Bituminous Materials Asphalt and Tars

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Bituminous Materials

  • Bituminous materials are used extensively for roadway construction, primarily because of their excellent binding or cementing power and their waterproofing properties, as well as their relatively low cost.
  • Bituminous materials consist primarily of bitumen, which, according to ASTM D8, is a class of black or dark colored solid or viscous cementitious substances composed chiefly of high-molecular-weight hydrocarbons;
  • By definition, Bitumen is soluble in carbon disulphide.
  • The term Asphalt is used in the American Technical Literature where as Bitumen is used in UK (Britian). In UK the term asphalt is reserved for the materials containing a mixture of bitumen and mineral matter.
  • Bitumen that are produced artificially from petroleum crudes are known as refinery bitumen where as bitumen from natural resources are native asphalt.

Bituminous materials are divided into two broad categories:

  1. Asphalt
  2. Tars

Asphalt and Tars

  • Asphalts are the residues of the petroleum oils. A great majority of asphalts used nowadays are the residues from the refinery of crude oils, although there are natural deposits called “native asphalt.”
  • Tars are residues from the destructive distillation of organic substances such as coal, wood, or petroleum. Tars obtained from the destructive distillation are crude tars, which must undergo further refinement to become road tars.

Comparison between Asphalt and Tars

  • Asphalts have no odor, are more resistant to weathering, and less susceptible to temperature than tars, which have a pungent odor and react to weathering and temperature.
  • Asphalt will be dissolved in petroleum oils whereas tars will not. Therefore, tars have been used to seat asphalt concrete surfaces, such as fog seals, to improve the oil resistance of asphalt surfaces. Asphalts are black in color, whereas tars are usually brown-black in color.
  • Today, tars are not used extensively as binders for highway pavements.

Sources of obtaining Asphalts

  • Petroleum Asphalts or Penetration-grade refinery bitumen

– Bitumen that are produced artificially from petroleum crudes (usually napthenicand asphaltic- base crudes) are known as Refinery Bitumen.

  • Natural Asphalt
  1. Lake Asphalt
  2. Rock Asphalt

1) Lake Asphalt

  • The largest natural deposit of lake asphalt occurs on the Island of Trinidad off the north-west coast of South America.
  • The main lake of asphalt covers an area of roughly 35 ha, has a depth of about 90 m, and is estimated to contain 10 to 15 million tonnes of material.
  • Following excavation, the asphalt is heated to 160°C to drive out gases and moisture, and then run through strainers to remove vegetable debris before being poured into wooden barrels for export under the name ‘Trinidad Epure’ or ‘Refined Trinidad Lake Asphalt’.
  • The refined lake asphalt product typically contains about 55 per cent bitumen, 35 per cent mineral matter, and 10 per cent organic matter.
  • Following the first commercial shipment to England (in 1840), it was widely used in road construction until the introduction of pitch- bitumen in the 1960s, i.e. a blend of 70-80 per cent bitumen with 20-25 per cent coal-tar pitch, which had similar qualities.

 

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Trinidad Lake Asphalt

2) Natural rock asphalts

  • Natural rock asphalts are mainly limestones and sandstones that are impregnated with, typically, 5-15 per cent of natural bitumen. Historically, the natural rock asphalt used in the UK was imported from Switzerland and France.
  • Natural rock asphalts are rarely employed in road construction in the UK today.

Natural rock asphalts

Production of Asphalt

  • A wide variety of refinery processes, such as the straight distillation process, solvent de- asphalting process, and solvent extraction process, may be used to produce asphalt of different consistency and other desirable properties.
  • Depending on the sources and characteristics of the crude oils and on the properties of asphalt required, more than one processing method may be employed.

Cutback Asphalt

  • Asphalt cement produced from the vacuum- steam distillation exists as a semisolid at room temperature, and usually proper workability can be attained by heating the asphalt cement to a temperature of 120 to 165°C (250°F to 330°F) to liquefy it.
  • In order for asphalt products to attain workability at room temperature, they must be rendered liquid at room temperature.

There are two ways to liquefy asphalt without resorting to heat:

  1. Dissolve asphalt in solvent
  2. Emulsify asphalt in water
  • When volatile solvents are mixed with asphalt cement to make a liquid product, the mixture is called “cutback asphalt.”
  • After a cutback asphalt is exposed to air, the volatile solvent evaporates, and the asphalt in the mixture regains its original characteristics (cured).
  • Depending on the volatility of the solvent used, the rate of curing of cutback asphalt can vary from a few minutes to several days.

Following are three types of cutback asphalt and the solvent used.

  1. Rapid-curing (RC): gasoline or naphtha
  2. Medium-curing (MC): kerosene
  3. Slow-curing (SC): road oils
  • The suffix numbers, for example MC-70, represent the minimum kinematic viscosity in centistokes at 60°C (140°F) for the particular grade.
  • Specifications for RC, MC, and SC are given in ASTM D2026, D2027, and D2028, respectively.
  • Cutback asphalt is increasingly being replaced by emulsified asphalt in commercial use.

Emulsified Asphalt

  • Emulsified asphalt is a mixture of asphalt cement, water and an emulsifying agent.
  • These three constituents are fed simultaneously into a colloid mill to produce extremely small globules (5-10 μ) of asphalt cement, which are suspended in the water.
  • The emulsified agent imparts the electric charges (cationic or anionic) to the surface of the asphalt particles, which causes them to repel one another; thus the asphalt particles do not coalesce.
  • • The emulsified asphalt thus produced is quite stable and could have a shelf life of several months.
  • • When an emulsified asphalt is exposed to the air, alone or mixed with an aggregate, it “sets” or “breaks,” because the asphalt globules react with the surface they are in contact with and coalesce, squeezing out the water between them.
  • The evaporation of water is the primary mechanism that finally causes the anionic emulsified asphalt to “break.”
  • Electrochemical processes are the primary mechanisms that cause the cationic emulsified asphalt to break.
  • Emulsified asphalt offers certain advantages in construction, particularly when used with moist aggregates or in wet weather.
  • An emulsified asphalt does not require a solvent to make it liquid and thus is relatively pollution-free.
  • Emulsified asphalt has low viscosity at the ambient temperature, it generally can be used without additional heat.
  • These factors tend to make emulsified asphalt more energy-efficient and less costly than cutback asphalt

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Penetration Grades of Bitumen

40 pen HD- Heavy duty penetration bitumen

Penetration Grades of Bitumen

  • Harder Grades (15-25 pen) – Mastic Asphalt
  • Medium Grades (35-70 pen)- hot rolled asphalt • Softer Grades (100-450 pen) – Macadams
  • 40 pen heavy duty (HD 40) is blown by passing air through it (oxidation process); this makes is less susceptible to temperature changes than other bitumen. Also, provide a stiffer asphalt which deform less under traffic.

Classification of Bituminous Material

  • Bituminous materials are commonly classified based on consistency.
  1. Penetration Grades
  2. Viscosity Grades
  3. Rolling Thin-Film Oven Test

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