table B.l- Detection limi
9、ts for the gel-pat test 6 10 O BSI 07-1999 i STD.BS1 BS 7943-ENGL 1999 Lb24669 079219b T7b I BS 7943:1999 Foreword This British Standard has been prepared by Technical Committee B/5022. The purpose of this guide is to mist in the selection of aggregate combinations which will minimize the risk of da
10、mage to concrete caused by W-silica reactivity (ASR). Current recommendations allow for the risk of damagmg ASR to be minimized by limiting the alkali content of the concrete, by using additions (such as pfa or ggbs) or by using an aggregate combination of low reactivity In many instances either the
11、 alkali control or the use of aditions may prove to be the most suitable option. However, there may be circumstances when it is considered necessary to specify the use of an aggregate combination of low reactivity. A method of test using concrete specimens has been developed (BS 812-123) and an acce
12、lerated test using mortar-bar specimens is currently being developed. However, a petrographical examination can be completed more quickly and so can be used for a relatively rapid assessment of an aggregate combination. This guide offers advice on the interpretation of the resuits of the petrographi
13、cal examiliation of aggregate samples carried out according to BS 812-104 in terms of the reactivity of a particular aggregate combination. The principle of the interpretation described in this guide is to ensure that a) the most reactive minerals (opal or opaline silica) are not present in aggregat
14、es used and b) aggregates may be classified as being of low, normal or high reactivity on the basis of their petrographic composition. Gened precision of the BS 812-104 petrographic examination method has been assessed by trials and the findings apposite to this guidance, in particular the quantific
15、ation of those constituents considered to be potentMy reactive, are summarized in annex A. The detection of those mineral constituents defmed in this guide as being the most reactive e. opal or opaline siiica) may require use of the gel-pat test in addition to petrographic examination. A suitable me
16、thod is provided in annex B. It is essential that the sample examined is adequately representative of the material being assessed. A procedure is given in BS 812-102 for sampling processed aggregates. In interpreting the results of a petrographical examhation the method of sampluig should be taken i
17、nto account. Annexes A, B and C are informative. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal oblig
18、ation. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 15 and a back cover. O BSI 07-1999 STD-BSI BS 7943-ENGL 1999 B 1624bb9 0791197 902 M BS 7943:1999 1 Scope This British Standard gives guidance on the interpretation of the results of petr
19、ographical examination of coarse and fine aggregates, carried out in accordance with BS 812-104, in tem of the susceptibility of the combined aggregate to alkali-silica reactivity (ASR) sufficient to cause damage to concrete in which it is used. The guidance is not applicable to alkali-carbonate rea
20、ctive materials. NOTE 1 Guidance on precision expected from the petrographical examination of coarse and ne aggregates, carried out in accordance with BS 812-104, is given in annex A. NOTE 2 It is recommended that a petrographical examination be carried out by a petrographer who is a member of the P
21、roficiency :heme for Aggregate Petrographers. 2 Characteristics of UK aggregates 2.1 Rocks are traditionally classified according to their mode of formation, grain size, texture and mineral composition. Crushed rock aggregates, with a few exceptions, retain the geological name of the parent rock. Na
22、tural aggregates are sands and gravel formed by geological erosion of rock and are composed of particles of rock and occasionally, particularly in the case of sand, discrete mineral grains. Sometimes, over-sized gravel material is crushed and then blended with the natural aggregate, the resulting ag
23、gregate being generally regarded as a natural aggregate. 2.2 Aggregate combinations (i.e. the total coarse and fine aggregate mixture for a given concrete) are mainly classified as being of low, normal or high reactivity on the basis of their petrographic :omposition (see notes 1 and 2). Aggregates
24、Lontaining detectable amounts of opal or opaline silica are not classified and, in most circumstances, should not be used in concrete (see 2.4). NOTE 1 A full explanation of this classification system may be found in BRE Digest 330 (1997 edition) i. NOTE 2 Experience indicates that most UK aggregate
25、 combinations will be classified as being of normal reactivity, with precautions to minimize any risk of ASR damage to concrete being taken in accordance with the appropriate advice provided by ES 5328, BRE Digest 330 i, Concrete Society Technical Report 1999 2 and the Specification for Highwau Work
26、s (Highways Agency) 3. NOTE 3 Minor concentrations of other constituents (e.g. shell) are permitted in aggregates for concrete, within the limits given in BS 882. 2.3 The mineral constituents of concern from the point of view of ASR are the following: a) opal or opaline silica; b) tridymite; c) cris
27、tobalite; d) quartz with crystal dislocations; e) microcrystalline quartz; f) highly strained quartz (see Table 1, note 3) g) volcanic glass (see Table 1, note 8); NOTE 1 Tables 1 and 2 give guidance for petrographers on rock types in which potentially alkali-silica reactive constituents occasionall
28、y occur. An indication of the reported occurrences in practice in the UK is included NOTE 2 Definitions of rock and mineral types are included in Tables 1 and 2. NOTE 3 The reactivity of silica constituents is dependent upon the degree of thermodynamic disorder of the molecular structure, with opal
29、being the most disordered form of silica and unstrained quartz being the least disordered form. 2.4 Opal or opaline silica is the most reactive of the mineral constituents listed in 2.3. NOTE Opal or opaline silica are very rarely encountered in UK aggregates. However, in situations where they are e
30、ncountered, they are usually found in low concentrations and often dispersed within aggregate particles. This may lead to difficulties in detecting these particles or constituents using petrography alone. If the presence of these exceptionally reactive constituents is suspected, it will be necessary
31、 to undertake a gel-pat test in addition to the petrographical examination (a gel-pat method is given in annex B). O BSI 07-1999 1 2.5 In the UK most cases of damage from ASR have involved flint and chert (rocks comprising mainly microcrystalline quartz) usually, although not exclusively, found in s
32、and and gravels. However, a few cases of ASR have been identified in which other aggregate types were involved. In these cases crushed greywacke type sandstone (usually referred to simply as greywacke) and greywacke type siltstones (usually referred to in this context as siltstone) have been identif
33、ied as the reactive aggregate, containing very fiiely distributed microcrystalline quartz and sometimes also dislocated quartz. NOTE 1 Investigations (Blackwell and Pettifer 4 and Blackwell et al 5) into crushed greywackes selected from UK sources have shown, when tested by the method described in B
34、S 812-123 and an accelerated mortar-bar method, that aggregates from about half of the sources would be considered possibly deleterious in use. These tests have shown that expansion increased more or less proportionally with the amount of crushed greywacke present. In some tests concentrations of 10
35、 % greywacke (mixed with ASR-inert aggregates) produced expansion. Concentrations of 30 % greywacke would be considered possibly deleterious in use. As the reactive constituent is concentrated in the fine grained matrix it is not possible to distinguish reactive and non-reactive greywacke type sands
36、tones/siltstones using petrography alone. NOTE 2 Experience to date indicates that greywacke type sandstone/siltstone particles contained within UK natural sands and gravels exhibit a much lower ASR potential than petrographically similar materials within a crushed rock aggregate. 3 Classification o
37、f aggregates 3.1 Although individual aggregates my be classified using the information in this guide, the selection of the appropriate measures for minimizing any risk of ASR will depend upon the classification of the total aggregate combination. 3.2 Any aggregate containing detectable opal or opali
38、ne silica should not be classified and, in most circumstances, should not be used in concrete. 3.3 An aggregate or aggregate combination should be classified as being of low reactivity if it wholly (see note) comprises rock and mineral constituents considered to be of low reactivity potential in Tab
39、les 1 and 2 and contains no detectable opal or opaline silica (see 3.2). NOTE In this context, “wholly“ is used to define aggregate containing not more than 3 % by mass of rock and any mineral constituents considered to be of normal or high reactivity in Tables 1 and 2. 3.4 An aggregate or aggregate
40、 combination should be classified as being of high reactivity if it comprises either more than 10 % crushed greywacke (i.e. greywacke type sandstone or siltstone, see note 1) or recycled demolition waste (see note 2) and contains no detectable opal or opaline silica (see 3.2). NOTE 1 Natural sand an
41、d gravel and gravel aggregates containing uncrushed greywacke material should be classified as being of normal reactivity (see 3.5). NOTE 2 There is expected to be increased usage of recycled aggregates consisting mainly of cmshed concrete and masonry. Pending greater experience of their use, at pre
42、sent it is considered prudent to regard these aggregates as being of high reactivity. 3.5 An aggregate or aggregate combination should be classified as being of normal reactivity if it cannot be classified as being of low (see 3.3) or high (see 3.4) reactivity and contains no detectable opal or opal
43、ine silica (see 3.2). NOTE 1 Aggregate combinations which are classified as being of normal reactivity by petrography may be reclassified as being of low reactivity on the basis of expansion results from the BS 812-123 concrete prism test method. Guidance on the interpretation of these results is gi
44、ven in BS 812-123 and BRE Digest 330 i. Preferably such expansion testing should be undertaken as part of an approved quality control procedure. NOTE 2 Aggregate combinations are sometimes associated with the greatest expansion in concrete when the reacting constituent forms significantly less than
45、100 % this is often termed “Pessimum behaviour“. In many cases, for reasons still being investigated, concrete expansion is decreased or even eliminated as the proportion of reactive constituent increases within the aggregate. In the UK, Pessimum behaviour has almost always been associated with reac
46、ting flint or chert particles in sand and gravel aggregates. Such flintchert bearing aggregates will usually be classified by petrography as being of normal reactivity. 2 O BSI 07-1999 STD-BSI BS 7943-ENGL It999 H Lb24bb9 0793399 785 BS 7943:1999 Table 1 - Guidance on rock types Definition Potential
47、ly alkali-silica reactive components that may sometimes be present Damage attributed in UK practice (see note 1) Suggested reactivity level (see note 2) Rock type Amphibolite Metamorphic rock characterized by a dominant content of amphibole minerals O L Highly-strained quartz and/or poorly crystalli
48、ne boundaries between quartz grains Microcrystalline or cryptocrystalline quartz Opaline or chalcedonic veination (see note 3) Glass or devitrified glass Opaline or chalcedonic veination or vugh-fillings See “Sandstone“ Andesite O L Fine grained volcanic rock of intermediate composition (differs min
49、eralogicaily from trachyte) Detrital sedimentary rock containing more than 25 % feldspar Fine-grained basic volcanic rock Arkose O N Basalt Glass or devitrified glass Opaline or chalcedonic veination or vugh-fillings O L Breccia Coarse detrital rock containing angular fragments See “Sandstone“ O N Chalk Very fine-grained white Cretaceous limestone Associated “Mnt“ O L N Chert Sedimentary rock vaxiously comprising microcrystalline quartz, cryptocrystalline quartz and chalcedony See “Flint“ * Conglomerate Coarse detrital rock containing rounded fragments See
