The distinctive new headquarters of the Abu Dhabi Financial Centre (ADFC) includes four high-rise office buildings comprising 285,000 square meters of dedicated office space. ADFC is part of Sowwah Square, a 570,000-square-meter office and retail space development on Sowwah Island. Construction for Sowwah Square commenced in the summer of 2007, with a phased completion date starting in 2010.
The main contractor for the ADFC wanted to strengthen several concrete corbels located in the podium level of four towers. After the structural engineer reviewed the design, it was determined that the existing concrete corbels' load capacity was not sufficient and there was a deficiency of reinforcement in the corbels. As a result, a solution was needed to increase the strength of pre-pour corbel. Further, this resolution had to be done parallel to and have minimal effect on existing construction work at ADFC.
Preliminary evaluations were compiled and the project team opted to use externally bonded Carbon Fiber Reinforced Polymer V-Wrap (CFRP) and concrete enlargements. CFRP V-Wrap system is an effective and economical solution that would keep the project on schedule and within the estimated budget. Externally bonded FRP was selected, because it offered ease of construction, minimal modification to the structure and minimal schedule impact. Structural was contracted to perform the corbel strengthening portion of the construction project. Structural began the strengthening phase by performing corbel strengthening to restore the integrity and improve the durability of the concrete corbels.
The surfaces of the corbels were prepped about 300 mm wide all around and then multiple plies of V-Wrap CFRP systems were used to wrap around the corbels. The corbel surface was roughened with the use of grinders utilizing segmented polycrystalline diamond (PCD) cups, optimizing the bond of the CFRP V-Wrap system. The objective of surface profiling was to achieve a concrete surface profile of three or better as recommended by the International Concrete Repair Institute (ICRI) guidelines. This is the minimum requirement to ensure that the FRP system is properly bonded to the existing concrete. V-Wrap CFRP sheets were bonded to the vertical side of the corbel on all sides. Twenty-six of the corbels were strengthened using CFRP V-Wrap for a total of approximately 200 square meters of CFRP installation. After the CFRP V-Wrap strengthening system was installed according to the manufacturer's recommended procedure, an additional coating of FRL intumescent paint was applied over the system to provide additional flame/smoke spread resistance as required by the local code.
Concrete Enlargement
Two corbels were strengthened using concrete enlargement techniques with pressurized form and pump techniques. For these corbels, a 150 mm to 200 mm concrete jacket was proposed with T 16 dowels and concrete was poured using form and pump techniques that utilized 0.86 bar pressure in fully enclosed formwork. The structure of the corbels was surveyed by chipping a slot across the corbel exposing the location of steel reinforcement. Embedment holes were then drilled to the required depth to allow for the new steel reinforcement. Structural then prepared the surface with a pointed chisel bit to achieve an open pour structure. Extra care was taken not to micro-facture the concrete through an aggressive chipping process. A grinder with wire wheels was used to eliminate any loose particles in the concrete. A brush-blow-brush method was used to ensure that the holes were cleaned. Dowels were embedded and anchored using epoxy in the existing corbel, tying the existing and new members. After the dowels were installed, the longitudinal reinforcement was tied and the formwork was installed. The enlargements were formed for pressurized pumping and concrete was placed. During the pumping operation, ice was used when mixing the concrete to keep the temperature low enough for flowability. Concrete enlargement was the necessary solution for some of the corbels, because the required strengthening could not be achieved using CFRP V-Wrap systems.
One of the main challenges on this project was that the as-built and field conditions differed from the design. Some corbels were smaller in width than shown on the design drawing. This required Structural to do an as-built survey of all 28 corbels to verify the field conditions and re-design some of them to accommodate the variation and strengthening requirements. Because of the high temperatures, the V-Wrap resins were kept in temperature controlled storage areas and were conditioned before use to make sure that enough working times were acquired so that the installation could be done smoothly.
All CFRP V-Wrap and concrete enlargement work was completed efficiently and on schedule. The curing nature of CFRP V-Wrap allowed the corbels to be turned over to the general contractor within 24 hours.