Page 5 More Binder the Key to Durability in AR OGFC

More Binder the Key to Durability in AR OGFC
The following article written by RPA Executive Director, Douglas Carlson offers some insight on reasons the AR OGFC Hot mix has proven to be more durable than the conventional AC OGFC in both cold and warm climates. Ultra Thin Blacktopping Asphalt-Rubber binder has been successfully used in open graded friction courses (OGFC) since 1975. OGFCs are applied in thin layers, usually only 1" (25 mm) thick. AR ORFCs can be placed as thin as 1/2" (12 mm). However, whenever the topic is brought up in areas outside of Arizona, California, Florida or Texas, a common obstacle is the perception that open graded frictions courses do not work because they lack durability. Many states have tried some type of open graded mix with limited success and usually have avoided their use since. Some poor experience with OGFC is related to the large, interconnected air voids in the mix. These voids are filled with debris over time, reducing the effectiveness of the open mix. The appearance of an open grade mix with interconnected voids can be seen in the photo, which was taken near Brussels, Belgium and used an AR binder.	Another common mode of distress is that the air voids are filled with water, which freezes and expands, breaking the bond between the aggregates. Asphalt-Rubber mixes only work where it's warm, right? Actually in Arizona and California the A-R mixes gave the DOTís confidence BECAUSE of their performance in the high altitudes of Flagstaff on I-40 and I-80 in Donner Pass, both above 7,000 ft, where freeze/thaw cycles are more extreme than the deep freeze experienced in the Northern states. The challenge is to get a high binder content into the mix to increase the durability but avoid the drain down where the binder runs off of the aggregate while the mix is being placed. By adding the rubber particles to open graded mixes, the high binder content and greater film thickness on the aggregate is achieved. Typical unmodified binder contents for an OGFC are near 6%; AR mixes commonly achieve a minimum of 8% and in some cases 10% binder content. The rubber makes the asphalt more viscous, preventing drain down and allows for film thickness as high as 36 microns, four times the typical film thickness. Additionally, OGFCs with AR binder are designed using a smaller stone aggregate, commonly 3/8" (9.5mm). Although the air voids are high, between 18-22%, they are not large, interconnected voids. (I-40 photo) Freeze damage is not experienced by these mixes. In fact, the Flagstaff District of ADOT prefers this mix due to its high durability and resistance to snow plow damage and good friction characteristic during inclement weather.

More Binder the Key to Durability in AR OGFC

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