Why different consistency tests, ductility

Types of consistency tests on bituminous binders

1. Penetration test - hardness at 25 degrees C (in units of 0.1 mm).
2. Softening point test (ring and ball) - temperature at which bitumen softens.
3. Viscosity test (Saybolt/tar viscometer or capillary) - flow time at the specified temperature.
4. Ductility test - elongation before breaking at 27 degrees C (25 degrees C in ASTM).
5. Float test - for semi-solid tars and cutbacks too soft for the penetration test (time taken for water to break through a plug of the binder at 50 degrees C).

Why different consistency tests are needed

• Bitumen is a visco-elastic material and its consistency changes with temperature, so one test at one temperature cannot describe its behaviour in all conditions.
• Penetration shows the hardness in service (25 degrees C); softening point gives the behaviour at high service temperatures (resistance to flow and bleeding); viscosity gives the behaviour at spraying, mixing and compaction temperatures (60 to 150 degrees C); ductility shows the cohesion and resistance to cracking.
• Two bitumens may have the same penetration but different temperature susceptibility, so combined tests are used (for example, the penetration index from penetration and softening point).
• Different types of binders (bitumen, cutback, tar, emulsion) have different consistency ranges, so different tests suit them.

Ductility test

The ductility of bitumen is the length in cm to which a standard briquette can be stretched before the thread breaks.

1. Heat the sample and pour it into a brass briquette mould (coated with glycerine-dextrin), then cool it and trim it level.
2. Keep the mould in a water bath at 27 degrees C for about 90 minutes.
3. Fix the specimen in the ductility machine in the water bath (27 degrees C), remove the side pieces of the mould, and pull the ends apart at a uniform rate of 50 mm per minute.
4. Record the distance, in cm, at which the thread breaks. Take the mean of three tests.

Significance

• A high ductility shows good flexibility and cohesion, so the bitumen can follow movements without cracking and binds the aggregate well.
• Specification: at least 75 cm for 60/70 and 80/100 penetration grades (IS:73:1992; a general minimum of 50 cm is often quoted). A low value means a brittle binder that cracks in cold weather and with age.