Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is stripping? In a bituminous road, every stone is coated with a thin film of bitumen. If water gets between the stone and the bitumen, the film peels off (strips). The road then loses strength and develops ravelling, potholes and loose stones, especially in the monsoon.
What does the test tell you? How well the bitumen sticks to the aggregate under water. Coated stones are kept in water at 40 °C for 24 hours and the bare area is estimated by eye.
This guide follows IS 6241.
What Stripping Looks Like
📏 Stripping value
The uncoated (bare) area of the stones as a percentage of their total area, judged by eye after the test.
✅ Retained coating
The area still coated = 100 − stripping value. Specifications usually ask for at least 95% retained coating.
Apparatus Required
Glass beaker (500 ml)
Heat-resistant; holds the coated stones under water.
Sieves 20 & 12.5 mm
Select stones passing 20 mm, retained on 12.5 mm.
Mixer / bowl & spatula
Mixes hot stones with hot bitumen until fully coated.
Balance (1 g)
Weighs aggregate and bitumen.
Oven & hot plate
Heat aggregate to 150 °C and bitumen to 160 °C.
Water bath (40 °C)
Thermostatically controlled; keeps beakers at 40 °C for 24 h.
Procedure
- Take 200 g of clean, dry aggregate passing 20 mm and retained on 12.5 mm.
- Heat the aggregate to 150 °C and the bitumen to 160 °C.
- Add bitumen equal to 5% of the aggregate weight (10 g for 200 g) and mix until every stone is fully coated.
- Put the coated stones into the 500 ml beaker and let them cool at room temperature for about 2 hours.
- Pour in distilled water to cover the stones completely. Cover the beaker.
- Place the beaker in the water bath at 40 °C, with the bath water at least halfway up the beaker. Leave it for 24 hours.
- Take out the beaker, let it cool to room temperature, and look at the stones while they are still under water. Estimate the percentage of bare area.
- Do three tests and report the mean, rounded to the nearest whole number.
How to Judge the Coating
The result is estimated by eye, so a visual guide helps. Compare your stones with these examples (black = bitumen, light = bare stone):
Calculation
Report the mean of three tests, rounded to the nearest whole number.
Try It: Stripping Value Calculator
Enter the retained coating (%) you observed in each test. The values filled in are a worked example.
Bitumen needed for the test
Precautions
🧼 Clean, dry stones
Dust and moisture on the stone stop the bitumen from sticking and give a false poor result.
🌡️ Right temperatures
Don’t overheat the bitumen; it hardens and bonds poorly. Keep to 150 °C (stone) and 160 °C (bitumen).
👁️ Judge under water
Estimate the bare area while the stones are still submerged, in good light. Have two people judge if possible.
🔥 Hot bitumen safety
Wear heat-resistant gloves and goggles; mix carefully to avoid burns and splashes.
Practical Tips for Site Engineers
1. Which Stones Strip More?
More likely to strip
Stones rich in silica, such as quartzite and some granites. They attract water more than bitumen.
Usually bond well
Basalt, limestone and dolerite generally hold bitumen better.
2. If the Coating Is Below 95%
- Add an anti-stripping agent (amine-based) to the bitumen at the dose recommended by the manufacturer and the specification, then retest.
- Use hydrated lime as part of the filler where specified; it improves adhesion with many stones.
- Clean the aggregate: wash or screen off dust and clay coatings.
- Change the source if the stone still strips after treatment.
3. Good Site Habits
- Test every source and every bitumen grade before mix design; the result depends on the pair.
- Check the anti-stripping dose at the plant: it must be mixed well into the bitumen, and some agents lose strength if kept hot for long.
- Keep water out of the road: good camber, drainage and sealed shoulders reduce stripping on site.
- Don’t lay on wet surfaces or with wet aggregate; moisture under the film starts stripping from day one.
- Watch early signs: loose stones, grey patches and ravelling after rain point to stripping. Take cores and check.
Key Takeaways
- Stripping = bitumen film peeling off the stone because of water.
- 200 g of 20–12.5 mm stone at 150 °C + 5% bitumen at 160 °C; cool 2 h; water bath 40 °C for 24 h.
- Estimate the bare area by eye under water; report the mean of three tests.
- Retained coating should be at least 95% (stripping value 5% or less).
- On site: use anti-stripping agents or hydrated lime for stones that strip; keep aggregate clean and dry and roads well drained.
References: IS 6241: Method of Test for Determination of Stripping Value of Road Aggregates; MoRTH Specifications for Road and Bridge Works (Section 500).