15 May 2001
and brittle. With some mixes the asphalt cement may tend to drain from the aggregate. As a general
rule, hot-mix asphalt mixtures should not be stored more than 4 hours regardless of the type of storage
silo used. If segregation of aggregate or draindown of asphalt cement occurs in the silo, use of the silo
should be disallowed or changes should be made to prevent segregation and draindown.
b. Placement Equipment.
(1) Asphalt spreader (Paver).
(a) Types of spreaders. An asphalt spreader is used to place most mixture types such as
hot mix, cold mix, and base course material. Spreaders currently in use operate on either tracks or
rubber tires and most have a vibrating screed to strike off and smooth the paving mixture. Some
spreaders use a tamping bar in conjunction with the screed, or an oscillating screed with a vibrating
compactor, and others use a vibrating screed for both strike-off and initial compaction. Conventional
paving machines are capable of placing hot-mix paving mixtures satisfactorily, provided they are
maintained in good mechanical condition, kept properly adjusted, and operated by experienced
personnel. Poor pavement surfaces result if the screed plates are worn or rusty or if the tamping bars
are worn or not properly adjusted.
(b) Automatic grade control. Asphalt spreaders should have a means of automatically
controlling the grade. If an automatic grade control device is used on the spreader for constructing
pavements that consist of two paving lanes, it should include a sensing device for grade control of one
end of the screed and a slope-control mechanism for control of the other end of the screed or a grade
control sensing device on each end of the screed. Where the paver is used for constructing pavements
with multiple paving lanes (more than two paving lanes), sensing devices will be used on each side of
the spreader for control of the screed. The slope-control mechanism should not be used for grade
control in multiple paving lane operation.
(2) Joint heaters. Joint-heating devices for attachment to asphalt spreaders have been used
on construction projects. They are used to heat the edge of an adjacent pavement lane during
placement so that a hot joint is obtained. Experience with joint heaters has shown that there is a danger
of overheating the existing asphalt mixture. Accordingly, it is the policy of the Army and the Air Force
that pavement joint heaters will not be used without the written authorization of the respective office. If a
contractor should desire to use a pavement joint heater, a request will be submitted to USACE
Transportation Systems Center (TSMCX) or the appropriate Air Force major command. To assure that
the asphalt mixture will not be detrimentally affected, the request will include a description of the controls
for the proposed joint heater.
(3) Asphalt distributor. Asphalt distributors are used to apply asphalt material evenly over a
surface. All nozzles should be free and open, and should be the same size and at the same angle with
reference to the spray bar to produce a uniform fan of bituminous material. The height of the spray bar
above the surface is important for uniform application. When the bar is too high or too low, a difference
in application rate in the middle of the spray fan and at the ends will occur, causing streaking. The
height of the spray bar should be adjusted so that a double or triple overlap of the spray fan is obtained.
The Asphalt Institute's Manual Series No. 13 offers guidance for calibrating and checking application
(a) Roller types. A number of roller types are being used for paving operations. Rollers
used to compact asphalt mixtures are static steel-wheel, vibratory steel-wheel, and rubber-tired rollers.