Welcome to the Washington Asphalt Pavement Association’s Asphalt Pavement Guide. This Guide will provide you with a general overview of hot mix asphalt (HMA) pavement from materials to design to construction to maintenance. It is intended to assist those who work in any way with HMA pavement including architects, engineers, contractors, government agencies, private consultants, students, and homeowners.
Hot mix asphalt (HMA) pavements have existed in their present form, as a mixture of angular aggregates and asphalt binder, since the beginning of the 20th century. However, HMA pavement can trace its roots back to ancient Roman roads and beyond.
The first recorded use of asphalt by humans was by the Sumerians around 3,000 B.C. Statues from that time period used asphalt as a binding substance for inlaying various shells, precious stones and pearls. Other common ancient asphalt uses were preservation (for mummies), waterproofing (pitch on ship hulls), and cementing (used to join together bricks in Babylonia). Around 1500 A.D., the Incas of Peru were using a composition similar to modern bituminous macadam to pave parts of their highway system. In fact, asphalt is mentioned several times in the Book of Genesis (Baird 2002).
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.
“Aggregate” is a collective term for sand, gravel and crushed stone mineral materials in their natural or processed state (NSSGA 1991). In 2009, the U.S. produced nearly 2 billion tons of aggregate at a value of about $17.2 billion. Roads and highways constitute31 percent of the total sand, stone and gravel market (NSSGA 2010). In HMA, aggregates are combined with a asphalt binding medium to form a compound material. By weight, aggregate generally accounts for between 92 and 96 percent of HMA and makes up about 25 percent of the cost of an HMA pavement structure. Aggregate is also used by itself or with a stabilizer for base and subbase courses.
Top down cracking appears to be a common mode of HMA pavement distress in at least several states and countries. Traditionally, pavement cracking is thought to initiate at the bottom of the HMA layer where the tensile bending stresses are the greatest and then progress up to the surface (a bottom-up crack). Most traditional transfer functions used in mechanistic-empirical structural design are based on this concept. However, the late 1990s saw a substantial focus on a second mode of crack initiation and propagation: top-down cracking.