1、Designation: C 457 08dStandard 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 666/C 666M Test Method for Resistance of Concrete toRapid Freezing and ThawingC 670 Practice for Preparing Precision and Bias Statementsfor Test Methods for Construction Materia
9、lsC 672/C 672M Test Method for Scaling Resistance of Con-crete Surfaces Exposed to Deicing ChemicalsC 823 Practice for Examination and Sampling of HardenedConcrete in ConstructionsC 856 Practice for Petrographic Examination of HardenedConcreteD92 Test Method for Flash and Fire Points by ClevelandOpe
10、n Cup Tester2.2 American Concrete Institute Standards:201.2R Guide to Durable Concrete4211.1 Recommended Practice for Selecting Proportions forNormal, Heavyweight, and Mass Concrete43. Terminology3.1 Definitions:3.1.1 air content (A)The proportion of the total volume ofthe concrete that is air voids
11、; 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 DiscussionThis term does not refer to voids ofsubmicroscopical dimensions, such as the porosity inherent tothe hardened-cement paste
12、. 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 thechords 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 rati
13、o of the volume ofhardened cement paste to the volume of the air voids in theconcrete.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 Dec. 15, 2008. Pub
14、lished January 2009. Originallyapproved in 1960. Last previous edition approved in 2008 as C 457 08c.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 Servic
15、e at serviceastm.org. For Annual Book of 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
16、the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.5 paste content (p)The proportion of the total volumeof the concrete that is hardened cement paste expressed aspercentage by volume.3.1.5.1 DiscussionWhen th
17、is 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 supplementary cementitious materialspresent (7, 8).3.1.6 spacing factor ( L )A parameter related to themaximum distance in the ce
18、ment 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 units so thatthe unit of specific surface is a reciprocal length.3.1.8 void frequency , nVoids per unit length of traverse;the
19、 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-not be directly determined by the paste-air ratio method as thisvalue refers to the voids per unit measure of traverse in the
20、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 particle or reinforcing bar. A water-void is usually identified by its irregular shape or evidence thata channel or cavity
21、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 determination of the volumetric composition ofthe concrete by summing the distances traversed across a givencomponent along a s
22、eries 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), the length traversed through paste (Tp), and thenumber of air voids intersected by the traverse line (N). Thesedata are used
23、 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-Count MethodThis pro-cedure consists of the determination of the volumetric compo-sition of the concrete by observation of
24、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 may be in one ormore planes intersecting the sample. The data gathered are thelinear distance between stops along the trave
25、rse (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 traverse over which the componentdata is gathered. From these data the air content and variousparameters of the air-void system a
26、re calculated. If only the aircontent is desired, only Saand Stneed be determined.4.3 Paste-Air Ratio ModificationIn some instances thesample is not representative of the concrete as a whole, so Ttand Stlose their significance and cannot be used as a basis forcalculations. The most common examples a
27、re concrete withlarge coarse aggregate and samples from the finished surfaceregion, for both of which the examined sample consists of adisproportionately large amount of the mortar fraction. In suchinstances the usual procedure must be changed, and thepaste-air ratio modification must be used (see 5
28、.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 susceptibility of the cement pasteportion of the concrete to damage by freezing and thawing.Hence, this test method can be used to dev
29、elop 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 development of products or proce-dures intended to enhance the resistance of concrete to cyclicfreezing and thawing (1).5.2 Values f
30、or 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 voids, entrained air voids, and water voids. Anysuch distinction is arbitrary, because the various types of voidsintergrade in
31、 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 beingirregular in shape, or both. The honey-combing that is aconsequence of the failure to compact the concrete properly isone type
32、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 contained excessive mixing water or in whichpronounced bleeding and settlement occurred. They are mostcommon beneath horizontal rein
33、forcing bars, pieces of coarseaggregate and as channelways along their sides. They occuralso immediately below surfaces that were compacted byfinishing operations before the completion of bleeding.5.5 Application of the paste-air ratio procedure is necessarywhen the concrete includes large nominal m
34、aximum 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 on air voids ortraverse across them. The ratio of the volume of aggregate tothe volume of paste in the original mix must be ac
35、curatelyknown or estimated to permit the calculation of the air-voidsystems parameters from the microscopically determinedpaste-air ratio.NOTE 1The air-void content determined in accordance with this testmethod usually agrees closely with the value determined on the freshconcrete in accordance withT
36、est 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 than thesample later examined microscopically. For concrete with a relatively highair content (usually over 7.5 %), the value de
37、termined microscopicallyC 457 08d2may be higher by one or more percentage points than that determined byTest Method C 231.SAMPLING AND SECTION PREPARATION6. Apparatus and Materials for Sample Preparation (foreither procedure)6.1 Apparatus and materials for the preparation of surfacesof concrete samp
38、les for microscopical observation are de-scribed in Practice C 856; other apparatus may be equallysuitable.NOTE 2Apparatus for measurement of prepared samples is describedin the two following procedures.7. Sampling (for either procedure)7.1 Samples of concrete can be obtained from specimenscast in t
39、he field or laboratory, or by coring, sawing, orotherwise removing concrete from structures or products. Theprocedure followed and the location from which the samplesare obtained will depend on the objectives of the program. Ingeneral, secure samples of hardened concrete in accordancewith Test Metho
40、d 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 For referee purposes or to determine the compliance ofhardened concrete with requirements of specifications for theair-void sy
41、stem, 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 uponthe objectives of the investigation.8. Preparation of Sections (for either procedure)8.1 Unless the objectives of the progr
42、am dictate otherwise,saw the section for observation approximately perpendicular tothe layers in which the concrete was placed or perpendicular tothe finished surface. Individual sections should be as large ascan be ground and examined with the available equipment. Therequired area may consist of mo
43、re 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 irregularities are present, begin the surfacepreparation by lapping (grinding on a flat surface) withnominal 150 m (No. 100) si
44、licon carbide abrasive. Lap thesurface with successively finer abrasives until it is suitable formicroscopical observation. An appropriate series of abrasiveswould include nominal 75, 35, 17.5 and 12.5 m grit sizes (No.220, 320, 600, and 800, respectively), and perhaps 5-maluminum oxide (Note 3). Fr
45、om 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 grinding compound. Use of ultra-sonic cleaners may be harmful to the surface. Such treatmentshould not be used without care and expe
46、rimentation. Cleaningwith a soft cosmetic brush under running water, or by apressurized dental spray has been successful. A surface that issatisfactory for microscopical examination will show an excel-lent reflection of a distant light source when viewed at a lowincident angle and there shall be no
47、noticeable relief betweenthe paste and the aggregate surfaces.Areas that are scratched orimperfect indicate the need for additional preparation; usespecial techniques if required (see 8.3). The edges of thesections of the air voids will be sharp and not eroded orcrumbled, and air-void sections inclu
48、ding 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 area. If needed to meet the require-ments of Table 1, prepare additional surfaces.NOTE 3Grit numbers of abrasives can denote sl
49、ightly differentparticle sizes, depending on the manufacturer. The suggested sizes willusually be appropriate, but others may be selected according to theexperience of the user.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. I
