AOS (ASTM D-4751) – A Straightforward Method a Confusing Result.
AOS (Apparent Opening Size) is a straightforward test. During the test, spherical, solid glass beads are dry sieved through a geotextile for a specified time and at a specified frequency of vibration. The amount of beads retained by the geotextile sample is then measured. The test is carried out on a range of sizes of glass beads. The apparent opening size is the pore size, measured in millimeters, at which 90% of the glass beads are retained on and within the fabric.
US Sieve in Relation to Millimeter Bead Size
Unfortunately, AOS is reported in a confusing manner. The problem lies in the relationship between the U.S. Mesh or Sieve number and the correlating millimeter bead size. For example, a 60 Sieve has a corresponding bead size of .250 mm. A 40 Sieve has a corresponding bead size of .400 mm. So the SMALLER the US Sieve number, the LARGER the particle size that will pass through the geotextile.
The Devil is in the Details
The most common way this property is reported is: “No. 60 sieve, maximum.” This is problematic. What is the intent here? Does the specifier want a fabric that will not pass any larger than a .250 mm bead? Or does he want a fabric with no larger than 60 Sieve number? In the first case, a 40 Sieve would not meet the specification, since the corresponding BEAD SIZE is .400mm, which is larger than the 60 Sieve .250 mm bead size. In the second case, a 40 Sieve is a smaller NUMBER than the required 60 Sieve, so it would be acceptable. How is one to know for sure? Leaving the intent of a specification to a geotextile company or contractor is certainly not what most specifiers have in mind.
Here’s How It Can Go Wrong
The attached specification is a perfect example of how the relationship between sieve number and bead size can really create confusion. In section 2.02 B, the specifier has required an AASHTO M-288, woven class 2 geotextile. The properties listed are taken verbatim from the AASHTO tables. However, the specifier has added the bead size in parentheses after the AOS Sieve Size. The M-288 specification does not include the bead size. While we believe this to be un-intentional, if interpreted as written, this specification now requires a maximum bead size of .250 mm. This would eliminate a 250# tensile strength woven slit-film; the very product the AASHTO M-288 specification was meant to require.
Always resist the temptation to add the millimeter bead size if the specification only lists the US Sieve number and vice-versa. However, it is our belief that specifiers should list only the bead size to eliminate any doubt as to the intent of the specification. In the case of the attached specification it should read: “AOS .250 mm, minimum.”
- US Mesh or Sieve is a NUMBER that correlates to a specific millimeter bead size
- The LARGER the US Sieve number, the SMALLER the bead size
- 60 Sieve = .250 mm
- 40 Sieve = .400 mm
- Avoid confusion by specifying only the millimeter bead size
- “AOS .250 mm, minimum.”
We hope you found this information helpful. Thanks for reading our blog!