Designers occasionally use concrete columns in buildings with structural steel beams supporting the floors. This reduces cost by about one fifth over steel columns, though concrete columns are bulkier and add weight to a structure. From an architectural perspective, concrete is more adaptable because it can be molded into interesting designs.
With sustainability an increasing issue for all building construction, concrete becomes an obvious choice. High-performance concrete (HPC) mixes typically replace some portland cement with fly ash, slag, or silica fume (or all three) to use what would normally become a waste product. These mixes also enhance the performance of the final concrete and reduce greenhouse gasses associated with cement production. But the long life and energy efficiency of a concrete structure are the keys to its sustainability. Concrete structures reduce the cost of heating and cooling—especially when insulated concrete panels are used for curtain walls—making them more energy efficient.
A number of the technology improvements have made high-rise concrete construction more competitive over the years, ranging from construction methods to enhancements to concrete mixes. Each has helped to increase concrete's advantages as the preferred building material for high-rise construction.
Flat-plate floors. Lawrence Novak, director of building structures for the Portland Cement Association, says that the development of flat-plate construction technology has had an impact on high-rise construction—especially for residential buildings. Without supporting beams, the height between floors is minimized and construction costs reduced, allowing the addition of floors without changing the height of the building. Novak adds that the introduction of headed shear stud assemblies that extend further out into floor plate areas from columns reduces punching shear. These devices can increase the allowable distance between columns to a degree and eliminate the cost of forming drop panels or capitals around the tops of columns.
Forming systems. The development of drop-head forming systems is a perfect fit with flat-plate construction. Form manufacturers have each developed systems that allow workers to quickly set floor forms from below—creating a safer, faster, and more efficient work environment. They also can be easily removed after concrete placement. Form tables by contrast are moved by crane and therefore are controlled more by crane schedules and wind conditions.
Today, it's common practice to construct building cores with cast-in-place concrete. Jump forms or self-jacking forming systems have become popular for core construction. After walls are cast, workers free the forms from the concrete and use hydraulic jacks built into the system to move interior forms to the next level, while exterior forms are lifted by crane to the next position.
The headed shear stud assembly increases punching shear around columns and can extend distance slightly between columns.
Concrete pumping. Concrete can be pumped great distances through vertical lines by single pumps—more than 2000 feet in the case of the Burj Dubai, the world's tallest building. Pumps allow tower cranes, which deliver concrete more slowly, to concentrate on other work. Key elements of the system include the pump, steel pipe slick lines that are anchored onto or cast into the structure for the duration of the project, and placing booms mounted on masts and attached to decks or on top of building core forms. When they are mounted on self-jacking core forms, they move upward with the forms and don't require crane assistance.
Concrete mixes. HPC and self-consolidating concrete (SSC) mixes meet the engineering requirements for tall building construction. SCC mixes solve placement challenges where there is congested steel reinforcement. On the Trump Tower in Chicago, 16,000-psi SCC mixes were used to ensure good consolidation at the transition levels of the building that contained as much as 1000 tons of rebar. HPC mixes are frequently used in column construction. Baker adds that specifying mixes with high modulus of elasticity is as important as specifying concrete compressive strength.
Reinforcement. The software used to design structural steel buildings is still more advanced than for structural concrete. But this is changing as software for designing steel reinforcement improves. There have been improvements in rebar too. Upgrading Grade 60 rebar to Grade 75 or even to Grade 100 can help reduce congestion. Using headed reinforcement—rebars with buttons or washers welded on the ends—eliminates the need for hooks. This makes placement easier in tight areas and reduces congestion. Threaded rebar splices also helps to reduce congestion.
Advancements in rebar fabrication makes concrete construction more efficient. Neal Anderson, vice president of engineering for the Concrete Reinforced Steel Institute, Schaumburg, Ill., says that rebar can be made on automatic table fabricators, increasing the accuracy of each piece. Often reinforcement is assembled on jigs on the ground, away from the focus of construction. Assemblies are moved by crane into location as they are needed and speeds up construction. In California, instead of tying bar, resistance welding is used to assemble cages and mats.