BItumen aka Asphalt

“Bitumen” is a dim tan to dark, remarkably viscous, hydrocarbon generated from petroleum distillation deposit. This distillation can happen indigenously, bringing about pavement bays, or happen in a petroleum refinery utilizing rough oil. In 2001, the U.S. prepared well-nigh 35 million tons of pavement at a difficult time of around $6 billion. Streets and interstates constitute the heftiest single utilize of pavement at 85 percent of the sum (Asphalt Institute, 2002[1]). In HMA, pavement roles as a waterproof, thermoplastic, viscoelastic cement. By weight, black-top for the most part explains in the middle of 4 and 8 percent of HMA and makes up about 25 – 30 percent of the price of a HMA asphalt structure relying on the sort and amount. The clearing industry in addition utilizes black-top emulsions, black-top reductions and frothed black-top.

“Black-top bond” points to black-top that has been ready for utilization in HMA and different clearing provisions. This segment utilizes the bland term, “black-top folio”, to speak for the chief tying operator in HMA resulting from the fact that “pavement folio” combines black-top concrete as well as any material put in alter the initial pavement concrete lands.

Black-top Physical Properties
Black-top might be ordered by its synthetic arrangement and physical lands. The asphalt industry ordinarily relies on physical lands for exhibition characterization even though a pavement's physical lands are a straight effect of its synthetic arrangement. Ordinarily, the most exceptionally critical physical lands are:

Toughness. Toughness is a measure of how pavement cover physical lands update with experience (now and then called experience solidifying). In customary, as a pavement fastener matures, its viscosity builds and it comes to be more hardened and fragile.
Rheology. Rheology is the study of twisting and course of matter. Deformity and flood of the pavement cover in HMA is noteworthy in HMA asphalt exhibition. HMA asphalts that misshape and rush too much might be helpless to rutting and draining, while those that are too firm could be powerless to exhaustion breaking.
Safeguard. Black-top concrete like most exceptionally alternate materials, volatilizes (gives off vapor) when warmed. At greatly heightened temperatures (well above those encountered in the production and development of HMA) pavement bond can discharge enough vapor to build the volatile focus quickly above the pavement bond to a focus where it will light (blaze) when uncovered to a spark or open fire. This is called the glimmer indicate. For safeguard explanations, the blaze purpose of pavement bond is tested and regulated.
Virtue. Black-top bond, as utilized in HMA clearing, ought to comprise of well-nigh perfect bitumen. Debasements are not engaged solidifying constituents and might be impeding to pavement appearance.

Evaluating Systems

Black-top covers are commonly sorted by one or more shorthand evaluating frameworks as per their physical aspects. The proposed frameworks extend from straightforward to complex and speak for an advancement in the fitness to describe pavement fastener. Today, most state firms utilize or are seting the groundwork to switch to the Superpave appearance evaluating (PG) framework.

Penetration Grading

Based on the depth a standard needle will penetrate an asphalt binder sample when placed under a 100 g load for 5 seconds (see Figure 3). The test is simple and easy to perform but it does not measure any fundamental parameter and can only characterize asphalt binder at one temperature (77°F). Penetration grades are listed as a range of penetration units (one penetration unit = 0.1 mm of penetration by the standard needle). Typical asphalt binders used in the U.S. are 65-70 pen and 85-100 pen.

Viscosity Grading

Measures penetration (as in penetration grading) but also measures an asphalt binder’s viscosity at 140°F and 275°F. Testing can be done on virgin (AC) or aged (AR) asphalt binder. Grades are listed in poises (cm-g-s = dyne-second/cm²) or poises divided by 10. Typical asphalt binders used in the U.S. are AC-10, AC-20, AC-30, AR-4000 and AR-8000. Viscosity grading is a better grading system but it does not test low temperature asphalt binder rheology.

Superpave Performance Grading (PG) System

The Superpave PG system was developed as part of the Superpave research effort to more accurately and fully characterize asphalt binders for use in HMA pavements. The PG system is based on the idea that an HMA asphalt binder’s properties should be related to the conditions under which it is used. For asphalt binders, this involves expected climatic conditions as well as aging considerations. Therefore, the PG system uses a common battery of tests (as the older penetration and viscosity grading systems do) but specifies that a particular asphalt binder must pass these tests at specific temperatures that are dependant upon the specific climatic conditions in the area of intended use. Therefore, a binder used in Hawai’i would be different than one used in, say, Alaska.

Superpave performance grading is reported using two numbers – the first being the average seven-day maximum pavement temperature (in °C) and the second being the minimum pavement design temperature likely to be experienced (in °C). Thus, a PG 64-16 is intended for use where the average seven-day maximum pavement temperature is 64°C and the expected minimum pavement temperature is -16°C. Notice that these numbers are pavement temperatures and not air temperatures. The typical PG grade used in Hawai’i is a PG 64-16. Realistically, pavement temperatures in Hawai’i will never dip down to -16°C, but the typical asphalt binder used will meet this standard so it is graded as such.

Asphalt Binder Modifiers

Some asphalt cements require modification in order to meet specifications. Asphalt cement modification has been practiced for over 50 years but has received added attention in the past decade or so. There are numerous binder additives available on the market today. The benefits of modified asphalt cement can only be realized by a judicious selection of the modifier(s); not all modifiers are appropriate for all applications. In general, asphalt cement should be modified to achieve the following types of improvements (Roberts et al., 1996^[2]):

Lower stiffness (or viscosity) at the high temperatures associated with construction. This facilitates pumping of the liquid asphalt binder as well as mixing and compaction of HMA.
Higher stiffness at high service temperatures. This will reduce rutting and shoving.
Lower stiffness and faster relaxation properties at low service temperatures. This will reduce thermal cracking.
Increased adhesion between the asphalt binder and the aggregate in the presence of moisture. This will reduce the likelihood of stripping. Figure 4 shows two aggregate samples from the same source after they have been coated with asphalt binder. The asphalt binder used with the sample on the left contain no anti-stripping modifier, which resulted in almost no aggregate-asphalt binder adhesion. The asphalt binder used with the sample on the right contains 0.5% (by weight of asphalt binder) of an anti-stripping modifier, which results in good aggregate-asphalt binder adhesion.

Other Forms of Asphalt Used in Paving

Besides asphalt cement, three other forms of asphalt are used prominently in the paving industry:

Emulsified asphalt. Emulsified asphalt is a suspension of small asphalt cement globules in water, which is assisted by an emulsifying agent (such as soap). Emulsions have lower viscosities than neat (plain) asphalt and can thus be used in low temperature applications. After an emulsion is applied the water evaporates away and only the asphalt cement is left. Emulsions are often used as prime coats and tack coats.
Cutback asphalt. A cutback asphalt is a combination of asphalt cement and petroleum solvent. Like emulsions, cutbacks are used because their viscosity is lower than that of neat asphalt and can thus be used in low temperature applications. After a cutback is applied the solvent evaporates away and only the asphalt cement is left. Cutbacks are much less common today because the petroleum solvent is more expensive than water and can be an environmental concern. Cutbacks are typically used as prime coats and tack coats.
Foamed asphalt. Foamed asphalt is formed by combining hot asphalt binder with small amounts of cold water. When the cold water comes in contact with the hot asphalt binder it turns to steam, which becomes trapped in tiny asphalt binder bubbles (World Highways, 2001^[3]). The result is a thin-film, high volume asphalt foam. This high volume foam state only lasts for a few minutes, after which the asphalt binder resumes its original properties. Foamed asphalt can be used as a binder in soil or base course stabilization, and is often used as the stabilizing agent in cold in-place recycling (CIPR).

Footnotes (↵ returns to text)

Asphalt Institute. (2001). HMA Construction. Manual Series No. 22 (MS-22). Asphalt Institute. Lexington, KY.↵
Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.↵
World Highways. (April 2001). Foamed Asphalt Takes Hold. World Highways, April 2001.↵