A Program Management Approach : Constructability Challenges (Part 2)
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2.?????Concrete Pumping
Concrete mix designs must be methodically detailed by the structural designer for a super tall.??With potentially 500 plus meter pump heights, concrete additives must be engineered to allow maximum flow-ability while still achieving design strength in a reasonable curing period.?Without attention to the concrete mix design, accelerated curing has the potential to develop uncontrolled concrete creep and shrinkage that could result in cracking.?
Another important consideration is early strength to meet schedule requirements. Formwork can only be stripped and jumped to the next level after placed concrete reaches a minimum value. For a typical 3-day cycle, the formwork must be on the next level shortly (within 12 hours) after the concrete is placed and, therefore, contractors will often use concretes whose strength exceeds the design strength to develop a higher early strength mix.??
Concrete Pumping at the Burj Khalifa.?Before the Burj Khalifa, pumping concrete more than 600 meters straight up a building had never been done.?The previous world record was 445 meters at Taipei 101 in 2003, pumped by a Schwing BP8000 HDR with pressures up to 22 bars.?One of the greatest challenges with the Burj Khalifa was ensuring that the concrete pumping methodology had appropriate redundancy and ability to perform at extreme heights. Extensive tests to select the concrete pumps was carried out at ground level using a series of long horizontally placed pipelines to simulate the pressure behavior and expected friction of the concrete. Two (2) Putzmeister super high pressure BSA 14000 HPD pumps and a normal BSA 14000 HDP pump as the backup pump were placed at the ground level pumping station. Even though it was proven that the selected pump could pump concrete 600 meters, further redundancy was implemented with a secondary pump at L147 (as a contingency plan).??
Burj Khalifa ground level concrete pumps
Steel In-Fill at Taipei 101.?Although Burj Khalifa has the title of world’s highest concrete pumping, Taipei 101 still holds the record for highest steel column in-fill concrete pumping.?Taipei 101’s frame consists of eight (8) mega columns, the largest one being 2.3 meters x 3 meters, comprised of 80 millimeter thick SM570 steel. These mega columns are composite members consisting of hollow steel boxes, stiffeners, studs, steel plates, rebar cages, and concrete in-fill. Concrete in-fill for these steel columns was cast from Basement 5 to Level 62 (270 meters) and Level 94 to Level 99 (429 meters).??
Taipei 101 steel infill
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10,000 psi self-compacting concrete was specified to ensure the concrete would fill up the space within the mega columns. Development of 10,000 psi self-compacting concrete was a challenge in itself. Such concrete existed only in the laboratories of universities or research institutes, and its use in a real construction project had never been attempted. The initial development of the required mix design for the Taipei 101 column in-fill started from a university and was refined by the main contractor, KTRT JV led by Kumagai, who had extensive experience in self-compacting concrete.?
A special concrete quality committee comprised of the main contractor, the concrete suppliers, the structural design consultants, and various professors was established to continuously review the concrete quality throughout the project. The design of this self-compacting concrete is still relatively unique.?Belite-rich cement was used to reduce the heat of hydration and improve the flow-ability.?The mix was designed for a 90-day strength, as early strength is not required for column in fill concrete. At the higher levels, fine lime stone powder was added in the concrete to increase the pump-ability.?
< Taipei 101 steel infill in progress
Three pumping tests were conducted for the full size steel columns to optimize the rate of placement and pump pressure. Column in-fill lagged behind the composite slabs and, at lower levels, mega columns were filled two floors at once.?At higher levels, the volume of concrete reduced and the mega columns?were filled four floors at once. In so doing, the pump would operate within 75% of its maximum capacity and push the concrete at the rate of 40 cubic meters per hour. Before conducting column in-fill at Level 94, trials were carried out at Levels 80, 88 and 93. The concrete mix was adjusted to increase the slump flow to 720 millimeters, and the pumping rate was reduced to 30 cubic meters per hour for the pump to operate at 75% of maximum capacity.?All in all, the activities were successful, as well as innovative.
Stay tuned for part 3, where we going to share about? Formwork Systems and Structural Steel Erection