November 2001

Marketing

Pavement Test Track Results May Affect Bid Specs

Pavement Test Track Results May Affect Bid Specs

DOT officials tell how results from Alabama’s test track could impact state specs

By Jai Wallace

Loading on Alabama¹s Pavement Test Track, above, began in September 2002. Drivers circle the track 16 hours a day, six days a week with the goal of applying 10 million equivalent single axle loads by November 2002. A 4,700-sq.-ft. testing lab, at right, serves as the site of the testing that takes place each Monday at the Pavement Test Track.

Officials from nine states traveled to the Pavement Test Track, Auburn, Ala., armed with their best asphalt mixes and a slew of questions about pavement performance. The idea behind this most recent track, managed and developed by the National Center for Asphalt Technology (NCAT), was this: Lay different mixes end to end, subject them to 10 million equivalent single axle loads (ESALs) from heavy trucks, a lifetime of truck traffic, and see how they fare. For the state agencies that sponsored sections on the track, the study provided the perfect opportunity to put their mixes under the microscope. According to NCAT’s Buzz Powell, test track manager, many of them came hoping to answer questions such as “How do fine mixes stand up against coarse mixes?”, “How do stone matrix asphalt (SMA) mixes measure up to Superpave mixes?” and “How does field performance correlate with lab performance?”
How does this testing relate to you as an aggregate supplier? Well, based on the data gleaned from the track, you could see spec changes or modifications in the future, according to state officials. For example, based on what happens with the Georgia Department of Transportation (GDOT) sections, state contractors could be laying fewer SMA mixes and more Superpave mixes, according to Don Watson, GDOT’s assistant materials engineer. Or, Florida aggregate producers, for instance, could be crushing more fine material in the future based on the outcome of the Florida DOT’s (FDOT’s) sections, according to Gregory A. Sholar, FDOT’s bituminous research engineer.
Although these changes will not happen overnight, it’s still important for aggregate producers and asphalt contractors to stay informed. That means knowing what’s being studied at the track and understanding how those factors could impact specs.

SMA takes on Superpave

Some state sponsors are interested in how Superpave mixes fare against SMAs, according to Powell. “We have numerous SMAs being compared to Superpave mixes,” he said. “We even have a gravel SMA, which is kind of an oxymoron in the conventional use of the terminology.”
The Alabama DOT (ALDOT) is one state agency comparing Superpave with SMAs, in addition to other pavement performance factors, according to Larry Lockett, the agency’s materials and test engineer.
Similarly, GDOT is comparing Superpave and SMA sections. According to Watson, the state developed two comparable mixes. “They both have a 0.5-in. NMA (nominal maximum aggregate) size,” he said, adding that both mixes include polymers as well.
Watson said that GDOT officials’ decision to study the two mix designs came down to cost factors. He explained that although officials are pleased with the performance they’ve seen from 10 years of laying SMAs on high-volume roads, they are also aware that these mixes can be expensive to produce and lay. He estimated the state spends 20 to 30 percent more of its available budget on SMA than on Superpave mixes. State officials hope the test track can shine some light on cost issues. “We want to know if the increase in performance with the SMAs justifies the increase in cost—or would we be just as well-served using our Superpave?” said Watson.
GDOT officials have their ideas about how the study will turn out. “We expect the SMA section to perform better as far as rut resistance and durability,” said Watson. “But that’s the need for the test sections—to verify this or prove that it’s not true.”
GDOT official base their beliefs that the SMA section will outperform the Superpave section on gradation issues, according to Watson. “SMAs are designed specifically for stone-on-stone contact,” he said. “Superpave mixes are not necessarily designed that way. They are, instead, designed within a gradation band, which may or may not ensure stone-on-stone contact.”
If the data from the test confirms GDOT officials’ prediction of SMA’s durability, Watson said the state’s contractors can expect more of the same. “We’ll be justified in our current policy of using SMA on high-traffic volume routes,” he said.
But if Superpave mixes outperform SMAs, state officials will take a serious look at specs, according to Watson. “We’ll have to determine whether the SMAs are worth the extra cost and possibly delete the use of SMA and stay with Superpave,” he said.
But, as Watson is quick to point out, that is only a possibility. “We may end up with a balance,” he said. “For instance, putting SMA on all of the interstate routes and routes over 50,0000 vehicles a day. Anything under 50,000 vehicles a day, or off the interstate system, gets Superpave.”
Such a spec would open the door for some of Georgia’s smaller contractors, according to Watson. “It could help them bid on work that, right now, they may not be inclined to do because of the extra SMA equipment that’s needed,” he said.

States Ask the Gradation Question

Beyond Superpave and SMAs, state sponsors want to know how gradation affects pavement performance. “For instance,” said Powell, “there are sections where states are looking at the percentage of stone in the mix to increase angularity and find out what impact that might have on rut resistance.”
Or take the South Carolina DOT (SCDOT) where officials put down a 0.5-in. NMA mix next to a 0.4-in. NMA mix. “They’re looking at the effect that a smaller topsize is going to have on the performance of the two sections,” said Powell.
Similarly, some state sponsors, such as ALDOT, are comparing coarse- and fine-graded mixes. “We’re trying to see if there’s a better gradation of asphalt that we can use,” said ALDOT’s Lockett. “We’re trying to measure their permeability and their rut resistance.”
Florida is interested in a similar study. The state sponsors two sections on the track. “We used a 0.5-in. coarse-graded and a 0.5-in. fine-graded mixture, both using the same aggregate components in different percentages,” said Sholar.
Sholar said the mix includes recycled asphalt pavement (RAP) and three types of stone. He explains that FDOT officials chose to compare coarse- and fine-graded mixes to help solve the debate of which performs better in high-traffic areas. “Superpave guidelines originally suggested that coarse-graded mixtures may perform better for high-traffic areas,” he said. “But subsequent studies suggested otherwise.”
FDOT’s current spec requires coarse-graded mixes on high-traffic highways, according to Sholar. But, he said, it could be advantageous if fine-graded mixes outperform coarse-graded mixes. “(Fine mixes) have an advantage because they are easier to construct and don’t have the permeability problems,” he said.
Sholar stops short of saying FDOT’s specs will change if the state’s fine-graded mixes outperform their coarse counterparts. Instead, he said state officials will take into account the results from other states that are doing similar studies. “That could provide enough information to warrant a specification change to allow fine-graded mixtures in high-traffic areas,” said Sholar.

Powell (center) discusses track matters with Sheila Hines (left) of the Georgia Department of Transportation (GDOT), and Kelly Fikes (right) of Vulcan Materials Co., Birmingham, Ala. Vulcan supplied the aggregates used to build the GDOT sections, as well as other sections on the track.

Correlation Questions Abound

Not all state sponsors are focusing on specific pavement performance factors, such as gradation, however. Some find that the greatest asset of the Pavement Test Track is that it can aid in establishing correlations between lab performance and performance on the track. This is the primary focus for Indiana Department of Transportation (InDOT) officials, according to Dave Andrewski, the agency’s materials engineer; and John Haddock, assistant professor at Purdue University, West Lafayette, Ind. Andrewski said InDOT’s primary goal is to study correlations between mix performance on the track and performance in the lab under torture testing devices, such as the accelerated pavement tester (APT) and the PUR Wheel.
Haddock noted: “What we’re basically trying to do is see if we can tie in results from the PUR Wheel and APT to the test track. We want to see how they relate to actual in-service pavements.”
InDOT officials used Superpave mixes in their test track sections. During the test track study, officials are also testing identical Superpave mixes in the lab using torture devices. They hope the track results will correlate with the lab results, according to Andrewski. He added that if this happens, contractors could benefit by having a device to use for quality control/quality assurance (QC/QA) measures.
Haddock agreed. “Hopefully, at some point down the road, (contractors will have) some type of test to determine whether a mix will impact the application in which it’s going to be placed,” he said.
That could provide contractors with a higher level of confidence when they go to bid a job, according to Lloyd Bandy, executive director of the Asphalt Pavement Association of Indiana Inc. “It (a correlation between the lab and the field) could provide the contractor with a higher degree of confidence in the predictions of performance of the pavement,” said Bandy. “And agencies have an even higher degree of confidence because we’re not dealing with hit and miss.”

Study Tracks Path to Future Changes

Beyond Superpave, SMAs, gradation and correlations, Powell said state sponsors are also curious about modifiers and open-graded friction courses (OGFC). “There’s a lot of interest in modified asphalts,” said Powell. “The perception is that you get better performance from them, but they’re difficult to work with, run through the plant and handle on the roadway. If they prove out, it could be that the DOTs move toward a more general use of modified asphalts. The track research will relate higher production costs to relative lifecycle costs so sponsors can make more informed decisions in the materials selection process.”
Powell adds that three of the track sections are constructed with OGFC, or permeable surface, mixes. “They’re of great interest,” he said.
When the Pavement Test Track study wraps up November 2002, the asphalt industry will know more about pavement performance. State sponsors will have answers to their questions. And, down the line, the aggregate industry may be producing stone for a different set of specifications.

Jai Wallace is a contributing editor for AggMan.