Compacting Hot-Mix Asphalt Pavements: Part II

James A. Scherocman, P.E. / December 28, 2000

Editor's note: This is the second installment of a two-part series on
asphalt compaction. In the August issue, Scherocman, a consulting engineer
in pavement design and construction located in Cincinnati, discussed the
importance of compaction and the relationship between density and the air-void
content of hot-mix asphalt (HMA) pavement. For an HMA to perform well under
traffic, it must be compacted to an air-void content in the range of 3%
to 8%. Proper use of rollers will help achieve this result.



Five primary variables are under the control of the roller operator during
the compaction process: roller speed, number of roller passes, rolling zone,
rolling pattern, and vibration frequency and amplitude for vibratory rollers.


The faster a roller moves over a particular point on a new HMA pavement
surface, the less time the weight of the roller dwells on that point. This
means that less compactive effort is imparted to the mixture at higher roller
speeds than at lower speeds. As roller speed increases, the density achieved
with each roller pass decreases.


For static steel-wheel rollers and vibratory rollers that have a maximum
frequency of 2,400 vibrations per minute (vpm), 21¦2 mph typically
is accepted as the maximum speed a roller should travel. For a vibratory
roller capable of applying compactive effort at a rate of 3,600 vpm, the
roller can be operated up to 4 mph. For a pneumatic-tire roller, the maximum
speed should also be 4 mph. Rollers can move faster or slower than the recommended
speed, but compaction varies directly with roller speed.


Roller speed also is governed by the lateral displacement or tenderness
of the HMA. If the mixture moves excessively under the rollers, the speed
of the compaction equipment should be reduced. Roller speed should be kept
constant. If the paver speeds up and the rollers also speed up, less density
will be obtained in the mix for the same number of passes of each roller
over each point of the pavement surface. It is very important that both
the paver and the roller maintain a consistent speed to obtain consistent
density.


The actual number of passes needed over a point in the pavement surface
by each of the rollers is a function of many variables. The type of compaction
equipment is one primary variable. Three-wheel, static steel-wheel rollers
apply different compactive effort than tandem static steel-wheel rollers;
pneumatic-tire rollers and single- or double-drum vibratory rollers all
apply different compactive effort, as well.


However, these capabilities vary with layer thickness, mix temperature,
mix design (asphalt content and aggregate gradation) and environmental conditions.
In addition, the number of roller passes required of a particular roller
depends on its position in the roller train-breakdown, intermediate or
finish rolling. It may be possible, for example, to obtain a significant
increase in the density of the mat for the same number of roller passes
when a pneumatic-tire roller is moved from the intermediate position behind
a vibratory roller to the breakdown position in front of that same vibratory
roller.


To determine the minimum number of roller passes needed to achieve the required
density level, a test strip should be constructed at the beginning of the
project. Typically, only one combination of rollers is tested with one
combination of passes of each roller. To pick the most efficient and economical
number of roller passes, it is suggested that more than one test strip be
constructed, with each test area using different rollers in different positions
behind the paver.


Roller passes must be distributed uniformly over the width and the length
of the HMA layer. Most often, the center of the paver lane receives more
passes from the rollers than do the outside edges of the lane. The number
of roller passes applied by each roller must be the same over each and every
point in the pavement surface to obtain consistent density.


Compaction must be achieved while the mix is still hot enough for the applied
compactive effort to reorient the mix's aggregate particles to reach maximum
density. If the HMA is stable under the rollers, the rollers should operate
as closely behind the paver as reasonably possible. Both the breakdown and
intermediate rollers should be within 500 ft of the laydown machine.


With a stable mix, fewer roller passes are needed to obtain a given level
of density when rolling is accomplished directly behind the paver-where
the mix is the hottest. More density is usually obtained with one pass of
the roller when the mix temperature is 250 F than with a similar pass when
the mat is at 220 F.


If the mix is tender and moves either longitudinally and/or transversely
under the compaction equipment, the breakdown rolling is often delayed to
avoid excessive shoving or checking of the mix by the rollers. This, however,
is the wrong solution to the problem.


The properties of a mix that cannot be compacted immediately behind the
paver need to be modified. It is very difficult to obtain the required degree
of density with a tender mix-the rollers can't compensate for a poor mix
design. When a tender mix is encountered, the mix design-not the compaction
process-should be changed.


Until the mix design can be changed, often the best way to compact a tender
mix is to use a pneumatic-tire roller in the breakdown position, directly
behind the paver. A vibratory roller can be operated in the intermediate
position, some distance away from the laydown machine. While most of the
required density is obtained with the pneumatic-tire roller, care should
be taken that the vibratory roller does not operate too close to the paver
and cause the mix to move-shove or check-and thereby cause a density reduction
instead of the desired increase in density.


The rolling zone for the static steel-wheel finish roller is the position
where marks from other rollers can be removed from the surface of the layer
without adding new marks by the finish roller itself. Finish rolling normally
takes place within a temperature range of 185 F down to 160 F. Finish rolling
for a stable mix is accomplished at higher temperatures than finish rolling
for a tender mix.


Rollers operate whenever the paver operates. Interestingly enough, when
the paver stops, often the breakdown and intermediate rollers also stop.
When the paver restarts, the rollers follow suit. While the paver and rollers
are stopped, the mix that has not been completely compacted is cooling.
Depending on the length of the shutdown, it may be difficult to obtain the
desired level of density if the mix has cooled too much. It is very important
that rollers continue their pattern, regardless of what the paver does,
until the required number of roller passes are applied to the pavement surface
and the compaction process is finished.


For each roller used on the project, the width of the paved lane should
be divided by the width of the compaction rolls on each roller to determine
the number of passes needed to cover each transverse point of the surface.
A tandem static steel-wheel roller, 41¦2 ft wide, for example, would
need to make at least four passes across the width of a 12-ft-wide lane.
This allows for a minimum overlap of 6 in. over each longitudinal edge
of the lane and a minimum 6-in. overlap between each roller pass.


However, a 7-ft.-wide double-drum vibratory roller could cover the full
12-ft-wide lane in only two passes across the width, still allowing for
a minimum overlap of 6 in. over each longitudinal edge and between each
roller pass. Thus, in terms of a roller pattern, the 7-ft-wide roller is
twice as efficient as the 41¦2-ft-wide roller. A roller that is 51¦2-ft-wide
would need to make three passes up and down the 12-ft-wide lane to obtain
complete transverse coverage of the surface, allowing for the minimum overlap
between passes.


In the longitudinal direction, the rollers should not stop at the same transverse
end point with each pass of the roller. The reversal points should be staggered
to prevent shoving of the mix. When paving is suspended for a period due
to a lack of haul trucks, for example, the roller should not sit on the
hot layer. The rollers should be parked either on an adjacent lane, on the
shoulder or at the back of the cold, fully compacted layer.


Vibratory rollers have two additional variables that the operator must control
during the compaction process. The first is the frequency of the vibration.
Most vibratory rollers have a range of vibratory frequencies available.


With very few exceptions, the maximum possible frequency setting available
should be chosen. This permits the roller to maximize the amount of compactive
effort applied to the mix by minimizing the spacing between impacts.


Frequency is measured in terms of vibrations per minute (vpm). At the same
roller speed, a vibratory roller operated at a frequency of 2,400 vpm will
provide more impacts per foot than will the same roller operated at a frequency
of 2,000 vpm.


More impacts per foot provides more compactive effort for each pass of the
roller. Frequency of vibration, in conjunction with roller speed, plays
a very significant role in the ability of the vibratory roller to efficiently
obtain density in the HMA material.


The amplitude setting (impact height) on a vibratory roller depends on the
thickness of the layer being compacted. For the vast majority of mixes placed,
the roller should be operated at the lowest amplitude setting. Only when
the lift thickness is greater than about 3 in. should the use of a higher
amplitude setting be considered.


The amplitude setting also is dependent, in part, on the characteristics
of the mix. If the mix is tender, only the lowest amplitude setting should
be used. If the HMA is stiff and stable, and the lift thickness is at least
21¦2 in., use of a higher amplitude may be possible. A high amplitude
setting on a thin lift (less than 2 in.) will typically cause the vibratory
roller to bounce, making it very difficult to obtain the desired density
level.


For very thin lifts, 1 in. or less in thickness, the vibratory roller should
not be used in the vibratory mode. Instead, operate the unit in the static
mode.


The compaction of an asphalt-concrete mix is really common sense. Because
density, or its inverse air-void content, is the single most important variable
affecting the long-term durability of an HMA material, it is very important
that proper attention is applied to those primary factors that affect the
time available to compact the mix-air temperature, base temperature, mix
laydown temperature, layer thickness and wind velocity.


In addition, the five main variables that can be controlled by the roller
operator during the compaction process should also be carefully monitored-roller
speed, number of roller passes, rolling zone, rolling pattern and vibration
frequency and amplitude.


The proper air-void content in the mix must be obtained at the time of construction
in order to prevent the mix from failing prematurely under the applied traffic
loads.

About the Author

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