BLEEDING AND FAT SPOTS
Description
Definition:
Bleeding of an
asphalt mixture (see Figure -1-) occurs when the asphalt cement flows to the
top of the mix surface under the action of traffic loading.
Bleeding is
often seen as two flushed longitudinal streaks in the wheel paths of the
roadway. Fat spots in an asphalt mixture (Figure 19-8) are isolated areas where
asphalt cement has come to the surface of the mix during the lay-down and
compaction operation or later under traffic. These spots can occur erratically
and irregularly, or they may be numerous and in a fairly regular pattern.
Causes
Fat spots are caused primarily
by excessive moisture in the mix . The problem is more common with mixtures
that contain a high percentage of fine aggregate (over sanded mixes) and those
that contain aggregates with a high porosity. If all the moisture in the coarse
and fine aggregate is not removed during the drying and mixing operation at the
asphalt plant, the moisture vapor will force asphalt cement to the surface of
the mix behind the paver as the moisture escapes from the mix and evaporates.
Fat spots occur more frequently when aggregate stockpiles are wet or when the
moisture content varies in different portions of the stockpiles. Fat spots
sometimes occur in areas where petroleum products, such as oil and diesel fuel,
were spilled onto the pavement surface prior to overlay (see Figure 19-9; see
also Section 14) or have contaminated the mix. In addition, fat spots can be
associated with segregated areas in the mix. If the mix deposited on the
roadway by the paver is segregated, areas in which excess asphalt cement is
present in the mix can result in free binder material on the top of the layer
being placed.
The causes of bleeding normally fall into
two categories. The first is an excess of fluids in the asphalt mixture—either
asphalt cement or moisture or both. Under traffic, the extra moisture and
asphalt cement will be pulled to the surface by the passage of vehicle tires.
This bleeding phenomenon usually occurs on new mix and during hot weather when
the viscosity of the asphalt cement is at its lowest level. Typically the
bleeding occurs shortly after traffic is allowed to travel over the fresh
mix—while there is still some moisture in the mix
and while the viscosity of the asphalt cement
binder is still relatively low.
Bleeding may also be associated with a lack of
adequate space in the mix for the asphalt cement. If the VMA content and air
void content of the mix do not provide enough room for the binder material,
bleeding can occur as the mix is densified by traffic, both shortly after
construction and later. The traffic compaction process will decrease the air
void content of the mix and may, in turn, squeeze some of the asphalt cement
out of the mix. The “extra” asphalt will appear as a longitudinal streak or fat
spot throughout the length of each wheelpath.
One additional possible
cause of bleeding is the condition of the pavement layer on which the new mix
is placed. If the underlying layer has excess asphalt on its surface or excess
crack seal material in the cracks and joints, some of this material may be
drawn up through a thin new mix layer. Further, if too much tack coat is
applied to the original pavement layer, the excess material may be pulled up through
a thin overlay and contribute to the bleeding problem.
Solutions
Variations in the asphalt mix temperature behind
the paver indicate that the moisture content of the mix may also be variable.
Where moisture has evaporated, the temperature is lower. This latter phenomenon
can contribute to both the bleeding of the mix later under traffic and the
generation of fat spots in the mix during construction. It is important,
therefore, that the aggregate used in the mix be relatively dry and that the
moisture content of the mix upon discharge from the asphalt plant be as low as
possible, but not more than 0.5 percent. Extra care needs to be taken in drying
when producing mixtures that incorporate highly absorptive aggregate.
Bleeding problems caused
by excess asphalt cement in the mix can most easily be solved by reducing the
asphalt content, consistent with other properties of the mix, such as air
voids, VMA, and strength or stability. Bleeding problems that occur in
conjunction with pavement rutting usually can be solved, however, only by a
complete redesign of the asphalt mixture, with emphasis on proper air void
content and VMA criteria.
Effects on Performance
Occasional fat spots in
the mix should not affect the ultimate durability of the pavement to a significant
degree. A large number of fat spots or bleeding in the wheelpaths does affect
pavement performance, however, because of variable asphalt and air void content
in different parts of
the mix. In addition,
other mix problems, such as shoving, rutting, and loss of skid resistance, may
occur in a mix that contains many fat areas or bleeding in the wheel-paths. The
design of the asphalt mixture, the operation of the asphalt plant (more
complete removal of moisture), or both should be checked to ensure that the mix
produced will provide adequate pavement performance under vehicular loading.
