Built in 1968 near the campus of Johns Hopkins University in Baltimore, Md., the Hopkins House Apartment Building is an 18-story apartment complex housing both university students and working professionals. Designed to have balcony access from all 271 apartment units, the building overlooks much of the Johns Hopkins campus with skyline views of downtown Baltimore. A five-level parking garage is connected to the apartment through a corridor, and an outdoor pool is located between the apartment building and garage.
The apartment balconies around the building were originally designed and constructed as conventionally reinforced concrete slabs, cantilevering from the building façade at an average of six feet. The balconies wrap around the building perimeter on every level with each apartment's balcony divided from the adjacent balconies with partition walls. Surface mounted aluminum handrails run along the entire balcony perimeter. The bare concrete surfaces of the original balconies offered limited protection against the elements.
The parking garage elevated decks were waffle-slab design, providing rigid support for both vehicle traffic and the swimming pool basin.
Damaging Environmental Conditions Cause Rebar Corrosion and Concrete Spalling
Since the building's original construction, minor repairs had been performed on the high-rise and parking garage structures. Most of these repairs had been performed on an as-needed basis. Because of damaging environmental conditions, including multiple freeze-thaw cycles and harsh winters requiring corrosive de-icing salts, rebar corrosion and concrete spalling were prevalent throughout both the high-rise balcony and parking garage structures.
In 2004, the owner was preparing to undertake a large building rehabilitation, which included:
- Installation of new windows
- New balcony handrails and divider panels
- Interior apartment renovations
- Large scale renovation of the building entrance.
Because the new balcony handrails and divider panels required anchorage into sound concrete decks, an investigation into the concrete structure was needed.
Several locations of visible concrete spalling and cracking of both the parking garage and balcony decks also helped to initiate this investigation. Many of these cracked locations were visible on the underside of the structures, raising concerns for both public safety and potential property damage.
Inspection/Evaluation Methods & Test Results
The engineer performed a sounding and visual delamination survey of the parking garage and balconies to determine the extent of concrete deterioration throughout both structures. This survey confirmed that significant concrete deterioration was present, resulting in the need for thorough concrete repairs to both structures.
Oxidation of Reinforcing Steel is Primary Cause of Deterioration
The primary cause of deterioration in both the balcony and parking garage structures was the oxidation of reinforcing steel. During the original construction of the structures, reinforcing steel at several locations was installed with insufficient concrete cover, accelerating the corrosion of the reinforcing steel. In the parking garage, the combination of cracks through the waffle slab and ribs along with the use of de-icing salts helped initiate and accelerate the corrosion of the reinforcing steel.
Other causes of deterioration of these structures included settlement cracking throughout the parking garage and vertical masonry walls, freeze-thaw damage to both the parking garage and balcony decks, and dissimilar metal corrosion of the existing handrail base plates.
Team Approach Selects Repair & Protection Plan
Once the pre-planning efforts were completed, the project team had the information it needed to finalize preparations for the repair system selection. The team determined that a repair and protection plan was needed to extend the useful life of these structures. The repair plan consisted of performing concrete spall, crack and masonry repairs. The waffle slab parking garage configuration made the many types of concrete repairs more challenging than they would be on a conventional flat slab. The repair contractor simplified these various challenging repairs by utilizing pre-molded fiberglass formwork matching the dimensions of the existing waffle pans and ribs.
The protection plan involved the installation of a cathodic protection system in the parking garage where extensive repairs were required. The selected method was a galvanic cathodic protection system using zinc anodes, designed to mitigate the new corrosion activity from occurring in the areas surrounding each concrete repair. Another component of the protection plan included the installation of a waterproof membrane coating system to protect the concrete from future freeze-thaw and de-icing salt deterioration. The team selected a traffic-duty polyurethane deck coating system for the elevated concrete decks in the parking garage and a pedestrian duty polyurethane coating for the topside of the balcony decks. To complete the balcony protection, the team selected a breathable acrylic coating to be applied to balcony soffits.
Preparing for the Repair (Site prep, demolition and surface prep)
Upon mobilization, the concrete repair contractor set up a staging location, site storage and office enclosure, site dumpster, swing-stage platform access to the exterior façade and balconies, and opened a new entrance door into the parking garage. The contractor then defined the locations and size of each concrete repair with a sounding delamination survey prior to beginning work in each phase. The demolition of the parking garage and balcony concrete slabs was performed using manual chipping hammers. Then, corrosion from the exposed reinforcing steel was removed and the contractor prepared the substrate surfaces per ICRI standards, using sandblasting equipment. New reinforcing steel was spliced into repair locations as necessary where remaining steel was not adequate. The concrete substrate was then brought to a saturated surface dry (SSD) condition prior to concrete placement.
A Successful Repair Process Execution
The repair and protection project included concrete spall and crack repairs; as well as installation of a cathodic protection system, protective coating systems and new drains and masonry repairs. The repair and protection project coincided with the larger rehabilitation of the apartment complex, which included the installation of new windows and balcony handrails, interior apartment renovations and a large scale renovation of the building entrance.
The balconies were repaired in six phases, with each phase strategically composed of multiple vertical tiers around the structure. Within each phase, the repair contractor performed:
- Removal of handrails and divider panels
- Shoring installation
- Demolition of deteriorated concrete
- Repair surface preparation
- Formwork installation
- Placement of repair material
- Coating surface preparation
- Top surface pedestrian waterproof coating and soffit coating installation
The parking garage was repaired in 14 phases, where public and contractor safety was an important consideration for phase boundaries. Within each repair phase the repair contractor performed:
Shoring installation
- Deteriorated concrete demolition
- Surface preparation
- Formwork installation
- Installation of galvanized cathodic protection around the perimeter of repairs
- Placement of the repair material
Upon completion of all concrete repairs in the parking garage, the contractor used another specialized phasing plan for work consisting of surface preparation for the protective coating, installation of traffic bearing membrane, and painting of the walls, columns, and underside of the waffle slab.
Because the balconies were continuous around the building and one work item included removal of the divider panels between apartments, the contractor was able to perform repairs horizontally in larger phases, thereby minimizing the daily use of the swing-stage work platforms. This new horizontal work sequence allowed the repair team to reduce the schedule by increasing the efficiency of the field crews performing the balcony repairs, while also improving safety by minimizing the use of swing-stage work platforms.
The repair contractor developed an innovative solution related to the parking garage access and phasing. The parking garage had narrow drive lanes, and the contract stipulated that the work be performed using only a small amount of parking spaces. The original garage layout would have required many phases with the potential for hazardous conditions for both the public and repair crews. Instead, an old abandoned garage entrance was identified by the planning team on one end of the garage. The repair contractor requested that this entrance be re-opened to help reduce the garage repair phases and improve safety of all parties involved. The owner granted this request, and the construction team demolished the cinder block fill to re-open the garage entrance and installed a remote-controlled garage door for tenant safety and ease of access.
Overcoming Challenges & Unforeseen Conditions
Many potential pitfalls were avoided by spending extra time planning and gathering information prior to construction. One issue identified early was the extensive concrete deterioration seen in the garage and balconies, believed to have progressed since the original concrete sounding survey was performed. The repair contractor working with the owner & engineer decided to perform repairs in one tier of balconies before moving to other tiers, and then compare these repairs to those identified in the survey. In essence, this portion of the work became a test location for the entire balcony project, similar to performing a "mock-up." Upon completion of this first phase, it was found that the corrosion had progressed into larger repair areas at many locations. The repair contractor agreed to report on the comparison after the completion of each phase and to actively work with the engineer to prioritize repair locations. These early efforts proved to be an important component in building the teamwork that would lead to a successful project.
Several logistical challenges arose throughout the Hopkins House project:
The repair contractor communicated daily with the owner and other subcontractors about coordination and safety concerns. The repair contractor assisted the other contractors with their access to the balcony levels and coordination of their storage locations on-site. A material hoist, brought to the site after the suggestion at a planning meeting, provided the opportunity to deliver construction materials to various balcony levels more efficiently and safely than the original process of using the swing-stage work platforms. Teamwork and communication between all trades made this project a success.
Another unforeseen challenge was that one level of the parking garage had significant water drainage problems, which led to the deterioration of a large portion of the slab. Over the years, a multitude of drains were installed as new areas collected water. The repair contractor corrected this issue by performing a large full-depth concrete repair at this parking level and then finished the concrete with a proper slope to the remaining drains.
Hopkins House ... A Project Worthy of an Award
This project utilized many of today's leading concrete protection technologies including galvanized cathodic protection and protective coating systems specifically designed and installed to protect each type of wearing surface.
Unique formwork and shoring designs were developed for the repair of the parking garage waffle slab as the ribs were spaced closer in one direction than the other, creating an unusual rectangular pattern. These unique designs consisted of engineered shoring plans, customized wood forms, and prefabricated fiberglass forms designed to match the dimensions of the existing waffle pan and rib sizes.
Project planning and teamwork by all of the involved parties were the keys to success. This project had many work items that needed to be coordinated around other renovation activities being performed by the owner. Coordination with the window and railing contractors was critical for success. In addition, the design and construction team had to be cognizant of the concerns of the building's tenants and minimize disruption to them.
Even with all the pre-construction planning performed for this project, the project would not have been successful without ongoing planning and flexibility throughout the project lifespan. The repair contractor held weekly progress meetings in which the project team reviewed safety, schedule, coordination, manpower and material issues. These meetings were also forums for team members to suggest new means and methods, to identify items that were deviating from the original project plan, and to review production rates as they related to the project budget. Also, since the site conditions continuously changed from those identified at the start of the project, the process of tracking the project's progress and refining the plan according to new information and changed project conditions was critical for success.