The high viscosity of penetration grade bitumens can be reduced by the addition of volatile diluents (cutters) to make cutback bitumens suitable for specific road building applications. In many countries the choice of cutbacks available commercially is very restricted but cutbacks can often be made locally by blending more readily available components. A range of laboratory blends have been tested to examine compliance with the widely-used American classification system and recommended compositions are given for cutbacks using 50 pen and 100 pen base bitumens. With the latter both medium and slow curing cutbacks could be made but with the former only medium curing blends satisfied the specifications. Rapid curing cutbacks could not be made with either grade. Blends were also produced which complied with the main requirements for the three grades specified in BS 3690 Part 1.

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