Bitumen is a solid at road temperature, which is a problem — you cannot spread a solid over a road surface. Every grade covered in this post is an answer to the same question: how do we make bitumen workable, and which method suits which job?
There are three answers. Heat it, dissolve it in a solvent, or disperse it in water. This post covers all three, plus tar, plus the IRC selection guidelines.
IRC Guidelines for Selecting a Bitumen Grade
| Type of Bitumen | Use |
|---|---|
| Penetration grade 30/40 | Hot-mix work in areas where the difference between maximum and minimum temperature is less than 25 °C, and on roads with high volume of traffic — expressways, urban roads and factory roads |
| Penetration grade 60/70 | Hot-mix work for bituminous macadam and bituminous concrete for superior roads with high traffic and normal summer temperatures |
| Penetration grade 80/100 | (i) Surface dressing (ii) Premix works in high altitudes (iii) Premix works on roads with less traffic intensity |
| Cutback bitumen | (i) Surface dressing in cold weather (ii) Premix in cold weather (iii) MC and SC cutbacks are used for priming |
| Emulsion | (i) Surface dressing in cold weather, wet conditions and maintenance works (ii) Premix works in wet weather (except dense and semi-dense carpet and maintenance works) (iii) Priming |
Reading the Pattern
Two variables run through the whole table — temperature and traffic.
Harder grades for heavier traffic and hotter conditions. Grade 30/40 is the hardest and goes to expressways and urban roads. Grade 80/100 is the softest and goes to high altitudes (where it is cold, so a soft binder stays flexible) and to lightly trafficked roads.
Cutbacks and emulsions for cold and wet conditions. Notice that both are specified for cold weather, and emulsion additionally for wet conditions. This is because hot-mix work needs heat, and heat is exactly what you cannot maintain in cold or wet weather. Cutbacks and emulsions get around the problem by making bitumen fluid without heating.
Cutback Bitumen
The viscosity of bitumen is reduced by a volatile diluent.
The principle is simple. Dissolve the bitumen in a solvent and it becomes liquid at ordinary temperature. After it is laid, the solvent evaporates and the bitumen is left behind, hard again. The solvent is a temporary passenger.
Three Types
- Rapid Curing (RC)
- Medium Curing (MC)
- Slow Curing (SC)
“Curing” here means the evaporation of the solvent. How fast that happens depends entirely on how volatile the solvent is.
| Type | Solvent Used | Notes |
|---|---|---|
| RC | Petroleum solvents such as naphtha or gasoline | Penetration value 80 to 120 |
| MC | Kerosene and light diesel oil | Have good wetting properties |
| SC | High boiling point gas oil, or obtained by controlling the rate of flow and temperature of crude during the first cycle of refining | Slowest to cure |
Line up the solvents and the logic is obvious. Gasoline evaporates fastest, kerosene slower, gas oil slowest of all — which is exactly why they produce rapid, medium and slow curing cutbacks respectively.
The Numbering System
Cutbacks are designated by numerals representing progressively thicker or more viscous cutback.
RC-2 is thicker than RC-1.
But RC-2, MC-2 and SC-2 all have the same viscosity.
This is an elegant system worth understanding. The letter tells you how fast it cures; the number tells you how thick it is. The two are independent, so you can pick the curing speed and the consistency separately.
Solvent Content
| Grade | Solvent | Bitumen |
|---|---|---|
| RC-0 and SC-0 | 45 % | 55 % |
| RC-5 and MC-5 | 15 % | 85 % |
The lower the number, the more solvent and the thinner the cutback. Grade 0 is nearly half solvent; grade 5 contains only 15 percent.
Bituminous Emulsion
An emulsion is a two phase system consisting of two immiscible liquids.
Here the two liquids are bitumen and water — which do not mix, since bitumen is insoluble in water. In an emulsion, tiny globules of bitumen are held suspended in water.
| Property | Value |
|---|---|
| Bitumen or tar content | 40 to 60 %, the remainder being water |
| Average globule diameter | About 2 microns |
Uses and the Key Advantage
- Emulsions are used especially in maintenance and patch repair works.
- The main advantage is that emulsion can be used in wet weather, even when it is raining.
- Emulsions can be used for soil stabilisation in deserts.
The wet-weather advantage follows directly from the composition. An emulsion is already mostly water, so rain cannot spoil it — there is nothing for the water to displace. Compare this with hot bitumen, which a shower would chill and a wet surface would prevent from sticking at all.
Once laid, the water evaporates or drains away, the bitumen globules coalesce, and a continuous film is left behind. This process is called breaking.
Cutback versus Emulsion
| Cutback Bitumen | Emulsion | |
|---|---|---|
| Bitumen made fluid by | Dissolving in a volatile solvent | Dispersing as globules in water |
| Second phase | Petroleum solvent | Water |
| Bitumen content | 55 to 85 % | 40 to 60 % |
| Works in wet weather | No — cold weather only | Yes, even while raining |
| Types | RC, MC, SC | — |
Tar
Production in Three Stages
- Carbonisation of coal to produce crude tar
- Refining or distillation of the crude tar
- Blending of the distillation residue with distillate oil fractions to give the desired road tar
Road Tar Grades
| Grade | Viscosity | Use |
|---|---|---|
| RT-1 | Lowest | Surface painting |
| RT-4 | Higher | Premix in macadam |
| RT-5 | Highest | Grouting |
The numbering runs the intuitive way here — higher number means higher viscosity, from thin painting grade RT-1 up to thick grouting grade RT-5.
Tar and Bitumen Compared
| Aspect | Bitumen | Tar |
|---|---|---|
| Origin | A petroleum product | Produced by destructive distillation of coal or wood |
| Solubility | Soluble in carbon disulphide and carbon tetrachloride | Soluble in toluene only |
| Water resistance | More resistant to water | Less resistant |
| Temperature susceptibility | Lower | More temperature susceptible, so viscosity varies greatly with temperature |
| Free carbon content | Lower | Higher, as seen from the solubility test |
| Specific gravity | 0.97 to 1.02 | 1.10 to 1.25 |
| Penetration test | Used for grading | Not used — tars are soft |
Why the Solubility Difference Matters
The solubility row explains the free carbon row. Tar contains more free carbon, and free carbon does not dissolve — so when a tar sample is tested for solubility, a larger insoluble residue remains. The solubility test is therefore how the free carbon content is revealed.
On the whole, bitumen outperforms tar on every count relevant to road work: better water resistance, more stable viscosity, less free carbon. That is why bitumen dominates modern road construction.
Quick Revision Notes
- 30/40 — hot mix where max–min temperature difference is less than 25 °C, high traffic roads.
- 60/70 — bituminous macadam and bituminous concrete, high traffic, normal summer temperatures.
- 80/100 — surface dressing, premix at high altitudes, premix on roads with less traffic.
- Cutback — surface dressing and premix in cold weather; MC and SC used for priming.
- Emulsion — surface dressing in cold and wet conditions, premix in wet weather, and priming.
- Cutback = bitumen viscosity reduced by a volatile diluent; three types RC, MC, SC.
- RC uses naphtha or gasoline (penetration 80–120); MC uses kerosene and light diesel oil (good wetting); SC uses high boiling point gas oil.
- RC-2 is thicker than RC-1, but RC-2, MC-2 and SC-2 have the same viscosity.
- RC-0 and SC-0: 45 % solvent, 55 % bitumen. RC-5 and MC-5: 15 % solvent, 85 % bitumen.
- Emulsion = two phase system of two immiscible liquids; bitumen 40–60 %, rest water; globules about 2 microns.
- Emulsion’s main advantage: usable in wet weather, even while raining; also used for soil stabilisation in deserts.
- Tar produced in three stages: carbonisation, refining or distillation, blending.
- RT-1 lowest viscosity for surface painting; RT-5 highest for grouting.
- Bitumen is a petroleum product; tar comes from destructive distillation of coal or wood.
Mistakes Students Commonly Make
- Thinking RC-2, MC-2 and SC-2 differ in thickness. They have the same viscosity; the letter indicates curing speed only.
- Assuming a higher cutback number means more solvent. It means thicker, so less solvent.
- Saying cutback bitumen can be used in wet weather. That is emulsion; cutbacks are for cold weather.
- Forgetting that MC and SC, not RC, are used for priming.
- Mixing up which grade suits high altitude. 80/100 — the softer grade — goes to high altitudes.
- Saying bitumen is made from coal. Bitumen is a petroleum product; tar comes from coal or wood.
- Swapping the solubility solvents. Bitumen: carbon disulphide and carbon tetrachloride. Tar: toluene only.
- Saying tar is less temperature susceptible. Tar is more susceptible, with greater viscosity variation.
Conclusion
Every grade in this topic answers the same problem in a different way. Penetration grades are used hot, and the harder ones go where traffic is heavy and temperatures are stable. Cutbacks dissolve bitumen in a solvent so it can be used cold, with rapid, medium or slow curing depending on how volatile the solvent is. Emulsions suspend bitumen in water, which is what allows work to continue in the rain. And tar, made from coal rather than petroleum, loses to bitumen on water resistance, temperature stability and purity — which is why bitumen is the standard binder today.
Frequently Asked Questions
Which bitumen grade is used for expressways and urban roads?
Penetration grade 30/40, for hot-mix work in areas where the difference between maximum and minimum temperature is less than 25 °C and traffic volume is high.
Which grade is used at high altitudes?
Penetration grade 80/100, which is also used for surface dressing and for premix works on roads with less traffic intensity.
What is cutback bitumen?
Bitumen whose viscosity has been reduced by a volatile diluent, so that it can be used without heating. The solvent evaporates after laying, leaving the bitumen behind.
What are the three types of cutback bitumen?
Rapid Curing (RC), Medium Curing (MC) and Slow Curing (SC).
What solvents are used in each type of cutback?
RC uses petroleum solvents such as naphtha or gasoline; MC uses kerosene and light diesel oil; SC uses high boiling point gas oil, or is obtained by controlling the flow rate and temperature of crude during the first refining cycle.
What does the number in a cutback designation mean?
It represents progressively thicker or more viscous cutback. RC-2 is thicker than RC-1, but RC-2, MC-2 and SC-2 all have the same viscosity.
Which cutbacks are used for priming?
MC and SC cutbacks.
What is a bituminous emulsion?
A two phase system consisting of two immiscible liquids, in which bitumen globules of about 2 microns are dispersed in water. Bitumen or tar content ranges from 40 to 60 percent.
What is the main advantage of emulsion?
That it can be used in wet weather, even when it is raining, which makes it valuable for maintenance and patch repair work.
How is tar produced?
In three stages — carbonisation of coal to produce crude tar, refining or distillation of the crude tar, and blending of the distillation residue with distillate oil fractions to give the desired road tar.
What are the main differences between tar and bitumen?
Bitumen is a petroleum product while tar comes from destructive distillation of coal or wood. Bitumen dissolves in carbon disulphide and carbon tetrachloride, tar only in toluene. Bitumen is more resistant to water, less temperature susceptible, and contains less free carbon.