There is little argument that today's owners have a vested interest in using innovative techniques that not only provide efficient and cost-effective construction solutions, but also provide long-term value in terms of life-cycle cost. One growing solution — the VSLABTM post-tensioning system — has proven to be a leading means of minimizing maintenance costs of concrete structures.
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. Benefits include greater clear spans for the same member depth, enhanced crack control, larger floor-ceiling heights or reduced building height, less cladding, smaller foundations because of the reduced structural dead load, and reduced maintenance.
Post-tensioning can be either an unbonded or bonded system. Unbonded tendons, comprised of 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.
A bonded tendon, however, is normally comprised of multiple post-tensioning strands. And, through its design, the bonded tendon forms a continuous bond along its length with the surrounding concrete. Bonding is achieved through cementitious grout that is injected after the tendons are stressed.
Using bonded post-tensioning in parking garages
Bonded systems offer a significant design advantage over unbonded systems in parking garages, and this factor leads to life-cycle savings. The key design feature is that the hardened grout locks the movement of the post-tensioning strands to that of the surrounding concrete. Hence, the force in a bonded strand is a function of the deformation of the surrounding concrete.
Also, bonded systems offer several practical benefits, such as reduction of mild steel, particularly at the top of slabs. Reducing reinforcing steel is especially important because most maintenance costs are associated with spalled concrete and corroded rebar. The bonded systems also offer more flexibility in terms of structural modification for stairwell openings, utility access, and future expansion.
Building on the benefits of bonded post-tensioning, the VSLAB system provides total encapsulation of the strands using a high-density plastic duct with watertight mechanical duct-toanchorage couplers. Developed by VSL — a firm specializing in the design, manufacture, and installation of posttensioning and special construction systems — this proprietary technology was created specifically to minimize longterm maintenance expenses. The system leverages the benefits of VSL bridge systems, including full encapsulation and bonded tendons, for other end-uses.
Permanent end-caps (for both beam and slab tendons) are included to completely seal the anchorages. High-performance grout pumped through the tendons provides an additional layer of protection.
The VSLAB Plus system increases durability while lowering life-cycle costs.
Further, the system can reduce substantially the amount of rebar in a slab, which eliminates the chance for corrosion and thereby increases durability.
Implementing the new technology
As one of the nation's fastest growing airports, the Baltimore Washington International Airport (BWI) developed a five-year improvement plan to upgrade the airport's functionality. The plan included a new, state-of-the-art parking garage that would consolidate eight rental car companies into one central location. Recognizing the bonded post-tensioning system to be optimal for BWI, a design concept was selected for the 3.5 million square feet of parking space, which included more than 1 million square feet of elevated,post-tensioned, cast-in-place concrete.
A single-level, elevated concept was selected to allow all rental car companies to operate from the same level. The structure is approximately 1,500 feet long by 840 feet wide and is founded on spread footings. The slab-on-ground is 6-1/2 inches thick, reinforced with welded wire fabric, and includes contraction joints at 20 feet on center, each way. The elevated deck consists of a post-tensioned concrete, one-way beam and slab system. The elevated slabs are 5-1/2 inches thick, and span 20 feet between beams, which span 60 feet to girders or columns.
The VSLAB post-tensioning system worked particularly well with garage functionality requirements. While standard commercial parking garages typically have 20- to 30-foot-wide by 54-foot-long bays, the use of posttensioning enabled column grid spacing for this project to be increased to 60 feet by 60 feet, allowing more flexibility for the rental companies. According to structural engineer Walker Parking Consultant's Project Manager Jason Gross, "The larger bay spacing was important because each rental group wanted as much clear space at the bottom level as possible to allow for staging and multi-directional traffic." Also, the floor-to-floor height was increased 19 feet in order to provide customers with the feel of an open structure.
Strict control of stressing and grouting operations was required because of the large quantity of strand involved (more than 3 million linear feet) and the aggressive placement schedule. Slab tendons were partially stressed to 10 kips per strand on the day after concrete placement to help control shrinkage cracking. They were completely stressed once concrete test cylinders, which were cured under jobsite conditions, reached a compression strength of 3,200 pounds per square inch (psi). Partial stressing of beam and girder tendons was not required. These members were fully post-tensioned when the concrete compressive strength reached 3,200 psi.
Most of the top mild steel reinforcement in the slab was eliminated because of the use of bonded post-tensioning.
With the exception of top rebar 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 alleviates concrete spalling that frequently occurs when rebar becomes exposed to harsh weather conditions over time. Additional savings were realized by the elimination of corrosion inhibitor admixture.
The garage now serves as a one-stop rental car center — freeing up more than 1,000 prime parking spaces in the terminal parking garage. And, the owner benefits from a garage that requires minimal maintenance and will have a cost-effective lifespan.
John Crigler, P.E., is vice president and technical manager of VSL, a Structural Group company that designs, manufactures, and installs post-tensioning and special construction systems. He has served the company for more than 25 years, and possesses extensive knowledge in the design and construction of prestressed concrete bridges, buildings, and tanks. He can be reached at 410-850-7000.
Donald Kline, P.E., has more than 15 years of experience in the design and construction of post-tensioned concrete structures. He currently serves as branch manager for VSL's Washington, D.C., operating center. He can be reached at 703-451-4300.