The most common type of flexible pavement surfacing in the U.S. is hot mix asphalt (HMA). Hot mix asphalt is known by many different names such as hot mix, asphalt concrete (AC or ACP), asphalt, blacktop or bitumen. For clarity, this Guide makes a conscious effort to consistently refer to this material as HMA. 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. 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. Reclaimed asphalt pavement (RAP) is generally considered a material within HMA, while forms of in-place recycling are considered separately. HMA can also be produced at lower than typical production temperatures (290 to 320°F) and is then categorized as Warm Mix Asphalt (WMA). WMA (see Sustainable Pavements page) can be produced using a variety of methods (e.g. asphalt foaming kits at the plant & chemical or wax-based additives) to reduce production temperatures by 15 to 50°F while maintaining or even increasing the time available to compact the mix. WMA is interchangeable with HMA in most paving applications.
“Superpave” is an overarching term for the results of the asphalt research portion of the 1987 – 1993 Strategic Highway Research Program (SHRP). Superpave consists of (1) an asphalt binder specification, (2) an HMA mix design method and (3) HMA tests and performance prediction models. Each one of these components is referred to by the term “Superpave”. This section provides a brief overview and background of Superpave.
Asphalt treated base (ATB) is a dense-graded HMA with a wide gradation band and lower asphalt content intended for use as a base course. ATB costs less than typical HMA mixes because it can be produced with less expensive aggregates and lower percentages of asphalt binder. In addition to the site paving benefits, ATB can be advantageous because it can provide:
HMA is one of the most recycled products in the U.S. As much as 100 million tons of HMA are milled off roads during resurfacing and widening projects each year. Of this amount, 80 million tons (80 percent) are recycled as “reclaimed asphalt pavement” (RAP) (Bloomquist et al. 1993). Additionally, in-place recycling techniques can also be used to resurface an existing pavement or pulverize an existing pavement for use as base material.
Hard surfaced pavements, which make up about 67 percent of U.S. roads and 70 percent of Washington State roads are typically categorized into flexible and rigid pavements:
- Flexible pavements. Those which are surfaced with bituminous (or asphalt) materials. These types of pavements are called “flexible” since the total pavement structure “bends” or “deflects” due to traffic loads. A flexible pavement structure is generally composed of several layers of materials which can accommodate this “flexing”.
- Rigid pavements. Those which are surfaced with portland cement concrete (PCC). These types of pavements are called “rigid” because they are substantially stiffer than flexible pavements due to PCC's high stiffness.