Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is the softening point? Bitumen doesn’t melt at one exact temperature; it slowly becomes softer as it warms. The softening point is the temperature at which a bitumen disc, carrying a small steel ball, sags 25 mm under standard heating.
Higher softening point = bitumen stays firm at higher temperatures, so the road resists softening, rutting and bleeding in summer.
This is the Ring and Ball test, as per IS 1205.
How the Test Works
The Ring and Ball Apparatus
Apparatus Required
Brass rings (2)
Shouldered rings, 6.4 mm deep, about 16–17.5 mm inside.
Steel balls (2)
9.5 mm diameter, 3.5 ± 0.05 g each.
Ball guides & support
Hold the rings; bottom plate exactly 25 mm below.
Glass beaker
Heat-resistant; at least 85 mm wide and 120 mm deep.
Thermometer & stirrer
Read temperature; keep the bath even.
Heater & stopwatch
Control a 5 °C per minute rise; check every minute.
Procedure
Step 1: Prepare the Specimens
- Heat the bitumen to 75–100 °C above its expected softening point. Stir until it is fully fluid and free of air bubbles and water (strain through IS sieve 30 if needed).
- Warm the two rings and place them on a metal plate coated with a glycerine–dextrin mixture (to stop sticking).
- Fill the rings with bitumen so there is a small excess above the top. Let them cool in air for 30 minutes.
- Cut off the excess with a warm knife so the bitumen is level with the ring top.
Step 2: Run the Test
- Assemble the rings, ball guides and thermometer in the support, with the bottom plate exactly 25 mm below the rings.
- Fill the beaker with freshly boiled distilled water (for softening points up to 80 °C) or glycerine (above 80 °C).
- Keep the bath at 5 °C for 15 minutes. Place the cooled steel balls (3.5 g each) in the guides on the bitumen.
- Heat the bath so the temperature rises at 5 ± 0.5 °C per minute, stirring all the time.
- Note the temperature when the bitumen with the ball touches the bottom plate, for each ring.
- If the two temperatures differ by more than 1 °C, repeat the test. Otherwise report the mean to the nearest 0.5 °C.
Try It: Softening Point Calculator
Enter the temperatures at which each ball touched the plate. The values filled in are a worked example.
Check your heating rate
Precautions
🔥 Steady 5 °C/min
Check the rise every minute. Heating too fast gives a falsely high softening point. Don’t “average out” a slow start with a fast finish.
🥄 Stir gently
Stir all the time so the temperature is even, but keep the stirrer away from the rings and balls.
📏 Exactly 25 mm
Check the gap between the ring bottoms and the base plate before starting.
💧 Right bath liquid
Use boiled distilled water up to 80 °C (no air bubbles) and glycerine above 80 °C.
Practical Tips for Site Engineers
1. Minimum Softening Point of Paving Bitumen (IS 73)
| Grade | Min. softening point |
|---|---|
| VG-10 | 40 °C |
| VG-20 | 45 °C |
| VG-30 | 47 °C |
| VG-40 | 50 °C |
Polymer and crumb rubber modified bitumens (IS 15462) have higher requirements. Follow your contract.
2. Good Site Habits
- Read it with penetration: low penetration with high softening point = harder bitumen; high penetration with low softening point = softer bitumen. Both should match the grade.
- Hot roads need a higher softening point: pavement surfaces in Indian summers can reach 60–70 °C. That’s why harder grades (VG-30/VG-40) or modified binders are used for heavy traffic and hot areas.
- Bleeding and rutting in summer? Check the softening point of the binder used, and the bitumen content of the mix.
- Test every tanker, along with penetration, and compare with the supplier’s certificate.
- Avoid overheating at the plant: it hardens bitumen and changes its properties. Keep storage and mixing temperatures within the specification.
Key Takeaways
- Softening point = temperature at which a bitumen disc with a 3.5 g ball sags 25 mm (Ring and Ball test).
- Fill brass rings, cool 30 min, trim; start the bath at 5 °C; heat at 5 ± 0.5 °C per minute.
- Water up to 80 °C, glycerine above; balls must agree within 1 °C; report the mean to 0.5 °C.
- Minimum: VG-10 40 °C, VG-20 45 °C, VG-30 47 °C, VG-40 50 °C.
- On site: read it with penetration; a higher softening point resists summer softening and rutting.
References: IS 1205: Methods for Testing Tar and Bituminous Materials – Determination of Softening Point; IS 73: Paving Bitumen – Specification; IS 15462: Polymer and Rubber Modified Bitumen.