The adage “time is money” is certainly relevant to many industries as it relates to building renovations and the cost of downtime, interruptions to services or inconveniences to customers. To the casino industry, the phrase is even more literal. Any interruptions to revenue generating operations come at great expense to the owner.
That is why Perini Building Company -- the largest builder of hotels and casinos in the United States and general contractor for new construction and renovation of the Resorts International Casino in Atlantic City -- and the owner of Resorts Casino looked to create a team that could bring together both engineering and construction expertise for a critical path concrete repair and strengthening project. They assembled a team that included Structural Group -- one of the nation’s leading specialty contractors in concrete repair -- and Lochsa Engineering -- one of the premier design groups for casino construction and renovations.
Challenging Renovation
As the first casino on the East Coast when it opened in 1978, Resorts Atlantic City has set the standard for casino gaming and entertainment. Situated on 11 acres of land with approximately 310 feet of boardwalk frontage overlooking the Atlantic Ocean, the 100,000-square foot casino features action 24-hours-a-day at more than 70 tables and nearly 3,000 slot machines. The original structure, constructed in the 1920s, consisted of two twin towers. Over the years, both towers have undergone various renovations.
Recently, Perini Construction demolished one of the two original towers and constructed a new, state-of-the-art hotel tower in its place. The final phase of this construction project was to connect the original hotel tower, lobby, check-in area and casino with the new hotel’s lobby by creating a new concourse between them. The access would allow customers to easily access all casino services without having to walk outside between the two towers.
Already a challenging task because of the complexities of seamlessly combining new construction with old, the different elevations of the two lobbies presented additional issues. As a result, the new promenade would need to be inclined to compensate for the change in elevation. With the new tower open, the need to create this access was immediate. However, there were many construction challenges that required imaginative solutions to address the structural aspects of the new walkway. The first had to deal with the fact that an existing intermediate slab would have to be removed in order for the new concourse to pass through from the lower elevation to the higher one. Although demolition of the old slab was not complex, the change in use could cause the columns supporting the original building load to buckle because the new unsupported length of the columns doubled. The second challenge related to a condition that was not discovered until construction of the promenade had begun. In the elevated areas of the new slab, supports that would bear down on the original base slab below would need to be constructed under the new walkway. The existing slab was thought to be a fully supported slab on grade. As the construction process progressed, an entrance hatch was discovered area on the original base slab - an unexpected occurrence in a slab-on-grade. After further investigation, it was discovered that part of this supporting base slab was actually elevated and had an old steam tunnel under it. Worse yet, the existing structural beams in this steam tunnel had deteriorated over time to the point that they would not be able to carry the new loads. Temporary shoring was installed immediately to improve the beams’ capacity.
The team also discovered a similar condition in the beams and slabs in a below-grade room called the Grease Recovery Unit Room (GRU). The GRU was adjacent to the steam tunnel and functioned as a processing room for the massive amounts of grease created daily in the many hotel restaurants. Shutting it down for structural repairs in any way would cause major disruptions.
Because these three areas needed to be repaired and or strengthened before any portion of the new concourse could be completed, a timely evaluation, repair strategy and installation process was essential. Recognizing the importance of proper and timely restoration, the owner tasked Perini Building Company to oversee the project, beginning with assembly of a team to develop the strategies to repair and or strengthen the deteriorated concrete. As such, the scope of the repair project was re-defined as:
- Strengthen the columns in the new promenade to resist the new buckling stresses
- Repair and strengthen to current codes the deteriorated beams in the steam tunnel
- Repair and strengthen to current codes the deteriorated beams, slabs and columns in the Grease Recovery Unit Room
Understanding Structural Repair vs. Strengthening
In order to gain an accurate sense of the work at-hand, first it is important to understand the difference between repairing and strengthening concrete structures. Structural repair describes the process of in-kind reconstruction of concrete members to bring them back to their original capacity and condition. In these cases, the cause of the deterioration is determined, deteriorated materials are removed, and repair materials designed to extend the structure’s useful life are selected and installed. Structural strengthening, on the other hand, describes the process of upgrading existing concrete members to improve their load carrying capacity for additional loads or stresses.
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For concrete strengthening projects, the design strategy must address the challenge that every member in the structure is already carrying a share of the existing loads. The effects of strengthening or removing any portion of a structural member -- whether permanent or temporary -- must be carefully analyzed to determine the influence on the global behavior of the structure. Failure to do so can easily overstress the members surrounding the affected area. Reinforced concrete systems, because of the continuity between members, tend to redistribute overstresses. This can lead to a reduction in capacity of surrounding elements. On the installation side, the construction sequence must address access and constructability issues as well as temporary shoring and repair material not typical to new construction processes. Also, with an occupied and operating structure, levels of dust and noise control are much more critical.
Although considered long-lasting and durable, buildings constructed using reinforced concrete, such as the Resorts Casino, have a finite service life. When exposed to harsh environments, chlorides, de-icing salts and chemicals, these structures will naturally experience significant deterioration - typically in the form of cracks, steel corrosion and concrete spalls. Interestingly, one of the most severe and widespread deterioration mechanisms in concrete is the internal damage created when external chlorides seep into the concrete and corrode the embedded reinforcing steel. Corrosion problems are basically caused by the fact that the corrosion process by-product (rust) expands the internal steel rebar up to eight times its original size. This expansion creates such high internal pressures that it will cause the concrete cover over the rebar to crack and then completely spall off. The process also results in a reduction of the effective area of steel rebar and, therefore, reduced structural capacity of the deteriorated member. If not addressed at early stages, corrosion will continue to grow rapidly - ultimately creating a safety issue due to falling concrete and loss of strength.
Bigger is not Better- Column Buckling Repairs
According to Bill Segal, General Superintendent on the Resorts Casino project for Perini Building Company, during the demolition phase, the existing intermediate slab was removed to allow for the creation of the new promenade slab. After removal, Jeff Baer of Lochsa Engineering conducted a survey and structural analysis, which showed that the columns were subject to excessive buckling because of the change in unsupported length (10-feet to 20-feet) caused by the removal of the original slab.
Traditional methods for strengthening the columns – which typically involve the use of steel and concrete in the form of an enlarged external jacket around the column -- need to be performed monolithically with the existing column in order to provide resistance to the higher buckling forces. Approximately 12-inches of additional concrete on all four sides of the columns would be required to handle the increased buckling forces. As such, this option was not considered viable because it would increase the overall column size such that the egress width of the new corridor would be below the code requirements.
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Accordingly, another strengthening option using very thin, high-strength carbon fiber sheets, 10 times the tensile capacity of steel, was considered. These paper-thin sheets would be bonded to the concrete columns with epoxy adhesive in both horizontal hoop sheets and continuous vertical sheets. While adding less than 1 inch to the column size, they effectively upgrade the column for both buckling and current load carrying capacity codes. As a non-corrosive material, the sheets are quick and easy to install and aesthetically easier to conceal. Structural Group was contracted to work with the structural engineer to design and install a carbon fiber reinforced polymer (CFRP) system on the columns to provide the necessary strength for eight columns and the floors above this 8,000-square-foot area.
As with any other externally-bonded system, the surface preparation and bond between the CFRP system and the existing concrete is very critical. On this project, proper installation criteria was achieved by sandblasting the entire surface, patching holes and deviations on the column and then applying an epoxy adhesive to the prepared surface. The CFRP reinforcement was then applied into the wet epoxy. The sheet application increased capacity to required levels. Two columns, which monitor fire alarm security systems for the entire facility, were located inside the Fire Command Control (FCC). To keep these systems in operation, two-hour rated falsework walls above the FCC were constructed to isolate the work from the equipment. One column, still attached to the original façade, could not be wrapped in CFRP due to lack of work access on all four faces. A large steel beam that was fabricated in one-third sections was installed at mid section of the column and attached to the adjacent column, hence acting like a horizontal brace for buckling. The steel beam was required to be installed in thirds to accommodate existing conduits that supply data information to the FCC.
It’s Getting Hot in Here: Concrete Repairs in the Steam Tunnel
With the column strengthening under control, attention was directed to the severe concrete deterioration in the steam tunnel and the GRU. In many of the beams, slabs and columns, the steel rebar was fully exposed or completely deteriorated. Not only would these members require repair, they would also have to be strengthened to meet today’s design codes.
In the steam tunnel, the working conditions and the upgrade design strategy presented unique challenges. Access to the beams was difficult at best – with the work area considered a confined space. Key safety issues needed to be addressed and, in addition to having a dozen super-heated high-pressure steam lines, the working area in the tunnel was very tight - giving new meaning to “back-breaking work.”
The design and construction team developed a creative approach to address the design requirements, limited access, tight working conditions, low headroom, air quality and ventilation issues. To support the floor above during removal and enlargement of the beams, the entire ceiling was shored. Also, the floor area over the steam lines was fully decked to protect both the workers and the steam lines during construction - giving the workers only 36-inches of working height. Workers, equipment and demolition debris would generally be transported in and out with low profile carts. In order to alleviate egress challenges, a second entrance in the tunnel was created. Also, through an engineered solution using forced air and airflow exchanges (approximately 30 per hour), the normally 90+degree Fahrenheit temperature was reduced to the high 70s.
When developing a repair strategy for the upgrade, field investigation of the existing rebar showed that all 10 beams had completely different rebar configurations, giving each a different in-place capacity. According to Tarek Alkhrdaji, Ph.D., Structural Group Engineering Manger, all of the beams in the steam tunnel required both flexural and shear upgrade to carry the new loads and all were in a state of deterioration. To strengthen the beams, it was decided to employ an enlargement technique using a reinforced concrete jacket that would be bonded to each existing beam. The challenge faced from a design perspective was defining the existing capacity of each beam as a starting point of reference for upgrade. For speed and simplicity, the design of the new jacket would be based on the worst case capacity found in the 10 existing beams. This plan addressed the upgrade in all the beams without 10 separate designs -- a common strengthening strategy. Also, for efficiency, all calculations were performed without any consideration of contribution from the original steel in the beams.
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To start the repair, temporary shoring was placed under the members and all deteriorated concrete was removed with small chipping hammers. The exposed concrete and steel were treated with abrasive blasting to create open surface pores that would promote a strong bond with the new material and guarantee composite behavior between the old and new concrete. U-stirrups for shear were doweled in every five inches along the beam and new bottom steel for flexural loads was placed. The enlargement area was formed such that the beam would be enlarged by five inches on all three sides. Specially designed formwork was then placed on all of the beams so that the new concrete could be sequentially pumped into all 10 areas on the same pour.
Self-consolidating concrete, or SCC, was selected as the repair material because it could be pumped the long distance from the concrete truck to the repair area and it would flow easily in and around the high concentration of reinforcement in the beam. Plexiglass windows were created in the formwork at the end of each beam to visually ensure that it was pumped to capacity. At the other end of the beam, a connection port for the concrete line was installed. Because the SCC repair material easily pumped and flowed, Structural Group used only a single port on the formwork – allowing pumping through the form for many feet without segregation. With the low ceiling height, this feature was beneficial because it eliminated many connections, reducing the amount of work and workers in the tunnel during placement.
GRU Room Concrete Repair and Strengthening
This below sea-level room contained the grease recovery and processing system for all of the casino restaurants. The amount of grease to process every day is far more than one would think and large tanks are required. The concrete ceiling was a two-way slab and beam construction which, over many years, had deteriorated to a point of structural concern because a constant combination of high temperatures and humidity in the room had caused corrosion of the reinforcement. Further, three very large columns in the room -- each supporting more than 12 floors of the building -- were deteriorated and did not meet the current codes.
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The general area was so congested with equipment and piping on the floor and ceiling that it was difficult to actually see the beams and slabs above. There were in excess of 100 pipe-hangers supporting approximately two tons of Mechanical/Electrical/ Plumbing in a 30-foot-by-15-foot area. Accessing the concrete repairs would have required removing the process equipment and associated piping – causing major disruptions to all of the restaurants. Instead the team developed an elaborate shoring plan to support the equipment and piping which allowed the entire beam and slab system to be removed and replaced without any operations losses.
Once the original ceiling structure was completely removed, formwork was placed and a new two-way slab and beam system was cast with new hangers and supports for the piping and the equipment. Next, reinforced enlarged sections were added to repair and strengthen the columns. First, six-inches of concrete were removed on all four faces of the 48-inch square columns. New vertical steel, doweled into the foundation, was installed along the faces of the column and new closed stirrups were placed around the columns before the columns were formed. Again, SCC was used to enlarge the columns.
Open for Business
Although the repaired areas were completely closed off to pedestrians, the owner wanted the Casino to stay operational during the reconstruction. Safety and minimizing interruptions and noise were paramount. All demolition, drilling and pumping operations adhered to stringent starting times for any noise-producing work. Brett Schneider, Structural Group Superintendent, said key to success for this fast-track project was extensive pre-planning between the owner, engineer and contractors.
“Prior to starting this project in September, the team had more than 100 man-hours pre-planning the work,” Schneider said. “This allowed an outline to be created for each of area of work and what challenges each area presented. We also scheduled a two-day safety training and orientation seminar prior to mobilizing the project. This allowed the beginning integration of our employees with the local trade unions.”
Alkhrdaji concurs that pre-planning was key to success for this project. ”Successful strengthening projects typically need to balance technical issues such as sound engineering concepts, temporary shoring and creating composite behavior between section enlargements and FRP and the existing structure,” he said. It was also important to address non-technical issues such as access, constructability, economics and aesthetics.
This fast-track project was completed ahead of schedule and was the result of a very open and cooperative relationship between the contractors, the engineer and the owner. The completion of Structural Group’s repairs allowed Perini to move forward with the remaining architectural changes and the Grand Opening of the new concourse occurred in January of 2006, ahead of schedule and on budget.
Authors
Jay Thomas is Vice President of Structural Group. Keith Eberhardt is Project Manager for Structural Group.