ASTM C426-23

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C426 − 22 C426 − 23
Standard Test Method for
Linear Drying Shrinkage of Concrete Masonry Units
This standard is issued under the fixed designation C426; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This test method covers a routine standardized procedure for determining the linear drying shrinkage of concrete masonry units
or related concrete units under specified accelerated drying conditions.
1.2 The text of this test method references notes and footnotes that provide explanatory material. These notes and footnotes
(excluding those in tables and figures) shall not be considered as requirements of the standard.
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 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.
2. Referenced Documents
2.1 ASTM Standards:
C490C490/C490M Practice for Use of Apparatus for the Determination of Length Change of Hardened Cement Paste, Mortar,
and Concrete
C1093 Practice for Accreditation of Testing Agencies for Masonry
C1232 Terminology for Masonry
2.2 ANSI Standard:
B94.11M—1993 Twist Drills
3. Terminology
3.1 Terminology defined in Terminology C1232 shall apply for this test method.
This test method is under the jurisdiction of ASTM Committee C15 on Manufactured Masonry Units and is the direct responsibility of Subcommittee C15.03 on Concrete
Masonry Units and Related Units.
Current edition approved June 1, 2022June 1, 2023. Published July 2022June 2023. Originally approved in 1958. Last previous edition approved in 20162022 as
C426 – 16.C426 – 22. DOI: 10.1520/C0426-22.10.1520/C0426-23.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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*A Summary of Changes section appears at the end of this standard
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C426 − 23
3.2 Definitions of Terms Specific to This Standard:
3.2.1 linear drying shrinkage, n—in this test method, the change in linear dimension of the test specimen due to drying from a
saturated condition to an equilibrium weight and length under specified accelerated drying conditions.
4. Significance and Use
4.1 This test method is intended to evaluate the drying shrinkage characteristics of a given unit. The results of this laboratory
method are considered in determining concrete masonry crack control provisions.
NOTE 1—The testing laboratory performing this test method should be evaluated in accordance with Practice C1093.
5. Apparatus
5.1 Strain Gauge—The instruments for measuring linear drying shrinkage shall be so designed as to permit or provide the
conditions described in 5.1.1 through 5.1.5.
NOTE 2—Strain gauges may be obtained with various gauge lengths. The 10-in. (254-mm) gauge length is recommended for use with regular concrete
masonry units, however, particular sizes of products may require other lengths. The length of the shrinkage specimen shall not be less than required for
a minimum gauge length (distance between gauge plugs) of 6 in. (152.4 mm).
5.1.1 A means of positive contact with the specimen that will ensure reproducible measurements of length.
5.1.2 Means for precise measurement, consisting of a dial micrometer or other measuring device graduated to read in 0.0001-in.
(0.0025-mm) units, and accurate within 0.0001 in. (0.0025 mm) in any 0.0010-in. (0.025-mm) range, and within 0.0002 in.
(0.0050-mm) in any 0.0100-in. (0.254-mm) range.
5.1.3 Sufficient range to allow for small variations in the gauge lengths.
NOTE 3—If the shrinkage reference points are set carefully to position, a dial micrometer with a travel of 0.2 or 0.3 in. (5.1 or 7.6 mm) provides ample
range in the instrument.
5.1.4 Means for checking the strain gauge at regular intervals against a standard reference bar. The standard reference bar shall
be protected from air currents by placing it inside a wooden box which should be closed except when the strain gauge is being
checked against it.
NOTE 4—A standard reference bar shall be furnished by the manufacturer of the instrument. A standard bar of ordinary steel is satisfactory, but corrections
must be made for variations in its length due to temperature changes. When a more nearly constant datum is desired, Invar is preferable because of its
low coefficient of thermal expansion.
5.1.5 Convenient and rapid measurement of specimens.
5.2 Comparator—When desirable to measure specimens end to end, a comparator conforming to the requirements of Practice
C490C490/C490M may be used as an alternative to the strain gauge for measuring linear drying shrinkage.
5.3 Gauge Plugs—The gauge plugs shall be made from metal that is resistant to corrosion. Plugs for use with strain gauges shall
3 1 1 1
be ⁄8 to ⁄2 in. (9.5 to 12.7 mm) in diameter and ⁄2 6 ⁄8 (12.7 6 3.2 mm) in thickness. Plugs for use with the comparator shall
consist of ⁄4-in. (6.4-mm) diameter stainless steel gauge studs shown in Practice C490C490/C490M.
5.4 Drying Oven—The oven shall be reasonably airtight and provide the features described in 5.4.1 through 5.4.4.
NOTE 5—One suggested oven construction is shown in Fig. 1.
5.4.1 A minimum storage capacity of three whole test specimens and a clearance of 1 in. (25.4 mm) on all sides of each test
specimen.
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NOTE 1—Provide access to heaters.
NOTE 2—Insulating fill is recommended in cabinets having outer covers of sheet metal
NOTE 3—The following materials are required:
Quantity Description
1 5-in. (127-mm) fan assembly, as shown
1 ⁄30-hp (25-W) shaded-pole, fan-cooled, stud-mounted electric motor
75 ft (22.9 m) 1-in. (25.4-mm) angle, steel or aluminum
2 2
60 ft (5.6 m ) Outer cover, ⁄2-in. (12.7-mm) plywood or equivalent, faced with sheet metal or other material to provide a positive vapor barrier
2 2 3
60 ft (5.6 m ) Inner cover, ⁄8-in. (9.5-mm) asbestos board or equivalent
1 Heat baffle, 25 by 34-in. (635 by 864-mm), sheet metal
16 ft (4.8 m) ⁄2-in. iron pipe
4 100-W porcelain light fixtures
1 500-W thermostat
1 24 × 30 × 1 ⁄2-in. (610 × 762 × 38-mm) tray, borosilicate glass or equivalent
1 pr 8-in. (203-mm) hinges and hasp
FIG. 1 Drying Oven Suitable for Determining Drying Shrinkage of Concrete Block
5.4.2 A constant, uniform temperature of 122 6 2°F (50 6 0.9°C) throughout the insulated cabinet attained by means of an
electrical heat source (Note 6).
NOTE 6—Direct heating of test specimens with the combustion products of gas or other carbonaceous fuels is not satisfactory due to the presence of carbon
dioxide and water and their possible effect on the drying characteristics of portland cement products.
5.4.3 A means of drying specimens to a condition of equilibrium with a relative humidity of 17 6 2 % (Note 7).
5.4.3.1 Calcium chloride (CaCl ), if used for this purpose, shall be in flake form. Suitable dishes or trays shall be provided to give
2 2 3
an exposed solution area of not less than 25 in. for each cubic foot (5800 cm for each m ) of oven volume. Dishes or trays shall
contain sufficient solid calcium chloride so that the crystals will be exposed above the surface of the solution throughout the test.
The calcium chloride solution shall be thoroughly stirred every 24 h, and more often if necessary, to prevent the formation of lumps
and crusting over.
NOTE 7—The air immediately above a saturated solution of calcium chloride (CaCl ) at 122°F (50°C) is approximately 17 %.
5.4.4 Moderate circulation of air within the oven, over and around all test specimens and the drying agent.
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FIG. 2 View of Sawed Block Showing a Suggested Sequence of Cuts and Location of Half-Face Shell Specimens
5.5 Cooling Chamber—An airtight enclosure of sufficient capacity for cooling a minimum of three whole specimens to a
temperature of 73.4 6 2°F (23 6 1.1°C).
5.6 Immersion Tank—A suitable container for completely immersing three whole test specimens in water maintained at 73.4 6
2°F (23 6 1.1°C).
5.7 Balance or Scale—The balance shall be sensitive to within 0.1 % of the weight of the smallest specimen tested.
6. Test Specimens
6.1 General:
6.1.1 Specimens shall comply with the requirements of 6.1.2 through 6.1.5 and the applicable requirements for the type of
specimen in 6.2 or 6.3.
6.1.2 A minimum of three specimens shall be tested.
6.1.3 Test specimens shall be representative of the lot from which they are selected. Specimens shall be free of visible cracks or
other structural defects, and shall not be exposed to external heat sources.
6.1.4 Specimens shall have a minimum height of 3 in. (76.2 mm), and a minimum thickness of at least 0.75 in. (19 mm). The
length of specimens shall conform to 6.1.4.1 or 6.1.4.2.
6.1.4.1 For full size units with a length of at least 12 in. (304.8 mm), use a test specimen with a minimum length of 12 in. (304.8
mm) and a gauge length of 10 in. (254 mm).
6.1.4.2 For full size units with a length of less than 12 in. (304.8 mm), do one of the following: (1) use a test specimen with a
minimum length of 7.5 in. (190.5 mm) and a gauge length of 6 in. (152.4 mm), or (2) butt two test specimens of approximately
the same thickness together and join with an epoxy resin cement to form a thin joint between units, and use a gauge length of 10
in. (254 mm). (See Note 8.)
NOTE 8—In tests of short units such as concrete brick by this method, use of a 10-in. (254-mm) Whittemore strain gauge is reported to be feasible when
two units are butted together and joined using an epoxy resin cement to form a thin joint between the units. The abutting ends of the units should be ground
to ensure intimate contact and a thin joint; these precautions are necessary to ensure the thinnest joints practicable and thereby avoid abnormal shrinkage
indications. Some laboratories have obtained satisfactory results using a 10-in. Whittemore strain gauge on specimens joined with unfilled epoxy cements.
6.1.5 When specimens are saw-cut from larger units, sawing shall be performed in an accurate, competent manner, subjecting the
specimen to as little saw vibration as possible. Use a diamond saw blade of proper hardness. Following cutting, residue from the
cutting operation shall be removed prior to continuing testing (see Note 9).
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NOTE 9—For specimens cut with a wet saw, rinsing with clean water is typically sufficient for removing cutting residue. For specimens cut with a dry
saw, brushing with a soft-bristle brush is typically sufficient for removing cutting residue.
6.2 Concrete Masonry Units:
6.2.1 Specimens shall be full-size units or portions of face shells saw-cut from full size units.
6.2.2 When specimens are portions of face shells, one specimen shall be removed from each of three full-size units.
6.3 Other Related Units:
6.3.1 For test specimens of units other than concrete masonry units, specimens shall be whole units, specimens saw-cut from larger
units, or specimens molded specifically for testing.
6.3.2 For specimens molded specifically for testing, the procedures used for molding and curing specimens shall be representative
and consistent with the procedure used to produce the actual units represented by the specimens.
6.4 Methods of Attaching Gauge Plugs to Specimens—Attach gauge plugs to specimens using one of the following procedures.
6.4.1 Gauge Plug Inserted into Specimen for Strain Gauge—Place a pair of gauge plugs at or near, and parallel to, the center line
in each of two opposite faces of the specimen. Drill plug holes with a drill that is slightly smaller in size than the plug diameter
so as to provide a snug fit (Note 10). The depth of the holes shall be such that the exposed surface of the inserted gauge plug is
approximately ⁄8 in. (2.5 mm) below the surface of the specimen. Prior to setting of the plug, plug holes shall be dry and dust free.
After the bonding agent (Note 11) has been placed in the hole, insert the gauge plug and prick punch the plug to proper gauge
length with the gauge bar provided. Wipe off excessive bonding agent and allow the remainder to cure in accordance with the
bonding agent manufacturer’s instructions. After the bonding agent is cured, drill receiving holes for strain gauge points with a No.
56 to 60 twist drill.
5 3
NOTE 10—A ⁄16-in. (7.9-mm
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