Credit: Photos: VSL
Top: Although post-tensioning has been used for nearly 50 years, older systems focused more on obtaining the desired prestress force and less on durability. The industry has evolved to offer systems that deliver the desired prestress force with improved protection for the prestressing steel. Bottom: The elevated deck of the Consolidated Rental parking garage at BWI Airport was constructed using a post-tensioned concrete beam and slab system.
With unbonded post-tensioning, the prestressing steel is installed on the jobsite just before concrete is poured. The prestressing steel is greased and encased in an extruded plastic sheathing to prevent it from bonding to the concrete.
Stressed to the Max
Stressing records and load-cell testing were used to ensure the initial force after friction loss in each tendon was within 7% of the calculated value. According to VSL project engineer Lee Siridumrongphun, the team estimated short- and long-term losses based on ACI 318 and other technical sources. He added that realistic estimates are important at service loading areas because overestimation of losses can result in excessive camber and horizontal movement. Load cell monitoring helped calibrate and further refine the friction coefficient calculations.
Most of the top mild steel reinforcement in the slab was eliminated because of the use of bonded post-tensioning. With the exception of the top rebar required in the slab at the first interior beam support to accommodate larger bending moments at the end-span, the slab is virtually free of the top mild steel typical of such projects. This elimination will alleviate the concrete spalling that frequently occurs from corroded reinforcing steel. Additional savings were realized by the elimination of corrosion inhibitor admixture.
Another interesting feature of this project was concrete placement sequencing. The original design layout called for 93 concrete placements at approximately 10,000 square feet each. Building on expertise and careful preplanning, the team opted to increase the placement size to 33,000 square feet. This equated to each of the 36 placements averaging about 1100 cubic yards of concrete, said Paul Barry of La Plata, Md.-based Facchina Construction. Because of the large volume, most of the concreting was scheduled for early morning hours to ensure steady concrete delivery and to take advantage of cooler temperatures during the summertime. This change allowed associated bulkheads and joints to be eliminated, decreased the project schedule, and streamlined the post-tension stressing process.
The garage saw its first business in December and now serves as a one-stop rental car center, freeing up more than 1000 prime parking spaces in the terminal parking garage. Beyond the cars, parking spaces, and more than 73,000 cubic yards of cast-in-place concrete used to construct the foundations and superstructure during the two-year fast-track construction cycle, the project boasts a long-term, cost-effective solution with minimal maintenance.
— Crigler is senior vice president of VSL; Kline is manager of the company's Washington, D.C., branch.