ASTM D8490-23a

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Designation: D8490 − 23 D8490 − 23a
Standard Test Method for
Determining the OpeningPore Size Characteristics of
Circular Knit Geotextiles Using an Optical Method
This standard is issued under the fixed designation D8490; 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 Image Opening Size (IOS) of a geotextile by pore size characteristics of
geotextiles using an optical method and image analysis.
1.2 This method has been developed for determination of the Image Opening Size (IOS) of knitted geotextiles by image analysis.
Other properties may be obtained based on the pore size distribution.
1.3 The applicability of this test method must be assessed on a product-by-product basis, as it requires light to pass through its
thickness to provide a useful observation. As a general rule, the tested product must be thin. Example of products which cannot
be tested using this test method is thick needle-punched nonwoven and woven with a complex three-dimensional structure.
1.4 This test method shows values in both SI units and inch-pound units. SI units is the technically correct name for the system
of metric units known as the International System of Units. Inch-pound units is the technically correct name for the customary units
used in the United States. The values in inch-pound units are provided for information only.
1.5 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.
1.6 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:
D123 Terminology Relating to Textiles
D4354 Practice for Sampling of Geosynthetics and Rolled Erosion Control Products (RECPs) for Testing
D4439 Terminology for Geosynthetics
D4751 Test Methods for Determining Apparent Opening Size of a Geotextile
E11 Specification for Woven Wire Test Sieve Cloth and Test Sieves
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 Feb. 1, 2023June 15, 2023. Published February 2023June 2023. Originally approved in 2023. Last previous edition approved in 2023 as
D8490 – 23. DOI: 10.1520/D8490-23.10.1520/D8490-23A.
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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8490 − 23a
2.2 Other Standards:
ISO 12956 Geotextiles and Geotextile-Related Products—Determination of the Characteristic Opening Size
CGSB 148.1 No. 10 Methods of Testing Geosynthetics Geotextiles—Filtration Opening Size
3. Terminology
3.1 Definitions—For definitions of other textile terms used in this test method, refer to Terminology D123. For definitions of other
terms relating to geosynthetics used in this test method, refer to Terminology D4439.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 circular knit, n—a structure produced by interlooping one or more ends of yarn or comparable material in a continuously
circular orientation around the fabric.
3.2.2 ellipse diameter, n—measured from the “best-fit” ellipse fitting inside a given opening.
3.2.2.1 Discussion—
The minimum and maximum ellipse diameter are the smallest and largest diameter of the ellipse fitting the inside of an opening
when varying the axis of the ellipse in the plane of observation.observation (Fig. 1).
3.2.3 Feret diameter, n—distance between two parallel lines tangent to the silhouette of the opening.
3.2.3.1 Discussion—
The minimum and maximum Feret diameter are the smallest and largest distance between two parallel lines tangent to the
opening’s silhouette when varying the angle of these lines in the plane of observation.observation (Fig. 1).
3.2.4 image opening size (IOS), n—for a knitted geotextile, a property that indicates the size of a soil particle which can be
effectively expected to pass through the geotextile.
3.2.5 opening, n—surface with an irregular shape, through which the light can pass.
FIG. 1 Feret’s and Ellipse Diameters
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from Canadian General Standards Board (CGSB), 11 Laurier St., Phase III, Place du Portage, Gatineau, Quebec K1A 0S5, Canada, http://www.tpsgc-
pwgsc.gc.ca/ongc-cgsb.
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4. Summary of Test Method
4.1 For circular knit geotextiles, a specimen is placed in a frame where its circumference will be controlled to reflect the outside
diameter of the pipe on which it will be installed. A stress is applied in the longitudinal direction to normalize the condition under
which the product will be observed.
4.2 An image of the surface of the geotextile is captured. The image is processed to determine relevant properties of each opening,
such as the minimum Feret diameter and the minimum ellipse diameter. A pore size distribution and a significant pore size defined
as the image opening size, IOS, are defined based on these observations.
NOTE 1—The minimum size of the image function should be selected considering the size of the openings of the product to be tested. For knitted geotextile
with openings up to 0.5 mm, an image 0.7 mm by 0.7 mm7.0 mm by 7.0 mm permitting the capture of 50 openings or more has been found satisfactory.
4.3 An equivalent Apparent Opening Size can be estimated based on the IOS.
5. Significance and Use
5.1 The significant opening size of geotextiles is usually determined using Test Method D4751, which involves sieving calibrated
beads through specimens of the geotextile being evaluated. However, Test Method D4751 includes many risks of errors due to
static electricity, precision of the glass beads, among other issues. This risk of error is even higher with knitted geotextiles which
exhibit a very low tensile modulus. This test method is proposed as an alternate to Test Method D4751 using a nondestructive
technique, where the stress conditions are controlled without manipulation of the specimen.
5.2 This test method has been found to provide representative results for products exhibiting a planar structure, such as
two-dimensional knits.
5.3 In case of a dispute arising from differences in reported test results when using ASTM D4751 Method A and this method,
ASTM D4751 Method A shall be considered the referee method. However, data obtained using ASTM D4751 Method A should
be reviewed considering the high risk of human error associated with the control of the stress condition of the geotextile.
5.4 Equivalency with the other pore opening size determined using other standards (for example, ISO 12956 and CGSB 148.1 No.
10) can also be considered using adequate correlations with test results obtained with these standards.
FIG. 12 Stretching Structure
Blond, E., Veermersch, O., Diederich, R., “A Comprehensive Analysis of the Measurement Techniques Used to Determine Geotextile Opening Size: AOS, FOS, O90,
and Bubble Point,” Proceedings of Geosynthetics 2015, Portland, OR, February 15–18, 2015.
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6. Apparatus
6.1 Stretching Structure—A When testing circular knit geotextiles, a structure able to control the circumference of the circular knit
geotextile, specimen and to uniformly apply a tensile stress in the machine direction. direction must be used. An example of such
a structure is presented in Fig. 12. This structure is not necessary when testing planar geotextiles where no stress needs to be
applied.
6.1.1 The structure permits clamping of the specimen on the upper end.
6.1.2 A free clamp is used on the bottom to apply a load using a dead weight.
6.1.3 Two vertical columns are installed to control the circumference of the specimen. The distance between these columns must
be adjustable and should be approximately equal distance from the centerline of the image viewing area.
6.1.4 It must be possible to stop the geotextile from creeping after some time, using a secondary clamp fixed on the frame itself.
6.1.5 The edges must be rounded to avoid accidental tear of the specimen.
6.2 Light Source, attached to the stretching structure, offering a diffused light on a surface larger than the image captured.
NOTE 2—The NERLITE BL 50×200 LED Backlight was found to be satisfactory.
6.3 Image Acquisition Device, positioned with its axis perpendicular to the plane of the geotextile. The sensor and optic used must
permit the capture of an image at least 7 mm by 7 mm, with a resolution of at least 250 pixels per millimeter length captured on
the test specimen for products with a mean opening size of 150 μm or more. Other image sizes may be considered for smaller or
bigger opening sizes.
NOTE 3—The criteria of 250 pixels per millimeter length is equivalent to one pixel for 4 μm. However, this measurement differs from the precision of
the opening size measurement considering the use of image analysis technique.
NOTE 4—The sensor Basler ace acA3800-14m 10Mp equipped with a Kowa 2/3” 10Mp LM35mm/F2.0 C-Mount lens was found to be satisfactory.
6.4 Image Processing Software, able to:
6.4.1 Process the image to define edges of the openings being measured and exclude stray yarns.
6.4.2 Determine the desirable property of each opening, such as the minimum Feret diameter, minimum ellipse diameter, area,
perimeter, and/or another property of interest.
6.4.3 Export the results for further processing.
NOTE 5—Image-J software with the Fiji extension was found to be satisfactory. It is available for free at https://imagej.net/software/fiji/downloads.
6.5 Image Analysis Routine.
NOTE 6—The routine provided in Annex A1 was found to be satisfactory for the Image-J software with the Fiji extension.
6.6 Calibrated Micrometric Ruler, 1.0 mm long or greater, graduated to 0.01 mm or less. The calibrated ruler should be on a glass
with a thickness of 1.0 6 0.1 mm.
7. Sampling and Test Specimens
7.1 Sampling:
NERLITE is a registered trademark of Microscan Systems, Inc., 700 SW 39th St. Renton, WA 98057.
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7.1.1 For manufacturer’s quality control (MQC) testing, divide rolls of circular knit geotextile fabric the product into lots and take
the lot sample as directed in Practice D4354, Section 7, Procedure B—Sampling for Manufacturer’s Quality Assurance Testing.
7.1.2 For specification conformance testing, sample as directed in Practice D4354, Section 8, Procedure C—Sampling for
Purchaser’s Specification Conformance Testing.
7.2 Test Specimens: Specimens, Circular Knit Geotextiles:
7.2.1 Cut at least three (3) specimens of the circular knit geotextile with a minimum length of 500 mm. This length may have to
be longer for products intended to be installed on pipes with a nominal diameter larger than 150 mm (6 in.).
7.2.2 Lay tubular fabric flat on a solid surface and make sure edges of fabric are parallel in the longitudinal direction. Make sure
there are no wrinkles or torque in the fabric. Draw a line perpendicular to the longitudinal edges and make sure the line is drawn
across the width of fabric touching both parallel edges. Draw a second line at the approximate location of the lower clamp (for
example, 300 to 500 mm depending on the height of the structure and how much the fabric will stretch) using the same technique
as above.
7.2.3 Make a cut 50 610 mm in the machine direction on both sides of the same end of the test specimen (Fig. 23).
7.3 Test Specimens, Planar Geotextiles:
7.3.1 Cut specimens with a size significantly larger than the observation area, to be able to hold it in place and control the distance
between the specimen and the image acquisition device. The specimen should entirely cover the light source.
8. Preparation of Apparatus
8.1 The When testing circular knit geotextiles, the stretching structure must be used. It must stand on a stable table or counter top,
exempt of vibrations, allowing enough space for the lower clamp to hang without touching a surface.
8.2 When testing planar geotextiles where no stress needs to be applied on the specimen, the image acquisition device may be used
in any direction as long as perpendicularity with the test specimen and working distance is controlled.
8.3 Optics of the image acquisition device must be clean, without dust or mold.
9. Calibration and Standardization
9.1 A calibration must be performed each time there is a change of distance between the plane in focus (that is, the plane of the
sample) and the optical acquisition device.
FIG. 23 Position of the Cuts
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9.2 Install the micrometric ruler with the graduation facing the image acquisition device in place of the test specimen, that is, by
taping it on the light source. Adjust the focus and proceed with the measurement of the number of pixels corresponding to a 1 mm
length of the ruler, using the image acquisition software calibration feature.
9.3 Turn the ruler around so the graduations face the light source, that is, are ~1.0 mm farther away from the image acquisition
device. Adjust the focus and proceed with the measurement of the number of pixels corresponding to a 1 mm length of the ruler,
using the image acquisition software calibration feature.
9.4 Average the measurements made in 9.1 and 9.2 to define the number of pixels per millimeter on the acquired image.
10. Conditioning
10.1 The samples must be conditioned to 21 6 2 °C (70 6 4 °F) for at least 24 h before proceeding with the optical measurement.
For manufacturing quality control, this duration can be reduced. Should differences arise between results obtained on
non-conditioned versus conditioned samples, the conditioned samples are to be considered the referee.
11. Stretching (for Circular Knit Geotextiles)
11.1 To obtain a labor
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