For every ton of asphalt paved, your reputation is on the line. Asphalt producers and paving contractors take pride in delivering a durable road that lasts for 15 or 20 years or longer. Yet, sending segregated mix to the jobsite or laying a mat with patches of segregation can ruin a company’s reputation, not to mention significantly affect the bottom line.
No matter where segregation occurs – in the aggregate stockpile, at the plant, on its way to the jobsite or behind the screed – it must be addressed. If not, the paved road suffers distresses such as raveling, rutting and cracking, which leads to costly rehabilitation ahead of schedule.
Manufacturers have been working with asphalt producers and contractors to develop equipment to help mitigate segregation occurrences. And there are several tactics that both producers and contractors can implement to reduce segregated mix.
Mixing it up
Prior to making the mix, the way producers handle aggregate has a profound impact on segregation. Building aggregate stockpiles in layers with radial stackers vs. large conical shaped stockpiles reduces aggregate segregation, especially when it comes to larger size stone. Loader operators can also help the process by blending the material from the fine and coarse areas of a segregated stockpile.
At the plant, operators should keep cold-feed bins as full as possible to avoid larger aggregate from separating from the fine material. “To promote proper aggregate gradation, many producers are now using more cold-feed bins to store different stone sizes,” says Joe Musil, senior engineering fellow with Terex Roadbuilding.
Today’s asphalt producers incorporate more additives in the mix, and they must pay attention to additive sizing and introduce these items at the right point in the mixing process. Reclaimed asphalt product (RAP) and shingles, baghouse fines and warm mix asphalt (WMA) all have their proper timing for introduction into the drum, and the mixer must have the flexibility to accommodate all the different additives.
“The drum is the ultimate commingler of the material,” says Musil. “Long mixing zones help to avoid asphalt ‘balling’ of fines and deliver more uniform temperatures throughout the mix when introducing RAP and shingles into the design.”
When loading transport vehicles from the silo, using the three-dump method – loading two batches at each truck end and the final batch in the middle – will avoid large particle run-off to the truck’s sides and ends.
Also, according to Terex Asphalt Paver Application Specialist Bill Rieken, the type of truck used to transport asphalt makes significant difference in reducing particle and thermal segregation.
Rieken says that end-dump trucks are the least preferred vehicle, especially with mainline and highway paving. “A dump truck is basically a box, where more of the asphalt comes into contact with the metal, which cools the asphalt at the ends,” he says. “The mix will segregate as the particles roll to the truck’s sides and ends during transport to the jobsite.”
Live-bottom trucks offer more protection against material and thermal segregation during transport. There is less surface area for the asphalt to come in contact with, so less cooling takes place, and the bed is shaped like a “V” to reduce particle segregation. As an added bonus, “Live-bottom trucks offer faster unloading into the paver’s hopper than end-dump trucks,” adds Rieken.
Smooth & consistent
No matter if it’s warm mix (WMA) or hot mix asphalt (HMA), paving contractors have equipment options to mitigate segregation occurrences at the site, and, in some regions, this additional equipment is a requirement to meet state specifications. “Some specifications are so strict for mainline paving, that they cannot be met with a traditional slat paver alone,” comments Rieken.