第二课沥青及沥青混合料Asphalt $ Mix AsphaltText: Asphalt &Asphalt Paving Materials1 Asphalt DefinedThe black cementing agent known as asphalt has been used for road construction for centuries. Although there are natural deposits of asphalt , or rock asphalt , most used today is produced during the refining of crude oil. Asphalt is a constituent of most petroleums and is isolated through the refining process.Asphalt is called a bituminous material because it contains bitumen, a hydrocarbon material soluble in carbon disulfate. The tar obtained from the destructive distillation of soft coal also contains bitumen. Both petroleum asphalt and coal tar are referred to as bituminous materials. Because their properties differ greatly , petroleum asphalt should not be confused with coal tar . Petroleum asphalt is composed almost entirely of bitumen while the bitumen content in coal tar is relatively low. The two materials should be treated as separate entities.One of the characteristics and advantages of asphalt as an engineering construction and maintenance materials is its versatility. Although a semi-solid at ordinary temperratures , asphalt may be liquefied by applying heat, dissolving it in solvents, or emulsifying it. Asphalt is a strong cement that is readily adhesive and highly waterproof and durable, making it particularly useful in road building. It is also highly resistant to the actions of most acids, alkalis, and salts.1.1 Asphalt BinderAsphalt is produced in a variety of types and grades ranging from hard-brittle solids to near water-thin liquids. The semi-solid form known as asphalt binder is the basic material used in Hot-Mix Asphalt(HMA) pavements. Liquid asphalt is produced when asphalt binder is blended or “cut back” with petroleum distillates or emulsified with water and an emulsifying agent.At ambient air temperatures, asphalit binder is a black, sticky , highly viscous material. It is a strong and durable binder with excellent adhesive and waterproofing characteristics. Applying heat, which facilitates mixing with mineral aggregates to produce HMA , can readily liquefy asphalt binders.The largest use of asphalt binder is for HMA . After compacting and cooling to air temperature, HMA is a very strong material with the ability to sustain heavy traffic loads while remaining flexible enough to withstand ambient environmental conditions and stress. Over 96 percent of the hard-surfaced roads in the United States are paved using HMA.1.2 Emulsified AsphaltsEmulsified asphalts(also known as emulsions) are low-viscosity mixtures of tiny asphalt binder droplets, water and emulsifying agents. The emulsifying agent coats the surfaces of the asphalt droplets and keeps them suspended in the water prior to application. After application, the asphalt emulsion breaks and the water separates and evaporates. Emulsions are brownish in color during application, but after breaking, the asphalt binder returns to its original black color. Emulsions are used for a Tack Coat between subsequent layers of HMA to aid in binding the layers together.Cut-back asphalts are low-viscosity liquid asphalt mixtures manufactured by diluting (cutting back ) Asphalt Binders with petroleum solvents (cutter stock or diluent). After application, the petroleum solvent evaporates, leaving the asphalt binder residue. Cut-back asphalts may be used as a tack coat between subsequent layers of HMA, particularly when ambient air temperatures are cool.2. Asphalt Binder GradingAsphalt binders appropriate for pavement construction were previously graded based on resistance to penetration and/or viscosity measures. Currently ,asphalt binders are graded based on the temperature range over which the binder retains certain desirable characteristics. These desirable characteristics include adequate flexibility to resist cold temperature cracking and sufficient rigidity to resist warm temperature rutting. The current grading system is known as the Performance Grading(PG) system.Performance grading specifications were developed as part of the Strategic Highway Research Program (SHRP) and are a major component of SUPERPAVE. Binders are specified on the basis of the climate andpavement temperatures in which the binder is expected to serve, pavement Age Performance graded (PG) binders used in Minnesota vary from north to south and with intended use; however, PG 58-28 is the most commomly used grade. The first number (58) represents the average 7-day maximum pavement design temperature in degrees Celsius. This maximum temperature establishes the upper temperature limit for the binder ti retain adequate rigidity to resist rutting .The second number (-28) represents the minimum pavement design temperature in degrees Celsius. The minimum temperature established the lower limit for the binder to retain sufficient flexibility to resist thermal cracking.Physical properties of the binders are measured at various temperatures both before