A post-tensioned track: expansive soil problems led one school to a monolithic solution

Concrete Construction, June, 2004 by Carol Carder

Due to expansive soils, the first asphalt running track built in 1987 at Horizon High School in Thornton, Colo., fell apart within 3 years. In 1990, the district installed corrective drainage and built a second asphalt track, but the 18-inch deep storm drain that also served the football field was no match for the area's expansive soil. Within a few years, wet expanding soil once more made the inside lane unusable. Looking for more durability, in July 2003 the Adams 12 Five Star School District installed a 400-meter, 8-lane post-tensioned concrete track in a 4-inch monolithic pour.

Adams 12 Five Star School District had previous experience with post-tensioned tracks, having installed its first at Five Star Stadium in 1999. That track has performed well, according to David Plute, Adams 12 Five Star School District construction manager. Unlike the new Horizon High track, the Five Star Stadium track has four construction joints, one at each radius. The construction crews filled the joints with surfacing material in the original construction and returned in 2001 and applied some caulk. Since then, the district's maintenance staff has found no movement at the joints in spite of the expansive soil, and no further treatment has been made.

Chris Valduga, post-tensioning designer, Post Tension Group LLC, Lakewood, Colorado, says, "To my knowledge, the Horizon High School project is the nation's first monolithic pour for a track." From a construction standpoint, the monolithic pour saved time and dollars with a single mobilization and placement of more than 700 cubic yards, rather than up to four mobilizations for four pours with joints at each radius as was done at the Five Star Stadium track. From a performance standpoint, eliminating the joints eliminates any possible independent movement of any section of the track. Even though the jointed Five Star Stadium track continues to perform well, the district wanted to improve the design by eliminating joints.

"The track we tore out looked like a roller coaster with cracks up to 2 inches wide and some sections 2 or 3 inches higher than a section 20 feet away," says Russ Wright, superintendent for Golden Triangle Construction (GTC), the prime contractor.

"In conversations with the architect, and the contractor, we realized [that] without significant soil removal and replacement, the asphalt wouldn't last," says Mike Gesie, Adams 12 Five Star School District project manager. Geotechnical engineers CTL|Thompson recommended overexcavating and replacing 5 to 6 feet of topsoil to build a non-expansive base. Plute determined that an asphalt track would actually have cost slightly more than the concrete post-tensioned alternative when one added the high costs of overexcavation and soil replacement.

"This site is too tight for heavy earth-moving equipment so we would have used smaller equipment, making it more expensive to take out the spoils and move in the structural fill," Plute explains. "If we were going to invest $300,000 to $500,000 in a track facility, it made sense to go for the benefit of that structural system." (The construction budget, without design and contingencies, totaled $560,000.)

Why post-tensioned concrete?

Explaining the advantages of post-tensioning, Valduga says "We have constant compression in either direction so the slab actually floats." If movement occurs in the expansive soil, the track performs independently rather than moving with the soil. "In Europe, post tensioning in concrete pavement extends the life of concrete roads practically indefinitely, so it makes sense to apply this same principle to concrete oval tracks," Valduga adds.

The 40,000 lineal feet of 1/2-inch cable of seven-wire tendons is unbonded. That is, it is encased in plastic sleeves, so the cables don't adhere to the concrete. The tendon spacing is 12 inches on center longitudinally and roughly 3 feet on center transversely. Crews applied half the stress 24 hours after concrete placement to prevent random cracking. Then, when the concrete reached 3000-psi compressive strength, the crews finished the post tensioning, first pulling the longitudinal cables, then the transverse cables, both to 33,000 pounds maximum and anchoring the ends in place. The resulting post-tensioning pressure is 27,000 pounds after some relaxation in the steel cables.

Valduga expects a long life for the post-tensioned concrete portion of the track--easily reaching 30 to 50 years based on the continued outstanding performance of the first post-tensioned concrete warehouse and housing slabs installed in Houston in 1962. The Post Tensioning Institute has checked the anchors on these projects and verified that they are still intact.

Preparation under the track surface

After removing, demolishing, and exporting the old asphalt, GTC installed the drainage system and prepared the subgrade for the track installation. To handle any water that migrates under the track into expansive soils, Loris and Associates, the civil engineers, designed a 3-tier drainage system An ACO sport drain that's an integral part of the track on the infield edges will catch runoff water from the 1-percent sloped track surface. Beneath the surface drain, a layer of 1-inch to 1 1/2-inch rock leads to a French drain. The French drain in the trench under this rock consists of a perforated pipe wrapped in geotextile. Water collected by the ACO drain and the French drains descends through 12-inch PVC pipes to catch basins tied into the existing 12-inch storm sewer. In the retrofitting, GTC also connected the French drains under the football field into the new catch basins.