Asphalt mixtures are commonly comprised of various ratios of aggregate, asphalt binder and air. While this is usually the case for standard dense-graded Superpave mix designs, specialty mixtures such as open-graded friction courses (OGFCs) and stone-matrix asphalt (SMA) mixes typically contain some type of fiber. Fibers are used to either increase the toughness or fracture resistance of asphalt mixtures (though this is rarely applied in practice), or to stabilize the asphalt binder in order to prevent binder drain-down while the asphalt mixes are hot. When binder drain-down occurs, it results in small flushed areas of the pavement with excess asphalt (sometimes called "fat spots"). The rest of the mix is lean, which results in premature raveling and cracking. The use of fibers in OGFC became common after the introduction of SMA to the U.S.
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