|
But the burden of responsibility still falls on the
operator, who must balance the demands of new mixes and equipment
technologies against the vicissitudes of weather, variations in plant mix,
and out-of-spec mixes caused by nothing more than traffic jams, and still
make the pavement come out perfect.
“Doing the right thing depends a lot on the material
type,” said Steve Wilson, marketing services/product manager,
Bomag
Americas, Inc., manufacturer of Bomag and Hypac compactors. “Today we’re
working with a lot of high-stability mixes, like the SMA and Superpave
mixes. We’re getting into polymer-modified mixes. We see a diversity of
materials being used, and one roller may be more applicable for one type of
material than another.”
Match roller to application
Selecting the right roller for today’s variety of
asphalt mixes and placements is a major challenge. The right roller must be
selected for the parameters of the job and the desired end result.
“The principal issue is selection of the right
machine to fit the application or material,” Wilson said. “The contractor or
agency has a good idea of the end results he wants to achieve, but there is
such a diversity of product that the selection can be confusing.”
If it’s a main line, high-level highway paving job,
production becomes paramount, Wilson told Better Roads. “They will look for
larger rollers, with a better ability to put down dynamic force or
compaction energy into the material,” he said. “They’re looking for width,
smoothness, and a high production rate. And they will need to have all these
factors rolled into one to do a good job, and hopefully achieve bonus points
and additional dollars.”
Alternatively, commercial and residential class work
will present different needs for roller selection. “It’s all in the type of
job,” he said.
Challenge of Superpave
Today’s Superpave, SMA, and OGFC mixes pose unique
compaction challenges to the contractor or government agency placing them.
Conventional asphalts from as little as five years
ago were blended differently from today, and the demands placed on the
roller were not as severe as now. “Today, compaction of Superpave and SMA
mixes demands more energy than before,” Wilson said. “You have to compact it
hard to get densities, and as a result you need a machine that can introduce
more centrifugal or dynamic force into the material, and that means a more
powerful machine, not just in terms of horsepower, but in the energy output
that the drum produces.”
Superpave (a registered trademark of the
Transportation Research Board) is a performance-based system of
specifications for designing asphalt pavements to hold up to the demands of
the new century. This performance-based approach offers more durable
pavements that are specifically designed with local temperature extremes and
traffic loads in the equation. Superpave designs are providing longer-lived
asphalt pavements that will stand up to local climate and traffic volumes at
lower, long-term costs.
Superpave has complicated compaction practices,
although contractors with Superpave experience appear to have mastered the
problems (see Does Superpave Have a Local Future?, Better Roads, July 2003).
“Superpave is a methodology of blending a material
type,” Wilson told Better Roads. “It isn’t necessarily consistent in mix
configuration, and is specific to the type of aggregate and asphalt that
goes into it. It’s a high-stability mix and its configuration is such that
it takes a higher degree of centrifugal force to get density and manipulate
the material to minimize air voids, which is the whole process of
compaction. So you will have to use a more powerful roller to do that.”
Superpave’s tender zone
Compaction must be done in a tighter time frame,
too. “The compaction of HMA must be done in a narrower temperature band with
Superpave — and even with SMA — compared to Marshall design mixes,” said
Dale W. Starry, Jr., marketing manager for Ingersoll-Rand compaction
products and applications. “In Superpave, a tender zone exists that
effectively takes the middle out of the rolling zone. The mixes are being
produced at a higher temperature, and placed and rolled at a higher
temperature, assuming we don’t cause excessive shoving with our compaction
equipment.”
“In some cases, the Superpave mixes are very stiff
and difficult to compact due to the large amount of crushed fine aggregate
incorporated into the mix,” said industry consultant Jim Scherocman, P.E.,
in a presentation, Innovation of Compaction Techniques for HMA, at the
Transportation Research Board meeting in January. His presentation was
co-authored by Dr. Yukinori Nose, Sakai America, and Todd Mansell, with
contractor Graniterock, Watsonville, California.
“In other cases, particularly for coarse-graded
Superpave mixtures — those designed with aggregate gradations below the
maximum density line — the mixtures are tender and move or shove under the
applied compactive effort, whether operated in the static or vibratory
modes,” Scherocman said.
At some surface temperatures, the tender mixes shove
forward as the steel drums approach, causing a bow wave to form, he said.
“Small transverse cracks, called checks, form in the top portion of the
mix,” he said. “These cracks significantly reduce the ability of the rollers
to achieve the desired level of density.”
Tender mixes tend to have three zones based on
approximate temperatures that impact a compactor’s ability to get specified
density. They are:
 |
The upper zone, from 150 degrees C (302 F) to 115
degrees C (239 F). In the upper zone, compaction can be achieved with steel
wheel rollers without shoving the mix. |
|
|
The middle zone, from 115 degrees C (240 F) to 90
degrees C (194 F). In this middle or tender zone, the mix actually moves
under a roller’s steel drums. |
|
|
The end zone, below 90 degrees C (194 F), the mix
again becomes stable and compactive effort can be applied without shoving.
|
Over the years the word has gotten out on when to
and when not to compact a Superpave mix, but operators and agencies new to
compacting Superpave still encounter the learning curve.
“The tender zone is never going to go away,” Wilson
said. “People today have a higher comfort level in working with Superpave,
and people understand it better and realize there is a temperature range
within that material where the material is more susceptible to pushing. In
turn, you try to avoid that. But Superpave mixes can vary in a lot of
parameters and still be classed as Superpave.”
The key is to identify the tender zone, and avoid it
until the temperature drops to the point at which compaction can resume. As
much as 30 to 40% of compaction may be achieved before the mix gets tender
and begins to push, Wilson said. “Better than 50% of the compaction will
take place following the tender zone,” Wilson said. “A lot of work will be
achieved after you’re back on the material.”
Selecting a roller with an onboard temperature gauge
— now very common — will help an operator know the temperature directly
beneath him and identify the tender zone.
Identifying overcompaction
How much density is acceptable? The density number,
or percent density, balances the volume of air voids against the volume of
solid material. Generally, the goal in a hot-mix asphalt pavement is
anywhere from 94 to 98% density, the remainder being air voids.
If the percentage of air voids is too low, a
pavement may have reduced flexibility. If air voids are too high it can
contribute to the pavement’s disintegration. In any case, the conventional
HMA mat must be compacted before its temperature drops to 80 degrees C (176
F), at which time the asphalt cement binder becomes so stiff it is no longer
possible to move the constituent aggregate with repeated passes of the
roller. If compaction is not achieved by then, the operator runs the risk of
shattering aggregate as he or she vibrates the mat over and over again.
“It all boils down to operator method or technique,”
Wilson said. “A skilled operator will recognize the signs of overcompaction.
Audibly, he will hear the drums ringing, he’ll feel the machine wanting to
slide or hop around a bit because the energy no longer is being transmitted
into the asphalt.”
Wilson compared this to dropping a steel ball and a
rubber ball on concrete. “The steel ball really doesn’t bounce very well,
but the rubber ball rebounds well,” he said. “That’s the reaction you get
when the material has come to density, to where it is solid. But now that
the densified material is less receptive to receiving vibration, it wants to
reflect that energy back into the roller, and the machine will begin to act
in an adverse way.”
Aggregate breakage has its own clues. As aggregate
breaks down, a chalking effect will be seen, in which the top surface
aggregate fractures and powders. Rather than having a sleek, black surface,
chalky white streaks will appear in the surface, indicating time to stop
immediately, because fractured aggregate exposes uncoated aggregate surfaces
to water, encouraging raveling of aggregate and break down of the pavement.
Establishing rolling pattern
For optimum productivity, the best operators will
achieve a rolling pattern early in the day and stick with it. Working with
density measurements, they will observe how the asphalt compacts after each
successive pass.
“They will watch the increase of density, the growth
curve, with each successive pass,” Wilson said. “Once they reach their
[percent] density target, they will know it takes three, four, or five
passes to get density and they will have their rolling pattern at that
point. Unless something changes with the material, such as temperature or
lift thickness, they should consistently get the same good results. But when
they exceed that pattern, and start to over-roll the material, they will
begin to hear, see, and feel overcompaction.”
Ingersoll-Rand’s Starry concurs. “Develop a rolling
pattern,” he said, “and unless there are mitigating circumstances which
require changes, the operator should stick with it, hour after hour, day
after day, so long as the mix doesn’t change or there are significant
environmental changes. Mix will cool more rapidly on a cloudy, windy day
than on a sunny, hot, calm day. So the time available for compaction will
change from environmental factors.”
To establish a rolling pattern, Starry urges
placement and compaction of a test strip under the expected conditions.
Sticking to a rolling pattern boosts productivity
because operators don’t have to keep monitoring their density on each pass.
But the wise operator will recognize changing conditions and adjust the
rolling pattern before problems develop. These conditions range from changes
in mix production that result in a changed or inferior mix being delivered,
to mixes that cool enroute due to traffic delays.
The operator’s first clue is a change in his ongoing
rolling pattern. The density point numbers may change suddenly (and the
operator may be blamed). Temperature changes are clues. If material was
being placed at 280 degrees F, and all of a sudden comes out at 220 degrees
F, the operator’s window of opportunity to compact is shortened.
“He will have to work harder and faster for perhaps
not the same results as with the previous loads,” Wilson told Better Roads.
“He also will expect to compact a continuous surface without joints, but if
a paver has to sit for a long period of time waiting for trucks, the surface
they will try to mate with will be cold compared to hot mix coming against
it, the same condition as though he were starting up fresh in the day. It
will be difficult for him to reach satisfactory densities.”
SMA, polymer modification
Stone matrix asphalt is a gap-graded (low,
medium-sized aggregate and fines) asphalt mix which combines strong, coarse
aggregate with a high content of asphalt cement — as much as 6 to 8% liquid
asphalt. The result is a strong mix incorporating a stone-on-stone skeleton
that is very resistant to rutting.
The stone-on-stone contact develops internal
friction and resistance to shear. Because the gap-graded SMA lacks
medium-sized aggregate, the low-penetration grade asphalt used
conventionally can drain out of the coarse aggregate skeleton. To avoid this
drain-down, the cement is modified by using cellulose fibers or similar
material to hold the binder in place. The percentage of fines is less than
in conventional U.S. mixes — about 15% of the aggregate weight. But because
they often use polymer modification, SMA mixes can pose difficulties for the
compactor operator.
“Polymer modification can make for a little more of
a challenge in the rolling process because of its stickiness,” Wilson said.
“They can exhibit a stringy aspect and have a tendency to stick like glue to
any surface that comes in contact with it.”
This is particularly a problem with rubber-tired
[pneumatic] rollers. A compactor may need a blended, silicone-based rubber
tire, or some form of release agent in the compactor water spray system to
counteract the affinity that rubber and asphalt have for each other.
“The appearance of the pavement when finished with a
pneumatic roller is dramatically different from one where only steel drums
have been used,” Ingersoll-Rand’s Starry said. “Surface texture is improved,
but not impermeability. But a pneumatic roller has a tendency to pick up
asphalt when the tires are not yet hot. Polymer-modified asphalt cements are
the worst for use of pneumatic compactors because they are so sticky, and
it’s almost impossible to keep a pneumatic roller from picking up that kind
of mix, even when the tires are hot. We’ve tried tire skirts, emulsions
mixed in with the spray system water, silicone, and release agents. Some
contractors have tried No. 2 diesel fuel, which we don’t support for
environmental reasons, nor for practical reasons because it leads to
shortened tire life.”
“For a steel drum roller, once the drum gets hot,
and water is kept on it, a sticking condition typically will not develop,”
Wilson said. “But with polymer-modified asphalts a release agent still may
be in order to maintain a clean drum surface, and work the material without
buildup of binder on the drum surface.”
Polymer-modified OGFCs
With their open character and polymer modification,
today’s open-graded friction courses also pose compaction challenges to
contractors. A conventional OGFC is a layer of asphalt that incorporates a
skeleton of uniform aggregate size with a minimum of fines. Typically, OGFCs
in the past have a void content as low as 12% and as high as 15 or 16%.
Most OGFCs are 0.75-inch thick, and never thicker
than 2 inches. The OGFC should be elevated above the shoulder, as the water
drains onto the shoulder and hence to a roadside ditch.
In dense-graded asphalt mixtures, a thin film of
asphalt — plus compaction effort — are required to keep the mix glued
together. In short, the final density of dense-graded mixes is a direct
measure of the strength and durability of the mix. But the OGFC mix uses a
grading of mostly 0.375-inch stone, the idea being to build up a thick film
of asphalt on the stone without the mixture draining or flushing. The
asphalt film thickness is usually four to six times that of a dense-graded
mix.
The new generation of OGFCs and OGFC pavements that
are being built in the U.S. and Europe have considerably higher air void
contents than before, up in the range of 17 to 22%. They are more open, with
more voids. Today’s OGFCs are polymer-modified and include spun mineral or
cellulose fibers.
The polymer modifier stiffens the asphalt binder,
while adding flexibility. Thus it better resists raveling of the top layer
of aggregate. As with SMA, this polymer modification can result in a sticky
binder which can complicate rolling if the machine is not prepared for it.
“The actual compaction process is not much different
from other asphalt mixes,” Wilson said. “You have to watch for the same
conditions of overcompaction and fractured material.”
Learning about warm mixes
To further complicate things, the new warm-mix
asphalts which are just now being popularized in North America will pose
their own rolling challenges.
These warm asphalt mixes are heated to a temperature
well below the 300-plus degrees F temperatures of conventional hot-mix
asphalt plants, and are attracting interest due to their potential for
reduced plant emissions in different stages of production, benefits in
construction in the field, and reduced energy consumption in the plant.
Their performance is achieved through introduction of modifiers (not
polymer) which lubricate or reduce viscosity of the mix through placement.
“Warm mixes likely will constitute a fourth kind of
mix,” Bomag’s Wilson told Better Roads. “Obviously there’s a new interest in
warm mixes, which require a different method of compaction. People are
beginning that education process as to what works best and what equipment to
use. We equipment manufacturers haven’t had a lot of experience with warm
mixes yet because there has not been a lot of it used at this time.”
Wilson said warm mixes may have a short window in
which to achieve density. “We are used to working with materials put down at
a much higher temperature, a variety of compactors used, and with a much
wider window of opportunity in which to work the material. “The warm mixes
may shorten that window and reduce the number of products you can get on the
material in the amount of time before it sets up,” Wilson said. “We will
have to achieve density and will have to do it quickly, so it will change
the rolling train, the method, and perhaps the product selection.”
Wilson added the variation of indigenous aggregate
materials among different states will force contractors to work warm mixes
differently from state to state. “It will be a learning curve for all of us
to find a method that is universally accepted and gets results across the
board.”
 |
 |
 |
|
With polymer modified asphalts, a release agent
still may be needed in the water system to maintain a clean drum surface.
|
Oscillation compaction is a new drum exciter
technology in which horizontal, not vertical, forces impinge on the mat.
|
Higher vibration frequency is an important criterion
for new compactors. |
Smoothness and compaction
Today’s focus on ultra-smooth HMA and portland
cement concrete pavements is putting new demands on contractors, but a
bonus-driven, carrot-and-stick approach by road agencies is helping change
minds.
At the end of the last decade, the Arizona
Department of Transportation changed its smoothness specifications so that
highways being built there must be 27% smoother than those constructed
previously. The means to this improvement were incentives for pavements that
were smoother than required. Contractors responded and gave the state what
it wanted: smoother pavements.
“After several contracts with the smoothness clause
had been completed,” said Arizona DOT’s Jim Delton in the FHWA’s FOCUS
newsletter, “we were able to show contractors that they could obtain
significant incentive payments by using the new technologies and equipment
available today.”
Smoothness of pavements is less a function of
rolling and more of best-practice paving and screed operation. For example,
the Arizona DOT and its contractors found the easiest way to boost
smoothness is to avoid stopping and starting the paver, which can result in
bumps in the mat. Crews should make sure that the paving machine always has
hot mix in front of it, so that there’s no need to stop and wait for another
load.
But compaction plays a role. Arizona found
smoothness can also be improved by making sure that the steel-wheeled
rollers are clean, track straight, and stay on the mat. If the compactor
fails to stay on the mat, it will pick up gravel and detritus at the margin
of the pavement and push it into the new mat.
While the function of smoothness clearly rests with
the paver and screed, and density and mat integrity with the compaction
equipment, there is a synergy between compactor and paver in attaining good
pavements that can’t be denied.
“Attaining smoothness and density are among the
greatest challenges to contractors today,” said Mike Wyss, marketing
manager, Caterpillar Global Paving Division of Caterpillar, Inc. “You can
have smoothness related to the paver, but if I have a smooth pavement that
doesn’t reach required density, then I really haven’t accomplished anything.
Conversely, there are jobs where the target density has been met or
exceeded, but if there are ripples in the mat, or it isn’t smooth, it won’t
meet the requirements of the owning agency or the driving public. We see
them intermingled; you can’t have one without the other.”
Vibration at screed
North American-style vibrating screeds can
complement a compaction train, but in their present form never will get such
high compaction numbers that it will eliminate a roller from the train,
experts say. But it will help the mat get a particular set before compaction
begins.
The amount of compaction off the screed varies by
manufacturer. Initially, a vibrating screed sets the material tighter, to
enhance compaction from the bottom of the lift up, but rollers always will
be needed to get the required density.
“It depends on the material and lift thickness as to
what vibration at the screed can accomplish,” Wilson said. “Because of
variables in the mix and operation, and the different types of screeds and
pavers out there, each will have a different value. They can enhance the
paving process, and may do even more in the future as technology develops.
But right now it’s a new condition for everybody.”
European-style compaction at the screed — as opposed
to vibrating screeds — is a robust application that achieves significant
density numbers right off the paver. But to make it work right the paver
must travel at a rate much slower than what most American contractors will
accept. As a result, Eurostyle pavers have limited distribution in the
United States and tend to be used for special applications.
High vibration enters the market
The advent of Superpave and realization of its
compaction peculiarities in 1992 led to a reconsideration of the compaction
equipment used for Superpave. One innovative equipment manufacturer, Japan’s
Sakai Heavy Industries, tested a 4,000 vibration-per-minute compactor in
Japan in summer 1998. This prototype had a weight of 9.4 tons, with drum
width of 67 inches, and diameter of 51 inches.
Then, at Conexpo/ConAgg 1999,
Sakai America, Inc.
introduced its new-design vibration system to North America. By the advent
of Conexpo/ConAgg 2002, most roller manufacturers were exhibiting their own
high-vibration frequency compactors to meet the needs of the new stiff
mixes. Two years later, with some significant exceptions, higher vibration
frequency is a critical criterion for high-performance compactors. For
example, today 4,000 vpm is standard on six different Sakai models, through
84-inch drum width.
“Equipment has had to compact with great expediency
and that has driven the industry to develop machines with greater drum
energy and higher vibration frequencies, so they can roll faster without
causing surface distress due to irregularly spaced drum impacts, or drum
impact spacing that is excessive, leading to a rough ride,” I-R’s Starry
said. “Just about everybody makes a high-frequency compactor and we have
seen the threshold of drum performance increasing with each new series of
machines produced.”
But there’s a potential drawback, Starry warned. “By
making these machines more powerful, they have the potential to be misused
and cause more damage than less-powerful compactors,” he said. That means
operators have to be trained for the new technologies. “I see a tremendous
need for training of paving crews and roller operators as to the right way
to use the equipment,” he told Better Roads.
“Training is critical for attaining highest quality
results,” said Kevin Mann, senior applications consultant and service
engineer, Caterpillar Global Paving. “At Caterpillar we are producing new
types of machines. But we are running into operators who don’t exactly know
the capabilities of the machines, or how to set them properly. Crews may go
through operators quickly. Operators may do their jobs very well, but still
may not understand how to achieve the best results.”
Some high-vibe machines
One such high-vibration machine is Bomag Americas’
new 131-horsepower BW161AD-4, introduced at the World of Asphalt 2004. It
delivers 36,000 pounds of centrifugal force in low frequency (2,700 vpm) and
27,225 pounds in high frequency (3,600 vpm), an increase of 23 and 60%,
respectively, compared with the previous model.
Bomag’s sister line, Hypac, presented its new C784
tandem-drum vibratory roller at the World of Asphalt. Featuring operating
frequencies up to 4,000 vpm, the C784 can achieve a maximum working speed of
4.5 miles per hour while maintaining 10 impacts per foot, and is ideal for
Superpave projects, Hypac said. The C784 weighs in at 26,500 pounds, and
generates 34,655 pounds of centrifugal force in low amplitude, and 41,235
pounds in high amplitude; it features 54-inch diameter drums to help
minimize shoving.
To ease compaction of stiff and tender Superpave
mixes, Sakai is marketing a high-frequency double-drum vibratory roller,
which it says will obtain the required level of density before the mix cools
to a level where density can’t be achieved.
“Use of a high vibratory frequency permits the
roller to be operated at a higher speed needed to obtain density, and still
obtain the required impact spacing to achieve smoothness in the asphalt
concrete layer,” said consultant Scherocman in his January 2004 TRB paper.
A vibratory, rubber-tire roller has also been
developed by Sakai. Research found that the combination of the kneading
action and the vibratory force of the rubber tires would increase the
uniformity of the density obtained at both the top and the bottom of the
asphalt pavement layer, Scherocman said.
Sakai recommends its new GW750 vibratory
rubber-tired roller for tender-mix Superpave applications, stiff HMA mixes,
and SMA. It develops an equivalent of up to 55,000 pounds of compaction
effort in a fast-moving 20,500-pound package.
New oscillation compaction
Even as high-vibration compactors ripple through the
market, an entirely new technology — oscillation compaction — is being
introduced to North America by
Hamm Compaction Division of
Wirtgen Group.
Some contractors are finding oscillation compaction works faster than
conventional vibration, especially when achieving high densities with a
minimum of passes.
Unlike traditional vibratory compactors that achieve
compaction by bouncing the drum on the ground, the firm says, Hamm’s
oscillation technology ensures that the roller drums maintain constant
contact with the ground for faster, more effective compaction.
In the oscillation drum, two eccentric masses
turning in the same direction cause a movement around the drum axle. The
movement changes its direction of effect during one turn so it generates an
oscillating or rocking movement of the drum. Horizontal forces are
transmitted from the drum into the pavement. The result, Hamm maintains, is
better compaction in fewer passes, with less vibration-related wear and tear
on operators and surroundings. Oscillation compaction in North America is
available only on the Hamm HD O90V, a 66-inch, double-drum, articulated
machine in which the front has a conventional vibratory drum, and the rear
incorporates oscillation technology.
“The oscillation roller will not overcompact,” said
Bruce Monical, Hamm marketing manager. “With the conventional vibratory
roller every pass within the compaction curve must be monitored. But with an
oscillatory roller, you don’t have to worry about that; you can run the
oscillation drum regardless of temperature or density achieved, and it won’t
break over. It will either stop completely, or if there is any viscosity or
movement left in the mat, it will continue to increase density.”
One contractor in Southern California has benefited
from the oscillation technology. Tom Maas, president of Santa Ana based
Hardy & Harper, said, “It appears that it’s at least saving an hour a day
for the roller man, enabling him to get off the job in a timely manner, and
leaving a nicer finish.”
Hardy & Harper has been able to reduce the number of
rollers on projects, rather than fielding the traditional breakdown roller,
intermediate roller, and finish roller. “We’ve been able to get by with only
two, or sometimes, the HD O90V does the whole job,” Maas said. “It will do
the finish rolling without vibratory after they’ve got compaction. That
works for us in eliminating that one roller; but we’ll always have a second
on the job as a backup.”
Intelligent compaction
The digital domain now is changing the way rollers
operate, automating some systems and providing real-time monitoring of
pavement conditions like temperature and density.
For example,
Bomag has introduced its Asphalt
Manager intelligent compaction system for its Variomatic tandem rollers. The
system is an automatic compaction control scheme, integrated with
temperature measuring. It physically monitors the temperature beneath the
roller, while monitoring the reaction of the drum on the mat as it densifies.
In North America, Bomag’s BW161AD-4 is available with the optional Asphalt
Manager technology.
“Rollers are being designed with automatic
compaction processes built into the electronics of the machine,” Bomag’s
Wilson told Better Roads. “They control the output energy of the drum, based
on the reaction of the drum to the material’s surface as it begins to
densify. It changes the amount of energy going into the material to
eliminate overcompaction. It will be of growing interest in the next one to
two years, and it’s something in which the Federal Highway Administration
has a strong interest. It will help contractors achieve correct densities
quicker, and quicker means more money in their pockets.”
Balancing vibration with amplitude
Caterpillar Global Paving
has introduced
high-vibration compactors, but is incorporating a system that balances
vibration frequency with power of amplitude.
“There was a tendency for people to think that the
only way to get density on Superpave was with higher frequency,” Cat’s Wyss
said. “We did a lot of testing and visited job sites, and we developed a new
vibration system called Versa Vibe, which gives the contractor a
high-frequency/low-amplitude compaction option, and a lower
frequency/high-amplitude option, in the same machine.”
With Versa Vibe, drum vibration is achieved by
rotating an unbalanced shaft mounted in a sealed housing inside each drum.
The technology permits the operator to match the machine’s vibratory system
to a wide range of applications, including high frequency compaction.
“That’s critical because contractors are changing
mixes on jobs,” Wyss said. “Some mixes, like an SMA, especially in the
breakdown mode, need high-amplitude compaction to get initial density behind
the screed. When you have only a high-frequency roller, it must have lower
amplitude to achieve an acceptable bearing life. Now the same machine can
run with a higher amplitude and lower frequency, or with higher frequency
and lower amplitude.”
This also is handy when dealing with softer
aggregates in thin overlays, as in Florida, Wyss said. “If a contractor used
a high-amplitude machine only, he could fracture the limestone,” he said.
“Now a contractor can use the same machine with Florida’s soft aggregates as
he would on a New England overlay with hard granite.”
Smart Drum technology
Ingersoll-Rand’s new HFA high-frequency compactors
employ a concept to Cat’s Versa Vibe. These units have eight amplitude
settings, each of which is automatically adjusted to the highest frequency
for the specified amplitude. The end result, I-R says, is two machines in
one, high frequency with lower amplitudes for thin lifts, and lower
frequency with high amplitude for thick lifts and stiff materials.
This Smart Drum technology is transparent to the
operator. “We were able to develop a technology with a supplier that enables
us — without any input from the operator — to reduce vibration frequency as
amplitude is increased, and control the speed of the roller to the proper
drum impact spacing, based on the amplitude selection,” Starry said. “The
operator doesn’t have any input; it’s all done invisibly to him.”
The company has incorporated a smart water system
that automatically adjusts the amount of water spray according to the speed
of the roller. At stop, there is no water; at full speed, the water is at
optimum flow.
Ingersoll-Rand compactors also feature drums with
chamfered, radiused drum edges to prevent drum edge marking. To match travel
direction, automatic reversing eccentrics are utilized. One model, the
DD-158HFA, is designed to deal with potential bow wave problems: both
vibrating drums are lifted up and over uncompacted material as the machine
makes its vibratory passes, rather than pushing it forward, I-R said. This
is said to reduce rolling resistance and minimize the size of the bow wave
in front of each drum, producing a smoother surface and eliminating cracks
or tears in asphalt.
The operator is key
While advances like automation, digital controls,
and oscillation technology may help an operator do his or her job better,
the responsibility for a successful compaction still remains with the
operator.
“The operator is one of the most important people in
the paving process,” Wilson said. “He has to make sure the finish he
achieves is what everyone is looking for, in terms of density, ride quality,
smoothness, and without marks.”
For More Information
Reams of additional information on compaction and
paving of asphalt pavements are available by inquiry and Internet downloads.
Here is a start:
An outstanding 20-page technical paper —
Compaction Research: Frequency, Amplitude and Speed — demonstrates how
frequency and amplitude of vibration can be varied to best compact a variety
of HMA pavements. It’s available at Cat Global Paving Products dealers; ask
for publication No. QEDQ9874.
The Baystate Massachusetts Roads Program Local
Technical Assistance Program Asphalt Paving Inspection Check List is
downloadable at
www.ecs.umass.edu/baystate_roads/technotes/28_asphalt_paving.pdf.
For a more complete overview of the Superpave
system, request a copy of the Federal Highway Administration’s brochure,
The Superpave System: New Tools for Designing and Building More Durable
Pavements (No. FHWA-SA-96-010); 301-577-0818, fax 301-577-1421.
The paper, Summary of Georgia’s Experience with
Stone Matrix Asphalt Mixes, is available from the Georgia DOT Office of
Materials and Research, as a .pdf file, at
www.dot.state.ga.us/dot/construction/materials-research/b-admin/research/onlinereports%5Cr-SMA2002.pdf.
Manufacturers of production-class asphalt rollers
publish excellent primers on paving and compaction, many of which include
information on materials and mixes as well as compaction techniques and
equipment selection. These publications are available through local dealers
as well as the manufacturer’s home office, and some are available on company
Web sites.
Reprinted from Better Roads Magazine
September 2004 |
