ASTM C457/C457M-23a

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C457/C457M − 23a
Standard Test Method for
Microscopical Determination of Parameters of the Air-Void
System in Hardened Concrete
This standard is issued under the fixed designation C457/C457M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 2. Referenced Documents
1.1 This test method describes procedures for microscopical
2.1 ASTM Standards:
determinations of the air content of hardened concrete and of
C42/C42M Test Method for Obtaining and Testing Drilled
the specific surface, void frequency, spacing factor, and paste-
Cores and Sawed Beams of Concrete
air ratio of the air-void system in hardened concrete. Three
C125 Terminology Relating to Concrete and Concrete Ag-
procedures are described:
gregates
1.1.1 Procedure A—Linear-traverse method.
C138/C138M Test Method for Density (Unit Weight), Yield,
1.1.2 Procedure B—Modified point-count method.
and Air Content (Gravimetric) of Concrete
1.1.3 Procedure C—Contrast enhanced method.
C173/C173M Test Method for Air Content of Freshly Mixed
Concrete by the Volumetric Method
1.2 This test method is based on prescribed procedures that
C192/C192M Practice for Making and Curing Concrete Test
are applied to sawed and lapped sections of specimens of
Specimens in the Laboratory
concrete from the field or laboratory.
C231/C231M Test Method for Air Content of Freshly Mixed
1.3 It is intended to outline the principles of this test method
Concrete by the Pressure Method
and to establish standards for its adequate performance but not
C666/C666M Test Method for Resistance of Concrete to
to describe in detail all the possible variations that might be
Rapid Freezing and Thawing
used to accomplish the objectives of this test method.
C670 Practice for Preparing Precision and Bias Statements
1.4 The values stated in either SI units or inch-pound units
for Test Methods for Construction Materials
are to be regarded separately as standard. The values stated in
C672/C672M Test Method for Scaling Resistance of Con-
each system are not necessarily exact equivalents; therefore, to
crete Surfaces Exposed to Deicing Chemicals (Withdrawn
ensure conformance with the standard, each system shall be
2021)
used independently of the other, and values from the two
C823/C823M Practice for Examination and Sampling of
systems shall not be combined.
Hardened Concrete in Constructions
1.5 This standard does not purport to address all of the
C856 Practice for Petrographic Examination of Hardened
safety concerns, if any, associated with its use. It is the
Concrete
responsibility of the user of this standard to establish appro-
D92 Test Method for Flash and Fire Points by Cleveland
priate safety, health, and environmental practices and deter-
Open Cup Tester
mine the applicability of regulatory limitations prior to use.
E691 Practice for Conducting an Interlaboratory Study to
For specific hazard statements see Note 9 and Note 12.
Determine the Precision of a Test Method
1.6 This international standard was developed in accor-
2.2 American Concrete Institute Standards:
dance with internationally recognized principles on standard-
201.2R Guide to Durable Concrete
ization established in the Decision on Principles for the
211.1 Standard Practice for Selecting Proportions for
Development of International Standards, Guides and Recom-
Normal, Heavyweight, and Mass Concrete
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This test method is under the jurisdiction of ASTM Committee C09 on Standards volume information, refer to the standard’s Document Summary page on
Concrete and Concrete Aggregates and is the direct responsibility of Subcommittee the ASTM website.
C09.65 on Petrography. The last approved version of this historical standard is referenced on
Current edition approved Dec. 1, 2023. Published January 2024. Originally www.astm.org.
approved in 1960. Last previous edition approved in 2023 as C457/C457M – 23. Available from American Concrete Institute (ACI), P.O. Box 9094, Farmington
DOI: 10.1520/C0457_C0457M-23A. Hills, MI 48333-9094, http://www.aci-int.org.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C457/C457M − 23a
3. Terminology void is usually identified by its irregular shape or evidence that
a channel or cavity has been created by bleed water trapped in
3.1 For definitions of terms used in this test method, refer to
the concrete at the time it hardened.
Terminology C125.
3.2 Definitions of Terms Specific to This Standard:
4. Summary of Test Method
¯
3.2.1 average chord length ~l!, n—the average length of the
4.1 Procedure A, Linear-Traverse Method—This procedure
chords formed by the transection of the voids by the line of
consists of the determination of the volumetric composition of
traverse; the unit is a length.
the concrete by summing the distances traversed across a given
3.2.2 binary image, n—formed by segmenting an image
component along a series of regularly spaced lines in one or
using only one threshold with the resulting image having only
more planes intersecting the specimen. The data gathered are
areas of black or white.
the total length traversed (T ), the length traversed through air
t
3.2.3 digital image, n—an image captured using a voids (T ), the length traversed through paste (T ), and the
a p
computer-based storage method where the information pre- number of air voids intersected by the traverse line (N). These
sented in the image can be seen visually, like a traditional data are used to calculate the air content and various param-
photographic image, but can also be extracted in a numeric
eters of the air-void system. If only the air content is desired,
form that can be used for additional analysis. only T and T need be determined.
a t
3.2.4 paste-air ratio (p/A), n—the ratio of the volume of
4.2 Procedure B, Modified Point-Count Method—This pro-
hardened cement paste to the volume of the air voids in the
cedure consists of the determination of the volumetric compo-
concrete.
sition of the concrete by observation of the frequency with
which areas of a given component coincide with a regular grid
3.2.5 paste content (p), n—the proportion of the total
system of points at which stops are made to enable the
volume of the concrete that is hardened cement paste expressed
determinations of composition. These points may be in one or
as percentage by volume.
more planes intersecting the specimen. The data gathered are
3.2.5.1 Discussion—When this parameter is calculated, it is
the linear distance between stops along the traverse (I), the
the sum of the proportional volumes of the cement, the net
total number of stops (S ), the number of stops in air voids (S ),
mixing water (including the liquid portions of any chemical t a
the number of stops in paste (S ), and the number of air voids
p
admixtures), and any supplementary cementitious materials
(N) intersected by the line of traverse over which the compo-
present.
nent data is gathered. From these data the air content and
3.2.6 pixel, n—the smallest definable point of a digital
various parameters of the air-void system are calculated. If
image that has an assigned value representing the brightness of
only the air content is desired, only S and S need be
a t
that component in an image.
determined.
3.2.6.1 Discussion—Typically a pixel will have the same
4.3 Procedure C, Contrast Enhanced Method—This proce-
aspect ratio as the overall image and will have an assigned
dure consists of the determination of the volumetric composi-
integer value occurring in the range 0-255. Multiple pixels are
tion of the concrete by summing distances measured in digital
arranged contiguously in two-dimensional arrays to form a
images of a prepared concrete surface using a series of
digital image.
regularly spaced lines in one or more digital images obtained
3.2.7 segment, v—the process of placing image pixels into
from one or more planes intersecting the specimen. The
classes or like-groupings using any number of thresholds.
specimen is prepared exactly as described for Procedures A and
¯
B with the additional steps of darkening the specimen surface
~ !
3.2.8 spacing factor L , n—a parameter related to the
and filling the air voids with a fine particle size white powder.
maximum distance in the cement paste from the periphery of
The data gathered are the total length measured (T ), the length
an air void, the unit is a length.
t
measured through air voids (T ), and the number of air voids
a
3.2.9 specific surface (α), n—the surface area of the air
intersected by the measurement lines (N). These data are used
voids divided by their volume, expressed in compatible units
to calculate the air content and various parameters of the
so that the unit of specific surface is a reciprocal length.
air-void system, except the paste-air ratio and spacing factor
3.2.10 threshold, n—a value used to discriminate pixels into
that require determination of the paste content (T ) as described
p
more than one class or like grouping.
in 16.1. If only the air content is desired, only T and T need
a t
3.2.11 void frequency (n), n—voids per unit length of be determined.
traverse; the number of air voids intercepted by a traverse line
4.4 Paste-Air Ratio Modification—In some instances the
divided by the length of that line; the unit is a reciprocal length.
specimen is not representative of the concrete as a whole, so T
t
3.2.11.1 Discussion—The value for void frequency (n) can-
and S lose their significance and cannot be used as a basis for
t
not be directly determined by the paste-air ratio method as this
calculations. The most common examples are concrete with
value refers to the voids per unit measure of traverse in the total
large coarse aggregate and specimens from the finished surface
concrete (including aggregate).
region, for both of which the examined specimen consists of a
3.2.12 water void, n—a space enclosed by the cement paste disproportionately large amount of the mortar fraction. In such
that was occupied by water at the time of setting and frequently instances the usual procedure must be changed, and the
found under an aggregate particle or reinforcing bar. A water- paste-air ratio modification must be used (see 5.5).
C457/C457M − 23a
5. Significance and Use SAMPLING AND SECTION PREPARATION
5.1 The parameters of the air-void system of hardened
6. Apparatus and Materials for Specimen Preparation
concrete determined by the procedures described in this test
6.1 Apparatus and Materials for All Procedures—
method are related to the susceptibility of the cement paste
6.1.1 Apparatus and materials for the preparation of sur-
portion of the concrete to damage by freezing and thawing.
faces of concrete speciemens for microscopical observation are
Hence, this test method can be used to develop data to estimate
described in Practice C856; other apparatus may be equally
the likelihood of damage due to cyclic freezing and thawing or
suitable.
to explain why it has occurred. The test method can also be
6.2 Materials for Procedure C—
used as an adjunct to the development of products or proce-
6.2.1 Opaque Permanent Black Ink, wide felt-tipped
dures intended to enhance the resistance of concrete to cyclic
marker, black ink stamp pad, or black ink roller, or similar.
freezing and thawing.
6.2.2 White Powder, barium sulfate, wollastonite, or tita-
5.2 Values for parameters of the air-void system can be
nium dioxide with a median particle size of 2-3 μm, or similar.
obtained by any of the procedures described in this test 6.2.3 Light Oil, light mineral oil, or similar.
method. The selection of which one of the three methods to be
NOTE 3—Apparatus for measurement of prepared specimens is de-
used shall be subject to agreement of the user and provider of
scribed in the three following procedures.
the determination
7. Sampling (for all procedures)
NOTE 1—Because Procedure C requires darkening the paste and
7.1 Specimens of concrete can be obtained from concrete
aggregate, its use must occur after other tests if the analyst is also
cast in the field or laboratory, or by coring, sawing, or
gathering petrographic data in addition to the measurements described in
this test method.
otherwise removing concrete from structures or products. The
procedure followed and the location from which the specimens
5.3 No provision is made for distinguishing among en-
are obtained will depend on the objectives of the program. In
trapped air voids, entrained air voids, and water voids. Any
general, secure samples of hardened concrete in accordance
such distinction is arbitrary, because the various types of voids
with Test Method C42/C42M or Practice C823/C823M or both.
intergrade in size, shape, and other characteristics. Reports that
Provide at least the minimum area of finished surface given in
do make such a distinction typically define entrapped air voids
Table 1 in each specimen. A sample may be composed of any
as being larger than 1 mm in at least one dimension being
number of specimens.
irregular in shape, or both. The honey-combing that is a
7.2 For referee purposes or to determine the compliance of
consequence of the failure to compact the concrete properly is
hardened concrete with requirements of specifications for the
one type of entrapped air void.
air-void system, obtain samples for analysis by this test method
5.4 Water voids are cavities that were filled with water at the
from at least three randomly selected locations over the area or
time of setting of the concrete. They are significant only in
mixtures that contained excessive mixing water or in which
TABLE 1 Minimum Area of Finished Surface for Microscopical
pronounced bleeding and settlement occurred. They are most
A B
Measurement ,
common beneath horizontal reinforcing bars, pieces of coarse
C
Total Area to be Traversed for
aggregate and as channelways along their sides. They occur
D ¯
Determination of p, A, α, or L, min, cm
Nominal or Observed 2
also immediately below surfaces that were compacted by
[in. ]
Maximum Size of Aggregate
finishing operations before the completion of bleeding.
Based on Direct Measurement of:
in the Concrete, mm [in.]
Total Air-Void
Paste-Air Ratio, p/A
5.5 Application of the paste-air ratio procedure is necessary
Content
when the concrete includes large nominal maximum size
150 [6] 1613 [250] 645 [100]
75 [3] 419 [65] 194 [30]
aggregate, such as 50 mm [2 in.] or more. Prepared sections of
37.5 [1 ⁄2 ] 155 [24] 97 [15]
such concrete should include a maximum of the mortar
25.0 [1] 77 [12] 77 [12]
fraction, so as to increase the number of counts on air voids or 3
19.0 [ ⁄4 ] 71 [11] 71 [11]
12.5 [ ⁄2 ] 65 [10] 65 [10]
traverse across them. The ratio of the volume of aggregate to
9.5 [ ⁄8 ] 58 [9] 58 [9]
the volume of paste in the original mix must be accurately
4.75 (No. 4) 45 [7] 45 [7]
known or estimated to permit the calculation of the air-void
A
The indicated values refer to reasonably homogeneous, well-compacted con-
systems parameters from the microscopically determined
crete. The microscopical measurement shall be made on proportionately larger
area of sections if the concrete is markedly heterogeneous in distribution of
paste-air ratio.
aggregate or large air voids. If more than one finished surface is taken from a
NOTE 2—The air-void content determined in accordance with this test
single portion of the concrete, the finished surfaces shall be separated by a
method usually agrees closely with the value determined on the fresh
distance greater than one half of the nominal or observed maximum size of
concrete in accordance with Test Methods C138/C138M, C173/C173M, aggregate.
B
See Section 3 for the interpretation of symbols employed.
or C231/C231M. However, significant differences may be observed if the
C
¯
When performing a point count to determine p, A, α, or L, the analysis points shall
sample of fresh concrete is consolidated to a different degree than the
be distributed evenly over the area to be traversed.
specimen later examined microscopically. For concrete with a relatively
D
high air content (usually over 7.5 %), the value determined microscopi- When p is determined, it shall be determined by analyzing the same area to be
¯
cally may be higher by one or more percentage points than that determined
traversed for determination of A, α, or L.
by Test Method C231/C231M.
C457/C457M − 23a
NOTE 4—Grit numbers of abrasives can denote slightly different
throughout the body of concrete to be tested, depending upon
particle sizes, depending on the manufacturer. The suggested sizes will
the objectives of the investigation.
usually be appropriate, but others may be selected according to the
experience of the user.
8. Preparation of Sections
From time to time during lapping, and when changing to a
8.1 Preparation of Sections for All Procedures—
finer abrasive and when lapping is complete, clean all surfaces
8.1.1 Unless the objectives of the program dictate
of the specimen gently and thoroughly to remove the grinding
otherwise, saw the section for observation approximately
compound. Use of ultrasonic cleaners may be harmful to the
perpendicular to the layers in which the concrete was placed or
surface. Such treatment should not be used without care and
perpendicular to the finished surface. Individual sections
experimentation. Cleaning with a soft cosmetic brush under
should be as large as can be ground and examined with the
running water, or by a pressurized dental spray has been
available equipment. The required area may consist of more
successful. A surface that is satisfactory for microscopical
than one prepared section. Spread the selected traverse length
examination will show an excellent reflection of a distant light
uniformly over the available surface so as to compensate for
source when viewed at a low incident angle and there shall be
the heterogeneity of the concrete.
no noticeable relief between the paste and the aggregate
8.1.2 If gross irregularities are present, begin the surface
surfaces. Areas that are scratched or imperfect indicate the
preparation by lapping (grinding on a flat surface) with
need for additional preparation; use special techniques if
nominal 150 μm (No. 100) silicon carbide abrasive. Lap the
surface with successively finer abrasives until it is suitable for required (see Note 9 and Note 12). The edges of the sections of
the air voids will be sharp and not eroded or crumbled, and
microscopical observation. An appropriate series of abrasives
would include nominal 75, 35, 17.5 and 12.5 μm grit sizes (No. air-void sections including those as small as 10 μm [0.0004 in.]
220, 320, 600, and 800, respectively), and perhaps 5-μm (No. in diameter will be clearly distinguishable. (See Fig. 1.) Do not
2500 grit) aluminum oxide. include scratched or broken portions of the surface in the
FIG. 1 Photographs of a Satisfactory Surface
C457/C457M − 23a
analyzed area. If needed to meet the requirements of Table 1, then complete the lapping operation as described above. Use
prepare additional surfaces. this surface as the reference plane, to which later measure-
8.1.3 Sometimes difficulty will be encountered in preparing ments are referenced. (3) Lap the back surface of the specimen
the lapped surfaces. The usual cause is a weak cement-paste
so as to produce a plane section. (4) Measure the thickness of
matrix. The problem is manifested by the plucking of sand
the specimen to the nearest 1 mm [0.05 in.] at four or more
grains from the surface during the lapping, with consequent
points uniformly spaced around the periphery. Average the
scratching of the surface, and by undercutting of the paste
results, and record the average to the nearest 1 mm [0.05 in].
around the harder aggregate particles. Friable particles of
(5) Determine the parameters of the air-void system on any
aggregate can also cause difficulty. In such instances the
plane desired or specified. If nearest surface values are desired,
following procedure is helpful. Heat the partially prepared
make the determination on the reference plane; if values for the
specimen of concrete to about 150 °C [300 °F] in an oven.
bulk concrete are desired, make the determination on the back
(Warning—If the specimen was sawn with a lubricant other
plane. If values for some other plane are desired, repeat the
than water, heating must be done so as to avoid inhaling the
grinding process to the desired depth. Redetermine the thick-
fumes and to preclude fire or explosion. Some lubricants have
ness of the specimen as specified above so that the parameters
a flash point as low as 140 °C [285 °F]. (The flash point of the
of the air-void system can be correlated with the distance of the
lubricant may be found by use of Test Method D92.) Unless
examined surface from the reference plane.
other precautions are taken, the temperature must not be
8.1.5 The composition of the near-surface zone differs from
allowed to approach the flash point. If this cannot be avoided,
that of the concrete as a whole. Therefore, whenever the design
heating must be done in the open air on a hot plate or in an
of the mixture is known, use the paste-air ratio method for the
explosion-proof hood.)
determination of the air-void system parameters in this region.
Remove the specimen from the oven and immediately brush
melted carnauba wax that was heated to the same temperature
8.2 Additional Preparation Required for Procedure C—
onto the surface. Repeat the application as the wax is absorbed
8.2.1 Prepare specimens in accordance with 8.1.
by the concrete, so that when the temperature of the concrete
falls below the melting point of the wax, a perceptible film
NOTE 6—If paste-air ratio and spacing factor are to be determined, it is
remains on the surface. After the specimen has cooled, scrape
necessary to provide a determination of the specimen paste content. If this
off any excess wax and repeat the lapping. After completion of
determination is to be made by point counting or another procedure
lapping, remove the residue of wax from the surface air voids requiring visually distinguishing aggregate from paste, such determina-
tions must be performed prior to blackening the specimen surface.
by reheating the concrete to about 150 °C [300 °F] to allow
absorption of the molten wax into the specimen. Again take
8.2.2 Apply an even layer of opaque, permanent, black ink
care to avoid approaching the flash point of the wax or of any
to the surface of the prepared specimen so the entire surface is
cutting oil present. Protect the surface from dust during
rendered uniformly black.
heating. The time to remove the wax from the surface air voids
varies with the properties and thickness of the specimen, but NOTE 7—Black markers have successfully been used as has black ink
applied with a roller such as that used in offset printing.
heating for about an hour is usually sufficient. Exceptionally
fragile concrete may require repetition of this process. Sub-
8.2.3 After the ink has dried, distribute a layer of white
stances other than carnauba wax have been used successfully to
powder over the blackened surface and gently press it into the
impregnate and strengthen the surfaces of concrete specimens
voids using a suitable implement such as a rubber stopper,
before grinding.
glass rod, or petrographic slide. Avoid abrading the surface by
NOTE 5—If performing a microscopic examination of the cement paste,
minimizing rubbing the powder over the surface.
using Practice C856, on the same test specimen that will be coated with
8.2.4 Remove excess powder by scraping it from the surface
wax or other paste strengthening media, perform the microscopic exami-
with a suitable implement (e.g., a sharp, new single-sided razor
nation prior to heating the specimen and application of the strengthening
materials. The application of such materials and exposure to oven
blade). Avoid abrading the ink thereby exposing cement or
temperatures of 150 °C [300 °F] will alter the physical characteristics and
aggregate grains.
appearance of the cement paste.
8.2.5 Inspect the darkened and filled surface. Areas where
8.1.4 If the parameters of the air-void system near a finished
ink has been abraded should be re-touched with a local, fresh
or formed surface are desired, then prepare the section exam-
application of ink. Porous aggregate grains should likewise be
ined in such a manner as to allow for the fact that the
re-touched with a local, fresh application of ink such that
parameters of the air-void system may vary greatly with the
internal pores in the aggregate are rendered black. Blacken
distance from such a surface. Therefore, measure the distance
areas filled in by white powder that are identified as pullouts
between the section to be examined and the original surface
occurring during specimen preparation. Likewise, blacken
accurately, to at least the nearest 1 mm [0.05 in.]. Use the
areas filled in with white powder that are identified as coarse
following procedure: (1) Prepare a specimen that includes a
cracks or other defects in the specimen not classified as part of
portion of the finished or formed surface to be investigated, and
1 the air-void system.
of convenient thickness, but not less than 12 mm [ ⁄2 in.] or
8.2.6 With a very lightly oiled finger or thumb, remove the
one-half of the nominal maximum size of the aggregate,
last remnants of white powder from the surface, leaving a shiny
whichever is greater. (2) Lap the surface with a coarse abrasive
until the last portion of the original surface is just removed, black surface without the presence of unfilled voids.
C457/C457M − 23a
PROCEDURE A—LINEAR TRAVERSE METHOD is mounted on three adjustable leveling screws and that
supports the specimen on the stage of the traverse device.
9. Apparatus for Measurement of Specimens
10. Procedure
9.1 The apparatus listed in 9.1.1 to 9.1.5 comprises a
10.1 Place the prepared specimen of concrete on the stage of
recommended minimum selection. Apparatus other than that
the linear-traverse device. Level the prepared surface with the
described has been used and may be equally satisfactory.
leveling device and the spirit level so that the surface may be
Apparatus that uses electronic switches and totalizers has been
traversed and microscopically examined with a minimum of
constructed. Computerized apparatus is commercially avail-
refocusing. Adjust the lamp so that the beam evenly illuminates
able. Image analyzers have frequently been used.
the field of view of the microscope and is incident on the
9.1.1 Linear-Traverse Device—Provide a platform, on
surface at a low angle, so the air voids are demarked by a
which the specimen is carried mounted on lead screws by
shadow. Superimpose the index point on the surface to be
means of which it can be smoothly translated in two perpen-
examined. Do not use a magnification of less than 50× and do
dicular directions. Provide one lead screw for movement in the
not change it during the course of the analysis. For a rectan-
N-S direction and at least two for movement in the E-W
gular section, place the index near an upper corner; for a
direction.
circular section, place it near the top and at one end of the
NOTE 8—In the descriptions of the linear-traverse and point-count
initial traverse. Engage and adjust all drives so as to remove all
devices the term “E-W direction” refers to the direction from the
play from the gear systems. Set all counters to zero. By
operator’s right to his left, and “N-S” means the direction perpendicular to
operation of the main lead screw move the assembly and
E-W, that is, the directions are analogous to those on a map.
specimen in the E-W direction while scrutinizing the prepared
One of these latter is called the “main” lead screw and the
surface as it moves beneath the microscope.
other(s) the “upper” lead screw(s). Ensure that the capacity of
NOTE 9—Surfaces examined may exhibit features that resemble air
the main (E-W) lead screw is at least 100 mm [4 in.], that of
voids, but are not: (a) Occasionally a transparent section of a grain of
each (E-W) lead screw at least 65 mm [2.5 in.], and that of the
quartz sand will look like an air void. (b) The socket left when a section
N-S lead screw at least 75 mm [3.0 in.]. Ensure that the pitch
of a nearly spherical and smooth sand grain is lost from the surface during
of the upper lead screw does not exceed 0.265 mm [0.0105 in.]
grinding looks much like an air void, but can be distinguished by
per revolution. Determine the pitch of all E-W lead screws to differences in the luster and sheen of the film lining the hole. (c)
Cenospheres, hollow particles of fly ash, or hollow plastic spheres will
the nearest 0.025 mm [0.001 in.]. Attach rotation counters
also have a different sheen, and are unaffected when the surrounding paste
readable to the nearest 0.01 of revolution to all E-W lead
is etched with dilute (10 %) hydrochloric acid.
screws. Provide a manually operated tally counter. For the
Warning—Do not acid-etch the specimen under the microscope, as the
determination of the paste content, provide a third E-W lead
effervescent spatter may damage the lens.
NOTE 10—Occasionally, air voids may become filled, during the service
screw complete with rotation counter, unless each traverse is to
exposure of the concrete, with secondary products. Whether such voids
be repeated, that is, performed once for the air content and
are counted as belonging to the air-void system or not depends on the
again for the paste content. Photographs of satisfactory linear-
purposes of the investigation.
traverse devices are shown in Figs. 2 and 3.
When the index point is exactly superimposed on the
9.1.2 Stereoscopic Microscope and Support, with objectives
periphery of a section of an air void in the prepared surface of
and eyepieces to give final magnification in the range from
the specimen, stop the movement of the carriage, actuate the
about 50× to about 125×. While it is possible to use a
tally counter once, and by means of the upper lead screw, move
microscope with a single, fixed magnification, it is more
the concrete under the microscope until the index point is
convenient to be able to vary the magnification within the
exactly superimposed upon the opposite periphery of the same
above range by replacing eyepieces or objectives or, better, by
air-void section. Stop the rotation of the upper lead screw, and
means of a zoom attachment. Arrange the microscope so as to
resume the movement by means of the main lead screw. Take
permit continuous observations of the surface of the specimen
extreme care to determine whether or not a section of an air
mounted on the platform of the linear-traverse device. Include
void is intersected by the index point when the line of traverse
cross hairs, scale, or some other reticle device to provide an
is nearly tangent to the void section. The results can be affected
index point in one eyepiece. Since an index point is dimen-
significantly by consistent error in this respect. If the periphery
sionless it shall be a point such as the intersection of one pair
of an air void is crumbled or rounded, estimate the position of
of edges of the cross hairs or one corner of the end of a line of
the true periphery in the plane of the surface by extrapolation
a scale. Use the same index point throughout any examination.
of the surface contour of the air void. If the paste content is
9.1.3 Microscope Lamp, spotlight-type, arranged to provide
being determined, as will usually be the case, carry out the
sufficient illumination at a low and variable incident angle to
above procedure for traverses across paste regions, except use
the surface. The spot of evenly lit area on the specimen surface
the second upper lead screw and do not use the tally counter.
should be slightly larger than the field of view of the micro-
Proceed in this way along the E-W traverse line, traversing all
scope.
chords across air voids with the upper lead screw, all sections
9.1.4 Spirit Level, the small circular type is convenient.
of paste with the second upper lead screw (if paste content is
9.1.5 Leveling Device—Provide a means to level the exam- being determined), and all other sections with the main lead
ined surface. This can be done by the insertion of small pieces screw. Stop the traverse at the end of the line, which should be
of modeling clay. A better way is by means of a platform that just within the examined area, not at its edge. By means of the
C457/C457M − 23a
A = Base plate.
B = Front and back rails supporting the middle plate C
C = Middle plate.
D = Upper front and back rails carrying the stage E
E = Stage.
F = Concrete specimen
G = Rectangular front groove in the base plate.
H = V-shaped back groove in the base plate.
I = Main lead screw.
J = Two bearing blocks for the main lead screw.
K = Revolution counter on main lead screw.
L = Manually operated knurled wheel.
M = Electric motor for driving the main lead screw.
N = Upper lead screw.
O = Revolution counter for upper lead screw.
P = Hand-driven wheel for moving the stage.
Q = Ratchet counter to tally the number of air voids encountered.
R = Stereoscopic microscope.
S = Microscope lamp.
NOTE 1—Not shown are a third lead screw and a disengaging clutch; the former is necessary if a determination of the air-paste ratio is required, and
the latter may be required (see 9.1.1).
FIG. 2 Photograph of a Linear-Traverse Device Meeting the Requirements of This Test Method
N-S lead screw shift the specimen an appropriate distance to the total rotation on each counter, or read and record each at the
the next traverse line. Space the segments of the traverse so as end of each traverse line, so that the total will be the
to cover the entire prepared surface with at least the minimum
summation of such records. If more than one specimen has
required traverse length. If the rotation counters operate in both
been prepared from the specimen of concrete, repeat the
directions, the next line of traverse can begin just below the
procedure on each such specimen as to comply with the
end of the previous one; if not, return the stage so the new line
requirements of Table 1. Electronic or computerized equipment
will begin just below the beginning of the previous one. Start
will require that the procedures specified by the fabricator be
each segment of the traverse just within the prepared area and
followed but the principles will remain as detailed above. The
on the satisfactory plane surface of the specimen rather than at
minimum length of traverse shall be as specified in Table 2.
the edge of the surface itself. The length of the segments of the
traverse may vary. Superimpose the index point at the begin-
11. Calculation
ning of the new line, and perform the traverse as before. Repeat
this process for all segments of the total traverse. Accumulate 11.1 When based on the air content of the total concrete:
C457/C457M − 23a
N
n 5 (5)
T
t
¯
11.1.5 Average Chord Length ~l!:
T
a
¯
l 5 (6)
N
or
A
¯
l 5 (7)
100n
11.1.6 Specific Surface (α):
α 5 (8)
¯
l
or
FIG. 3 Photograph of a Computerized Linear-Traverse/Point-
4N
Count Device Meeting the Requirements of This Test Method
α 5 (9)
T
a
TABLE 2 Minimum Length of Traverse for the Linear Traverse
11.1.7 Paste Content (p),in %:
A
Method
T ·100
p
Length of Traverse for
p 5 (10)
Nominal or Observed T
Determination of A, α, or
t
Maximum Size of Aggregate
¯
L,
in the Concrete, mm [in.]
11.1.8 Paste-Air Ratio (p/A):
min, mm [in.]
p T
150 [6] 4064 [160]
p
5 (11)
75 [3] 3048 [120]
A T
a
37.5 [1 ⁄2 ] 2540 [100]
25.0 [1] 2413 [95]
¯
11.1.9 Spacing Factor ~L!:
19.0 [ ⁄4 ] 2286 [90]
12.5 [ ⁄2 ] 2032 [80]
11.1.9.1 If p/A is less than or equal to 4.342
9.5 [ ⁄8 ] 1905 [75]
4.75 (No. 4) 1397 [55] T
p
¯
L 5 (12)
A
The limits of uncertainty of results obtained for air-void content depend upon the 4N
length of traverse and the air-void content of the concrete. Based on experience,
the recommended minimum length of traverse shown in this table should produce
11.1.9.2 If p/A is greater than 4.342
limits of uncertainty such that up to 3 % air-void content the standard deviation is
1/3
3 p
not greater than 0.5 %, which at 3 % air-void content corresponds to a coefficient
¯
L 5 1.4 11 2 1 (13)
F S D G
of variation of 17 %. For traverse lengths greater than 1375 mm [55 in.] and
α A
air-void contents greater than 3 % the coefficient of variation is correspondingly
reduced. The data obtained can be analyzed by statistical methods to determine
11.2 If the calculations are based on the paste-air ratio
the limits of uncertainty to be applied.
method, the design of the mixture must be known. The
microscopically determined data will consist of T , T , and N.
p a
Proceed as follows:
11.2.1 Calculate the paste-air ratio (r):
11.1.1 The data will consist of:
T
p
r 5 (14)
T
a
where:
11.2.2 From the mixture design calculate the ratio of aggre-
N = total number of air voids intersected,
gate volume to paste volume (M):
R = number of rotations of the respective lead screws, and
i
P = pitch of the corresponding lead screws.
i G
c
M 5 (15)
p
c
where:
11.1.2 Calculate:
G = sum of the masses of the aggregates; each divided by
c
T 5 total length of traverse 5 sum of P × R (1)
t i i
its specific gravity, and
T 5 traverse length through air 5 P × R (2) p = sum of mass of cement divided by specific gravity of
c
a a a
cement + mass of mineral admixture divided by the
T 5 traverse length through paste 5 P × R (3)
p p p
specific gravity of the mineral admixture + mass of
11.1.3 Air Content (A),in %:
water (including the fluid portion of any admixture)
divided by the specific gravity of the fluids:
T ·100
a
A 5 (4)
NOTE 11—The values used for the calculated G and p must accurately
T c c
t
reflect the differing densities of the constituents and the proportions of the
11.1.4 Void Frequency (n): masses of each.
C457/C457M − 23a
11.2.3 The percent air (A): at a low angle, so the air voids are demarked by a shadow.
Superimpose the index point of the cross hairs (or other reticle
A 5 (16)
device) on the surface to be examined. Use a magnification not
r 11M 11
~ !
less than 50× and do not change it during the course of the
11.2.4 Paste expressed as percent (p):
analysis. For a rectangular section, place the index near an
upper corner; for a circular section, place it near the top and at
p 5 A·r (17)
one end of the initial traverse. Position the stopping device at
¯
11.2.5 Average chord length ~l!:
a stop or click position at the beginning of the traverse. Do not
include the initial stops for each traverse line in the total
T
a
¯
l 5 (18)
number of stops or in the number of stops for any component.
N
Zero all counters. By operation of the E-W lead screw, cause
11.2.6 Specific Surface (α): Use Eq 8 and 9.
movement of the stage and specimen while simultaneously
¯
11.2.7 Spacing Factor ~L!:
scrutinizing the surface. At each click stop, except not at the
11.2.7.1 When r is equal to or less than 4.342 use Eq 12. beginning of any traverse line, pause and examine the field of
view, and record on the appropriate counter the material or
11.2.7.2 When r is greater than 4.342 use Eq 13 with the
substitution of r for the p/A ratio. phase on which the index point is superimposed.
NOTE 12—Surfaces examined may exhibit features that resemble air
PROCEDURE B—MODIFIED POINT-COUNT
voids, but are not: (a) Occasionally a transparent section of a grain of
METHOD
quartz sand will look like an air void. (b) The socket left when a section
of a nearly spherical and smooth sand grain is lost from the surface during
12. Apparatus for Measurement of Specimens
grinding looks much like an air void, but can be distinguished by
12.1 The apparatus listed in 12.1.1 to 12.1.5 comprises a differences in the luster and sheen of the film lining the hole. (c)
Cenospheres, hollow particles of fly ash, or hollow plastic spheres will
recommended minimum selection. Apparatus other than that
also have a different sheen, and are unaffected when the surrounding paste
described has been used and may be equally satisfactory.
is etched with dilute (10 %) hydrochloric acid.
Apparatus that uses electronic switches and totalizers has been
Warning—Do not acid-etch the specimen under the microscope, as the
constructed. Computerized apparatus is commercially avail-
effervescent spatter may damage the lens.
able. Image analyzers have frequently been used.
NOTE 13—Occasionally, air voids may become filled, during the service
12.1.1 Point-Count Device, comprising a stage or platform exposure of the concrete, with secondary products. Whether such voids
are counted as belonging to the air-void system or not depends on the
connected to E-W and N-S lead screws (see 9.1.1) and
purposes of the investigation.
designed in such a way that a specimen of concrete placed on
the stage can be moved smoothly and uniformly through equal Normally use one counter for air voids, one for paste, and
distances by turning of the screws. Ensure that the total one for all other phases (or a totaling counter). Other compo-
possible translation of the stage is at least 100 mm [4.0 in.] in nents (fine and coarse aggregate, for example—if they are
each direction. Fit lead screws with notched wheels and lithologically distinguishable) of the concrete can be deter-
stopping devices, such that with each rotation of the screws a mined with the use of additional counters. Continue in this way
click can be detected by the operator when a stop position is along the line until a last stop is reached just within the
reached. Ensure that the intervals between the stops correspond prepared area, but close to its edge. When the end of the line is
to a translation of the stage a distance of 0.6 to 5.0 mm [0.025 reached, turn off the totaling counter. Reverse the E-W lead
to 0.200 in.]. Determine the magnitude of the average transla- screw and proceed back along the same line, recording on
tion of the stage between stops to the nearest 0.025 mm [0.001 another counter each air void intersected, whether or not a stop
occurred within the air void. Terminate the void counting just
in.]. Provide at least four digital counters; more may be better.
It may be convenient to attach one counter to the stopping before the initial stop. Counting of the air voids intersected
during the same traverse used for counting the phases at each
device of the E-W lead screw, so as to register automatically
the total number of stops in that direction. A photograph of a click stop is not prohibited. Take extreme care to determine
satisfactory device for the modified point-count method is whether a section of an air void is intersected by the movement
given in Fig. 4. of the index when the line of traverse is nearly tangent to the
12.1.2 Stereoscopic Microscope and Support, as described void section. The results can be affected significantly by
in 9.1.2. consistent error in this respect. If the periphery of an air void
12.1.3 Microscope Lamp, as described in 9.1.3. is crumbled or rounded, estimate the position of the true
12.1.4 Spirit Level, as described in 9.1.4. periphery in the plane of the surface by extrapolation of the
12.1.5 Leveling Device, as described in 9.1.5. surface contour of the air void. If the examination is being
made to determine only the air content of the concrete, the
13. Procedure
number of air voids intersected by the line of traverse need not
13.1 Place the prepared surface of concrete on the stage of be determined. By means of the N-S lead screw, shift the
the point-count device. Using the spirit level, level the prepared concrete specimen at right angles to the direction of traverse an
surface with the leveling devi
...