Prep Coat Primer

Optimum Base Conditions

Optimum conditions that a base should have before a sprayed on prime is applied. As observed by Senadheera and Vignarajah ( TEXDOT 2007)

1. A Reasonably Smooth Surface - Not too smooth as that can make it difficult for the prime to penetrate - no slush rolling
2. Reasonable Prorosity - this is best achieved by blading and rolling the base at or slighly above optium moisture content. 
3. No Loose Dust on the Surface - brooming must be done carefully to avoid distrubing larger aggregate particles at the surface of the compacted base. if the base is too fragile for aggressive brooming, compressed air can used.
4. Adequate Structural Strength- The base should be adequately, but only partially, cured such that is can support construction traffic.

Application Rates

Prime coat materials are either cutback asphalts or emulsified asphalts, which are diluted or cut or with a petroleum solvent or emulsified with water, respectively. Further dilution is usually not required. Therefore, most agencies specify application rates based on the volume of the delivered product per unit area. Confusion rarely occurs unless the prime is diluted further to aid in application. To avoid confusion, the Hot-Mix Asphalt Paving Handbook 2000 (5) recommends application rates be based on residual asphalt content. The shot rate or application rate to achievethe specified residual asphalt content can be determined using the following formula:

AR = RAR / RAC [1]

Where: AR = application or shot rate of undiluted prime

RAR = specified residual application rate

RAC = residual asphalt content of prime

There is good agreement in the literature on application rates. The Asphalt Institute recommends application rates of 0.9 to 2.3 L/m2 (0.2 to 0.5 gal/yd2) for MC cutbacks and 0.5 to 1.4 L/m2 per 25 mm of depth (0.1 to 0.3 gal/yd2 /in depth) for asphalt emulsions. Others recommend from 0.65 L/m2 to 2.0 L/m2 (0.15 to 0.45 gal/yd2). Application rates should vary based on the openness of the base and no more prime should be placed than can be absorbed by the granular base in 24 hours. Any excess should be removed with blotter sand.

Prime Coat Curing

Prime coat must cure completely to function properly. Prime coats generally take several days to properly cure so they can withstand construction traffic.

• The curing of prime coat depends upon the weather. If the weather is hot, the prime coat will cure quickly but if the weather is cool and damp the prime coat will cure slowly.
• Emulsified products generally cure faster than cutback asphalts. Asphalt emulsions require a minimum of 24 hours to fully cure. Cutbacks require a minimum of 72 hours to fully cure.
• It is riskier to place an HMA layer over an uncured prime coat than an unprimed base, because the uncured prime can cause more base movement than construction on an unprimed base. Excessive prime remaining on the surface can be absorbed into overlying asphalt layers and the solvents in the prime used to liquefy the asphalt, typically kerosene or diesel fuel, can damage the asphalt layer quickly, contributing to pavement slippage or rutting and lateral movement of the asphalt concrete during rolling operations.
• At a minimum, construction traffic should be kept off a fresh prime coat until cured sufficiently to prevent tracking and rutting of the prime.

Blotter Sand

Any excess prime that is not absorbed into the aggregate base course after 24 hours should be removed with blotter sand to prevent wash off into waterways and tracking and pickup of the material by traffic.

• Recommended blotter sand application rates are 2.2 to 4.4 kg/m2 (4 to 8 lbs/yd2).
• Blotter sand should be applied using a mechanical devise such as a salt or chip spreader. Dumping blotter sand with a loader and spreading with a shovel should be avoided.
• Excess blotter sand should be broomed from the surface before HMA placement to ensure a proper bond.