Permeable pavements can be hydraulically engineered such that there is a retention volume under the pavement (usually a layer of uniformly graded aggregate so the water can occupy the space between the aggregates) where the water can reside as it infiltrates into the subgrade below. The intent is to let water drain through the pavement structure itself into the underlying material. An open-graded mix is used for the entire pavement hard surface. Open-graded mixes can be used in these ways: Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands. Unlike dense-graded mixes and SMA, an open-graded HMA mixture is designed to be water permeable. Figure 5: Placing SMA at Hilo Harbor Open-Graded Mixes Mineral fillers and additives are used to minimize asphalt binder drain-down during construction, increase the amount of asphalt binder used in the mix and to improve mix durability. Information: Other reported SMA benefits include wet weather friction (due to a coarser surface texture), lower tire noise (due to a coarser surface texture) and less severe reflective cracking. Materials: Gap-graded aggregate, modified asphalt binder, fiber filler SMA is almost exclusively used for surface courses on high volume interstates and U.S. Purpose: Improved rut resistance and durability. Figure 4: SMA Surface Figure 5: SMA Lab Sample Figure 6. In the right situations it is cost-effective because of its increased rut resistance and improved durability. SMA is generally more expensive than a typical dense-graded HMA because it requires more durable aggregates, higher asphalt content, modified asphalt binder and fibers. Since aggregates do not deform as much as asphalt binder under load, this stone-on-stone contact greatly reduces rutting. The mix design goal is to create stone-on-stone contact within the mixture. Stone matrix asphalt (SMA), sometimes called stone mastic asphalt, is a gap-graded HMA originally developed in Europe to maximize rutting resistance and durability. Information: Particulars about dense-graded HMA are covered by the rest of this Guide. Materials: Well-graded aggregate, asphalt binder (with or without modifiers), RAP Works well for structural, friction, leveling and patching needs. Purpose: Suitable for all pavement layers and for all traffic conditions. Figure 1: Dense-Graded HMA Up Close Figure 2: Dense-Graded Cores Figure 3. Fine-graded mixes have more fine and sand sized particles than coarse-graded mixes. Dense-graded mixes are generally referred to by their nominal maximum aggregate size and can further be classified as either fine-graded or coarse-graded. When properly designed and constructed, a dense-graded mix is relatively impermeable. Dense-Graded MixesĪ dense-graded mix is a well-graded HMA intended for general use. Reclaimed asphalt pavement ( RAP) is generally considered a material within HMA, while forms of in-place recycling are considered separately. Typically agencies consider other types of asphalt-based pavement surfaces such as fog seals, slurry seals and BSTs to be maintenance treatments and are therefore covered in the Maintenance & Rehabilitation section. HMA is distinguished by its design and production methods (as described in this Guide) and includes traditional dense-graded mixes as well as stone matrix asphalt (SMA) and various open-graded HMAs. For clarity, this Guide makes a conscious effort to consistently refer to this material as HMA. Hot mix asphalt is known by many different names such as hot mix, asphalt concrete (AC or ACP), asphalt, blacktop or bitumen. The most common type of flexible pavement surfacing in the U.S.
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