Xerox Corporation's 33-storey Xerox Square Tower stands high along the Rochester, New York skyline. The elegant façade is constructed of cast-in-place exposed garnet aggregate concrete, which offers extremely high compressive strengths and a dramatically decorative finish.
On 25 September 1998, a mild earthquake created vibrations reaching Rochester. The tremors caused delaminated concrete to fall from the building's third floor façade, revealing an undetermined quantity of deterioration as well as other defects associated with the original construction. In 1999, a façade inspection using hammer soundings and minor destructive testing together with a detailed mapping survey was conducted. The investigation revealed corrosion-induced delaminations and cracks in the concrete as well as extensive interior honeycombs. These honeycombs were presumed to be contributing factors to moisture transfer to the steel reinforcement which caused the subsequent corrosion. Water was also penetrating the building through sealant joints at various locations including along failures in the gasket system at the window perimeters. Subsequently, recommendations were made for the repair of the building's concrete structure, sealants, curtain wall system and other remedial repairs.
The project
The project began in June 2002 and concluded in October 2005. Mobilization took place each spring and demobilization each autumn for four consecutive years. During each phase, crews operated 24 hours per day, five days a week. In addition to the magnitude of the work, the difficulty in matching the exposed garnet aggregate, the extreme height of the work areas and the diligence required to protect the public and property presented challenges and learning opportunities. However, through careful planning and teamwork, the repair team provided Xerox with the continued beauty and structural integrity of their landmark building.
Concrete repair and waterproofing
Timing and safety were the first tasks for the selected repair contractor, Structural Preservations System (SPS). SPS was selected for the repair project because of its extensive experience in concrete and masonry restoration of high-rise structures. The safety of building occupants, the general public and the contracting team was paramount. It also was important to phase the project so that only one building elevation would be affected at a time, leaving the rest of the building accessible. The safest way to accomplish this task was to work from June to October over four consecutive years
The project operated 24 hours a day, five days a week, with Saturdays as ‘rain make up' days. Each year, the project was mobilized in the spring and completely demobilized in the autumn. In total, the crews worked for more than 100,000 hours.
Concrete repairs were necessary from the second floor entablature to the roof level, replacing portions of deteriorated cast-in-place columns and precast sections. Each repair site was sounded; the concrete with compressive strengths ranging from 80-100MPa was chipped with 7kg pneumatic chipping guns and sounded again. The edges were saw-cut and rusted steel was either cleaned and coated or supplemented with stainless steel reinforcement. The engineer then inspected and sounded each site to ensure that no loose or delaminated areas remained before each patch was formed and the concrete was hand poured. In total, 46m3 of concrete were replaced.
Sealants were replaced at horizontal construction joints in the columns from the third floor to the roof level. Vertical control joints were replaced between precast concrete sections at the second-floor level entablature. SPS also installed a silicone sealant system between the glass/metal panels and concrete columns from the second floor to the roof level.
SPS also inspected and repaired the curtain wall system from the third to the thirtieth floor. In the process, 100% of the lockstrip T-gasket system was inspected and 11,000 linear metres were replaced. The area between the concrete column and glass of the curtain wall was sealed with a new sealant system. Finally, new flashing was installed at the seventh and twenty-second floor louvers and at metal panels at the thirtieth floor mechanical rooms. All repairs and inspections were documented on the as-built drawings and quantities sheets and were verified by the engineer.
Repair challenges
A further challenge was containment of falling debris and dust in this city location. Along with large nets placed where required, SPS installed tight rubber gaskets at the access equipment interface to contain the debris. Additionally, the entire plaza area was fenced and covered with a flooring system to protect the pavers system.
As work commenced, access equipment used to reach the height of the repairs became a challenge. To perform concrete repairs, SPS needed access all around the column perimeters protruding 1.2m from the face of the windows. Additionally, to keep debris from falling, SPS needed a swing stage that conformed to the configuration of the columns and windows. Initial plans used the existing window washing swing stages. After recognizing challenges with the swing stage system, the team decided to purchase mast climbing platforms to complete the project. By using mast climbers, SPS could better control power requirements and issues with the wind. The mast climbing platforms used large motorized work decks attached to steel frames anchored to the building at 6m intervals. The work platform selected was chosen based on gross weight, platform size and capacity. Overall loading was a major consideration since the existing construction consisted of a supported concrete slab over the plaza deck and could not be used for bearing the load of the equipment. In addition, the equipment had to be anchored to the building's exposed columns with designed steel platforms attached to the columns using all-thread steel and epoxy anchors. Access to the platform and the mast climbers above the second floor entablature required construction of 12m-high stair towers from the plaza level.
In addition to addressing the demands of accessing and working in the repair areas, tremendous effort was put into matching the architectural finish of the original concrete surface. With its exposed garnet tailing aggregate finish, an incredible amount of experimentation was required to develop an end-product that blended seamlessly with original aesthetics. After securing a supply of the ‘hard-to-find' aggregate, SPS worked through numerous trials, batches and field tests to establish a color and concrete mix consistent with the original construction. Preparation of the aggregate proved critical. This included washing, cleaning and sifting it so only appropriately-sized pieces remained. At the end of the testing, the final result was a field-placed concrete that included a pre-packaged repair mortar, precisely measured water and the garnet aggregate.
A crucial step in ensuring the integrity of the mix was the process of pre-washing and drying the garnet tailing. Because garnet has a specific gravity of 3.01 and can absorb 3-4% of its own weight in water, it had to be pre-washed and soaked for 24 hours and dried so it was moist on the surface. Failure to do so would cause the garnet to absorb water from the concrete mixture. Such absorption would lead to a low slump and, ultimately, difficulty in placing the concrete. Prior to each placement cycle, the mixture was tested by the on-site engineer. Once approved, batches were mixed on the platforms for use on individual patches.
Key to ensuring a consistent finish was coating each custom-built mixture with a cementitous retarder. The retarder, along with favorable weather, temperature and humidity, allowed the concrete at the surface of the patch to strengthen at a slower pace. When the formwork was removed, the surface layer of the concrete could be removed with a combination of mechanical means and power washing. The result was a beautifully exposed garnet aggregate finish.
Concluding remarks
Projects of the size of the Xerox Square Tower façade restoration project provide many learning opportunities. The choice of the mast climbing equipment proved a very effective tool for safety and functionality. SPS also demonstrated that significant preplanning, flexibility in construction methods and thorough coordination of continuous three shift operations creates a unified team with pride in and dedication to craftsman-inspired work.
The commitment demonstrated by Xerox also set an admirable example. Effectively, repairing this signature structure required dedication to a demanding repair schedule that affected the building for several months of the year. However, by effectively teaming with the design and construction team, Xerox Corporation ensured the continued beauty and structural integrity of a landmark building.
About the Author
Pete Wallace is Branch Manager of Structural Preservation Systems' (SPS) Detroit Office.