Bitumen is the material that turns a heap of stone into a road. It makes up only a few percent of a bituminous mix by volume, yet without it the aggregate would simply scatter under the first wheel that passed.
This post covers where bitumen comes from, the six properties road engineers look for, and its nine physical properties.
Why Bitumen Matters
Bituminous materials are widely used in road construction and maintenance. After experience was gained in using them to obtain a smooth riding surface, bituminous mixtures began to be used as structural layers as well.
These materials are considered flexible from a structural point of view.
That word flexible is the origin of the term “flexible pavement”. A bituminous layer bends under load and recovers, distributing the wheel load down to the layers beneath rather than bridging over them as a rigid concrete slab does.
Sources of Bitumen
| Source | Where Found |
|---|---|
| Petroleum crude | The most common source |
| Rock asphalt | Some parts of Europe |
| Lake asphalt | Trinidad |
The Indian Situation
Indian crude does not yield good bitumen suitable for road work, except Digboi bitumen in Assam. India therefore gets its entire bitumen through imported crude.
This single fact explains a great deal about Indian road practice.
Because bitumen must be imported, it is expensive in foreign exchange terms — and that is precisely why the Lucknow Plan listed the need to conserve bitumen among its criteria for selecting specifications, alongside the use of alternate binders and cement concrete pavements wherever economically feasible. When a material has to be bought from abroad, using less of it becomes national policy, not just good engineering.
Six Desirable Properties of Bitumen
(a) Good Affinity to Aggregates
Bitumen must stick to the stone and stay stuck. This is the other half of the adhesion problem discussed under aggregate properties — the aggregate must prefer bitumen to water, and the bitumen must have good affinity for the aggregate. If either fails, stripping follows.
(b) Fluid Enough to Coat All Particles
In a premix process, bitumen must be fluid enough to coat all particles of the aggregate. This is achieved by heating the bitumen and the aggregates too.
Note that both are heated. Pouring hot bitumen onto cold stone would chill it instantly and stop it flowing before it had coated anything.
(c) Sprayable to a Thin Film
In spraying work, bitumen should be capable of being sprayed as a thin film. This is achieved in one of three ways:
- Heating it
- Fluxing it — adding a solvent, producing cutback bitumen
- Emulsifying it — dispersing it in water, producing bituminous emulsion
These three methods are the entire basis of the next topic on grades of bitumen. Each is a different way of temporarily making bitumen thin enough to work with.
(d) Hardens on Cooling
When the binder cools to atmospheric temperature it should harden to hold the aggregates together.
Properties (b), (c) and (d) together describe a genuine engineering demand: bitumen must be fluid when we want to place it and solid when we want it to work. That is a contradiction resolved entirely by temperature.
(e) Low Temperature Susceptibility
Its susceptibility to change viscosity when temperature varies should be low. In particular, viscosity characteristics should be reasonably constant within the range of temperatures the road experiences.
Read this alongside property (d) and the balance becomes clear. We want bitumen to change with temperature — soft when hot for mixing, hard when cool for service. But within the everyday range a road actually experiences, we want it to stay stable. A binder that turns runny on a hot afternoon and brittle on a cold night will fail at both extremes.
(f) Durability
Bitumen should retain its properties over a long period. It should be a durable binder and should not lose its properties too soon.
Nine Physical Properties of Bitumen
| No. | Property |
|---|---|
| (a) | It is a viscous liquid, black or brown in colour |
| (b) | It consists predominantly of hydrocarbons derived from petroleum crude |
| (c) | It is soluble in carbon disulphide |
| (d) | It is insoluble in water |
| (e) | Its specific gravity is around 1.00 |
| (f) | It has water-proofing properties |
| (g) | It is thermoplastic — it becomes soft on heating and hard on cooling |
| (h) | It oxidises slowly |
| (i) | It is chemically inert |
The Three That Matter Most
Thermoplastic (g). This is the property that makes bitumen usable at all. Heat it and it softens; cool it and it hardens; and crucially, the change is reversible. The same material can be melted, mixed, laid, cooled and later reheated for maintenance. Contrast this with cement concrete, where setting is a chemical reaction that happens once and cannot be undone.
Insoluble in water and water-proofing (d and f). These two go together and explain why bitumen protects a pavement. Water cannot dissolve it and cannot pass through it, so a well-sealed bituminous surface keeps the sub-grade beneath it dry — which, as the soil topic showed, is the single most important thing you can do for a road’s foundation.
Oxidises slowly (h). Bitumen does not last forever. Over years, oxygen from the air reacts with it and it gradually hardens and grows brittle — a process called ageing. It is slow, which is why roads last a decade or more, but it is the reason property (f) in the desirable list demands durability.
The Solubility Point
Property (c) is more useful than it first appears. Solubility in carbon disulphide is one of the ways bitumen is distinguished from tar — bitumen dissolves in carbon disulphide and carbon tetrachloride, whereas tar dissolves only in toluene. Solubility tests also reveal the free carbon content, which is higher in tar.
Desirable Properties versus Physical Properties
Students sometimes merge these two lists. They are different in kind:
| Desirable Properties | Physical Properties | |
|---|---|---|
| What they describe | What we want bitumen to do for road work | What bitumen is, as a material |
| Number | Six | Nine |
| Example | Should have good affinity to aggregates | Specific gravity is around 1.00 |
Quick Revision Notes
- Bituminous materials are flexible from a structural point of view.
- Most common source is petroleum crude; also found as rock asphalt in Europe and lake asphalt in Trinidad.
- Indian crude does not yield good road bitumen except Digboi bitumen in Assam; India relies on imported crude.
- Six desirable properties: good affinity to aggregates; fluid enough to coat particles; sprayable as a thin film; hardens on cooling; low temperature susceptibility; durable over a long period.
- Thin films are achieved by heating, fluxing or emulsifying.
- Nine physical properties: viscous black or brown liquid; predominantly hydrocarbons from petroleum crude; soluble in carbon disulphide; insoluble in water; specific gravity around 1.00; water-proofing; thermoplastic; oxidises slowly; chemically inert.
- Thermoplastic means soft on heating, hard on cooling — and the change is reversible.
Mistakes Students Commonly Make
- Saying India produces its own bitumen. Only Digboi in Assam yields suitable bitumen; the rest comes from imported crude.
- Confusing the six desirable properties with the nine physical properties.
- Stating that bitumen should have high temperature susceptibility. It should be low, so viscosity stays reasonably constant in service.
- Forgetting that both bitumen and aggregate are heated in a premix process.
- Saying bitumen is soluble in water. It is insoluble in water and soluble in carbon disulphide.
- Giving the specific gravity as much above or below 1.00. It is around 1.00.
- Overlooking that oxidation is what eventually ages and embrittles a bituminous pavement.
Conclusion
Bitumen has to perform a contradiction — flow freely enough to coat every stone during construction, then set hard enough to hold those stones for years, while resisting water, weather and slow oxidation throughout. Thermoplasticity is what makes that contradiction possible, since temperature alone switches the material between its two states. And because India imports almost all of it, conserving bitumen has been an explicit aim of national road policy since the Lucknow Plan.
Frequently Asked Questions
What is the most common source of bitumen?
Petroleum crude. It is also found as rock asphalt in some parts of Europe and as lake asphalt in Trinidad.
Does India produce bitumen from its own crude?
Indian crude does not yield good bitumen suitable for road work, with the exception of Digboi bitumen in Assam. India therefore obtains its entire bitumen supply through imported crude.
What are the desirable properties of bitumen?
Good affinity to aggregates; sufficient fluidity to coat all aggregate particles in a premix process; the ability to be sprayed as a thin film; hardening on cooling to hold the aggregates; low susceptibility of viscosity to temperature change; and durability over a long period.
How is bitumen made thin enough to spray?
By heating it, by fluxing it with a solvent, or by emulsifying it.
Why should temperature susceptibility be low?
Because viscosity should remain reasonably constant within the range of temperatures a road actually experiences, so the binder does not become runny in hot weather or brittle in cold.
What does thermoplastic mean?
That bitumen becomes soft on heating and hard again on cooling, and that the change is reversible.
What is the specific gravity of bitumen?
Around 1.00.
Is bitumen soluble in water?
No. Bitumen is insoluble in water and has water-proofing properties. It is soluble in carbon disulphide.
Why does bitumen age?
Because it oxidises slowly, gradually hardening and becoming more brittle over the service life of the pavement.
Why is bitumen conservation a national concern in India?
Because almost all of it is produced from imported crude, so reducing consumption saves foreign exchange. This is why bitumen conservation appears among the Lucknow Plan criteria for selecting road specifications.