Have you ever wondered about asphalt? Me neither.
Asphalt is a dark brown to black, highly viscous, hydrocarbon produced from petroleum distillation residue. This distillation can occur naturally, resulting in asphalt lakes, or occur in a petroleum refinery using crude oil. In 2001, the U.S. produced almost 35 million tons of asphalt at a rough value of around $6 billion. Roads and highways constitute the largest single use of asphalt at 85 percent of the total. In hot mix asphalt (HMA), asphalt functions as a waterproof, thermoplastic, viscoelastic adhesive. By weight, asphalt generally accounts for between 4 and 8 percent of HMA and makes up about 25–30 percent of the cost of an HMA pavement structure depending upon the type and quantity. The paving industry also uses asphalt emulsions, asphalt cutbacks and foamed asphalt.
Asphalt cement refers to asphalt that has been prepared for use in HMA and other paving applications. The generic term, “asphalt binder”, represents the principal binding agent in HMA because “asphalt binder” includes asphalt cement as well as any material added to modify the original asphalt cement properties.
Asphalt can be classified by its chemical composition and physical properties. The pavement industry typically relies on physical properties for performance characterization although an asphalt’s physical properties are a direct result of its chemical composition. Typically, the most important physical properties are:
- Durability. Durability is a measure of how asphalt binder physical properties change with age (sometimes called age hardening). In general, as an asphalt binder ages, its viscosity increases and it becomes more stiff and brittle.
- Rheology. Rheology is the study of deformation and flow of matter. Deformation and flow of the asphalt binder in HMA is important in HMA pavement performance. HMA pavements that deform and flow too much may be susceptible to rutting and bleeding, while those that are too stiff may be susceptible to fatigue cracking. HMA pavement deformation is closely related to asphalt binder rheology. Rheological properties of asphalt binder vary with temperature, so rheological characterization involves two key considerations. First, to fully characterize an asphalt binder, its rheological properties must be examined over the range of temperatures that it may encounter during its life. Secondly, to compare different asphalt binders, their rheological properties must be measured at some common reference temperature.
- Safety. Asphalt cement like most other materials, volatilizes (gives off vapor) when heated. At extremely high temperatures (well above those experienced in the manufacture and construction of HMA) asphalt cement can release enough vapor to increase the volatile concentration immediately above the asphalt cement to a point where it will ignite (flash) when exposed to a spark or open flame. This is called the flash point. For safety reasons, the flash point of asphalt cement is tested and controlled.
- Purity. Asphalt cement, as used in HMA paving, should consist of almost pure bitumen. Impurities are not active cementing constituents and may be detrimental to asphalt performance.
Now you can say you know more about asphalt then just what its like to drive on. I should probably raise the difficulty of this cache just for making you read all that!