Hot Mix Asphalt for Intersections in Hot Climates
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Hot Mix Asphalt for Intersections in Hot Climates

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English

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    Rutting of hot mix asphalt (HMA) pavement at or near intersections is very common both in cold and hot climates. Obviously, the problem is more acute in hot climates compared to cold climates because the stiffness of HMA decreases with increase in pavement temperature. In most cases, there is no significant rutting in the same asphalt pavement structure away from the intersections under fast moving traffic. This indicates a need for a different asphalt mix at or near intersections, which can sustain slow or standing traffic loads without any permanent deformation. A field investigation of rutting near five signalized intersections in Pittsburgh, Pennsylvania indicated the following causes related to in-place HMA: (a) low voids in the mineral aggregate (VMA), (b) low air voids, and (c) use of subrounded to subangular sand. Although the mixes were designed in the laboratory with high VMA and air void content, the asphalt pavements densified significantly in the field to yield very low VMA and air voids. It was felt that the HMA at the intersections should have the following attributes: (a) should maintain adequate VMA to ensure durability, (b) should not densify below 4% air voids under slow and standing traffic during hot summer days, and (c) should contain stiff asphalt binder to resist creep behavior. Based on the documented experience in the United States with the performance of stone matrix asphalt (SMA) and Superpave HMA mixes, the following recommendations have been made for asphalt pavements at intersections: (1) Wearing Course - use 50 mm thick SMA wearing course with a maximum nominal aggregate size of 12.5 mm; (2) Binder Course - use 50 mm thick SMA binder course with a maximum nominal aggregate size of 19.0 mm; and (3) Base Course - use 150 mm thick dense-graded large stone mix with a maximum nominal aggregate size of 25 mm; this mix should be designed either by Superpave mix design method or by Marshall method modified for 6-in. (151.8-mm) diameter specimens. Various rutting studies have shown that rutting is usually confined in the top 100 mm of asphalt pavements. Therefore, the use of SMA has been recommended in the top 100 mm. The SMA has a stone-on-stone contact and, therefore, it has a very high stability and it does not densify to unacceptably low air voids under traffic. Specifications and mix designs for the recommended three asphalt pavement courses are included in the paper.
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