Bitumen Penetration Test: Hardness, Grades, and Equipment

What Does the Penetration Test Measure?

The penetration test measures the consistency (hardness) of bituminous binders by recording how far a standard needle penetrates a bitumen sample under a 100 g load, at 25°C, for 5 seconds. The result is expressed in units of 0.1 mm (dmm) — called penetration units.

A penetration of 70 means the needle sank 7 mm into the bitumen. Lower penetration = harder bitumen = better for hot climates. Higher penetration = softer bitumen = better for cold climates.

Standard Penetration Grades

Equipment Required

• Penetrometer: Motor-driven or manual instrument. Holds the standard needle, applies 100 g total load, and times 5 seconds precisely. Digital penetrometers automatically release, time, and read penetration.
• Standard Needle: Stainless steel, hardened, 1.00 ± 0.02 mm diameter at tip, polished.
• Water Bath: Thermostatically controlled at 25.0°C ± 0.1°C. The bitumen sample container sits in water during conditioning.
• Bitumen Container: 55 mm diameter, 35 mm deep cylindrical tin for the bitumen sample.
• Transfer dish: Larger water bath dish that holds the sample container during testing.

Procedure (ASTM D5 / EN 1426)

1. Heat bitumen sample to pouring temperature. Fill container to within 5 mm of top. Allow to cool at room temperature 1–1.5 hours, then condition in water bath at 25°C for 1–1.5 hours.
2. Place container in transfer dish filled with water at 25°C.
3. Position penetrometer needle just touching the bitumen surface. Zero the gauge.
4. Release needle for exactly 5 seconds under 100 g total load.
5. Read penetration depth in 0.1 mm units. Perform three measurements at different points. Report mean.