That’s because EVA is a polymer that approaches elastomeric materials in softness and flexibility, yet can be processed like other thermoplastics. EVA typically is added at concentrations between 2 to 5 percent by weight of asphalt binder, and is typically dispersed to the hot asphalt binder at temperatures between 300 to 340 degrees F.

“Benefits of this polymer modifier include the fact that only moderate agitation is necessary,” says blending equipment manufacturer Ecopath Holdings. “The blends can be stored for weeks without succumbing to separation. Consistent with many other polymer modified asphalts, the compatibility of the EVA and asphalt binder is vital for achieving the desired properties.”

Studies have shown that at lower polymer contents (3 percent by weight), EVA modified binders exhibit dispersed polymer particles in a continuous bitumen matrix, Ecopath reported. “EVA modified binder properties, such as morphology and storage stability, are influenced by the characteristics of the base bitumen and binders,” Ecopath says. “Generally, increases in EVA concentration yield greater improvements in the binder; however, these increases also lead to reductions in storage stability. Other studies have shown that, when EVA and SBS modified binders are compared to neat binders, SBS binder exhibits a significantly higher elastic recovery than neat binders. Also, EVA binders tend to exhibit fewer improvements in elastic recovery while losing ductility and elastic recovery at a greater rate.”

And that’s not all. “There are chlorinated polyethylenes, oxidized polyethylenes and polypropylenes,” Blacklidge Emulsions’ Daranga says. “Depending on how long or short the molecules are, you get into the waxes or paraffins. There are ring-based polymers such as DuPont’s Elvaloy. Cellulose-based polymers can be used as reinforcers. The possibilities are limited only by our imagination.”

Response to Superpave

The advent of the PG binder rating system after the first Strategic Highway Research Program (SHRP) and the Superpave system of asphalt mix design was the prime driver of the current interest in asphalt modifiers.

“It definitely highlighted the need for us to modify asphalt to get the better-performing binder properties that we needed out of the same old asphalts we were using,” Daranga says.

In central California, terminal blend of rubberized asphalt liquid is injected into production of warm mix asphalt

Polymer modified binders have allowed the use of techniques previously not practicable, such as microsurfacing and use of asphalt emulsion chip seals on high-volume roads. And specifying agencies are finding that many of the Superpave binder grades require polymer modification to concurrently meet the requirements for high temperature resistance to rutting, and low temperature resistance to thermal cracking.

“Superpave did a lot to accelerate modified asphalts,” Kluttz says. “Up to then we just had pen-graded asphalts, and viscosity-graded asphalts. Both those parameters were generic single-points, so pretty much any crude source could produce straight run asphalt to meet most of the required pen and viscosity grades. Most of the states were developing their own tests and requirements to make sure polymers were included, but the Superpave specs to some degree built that in using the temperature range.”

That didn’t capture everything. When Superpave came out the majority of the states, even today, have kept some or all of their additional tests they use, along with Superpave, such as elastic recovery, the most common one. They’ve maintained those tests to give them the performance they expect out of polymer-modified asphalt, the so-called Superpave Plus specs.

“The liquid asphalt cement by itself is a material that is not very easy to work in ambient conditions, but value can be added to the asphalt,” Daranga says. “The typical asphalt that comes from the refinery has only a certain useful temperature interval, that is, the temperature limits during which that particular material can most probably perform without failure. Quite often that range of temperature is too narrow, and lots of times we will need to ‘stretch’ it, or expand it to allow it to stand up to heavy traffic loads or the weather. The only way we know how to do that today is by adding compounds that enhance that ability.”

The Missouri experience demonstrates the embrace of modified asphalts in the post-Superpave climate. In a presentation at the 2011 conference of the Association of Modified Asphalt Producers – Missouri’s Full Depth Pavement with Modified Binders – MoDOT’s state construction and materials engineer Dave Ahlvers says in 2000 the Show-Me State placed just 2,800 tons of polymer modified asphalt mix on two centerline miles of major roads. But in 2006, the state placed 3 million tons of polymer modified mix on 1,500 centerline miles of major roads. “MoDOT is willing to pay more for polymer modified asphalt to provide a longer lasting pavement,” Ahlvers says.