Slipformed Concrete

Slipformed Concrete

In the slip forming method, the form work is moved continuously in sync with the concreting process in a 24-hour operation. The form work  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 form work 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.

Slip forming 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 form work 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 form work (“plucking”). The main requirement for slip forming 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.