Prime Coat Quality Trouble

Poor Distributor Performance

The improper configuration of the distributor can lead to bond coat applications that are streaky or striped in appearance.  This is refered to as  “corn rows” or "Zebra Tack".  This does not produce good bond strengths. If this is observed in the field, it should be stopped until the spray pattern can be corrected to produce an uniform application.

Silty or High Clay Soils

The Hot-Mix Asphalt Handbook (2000) states there is no need to place a prime coat on a silty clay or clay subgrade soil because the low permeability of silty clay and clay soils would prevent absorption of the prime coat material.  The photo shows the lack of penetration of an asphalt emulsion prime placed on a silty clay subgrade.

Wrong Prime Material Viscosity

Adequate viscosity of the liquid asphalt is required for proper spray application. This is achieved by heating MC cutbacks and occasionally emulsions or diluting emulsions with water. The photo shows the results of applying prime at too high a viscosity.

Rain Before Cure

Both emulsions and cutbacks need time to cure before they are resistant to rain. A general rule of thumb is 48 hours, but this can change depending on humidity and temperature.  The photo shows the results of a rain incident on a prime before it had time to cure.