Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is the penetration test? It measures how hard or soft bitumen is. A standard needle carrying 100 g is allowed to sink into bitumen at 25 °C for 5 seconds. The depth is measured in units of 0.1 mm.
Higher penetration = softer bitumen. A penetration of 65 means the needle went 6.5 mm deep.
This guide follows IS 1203.
Soft vs Hard Bitumen
The Penetrometer
Apparatus Required
Penetrometer
Holds the needle; dial reads in 0.1 mm.
Standard needle
Hardened, polished steel; total moving weight 100 g.
Sample containers
55 × 35 mm (penetration below 225); 70 × 45 mm (225–350).
Water bath (25 ± 0.1 °C)
At least 10 litres; keeps the exact test temperature.
Transfer dish
Keeps the sample under water during the test.
Stopwatch & thermometer
Time exactly 5 seconds; check bath temperature.
Procedure
Step 1: Prepare the Sample
- Heat the bitumen gently until it can be poured, not more than 90 °C above its softening point (60 °C for tars). Stir until it is uniform and free of air bubbles and water. Use an air oven or sand bath so the bottom doesn’t overheat.
- Pour it into the container to a depth at least 10 mm more than the expected penetration. Cover it against dust.
- Let it cool in air (15–30 °C): 1–1.5 hours for the small container (35 mm deep), 1.5–2 hours for the large one (45 mm deep).
- Then keep it in the water bath at 25 ± 0.1 °C for the same time.
Step 2: Take Readings
- Place the container in the transfer dish, fully covered with water from the bath, on the penetrometer stand.
- Clean the needle (with benzene or a suitable solvent) and dry it. Lower it until the tip just touches the bitumen surface (use the reflection to judge).
- Set the dial to zero. Release the needle for exactly 5 seconds, then lock it and read the dial.
- Take at least three readings, each at least 10 mm apart and 10 mm from the container wall. Clean the needle before each reading.
Result
Maximum Allowed Difference Between Readings
| Penetration | Max. difference (highest − lowest) |
|---|---|
| 0–49 | 2 |
| 50–149 | 4 |
| 150–249 | 6 |
| 250 and above | 8 |
If the readings differ by more than this, repeat the test.
Try It: Penetration Calculator
Enter the three dial readings (0.1 mm units). The values filled in are a worked example.
Precautions
🌡️ Exactly 25 °C
Penetration changes a lot with temperature. Keep the bath at 25 ± 0.1 °C and test with the sample under water.
📍 Touch, don’t press
The needle tip must just touch the surface at the start. Starting above or pressed into the surface gives a wrong reading.
🧽 Clean, straight needle
Clean and dry the needle before every reading; replace it if it’s bent or damaged.
🚫 Don’t overheat
Overheating while melting hardens the bitumen and lowers penetration. Discard a reading if the container moves.
Practical Tips for Site Engineers
1. Minimum Penetration of Paving Bitumen (IS 73)
| Grade | Min. penetration at 25 °C | Typical use |
|---|---|---|
| VG-10 | 80 | Spraying, cold regions, light traffic |
| VG-20 | 60 | Cold and hilly regions |
| VG-30 | 45 | Most roads in India; heavy traffic |
| VG-40 | 35 | Hot climates, very heavy traffic, junctions |
VG grades are specified mainly by viscosity; penetration is one of the other requirements. Follow IS 73 and your contract.
2. Good Site Habits
- Test every tanker: a quick penetration test confirms the grade before unloading; compare with the supplier’s certificate.
- Too low = overheated or aged: bitumen held too hot for too long in plant tanks hardens and loses penetration, leading to brittle, cracking roads. Watch storage temperatures.
- Too high = wrong grade or diluted: unusually soft bitumen may be a softer grade or mixed with oil; it causes rutting and bleeding in summer.
- Match grade to climate: harder grades (VG-30, VG-40) resist rutting in hot regions; softer grades suit cold areas.
- Keep samples: keep a sealed, labelled sample from each tanker until the work is accepted, for re-testing if problems appear.
Key Takeaways
- Penetration = depth (0.1 mm) a 100 g needle sinks in 5 seconds at 25 °C; higher means softer.
- Heat gently, pour, cool in air, then condition in the water bath at 25 ± 0.1 °C.
- At least 3 readings, 10 mm apart and 10 mm from the edge; check the allowed spread.
- Minimum penetration: VG-10 80, VG-20 60, VG-30 45, VG-40 35.
- On site: test every tanker; low values mean overheating or ageing, high values mean the wrong grade or dilution.
References: IS 1203: Methods for Testing Tar and Bituminous Materials – Determination of Penetration; IS 73: Paving Bitumen – Specification; MoRTH Specifications for Road and Bridge Works (Section 500).