1、Designation: C 457 08aStandard 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 volume ofthe 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 Discussion
11、This 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 thecho
12、rds 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 c
13、ement 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 May 1, 2008. Published June 2008. Originallyapproved in 196
14、0. Last previous edition approved in 2008 as C 457 08.2The 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 ASTMSt
15、andards 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 Interna
16、tional, 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 supplementar
17、y 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 units so
18、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-not be di
19、rectly 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 partic
20、le 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 determinat
21、ion 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), the len
22、gth 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-Count
23、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 may be
24、 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 traverse ove
25、r 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 ModificationIn some instances thesample is not representative of the concrete as
26、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 mortar fr
27、action. In suchinstances the usual procedure must be changed, and thepaste-air ratio modification must be used (see 5.7).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
28、 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 development of
29、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 voids, en
30、trained 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 beingirregular
31、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 contained exces
32、sive 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 completio
33、n of bleeding.5.5 For air-entrained concrete designed in accordance withACI 201.2R and ACI 211.1, the paste-air ratio (p/A) is usuallyin the range 4 to 10, the specific surface (a) is usually in therange 24 to 43 mm1(600 to 1100 in.1), and the spacing factor( L ) is usually in the range 0.1 to 0.2 m
34、m (0.004 to 0.008 in.).5.6 The air-void content determined in accordance with thistest method usually agrees closely with the value determinedon the fresh concrete in accordance with Test Methods C 138,C 173, or C 231 (11). However, significant differences may beobserved if the sample of fresh concr
35、ete is consolidated to adifferent degree than the sample later examined microscopi-cally. For concrete with a relatively high air content (usuallyover 7.5 %), the value determined microscopically may behigher by one or more percentage points than that determinedby Test Method C 231.5.7 Application o
36、f 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 on air voids ortraverse across them. The
37、 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.C 457 08a25.8 Of the parameters determined with this test method, thespacing
38、 factor ( L ) is generally regarded as the most significantindicator of the durability of the cement paste matrix tofreezing and thawing exposure of the concrete. The maximumvalue of the spacing factor for moderate exposure of theconcrete is usually taken to be 0.20 mm (0.008 in.). Somewhatlarger va
39、lues may be adequate for mild exposure, and smallerones may be required for severe exposure, especially if theconcrete is in contact with deicing chemicals. Care should beexercised in using spacing factor values in specifications sincethe standard deviation of that property has been found toapproach
40、 one-fifth of the average when determinations aremade in different laboratories. Hence, substantial differences inspacing factor may be caused solely by sampling and betweenlaboratory variation. The factors affecting the variability of thetest method are discussed in the section on Precision and Bia
41、s.5.9 The air content and the parameters of the air-voidsystem in hardened concrete depend primarily on the kind anddosage of the air entraining agent used, the degree of consoli-dation of the concrete, and its water-cement ratio. The valuesof the specific surface (a) and the void frequency (n) decr
42、easerapidly with an increase of the water-cement ratio or the pastecontent if other conditions are not altered. Satisfactory valuesof specific surface (a) and spacing factor ( L ) require that thevoid frequency be larger than about 315/m (8/in.). An increasein the water-cement ratio or the paste con
43、tent must be accom-panied by an increase in the air content, if the spacing factor( L ) is not to increase. The air content can be reducedsubstantially by extended vibration of the concrete, without asignificant increase of the spacing factor ( L ), provided theconcrete was adequately air entrained
44、originally. Extendedvibration is not, however, recommended as a field practicebecause of the dangers of excessive bleeding and segregation.5.10 The void frequency (n) is a critical parameter indetermining the magnitude of the specific surface (a) and thespacing factor ( L ). Consequently, utmost car
45、e must be takenin conducting either microscopical method to observe andrecord all air-void sections intersected by the line of traverse.Recognition of air-void sections of small size, for example, 10m (3.94 by 105in.) is essential to securing a correctevaluation of these parameters. For this reason,
46、 care must betaken to prepare extremely smooth and plane sections, themagnification employed should be not less than 503, and theindex point in the cross hairs (or other reticle device) must beobserved precisely in relation to the area and periphery of theair-void section.5.11 Provided the value of
47、the specific surface (a)orthevoid frequency (n) is sufficiently high, a suitable spacing factor( L ) will be obtained even when the air content is low.However, in order to obtain an air-void system that has both thevolume capacity and the geometric parameters necessary toprotect saturated mature cem
48、ent paste during exposure tofreezing, it is important to obtain concrete with an acceptablyhigh air content (A) and a low enough spacing factor ( L )toprovide protection (12).5.12 For concrete exposed to freezing and thawing whilecritically saturated, a minimum-compressive strength must bedeveloped
49、prior to the freezing exposure, in addition to thesecuring of adequate air entrainment if the concrete is to beprotected properly. Such compressive strength must be at least28 MPa (4000 psi).SAMPLING AND SECTION PREPARATION6. Apparatus and Materials 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 1Apparatus for measurement of prepared samples is describedin the two following procedu
