Ductility test of bitumen

Concept

Ductility of bitumen is the distance, in cm, to which a standard briquette of bitumen can be drawn out before the thread breaks, under specified conditions (27 degrees C in IS:1208, 25 degrees C in ASTM; rate of pull 50 mm/min). It shows the cohesion, elasticity and flexibility of the binder.

Apparatus

• Standard briquette mould (brass, 75 mm long, with a minimum cross-section of 10 mm x 10 mm at the neck).
• Ductility machine with a water bath at 27 (or 25) degrees C, in which one clip is fixed and the other moves at 50 +/- 2.5 mm/min.
• Thermometer, knife, and glycerine-dextrin release agent.

  fixed end                      moving end
   [clip]=====\            /=====[clip]
   | |         \___thread___/         | |
   water bath 27 C, pulled at 50 mm/min

Procedure

1. Heat the bitumen to 75-100 degrees C above its softening point so that it is fluid, and pour it into the mould (coated with the glycerine-dextrin release agent) placed on a brass plate; slightly overfill it.
2. Cool in air for 30-40 minutes, then trim the excess with a hot knife so that it is level with the mould.
3. Place the specimen, with the mould and plate, in the water bath at the test temperature for 85-95 minutes.
4. Remove it from the plate, take off the side pieces of the mould, and fix the rings to the clips of the machine; the water in the bath, at the test temperature, should cover the specimen by at least 25 mm.
5. Pull at 50 mm/min; the distance from the start to the breaking point is the ductility value (cm). Report the average of three tests.

Significance and engineering application

• A high ductility means that the binder can stretch without cracking, so the pavement stays flexible under temperature changes and traffic.
• Bitumen with low ductility cracks, especially in cold weather and after ageing; ductility also indicates the adhesive strength with the aggregates.
• Minimum values: 75 cm for 60/70 and 80/100 penetration grades (50 cm for 30/40) under IS:73:1992, and 40 cm for VG-30 after thin-film oven ageing (IS:73:2013); a general minimum of 50 cm is often quoted in textbooks.
• Used in specifying and controlling the quality of bitumen for surface dressing, bituminous concrete and DBM, and to reject bitumen with low ductility.