Learn the Softening Point Test of Bitumen (Ring and Ball Method) as per IS 1205:1978. Step-by-step procedure, apparatus, formula, and significance for highway QA/QC engineers in NHAI projects.
The test is conducted as per:
IS 1205:1978 – “Methods for Testing Tar and Bituminous Materials: Determination of Softening Point (Ring and Ball Method)”
ASTM D36 – Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus)
Ring and Ball Apparatus (Brass rings + steel balls)
Thermometer (Range: 0°C to 110°C, accuracy ±0.5°C)
Beaker – 800 ml capacity
Heating device – to maintain uniform temperature rise (5 ± 0.5°C per minute)
Glycerin or distilled water (as heating medium depending on temperature range)
The Ring and Ball Method involves a series of steps to determine the softening point of bitumen precisely. Each stage must be executed carefully to avoid errors in temperature reading or material preparation.
As per Clause 4.1 of IS 1205:1978, the preparation must be done carefully to ensure uniformity and eliminate air or moisture contamination.
Step-by-step process:
Heat the bitumen sample to a temperature between 75°C and 100°C above its expected softening point.
This ensures the bitumen becomes completely fluid without overheating.
Stir thoroughly until the material is homogeneous, free from air bubbles, and moisture.
If necessary, filter through IS Sieve 30 (as per IS 460:1962) to remove any impurities or coarse particles.
Prepare the brass rings by preheating them to approximately the same temperature as the molten bitumen.
This prevents premature cooling during filling.
Coat the metal base plate with a mixture of equal parts of glycerine and dextrine to avoid adhesion between bitumen and plate.
Fill the rings completely with the molten bitumen so that there is a slight excess above the top edge after cooling.
Allow the filled rings to cool in air for 30 minutes, then trim the excess using a warm, sharp knife to obtain a perfectly level surface.
Avoid overheating; prolonged heating can cause bitumen hardening or oxidation.
Ensure no entrapped air remains, as it affects the actual softening temperature.
Filtering is recommended especially for field-extracted binder samples (e.g., from RAP or plant tanks).
After preparing the bitumen sample (as described in Clause 4.1), the next steps involve setting up the apparatus, heating, and observing the softening behavior of the material.
Place the metal rings containing the prepared bitumen samples into the ring holder of the apparatus.
Fix the ball centering guides above the rings and ensure the steel balls (9.5 mm diameter, approx. 3.5 g each) are clean and dry.
The assembly is then suspended in a liquid bath (beaker) filled with distilled water or glycerin depending on the expected softening point:
Water bath for softening points below 80°C.
Glycerin bath for softening points above 80°C.
The liquid level in the bath should be at least 50 mm above the upper surface of the rings to ensure uniform heating.
Insert a calibrated thermometer (range 0°C–110°C, accuracy ±0.5°C) into the bath so that its bulb is level with the ring sample.
Adjust the temperature of the bath to 5°C below the expected softening point before starting the actual heating process.
Ensure the setup is stable and free from vibration to avoid premature displacement of the steel ball.
Heat the bath uniformly at a rate of 5 ± 0.5°C per minute.
A controlled rate of temperature rise is crucial for accuracy.
Rapid heating can cause false readings.
Observe each ring carefully — the bitumen will gradually soften, allowing the steel ball to sink through the material.
Record the temperature shown by the thermometer at the instant the ball touches the metal base plate.
Conduct the test on two samples and take the average temperature as the Softening Point of the bitumen.
Observation: Record the temperature (°C) at which the bitumen sample surrounding the ball touches the bottom plate or bath.
Repeatability: The difference between two test results should not exceed 1.0 to 3.0°C, depending on the grade.
Reproducibility: Consistent results should be within ±5.5°C.
Final Result: Report the average of two readings, rounded to the nearest 0.5°C, as the Softening Point (without thermometer stem correction).
Maintain a consistent heating rate using a temperature-controlled water or oil bath.
For VG-40 or modified binders, ensure glycerin is used as the bath medium due to higher temperature range.
Avoid drafts and sudden movements around the apparatus during testing.
Verify thermometer calibration before and after every test series.
| Sample No. | Medium Used | Temperature at Drop (°C) | Remarks |
|---|---|---|---|
| 1 | Water | 54.0 | Normal drop observed |
| 2 | Water | 53.5 | Consistent result |
| Average Softening Point | 53.75°C |
Softening Point (°C) = Average of two observations.
Report to the nearest 0.5°C, along with:
Type and grade of bitumen
Bath medium used
Heating rate maintained
Reference standard: IS 1205:1978 – Ring and Ball Method
For NHAI and MoRTH projects, bitumen batches must show consistent softening point values within the limits prescribed by IS 73:2013 for each viscosity grade (VG).
Regular testing ensures temperature stability, resistance to rutting, and longer pavement life.
| Bitumen Grade | Softening Point (°C) | Recommended Use |
|---|---|---|
| VG-10 | 40 – 55 | Cold Climate Roads |
| VG-20 | 45 – 60 | Moderate Climate |
| VG-30 | 47 – 63 | Hot Climate Regions |
| VG-40 | 50 – 70 | Heavy Traffic & High Temp. Roads |
Helps determine bitumen suitability for different climatic zones.
Prevents bleeding, rutting, and deformation in asphalt pavements.
Aids in bitumen grade selection (VG-10, VG-20, VG-30, VG-40).
Essential for bituminous mix design verification as per IS 73:2013.
Required in NHAI/MoRTH quality records during construction audits.
Maintain heating rate of 5°C/min throughout.
Avoid air entrapment while filling the rings.
Use clean water or glycerin for every test.
Calibrate thermometer and apparatus regularly.
In hot climates or heavy traffic zones, prefer bitumen with a higher softening point (VG-30 or VG-40).
For quality assurance, compare softening point results with penetration and viscosity tests to ensure compliance with IS 73:2013.
The Softening Point Test (Ring and Ball Method) provides a reliable measure of the temperature susceptibility of bitumen.
Accurate testing and proper interpretation ensure durable, rut-free pavements and enhance overall pavement performance under varying temperatures.
