Constructed over 20 years ago, the Atrium is a twenty-story contemporary condominium complex convenient to Manhattan. Surrounding the Atrium is a 153,200 square foot outdoor concrete plaza area sitting atop a two-level parking garage. The plaza deck is home to many amenities – tennis courts, a pool, basketball courts, a childrens’ play area and a running track. Below the deck, the 200,000 square foot parking garage accommodates approximately 700 vehicles.
Over time, these areas suffered severe deterioration. In the garage, extensive shear cracking on the column corbels and beams was visible, as were spalls and failed post-tensioning tendons. Over the garage, the entire plaza deck topping slab was severely cracked and heaved. Water was leaking through the waterproof membrane to the lower garage levels. The cast-in-place concrete pool was also leaking water to the lower level of the garage and the plaza deck drainage system was clogged and blocked. Expansion joints had failed and created huge gaps in the slab.
Intensive repairs were required to restore the structures to their intended use, but significant consideration had to be given to the development of a complex phasing plan to no more than150 parking slots were displaced at any one time.
The plaza deck and parking structure were built on cast-in-place concrete foundations stemming from solid bedrock. Cast-in-place concrete reinforced by 0.5” unbonded monostrand post-tensioning with mild steel reinforcement was used to construct the parking structure. The plaza deck was comprised of a structural deck with a sheet-applied waterproof membrane that served as the top of the underground parking garage. A thin, unreinforced three-inch topping slab acted as the plaza deck’s wearing slab and expansion joints were located midway in the plaza deck/parking garage.
In total, there were over 50 work items ranging from 32,000 square feet of reinforced concrete topping slab; 153,200 square feet of plaza waterproofing; the installation of a new pool liner and new basketball courts to post-tensioning beam repairs.
With this plan in place, crews began shoring the upper plaza deck level around compromised corbels and beams to allow for the demolition equipment to remove the deteriorated topping slab. Upon completion of each phase of topping slab demolition, the membrane was removed. The clean, exposed structural deck was hammer sounded and chain dragged to determine the extent of deterioration. Spalls and areas of delamination were repaired in accordance with ICRI guidelines.
The plaza’s structural deck was waterproofed using reinforced 180 mil hot-applied modified bitumen membrane and protection board was placed in all areas scheduled for new concrete topping. In parts of the plaza that would support planters, the Contractor used quarter-inch protection board. A drainage board was placed prior to installation of the concrete topping slab. Sub-surface drains were located in low areas of the structural deck to allow proper drainage from the new topping slab. New expansion joint headers were constructed and tied into the structural deck. A gland was installed in the concrete headers to allow for movement and waterproofing integrity.
The topping slab was reconstructed using a combination of reinforced integral colored concrete and stamped concrete. In total, 24,800 square feet of stamped concrete and 8,200 square feet of colored concrete were installed. Concrete was placed in phased areas to allow for proper isolation joints. With strict attention to reinforcing steel placement, control joints, concrete quality control, and good concrete placement practice, there were no visible cracks in the concrete topping slab.
To ensure the quality of the repairs as they happened, each phase of the waterproofing was water tested for 72 hours prior to progressing to the next section.
Concurrent with each phase, deteriorated post-tensioning tendons in the garage were repaired. Deteriorated corbels and beams were also repaired and the deck was waterproofed using a low-odor traffic-bearing urethane membrane.