A 23.8-mi. (38 km) stretch of Interstate 75 between Atlanta and Macon, GA, has been resurfaced twice in three years, and the rate of deterioration has not been pleasing to taxpayers, motorists and the transportation budget.
But Georgia Department of Transportation (GDOT) officials said they have learned an important lesson from the episode, a lesson that they are applying around the state.
The cause of the deteriorating roadway was determined to be an unstable bottom course of pre-existing asphalt, a condition that was not readily apparent to GDOT superintendents and engineers. To hereafter detect it, GDOT engineers are running a more thorough test on old asphalt.
“It’s embarrassing,” said Harold Linnenkohl, GDOT deputy commissioner. Besides embarrassing, failure to pave I-75 successfully the first time has been costly to taxpayers and aggravating to local drivers and travelers. The embarrassment came to light in June of last year when GDOT officials monitoring the work noticed that rutting had begun to occur in two of three repaved lanes. The discovery came near the end of the mill and inlay project that spanned Henry, Butts and Spalding counties. The contract had been let in August 2000.
“We were very near to finishing that project,” Linnenkohl said. Rather than wrap it up, GDOT engineers had to open it up to new scrutiny. Engineers had tested the old pavement before the contractor’s milling machine ever started sinking its teeth into it. That testing revealed no structural problems in the asphalt, which dated from the 1970s when north and southbound lanes were laid atop and alongside some original concrete paving.
“We determined from our testing that we needed to do just a minor mill and inlay, not take it clear down to the pavement,” Linnenkohl said. The conclusion was reached after drilling core samples 4 in. (10 cm) deep.
But when the rutting began and engineers returned to take more core samples, they drilled deeper. They found a bottom layer of asphalt mix that would not firmly adhere to the stone. It would “strip out,” or disaggregate, which meant that the new surface of the road was being laid atop a shifting base. Only the two lanes of asphalt overlaid on the old concrete pavement were failing. A third lane laid alongside was good all the way down.
Engineers concluded that the weight of construction traffic on segments of the still-uncompleted roadway weakened the loosely-bonded stone and asphalt mix next to the concrete bed.
But more than gravity was responsible for the deterioration. Engineers have reached several tentative conclusions about why the situation developed. First, they believe a liquid anti-strip additive used during the 1970s and early 1980s did not adequately bond with the granite stone primarily used in Georgia asphalt plants.
“For that time period, it was the best there was,” the deputy commissioner said of the additive. But the hard stone used in Georgia is susceptible to stripping action and the additive does not seem to securely affix asphalt to the stone.
Hydrated lime now is the binding agent of choice for the materials. It seems to hold up in tests in which the liquid additive fails. Linnenkohl said engineers also believe the asphalt might not have come apart if the removal of the upper layers of asphalt hadn’t exposed the lower layer. The pressure of construction equipment rolling atop it, combined with the heat radiated from the new hot mix, seriously destabilized the old asphalt and left it vulnerable to subsequent pounding by compaction equipment.
With an annual increase of approximately 10,000 cars per day, Linnenkohl also noted that the traffic load I-75 carries now is far more than when it was originally designed.
Numerous factors came into play, Linnenkohl said, and GDOT engineers have changed testing procedures in hopes of not being fooled again. A lot more core samples are being taken, for one thing, and they are 2 in. (5 cm) deeper than required previously.
The sampled material also is being more rigorously tested. Engineers fabricated a mold in which a sample is placed and subjected to increasing amounts of moisture, pressure and heat.
“We want to get some kind of determination if the asphalt is going to slip out,” Linnenkohl said. “This test is giving us a better indication.”
In the end, the failed I-75 project was a $10-million mistake, which is how much money was expended on the first resurfacing. That was fewer dollars than would have been spent had the project been totally finished before the problem was discovered, but not by much.
While the new asphalt laid the first time around is being reclaimed, the time and labor used to lay it will not. The contract for the second resurfacing is twice as large — $22 million. That reflects deeper milling but also includes paving some pull-off areas for law enforcement patrol vehicles, upgraded guard rails and a more expensive friction course.
A new contract was let in March to C.W. Matthews Contracting Co., of Marietta, GA, and APAC-Georgia out of Stockbridge and is going “real well,” Linnenkohl said. Matthews is reworking the southbound lanes, APAC the northbound. Contractors are finishing the mill and inlay course months early and hope to complete the final course this year. If not, the task still is expected to be wrapped up well before the June 2004 end date, Linnenkohl said. He and contractors credit a good working relationship for the speedy work.
Most of the second resurfacing has been done at night, with double-lane and single-lane closures allowed at different stages of construction and on certain days of the week.
Illuminated by halogen lighting, the Matthews crew of approximately 20 is cleaning up and paving behind a milling crew from Miller Group Inc., of Morrow, GA. Both Matthews and APAC subcontracted with Miller to remove the old asphalt. CMI Terex milling machines are used.
Matthews pavers employ a Roadtec Shuttle Buggy system and Caterpillar rollers. It is hauling asphalt from the company plant at Forest Park near Atlanta, GA.
C.W. Matthews Contracting Co. dates from the 1940s and laid miles of interstate highway during the period when the unstable additive was used by the industry. Travis Padgett, Matthews vice president, said his company does not appear to have laid the original I-75 asphalt during that period, though it used similar technology on interstate work north of Atlanta.
The I-75 situation was just part of a pattern of unstable asphalt that GDOT engineers recognized and acted upon. A similar problem had developed on a segment of the Interstate 285 belt around Atlanta.
Recognizing the common thread running through those two projects might have saved everyone a headache on a third project — resurfacing of 6 to 8 mi. (10 to 13 km) of westbound lanes of Interstate 20 near Atlanta. When material disintegration was found at that location, GDOT officials reacted quickly.
“We’d already let the contract when they found deterioration of a similar sort there,” Linnenkohl said. “So we said, well, before the contractor gets started, let’s check.”
It proved to be a good decision. Testing found the same bottom level problem. GDOT officials modified the contract, changing it to a deeper milling project that eliminated the threat.
Highway officials have estimated as many as 190 mi. (306 km) of Georgia interstate have the seed of asphalt instability planted in them. Apparently, the problem is not just found in Georgia. Linnenkohl said he is aware of similar difficulties in at least two other states farther west.
Dorothy Daniel, the communication specialist for District Three of GDOT, which contains the troubled stretch of I-75, said in a news release that “an identification project is under way to pinpoint other possible interstate locations [in Georgia] that may have this problem.
“We learned a great deal from the I-285 project and are learning even more with this one, so we anticipate being able to identify the problem even earlier,” she said in the release.
That is good news for hundreds of thousands of motorists who travel Georgia’s interstates every day. Linnenkohl acknowledged that the back-to-back I-75 resurfacings were especially aggravating. “People were inconvenienced very heavily.”