A cutback bitumen or asphalt is simply a solution of bitumen in a solvent. This was one of the primary ways of delivering bitumen to a road or other surface.

The classifications of cutbacks are many, depending on the local specification; usually they are classified in one of three ways:

Slow cure (SC)
Medium Cure (MC)
Rapid cure (RC)
They are designated by their viscosity and binder content. In ASTM the following grades are usual. The number refers to the viscosity at 60C ( Kinematic- mm2/s) ( normally base bitumen is 80 /100). The most used grades are MC250 and SC250 for priming. The higher viscosity grades are used for chip sealing and the lower as utility or waterproofing products.

Cutbacks may be manufactured as finished products or made up in the field by adding solvent.
The most used cutbacks are MC type this has kerosene as the diluent. In USA SC are only used as they contain diesel and this has lower volatility. RC types are seldom used as they contain naphtha and pose a significant fire risk. Cutbacks also can generate significant VOC when sprayed hot.

If the wrong cutback is used or it is used at too high a level bleeding too may occur.

Cutbacks have their main application for priming granular road surfaces as they can penetrate and bind the surface layers and create waterproofing.

SC cutbacks usually give the best results for this but MC is satisfactory too. Specialized cutbacks with additives or special solvent systems may be made for industrial applications. Use of chlorinated solvents can remove the fire risk.

Manufacture:

Cutbacks may be manufactured in several ways. The simplest is with circulation in a bitumen tank or tanker, the temperature should be kept to about 140C and the solvent added below the level of the binder. This must be done slowly.

The best way is to use a blend system employing a colloid mill or other inline mixer. VSSEmultech® plants are ideal when modified.

The system requires, above the usual emulsion plumbing, an inline injection of solvent into the mill or bitumen line.
The colloid mill makes a rapid and well dispersed cutback.

Cutbacks have a place in road building in many countries, emulsions generally may replace them e.g. EMULPRIME® 1000 but VSS has the technology and materials to assist the client in making the best cutbacks in the most efficient way.