introduce diffrent methods for asphalt pavement construction

Discussion in 'Transportation Engineering' started by nooshin-kh, Aug 8, 2016.

  1. nooshin-kh

    nooshin-kh Novice


    Asphalt paving mixtures or hot mix asphalt is manufactured in a central mixing facility. The workhorse of the asphalt industry is the dense-graded HMA mixture. It consists of a well-graded, high-quality aggregate and asphalt cement. The two components are mechanically mixed while hot to ensure good coating of the aggregate and consistency of the mix. In order to ensure the quality of the resultant mix it is necessary to carefully proportion the blend of several different aggregates to consistently produce a uniform final gradation that meets the requirements of local specifications.Typically, the asphalt and aggregate are heated to 250˚-325˚F (121˚-163˚C). The hot mixture, kept hot during transit, is hauled to the construction site, where it is spread on the roadway by an asphalt-paving machine. The smooth layer placed by the paver is compacted by rollers to proper density before the asphalt cools.
    In the 1990s, stone matrix asphalt (SMA) was introduced in the united Stases and the use of open-graded friction courses (OGFC) was expanded.
    Stone matrix asphalt is a durable mixture that provides high resistance to permanent deformation. An SMA mixture is generally used on very high volume roadways or in high-volume intersections. SMA is a gap-graded mixture containing a large proportion of coarse aggregate (about 70 percent) and just enough fine aggregate to fill the voids in the coarse aggregate. The absence of mid-sized aggregate in this type of mix results in stone-to-stone contact, which gives these mixes a high degree of resistance to plastic flow under traffic (rutting). In addition, inorganic filler (typically lime) and fibers (typically cellulose) are added to the mix to make the fine aggregate portion of the mix into mastic. The mix is also typically made with polymer modified asphalt (PMA). The PMA improves both the strength and the durability of the SMA mix. The inclusion of PMA, fibers and filler is required to prevent drainage of the asphalt cement during transport and placement. This mixture has a surface appearance similar to that of an open-graded friction course; however, it has low in-place air-voids similar to that of a dense-graded HMA. In addition, it has a thicker film of asphalt cement on the coarse aggregate particles, which enhances durability.
    An open-graded friction course is another gap-graded mix that is not dense graded but has permeable voids in the range of 16 to 23 percent.
    These voids allow drainage through the surface of an HMA pavement during heavy rainstorms. The rainwater will drain vertically though the OGFC to an impermeable, underlying layer and then laterally to the day-lighted edge of the OGFC. In addition to minimizing hydroplaning and providing a high friction resistance on wet pavements, an OGFC layer will also provide a reduction of splash and spray; enhanced visibility of pavement markings; reduced night-time surface glare in wet weather; and reduced tier-pavement noise. The aggregate gradation for an OGFC is generally of one size.

    1.3.2 Surface Treatments
    Asphalt surface treatments are widely used methods for asphalt pavement construction and maintenance. There are several forms of surface treatments. Properly designed, they are economical, easy to place, and long lasting. They seal and add life to road surface, but each type has one or more special purposes. A surface treatment is not a pavement in itself. Rather, it provides both a protective cover that helps to resist traffic abrasion and a waterproof cover over the underlying pavement structure. Surface treatments add little or no load-carrying strength and, therefore, they are not normally taken into account in computing the load-carrying capacity of a pavement. Although a surface treatment can provide an excellent surface if used for the correct purpose, it is not a solution for all pavement problems.
    Asphalt surface treatment is a broad term embracing several types of asphalt applications with or without a cover of mineral aggregate to any type road or pavement surface. It is usually less than 1 inch (25 millimeters) thick. An asphalt seal coat, or simply seal coat, is a thin asphalt surface treatment applied to a paved surface. The types of surface treatments include:
    •Spray-applied seal. This is an application of a diluted asphalt emulsion without a cover aggregate used to seal and enrich the asphalt pavement surface and to seal minor cracking.
    •Chip seal. This is an application of asphalt followed immediately by an aggregate cover with two layers (referred to as double-chip seal), or three layers, which is called a triple-chip seal.
    •Slurry seal. This is a mixture of fine aggregate, asphalt emulsion, water, and mineral filler (generally Portland cement).
    •Microsurfacing. This is sometimes referred to as a polymer-modified slurry seal. It differs from a slurry seal, which is cured through a thermal process. With microsurfacing, the curing process is chemically controlled.

    source: bitumen

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