The building is supported by 194 caissons, 5 feet in diameter and approximately 150 feet deep. Baker says that soils in the region have high chloride and high sulfate contents and are composed of silt-formed calcium type rock. The caissons depend on “skin friction”—the resistance between the concrete caisson and the surrounding soil—to provide the necessary support. High-performance, dense concrete resists the high sulfate soils.
Resting on the caissons is a 12-foot-thick mat slab cast with SCC placed in three lifts. The core structure, the buttresses, and the columns are all supported by the mat slab. The core walls start at 26 inches thick, diminishing to a 20-inch thickness at the top of the structure. Floor thicknesses for the bottom floors are typically 8 inches thick and mechanical floors are 12 inches thick. There is no post-tension (PT) reinforcement used anywhere in the building.
SOM believes that the modulus (E) number of concrete is as important as its compressive strength for super-tall building construction. Normal-weight concrete has an E of 2000 to 6000 ksi, with the requirement for the Burj being 6300 ksi at 90 days. The quality of aggregate material has much to do with E but SOM decided to specify the E they required and let the contractor be responsible for the mix details.
When there are hundreds of concrete placements over the course of construction, shrinkage and creep, occurring at different rates over time, can be critical to a building like the Burj Dubai. For that reason, Novak says they decided to use one mix for all the vertical work on a given floor level, keeping surface-to-volume ratios the same for columns and core walls (column and wall thicknesses are the same). This way shrinkage and creep would be the same and have minimal influence on the structure.
Shown in the plan diagram is the “buttressed core system” that increases wind shear resistance and reduces torsional movement of the core structure.
James Aldred from the Independent Verification and Testing Agency (IVTA) says that most of the mixes are “triple blends” including portland cement, fly ash, and silica fume. They have a relatively high fine aggregate fraction as well as containing up to 650 pounds/cubic yard of cementitious content. Flowability is increased with polycarboxylate superplasticizers while keeping water-cement (w/c) ratios below 0.32 for higher strength concretes. Although some vibration was used during casting, the concrete could be considered to be SCC, according to Aldred. Three-quarter inch maximum aggregate was used up to the 100th floor and 9/16 inch at higher elevations to reduce pumping pressures. Significant amounts of ice were added to keep concrete temperatures between 75° F to 90° F. Even with placements conducted at night, ambient temperatures could be up to 105° F from daily highs of 120° F in the middle of summer.
Concrete cube strengths specified for building elements includes the following:
- Caissons: 9000 psi minimum strength
- Mat slab: SCC with 6000 psi minimum cube strength
- Core walls and columns < 126th floor and floors 154 and 155: 11,600 psi minimum strength
- Core walls and columns > 126th floor: 9000 psi minimum strength
- Floors: 5000 psi minimum strength
Novak says the quality of the concrete was excellent. Column and core wall mixes specified for 11,600 psi compressive strengths were actually developing an average 56-day strength of 15,000 psi with a modulus of 7000 ksi.
Forming and placing
Baker says that recent advancements in forming technology have helped to make structural concrete construction attractive. The same is true for concrete pumps because they effectively deliver concrete—1900 feet straight up in this case.
Doka, Lawrenceville, Ga., supplied the vertical formwork for the 4.6 million square feet of walls. Core walls were constructed with self-rising or “jump forms” with the concrete placing boom mounted on the top of the forms. The boom advances as the forms move upward.
Winds in the region didn't permit the use of table forms for floor construction so workers used an efficient handset forming system manufactured by Meva Formwork Systems, Springfield, Ohio, for the floor construction.
The concrete pump for the project is the largest one Putzmeister, Aichtel, Germany, manufactures. Bill Carbeau, a sales and product manager for Putzmeister, says it can develop as much as 5500 psi pressure on the material, although 3000 psi is all that's needed for this project; the rest being reserve capacity. At the placing boom, the pressure is approximately 50 psi to ensure safe delivery. The pump weighs 22,000 pounds and is powered by a 630-hp Caterpillar engine. Carbeau says they decided to use a 6-inch-diameter pipeline instead of the customary 5-inch to reduce friction in the line and decrease the chance for plug-ups (there was a reduction back to a 5-inch line at the placing boom). This decision, along with the well-designed concrete mixes, significantly reduced pipeline wear. Putzmeister also developed a special tool that could be wheeled onto any floor in the building to quickly lift vertical lines, assisting in the process of removing blockages should they occur.