ASTM D7971-20

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Designation: D7971 − 15 D7971 − 20
Standard Guide for
Measuring Roundness of Glass Spheres Using a Flowing
Stream Digital Image Analyzer
This standard is issued under the fixed designation D7971; 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 guide covers the determination of the roundness of glass spheres used in pavement marking systems using a flowing
stream digital analyzer. Typical gradations for pavement marking systems are defined in ranges from Type 0 through 5 in AASHTO
M247.
1.2 This guide provides for the presentation of roundness data in a variety of formats to the requirement of the agency pavement
marking material specification. For most specifications the standard format is to present the roundness data as Percent True Spheres
relative to a series of standard ASTM sieve sizes.
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 safety, health, and healthenvironmental 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:
B215 Practices for Sampling Metal Powders
2.2 AASHTO Standard:
AASHTO M247 Standard Glass Beads Used in Traffic Markings
2.3 ISO Standards:
ISO 9276–6 Representation of results of particle size analysis — Part 6: Descriptive and quantitative representation of particle
shape and morphology
ISO 13322–1 Particle size analysis — Image analysis methods — Part 1: Static image analysis methods
3. Terminology
3.1 Flowing Stream Digital Analyzer:
3.1.1 A computer controlled particle analyzer employing a high resolution digital imaging device and computer image
processing software to do photo optical single particle counting and particle size analysis.
3.2 Roundness of Glass Beads:
3.2.1 Roundness, in the context of this guide, refers to the percentage of true spheres in a sample as a ratio of the total number
of particles measured.
3.2.2 Methods:
This guide is under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials, and Applications and is the direct responsibility of Subcommittee
D01.44 on Traffic Coatings.
Current edition approved April 1, 2015June 1, 2020. Published April 2015July 2020. Originally approved in 2015. Last previous edition approved in 2015 as D7971 – 15.
DOI: 10.1520/D7971-15.10.1520/D7971-20.
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volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American Association of State Highway and Transportation Officials (AASHTO), 444 N. Capitol St., NW, Suite 249, Washington, DC 20001,
http://www.transportation.org.
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D7971 − 20
3.2.2.1 These are specific observations and calculations of the streaming particles that combine to form a protocol for measuring
the percentage of true spheres within the sample.
3.3 Aspect Ratio:
3.3.1 Aspect ratio, often referred to as b/l or w/l, is one of the several methods of determining roundness and is illustrated in
Fig. 1.
3.3.2 Aspect ratio will be the recommended method for the majority of roundness measurements, especially when there exists
a reasonable expectation that all of the particles being measured have rounded surfaces and mostly resemble the shape of a sphere.
3.4 Sphericity (also referred to as circularity):
3.4.1 Sphericity is one of a number of methods of determining roundness and is illustrated in Fig. 2.
3.4.2 SPHT-value or sphericity is calculated from the measurements of
1. the area* A* of the particle projection (particle image),
2. multiplied by four · Pi (4π) and
3. divided by the perimeter P of the particle projection squared (P ).
3.4.3 SPHT result values are between 0 and 1 (including 1) and following ISO 9276–6.
NOTE 1—A high percentage of the volume of glass beads are produced from crushed and sized recycled glass. The most common method of production
involves passing the particles in a reverse free fall through a gas-rich, highly luminous, natural gas flame wherein they reach their melting point. Due
to nature’s insistence that all liquids exhibit tension at their surface, the particles are then forced to reconfigure into a shape which maintains the smallest
ratio of surface area to volume. This shape is a sphere. One of the main quality control concerns of this production method is ensuring that all the particles
pass through the flame and become spheres. If not, the particles will remain in the state in which they are introduced and will consist of irregular shapes
with sharp edges. A large variance between sphericity and aspect ratio could reveal that sphericity is the better method in this particular case.
3.5 Nominal Covered Area:
3.5.1 Nominal covered area (Fig. 3) (set value of obscuration percentage) = A / A .
obscured total measurement field
3.5.2 Nominal covered area (set value of obscuration) is calculated using the obscured area by particles divided by the total
measurement area.
3.5.3 The higher the nominal covered area, the more coincidental particles that are captured. With more coincidental particles
measured, the more non-round particles detected.
3.5.4 The task file setting for nominal covered area should not exceed a certain percentage. Values of nominal covered area (fld
= field density) of 0.8 % for smaller grades (20 to 100 mesh, Type 0 and Type 1 outlined in AASHTO M247) and 1 % for larger
grades (10 to 25 mesh, Type 2, Type 3, Type 4 and Type 5) should not be exceeded. Lower values are possible. Values of 0.3 %
to 0.5 % will lead to more accurate results. For example, a measurement with 0.7 % nominal covered area leads to 18 % non-round
particles, a measurement with 0.3 % nominal covered area would show 17 % non-round particles.
4. Significance and Use
4.1 The roundness of glass beads has a significant influence on the retroreflective efficiency of a pavement marking system.
4.2 The guide is for the characterization of the roundness of glass beads for the purpose of compliance testing against standard
specification for glass beads in pavement marking applicatio
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