The Richter scale has not twitched in the state of Tennessee for decades. Until a year ago it served as little reassurance for the people of Bristol, who were looking at a growing fault line and a surface that was shaky at best.
The weakening pavement at Bristol Motor Speedway was indeed starting to tremble under the consistent strain of 150-mph race cars every year. It was losing its fight against age and gravity. The concrete track, built in 1992, was being held together by an undersized lug and cable system that was quickly losing its grip. With the failing wires responsible for holding 31,000 lb of track, which turned steep around the turns, heave and spalling began to set in, and chunks of concrete were being spit up unpredictably.
Then there was that fault line, which was as wide as 10 in., splitting the track from the concrete wall. It was an ominous sign that something needed to be done.
“They had a small lug that wasn’t sufficient to hold the track up and overcome the pull of the cables and the gravity,” Stephen Swift, construction manager for Speedway MotorSports Inc., told Roads & Bridges. “What would happen is we had four joints that pulled the post tension, and it would heave those joints and create bumps in the track, and then it would spall those areas out and we had a chunk of concrete come out when one of the cables failed two years ago. It was just a ticking time bomb until that thing fell apart.”
It was a time bomb that racing officials had to hold and cringe for the next two years. Projects at other tracks pushed Bristol’s rebirth a couple of years down the line. In the meantime, conditions were falling further south. Cables were cut loose in March 2006.
“Once we cut the cables we knew we had to redo the track because there wasn’t anything holding the concrete together and the cracks were opening up bigger,” said Swift.
The concrete mix design also was in need of a redo. A heavy amount of clay was used in the 1992 creation, causing the concrete to turn brittle.
“You could look at it prior to repave and there were patches everywhere where the concrete had failed and spalled,” observed Smith.
The original track design also called for a 6 in. slab, which was eventually ground down to 3 to 4 in., leaving only 1 to 1½ in. of cover over the cables. As the thinner concrete deteriorated, cracks would form and moisture would seep in, creating a freeze-thaw effect.
A track with twinkle
Bristol detonated its time bomb with concrete breakers and saws starting on March 26. Using a squadron of excavators—three Caterpillar 330s, two Caterpillar 345s and a Caterpillar 325—Baker Construction Services, a hometown demolition contractor, began removing the troubled surface. On the straightaways, where slope was not a factor, crews saw cut 10-ft x 10-ft areas and used a Cat excavator armed with a crab bucket for removal.
Concrete on the turns did not give up as much of a fight as was expected. Cat excavators with concrete breakers handled the work. The old surface contained 3-in. dowel bars on a 4-in. x 4-in. pattern for reinforcement on top of the cables. Swift said the setup made demolition a breeze, and work was completed in about 2½ weeks.
“When we started beating on it, it removed pretty quick,” he said. “It was faster than we thought it would be with those cables.
“If you had a perfect world, that demo went perfect for us.”
Prior to 1992, asphalt was the racing platform of choice at Bristol. Once Baker Construction Services pulled all the concrete, it discovered about 10-14 in. of asphalt. The contractor milled everything that could be reached, and recycled it for new parking lot pavement outside the racetrack.
The problems at Bristol, however, ran deep. Once all the pavement was removed, crews discovered a series of drainage issues. Surrounded by hillsides, Bristol’s sunken infield became a haven for runoff.
“Once we got to dirt we found a lot of wet dirt,” said Swift. “We would hit shell seams that water would run out of for five hours. A 2-ft by 1-ft ditch had a nonstop water stream.”
The decision was to undercut an average of 4 ft of material all the way around the track. Geotech fabric was placed along with 12 in. of 4- to 5-in. stone and another 6 in. of ½- to ¾-in. stone. A French drain system covering every 100 ft also was installed, which was connected to the fabric and 12 in. of aggregate. Aiding in the installation was a 36-in. bore, which was installed in turns three and four 1½ years prior to reconstruction.
Brown shell material, which contained some clay and according to Swift, “compacted really tight,” was placed on top of the drainage system. “We were getting anywhere from 98 to 99% modified density [with the shell layer]. You could not drive a stake through it,” he said.
Six inches of pug mix followed. About 13% of the mix was of crushed-up limestone material, which allowed the layer to dry like concrete against the sun. Manufactured sand was the main ingredient, along with ¼-in. to 3/8-in. stone and smaller.
The track was now ready for its tough skin, skin that was perfected months earlier at the site of paving contractor Baker Concrete in Monroe, Ohio. Using mock-ups of the slopes of the track, different mixes were tested. Crews discovered wet concrete poured on 3- to 4-in. slopes ran down the hill. Drier concrete was the answer. Using less water, however, required more consolidation around the rebars, so it was decided that crews would run vibrators in front of the paving operation to make sure the right amount of consolidation was achieved.
“We own six tracks and all the rest are asphalt,” said Swift. “With concrete you just get one shot. What we ended up doing here was to make sure we had our stuff right on the first go-around.”
With tests completed the contractor was ready to make a circle of success. Two lifts, approximately 42 ft wide, were placed. The first was a 7-in. layer of lean concrete. Consisting of 100 lb of cement per yd; 250 lb of fly ash; 2,860 lb of sand; and a 1.5 water/cement ratio, the lean concrete, which did not contain any reinforcement, was poured on a 3-in. slump and contained 0-2% air voids. It strengthened to 1,200 psi in seven days. The 5-in. surface course contained silica sand from a mountain source instead of a river source, making it more of a dry mix. Also included were 560 lb of cement per yd; 1,500 lb of ½- to ¾-in. stone; 1,400 lb of sand; and a .42 water/cement ratio. Baker was looking for a 2- to 3-in. slump and 3-5% air voids. A spray cure called Confilm was applied, and after 21 days a 5,800 psi was achieved.
Rebar reinforcement was used instead of cables. Laterally, No. 5 steel rebar ran on 5 in. centers. Transversally, Baker ran the No. 5 steel rebar at 2 ft. centers.
Variable banking ranging from 24 to 30° was supposed to serve as an exciting new element to the track, and it certainly kept Baker on edge during installation. To handle the steep angles, a GOMACO SL-450 cylinder finisher and two work bridges were used. The equipment ran on tracks.
“The machine had a roller drum that ran up and down the slope,” said Swift. “They would move 2 ft and the roller would run up and down.”
Baker had a limited amount of rail. It covered only half of the track, so crews would lock it down, place the layer of lean concrete, move the paver back, raise it 5 in. and drop the surface course. The setup called for paving one day and executing site prep the next. The operation was a smooth one. Baker started paving the first week of June and had the job complete on July 3.
Work also was done on pit row, where 6 in. of continuous reinforced concrete was placed. They also placed two lugs—4 ft tall, 20 in. wide—below the track. Connecting the two lugs is an 8-in. structural slab, which was the final resting place for the track.
“When we did analysis on the stress and strain the track would have due to the gravity and the pull as it was expanding and contracting, we found a lot of the force was pushed at the bottom of the track,” said Swift. “The track wanted to slip down that slope. What those lugs do is push the pressure away from that point and make it go to four control joints instead of at the bottom of the track.”
And that fault line will never again rattle anyone’s nerves.
Crews used forms to place three types of walls—52-in.-high exterior, 42-in.-high interior and and 24-in.-high pit walls. A total of 7,200 ft of wall was installed.
Road rave
Six races christened the new track this year, and all the talk from racecar drivers has been positive. Prior to the new concrete being place, Bristol was known as a one-groove track. This restricted passing and usually resulted in single-file racing. After gathering feedback from drivers, racing officials pulled in a design engineer, Eberley and Associates, which specializes in racetrack design. Using a computer program, multiple grooves were figured into the track, and breath taking racing was born.
“[Eberley and Associates] figure out what the fastest way around the track is, and what is the best feel,” said Swift. “The main thing in racing is how the transitions form as you are coming out of the turn.”
The new transitions are now smoother and longer than their predecessors.
