ASPHALT ANSWERS

Asphalt Article June 11, 2001
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New generation open-graded asphalt friction courses


New generation open-graded asphalt friction courses


Open-graded friction course (OGFC), also called porous asphalt in Europe, is an open-graded, hot-mix asphalt (HMA) mixture with interconnecting voids that provides drainage during rainfall. In addition to minimizing hydroplaning during rainfall, the OGFC offers the following advantages:


  • High skid resistance on wet pavements;


  • Reduced splash and spray;


  • Enhanced visibility of pavement markings; and


  • Reduced tire pavement noise.

According to a 1998 survey by the National Center for Asphalt Technology (NCAT), the states which had excellent performance of OGFC generally use polymer-modified asphalt binders and use a relatively coarse gradation. For example, the OGFC is being used successfully in Oregon, Washington, California, Nevada, Arizona, Georgia and Florida. Its use is quite common in several European countries.


NCAT has now developed a new generation OGFC based on a research project, experience of some states in the U.S. that have placed durable OGFC pavements and experience in Europe. The mix design for this OGFC is conducted using 50 gyrations of a Superpave gyratory compactor. High quality, angular aggregates are used as well as high stiffness binders such as PG 76-XX made with polymers. The use of cellulose or mineral fiber is recommended.


The selected gradation should be within a recommended band of gradation and should provide stone-on-stone contact in the coarse aggregate (material retained on 4.75 mm sieve). The optimum asphalt content for the selected OGFC gradation is established to meet the following criteria: (a) 18% minimum air voids; (b) abrasion loss from the Cantabro test (conducted in a Los Angeles abrasion machine) not to exceed 20% for unaged compacted specimens and 30% for aged specimens; (c) draindown not to exceed 0.3%; and (d) the retained tensile strength of the compacted specimens subjected to five freeze/thaw cycles in modified Lottman method (AASHTO T283) should be at least 80%.


The report on this new generation OGFC can be downloaded from the NCAT website (www.eng.auburn.edu/center/ncat).


About the author: 
Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.
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