Asphalt Physical Properties
As asphalt pavements meet the safety and performance needs of road users as well as provide road owners and designers with the innovative technologies to make smart decisions. Well-maintained asphalt roads can last indefinitely and never need to be reconstructed.
In addition, there are asphalt contractors in every community, so competitive bidding ensures road owners get the best price for their project. Asphalt pavements are quick to construct and maintain, minimizing disruption for drivers and local businesses. Robbins, M. Tran When an asphalt pavement is maintained, the in-place material removed from the site becomes part of the raw materials for new pavement layers. A percent reusable material, reclaimed asphalt pavements RAP perform as well or better than virgin mixes.
In , more than That year about Reusing RAP in future pavements saved nearly 50 million cubic yards of landfill space during Asphalt engineers are constantly innovating to make roads perform for today and the future. With time, the asphalt-cement binder in an asphalt-concrete mix will oxidize and become more brittle.
This oxidation or aging process causes the asphalt cement to decrease in penetration and increase in viscosity. The rate of oxidation is directly related to the air-void content of the mix. The lower the air-void content, the less quickly the HMA material will age and become stiffer.
Moisture damage or stripping occurs when water is able to enter the mix and, under the repetitive action of traffic, works its way in between the asphalt coating on the aggregate and the surface of the aggregate. The degree of moisture damage is primarily related to the characteristics of the aggregate used in the mix but also is directly related to the air-void content of the mix.
As the air-void content in the mix decreases, the amount of moisture damage also decreases. Distortion or shoving is the displacement of the mix, typically in the longitudinal direction, under the action of traffic. Distortion is primarily related to the design and properties of the mix but also is related to the air-void content. For a given mix, a decrease in the air-void content at the time of construction will decrease the amount of distortion that the mix will undergo when exposed to traffic loads, particularly stopping or turning movements.
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An increase in the density of the mix a decrease in the air-void content will increase the internal stability and strength of the mix and may significantly reduce the amount of distortion that occurs under load. Disintegration or raveling is directly related to the air-void content of the mix. If the same mix is compacted to a high air-void content, however, major raveling may occur under the applied traffic loads. As the air-void content of the mix decreases, the amount of raveling also will decrease.
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An asphalt-concrete mix must be fully compacted before it cools to a temperature of about F. At temperatures above this value, the mix is normally still warm enough for the compaction equipment to reorient the aggregate particles into their densest configuration. Below that temperature, however, the mix is generally too stiff to increase in density any significant amount with continued rolling, although roller marks can often be removed below this compaction cutoff temperature.
The mix must, therefore, be compacted while it is still hot.
Asphalt Pavement Construction
Five factors directly affect the rate of cooling of the asphalt concrete mix when that material is placed on top of another existing layer of the pavement structure. Those variables are: air tem- perature; base temperature; mix laydown temperature; layer thickness; and wind velocity. All other factors being equal, as the ambient air temperature increases, the time available for compaction also increases. The mix will take longer to cool to the cutoff temperature of F on a warm day than on a cool day. An increase in the air temperature allows more time for the compaction equipment to achieve the desired density level in the mix.
More important than air temperature in the rate of cooling of the HMA mix is the temperature of the surface of the layer on which the new mix is placed. It is well known that heat in an asphalt concrete layer is lost in two directions. The surface of the mixture cools as heat is transferred to the air. The bottom portion of the mixture also cools as heat is transferred to the underlying base material.
Asphalt Pavement Construction
There is more rapid cooling of the mix downward into the base than upward into the ambient air. Base temperature-the temperature of the layer on which the new asphalt concrete mix is placed-is actually more important than air temperature in determining the time available for compaction. An increase in the base temperature allows more time for compaction. It is intended to assist those who work with asphalt pavements and pavement treatments, including architects, engineers, contactors, students and homeowners.
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