Streets professionals view warm mix asphalt at APWA Congress workshop
Buffalo Grove, Illinois
Over 200 municipal public works and streets personnel from across the nation got a hands-on look at the cutting edge of "green" asphalt technology late last year when they observed warm mix asphalt (WMA) being placed in San Antonio.
The warm mix asphalt was being placed as part of a demonstration of all phases of a full-depth municipal street reconstruction project, including lime-slurry base soil stabilization, asphalt emulsion-stabilization of the pavement base using reclaimed asphalt pavement from the existing pavement, and stabilized base compaction.
|San Antonio Congress workshop welcome sign|
The workshop was held in conjunction with the American Public Works Association's International Public Works Congress & Exposition held in San Antonio in September 2007, and included a classroom session which brought together materials experts Larry Peirce, Executive Director, Lime Association of Texas, Austin, Tex.; Gary Fitts, P.E., Senior District Engineer, The Asphalt Institute, San Antonio, Tex.; and Harry H. Bush Jr., P.E., Manager Technical Services, Vulcan Materials Company Southwest Division, San Antonio, Tex.
"Our goal was to illustrate new and innovative techniques in roadbuilding," said Richard E. Martinez, Street Operations Manager, City of San Antonio. "Although the reclamation process we demonstrated has been a mainstay of San Antonio for many years, warm mix asphalt is just becoming popular in this market."
The streets reconstructed were residential drives just east of downtown San Antonio. Their existing asphalt surfaces and black bases sat directly on top of highly expansive clay subgrades, which have caused many problems with performance, ultimately leading to premature failure.
These poor subgrade soils were lime-stabilized. For the overlying pavement structure, a reclaimed asphalt emulsion-stabilized base was constructed using existing surface and base materials milled up from the original roadway. And the pavement surface was the new "warm-mix" design incorporating Evotherm WMA technology from MeadWestvaco Asphalt Innovations of Charleston.
What is warm mix asphalt?
With the introduction in 2004 of warm asphalt mixes into North America, public works officials are closer to a low-emissions asphalt mix that reduces volatile organic compound (VOC) emissions for residents and workers, makes placement in summer easier with substantially less heat emitted from the mat, makes siting of asphalt plants easier, expands the construction season, and requires less fuel to bring mixes to temperature.
|Placement of warm mix asphalt|
Warm asphalt mixes are just that; they're heated to a temperature well below the 300 degree F-plus temperatures of conventional hot-mix asphalt plants. They attract interest because of their potential for reduced plant emissions in different stages of production, including greenhouse gases such as carbon dioxide and water vapor, and benefits in construction in the field.
"This technology could have a significant impact on transportation construction projects in and around non-attainment areas such as large metropolitan areas that have air quality restrictions," reports the Federal Highway Administration's Office of Pavement Technology.
Warm asphalt mixes produce emissions at a greatly reduced level from conventional hot-mix asphalt plants, easing the permitting of asphalt plants in air-pollution non-attainment areas, or where there is local opposition. "We have reached non-attainment for air quality in San Antonio, and we are looking closely at ways to lower the emission levels here," San Antonio's Martinez said. "Warm mix does that, and also makes placement of asphalt much more comfortable for our personnel because it's 100 degrees cooler than conventional hot mix. And that's significant when it's over 100 degrees outside. It also allows us more time to roll to get a better quality material."
And because less heat is required to bring the mix to temperature, warm asphalt mixes can save money in the plant through reduced energy costs. This is balanced by marginally higher material costs of the WMA technology.
Warm mixes can allow faster construction of pavements made up of deep lifts of asphalt, for example intersections, which need to be opened as soon as possible. Because the mix is not so hot to begin with, less time is required to cool the mix before the next lift is placed. Warm-mix asphalts are compatible with Superpave mix designs utilizing PG-graded binders.
"In the United States the bulk of our applications have been dense-graded surface mixes," said Everett Crews, Ph.D., MeadWestvaco Technical Manager for Evotherm. "These typically have 19mm (3/4-in.) or 12.5mm (1/2-in.) nominal maximum aggregate size, sometimes down to 9.5mm (1/3-in.) NMAS. In other countries we are seeing WMA being used in project-scale quantities, as opposed to demo-scale, and we expect the U.S. market to move in that direction this year."
Evotherm is different from other warm mix technologies in its ability to produce mixes around 200 degrees F, Crews said. "Other warm mix technologies produce material at temperatures up to 260 degrees F," he said. "Evotherm is unique in that we can push the lower limit lower. We have made mixes at below 200 degrees F that have performed very well."
The reduced heat is a big bonus for sun-drenched cities like San Antonio, where temperatures routinely hit the high-90s and higher on summer afternoons.
"City crews report the mix is much cooler, and that they like laying it when the temperature is 95 or 100 degrees in summer," Vulcan's Bush said. "Evotherm WMA also lacks the asphalt odor that you might smell. You can't smell it because it's just not there. Most crews say placement of the mix is the same as conventional HMA, with the WMA sometimes being a little stiffer. They say it may take a little more energy to get the paver moving, but once it's going, the mix is as fluid as conventional mix."
Warm mixes also permit trucking of loads of asphalt over longer distances, without fear of critical loss of temperature, allowing contractors to expand market areas. "We sometimes have jobs in south and west Texas that may be as long as a 150-mile haul," Bush said. "For these we hope to use warm mix technology, because in the past we've run the mix at 350 degrees F, as hot as allowed. We tarp the trucks, but it still cools over the three- to four-hour drive, making compaction difficult. WMA should improve that situation."
"By lowering the temperature of production we are avoiding some of the potential degradation to binder that can occur at production higher temperatures," Crews said.
Similarly, warm mixes may allow construction of pavements in colder weather, because contractors may no longer fear critical loss of temperature in the cold. The result may be a longer construction season extending into the winter in some regions of the country.
WMA in San Antonio
WMA mixes in San Antonio using Evotherm warm mix technology, including that for the workshop, have been produced by Vulcan Materials Company at its Helotes, Tex., plant.
"The typical temperature of a U.S. hot mix is 300 to 340 degrees F, and some specialized mix types in Europe will go up to 400 degrees F," says Vulcan's Bush. "The higher you go the more emissions that are released. Warm mix ranges from 200 to 275 degrees F. We're producing the warm mix for this project at about 220 to 240 degrees F, and it will be laid at about 190 to 200. We've actually produced warm mix at lower temperatures but wanted to keep it higher because of additional time delay on this project."
The Evotherm process is based on additives designed to improve aggregate coating, mix workability, and resistance to moisture-related damage. Evotherm's technology can be used for all traditional hot-mix applications, including surface, binder and polymer-modified mixes; mixes containing reclaimed asphalt pavement (RAP); and thin and ultra-thin lift asphalt mixes.
"Throughout test placements we are looking at the long-term performance of warm mix asphalt in our city," Martinez said. "Previous to the workshop we have done small blocks, and our P.E.'s are examining and sampling them. This is the first, major four-block area that we've paved with WMA. We are looking at how neighborhood streets react as well, and how the WMA will perform with the high plasticity-index soils we have here."
With the WMA lift as the final act, workshop delegates saw all three reconstruction processes taking place simultaneously.
Full-depth lime stabilization took place on one street that had been milled most recently. Construction of the reclaimed asphalt emulsion base took place on an adjacent street that already had been stabilized. There, original pavement material that had been milled and stockpiled was brought back out to the jobsite and spread evenly over the roadway. Then, using the City's reclaimer/stabilizer, an asphalt emulsion was mixed into the material, which was then compacted with sheepsfoot and pneumatic rollers.
And on another intersecting street that had been base-stabilized and reclaimed base course mixed and compacted, warm mix asphalt was being placed using San Antonio equipment and forces.
|Workshop crowd with emulsion stab base course compaction|
At the workshop San Antonio used a Type D mix using a PG 64-22 asphalt with top size aggregate 3/8-in. size. "We inject the low-water Evotherm material as a percent of the liquid asphalt cement added, right into the feed line to the drum mixer," Bush said.
Bush said he sees a strong future for WMA in Texas. "The Texas Asphalt Pavement Association is implementing a task force, and all the contractors are very interested," he said. "Contractors are volunteering demo jobs at their local municipalities because they want to see and touch it themselves." Three TxDOT projects totaling over 100,000 tons of WMA are underway in 2008.
Tom Kuennen can be reached at (847) 229-1839 or firstname.lastname@example.org.