o Research into Perpetual Pavement design deepened as the Pavement Test Track of the National Center for Asphalt Technology (NCAT) at Auburn University began a second iteration of pavement testing, including Perpetual Pavement designs.

o Building of durable HMA pavements by rubblizing portland cement concrete (PCC) pavements to create a super-strong base that eliminates reflective cracking in HMA overlays is exciting contractors anew, as seen in a recent demonstration.

o New low-temperature "warm mix" designs from Europe -- beneficial due to their lower-VOC ("fume") emissions and lower fuel costs -- are getting attention nationally, and will be a focus of the World of Asphalt 2004 show and conference in Nashville March 15-18.

o In 2004 the industry is ramping up its vision of HMA as a paving material as durable as portland cement concrete via the promotion of super-strong stone matrix asphalt (SMA) as a companion to Superpave mix designs. A new permutation is SMA pavements that can be "gritted" with fine-graded screenings mixed with only 0.5 percent liquid asphalt to fill surface voids.

o Superpave mix designs continue being adopted throughout the United States, have become the new standard for most state DOTs, and now are penetrating the county and local levels of government. The Federal Highway Administration (FHWA) is validating new "Superpave Plus" mix designs that incorporate polymer modifiers.

o Counterbalancing the Green movement, HMA is positioning itself as THE environmentally friendly pavement, 100 percent recyclable, at 73 million tons by far the most-recycled product in terms of volume in the United States. Asphalt plants have greatly reduced their particulate and VOC emissions, on the order of 97 percent in the last 50 years, according to the Asphalt Pavement Alliance.

o The new open-graded friction courses (OGFCs) -- now feasible due to the increased use of polymer modifiers -- are promoted as environmentally friendly due to their enhanced tire-pavement noise attenuation, on the order of 3 to 5 decibels on high-speed roadways, while eliminating wet-pavement tire spray, with enhanced visibility and pavement friction.

o And the new "porous asphalt" pavement designs -- not to be confused with OGFCs -- permit quick drainage of storm water to catchment areas and help urbanized areas meet EPA Phase II stormwater pollution regs now impacting smaller municipalities. Portland cement concrete also is promoting "pervious concrete" designs for the same purpose.

As 2004 begins, HMA is accelerating its transition from being a reliable, commonplace overlay medium, to a sophisticated, versatile, engineered product useful for many different purposes.

"From the ultra-thin HMA gritting mix, to the thin smooth seal preventive maintenance  mixes used in Maryland and Ohio, to thick full-depth structural sections used for heavy duty applications, asphalt can do it all," said Mike Acott, president of the National Asphalt Pavement Association. "This versatility in our product is a major reason that HMA is the pavement selected by most pavement specifiers, not because of the industry's marketing efforts."
 

Perpetual Pavement promotion

Notwithstanding Acott's comments about marketing, the industry is indeed pumping prodigious resources into promotion, with Perpetual Pavement at the forefront of the efforts of the Asphalt Pavement Alliance (APA), a relatively new coalition made up of NAPA, The Asphalt Institute, and the State Asphalt Associations in 36 states.
 Perpetual Pavement is a three-layer pavement design which should provide pavement life spans of up to five decades, with follow-up asphalt overlays as a sacrificial friction course to continue smoothness.

Perpetual Pavements use multiple layers of engineered, durable asphalt to produce a safe, smooth, long-lasting road, APA said. Most critical is a sold base which resists cracking that could be perpetuated to the surface.

o Perpetual Pavement begins with a strong, yet flexible bottom layer that resists tensile strain caused by traffic, and thus stops cracks from forming in the bottom of the pavement.

o A strong intermediate layer completes the permanent structural portion.

o A final layer of rut-resistant HMA yields a surface that lasts many years before scheduled restoration.

"The concept of perpetual asphalt pavements is rapidly gaining acceptance in the United States," said the Transportation Research Board (TRB) Committee on General Issues in Asphalt Technology in December 2001. "The idea is to extend the 20-year life expectancies of hot-mix asphalt pavement to greater than 50 years."

To do so requires combining a rut-resistant, impermeable, and wear-resistant top structural layer with a rut-resistant and durable intermediate layer and a fatigue-resistant and durable base layer, the TRB committee said. "With these pavements, only periodic surface restoration is necessary, offering advantages in speed of construction (user delay costs) and construction costs," it said.

One way APA is promoting Perpetual Pavements is through an awards program. Now in its third year, the program extols pavements which meet strict criteria and demonstrate long-life characteristics of HMA, in addition to excellence in design, quality in construction, and value to the traveling public. All winning pavements were constructed at least 35 years ago.

Another way is through technology transfer. via its web site, http://www.asphaltalliance.com, APA distributes an interactive CD-ROM titled "What if Roads Could Last a Lifetime?", and a variety of technical papers on Perpetual Pavements.
 

New research at NCAT's track

Field validation of an HMA mix which incorporates elements of Perpetual Pavement design began in October at NCAT's Pavement Test Track in Alabama. One test section has a rich HMA base course with a binder content of optimum plus 0.5 percent. "The rich base has a potential of minimizing propagation of fatigue cracks from the bottom upwards," NCAT said.

"This work will substantiate some of the Perpetual Pavement issues," said Dr. Marvin Traylor, P.E., director of engineering & research, Illinois Asphalt Pavement Association. "The original test track looked at how various mix designs stood up under heavy traffic to resist rutting. The second phase is taking a look at the total pavement structure to find out how stresses and strains develop in thick, full-depth HMA pavement, and add to the body of knowledge that suggests properly constructed full-depth pavements don't fail structurally, but merely wear out at the top.

"This means they have a perpetual structure, and that they can be repair economically and rapidly at the top, which makes them ideal for urban expressways," Traylor said. "When it comes time to fix them, you don't have to dig them out and patch them with barricades up forever; you can send in a crew at night, mill off the top few inches, pave it back, and by morning rush hour the road is open."

In December 2002 the test track completed some two years of field testing of asphalt pavement sections using real-world loadings in test rigs driven by professional drivers. At the end of 10 million equivalent single axle loads (ESALs), NCAT found Superpave mixes do indeed stand up to truck loads with very minimal rutting, thus validating laboratory testing. Following reconstruction in summer 2003, the track began another 10 million-ESAL test phase in October which will last through 2004.
 

Perpetual Pavements in Illinois, Texas

In spring 1999 the Illinois General Assembly passed legislation that required Illinois DOT demonstrate construction of pavements designed for a 30-year life cycle. In partnership with private sector interests, IDOT selected a section of I-70 in Clark County near Casey, Ill., to be reconstructed with an HMA pavement design for 30 years of traffic.

In addition to the structural design of the pavement, this section included several new specifications to enhance the performance and durability of the asphalt pavement and concrete bridge structures.

"Our state legislature has indicated interest in extended-life pavements," said Steve Lackey, P.E., District 5 materials engineering, Illinois DOT. "A 20-mile stretch of I-70 had continuing problems with rutting, and the pavement exceeded its life span. The asphalt industry brought forth the idea of using extended life concepts on this section."

In this instance, rubblization -- the in-place shattering and compaction of the existing PCC pavement as a super-strong base course -- became part of the project. "The existing PCC pavement was a good candidate for rubblization and the contract was awarded," Lackey said.

And in July 2003, a open house was conducted to demonstrate these concepts to contractors and agency personnel alike. This pavement showcase, co-hosted by IDOT, the FHWA, and IAPA, attracted DOT and industry personnel from several states and as far away as Ontario. Speakers from IDOT emphasized the series of previous successful rubblization projects in Illinois and pavement design, material selection, mix design, and rubblization techniques were 
covered.

The 11-mile project required 550,000 tons of HMA for use on this heavily traveled truck route, burdened with 49 percent truck traffic.
 In Texas, the Texas DOT continues to apply the Perpetual Pavement approach to major interstate reconstruction projects, having let three additional projects in late 2003. Two of these projects, I-35 in Hill County and I-35 in Webb County, were designed using HMA instead of concrete at least partly due to the successful application of this approach on previous projects in those two TxDOT districts, reported the APA.

Another project (SH 114, Fort Worth District) includes segments where a Perpetual Pavement and a thick, continuously-reinforced concrete pavement are being constructed where there is extremely heavy truck traffic on a route connecting major aggregate quarries to the Dallas-Fort Worth Metroplex. This project was originally designed in 1997-98 and was only recently let to contract. It will provide an apples-to-apples comparison between the two major approaches that TxDOT is using for "heavy-duty" pavements.
 

Warm HMA from Europe

While today's Superpave is a U.S.-developed product, some of the most innovative improvements in HMA mix technology have migrated from Europe. These include stone matrix asphalt (SMA) and open-graded friction courses (OGFCs). But the latest import from Europe is so-called "warm" asphalt mixes, mixed and placed at temperatures 100 deg below that considered feasible.

Warm mixes were previewed by U.S. delegates on an international technology scanning tour in 2002, but they were introduced to these shores at the 2003 annual meeting of NAPA, where a blue-ribbon panel of experts described European practice.

Immediate benefits of warm mixes include lower costs of fuel required to heat the asphalt mix in production, reduction in asphalt fumes produced,  change in the chemical nature of the fumes, and big reduction in emissions from asphalt plants both in volatile organic hydrocarbons (VOCs) and greenhouse gases such as carbon dioxide and water vapor.

Warm mix is possible through three different processes. One involves addition of a synthetic aggregate during HMA mixing, which releases a small amount of moisture which creates a foaming effect in the binder, reducing viscosity and the need for higher temperatures. Another incorporates a foamed hard binder which enables lower mixing temperatures. A third method introduces organic additives with the liquid asphalt to reduce viscosity at mixing and compaction temperatures.

Now warm mixes are being studied to determine if warm mix technology is compatible with U.S. practices, and whether there will be a tradeoff in mix performance. "There are questions about whether the processes and products are compatible with the mix designs, equipment, climate conditions and work practices in the United States," NAPA's Acott said in April. "We must not lose any of the significant performance benefits of SMA, Superpave, or mixtures containing reclaimed asphalt pavement (RAP)." A research program initiated by NAPA at NCAT may answer some of those questions.

A demonstration of warm mix technology took place, on Thursday, March 18, at the conclusion of the World of Asphalt 2004 show and conference March 15-18 in Nashville.
 

SMA gets new attention

In the midst of warm asphalts and Perpetual Pavements, durable stone matrix asphalt (SMA) is getting a close look as states such as Maryland and Virginia use performance-graded asphalt binders (as are used for Superpave) with SMA design.

SMA is a gap-graded mix, a German import which brings together coarse, durable, premium aggregate with as much as 6 to 8 percent liquid asphalt. This high asphalt content contributes to a durable HMA mix with a stone-on-stone structure which readily fights rutting.

This stone-on-stone contact develops internal friction and opposition to shear. SMA was brought to the U.S. in 1990, one year ahead of Superpave, following an FHWA technology scanning tour.

Now states such as Maryland, Virginia and Georgia are setting the pace by melding Superpave design principles into SMA mixes, making a "gap-graded Superpave" that brings together the best of both worlds.

While SMA carries a cost premium over Superpave, new research presented at last year's TRB meeting demonstrated that SMA is more durable than Superpave mixes. The paper, An Updated Review of SMA and Superpave Projects, by Don Watson, P.E., research engineer at NCAT, said SMA mixes appeared to be more durable than the Superpave mixtures seen, for example, showing no cracking where the Superpave mixes had cracked.

SMA's gap-grading also results in a surface that provides more friction and less noise created at the tire/pavement interface. "Noise reduction with SMA and open-graded friction courses is a big issue for the HMA industry," said IAPA's Traylor. "This is an issue in urban areas where there is opposition to widening highways. Some in the industry are making the point in which we can top the pavement with HMA and lower the decibels to where they've never been before, whether or not a lane is added."
 

Superpave aims at local agencies

In the meantime, Superpave has spread to all but two states of the union, with 48 state DOTs permitting or requiring its use (California and Nevada being the hold-outs). Now, industry attention is turning to educating municipal and counties agencies to Superpave's benefits.
 "There seems to be a special interest in educating local agencies," Traylor said. "Some local agencies seem to be perplexed about Superpave even as the state DOTs have folded it in and have a lot of expert people. But city personnel have to be 'jacks of all trades'.

"So the amount of information being filtered down on understanding of Superpave is not what we would like," Traylor said. "But now we're starting to focus on helping them understand the terminology and concepts so they can specify Superpave properly and get the best bang for their buck."

Superpave (R) -- a registered trademark of the TRB -- is a performance-based system of specifications for designing asphalt pavements to hold up to the demands of 100 years.

This performance-based approach offers more durable pavements that are specifically designed with local temperature extremes and traffic loads in the equation. Superpave designs are providing longer-lived asphalt pavements that will stand up to local climate and traffic volumes at lower long-term costs.

A research project at the University of Texas at El Paso found that for the 2002 construction season, the most recent year for which firm data are available, some 4,726 scheduled projects were designed using Superpave procedures.

That's about three times the volume of Superpave in 1998, and some 60 percent of the state HMA projects planned for 2002, said the TRB Superpave Committee. And now that the price differential between conventional Marshall mixes and Superpave mixes is narrowing -- to within $1 per metric tonne (1.1 short tons) -- the enhanced durability of performance-graded Superpave mixes means that Superpave now is competitive in price.

The shift to Superpave is so complete that local governments may be receiving Superpave mixes from asphalt plants even when they don't specify them. For example, in the upper Midwest, if an AC-20 unmodified asphalt binder is specified, it likely will receive a PG 64-22.