Penetration Test of Bitumen: Procedure, Grades and Results

The penetration test of bitumen answers one question: how firm is this binder at room temperature? A weighted needle rests on a cup of bitumen for a few seconds, and the depth it sinks is the answer. It sounds crude, but it has been the backbone of bitumen grading for about a century, and grades such as 60/70 and 80/100 still come from it. This guide explains the procedure in ASTM D5 and AASHTO T 49, the units, the common grades and how penetration fits alongside newer tests.

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Quick facts

Standards: ASTM D5, AASHTO T 49, IS 1203 (India), EN 1426 (Europe).
Standard conditions: 100 g total load, 25 degrees C, 5 seconds.
Unit: 0.1 mm. A reading of 80 means the needle went 8 mm deep.
Meaning: a higher number is a softer binder.
Used for: penetration grades, quality control, and the penetration limits inside viscosity grade specs.

Equipment

Penetrometer: a frame with a spindle that drops freely and a dial or digital readout to 0.1 mm. In ASTM D5 the needle plus spindle weighs 50 g, and an extra 50 g weight brings the total to 100 g.
Needle: a hardened stainless steel needle with a precisely ground tapered tip.
Sample containers: flat-bottomed metal or glass cups sized for the expected depth.
Water bath and transfer dish: to hold the sample at exactly 25 degrees C during the test.
Timer: readable to a tenth of a second for manual machines.

Step-by-step procedure

Melt the sample gently. Warm it only until it pours, stirring to remove air, and pour it into the container to the required depth.
Cool, then condition. Let it cool in air for the set period, then place it in the 25 degree C water bath long enough for the whole mass to reach that temperature. Bigger cups need more time.
Set the needle. Clean the needle, load the spindle and lower the tip until it just touches the bitumen surface. A light reflecting off the surface helps you see contact.
Release for 5 seconds. Free the spindle, let the needle sink for 5 seconds, then lock it and read the dial.
Repeat. Make at least three penetrations at points well apart from each other and from the container edge, cleaning the needle between pushes.
Report. Average the readings, rounded to a whole unit, if they fall within the spread the standard allows.

Penetration grades

Grade	Penetration range (0.1 mm)	Relative stiffness
40/50	40 to 50	Hard
60/70	60 to 70	Medium hard
80/100	80 to 100	Medium soft
120/150	120 to 150	Soft

As a rule of thumb, harder grades go where pavements get hot and traffic is heavy, and softer grades go where winters are cold. India's IS 73 viscosity grades also carry a penetration window; VG-30, for example, must fall between 50 and 70.

Worked example

Say three pushes on a sample read 6.3 mm, 6.6 mm and 6.4 mm. In penetration units that is 63, 66 and 64. The average is about 64, so the sample sits inside the 60/70 band. These numbers are made up to show the math; your lab sheet is the record that counts.

What can go wrong

Wrong temperature. Penetration changes quickly with temperature. A sample that is a degree or two warm reads noticeably soft.
Dirty or bent needle. Residue on the tip drags and gives short readings.
Too close to the wall. Near the rim the bitumen is restrained, so the needle stops early.
Overheating during prep. Too much heat ages the binder and makes it harder before the test even starts.

Penetration before and after aging

Many specs ask for "retained penetration": the penetration after a short-term aging test, expressed as a percentage of the original. It shows how much the binder hardens in the plant and paver. Our guide to the rolling thin film oven explains that aging step. Penetration is also paired with the softening point to estimate temperature sensitivity.

Penetration vs PG grades

Penetration describes the binder at one temperature only. The Superpave PG system tests at the hot and cold temperatures the pavement will actually see, which is why US agencies adopted it. For the basics, see asphalt binder and our explainer on Superpave mix names.

Planning a paving job?

The plant handles binder testing. For your own numbers, the driveway asphalt calculator converts area and thickness to tons, the asphalt base calculator covers the stone underneath, and the crushed stone calculator handles any extra aggregate.

Questions people ask

What does the penetration test of bitumen measure?

It measures consistency, meaning how hard or soft the binder is at a set temperature. A standard needle is loaded and allowed to sink into the sample for a fixed time, and the depth it reaches is the result. Deeper means softer.

What are the standard test conditions?

The usual conditions are a 100 g total load on the needle, a sample temperature of 25 degrees C (77 degrees F) and a 5 second loading time. The same method, ASTM D5 or AASHTO T 49, also allows other loads and temperatures when a specification calls for them.

What unit is penetration reported in?

Tenths of a millimeter, often written as dmm or simply as penetration units. If the needle goes in 6.5 mm, the penetration is 65.

What does 60/70 bitumen mean?

It is a penetration grade. The binder must give a penetration between 60 and 70 at standard conditions. Grades like 40/50 are harder, and 80/100 or 120/150 are softer.

Why take several readings?

One push can hit a small bubble or a stiff spot. The standard calls for at least three penetrations at points spaced apart and away from the container wall, then reports the average, provided the readings agree within the allowed spread.

Is the penetration test still used?

Yes, in much of the world penetration grades are still how paving bitumen is bought, and India specifies penetration limits inside its viscosity grades. US highway work moved to the PG system, but penetration remains common for roofing, sealants and quality checks.

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