Slipformed Concrete

In the slipforming method, the formwork is moved continuously in sync with the concreting process in a 24-hour operation. The formwork, including the working platform and the hanging scaffold mounted internally or on both sides, is fixed to the jacking rods in the center of the wall. The hydraulic oil operated lifting jack raises the formwork by 15 to 30 cm per hour depending on the temperature. The jacking rods are located in pipe sleeves at the top and are supported by the concrete that has already hardened. The rods and sleeves are also raised continuously. These
works are carried out almost entirely by specialist contractors.

Slipforming is quick and efficient. The method is particularly suitable for simple, consistent ground plans and high structures such as:

• High bay warehouses, silos
• Tower and chimney structures
• Shaft structures

Because the height of the formwork is usually only around 1.20 m and the hourly production rate is 20 to 30 cm, the concrete underneath is 4–6 hours old and must be stiff enough to bear its own weight (green strength). However, it must not have set enough for some of it to stick to the rising formwork (“plucking”). The main requirement for slipforming without problems is concreting all areas at the same level at the same time, and then the simultaneous setting of these layers. Therefore the temperature has a major influence, along with the requirement for the
consistently optimum w/c ratio.

Composition:

• Aggregate 0–32 mm, or 0–16 mm for close reinforcement
• Although slipformed concrete is mainly crane handled concrete, the fines content should be as for pumped concrete
• Cement Min 300 kg/m³
• CEM I 42.5 for close reinforcement and large dimensions, CEM I 52.5 for smaller dimensions (towers, chimneys)

Workability

The best workability has proved to be a stiff plastic concrete having a flow diameter of 35–40 cm and a low water content. Note in particular that a wall thickness of less than 14 cm can be a problem (plucking, anchorage of jacking rods etc.). The newly struck surfaces should be protected as much as possible from wind, sunlight etc.