Prime Coat vs Tack Coat: What's the Difference?
Two different bituminous applications, used at two different stages
Prime coat: on the granular base
A prime coat is liquid asphalt sprayed onto an untreated granular base before the first layer of hot-mix asphalt (HMA). FHWA's guidelines on prime and tack coats give its major purpose as protecting the layers below from wet weather with a temporary waterproofing layer; it also binds the surface fines together and promotes bond to the asphalt above.
To work, prime has to soak into the base. Medium-cure cutback asphalts are normally used because they penetrate deeper than ordinary emulsions; emulsions generally have to be mixed into the base. Prime must cure completely before paving, and prime that hasn't soaked in after 24 hours is blotted with sand and removed.
Tack coat: between asphalt layers
A tack coat is a thin coat of asphalt, usually a diluted slow-set emulsion, sprayed onto an existing asphalt or concrete surface before the next layer goes down. Its purpose is to bond the existing surface to the new one. Losing that bond can cause crescent-shaped slippage cracks or debonding and shorten pavement life.
It should cover the whole surface in a thin, even film, including the vertical faces of joints. Too little can cause debonding; too much can cause slippage.
Prime coat vs tack coat at a glance
The table sets the two side by side.
| Prime coat | Tack coat | |
|---|---|---|
| Applied to | Untreated granular base | Existing asphalt or concrete surface, before the next layer |
| Main purpose | Temporary waterproofing; binds surface fines; promotes bond | Bond between the existing and new layers |
| Usual material | Medium-cure cutback (emulsions need mixing in) | Diluted slow-set emulsion |
| Rate usually quoted as | Volume of product delivered per area | Often residual asphalt per area — check the specification |
| When it may be left out | No wet weather expected and the base covered within seven days; may not be needed under asphalt thicker than 4 in | Has been left out successfully between new lifts that are still clean and tacky |
Application rates: delivered vs residual
Rates are given in gallons per square yard, but not always on the same basis. A delivered (or 'shot') rate is the volume of material sprayed. A residual rate is the asphalt left once the water or solvent has gone. FHWA converts between them as: application rate = specified residual rate ÷ residual asphalt content of the material, after allowing for any dilution.
For prime, FHWA reports Asphalt Institute rates of 0.2–0.5 gal/yd² for medium-cure cutbacks and 0.1–0.3 gal/yd² per inch of depth for emulsions, and notes that most agencies specify prime by the volume of delivered product. No more prime should go down than the base can absorb in 24 hours.
For tack, FHWA's tack-coat checklist gives residual rates by surface. Dense surfaces need less than open-textured, raveled or milled ones.
| Surface | Residual tack rate (gal/yd²) |
|---|---|
| New asphalt | 0.020–0.045 |
| Existing asphalt | 0.040–0.070 |
| Milled surface | 0.040–0.080 |
| Portland cement concrete | 0.030–0.050 |
Why the distinction matters for estimating
Prime and tack go on different surfaces at different rates, so they are calculated and ordered separately: prime over the base area, tack over each asphalt or concrete surface that gets another layer. In a pavement with several lifts, the tack area can exceed the prime area.
The Bituminous Prime & Tack Coat Calculator multiplies the area by the rate you enter, so the total is on the same basis as that rate: a delivered rate gives delivered gallons, and a residual rate gives residual asphalt.
FAQ
- What is the difference between a prime coat and a tack coat?
- Prime coat goes on an untreated granular base and soaks in, mainly to protect it from wet weather and help the first asphalt layer bond. Tack coat is a thin film on an existing asphalt or concrete surface that bonds it to the next layer.
- Is a prime coat always required?
- No. FHWA notes prime is often left out when no wet weather is expected and the base will be covered within seven days, may not be needed under asphalt thicker than 4 in, and should not be applied to stabilized bases. Your agency's specification decides.
- What does residual application rate mean?
- It is the amount of asphalt left on the surface after the water in an emulsion, or the solvent in a cutback, has gone. The sprayed rate is higher, depending on the material's residual asphalt content and any dilution.
Related Calculators
Related Guides
From the Blog
Sources
FHWA-CFL/TD-05-002, Guidelines for Using Prime and Tack Coats
Purposes of prime and tack coat, materials, curing, when prime is omitted, prime application rates (Asphalt Institute) and the residual-rate conversion.
FHWA-CFL/TD-05-002, Guidelines for Using Prime and Tack Coats — referenceFHWA-HIF-19-039, Asphalt Emulsion-Based Tack Coat Checklist
Residual tack coat application rates by surface type, and accounting for dilution and water content.
FHWA-HIF-19-039, Asphalt Emulsion-Based Tack Coat Checklist — reference