Category:408 Prime Coat

408 Prime Coat 408.1 Description. This is not a contract document and is to be used solely as a commentary on Standard Specification Section 408. There will be no commentary for sections in which the intent of the specification, as written, is clear. 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 4 in. 408.2 Material. See also Field Sections 408 and 1015 of the Materials Manual. 408.3 Equipment. See also GCM Section 400.5.1 “Distributor.” on page 17. 408.4 Construction Requirements. See also GCM Section 400.7.4 “Spreading.” on page 26. 408.4.1 Preparation of Surface. 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. 408.4.2 Application. 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 Gc = 154.5 Gallons Gc = 150 Gallons General Construction Manual © Missouri Department of Transportation 2006 400-83 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. 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. 408.5 Method of Measurement. 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 GCM Section 407.5 “Method of Measurement.” on page 81). The quantity will be rounded to the nearest 10 gallons. 408.6 Basis of Payment.