Slipforming is a technique for rapid construction of tall concrete structures, it involves the extrusion of the concrete in situ rather than casting in short lifts. It offers the advantages of speed, ability to produce a monolithic structure, and economy of operation. The disadvantages are the high cost of the initial set-up and the need for specialist expertise and equipment. The dimensional accuracy that can be achieved may also be less than by conventional techniques.
This method can be applied to a wide variety of structures, including service cores of high-rise buildings and chimneys. Tapering shapes or wall thicknesses can be accommodated. The complete plan of the structure is set out in formwork normally 1 to 1.2 m high, and lifted in small increments by hydraulic jacks at a rate, typically, of between 150 mm to 350 mm per hour.
The jacks climb on smooth rods or structural tubing embedded in the hardened concrete. Reinforcement is fixed just ahead of the form, which is then filled in layers with concrete. The rate of climb is regulated to ensure that the concrete is self-supporting on emerging yet not so slow that the concrete hardens and binds on the forms. There is virtually no limit to the height and size of a structure that can be slipformed.
The constraints lie in matching the resources to the production requirements of the process. The latter is determined by the size and complexity of the structure and by the properties of the concrete mix chosen. The logistics of reinforcement fixing, concrete placement, etc on a 24-hour, 7 days-a-week basis must be adequate to maintain the rate of climb and have sufficient flexibility to cope with all contingencies.