Alternatives that greatly reduce the release of VOCs are allowed on a limited basis, but the Texas Natural Resource Conservation Commission has begun a program to lift some restrictions on prime coat alternatives that pass specific environmental tests. However, not many alternatives exist that release less VOCs and are proven to be cost-effective and efficient. Engineers need a prime coat that protects the base course from weather and light traffic damage. First, they measured the effectiveness of prime coats by developing tests to measure the penetration capabilities of prime coat materials, investigate the benefits of using prime coats to bond the surface particles of a base material together, and compare the strength of the bond between surface courses and bases with and without different prime coats. TTI researchers next measured the ability of various emulsified asphalt products and emulsified oils containing little or no asphalt to penetrate into the base material.
Emulsified asphalt is a mixture of finely ground asphalt suspended in water by stabilizing agents.
For prime coating to work, it must penetrate into the top layer of the base course to a depth between 5 and 10 mm. Tests showed that just spraying an emulsified asphalt prime coat onto a base material did not provide adequate penetration. However, mixing the emulsified asphalt into the top two inches of the base material before compacting it ensures the asphalt is distributed into the base course to an adequate depth. The stabilized layer that results from mixing the aggregate and prime coat contributes significantly to the strength of the pavement. Using emulsified rather than cutback asphalt prevents the release of VOCs into the air.