A bitumen tack coat is a thin sprayed film of bitumen, usually a bitumen emulsion, applied to an existing road surface just before a new asphalt layer is laid on it. Its job is to glue the two layers together so that the pavement behaves as one structure. A prime coat is different: it is sprayed onto an unbound granular base, before the first asphalt layer, and is meant to soak into the stone.
Both use sprayed liquid bitumen, which is why they are confused. They use different materials, go on different surfaces and are specified at different rates.
What does a bitumen tack coat do?
The US Federal Highway Administration (FHWA) defines a tack coat as a sprayed application of asphalt binder on an existing asphalt or Portland cement concrete surface before an overlay, or between layers of new asphalt. ("Asphalt binder" is the American term for bitumen.)
A pavement is designed on the assumption that its layers work together. If the bond between layers fails, the top layer can slip and shove under braking traffic, and the pavement cracks from fatigue earlier than designed. The FHWA brief on tack coat practice cites research in which a complete loss of bond between layers was associated with a 60 to 75% loss of pavement life.
What does a prime coat do?
A prime coat is a low-viscosity bitumen sprayed on a prepared granular base course. The Ohio Department of Transportation describes it as a material intended to penetrate the granular surface and bind the particles together to some depth, which adds stability before the asphalt is placed. It also protects the finished base from rain and construction traffic until paving starts.
A prime coat only works if it penetrates. Ohio's guidance is that the application rate should be one the base absorbs within 24 hours. If the surface is tight and the prime sits on top as a film, it can cause the asphalt layer above to slip. Where traffic has to use a primed surface before the prime has soaked in, a light cover of fine aggregate is spread to blot the excess.
Practice on prime coats varies. Some road authorities require one on every granular base. Others omit it where the asphalt goes down soon after the base is finished, or where the pavement is thick. The contract specification decides.
Prime coat vs tack coat: what is the difference?
| Tack coat | Prime coat | |
|---|---|---|
| Surface it goes on | Existing asphalt, milled asphalt, concrete, or a fresh asphalt layer | Unbound granular base course |
| Purpose | Bond between two bound layers | Penetrate and bind the top of the base; protect it before paving |
| Should it soak in? | No. It stays on the surface as a thin film | Yes. It must penetrate |
| Usual materials | Slow-setting emulsions SS-1, SS-1h, CSS-1, CSS-1h; also rapid-setting grades and hot paving bitumen | Medium-curing cutbacks such as MC-30 and MC-70; diluted emulsions and emulsion primes |
| Ready for paving when | The emulsion has broken and set | The prime has been absorbed and cured |
| Main failure if wrong | Slippage, delamination, early fatigue cracking | Film left on the surface, slippage of the layer above |
Which materials are used for a bitumen emulsion tack coat?
Most tack coats are bitumen emulsions. The FHWA brief states that slow-setting emulsions, SS-1, SS-1h, CSS-1 and CSS-1h, are the most common tack coat materials, and that rapid-setting grades such as RS-1, RS-2, CRS-1 and CRS-2 are increasingly used. Hot paving-grade bitumen is also used as tack, because it has no water to evaporate.
An emulsion is bitumen dispersed in water. After spraying, two things happen:
- Break. The water separates from the bitumen and the film changes colour from brown to black.
- Set. All the water has evaporated and only the bitumen remains. This remaining bitumen is called the residual. It is typically 57 to 70% of an undiluted emulsion.
Slow-setting emulsions are sometimes diluted with water, most often at 1:1, to make it easier to spray a light, even film. The FHWA brief advises against dilution in the field, because the residual rate is then hard to control. If dilution is allowed it should be done by the supplier.
What does CSS-1h mean?
The US emulsion grade names are codes, read left to right:
- C means cationic: the bitumen droplets carry a positive charge. Grades without the C are anionic.
- SS means slow-setting. RS is rapid-setting and MS is medium-setting.
- 1 is the viscosity class of the emulsion. The trailing number marks relative viscosity, and 1 is the low-viscosity class.
- h means the emulsion is made from a harder base bitumen.
So CSS-1h is a cationic, slow-setting, low-viscosity emulsion with a hard residue. SS-1h is the anionic equivalent. A freshly sprayed tack coat can be picked up and tracked away on truck tyres, and the FHWA notes that emulsions are increasingly formulated with stiffer base bitumen to reduce this.
What materials are used for a prime coat?
The traditional prime is a medium-curing cutback, which is bitumen thinned with a petroleum solvent so that it is fluid enough to soak into the base. Ohio's specification lists MC-30, MC-70 and MC-250. The lower the number, the thinner the cutback, and thin grades such as MC-30 are preferred for dense bases that are hard to penetrate.
Cutbacks release their solvent to the air as they cure. Environmental rules have restricted them in many places, and emulsion-based primes, applied diluted, are used in their place.
Typical tack coat application rates
The FHWA brief reproduces the Asphalt Institute's recommended tack coat rates. They are residual rates: the bitumen left on the road after the water has gone, not the volume of emulsion sprayed.
| Surface | Residual rate, US gal/yd² | Residual rate, L/m² (converted) |
|---|---|---|
| New asphalt | 0.02 to 0.05 | 0.09 to 0.23 |
| Existing asphalt | 0.04 to 0.07 | 0.18 to 0.32 |
| Milled surface | 0.04 to 0.08 | 0.18 to 0.36 |
| Portland cement concrete | 0.03 to 0.05 | 0.14 to 0.23 |
Rougher and more worn surfaces need more. The Asphalt Institute encourages use of the upper end of each range.
The point that causes most disputes is the basis of the rate. On the Asphalt Institute's assumption of an emulsion that is 67% bitumen, a residual rate of 0.04 gal/yd² needs about 0.06 gal/yd² of undiluted emulsion, or 0.12 gal/yd² of emulsion diluted 1:1. A specification that says only "apply at 0.04" can be read three ways. The FHWA recommends that every rate be written as residual bitumen.
We have not quoted a prime coat rate here. Prime rates depend on how open the base is and are set by the road authority's specification and, often, a site trial.
What a buyer of tack coat emulsion should specify
- The grade (for example CSS-1h) and the standard it is to meet.
- Cationic or anionic, matched to the aggregate and the contractor's practice.
- The residue content, since that is the bitumen being paid for.
- Packing: drums, IBCs or bulk tank containers. Emulsion contains water and must be protected from freezing.
How bitumen emulsion is traded
Emulsions are made by specialist plants from paving-grade bitumen and are sold per tonne against a named grade and standard. Because the product is partly water, long sea voyages are less common than for bitumen, and many buyers import paving bitumen and emulsify it locally. Grades and packing are set out on our bitumen emulsion page, and the base material in What is bitumen?.
CBIL is a commodity brokerage company. We broker bitumen and bitumen emulsion between a named producer or seller and a named buyer, against independent inspection by SGS, Intertek or a similar company at loading.
Request a quote with the emulsion grade, standard, quantity, packing and destination port.