Bitumen is graded, bought and specified almost entirely on the basis of test results. This post covers all ten tests — what each measures, the exact test conditions, and the values you need to remember.
1. Viscosity
Viscosity is the property of bitumen which resists flow due to internal friction.
Absolute or Dynamic Viscosity
Measured by a sliding plate viscometer. A thin film of bitumen of thickness d (20 to 50 microns) is held between two plates with contact area A. The tangential force F needed to move the top plate at velocity V is measured.
η = F d / (A V)
In CGS units, dynamic viscosity is measured in Poise.
Kinematic Viscosity
Kinematic viscosity = Dynamic viscosity / Density = η / ρ
Measured in CGS units in Stokes (cm2/sec).
Efflux Viscometers
The viscosity of liquid bitumen is measured by efflux viscometers. The liquid is kept at a constant temperature and made to pass through an orifice; the time required for a measured quantity to pass through is an indirect measure of viscosity.
| Item | Detail |
|---|---|
| Methods used | STV (Standard Tar Viscometer), Saybolt Furol, Redwood, Engler |
| Orifice diameter | 3 to 10 mm |
| Volume of fluid | 50 to 200 ml |
| Standardised test | Furol viscosity |
Note the neat idea behind an efflux viscometer: rather than measuring force directly, it measures time. A thicker liquid takes longer to drain, so the stopwatch does the work.
2. Ductility
Bitumen should be sufficiently ductile, that is, capable of being stretched without breaking. Ductility is the opposite of brittleness.
Test Conditions
| Item | Value |
|---|---|
| Specimen | Standard briquette, cross-sectional area 1 sq cm |
| Temperature | 27 °C |
| Rate of pull | 5 cm/min |
| Result | The distance in cm the briquette stretches before breaking |
Values
- Values vary from 5 to 100.
- A minimum value of 50 is commonly specified.
- For waxy bitumen, the value may be as low as 15.
- ISI recommends a minimum ductility of 75 cm for grades of 45 and above.
Why does ductility matter on a road? Because a pavement flexes under every wheel. A brittle binder cracks at the first bending; a ductile one stretches and recovers.
3. Penetration
A measure of the hardness of bitumen, obtained indirectly.
Test Conditions
It measures the distance a standard blunt-pointed needle vertically penetrates a sample:
| Item | Value |
|---|---|
| Temperature | 27 °C |
| Load | 100 g |
| Time of application | 5 seconds |
| Unit of penetration | 1/10 mm |
The unit is one tenth of a millimetre. So 80/100 pen means a penetration of 8 to 10 mm.
That unit conversion is where marks are most often lost. A penetration reading of 80 does not mean 80 mm — it means 8.0 mm.
Grades
- Penetration limits allow bitumen to be classified on the basis of consistency.
- Common grades: 30/40, 60/70 and 80/100.
- Tars are soft and the penetration test is not used for them.
Note the direction: a higher penetration number means softer bitumen. Grade 30/40 is harder than 80/100, because the needle sinks less far into it.
4. Softening Point
Measured by the “Ring and Ball” test. The softening point is the temperature at which bitumen attains a particular degree of softness under standardised test conditions.
How the Test Works
The softening point is the temperature at which a standard steel ball, placed on a layer of bitumen held in a standard ring, passes through the bitumen layer and touches a bottom plate kept 2.54 cm below.
The bath liquid is water or glycerine.
Values and Significance
| Item | Value |
|---|---|
| Softening point of 80/100 paving grade bitumen | 35 to 50 °C |
| Ball-to-plate distance | 2.54 cm |
- Its significance is that it indicates the temperature at which bitumen passes from solid to liquid consistency.
- For satisfactory performance and to avoid bleeding, bitumen should have a softening point 5 to 10 °C above the maximum atmospheric temperature.
What Bleeding Is
If the softening point sits below the road surface temperature on a hot day, the binder softens in service and works its way up through the mix, leaving a shiny black film on the surface. That is bleeding — the road becomes slippery and the mix loses binder from within.
The 5 to 10 °C margin above maximum atmospheric temperature is the safety allowance that prevents it, and it is why hotter regions need harder grades of bitumen.
5. Flash and Fire Point
At high temperature bitumen becomes volatile and catches fire, which is very hazardous. These temperatures must therefore be quantified for each grade.
| Term | Definition |
|---|---|
| Flash point | The lowest temperature in °C at which application of a test flame causes the vapour from bitumen to catch fire momentarily, in the form of a flash |
| Fire point | The lowest temperature in °C at which application of the test flame causes the bitumen to ignite and burn for at least 5 seconds under specified test conditions |
The safe limit for heating bitumen is normally 50 °C below the flash point.
Flash versus Fire — The Distinction
At the flash point, only the vapour above the bitumen ignites, and only momentarily. At the fire point, the bitumen itself ignites and keeps burning for at least 5 seconds.
The fire point is therefore always the higher of the two — and the flash point is the one used for safety limits, because it marks the first appearance of danger.
6. Specific Gravity
- Determined by the pycnometer method.
- Measurements are taken at 27 °C.
- Specific gravity is generally around 1.00.
| Material | Specific Gravity Range |
|---|---|
| Pure bitumen | 0.97 to 1.02 |
| Tars | 1.10 to 1.25 |
The two ranges do not overlap, which makes specific gravity a quick way of telling bitumen from tar.
7. Solubility
- The solubility of bitumen in trichloroethylene is a measure of its purity.
- A minimum value of 99 percent is generally desired.
The reasoning is straightforward — pure bitumen dissolves completely, so anything that refuses to dissolve is an impurity. A 99 percent requirement allows only one percent contamination.
8. Float Test
- Normally the consistency of bituminous material is measured by the penetration test or viscosity test.
- But for certain ranges of consistency these tests are not applicable, and the float test is used instead.
- It is a modified viscosity test adopted for small quantities of very viscous bituminous materials.
The float test result is the time in seconds required for a small plug of chilled bitumen, held in an open mould attached to the bottom of a saucer, to become sufficiently fluid when the saucer is floated in water at 50 °C.
The float test therefore fills a gap. Some materials are too stiff for the penetration needle to give a useful reading, and too thick to flow through a viscometer orifice — so a third method is needed.
9. Loss on Heating
- When a bituminous material is heated, it loses its volatiles and therefore hardens.
- The less the loss on heating, the better the bitumen.
Test Conditions
| Item | Value |
|---|---|
| Sample | 50 g of bitumen |
| Shelf rotation | 5 to 6 revolutions per minute |
| Duration | 5 hours |
| Oven temperature | 163 °C |
| Permissible loss | Not more than 1 percent loss in weight is desirable |
This is an accelerated ageing test. It compresses into five hours the hardening that would otherwise take place slowly during mixing, laying and early service.
10. Water Content Test
It is desirable that bitumen contains minimum water content, to prevent foaming of bitumen when it is heated above the boiling point of water.
Maximum water content should not exceed 0.2 percent by weight.
The danger is practical and immediate. Bitumen is heated well above 100 °C for mixing, so any water in it flashes to steam and the whole batch foams violently — dangerous for the workers and destructive to the mix.
All Tests and Their Key Values
| Test | Measures | Key Conditions and Values |
|---|---|---|
| Viscosity | Resistance to flow | η = Fd/AV; Poise (dynamic), Stokes (kinematic); film 20–50 microns |
| Ductility | Stretch before breaking | 27 °C, 5 cm/min, 1 sq cm briquette; min 50; ISI 75 cm for grades 45 and above |
| Penetration | Hardness | 27 °C, 100 g, 5 sec; unit 1/10 mm; grades 30/40, 60/70, 80/100 |
| Softening point | Solid-to-liquid temperature | Ring and Ball, 2.54 cm; 35–50 °C for 80/100; 5–10 °C above max atmospheric temp |
| Flash point | Momentary vapour flash | Safe heating limit = 50 °C below flash point |
| Fire point | Sustained burning | Burns at least 5 seconds |
| Specific gravity | Density ratio | Pycnometer at 27 °C; bitumen 0.97–1.02, tar 1.10–1.25 |
| Solubility | Purity | In trichloroethylene; minimum 99 % |
| Float test | Consistency of very viscous materials | Water at 50 °C; result in seconds |
| Loss on heating | Volatile loss and hardening | 50 g, 163 °C, 5 hours, 5–6 rpm; max 1 % loss |
| Water content | Foaming risk | Maximum 0.2 % by weight |
Quick Revision Notes
- Viscosity resists flow due to internal friction; dynamic in Poise, kinematic in Stokes.
- Efflux viscometers measure the time for a fixed volume to pass an orifice; Furol viscosity is the standardised test.
- Ductility: 27 °C, 5 cm/min, 1 sq cm briquette; range 5 to 100; minimum 50 commonly specified; ISI minimum 75 cm for grades 45 and above; waxy bitumen may be as low as 15.
- Penetration: 27 °C, 100 g load, 5 seconds, unit 1/10 mm. Higher number = softer bitumen. Not used for tars.
- Softening point: Ring and Ball test, ball falls 2.54 cm; 80/100 grade gives 35–50 °C; should be 5–10 °C above maximum atmospheric temperature to avoid bleeding.
- Flash point = momentary flash of vapour; fire point = burns at least 5 seconds. Safe heating limit = 50 °C below flash point.
- Specific gravity by pycnometer at 27 °C; bitumen 0.97–1.02, tar 1.10–1.25.
- Solubility in trichloroethylene measures purity; minimum 99 %.
- Float test used where penetration and viscosity tests do not apply; water at 50 °C.
- Loss on heating: 50 g, 163 °C, 5 hours, 5–6 rpm; not more than 1 %.
- Water content must not exceed 0.2 %, to prevent foaming.
Mistakes Students Commonly Make
- Reading penetration as millimetres. The unit is 1/10 mm, so 80/100 pen means 8 to 10 mm.
- Thinking a higher penetration means harder bitumen. It means softer.
- Confusing flash and fire point. Flash is a momentary flash of vapour; fire point requires at least 5 seconds of burning.
- Taking the safe heating limit as 50 °C below the fire point. It is below the flash point.
- Forgetting that ductility and penetration are both conducted at 27 °C.
- Giving 50 cm as the ISI ductility requirement. ISI recommends 75 cm for grades 45 and above; 50 is the commonly specified general minimum.
- Confusing the softening point rule. Bitumen should be 5–10 °C above maximum atmospheric temperature, not below.
- Swapping the specific gravity ranges of bitumen and tar. Tar is heavier.
- Using 100 °C as the loss on heating temperature. It is 163 °C.
Conclusion
Ten tests, each answering one question about the binder. How thick is it (viscosity), how far will it stretch (ductility), how hard is it (penetration), when does it soften (softening point), when is it dangerous to heat (flash and fire point), is it pure (solubility), how much does it age (loss on heating), and will it foam (water content). The two most heavily examined are penetration — remember the 1/10 mm unit — and softening point, with its 5 to 10 °C margin above maximum atmospheric temperature that prevents bleeding.
Frequently Asked Questions
What is viscosity of bitumen?
The property of bitumen which resists flow due to internal friction. Dynamic viscosity is measured in Poise and kinematic viscosity in Stokes.
What is the unit of penetration?
One tenth of a millimetre. A grade of 80/100 pen therefore corresponds to a penetration of 8 to 10 mm.
What are the conditions for the penetration test?
A standard blunt-pointed needle penetrating the sample at 27 °C, under a load of 100 g, applied for 5 seconds.
Does a higher penetration value mean harder or softer bitumen?
Softer. The needle sinks further into a softer binder, so grade 30/40 is harder than 80/100.
What are the conditions for the ductility test?
A standard briquette of 1 sq cm cross-section is stretched at 27 °C at a rate of pull of 5 cm per minute, and the distance stretched before breaking is the ductility.
How is the softening point measured?
By the Ring and Ball test. It is the temperature at which a standard steel ball placed on a bitumen layer in a standard ring passes through and touches a plate 2.54 cm below.
Why should the softening point be above the maximum atmospheric temperature?
To avoid bleeding. Bitumen should have a softening point 5 to 10 °C above the maximum atmospheric temperature, so it does not soften and rise to the surface in service.
What is the difference between flash point and fire point?
At the flash point, the vapour from bitumen catches fire momentarily in the form of a flash. At the fire point, the bitumen itself ignites and burns for at least 5 seconds.
What is the safe limit for heating bitumen?
Normally 50 °C below the flash point.
What does the solubility test indicate?
The purity of bitumen. Solubility in trichloroethylene should generally be a minimum of 99 percent.
When is the float test used?
For certain ranges of consistency where the penetration and viscosity tests are not applicable. It is a modified viscosity test for small quantities of very viscous bituminous materials, conducted in water at 50 °C.
Why must the water content of bitumen be limited?
To prevent foaming when the bitumen is heated above the boiling point of water. The maximum permitted is 0.2 percent by weight.