Prime Coat Materials

Historically, a prime coat has usually consisted of a spray-applied cutback bituminous material (e.g., MC-30) using a distributor truck for application. More recently, to enhance safety and reduce VOC emissions, emulsified asphalt specialty products (e.g., AEP, EAP,and PCE) have been successfully used. However, ordinary emulsified asphalt (e.g., SS-1,SS-1h, and MS-2) have also been mechanically mixed into the surface of base layers primarily because this is the most effective technique for minimizing VOCs. It should be noted that, these ordinary emulsified asphalts, merely sprayed onto the surface, will not adequately penetrate most compacted bases.

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.  

There are several environmental issues related to the use of prime and bond coat which are not solely related to the use of asphalt emulsions versus cutback asphalts. Numerous references were found stating that asphalt emulsions are replacing cutbacks due to environmental concerns. NCAT lists the following four reasons that asphalt emulsions should be used in lieu of cutbacks:

1. Environmental regulations. Emulsions are relatively pollution free. Unlike cutback asphalts, there are relatively small amounts of volatiles to evaporate into the atmosphere other than water.

2. Loss of high energy products. When cutback asphalts cure, the diluents which are high energy, high price products are lost into the atmosphere.

3. Safety. Emulsions are safe to use. There is little danger of fire as compared to cutback asphalts, some of which have very low flash points.

4. Lower application temperature. Emulsions can be applied at relatively low temperatures compared to cutback asphalt, thus saving fuel costs. Emulsions can also be applied effectively to a damp pavement base, whereas dry conditions are required for cutback asphalts.

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.

Asphalt emulsions are typically used in place of cutback asphalts to eliminate VOC emissions. The use of cutback asphalt is regulated in many jurisdictions to help reduce VOC emissions. Prohibitions on the use of cutback, either permanently or during certain times of the year, are common in jurisdictions that have either reached, or are nearing non-attainment for the ozone requirements of the Clean Air Act.

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, emulsified naphthenic oils, and EcoPrime.  The results and effectiveness 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.