In 2000, the owner of a 30-year old, 180-car parking garage in Cleveland asked us to investigate the cause and implication of two pairs of post-tensioning tendons that had broken through the underside of the 8-inch-thick post-tensioned flat-plate slab. Analysis indicated that the loss of these tendons was not significant, but it led us to initiate a surveillance program to monitor the structure. Three more tendons popped through in 2003 and one more in 2006. By late 2009, there were eleven broken tendons requiring shoring in some areas. Testing and inspection revealed deterioration of some tendons from corrosion as well as from damage that occurred during previous repairs.
We evaluated various schemes for dealing with this problem. The obvious first option was to demolish and replace the entire garage. This approach seemed wasteful because the walls, footings, slab on ground, and roof were all in good condition. Site limitations didn’t permit converting this parking structure into open or covered surface parking and building a new structure elsewhere on the site, so we began looking at repair options, which included replacing the strands in the tendons, adding an external tendon system, and installing steel beams. None of these suited the owner’s concerns of having an aesthetically pleasing final appearance and minimizing downtime. Ultimately the owner decided to remove just the PT slab and replace it with one using a contemporary design and materials that conform to current practices.
The 125x225-foot, two-story parking structure features parking on two levels: the first floor slab on ground and the second floor post-tensioned slab, which is covered by a steel-framed roof. The perimeter walls are load-bearing masonry that support the PT slab and the roof. The sloping exterior grade allows entry and exit directly to each floor, so there are no interior ramps. The first floor slab is a 5-inch-thick slab on ground. The perimeter walls are supported on continuous strip footings, and the interior foundations are spread footings on soft sandstone.
The second-floor was an 8-inch-thick PT slab with a 2- to 3-inch-thick topping. The tendons were proportioned between column and middle strips, in keeping with customary practice at the time of design in 1969, with an average compression of 220 psi in the north-south direction and 250 psi in the east-west direction. The slab was placed in three full-width areas so there were two east-west construction joints. The tendons were common to the era: unbonded, with “cigarette wrap” sheathing and unprotected anchors. In general, the concrete in the slab below the topping was in good condition with no cracks.
Although the concept of removing the PT slab was simple, the execution was more complicated. The challenges included:
- The slab provided bracing for the exterior walls and for the interior roof columns.
- The roof was to remain intact.
- Limited headroom meant that all materials and equipment had to go in and out through a 7-foot-high door.
- The edges of the slab were concealed by load-bearing brick masonry.
Demolishing the slab by breaking it up and letting the pieces fall was impractical, so a plan was devised that included sawcutting the slab into manageable pieces and transferring them using a fork lift that stayed inside the building to one stationed outside the building. Here are the steps in removing the damaged PT slab:
- Remove lights, pipes, and ductwork from the underside of the slab.
- Brace the roof pipe columns to the roof joists and beams.
- Brace the perimeter walls to resist earth, wind, and seismic forces.
- Reroute the roof drains to discharge through the walls.
- Remove first floor diamonds at columns and excavate to the existing footings.
- Patch column concrete voids and spalls.
- Drill and grout new vertical dowels into the existing footings.
- Fill the excavated area around each column with concrete up to the level of the first floor slab.
- Shore the second floor slab in a pattern to fit the sawcutting.
- Sawcut the slab along the perimeter walls to detension the tendons.
- Make north-south cuts through the slab at the proper intervals.
- Make east-west cuts to detach the sections.
A forklift was brought into the building on the first floor. It was crucial that this machine was as large as possible to minimize the amount of sawcutting. The selected machine was able to handle the 6x7-foot slab sections, which weighed up to 6800 pounds. Shoring was positioned and tightened under each slab section prior to the cut that severed the section. The walk-behind saw used good, appropriately sized blades. Tendons and rebar were easily cut as part of the process. The section was lifted from the shoring and placed on dunnage just inside an overhead door opening on the north side. Then the outside machine lifted the section from the dunnage and brought it outside where it was stacked until time for a truckload of sections to be removed from the site. The neat cuts eliminated the need to handle broken concrete and tangles of reinforcing bars and tendons. It also meant the slab pieces could be reused—they ended up as pavement for part of a contractor’s yard.
The contractor needed to take special care to avoid hitting the wall braces and the braced roof columns but the demolition was completed without incident. No repair work was necessary for the slab on ground, which was in good condition initially and remained undamaged by the work.
The new slab is an 8-inch flat plate with unbonded, encapsulated tendons in a banded layout, designed according to ACI 318-08. Concrete was specified to have a 28-day compressive strength of 5000 psi with appropriate entrained air and a corrosion inhibitor. The drainage on the slab was changed to have twice as many floor drains as the original slab and use a slope of 1/4 inch per foot. Inserts were placed to support the suspended items to assure an orderly arrangement of utilities and to avoid the risk of damaging tendons from drilling for expansion anchors.
The existing columns supporting the slab were encased in a concrete jacket, which simplified shear and bearing conditions and delivered the new floor loads directly to the footings. New columns were added along the perimeter of the new slab. The columns along the south and west walls were located four inches clear of the walls while those along the north and east walls were 4 feet clear to accommodate the closure strips needed for room to stress the tendons. This arrangement eliminated the need to have closure strips within the slab drainage areas, which is important because closure strip construction joints are not compressed by the PT and are susceptible to leakage. Two east-west construction joints, compressed by the post-tensioning tendons, were used for the three placement areas, and the replacement slab was installed from south to north. The walls were attached to the new slab with details that provided minimal restraint to shortening from creep and drying shrinkage. The perimeter slab-to-wall joint was left open for several months to accommodate shrinkage before being sealed.
This project showed that a 40-year-old structure can be economically recycled if the owner is receptive to nontraditional solutions, and if proper attention is paid to details and procedures that are different from those normally found in construction. The owner, the structural engineer, and the contractor worked together to come up with creative procedures that enhanced the project and allowed the work to move along at a steady pace.
The work, originally scheduled to use two-shifts per day, was accomplished using a single-shift schedule. The project went smoothly, had fewer than the usual number of heated discussions, was executed without any increases to the scope of the work, and came in just under the negotiated maximum cost. Surprises were few and were dealt with promptly.
Demolishing and replacing the slab took a few days less than the 90 days scheduled for the work. The garage was returned to service on schedule, as promised, before the onset of winter. The facility is clean, well drained, functions as well as a totally new one, and the tenants are happy to have their parking spaces back.
Gregory P. (Greg) Chacos, PE, has more than 40 years experience in the design of structural prestressed concrete, with a special interest in post-tensioning. He can be contacted at www.gpchacos.com.
Parking Structure Durability
It’s no secret that many cast-in-place concrete parking structures have suffered from durability issues due to corrosion of standard steel rebar or post-tensioning tendons. But the knowledge exists today to design and build durable structures. Use some combination of these tips:
- Start with the requirements in ACI 318, Chapter 4, Durability.
- Use epoxy-coated reinforcing steel (stainless or galvanized steel also are useful in areas that are repeatedly saturated with salty water).
- The most common reinforcing system is unbonded post-tensioning tendons with watertight encapsulation and corrosion-resistant grease. These allow tendon remediation rather than complete replacement of the slab as would be necessary with bonded tendons. Stress to minimum compression in the concrete of 200 psi. For details, get the PTI publication “Design, Construction and Maintenance of Cast-in-Place Post-Tensioned Concrete Parking Structures.”
- Assure proper placement tolerances on reinforcement and maintain minimum cover (concrete cover of 2 inches for slab top reinforcement, 1 inch for bottom, and 2 1/2 inches for other members).
- Avoid high early strength, fast setting, and excessively strong concrete mixes in slabs, because these tend to crack more. Use low-permeability concrete with a low water-cementitious materials ratio (below 0.45), SCMs, sound aggregate, and well-graded sand. Get good entrained air at the percentages recommended in ACI 318 for the aggregate used (6% for 1-inch aggregate).
- Use a corrosion-inhibiting admixture.
- Apply and maintain drivable membrane-type deck surfaces.
- Provide good slab flatness, and adequate slope and drainage to prevent the formation of persistent puddles. Avoid excessive span-to-depth ratios that encourage deflection and vibration and may result in crack propagation.
- Seal cracks and use high-quality sealants at construction and expansion joints.
Thanks to John Turner and Mike Mota at the Concrete Reinforcing Steel Institute and Ted Neff at the Post-Tensioning Institute for these tips.