TM 5-822-10/AFM 88-6, Chap. 6
The equipment required for pavement removal and crushing will include either conventional equipment for ripping
and crushing or a cold-milling machine. A batch or drum plant, either designed or modified to mix recycled materials is
also required. Placement is with a conventional paver, and trucks, front-end loader, and asphalt distributor are also
Recycling hot-mix procedures.
Recycling hot mix consists of removing the existing pavement; crushing the reclaimed mix, if necessary; mixing the
reclaimed mix with virgin aggregate, virgin asphalt, and recycling agent; and placing the recycled mix by the same
procedures as those used for a conventional mix (fig 4-1).
Removal and sizing.
The asphalt concrete pavement should be removed by a cold-milling machine or with a ripper tooth and crushed. The
cold-milling machine is a self-propelled, power-operated planing machine capable of removing, in one pass, a layer of
bituminous material up to 12 feet wide and 2 to 4 inches deep. The equipment should be capable of establishing grade
control by referencing from existing pavement or from independent grade control and should have a positive means of
controlling transverse slope elevations. The equipment should have an effective means of preventing dust from the
operation from escaping into the air. The milled material should pass through a 2-inch sieve. The teeth on the cutting drum
must be in satisfactory condition at all times to prevent shearing off chunks of the asphalt concrete and creating oversize
particles or a rough surface. If oversize particles are present, they should he removed by screening.
Virgin aggregates are added to the recycled hot mix for a number of reasons.
a. Pollution control. Without the addition
of new aggregate, air pollution during mix production for most plants
would exceed the allowable levels. With the addition of new aggregate, an aggregate shield can be used to prevent the
flame from having direct contact with the reclaimed asphalt pavement (RAP) and causing the burning of the asphalt in
the reclaimed asphalt pavement, which is the main source of air pollution in hot recycling.
b. Gradation. The gradation
of the aggregate
in the existing mix can
by adding virgin aggregates. Many
times existing pavements do not contain the desired aggregate gradation, and if they do contain a satisfactory gradation,
it may be changed during the milling or crushing operation. Therefore, the addition of new aggregate allows the gradation
of the recycled mix to be modified to an acceptable range.
c. Aggregate quality. Many times the quality of the aggregates in an existing mix is not acceptable, even though
the gradation is satisfactory. One cause of poor quality in an aggregate blend is the use of an excessive amount of natural
rounded sand. Rounded sand is a poor aggregate for asphalt concrete, but because of its abundance and low cost, it is often
used in excess in asphalt concrete mixtures. The addition of a new high-quality aggregate can reduce the percentage of
rounded sand in the mixture and thus improve the overall quality of the mix. The amount of natural sand added to a
recycled mixture should not exceed 15 percent of the new aggregate for airfields.
d. Excess filler material. Existing asphalt concrete pavements were generally constructed with the amount of filler
material passing the No.200 sieve near or above the maximum allowed by specifications. The amount of filler in the
reclaimed mixture most often varies between 8 and 12 percent whereas the maximum amount of filler allowed is 6 percent.
During the milling or crushing operation approximately 1 to 3 percent additional filler will be manufactured. Thus, in order
to control the amount of filler, the new aggregates must be limited to very little or no filler. The virgin aggregates may
have to be washed to minimize the amount of filler material. In addition, the percent of virgin aggregate in the recycled
mixture may have to be adjusted to help control the filler content.
e. Asphalt binder. The asphalt binder in existing pavement is usually oxidized and requires some modification
during recycling to produce an acceptable asphalt binder and mixture. If no new aggregate is added to the mix, the addition
of asphalt or recycling agent needed to produce satisfactory asphalt cement properties may result in a mixture that is too
rich. The asphalt cement content of the existing pavement mixture is generally near the optimum asphalt content; hence,
the addition of more asphalt cement or recycling agent may result in an excessive asphalt content. If the existing asphalt
binder is not modified with a low viscosity asphalt or recycling agent, a brittle mixture will be produced.