Battling Segregation in Asphalt Paving

Asphalt Article December 28, 2000
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Segregation of hot-mix asphalt (HMA) is a
problem facing road builders. When the aggregate becomes
segregated prior to laydown, weak spots can form in the road.
This will eventually lead to a quicker deterioration of the
road.

Segregation can first occur at the plant, when the mix
material is loaded into the HMA storage silos. As the mix falls
to the bottom of the silo, the larger size aggregate rolls to
the edges of the pile and the smaller aggregate remains in the
center of the pile.

The segregation problem does not end in
the storage silo but continues when the HMA is loaded into the
trucks, which transport the material to the work site. The
separation process is the same as in the silo.

The larger
sized aggregate rolls to the corners of the dump truck's bed
while the smaller aggregate stays in the center of the loaded
pile. "Any segregation in the truck will get transferred to the
paver screed," says Mike Kvach, national sales manager for
pavers, Cedarapids, Cedar Rapids, Iowa.

Kvach elaborates on
the problem, "Segregation occurs when you have a top grade
aggregate and a very small fine aggregate, with very few medium
aggregates in between. When you handle this material the large
stones have a tendency to roll away from the smaller stones and
group together in bunches. This is a problem in some mix designs
such as a gap-graded mix."

"Everyone is having some problems
with segregation. It is the loading and unloading that causes
it. You have to be careful with the loading," adds Ed Hanna,
project manager, R.A. Cullinan and Sons Inc., Tremont, Ill.

Upon arriving at the construction site there is not much that
can be done to integrate the altered mix. In its segregated
state it is paved onto the road bed, resulting in a pavement,
which contains areas with a surplus of large aggregate and areas
composed mostly of smaller aggregate.

Segregation weakens
the road

It is important to prevent segregation in order to
maintain the true matrix of the material. Kvach explains,
"Engineers design a particular matrix using the type of
aggregate. Anytime you can maintain a cohesiveness throughout
the material, as you are laying it, then you are adhering more
to the design of the material and the road will perform like it
is suppose to. When you have material segregating you are
breaking down the matrix and you will have weak spots."

Hanna agrees, "Segregation causes weak spots in the road."

Eventually traffic will wear away these weak spots leaving
behind potholes and cracks. The problem often is not detected
until years later.

"You may not see deterioration on the
road for two to three years," explains Hanna.

Segregation
reduces the wear life of a road. Problems are most serious on
interstates or other heavily trafficked roads.

"Segregation
can result in a early break down or premature failure of the
road," states Kvach.

A new idea

There has been much
experimentation in an attempt to solve the segregation problem.
Some methods involved confining the material. "Anytime you can
confine the material you can prevent segregation," states Kvach.
In order to achieve this, hopper inserts were placed in the
paver. Other methods used transfer vehicles or a windrow pick-up
machine.

One way to prevent segregation is to remix the HMA
right before final laydown, but how? John Trygg, president,
Konza Construction, Junction City, Kan., had an idea. Trygg
theorized that if augers are used instead of slat bars to
transfer and remix the HMA to the spreader screws, segregation
can be eliminated.

Working with Cedarapids on the theory, a
prototype paver unit equipped with remixing augers was
constructed. Testing on the prototype was done in conjunction
with the Kansas DOT, (KDOT) the Kansas Asphalt Pavement
Association, Cedarapids and Trygg.

After KDOT field testing,
the department approved a specification for the 1996 season,
which allows a paver equipped with the remix conveyor system as
an alternative to conventional slat conveyor type asphalt
pavers.

What resulted from the experimentation was a paver
with screw augers in its hopper instead of slats. The augers act
as conveyors, pulling the material from the hopper to the
spreading screws in front of the screed. While the material is
being conveyed the augers remix it.

Kvach explains, "We
eliminated the slats in the paver and replaced them with two
augers per slat. The augers counter rotate against one another
and reblend the material. As the material is reblended, the
augers pull it into the feed tunnels and spread it out to the
screed. The process leads to a more homogeneous mix."

The
speed of the remixing augers is controlled by the hydraulic
output flow from the system's pump, which is in turn controlled
from the operator's console.

The machine catches on

Since its introduction in Las Vegas, at ConExpo 1996 the new
remixing paver has been catching on among roadbuilders. "Several
contractors in Kansas are using it, and several in both North
and South Carolina, and several in Canada also are using it,"
says Kvach.

Another contractor who is using the new remixer
paver is R.A. Cullinan and Sons Inc. located in Tremont, Ill.
The machine is being used on a paving job on I-474 near Peoria,
Ill.

Ed Hanna is a veteran roadbuilder. He began working in
construction in 1957 for Illinois' highway division, the
predecessor of the Illinois DOT. After college he joined
Cullinan and has worked there for the last 33 seasons.

As
project manager for the I-474 job he explains what the work
involves. "We're doing reinforced paving with two lifts of
1-3/4-in. binder and a 1-1/2-in. surface. We're putting a total
of 5 in. of mix on the road. We're also doing some patching,
under drain work, repair of erosion and paved ditches and
guardrail work."

The work covers four lanes and about a
5-1/2 mile length of I-474. In mid-August crews had paved a
two-mile stretch of center line, and according to Hanna, "the
new machine seems to be helping." However, he went on to say,
"we will know more about how it is working when we get to the
surface. We'll also know more about three years down the road."
It may take this long because roads that are paved with
segregated HMA last about two to three years before
deterioration occurs. Despite the wait, Hanna believes the new
machine will catch on in the industry.

Kvach also is
optimistic, "The new paver is something we are just coming out
with and it is proving itself. It is exciting because it is
attacking the problem of segregation head on. It is giving
contractors a viable option against segregation."

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