How to Spray Prime Coat on the Surface of Aggregate Base Course?

Prime coat is an application of a low viscosity asphalt to a granular base in preparation for an initial layer (or surface course layer) of asphalt. The purpose of the prime coat is; to coat and bond loose material particles on the surface of the base, to harden or toughen the base surface to provide a work platform for construction equipment, to plug capillary voids in the base course surface to prevent migration of moisture, and to provide adhesion between the base course and succeeding asphalt course. After applying the prime coat, it must cure for a minimum of 48-72 hours before asphalt is placed, with no rain in the forecast.

There are four primary purposes for the application of a prime coat on an aggregate base course;
1-Coat and bond loose material particles on the surface of the base.
2-Harden or toughen the base surface to provide a work platform for construction equipment.
3-Plug capillary voids in the base course surface to prevent migration of moisture.
4-Provide adhesion between the base course and succeeding asphalt course.

MATERIALS FOR PRIME COAT
(a) The materials for Prime Coat shall be either AC-20 grade asphalt cement (which is approximately equivalent to 60/70 Pen.) diluted with kerosene or medium curing cut-back asphalt grade MC-70.
(b) The AC-20 grade asphalt cement shall conform to AASHTO M226 standard for viscosity graded asphalt cement. The MC-70 shall conform to AASHTO M 82 for medium curing cut-back asphalt.
(c) When AC-20 is to be diluted with or as otherwise approved by the Engineer.

RATE OF APPLICATION
The Contractor shall carry out field trials under the supervision of the Engineer to establish the appropriate application rate and such trials will be repeated, as directed by the Engineer, whenever the type of bituminous material or the type of
surface changes. The rate of application shall be within the range 0.45 – 2.25 liters/m2 and/or as otherwise approved by the Engineer.

PRECAUTIONS
Extreme care should be used when heating any cut back asphalt cement. Open flames or sparks must not be permitted close to these materials. Controlled heat should be applied in heating kettles, mixers, distributors, or other equipment
designed and approved for the purpose. Open flames must not be used to inspect or examine drums, tank cars or other containers in which these materials have been stored. All vehicles transporting these materials must be properly vented. Only experienced personnel must be permitted to supervise the handling of these
materials.

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