When Prescriptive Specifications Fail
Prescriptive specifications work only when users have years – or even decades or centuries – of experience with a product/system. If it remains unchanged over that long time period, you can have a high degree of confidence that the product/system will still work today. A graphic illustration of how this can go wrong is ASTM D41 for asphalt roof primer. The specification was originally issued in 1917. There are no performance requirements in it and no tests to verify that a particular product is a primer, that it improves adhesion to a variety of substrates, or that using the primer is better than not using one at all. Basically, D41 just calls for no less than 35-percent solvent and the rest asphalt, and that it be thin enough to flow through a calibrated orifice in not less than 25, or more than 125, seconds (i.e. not be too thin or too thick). Since the product had only two components, and experience has proven that primer improves adhesion to cementitious, porous, greasy, oily, or dirty surfaces, it’s used regularly both in new construction and in the maintenance, repair, and recoating of most substrates. Unfortunately, concerns over volatile organic compounds (VOCs) have become an environmental issue. VOCs contribute to smog generation, and the entire coatings industry (not just roofing) has had to find ways to comply with new regulations regarding VOCs. Catalyzed, solvent-free liquids are now available, as are water-based materials. For asphalt primer, however, if we reduce the volatile organic component (i.e. the solvent), the outcome is a thicker material (with more asphalt) that’s more of a coating than a penetrating primer. It no longer meets its intended use. ASTM D1079 defines a primer as “thin liquid bitumen applied to a surface to improve the adhesion of heavier applications of bitumen and to absorb dust.” In 2005, the ASTM Subcommittee D08.05 that has jurisdiction over D41 decided, without any performance test data or documentation, that a thicker primer that might meet the VOC requirements was just fine. Rather than retain the maximum flow limit using the ASTM Standard Test Method for Saybolt Furol Viscosity (see Fig. 1) to 125 seconds (maximum) to drain the cup, the subcommittee decided to introduce a new Type II primer that increased the maximum allowable flow time to 800 seconds (more than 13 minutes). The committee never developed any performance test methods to confirm that this change to a much higher viscosity was not detrimental to the primer. No testing was done on dramatically changing a standard that had existed for 82 years, and there was no data to prove that it’s still a primer, or that its use as a thick syrup is better than no primer at all. 
Fig. 1 - Saybolt Viscometer with Universal and Furol Orifice
All dimensions are in millimeters (inches)
Perhaps the industry is at fault for not having performance requirements for a primer in the past. But, creating a new D41 Type II (thick) primer solves nothing, and legitimizes a product that should not be used. The ASTM D08.05 subcommittee should either prove through performance tests that this Type II primer is acceptable, or delete it from D41. Where compliance with VOC regulations are absolutely necessary, we should either skip the primer or indicate that VOC-compliant primer does not meet D41. In the real world, where D41 Type II primer is shipped to a roof project, the roofing contractor will find it too thick to apply. Hopefully, the ASTM subcommittee will reconsider its efforts and will either withdraw Type II primer from its standards or develop performance tests to prove that this substance is fit for its intended use. When the prescription for a prescriptive specification is changed without documentation, it has no value at all. Would you take aspirin if a company changed its formula without comprehensive FDA testing? Resources: |