DO NOT USE AASHTO SPECIFICATION WITH QUALIFIERS.
An oft repeated mistake of the specifying community is to add qualifiers to the AASHTO M-288 standard specification. It’s important to keep in mind that each AASHTO Class refers to a set of specific physical and hydraulic properties. These are not “stand alone” properties. For example, increasing the tensile strength or mass per unit of a nonwoven geotextile will negatively impact the hydraulic properties of that geotextile.
A Specification That Cannot Be Met
The attached specification is a good example of this issue. Here the specifier is requiring an “AASHTO M 288 Class 1”. However, the qualifier of “and weighs at least 15 oz/yd2” is added. It is important to note that AASHTO does not currently include ASTM D-5261 Mass Per Unit Area in its specification. However, we can determine a weight. An AASHTO Class 1 nonwoven has a tensile strength of 205 pounds. A typical weight for a 205 pound nonwoven is 8 oz/sy2.
A second issue arises because a 15 oz/sy2 nonwoven is not a standard product. The closest standard product is a typical 16 oz/sy2. The chart below compares the physical and hydraulic properties of a typical 16 oz nonwoven with an AASHTO Class 1 nonwoven. While the 16 oz will surpass the strength requirements of AASHTO Class 1, notice what happens to the hydraulic properties:
|PROPERTY||TEST METHOD||16 oz/yd2||AASHTO CLASS 1|
|Weight – Typical||ASTM D-5261||16.0 oz/yd2||8 oz/yd2|
|Tensile Strength||ASTM D-4632||380 lbs||205 lbs|
|Elongation @ Break||ASTM D-4632||50%||50%|
|CBR Puncture||ASTM D-6241||1,080 lbs||535 lbs|
|Trapezoidal Tear||ASTM D-4533||145 lbs||85 lbs|
|Apparent Opening Size||ASTM D-4751||100 US Sieve||80 US Sieve|
|Permittivity||ASTM D-4491||0.70 Sec-1||1.35 Sec-1|
|Water Flow Rate||ASTM D-4491||50 g/min/sf||90 g/min/sf|
A 15 oz/sy2 nonwoven geotextile cannot meet the Permittivity and Water Flow Rate requirements of a Class 1 AASHTO nonwoven. The oz/sy2 qualifier has created a contradictory specification that cannot be met.
What Was The Intent of the Specifier?
So what is important? Is it weight, hydraulic properties or something else? It falls on the geotextile supplier to answer these questions and determine the intent of the specification. This is a scenario specifiers want to avoid. Many suppliers do not have the product knowledge to understand the issues with this specification since geotextiles are an auxiliary item they offer along with their main product line. Many would simply offer a Class 1 nonwoven. Others may offer a typical 16 oz/yd2. Neither of these would be correct for the intended use, but could be reasonably perceived as meeting the specification.
We believed this to be a “bond-breaker” application since a common specification for a bond-breaker requires a 15.0 oz/yd2 weight. We were able to verify this with the contractor. As such, we took exception to the specification and submitted product data for a special manufactured bond-breaker product. That data included the properties of Thickness (under load), Permeability (under load) and Hydraulic Conductivity (under load) which are not part of the AASHTO specification, but important for a bond breaker application. So in this case, the important properties were weight as well as something else not addressed in the specification. Adding 15 oz/sy2 to the AASHTO Class 1 specification did not create a bond breaker specification. It created confusion and left room for serious error.
What Product Was Installed?
Since our contractor was not awarded the job, we do not know if the desired product was actually installed. More than likely, neither does the specifier.
- Do Not Attach Qualifiers to AASHTO Spec
- Do Not Leave Interpretation of Intent to Suppliers or Contractors
- Call US Fabrics For Guidance
- Reduce Possibility of Utilizing the Wrong Geotextile
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