Located along the western banks of the Hudson River in Cliffside Park, N.J., (approximately two miles south of the George Washington Bridge and seven miles north of the Lincoln Tunnel), 300 Winston Towers is a popular residential building with approximately 600 units. The main entrance to the building is accessed via a 28,000-square-foot entrance plaza, which consists of driveways, walkways, and a visitors' parking lot, and sits directly above the 500-space residents' parking garage. Over the years, pooling water on the entrance plaza had leaked into the parking garage below, leading not only to the deterioration of the entrance plaza structural slab, but also to spalling of some of the concrete beams and columns in the garage. The solution came in the form of a waterproofing membrane.

Preconstruction planning

The original plaza was composed of various toppings over a precast concrete double-tee structural slab. The driveway and visitors' parking area were paved with asphalt, and the walkways were topped with pavers. The north perimeter of the entrance plaza was landscaped, containing various soil depths. When the owner identified the deterioration, an initial condition survey performed by the engineering firm The Falcon Group was used to develop project specifications and drawings. Structural Preservation Systems, hired to demolish and reconstruct the entrance plaza, corrected these problems. After the existing toppings were removed, the contractor and engineer performed another condition survey to determine the extent of deterioration.

Considerable effort was devoted to preconstruction planning and site phasing to ensure the demolition and reconstruction process went smoothly and safely for both the onsite crews and the building's residents. The project team -- consisting of a project manager, superintendent, and foreman -- collaborated closely on all aspects of the project, including budget, working restrictions, phasing/site logistics, and, most importantly, expectations and procedures. Communication of the contractor's safety culture was also an important focus of planning meetings, in order to keep all parties safe.

The importance of safety also extended to the residents of the building, who needed to be able to use the entrance plaza during the project. Working closely with the owner and engineer, Structural Preservation Systems developed a phasing plan that identified which parking spots would be closed and where the barricades (in this case, post shores and polyethelene plastic) would be placed. To protect vehicles that would remain in the garage below, ethylene propylene diene monomer (EPDM) was installed over some of the repaired tee-to-tee joints as temporary waterproofing. Further, to maintain free and clear access to the front door, a site logistics plan was created to show how tenants could safely navigate the work area. Jersey barriers, orange snow fencing and post fencing were clearly color-coded so everyone would be aware of the site restrictions. The plan also highlighted the egress path for the tenants from the front main entrance. The owner informed the tenants about these phasing and logistics plans by distributing them and posting signs throughout the building.

Demolition

The first phase of construction involved demolishing the various plaza toppings to make way for the new toppings and a waterproofing membrane. With the large volume of debris generated, it was critical to avoid point loading of the structural deck. The deck could support lighter machinery such as skidsteers, but heavier trucks and containers were set on grade to avoid any structural problems. The existing soil was not piled too high to avoid structural damage to the deck. Once the toppings were disposed of completely, the existing waterproofing membrane was removed with the aid of ride-on scrapers, as well as handheld grinders for hard-to-reach areas. Any membrane still bonded to the deck after this process was removed by hydrodemolition. It also was determined that it was necessary to install additional drains to help prevent the water from ponding on the plaza.

With a clean structural deck, the engineer and contractor performed a condition survey to determine the extent of the concrete restoration. A few spall repairs were required, and most of the grout in the tee-to-tee joints had to be replaced. New drains were installed in the structural deck to allow for better drainage and prevent the pooling that had led to the failure of the waterproofing membrane.

Waterproofing

The waterproofing solution American Hydrotech MM6125 was selected for a number of reasons. To begin, the owner wanted the plaza to have certain landscaped toppings including various plantings and pavers. A buried or subsurface membrane was needed and an asphaltic buried membrane is ideal because it is thick and durable enough to withstand the installation of the toppings such as asphalt, pavers, soil, concrete, and plantings, as well as the wear and tear of plaza deck traffic over these toppings. It is also flexible enough that it can still perform even if a small crack develops in the structural deck. Although sheet membranes typically have seams, a hot-applied membrane is monolithic and there are less areas where water can potentially infiltrate.

After the demolition process, the new waterproofing membrane was applied to the now-sound structural substrate. To prepare it for installation, the surface of the structural deck was shot-blasted. Nearby balconies were covered with plastic and sheets of plywood to protect them from the ample amounts of dust and debris generated. This protection also was left in-place during the waterproofing installation to prevent any staining from the new membrane. The rubberized asphalt membrane was heated to approximately 350° F in an oil kettle before being applied. Field technicians were trained on the proper use of the kettle and the proper handling of the material to avoid any safety issues. To prevent any future problems with the waterproofing membrane, the engineer designed specific details for membrane reinforcing at the expansion joints and tee-to-tee joints.

After the membrane was applied, a water test was performed on the plaza deck to determine that there were no imperfections, after which drainage board was installed to direct water into the drains and further prevent it from pooling. A special protection board was used that was strong enough to withstand root penetration. This was important especially with the elaborate landscaping that would be installed over the plaza deck.

New toppings and landscaping

The visitors' parking area followed the original design plans for a cast-in-place concrete topping, but a design change was made with regard to the paver installation for the walkways. Originally, the pavers were to be set in an asphalt base course, which would hold them securely in place, but make it difficult to remove any broken or chipped pavers. The asphalt base course also would be subject to freeze/thaw cycles, which would eventually cause some of the pavers to dislodge. To avoid these problems, a stone dust setting bed was used instead. A layer of stone dust and sand was laid over the deck, and the pavers were then set in place according to the landscape architect's design. Next, another thin layer of stone dust and sand was spread evenly over the top and was worked into all of the paver joints. After the pavers were placed, new landscaped toppings (with a specific soil mix that would prevent any overloading of the structural deck) were installed by a landscaping subcontractor.

Lessons learned

Key to success for this project was detailed phasing and site logistic planning, as well as the ability to adapt to changing conditions. It is fairly typical that original construction drawings may not be available for a plaza deck project. As such, existing conditions below the toppings often are unknown. Although some assumptions can be made by the engineer when they prepare the bid documents, in the event that the conditions change or are unforeseen, it is important to keep the engineer involved in the project. Such was the case for this project, and good communication contributed to project success.

Another key consideration is a reminder about the importance of owners evaluating their structure to identify any water that may be leaking to levels below their plaza. Such a condition may indicate that existing waterproofing membranes may not be performing as intended. Further, plaza decks should be continually maintained, and it is very important to make sure that any drains are working properly and are not clogged. If the drains are clogged, the water will end up ponding on the deck and will eventually find a way to get under the waterproofing membrane. Owners or property managers also should check to see if they have any cracks or spalls in the plaza toppings, as this could indicate problems below the toppings.

Next steps

If an owner or property manager does suspect that they may need a plaza restoration, they should contact an engineer to perform an initial conditions survey to determine the cause of deterioration as well as the appropriate restoration solution. The next step is hiring a specialty contractor with specific experience in these types of projects. There are many key details, such as surface preparation, flashing, shoring, concrete, and masonry removal and restoration, that would be best performed by a specialty contractor. The contractor should be accustomed to working in active structures, as the property managers most likely will not be able to completely shut down the section of the complex that needs restoration. Further, a specialty contractor will have experience with site logistics and phasing that will be required for a plaza restoration project.

Thanks to precise preplanning and a concerted effort to make the 300 Winston Towers safe for both crews and tenants, construction as a whole went very smoothly and the project was completed safely within eight months.