ASTM C457-98

Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete

SCOPE
1.1 This test method describes procedures for microscopical determinations of the air content of hardened concrete and of the specific surface, void frequency, spacing factor, and paste-air ratio of the air-void system in hardened concrete (1).  Two procedures 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 method is based on prescribed procedures that are applied to sawed and lapped sections of specimens of concrete from the field or laboratory.
1.3 It is intended to outline the principles of this test method and to establish standards for its adequate performance but not to describe in detail all the possible variations that might be used to accomplish the objectives of this test method.
1.4 The values stated in inch-pound units are to be regarded as the standard.
1.5 This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements see Notes 3 and 6.

General Information

• Status: Historical
• Publication Date: 09-Jul-1998
• ICS: 91.100.30 - Concrete and concrete products
• Technical Committee: C09 - Concrete and Concrete Aggregates
• Drafting Committee: C09.65 - Petrography

Relations

• Replaced By: ASTM C457-06 - Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete
• Effective Date: 01-Jan-2006
• Referred By: ASTM C856/C856M-20 - Standard Practice for Petrographic Examination of Hardened Concrete
• Effective Date: 10-Jul-1998
• Referred By: ASTM C823/C823M-12(2017) - Standard Practice for Examination and Sampling of Hardened<brk/> Concrete in Constructions
• Effective Date: 10-Jul-1998

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: C 457 – 98
Standard Test Method for
Microscopical Determination of Parameters of the Air-Void
System in Hardened Concrete
This standard is issued under the fixed designation C 457; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope C173 Test Method for Air Content of Freshly Mixed
Concrete by the Volumetric Method
1.1 Thistestmethoddescribesproceduresformicroscopical
C231 Test Method for Air Content of Freshly Mixed
determinations of the air content of hardened concrete and of
Concrete by the Pressure Method
the specific surface, void frequency, spacing factor, and paste-
C670 Practice for Preparing Precision and Bias Statements
air ratio of the air-void system in hardened concrete (1). Two
for Test Methods for Construction Materials
procedures are described:
C823 Practice for Examination and Sampling of Hardened
1.1.1 Procedure A, the linear-traverse method (2, 3).
Concrete in Constructions
1.1.2 ProcedureB,themodifiedpoint-countmethod(3,4,5,
C856 Practice for Petrographic Examination of Hardened
6).
Concrete
1.2 This test method is based on prescribed procedures that
D92 Test Method for Flash and Fire Points by Cleveland
are applied to sawed and lapped sections of specimens of
Open Cup
concrete from the field or laboratory.
2.2 American Concrete Institute Standards:
1.3 Itisintendedtooutlinetheprinciplesofthistestmethod
201.2R Guide to Durable Concrete
and to establish standards for its adequate performance but not
211.1 Recommended Practice for Selecting Proportions for
to describe in detail all the possible variations that might be
Normal, Heavyweight, and Mass Concrete
used to accomplish the objectives of this test method.
1.4 The values stated in SI units are to be regarded as the
3. Terminology
standard. The values in parentheses are provided for informa-
3.1 Definitions:
tion purposes only.
3.1.1 air content (A)—Theproportionofthetotalvolumeof
1.5 This standard does not purport to address all of the
the concrete that is air voids; expressed as percentage by
safety concerns associated with its use. It is the responsibility
volume.
of the user of this standard to establish appropriate safety and
3.1.2 air void—A space enclosed by the cement paste and
health practices and determine the applicability of regulatory
that was filled with air or other gas prior to the setting of the
limitations prior to use. For specific hazard statements see
paste.
Note 3 and Note 6.
3.1.2.1 Discussion—This term does not refer to voids of
2. Referenced Documents submicroscopical dimensions, such as the porosity inherent to
the hardened-cement paste.Air voids are usually larger than a
2.1 ASTM Standards:
fewmicrometersindiameter.Thetermincludesbothentrapped
C42 Test Method for Obtaining and Testing Drilled Cores
3 and entrained voids.
and Sawed Beams of Concrete
3.1.3 average chord length ( l )—The average length of the
C138 Test Method for Unit Weight,Yield, andAir Content
chords formed by the transection of the voids by the line of
(Gravimetric) of Concrete
traverse; the unit is a length.
3.1.4 paste-air ratio (p/A)—The ratio of the volume of
This practice is under the jurisdiction of ASTM Committee C-9 on Concrete
hardened cement paste to the volume of the air voids in the
andConcreteAggregatesandisthedirectresponsibilityofSubcommitteeC09.65on
concrete.
Petrography.
Current edition approved July 10, 1998. Published December 1998. Originally
e1
published as C457–60T. Last previous edition C457–90 (82a ).
2 4
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Annual Book of ASTM Standards, Vol 05.01.
this test method. American Concrete Institute of Concrete Practice, issued annually, available
Annual Book of ASTM Standards, Vol 04.02. from ACI, 38800 Country Club Drive, Farmington Hills, MI 48331.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 457–98
3.1.5 paste content (p)—Theproportionofthetotalvolume disproportionately large amount of the mortar fraction. In such
of the concrete that is hardened cement paste expressed as instances the usual procedure must be changed, and the
percentage by volume. paste-air ratio modification must be used (see 5.7).
3.1.5.1 Discussion—When this parameter is calculated, it is
5. Significance and Use
the sum of the proportional volumes of the cement, the net
5.1 The parameters of the air-void system of hardened
mixing water (including the liquid portions of any chemical
concrete determined by the procedures described in this test
admixtures), and any mineral admixtures present (7, 8).
method are related to the susceptibility of the cement paste
3.1.6 spacing factor ( L )—A parameter related to the
portion of the concrete to damage by freezing and thawing.
maximum distance in the cement paste from the periphery of
Hence,thistestmethodcanbeusedtodevelopdatatoestimate
an air void, the unit is a length.
the likelihood of frost damage to concrete or to explain why it
3.1.7 specific surface (a)—The surface area of the air voids
hasoccurred.Thetestmethodcanalsobeusedasanadjunctto
divided by their volume, expressed in compatible units so that
thedevelopmentofproductsorproceduresintendedtoenhance
the unit of specific surface is a reciprocal length.
the frost resistance of concrete (1).
3.1.8 void frequency , n—Voids per unit length of traverse;
5.2 Values for parameters of the air-void system can be
the number of air voids intercepted by a traverse line divided
obtained by either of the procedures described in this test
by the length of that line; the unit is a reciprocal length.
method.
3.1.8.1 Discussion—The value for void frequency (n) can-
5.3 No provision is made for distinguishing among en-
not be directly determined by the paste-air ratio method as this
trapped air voids, entrained air voids, and water voids. Any
valuereferstothevoidsperunitmeasureoftraverseinthetotal
such distinction is arbitrary, because the various types of voids
concrete (including aggregate).
intergradeinsize,shape,andothercharacteristics.Reportsthat
3.1.9 water void—A space enclosed by the cement paste
do make such a distinction typically define entrapped air voids
thatwasoccupiedbywateratthetimeofsettingandfrequently
as being larger than 1 mm in at least one dimension being
found under an aggregate particle or reinforcing bar. A water-
irregular in shape, or both. The honey-combing that is a
voidisusuallyidentifiedbyitsirregularshapeorevidencethat
consequence of the failure to compact the concrete properly is
a channel or cavity has been created by bleed water trapped in
one type of entrapped air void (9, 10).
the concrete at the time it hardened.
5.4 Watervoidsarecavitiesthatwerefilledwithwateratthe
4. Summary of Test Method
time of setting of the concrete. They are significant only in
4.1 Procedure A, Linear-Traverse Method—This procedure mixtures that contained excessive mixing water or in which
consists of the determination of the volumetric composition of pronounced bleeding and settlement occurred. They are most
theconcretebysummingthedistancestraversedacrossagiven common beneath horizontal reinforcing bars, pieces of coarse
component along a series of regularly spaced lines in one or aggregate and as channelways along their sides. They occur
more planes intersecting the sample. The data gathered are the also immediately below surfaces that were compacted by
total length traversed (T), the length traversed through air finishing operations before the completion of bleeding.
t
voids (T ), the length traversed through paste (T ), and the 5.5 For air-entrained concrete designed in accordance with
a p
number of air voids intersected by the traverse line (N). These ACI 201.2R andACI 211.1, the paste-air ratio (p/A) is usually
data are used to calculate the air content and various param- in the range 4 to 10, the specific surface (a) is usually in the
−1 −1
eters of the air-void system. If only the air content is desired, range24to43mm (600to1100in. ),andthespacingfactor
only T and T need be determined. ( L ) is usually in the range 0.1 to 0.2 mm (0.004 to 0.008 in.).
a t
4.2 Procedure B, Modified Point-Count Method—This pro- 5.6 The air-void content determined in accordance with this
cedure consists of the determination of the volumetric compo- test method usually agrees closely with the value determined
sition of the concrete by observation of the frequency with on the fresh concrete in accordance with Test Methods C138,
which areas of a given component coincide with a regular grid C173,or C231 (11). However, significant differences may be
system of points at which stops are made to enable the observed if the sample of fresh concrete is consolidated to a
determinations of composition. These points may be in one or different degree than the sample later examined microscopi-
more planes intersecting the sample. The data gathered are the cally. For concrete with a relatively high air content (usually
linear distance between stops along the traverse (I), the total over 7.5%), the value determined microscopically may be
number of stops (S), the number of stops in air voids (S ), the higher by one or more percentage points than that determined
t a
number of stops in paste (S ), and the number of air voids (N) by Test Method C231.
p
intersected by the line of traverse over which the component 5.7 Application of the paste-air ratio procedure is necessary
data is gathered. From these data the air content and various when the concrete includes large nominal maximum size
parameters of the air-void system are calculated. If only the air aggregate, such as 50 mm (2 in.) or more. Prepared sections of
content is desired, only S and S need be determined. such concrete should include a maximum of the mortar
a t
4.3 Paste-Air Ratio Modification—In some instances the fraction, so as to increase the number of counts on air voids or
sample is not representative of the concrete as a whole, so T traverse across them. The ratio of the volume of aggregate to
t
and S lose their significance and cannot be used as a basis for the volume of paste in the original mix must be accurately
t
calculations. The most common examples are concrete with known or estimated to permit the calculation of the air-void
large coarse aggregate and samples from the finished surface systems parameters from the microscopically determined
region, for both of which the examined sample consists of a paste-air ratio.
C 457–98
5.8 Of the parameters determined with this test method, the securing of adequate air entrainment if the concrete is to be
spacingfactor( L)isgenerallyregardedasthemostsignificant protected properly. Such compressive strength must be at least
indicator of the durability of the cement paste matrix to 28 MPa (4000 psi).
freezing and thawing exposure of the concrete. The maximum
SAMPLING AND SECTION PREPARATION
value of the spacing factor for moderate exposure of the
concrete is usually taken to be 0.20 mm (0.008 in.). Somewhat
6. Apparatus and Materials for Sample Preparation (for
larger values may be adequate for mild exposure, and smaller
either procedure)
ones may be required for severe exposure, especially if the
6.1 Apparatus and materials for the preparation of surfaces
concrete is in contact with deicing chemicals. Care should be
of concrete samples for microscopical observation are de-
exercised in using spacing factor values in specifications since
scribed in Practice C856; other apparatus may be equally
the standard deviation of that property has been found to
suitable.
approach one-fifth of the average when determinations are
madeindifferentlaboratories.Hence,substantialdifferencesin NOTE 1—Apparatus for measurement of prepared samples is described
in the two following procedures.
spacing factor may be caused solely by sampling and between
laboratory variation.The factors affecting the variability of the
7. Sampling (for either procedure)
test method are discussed in the section on Precision and Bias.
7.1 Samples of concrete can be obtained from specimens
5.9 The air content and the parameters of the air-void
cast in the field or laboratory, or by coring, sawing, or
system in hardened concrete depend primarily on the kind and
otherwise removing concrete from structures or products. The
dosage of the air entraining agent used, the degree of consoli-
procedure followed and the location from which the samples
dation of the concrete, and its water-cement ratio. The values
are obtained will depend on the objectives of the program. In
of the specific surface (a) and the void frequency (n) decrease
general, secure samples of hardened concrete in accordance
rapidly with an increase of the water-cement ratio or the paste
with Test MethodC42 or Practice C823 or both. Provide at
content if other conditions are not altered. Satisfactory values
least the minimum area of finished surface given in Table 1 in
of specific surface (a) and spacing factor ( L ) require that the
each sample. A sample may be composed of any number of
void frequency be larger than about 315/m (8/in.).An increase
specimens.
in the water-cement ratio or the paste content must be accom-
7.2 For referee purposes or to determine the compliance of
panied by an increase in the air content, if the spacing factor (
hardened concrete with requirements of specifications for the
L ) is not to increase. The air content can be reduced
air-voidsystem,obtainsamplesforanalysisbythistestmethod
substantially by extended vibration of the concrete, without a
fromatleastthreerandomlyselectedlocationsovertheareaor
significant increase of the spacing factor ( L ), provided the
throughout the body of concrete to be tested, depending upon
concrete was adequately air entrained originally. Extended
the objectives of the investigation.
vibration is not, however, recommended as a field practice
because of the dangers of excessive bleeding and segregation.
8. Preparation of Sections (for either procedure)
5.10 The void frequency (n) is a critical parameter in
8.1 Unless the objectives of the program dictate otherwise,
determining the magnitude of the specific surface (a) and the
sawthesectionforobservationapproximatelyperpendicularto
spacing factor ( L ). Consequently, utmost care must be taken
thelayersinwhichtheconcretewasplacedorperpendicularto
in conducting either microscopical method to observe and
the finished surface. Individual sections should be as large as
record all air-void sect
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