Primate Coat - Meaning – Thickness – Temperature – Tack coat
A prime coat is a low viscosity asphalt waterproofing layer between asphalt course and aggregate base or WBM. But there’s far more than just the meaning. So, in today’s blog article; we’ll cover all the specifications for primate coat i.e thickness, temperature, and will answer primate vs 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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