Bitumen Cutback or Asphalt cutbacks use petroleum solvents for dissolving asphalt cement. The solvents are variously called distillate, diluents, or cutter stock. If the solvent used in making the cutback asphalt is highly volatile, it will quickly escape by evaporation. Solvents of lower volatility evaporate more slowly. On the basis of the relative speed of evaporation, cutback asphalts are divided into three types:

1. Rapid Curing (RC)
2. Medium Curing (MC)
3. Slow Curing (SC)

Rapid Curing (RC) Cutback Bitumen:

Rapid Curing (RC) Cutback asphalt cement is a combination of light diluents with high volatility, typically in the gasoline or naphtha boiling point range, and asphalt cement. Also, the level of kerosene will usually be between 28% – 33% depending on the application. As much as the level of kerosene increases in the rapid curing, the level of viscosity of asphalt will come down. Rapid Curing Cutback grades include RC–30, RC–70, RC–250, RC–800, and RC–3000. Cutback agents are used to lower the viscosity of bitumen when it is applied as a primer to the surface of a road pavement aggregate base course or substrate. Kerosene is used as a bitumen cutback agent at different concentrations according to local conditions and requirements.

The cutback asphalt RC–70 is applied for the waterproofing of surfaces like plug capillary voids, coat, and bond loose mineral particles.

The degree of fluidity developed in each cutback case depends principally on the ratio of solvent to asphalt cement. To a minor degree, the liquidity of the cutback may be affected by the hardness of the base asphalt from which the cutback is made. The degree of fluidity results in several grades of cutback asphalt, some quite fluid at ambient temperatures and others somewhat more viscous. The more viscous grades may require a small amount of heating to make them fluid enough for construction operations.

Rapid curing cutback grades are designed to react quickly and are primarily used in spray applications such as bond/tack coats, aggregate chips seals, sand seals, granular priming, and similar surface treatments.

The degree of liquidity developed in each case depends principally on the proportion of solvent to asphalt cement. To a minor degree, the liquidity of the cutback may be affected by the hardness of the base asphalt from which the cutback is made. The degree of fluidity results in several grades of cutback asphalt and some quite fluid at ordinary temperatures and others somewhat more viscous. The more viscous grades may require a small amount of heating to make them fluid enough for construction operations.

The rapid–setting grades are designed to react quickly primarily for spray applications, such as bond/tack coats, aggregate chips seals, sand seals, and similar surface treatments.

Medium Curing (MC) Cutback Bitumen:

Medium Curing grades cutback asphalt is simply a combination of asphalt cement and petroleum solvent. Like emulsions, cutbacks are used because they reduce asphalt viscosity for lower-temperature uses (tack coats, fog seals, slurry seals, and stabilization material). Similar to emulsified asphalts, after cutback asphalt is applied the petroleum solvent evaporates leaving behind asphalt cement residue on the surface to which it was applied. Rapid curing cutback grades include MC–30, MC–70, MC–250, MC–800, and MC–3000. The use of cutback asphalts is decreasing because of:

1. Environmental regulations: Cutback asphalts contain volatile chemicals that evaporate into the atmosphere. Emulsified asphalts evaporate water into the atmosphere.
2. Loss of high-energy products: The petroleum solvents used require higher amounts of energy to manufacture and are expensive compared to the water and emulsifying agents used in emulsified asphalts.
3. Slow Curing Cutback Bitumen and oils of low volatility are generally in the heavy distillate range (SC–70, SC–250, SC–800 & SC–3000). The degree of liquidity developed in each case depends principally on the proportion of solvent to asphalt cement. To a minor degree, the liquidity of the cutback may be affected by the hardness of the base asphalt from which the cutback is made. The degree of fluidity results in several grades of cutback asphalt–some quite fluid at ordinary temperatures and others somewhat more viscous. The more viscous grades may require a small amount of heating to make them fluid enough for construction operations. Slow Curing often called “Road Oil” is usually a residual material produced from the fractional distillation of certain crude petroleum. Traditionally any kind of aromatic, naphthenic, and paraffinic oils are used. Slow Curing liquid bitumen materials can be prepared by blending bitumen with an oily petroleum fraction.

Slow Curing (SC) Cutback Bitumen:

Current common uses are in penetrating prime coats and in producing patching or stockpile mixtures. Cutback asphalt used in mixing with aggregate will usually contain an adhesion agent to assist in the coating of the aggregate surface.

Cutback agents are used to lower the viscosity of bitumen when it is applied as a primer to the surface of a road pavement aggregate base course or substrate. Kerosene is used as a bitumen cutback agent at different concentrations according to local conditions and requirements. The cutback bitumen is ideal for prime coat and cold application because of its easy uses and no need for thinning and heating.

Slow Curing (SC) Cutback Bitumen consists of the initial incorporation of asphalt into the surface of non–an asphalt-based course preparatory to any superimposed treatment of construction. The cutback asphalt SC applies to waterproofing surfaces, plugging capillary voids, coats, and bonds to lose mineral particles. Rapid curing cutback grades include SC–70, SC–250, SC–800, and SC–3000.

STANDARD SPECIFICATION OF BITUMEN CUTBACK GRADE RC – 30