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.
By industry association figures, it costs $10,000 to $12,000 per space to build a new multi-level garage. Even more staggering is the estimated annual cost of $650 per space to simply operate the structure, including lighting, cleaning, employees, elevators, and gate equipment.
Based on these estimates, there is little doubt the parking industry has a vested interest in employing efficient, cost-effective construction solutions and long-term value relating to life-cycle cost. In the case of one of the nation's fastest-growing airports, a unique post-tensioning system was chosen to minimize maintenance expenses.
Located between Washington, D.C., and Baltimore, Baltimore Washington International Airport (BWI) serves as a powerful economic engine for the region. More than 52,000 people traveled through BWI each day in 2002, and business revenues for central Maryland exceeded $7 billion that year, with a significant portion of revenue coming from rental car usage.
Growth creates the need for greater infrastructure. By the late 1990s, this rapid increase in service led to growing concerns about parking availability at the airport. In 2000, the Maryland state government and the Maryland Aviation Administration announced a five-year improvement plan to upgrade the airport's functionality; it included a new state-of-the-art parking garage that would consolidate eight rental car companies into a central location.
One result of the plan was the assembly of an expert team in Maryland to design the optimal solution for the new BWI Consolidated Parking Garage. Structural engineering firm Walker Parking Consultants, based in Kalamazoo, Mich., and design firm Michael Baker Corp., Moon Township, Pa., were selected. According to Bill Reitner of Walker Parking, the first priority was selecting the most advanced design and construction materials to maximize durability and minimize maintenance for the structure's life cycle.
“After considering various other options such as precast concrete and unbonded post-tensioned concrete,” said Reiter, “we selected bonded, post-tensioned, cast-in-place concrete to minimize joints and promote long-term durability.”
The design consultants turned to VSL, Hanover, Md., a firm specializing in design, manufacture, and installation of post-tensioning and special construction systems. In addition to the company's VSLAB post-tensioning system, plans for the project included fast-track scheduling, pre-construction scale mock-ups, large-area concrete placements, and architectural precast wall panels.What is Post-Tensioning?
Post-tensioning is a technique used to counteract tensile stresses and deflections from externally applied loads. Unlike mild steel rebar, post-tensioning provides “active” reinforcement. This is accomplished by introducing a prescribed magnitude and distribution of internal loading using seven-wire, grade 270 ksi post-tensioning strand systems. Post-tensioned structures are known for their strength and durability. Benefits include greater clear spans for the same member depth, enhanced crack control, larger floor-to-ceiling heights or reduced building height, less cladding, smaller foundations, and reduced maintenance.
Used in the United States for more than 40 years, post-tensioning design for parking garages can be either unbonded or bonded. Unbonded tendons, comprising single strands (monostrands) covered with a grease coating and enclosed in high-density plastic extruded sheathing, do not form a bond along their length in the concrete. The force in the stressed tendon is transferred primarily to the concrete by end anchors.
A bonded tendon, however, is made of multiple post-tensioning strands and, by design, forms a continuous bond along its length with the surrounding concrete slab, beam, or girder. Bonding is achieved through cementitious grout that surrounds the strands. The grout acts with the duct that is encased in the concrete member to complete the bond path between the post-tensioning strands and the concrete member.
Flat, corrugated high-density polyethylene (HDPE) ducts that house between two and five strands are used for bonded systems in thinner members such as slabs; larger, round ducts (HDPE or galvanized metal) are used in beams and girders. Today, bonded post-tensioning systems are the industry standard for concrete bridge construction and are gaining popularity in the parking garage market.