ASTM D6767-21

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Designation: D6767 − 20a D6767 − 21
Standard Test Method for
Pore Size Characteristics of Geotextiles by Capillary Flow
Test
This standard is issued under the fixed designation D6767; 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
1.1 This test method covers the determination of the pore size distribution of geotextile filters with pore sizes ranging from 1 to
1000 μm.
NOTE 1—The accuracy of this procedure has been verified up to a maximum pore size of 500 μm. Above this value, accuracy has been found to be
equipment dependent and should be verified by the user through checks on materials with known opening sizes.
1.2 The test method measures the entire pore size distribution in terms of a surface analysis of specified pore sizes in a geotextile,
defined in terms of the limiting diameters.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this 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:
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D4354 Practice for Sampling of Geosynthetics and Rolled Erosion Control Products (RECPs) for Testing
D4439 Terminology for Geosynthetics
3. Terminology
3.1 Definitions—For definitions of other terms used in this test method, refer to Terminologies D4439 and D1129.
3.2 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee D35 on Geosynthetics and is the direct responsibility of Subcommittee D35.03 on Permeability and
Filtration.
Current edition approved Dec. 1, 2020May 1, 2021. Published December 2020May 2021. Originally approved in 2002. Last previous edition approved in 2020 as
D6767 – 20.D6767 – 20a. DOI: 10.1520/D6767-20A.10.1520/D6767-21.
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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D6767 − 21
3.2.1 bubble point pore size (O ), n—the largest effective pore diameter detected by the sudden increase of flow rate at the
beginning of the wet test.
3.2.2 pore constriction (O), n—diameter of a circle having the same area as the smallest section of a given pore.
3.2.3 pore size (O ), n—capillary equivalent pore diameter for which the percent of total pore diameters i in a given geotextile
i
based on the surface occupied by the pores is smaller than or equal to that diameter.
3.2.4 pore size distribution (PSD), n—percent cumulative distribution of the complete range of pore sizes with in a given
geotextile based on the surface occupied by the pores.
3.2.5 wetting fluid, n—fluid used to wet the geotextile test specimen for the capillary porometry portion of the test method, that
is, the “wet” test. The verb “wet” is intended to mean to completely saturate the geotextile specimen with the wetting fluid
thoroughly such that the entire thickness of the specimen contains fluid with no entrained air bubbles.
4. Summary of Test Method
4.1 Geotextile filters have discrete pores from one side to the other of the geotextile. The bubble point test is based on the principle
that a wetting fluid is held in these continuous pores by capillary attraction and surface tension, and the minimum pressure required
to force fluid from these pores is a function of pore diameter.
4.2 A fluid-wet geotextile will pass air when the applied air pressure exceeds the capillary attraction of the fluid in the pore
constriction. Smaller pore constrictions will exhibit similar behavior at higher pressures. The relationship between pore size and
pressure has been established for the wetting fluid.
4.3 By comparing the gas flow rates of both a wet and dry test at the same pressures, the percentage of the flow passing through
the filter pores larger than or equal to the specified size may be calculated from the pressure-size relationship. By increasing
pressure in small steps, it is possible to determine the flow contribution of very small pore size increments by difference.
5. Significance and Use
5.1 This test method may be used to:
5.1.1 Determine the pore size distribution of a geotextile,
5.1.2 Determine the maximum pore size of a geotextile,
5.1.3 Determine the mean flow pore size of a geotextile,
5.1.4 Determine the effect of processes such as calendaring or needle punching upon the pore size distribution,
5.1.5 Determine the effect of compression upon the pore size distribution of a geotextile, if the test equipment allows, and
5.1.6 Determine the gas flow rate of a geotextile, and thereby its gas flow capability.
5.2 The pore size distribution test may also be used for research, material development, or to assess clogging on field-retrieved
samples.
6. Apparatus
6.1 Clean Gas Pressure Source, with regulation (filtered air).
6.2 Pressure Sensor—Pressure measurements may be obtained with a digital pressure transducer, a U-tube manometer, or an
inclined manometer covering the necessary pressure range for the pore sizes under study and the wetting fluid used. The pressure
D6767 − 21
sensor sensitivity shall be dictated by the range of pressures associated with the openings sizes. The bubble point pressure should
be measured with an accuracy of 61 % or 65 Pa, whichever is larger.Pressure measurements must be accurate to 65 Pa to
1000 Pa, and 61 % above 1000 Pa.
6.2.1 Pressure sensor(s) must be installed immediately upstream (for example, within 5 mm) of the sample holder.
6.3 Closed Filter Holder, (see Fig. 1).
6.3.1 Filter holder for the test specimens that fully confines the perimeter of the specimen to prevent any lateral pressure losses.
6.3.2 The specimen flow area shall be 25 mm (1 in.) or 50 mm (2 in.) in diameter. Both the 50-mm (2-in.) diameter and other
diameters shall be verified with comparative tests with the standard 25-mm diameter.
6.3.3 The filter holder should be checked for leaks by placing an impermeable membrane in the holder and increasing the pressure
to the maximum capacity of the pressure sensor and holding it for a period of 1 min. The flow rate measured during this period
must be zero.
6.4 Metal Punch, used to cut a suitable size geotextile from the test sheet to fit the test filter holder.
6.5 Flow Rate Measurement Sensors—The porometer should be equipped with sensors to measure the flow rate that are high
enough to derive the desired pore size distribution. The maximum flow rate measurement required will depend on the opening
diameter and the dry air flow that corresponds to the smallest opening that can be determined with this method on the geotextile
type under test. The minimum sensitivity, that is, the detection threshold, is dictated by the flow rate that corresponds to the onset
of flow at the bubble point. For some geotextiles, this value may be as low as 0.1 L/min.
6.5.1 A series of floating ball-type flow meters placed in a parallel arrangement to cover the ranges of flow rates is acceptable,
provided the minimum and maximum flow rate measurements can be obtained with an accuracy of 5 % or less of the measured
value.
6.5.2 Digital flow meters are preferred for measurement of flow rates. Two or more digital flow meters of different capacities and
sensitivities may be necessary to cover both the minimum detection value and the maximum dry specimen value. The accuracy
of digital flow meters shall be at least 60.5 lpm or 62 % of the measured value, whichever is larger.
6.6 In-Line Fluid Trap, for porometers which have the flow rate sensors downstream from the test specimen to protect the flow
meters from being contaminated by the exhausted fluid.
6.7 Appropriate Fittings, Hose, Connectors, Piping, to assemble apparatus.
6.8 Balance, with a precision of 0.001 g.
7. Sampling
7.1 Lot Sample—For routine quality control testing, divide the product into lots and take the lot sample as directed in Practice
FIG. 1 Closed Filter Holder
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D4354, Section 7, Procedure B—Sampling for Manufacturer’s Quality Assurance Testing. For specification conformance testing,
sample as directed in Practice D4354, Section 8, Procedure C—Sampling for Purchaser’s Specification Conformance Testing.
7.2 Laboratory Sample—As a laboratory sample for acceptance testing, take a full-width swatch 1 m long from t
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