The Tower at PNC
The Tower at PNC Plaza is a 30 floor skyscraper and is the newest building to touch the Pittsburgh skyline. The projects goals were to exceed LEED Platinum status by utilizing green building products and minimizing environmental impacts that are typical to such a large building.
In an effort to help the project attain the LEED goals they hoped to achieve, we wanted to develop specific concrete mix designs that provided higher concentrations of post consumer and post industrial products while still meeting all quality requirements on the project. It is always our intention to work with the contractor to provide any and all support necessary to meet the project goals.
Auger cast grout is a construction method used to support the building foundations. The auger piles are deep foundations that are drilled into the ground and filled with concrete as they are being drilled. The concrete is pumped through the hollow stem located on the drilling rig. As the auger is extracted from the ground the concrete is pumped in to fill the void. Pilings are located at very specific locations to provide support for the building elements.
The concrete mixture needs to have proper slump and flow characteristics to enable the contractor to pump through the small pump line and into the piling. The utilization of various admixtures give us the ability to provide high fluidity concrete while maintaining a cohesive mix. We need to ensure that the concrete will flow property but not segregate once in place.
115
Concrete unit weight requirement in pounds per cubic foot
The unit weight requirement of the slab on deck concrete reduced the dead load on the structure and more importantly provided the fire rating requirements without the need for fire proofing insulation that is typically used.
5,000
Compressive strength requirement for the lightweight in pounds per square inch
The design strength for the lightweight concrete was achieved by utilizing a specific combination of cementitious materials. Special consideration was given to the LEED requirements and the utilization of post industrial recycled materials.
A significant amount of lightweight concrete required on the project that needed to be supplied throughout the winter.
This can be challenging because lightweight needs “conditioned” or saturated with water to ensure the mix performs as intended. Fortunately, at our Frank Bryan concrete plant, we have the unique ability to store significant amounts of material in heated bins. This enabled us to have warm, conditioned lightweight ready at any time and helped us procure the job with P.J. Dick.
Another interesting challenge was that the project goal was to exceed LEED Platinum status, as mentioned above. This gave us the opportunity to participate through utilizing high volumes of post industrial recycled materials in the for of Class C Flyash (C-Ash) and
Ground Granulated Blast Furnace Slag (GGBFS). We began developing mix designs for the project early on, in an effort to optimize the percent concentrations of C-Ash and/or GGBFS.
At our Research and Development Center we ran multiple mix designs with various concentrations of cement replacements in an effort to optimize the mix while maintaining the specified compressive strength and unit weight requirements. Another consideration that is always important to our design process is the pump-ability and finish- ability of the concrete. Since the concrete was going to be pumped through a centrally located stand pipe we wanted to ensure that the mix had the fluidity to pump easily while providing good finishing characteristics for the contractor.