Battling Segregation in Asphalt Paving

By: David Banasiak

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."