UFC 3-250-03
15 May 2001
c. 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 (figure 7-3).
d. Removal and sizing. The asphalt 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 4 meters (12 feet) wide
and 50 to 100 millimeters (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 50 millimeter (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 be removed by screening.
e. Virgin aggregates. Virgin aggregates are added to the recycled hot mix for a number of reasons.
(1) Gradation. The gradation of the aggregate in the existing mix can be improved 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.
(2) 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 in the plant's mixing drum to prevent the flame from having direct contact with the
reclaimed asphalt pavement (RAP). Such direct contact between the heating flame and RAP would
cause the asphalt in the reclaimed asphalt pavement to burn, which is the main source of air pollution in
hot recycling.
(3) 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.
(4) Excess filler material. Many existing asphalt pavements have been constructed with the
amount of filler material passing the 75 Fm (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.
(5) Asphalt binder. The asphalt binder in the existing pavement is usually oxidized and requires
some modification during recycling to produce an acceptable asphalt binder and resulting 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
7-15
Previous Page
