Special formwork avoids scabbling

The task

Construction of large concrete structures.

The problem

A construction company was awarded a contract to build one of the stations for the London Underground Jubilee Line Extension. The station design included a reinforced concrete base slab 300 m long, 25 m wide and 3 m deep. The slab was cast in situ in 43 sections, each 7 m long. The sections were cast one after another using the previous section to support one side and a specially constructed formwork stop end to support the other. With this method of construction it is important that the new concrete makes an effective bond with the old. When wooden formwork is used, this bond can only be achieved by removing the top surface of the concrete to reveal the aggregate underneath. This is often done with impulsive tools in a process known as scabbling. In this case, the tools were used for about 2 hours per day and exposed operators to high vibration magnitudes (typically 18 m/s²). At that work rate, each stop end would have taken approximately eight worker shifts to scabble with a vibration exposure well above the limit value.

The solution

The company used an expanded metal material to construct the formwork for the stop end mating surfaces. The material was ribbed and featured bent tabs of mesh which, when concrete was poured behind it, become embedded in the concrete, forming a bond. The formwork was left in place once the concrete had cured and when the next section was poured, it formed a bond with the outer surface of the expanded metal that was as strong as a traditional scabbled joint. No scabbling was necessary.

The result

• The operators were not exposed to any vibration.
• It was installed more quickly than using wooden formwork as it allowed the next section of concrete to be poured before the previous section was fully cured (set).
• Noise and dust levels were also reduced.