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

Concrete Construction, June, 2004 by Carol Carder

"The previous system was buried and forgotten," says Curt Parker, chief civil engineer, Loris and Associates. "The new catch basins provide access for cleaning and maintenance."

To prepare the subgrade, GTC excavated 6 inches of the expansive soil and exported it offsite. Then the crews scarified the native soil to a depth of 12 inches, added 17% optimum moisture to expand the soil to its limit, and compacted it. Work crews covered the subgrade with a polyurethane liner and laid down the post-tensioning grid. Finally work crews installed the 4-inch-deep concrete slab in a monolithic pour.

Concrete mix and placement

Although the concrete specifications required only a 3000-psi mix, the track installer insisted on 4000-psi compressive strength. John McMillan at ASCI Readi-Mix worked with the contractor to design a 28-day 4500-psi mix with a midrange water reducer that gave the required finishability and desired set time. Since temperatures hovered in the high 90s on the day of the pour, ASCI used retarders to adjust the set time to approximately 3 hours.

The pour started at 6:15 a.m. with the first concrete pump and laser screed, and at 6:40 a.m. with the second concrete pump and laser screed. To keep the trucks off the post-tensioning grid as much as possible, trucks dumped their loads into two 28-meter concrete pumps that distributed the concrete to a 52-meter pump on one side of the track and a 58-meter pump on the opposite side.

The goal on the monolithic pour was 100 cubic yards an hour, 50 cubic yards on each side. By mid-morning, Luchts Concrete Pumping moved out the 28-meter pumps, and the concrete trucks carefully backed over the post-tensioning grids to unload into the hoppers for the larger pumps on the northeast and southeast sides of the track. Laying down and finishing the 700 cubic yards turned into an all-day job in the blistering heat. At just over 3 hours after placement and screeding, workers were able to get on the slab and begin finishing operations with riding trowels. The final finish was a light broom. The finishers met the smoothness criteria of no more than 1/8-inch variance in 10 feet, a standard set by the National Federation of State High School Associations and followed by its state affiliate, the Colorado High School Activities Association. In September, after the concrete had cured for more than a month, crews returned and installed the 12-mm-thick red polyurethane surfacing and the final striping.

Normally, owners replace the rubberized surface every 5 to 10 years. When the track is asphalt, this replacement includes milling off the top 2 inches and laying a new lift of asphalt beneath the topping surfacing. With the concrete, the extra expense of milling and replacing asphalt will be eliminated as only the rubberized surface will need replacing.

In replacing the track with post-tensioned concrete, the district is looking for long wear and lower maintenance costs. "We're always looking at fiscal impact on our physical assets with the tight operating and maintenance budgets, so we hope the surfacing material will last 10 to 15 years and the track much longer," Plute concludes.