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Is ‘green’ the new look of pavement?
Asphalt is black and concrete is gray, but “green” is becoming the new look of pavement as environmental concerns grow in importance. Speakers at the 12th Annual Minnesota Pavement Conference described emerging areas such as “green” roads, warm-mix asphalt, and sustainable innovations in concrete pavements.
The conference, held February 14 in St. Paul, was sponsored by Mn/DOT, Minnesota LTAP, and a number of other organizations.
Coverage of several presentations from the conference follows. Additional coverage is in the 2008 Minnesota Pavement Conference Session Summaries, available for download on the Minnesota LTAP Web site at www.mnltap.umn.edu/Publications.
Figure 1: Conventional asphalt mix placed at 320° F.
Be cool with warm-mix asphalt
As an industry, we already have moved toward environmental responsibility by recycling asphalt, said Dave Newcomb of the National Asphalt Pavement Association. But we can do more, he said, by paying attention to the temperature of asphalt mixes. By reducing temperature, we will decrease both our fuel usage and emissions. Furthermore, he pointed out that lower temperatures will benefit the industry by providing:
- Earlier start dates
- Earlier start times
- Longer haul distances when needed
- Cooler working conditions
- Reduced plant wear
- Decreased binder aging
- Improved compaction with stiff mixes
Figure 2: Asphalt with Advera additive placed at 245° F.
Although the Europeans had a head start on the technology, the United States has rapidly gained ground with 10 warm-mix technologies currently available and more in development. As a signatory to the Kyoto Accord, a treaty whereby countries have agreed to reduce their greenhouse gas emissions, the European Union was compelled to look for ways to reduce emissions in many ways—including in its paving methods. By the year 2000, the EU had introduced new paving strategies, including warm-mix asphalt (WMA), which Newcomb defined as mixes produced and placed between 200° and 275° F. He said at least 72 WMA field trials have now been conducted in the United States.
“High temperatures are brought on by polymer-modified binders,” Newcomb said. He went on to discuss several strategies for reducing temperature while maintaining acceptable physical properties. One of these is to reduce viscosity by use of an additive.
Figure 3: Truck at right is being loaded via ASTEC’s Double Barrel Green process, in which water is introduced into the asphalt mix to decrease viscosity.
Figures 1 and 2 illustrate a project that evaluated one such additive: Advera®, a zeolite product manufactured by PQ Corporation. The zeolite releases a small amount of steam into the mix during production. Figure 1 shows a control project in which conventional HMA was placed at 320° F. Figure 2 shows WMA with Advera being placed at 245° F in the same location (Yellowstone National Park). The difference in emissions is obvious in the photographs.
Another successful technique is to achieve lower viscosity by mechanically foaming the asphalt. One example is ASTEC’s Double Barrel Green process that introduces water into the mix at the plant. Figure 3 illustrates the difference in visible emissions between a conventional mix on the left and the Double Barrel Green process on the right. This particular mix contained 50 percent RAP. The Double Barrel Green process reduces smoke and odor because the lighter oils in the liquid asphalt are not volatilized.
But an obvious question is: How do WMAs perform? To answer that question, Newcomb provided preliminary data from a research project on I-70 in Colorado in which WMA was compared side-by-side with conventional HMA. Results showed that the WMA had about the same optimum asphalt content, fewer air voids in field-produced mixes, comparable tensile strength, and generally favorable rutting characteristics.
Newcomb recommended the following practices with WMA:
- Work to minimize aggregate moisture through proper handling and stockpiling practices.
- Make sure the burner is tuned for the temperature.
- Keep baghouse temperature above the condensation point.
- Consider superheating aggregate ahead of RAP addition.
- Follow normal placement practices.
For more information on warm-mix asphalt techniques, go to the Web site of the WMA Technical Working Group: warmmixasphalt.com. The group is cosponsored by multiple agencies, organizations, and companies. Newcomb also announced a NAPA publication titled Warm Mix Asphalt: Best Practices and an international WMA conference to be held November 11–13, 2008, in Nashville, Tennessee.
The Green Roads initiative
Joe Mahoney, professor of civil engineering at the University of Washington, provided an overview of “Green Roads,” a proposed rating system being developed by the University of Washington and CH2M Hill to improve the sustainability of our roadway design and construction practices.
Road-building: the hungry consumer
Mahoney began by laying out the problem, which is that our current road-building and road-use practices are primary sources of environmental degradation. For example, he used the statistics in Table 1 to dramatize just how much material we consume every year in road construction.
Table 1: Annual Material Use in U.S. Road Construction
Mahoney then went on to show that our transportation activity, much of which obviously involves road use, accounts for about 29% of the energy consumed in the United States. He also cited statistics from the California Integrated Waste Management Board showing that waste from all forms of construction, including road building and maintenance, accounts for between 20% and 40% of the municipal waste stream.
Mahoney’s conclusion was that by focusing on our road-building practices, we can contribute significantly to solving the world’s environmental problems. Our goal, he said, should be sustainability, which he defined as “maintaining economic prosperity and a high quality of life for all while protecting the natural systems of the planet.”
Aren’t we already recycling?
Like Newcomb, Mahoney acknowledged and praised pavement recycling as an environmentally responsible practice. He showed that the combined amount of recycled HMA and PCC from pavements in Washington State has quintupled since 2000. While that sounds impressive, he also pointed out that in this country, we still derive only 6% of our aggregate from recycled sources; 94% is virgin material. He challenged the road-building community to do better. For example, he urged us to get more value from recycled material by putting it into a road’s surface rather than into its base.
Green Roads: A roadway sustainability rating system
Table 2: Green Roads Credit System
Another part of the solution, Mahoney predicted, will be the Green Roads initiative, which has been developed and promoted in Washington State by Steve Muench, also a professor at the University of Washington. The Green Roads initiative is a systematic way to rate the sustainability of roadway design and construction—in other words, said Mahoney, a way to quantify sustainability.
Table 3: Materials & Resources Category
The Green Roads system is patterned after the highly successful LEED (Leadership in Energy and Environmental Design) rating system, which is administered by the U.S. Green Building Council (USGBC). Since 1998, USGBC inspectors have certified more than 800 new and rehabilitated buildings throughout the United States. In the LEED program, buildings are certified at four levels: certified, silver, gold, and platinum. To achieve these levels, building designers and builders can apply for certification by designing for energy efficiency, designing building components that can be re-used rather than discarded, choosing recycled materials, employing energy-efficient construction methods, and reducing construction waste.
Mahoney presented the information in Table 2, which outlines a similar credit system for the proposed Green Roads program.
Mahoney also provided a more detailed view of the categories. For example, Table 3 shows a break-down of the Materials & Resources category.
The promoters of the Green Roads system hope to gain support throughout the pavement industry. For more information, go to: www.greenroads.us.
Sustainable innovations in concrete pavements
The long-term performance of projects involving recycled concrete aggregate (RCA) has been mixed, said Mark Snyder, an independent pavement engineering consultant from Bridgeville, Pennsylvania. He reported on his work in a 2006 FHWA-sponsored project titled “RCA Concrete Pavement Survey Outreach ” through the University of New Hampshire’s Recycled Materials Resource Center. The project produced a more long-term assessment of test sections first surveyed in the 1990s. The sections studied included jointed plain, jointed-reinforced, and continuously reinforced concrete pavements.
Most common failure modes
Snyder said the most common failure modes in pavements with RCA are deteriorated mid-panel cracks and loss of load transfer. His group did a literature search and performed an array of field evaluations and laboratory testing with the following goals:
- Determine the causes of these problems.
- Develop guidelines for concrete mix designs that include RCA.
- Identify pavement designs that work with RCA.
Snyder focused his remarks mostly on pavement sections in the Upper Midwest. For example, he showed the results for a pair of 18-year-old sections of I-94 in northwestern Minnesota near Brandon—one with RCA and a control with no RCA. The RCA section has 77% mortar content, while the section without RCA has 66% mortar content; other RCA and non-RCA pavements in the study had similar percentages. Snyder reported that, predictably, higher mortar content often yielded a somewhat higher coefficient of thermal expansion and contraction. Though in 1994 both of these sections had been assessed as performing well, the RCA section had significantly greater distress when it was re-evaluated in 2006 (as shown in Table 4).
Table 4: Comparison of two sections of I-94 near Brandon, Minn.
Also, though testing on the two sections in 1994 produced relatively similar results, 2006 tests showed that the RCA section had consistently lower— though adequate—strength. Other Minnesota RCA sections evaluated in the 2006 study showed similar results: more distress and decreased (but adequate) strength compared with the non-RCA sections. However, sections that had been rehabilitated (mostly via retrofitted dowel bars, crack repair, and joint repair) were doing about as well as non-RCA sections.
Mix-design and structural-design guidelines
So what are the critical factors in the deterioration of RCA pavements? Snyder pointed to issues related to both mix design and structural design.
On the mix-design side, he said RCA sections performed equal to or better than control sections when the water-cement ratio was reduced in the recycled mix. He also mentioned that, in one case, concrete that had experienced significant alkali-silica reactivity (ASR) was used as recycled concrete. In that project (in Wyoming), the contractor used Class F fly-ash as an ASR mitigator and there has been almost no recurrence of ASR distress.
On the structural-design side, Snyder said panel length appears to be a major factor. Since high mortar content yields greater shrinkage (due to increased coefficient of linear expansion and less natural aggregate for restraint), shorter panels produce fewer cracks. He recommended panel lengths of approximately 15 feet (perhaps shorter on stabilized bases), showing that panels more than 18 feet long were more prone to cracking.
He also noted that controlling the process of crushing the recycled product can help to improve the resulting RCA pavement. He noted that impact crushing removes more mortar from the aggregate, making it perform more like natural aggregate; the removed mortar can then be used as fines or as backfill in other applications.
In summation, Snyder said any problems related directly to the physical properties of the mortar can be controlled by treating RCA as an engineered material.
Selecting the right fix: overlays, FDR, CIR
Gene Skok, retired from the Department of Civil Engineering, University of Minnesota, described a project he and Shongtao Dai of Mn/DOT conducted with funding from the Minnesota Local Road Research Board (LRRB). The objective was to lay out best practices for the selection of asphalt concrete recycling techniques: full-depth reclamation (FDR), cold in-place recycling (CIR), and mill and overlay (M&O).
For the project, the research team created a database of more than 100 rehabilitation projects by Mn/DOT and some cities and counties around the state. The surveys collected the location, original pavement construction, pre- and post-rehab conditions, and rehabilitation method.
The researchers then developed a set of decision checklists—covering geometrics, pavement condition, surface rating, and structural adequacy—for choosing a rehabilitation procedure. For example, the structural checklist asks if an existing HMA thickness and subgrade stiffness are adequate to support CIR equipment; if not, FDR would be recommended.
The report, MnPavement Rehabilitation Best Practices (2008-06), will be available on the LRRB Web site: www.lrrb.org.