Primate Coat – Meaning – Thickness – Temperature – Tack coat

So, grab a cup of coffee and read on.

Before I tell you the exact details about prime coat let me put up an example to help you in understanding what really the need of prime coat is.

We lay flexible asphaltic pavements in layers in which asphaltic wearing and base course is laid on top of the crushed aggregate. You can also call this base course as a granular aggregate layer or water bound macadam. This layer of aggregate has capillaries in there so it is very important that any underground water seepage may not go up to damage asphaltic layer.

So, let’s now define what prime coat is.

Definition – Meaning

A prime coat is a low viscous bituminous layer of cutback or emulsified asphalt that is applied on a prepared layer of granular base. A prime coat is also termed as initial layer of surface course layer of asphalt that tends to coat and bond the loose surface material of the base.  

Purpose

The purpose of applying prime coat is:

·         It provides a tough and tight surface for the asphalt layer by filling and penetrating into voids of the aggregate base.

·         It provides a water proofing layer underneath the asphalt coat to prevent any deterioration from water or capillary action.

·         It enhances the bond between the aggregate base and the asphalt binder layer.

·         It ensures lesser damage to the compacted base course, in case the laying of asphalt is delayed.

Curing of prime coat

The prime coat requires apt amount of curing period to ensure sufficient penetration into the aggregate base and to be effective. Although no clear guideline is present about curing time but the minimum is 24 hours. But the curing time depends on the type of prime coat, the weather conditions, and type of base layer. Before applying the coat, the site manager must be aware of the weather conditions to avoid any problem.

In case you use cutback asphalt (asphalt cement by combining it with light petroleum oil) the curing time is more as it hardens when the light oils evaporate into the atmosphere. The other type is emulsified asphalt that is made by liquefying by suspending it in water. The curing time is lesser for emulsified asphalt. For cutback it is 24 to 72 hours but for emulsified it is 24 hours. Similarly, the curing time will be more in cold weather areas and will lesser for hot weather areas.   

Excessive Prime Coat – What to do?

In case you’ve applied excessive prime coat on the granular base, you must have to remove the excessive coat otherwise it can cause base movement, rutting and slippage. To remove the excessive prime coat, apply blotter sand on the top and remove the sand afterwards.

When to omit prime coat?

The prime coat can be omitted in some cases. According to Hot-mix asphalt handbook 2000, the purpose of using a prime coat is to protect the underlying material from moisture in wet conditions. If the underlying material can be protected from rainfall, the prime coat will not be required. The prime coat may not be allowed in cold weather because the curing time would be long. According to the asphalt institute, a prime coat should be used when the HMA layer is less than 100 mm. Erdminger, in his guideline, suggested that the using of the prime coat is not necessary when the HMA is thicker than 100 mm.

Specification

For priming hard, dense, and difficult to penetrate bases low viscosity medium curing cutbacks (MC) are used. In general, MC-30 or MC-70 used. Higher viscosity MC grades or low viscosity rapid curing RC may used for the surface that sufficiently open. OCAPE recommended the using of MC over RC because of MC cutbacks distillate with Kerosene, which safer than RC distillates (gasoline or naphtha).

Recommended Rate of application

The Asphalt Institute recommends application rates of 0.9 to 2.3 L/m2 (0.2 to 0.5 gal/yd2) for MC cutbacks and 0.5 to 1.4 L/m2 per 25 mm of depth (0.1 to 0.3 gal/yd2 /in depth) for asphalt emulsions. Others recommend from 0.65 L/m2 to 2.0 L/m2 (0.15 to 0.45 gal/yd2) (5,19). Application rates should vary based on the openness of the base, and no more prime should be placed than can be absorbed by the granular base in 24 hours. Any excess should be removed with blotter sand.

Recommended method of application

The prime coat shall be distributed uniformly and adequately. To prevent streaking and ensure proper application and coverage of prime coat. The spray nozzles shall be set at 15-30 from the horizontal axis of the spray bar, as shown in figure no:2. Proper setting spray nozzles will prevent the prime coat from interfering with adjacent spray nozzles. The height of the spray bar should be adjusted to ensure at least a single, double, or triple overlap. The double overlap is recommended for the most prime coat. For uniform applying of prime coat, the spray bar height must be maintained. Figure no: 3 shows the effects of spray bar height on prime coat coverage.

Recommended Temperature Range

Adequate viscosity should be maintained to ensure proper penetration and spray applications of liquid asphalt. The cutbacks are heated to achieve the required viscosity and occasionally emulsions or diluting emulsions with water. Table no.1 showing the recommended temperature for a typical prime coat.

Prime Coat Vs Tack Coat

A tack coat is applied between two subsequent layers of asphalt in flexible pavement. An asphalt binder is placed on the existing asphalt layer prior to overlay to have a proper bond among different layers.  Tack coat comprises of thin bituminous asphalt emulsion or cutback to promote bonding. In most of the areas around the world, tack coat is sprayed by a distributor truck.

The tack coat is vital for structural integrity and monolithic behaviour among various lifts of asphalt. Without a tack coat, the asphalt layers may slip under traffic loadings the application rate of tack coat has to be as per specifications of the projects.

Generally, the application rate of prime coat is more as compared with that of a tack coat. If you’re confused whether you can use tack coat instead of prime coat. So, it’s necessary that you consult your project specifications that will dictate what constitute a prime coat and a tack coat.

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