Prime Coat

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There are two main variations of the Prime Coat process:

Sprayed Prime Coat - Cutbacks, Prime Asphalt Emulsions, Non-asphalt Emulsions

  1. The road base is prepared according to the design specifications
  2. Finish rolling is completed
  3. Free dust is swept off 
  4. Prime coat is applied at the rate of 0.20 gallons per square yard

Mixed Prime Coat - Mixing Grade Emulsions 

  1. The road base is prepared according to the design specifications
  2. The upper portion of the base is lightly scarified
  3. A mixing grade emulsion is applied
  4. The base is rolled

Waiting Period Before Paving

Following application of the prime material and prior to application of the succeeding layer of asphalt mixture, allow the prime coat to cure and water or volatiles to evaporate prior to overlaying.  This may take 24 hours for emulsion primes and 72 hours for cutback primes depending on site and weather conditions.

Prime coats need to be allowed to cure completely before covering with HMA or chip seal.

Cutbacks generally take longer to cure than asphalt emulsions.

Excess prime not absorbed into the base or cured should be absorbed with blotter sand and removed from the surface.


Equipment Variations

Most prime coats can be applied with just a spray distributor.  However with dense granular bases, or stabilized bases the surface may need to be broken up by scarifying before application, to ensure good penetration.  In this case, the surface should be prepared with a shallow scarification (1 - 3 inches) with equipment similar to a field cultivator or dirt track grooming attachment.  The primed base would need to be rolled again using a steel drum roller.

Material Variations

Prime Emulsions - EAP, AEP, ePrime, PEP, IEP

Prime Emulsions are emulsions that have been engineered specifically for use as prime coats.  They have properties that increase penetration into the base without all of the drawbacks of a traditional cutback.  There is not an universal nomenclature for these emulsions. Most large emulsion manufactures have developed their own prime emulsion products.  

Mixing Grade Emulsions SS-1, SS-1h, CSS-1, CSS-1h

SS and CSS emulsions are designed to work with fine aggregates to allow for maximum mixing time and extended workability.  They are the most stable emulsions and can be used  in dense-graded aggregate bases. SS and CSS emulsions can be diluted with water. 

For most applications, cationic and anionic emulsions will perform equally well. Anionic emulsions such as SS-1 may provide better adhesion to calcareous aggregates with pH less than 7, such as limestone. Cationic emulsions such as CSS-1 may provide better adhesion to siliceous aggregates with pH greater than 7, such as granite.


Non-Asphalt Emulsions

A wide variety of non-asphalt emulsion have been used including coal tar emulsion primes, polymer-modified coal tar emulsion, emulsified wood pitch and emulsified naphthenic oils.  The results and effectiveness of these products have not been widely evaluated.


Cutbacks - MC-30, MC-70, RC-250

A cutback is an asphalt cement that has been diluted with a cutter stock to make it more workable as a construction material.  Cutbacks have been the traditional choice for prime coats; however due to environmental concerns, cutbacks have been restricted in some states and dramatically reduced in others.  

Common Combinations

One of the main reasons to place prime coat is to provide some protection to the base until the pavement can be finished.  In some cases the project may have to accept traffic for an extended period of time.  In this case, the prime coat (applied at 0.17 to 0.20 gallons per square yard) may be covered with a fine aggregate (equivalent to Texas Grade 5). This is effectively a chipseal with a very small aggregate.  This process is called an "inverted prime" or a "covered prime." 

Gray (1982) found that the use of emulsified asphalt instead of cutback asphalt for a covered prime or inverted prime saved significant money and yet exhibited good performance.