Category:408 Prime Coat

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408.1 Construction Inspection for Sec 408

This item is used for pervious surfaces such as earth, aggregate base or cement treated base over which a seal coat or bituminous mixture less than 3-3/4 in. thick is to be placed. Generally RC-70 or MC-30 liquid asphalt is used.

408.1.1 Description (Sec 408.1)

The purposes of a prime coat are to protect the base course from the weather and increase the bond between the base and the mat, preventing thin lifts from slipping. The base course is not primed if the thickness of the succeeding pavement is greater than or equal to 3-3/4 in.

408.1.2 Material (Sec 408.2)

See also Materials Inspection and EPG 1015 Bituminous Material.

408.1.3 Equipment (Sec 408.3)

See Distributor in Paving Equipment.

408.1.4 Construction Requirements (Sec 408.4)

See Spreading in Paving Operations.

Preparation of Surface (Sec 408.4.1)

The base course will absorb the prime more rapidly and completely if its surface is damp. However, excessive water must not be used and free water should not be standing on the surface of the base course.

Application (Sec 408.4.2)

If the prime coat is too heavy or too light, an inadequate bond is created and the mat may slip during compaction, which results in shoving and checking. In addition, a slippage failure (cracking) may occur after the pavement has been subjected to traffic. Bleeding may also occur if the prime coat is too heavy.

If the prime coat is too heavy, it will not be completely absorbed by the base course. Excess prime must be removed. If the prime coat is too light, it will have a light brown color and will be absorbed quickly and will not sufficiently penetrate into the base.

The prime coat must be uniformly applied to the base course at the rate specified in the contract. The application rate may need to be adjusted in the field depending on the nature of the base course. The type of material, amount of fines, moisture content, and surface texture of the base course are all factors that affect the application rate. The anticipated weather conditions should also be considered. Adjusting the application rate is acceptable as long as approval is obtained from the RE and the rate is within the range given in Standard Specification Section 408.4.2.

Sec 408.4.2.1 The spray bar on the distributor must be set at a height that results in no overlapping of the material.
408.4.2.2 The prime coat, including any repairs that have been made to the prime coat, must be allowed to cure prior to spreading mix. The prime has cured when it has hardened and penetrated into the base course to the extent that it cannot be picked up.

408.1.5 Method of Measurement (Sec 408.5)

If the prime coat consists of a cutback (Type RC or MC liquid asphalt), the quantity to be paid for will be determined in accordance with Standard Specification Section 1015.7.1. The application temperature (rounded to the nearest 5°F) is found in Table 24b of ASTM D1250 and the corresponding correction factor is multiplied by the total gallons of bituminous material used. Assume that the specific gravity of the material is above 0.966. The quantity will be rounded to the nearest 10 gallons.

If the prime coat consists of an emulsion, the quantity to be paid for will be determined in accordance with Standard Specification Section 1015.7.2 (see Method of Measurement in Tack Coat. The quantity will be rounded to the nearest 10 gallons.

408.2 Materials Inspection for Sec 408

Prime Coat material shall be sampled and inspected in accordance with EPG 1015.

408.3 Maintenance Priming

Maintenance priming consists of applying a light grade of cutback or emulsified asphalt to an aggregate surface to provide a bond for the wearing surface.

408.3.1 Purpose

Priming coats and bonds loose mineral particles on the surface of the aggregate base, hardens or toughens the surface of the aggregate base, waterproofs the surface of the aggregate base by plugging capillary or interconnected voids, or provides adhesion/bonding between the aggregate base and the asphalt mixture.

408.3.2 Application Rates

For cutback asphalts, the application rate is 0.4 to 0.5 gal/yd2. If the surface is smooth and tight 0.4 gal. is usually sufficient. Emulsions used for priming almost always require dilution with water. The dilution rates range from 1:1 to 10:1 (water to emulsion). The dilution rate depends on the base material characteristics and method of treatment. The application rates can vary for a 1:1 dilution 0.5 gal/yd2 for high fines and tight bases up to 1.5 gal/yd2 for loose sands and very porous surfaces.

On all work, only one-half the roadway should be primed at one time and it must be allowed to cure before the other half is primed. The width of prime applied on an aggregate surface may be 6 in. to 12 in. wider on each side than the desired wearing surface. After prolonged periods of dry weather a light application of water to the surface will aid in the penetration of the prime. If penetration of the prime is extremely slow due to wet or cold weather, it may be necessary to light blade a small amount of blotter material over the prime to absorb the excess oil. This should be done with care and should not include soil. If no material is available on the roadway edge, it may be applied lightly with truck spreader.

408.3.3 Products

Asphalt emulsions commonly used for priming are SS-1, SS-1h, CSS-1 and CSS-1h. Cutback asphalt used for priming are usually a low viscosity, medium or rapid curing (MC or RC) cutback asphalt such as SC-70 or MC 30. In general cutbacks may work better than emulsions for priming due to the ability of the cutback’s ability to penetrate and soften the asphaltic surface. Refer to Standard Specification Section 408 for additional information regarding prime coats.