1、Designation: C 457 08cStandard Test Method forMicroscopical Determination of Parameters of the Air-VoidSystem in Hardened Concrete1This standard is issued under the fixed designation C 457; the number immediately following the designation indicates the year oforiginal adoption or, in the case of rev
2、ision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method describes procedures for microscopicaldeterminations of the air content of harden
3、ed concrete and ofthe specific surface, void frequency, spacing factor, and paste-air ratio of the air-void system in hardened concrete (1).2Twoprocedures are described:1.1.1 Procedure A, the linear-traverse method (2, 3).1.1.2 Procedure B, the modified point-count method (3, 4, 5,6).1.2 This test m
4、ethod is based on prescribed procedures thatare applied to sawed and lapped sections of specimens ofconcrete from the field or laboratory.1.3 It is intended to outline the principles of this test methodand to establish standards for its adequate performance but notto describe in detail all the possi
5、ble variations that might beused to accomplish the objectives of this test method.1.4 The values stated in SI units are to be regarded asstandard. The values given in parentheses are mathematicalconversions to inch-pound units that are provided for informa-tion only and are not considered standard.1
6、.5 This standard does not purport to address all of thesafety concerns associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use. For specific hazard statemen
7、ts see 8.3and 10.1.2. Referenced Documents2.1 ASTM Standards:3C 42/C 42M Test Method for Obtaining and Testing DrilledCores and Sawed Beams of ConcreteC 138/C 138M Test Method for Density (Unit Weight),Yield, and Air Content (Gravimetric) of ConcreteC 173/C 173M Test Method for Air Content of Freshl
8、yMixed Concrete by the Volumetric MethodC 231 Test Method for Air Content of Freshly MixedConcrete by the Pressure MethodC 670 Practice for Preparing Precision and Bias Statementsfor Test Methods for Construction MaterialsC 823 Practice for Examination and Sampling of HardenedConcrete in Constructio
9、nsC 856 Practice for Petrographic Examination of HardenedConcreteD92 Test Method for Flash and Fire Points by ClevelandOpen Cup Tester2.2 American Concrete Institute Standards:201.2R Guide to Durable Concrete4211.1 Recommended Practice for Selecting Proportions forNormal, Heavyweight, and Mass Concr
10、ete43. Terminology3.1 Definitions:3.1.1 air content (A) The proportion of the total volumeof the concrete that is air voids; expressed as percentage byvolume.3.1.2 air voidA space enclosed by the cement paste andthat was filled with air or other gas prior to the setting of thepaste.3.1.2.1 Discussio
11、nThis term does not refer to voids ofsubmicroscopical dimensions, such as the porosity inherent tothe hardened-cement paste. Air voids are usually larger than afew micrometers in diameter. The term includes both entrappedand entrained voids.3.1.3 average chord length ( l )The average length of thech
12、ords formed by the transection of the voids by the line oftraverse; the unit is a length.3.1.4 paste-air ratio (p/A)The ratio of the volume ofhardened cement paste to the volume of the air voids in theconcrete.3.1.5 paste content (p)The proportion of the total volumeof the concrete that is hardened
13、cement paste expressed aspercentage by volume.1This test method is under the jurisdiction of ASTM Committee C09 onConcrete and Concrete Aggregates and is the direct responsibility of SubcommitteeC09.65 on Petrography.Current edition approved Oct. 1, 2008. Published October 2008. Originallyapproved i
14、n 1960. Last previous edition approved in 2008 as C 457 08b.2The boldface numbers in parentheses refer to the list of references at the end ofthis test method.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of
15、ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available fromAmerican Concrete Institute (ACI), P.O. Box 9094, FarmingtonHills, MI 48333-9094, http:/www.aci-int.org.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM I
16、nternational, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.5.1 DiscussionWhen this parameter is calculated, it isthe sum of the proportional volumes of the cement, the netmixing water (including the liquid portions of any chemicaladmixtures), and any supple
17、mentary cementitious materialspresent (7, 8) .3.1.6 spacing factor ( L )A parameter related to themaximum distance in the cement paste from the periphery ofan air void, the unit is a length.3.1.7 specific surface (a)The surface area of the air voidsdivided by their volume, expressed in compatible un
18、its so thatthe unit of specific surface is a reciprocal length.3.1.8 void frequency , nVoids per unit length of traverse;the number of air voids intercepted by a traverse line dividedby the length of that line; the unit is a reciprocal length.3.1.8.1 DiscussionThe value for void frequency (n) can-no
19、t be directly determined by the paste-air ratio method as thisvalue refers to the voids per unit measure of traverse in the totalconcrete (including aggregate).3.1.9 water voidA space enclosed by the cement pastethat was occupied by water at the time of setting and frequentlyfound under an aggregate
20、 particle or reinforcing bar. A water-void is usually identified by its irregular shape or evidence thata channel or cavity has been created by bleed water trapped inthe concrete at the time it hardened.4. Summary of Test Method4.1 Procedure A, Linear-Traverse MethodThis procedureconsists of the det
21、ermination of the volumetric composition ofthe concrete by summing the distances traversed across a givencomponent along a series of regularly spaced lines in one ormore planes intersecting the sample. The data gathered are thetotal length traversed (Tt), the length traversed through airvoids (Ta),
22、the length traversed through paste (Tp), and thenumber of air voids intersected by the traverse line (N). Thesedata are used to calculate the air content and various param-eters of the air-void system. If only the air content is desired,only Taand Ttneed be determined.4.2 Procedure B, Modified Point
23、-Count MethodThis pro-cedure consists of the determination of the volumetric compo-sition of the concrete by observation of the frequency withwhich areas of a given component coincide with a regular gridsystem of points at which stops are made to enable thedeterminations of composition. These points
24、 may be in one ormore planes intersecting the sample. The data gathered are thelinear distance between stops along the traverse (I), the totalnumber of stops (St), the number of stops in air voids (Sa), thenumber of stops in paste (Sp), and the number of air voids (N)intersected by the line of trave
25、rse over which the componentdata is gathered. From these data the air content and variousparameters of the air-void system are calculated. If only the aircontent is desired, only Saand Stneed be determined.4.3 Paste-Air Ratio Modification In some instances thesample is not representative of the conc
26、rete as a whole, so Ttand Stlose their significance and cannot be used as a basis forcalculations. The most common examples are concrete withlarge coarse aggregate and samples from the finished surfaceregion, for both of which the examined sample consists of adisproportionately large amount of the m
27、ortar fraction. In suchinstances the usual procedure must be changed, and thepaste-air ratio modification must be used (see 5.5).5. Significance and Use5.1 The parameters of the air-void system of hardenedconcrete determined by the procedures described in this testmethod are related to the susceptib
28、ility of the cement pasteportion of the concrete to damage by freezing and thawing.Hence, this test method can be used to develop data to estimatethe likelihood of damage due to cyclic freezing and thawing orto explain why it has occurred. The test method can also beused as an adjunct to the develop
29、ment of products or proce-dures intended to enhance the resistance of concrete to cyclicfreezing and thawing (1).5.2 Values for parameters of the air-void system can beobtained by either of the procedures described in this testmethod.5.3 No provision is made for distinguishing among en-trapped air v
30、oids, entrained air voids, and water voids. Anysuch distinction is arbitrary, because the various types of voidsintergrade in size, shape, and other characteristics. Reports thatdo make such a distinction typically define entrapped air voidsas being larger than 1 mm in at least one dimension beingir
31、regular in shape, or both. The honey-combing that is aconsequence of the failure to compact the concrete properly isone type of entrapped air void (9, 10).5.4 Water voids are cavities that were filled with water at thetime of setting of the concrete. They are significant only inmixtures that contain
32、ed excessive mixing water or in whichpronounced bleeding and settlement occurred. They are mostcommon beneath horizontal reinforcing bars, pieces of coarseaggregate and as channelways along their sides. They occuralso immediately below surfaces that were compacted byfinishing operations before the c
33、ompletion of bleeding.5.5 Application of the paste-air ratio procedure is necessarywhen the concrete includes large nominal maximum sizeaggregate, such as 50 mm (2 in.) or more. Prepared sections ofsuch concrete should include a maximum of the mortarfraction, so as to increase the number of counts o
34、n air voids ortraverse across them. The ratio of the volume of aggregate tothe volume of paste in the original mix must be accuratelyknown or estimated to permit the calculation of the air-voidsystems parameters from the microscopically determinedpaste-air ratio.NOTE 1The air-void content determined
35、 in accordance with this testmethod usually agrees closely with the value determined on the freshconcrete in accordance with Test Methods C 138/C 138M, C 173/C 173M,or C 231 (11). However, significant differences may be observed if thesample of fresh concrete is consolidated to a different degree th
36、an thesample later examined microscopically. For concrete with a relatively highair content (usually over 7.5 %), the value determined microscopicallymay be higher by one or more percentage points than that determined byTest Method C 231.C 457 08c2SAMPLING AND SECTION PREPARATION6. Apparatus and Mat
37、erials for Sample Preparation (foreither procedure)6.1 Apparatus and materials for the preparation of surfacesof concrete samples for microscopical observation are de-scribed in Practice C 856; other apparatus may be equallysuitable.NOTE 2Apparatus for measurement of prepared samples is describedin
38、the two following procedures.7. Sampling (for either procedure)7.1 Samples of concrete can be obtained from specimenscast in the field or laboratory, or by coring, sawing, orotherwise removing concrete from structures or products. Theprocedure followed and the location from which the samplesare obta
39、ined will depend on the objectives of the program. Ingeneral, secure samples of hardened concrete in accordancewith Test Method C 42 or Practice C 823 or both. Provide atleast the minimum area of finished surface given in Table 1 ineach sample. A sample may be composed of any number ofspecimens.7.2
40、For referee purposes or to determine the compliance ofhardened concrete with requirements of specifications for theair-void system, obtain samples for analysis by this test methodfrom at least three randomly selected locations over the area orthroughout the body of concrete to be tested, depending u
41、ponthe objectives of the investigation.8. Preparation of Sections (for either procedure)8.1 Unless the objectives of the program dictate otherwise,saw the section for observation approximately perpendicular tothe layers in which the concrete was placed or perpendicular tothe finished surface. Indivi
42、dual sections should be as large ascan be ground and examined with the available equipment. Therequired area may consist of more than one prepared section.Spread the selected traverse length uniformly over the avail-able surface so as to compensate for the heterogeneity of theconcrete.8.2 If gross i
43、rregularities are present, begin the surfacepreparation by lapping (grinding on a flat surface) withnominal 150 m (No. 100) silicon carbide abrasive. Lap thesurface with successively finer abrasives until it is suitable formicroscopical observation. An appropriate series of abrasiveswould include no
44、minal 75, 35, 17.5 and 12.5 m grit sizes (No.220, 320, 600, and 800, respectively), and perhaps 5-maluminum oxide (Note 3). From time to time during lapping,and when changing to a finer abrasive and when lapping iscomplete, clean all surfaces of the specimen gently andthoroughly to remove the grindi
45、ng compound. Use of ultra-sonic cleaners may be harmful to the surface. Such treatmentshould not be used without care and experimentation. Cleaningwith a soft cosmetic brush under running water, or by apressurized dental spray has been successful. A surface that issatisfactory for microscopical exam
46、ination will show an excel-lent reflection of a distant light source when viewed at a lowincident angle and there shall be no noticeable relief betweenthe paste and the aggregate surfaces.Areas that are scratched orimperfect indicate the need for additional preparation; usespecial techniques if requ
47、ired (see 8.3). The edges of thesections of the air voids will be sharp and not eroded orcrumbled, and air-void sections including those as small as 10m (3.94 105in.) in diameter will be clearly distinguishable.(See Fig. 1.) Do not include scratched or broken portions of thesurface in the analyzed a
48、rea. If needed to meet the require-ments of Table 1, prepare additional surfaces.NOTE 3Grit numbers of abrasives can denote slightly differentparticle sizes, depending on the manufacturer. The suggested sizes willusually be appropriate, but others may be selected according to theexperience of the us
49、er.8.3 Sometimes difficulty will be encountered in preparingthe lapped surfaces. The usual cause is a weak cement-pastematrix. The problem is manifested by the plucking of sandgrains from the surface during the lapping, with consequentscratching of the surface, and by undercutting of the pastearound the harder aggregate particles. Friable particles ofaggregate can also cause difficulty. In such instances thefollowing procedure is helpful. Heat the partially preparedspecimen of concrete to about 150 C (300 F) in an oven.(WarningIf the specimen was sawn with
