Slipformed cores

How does Slipformed cores work, step by step?

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   Step 1: Design the system and plan the continuous operation

   Slipform is a temporary works design job before it is a concreting job. The temporary works designer works from the core geometry, the wall arrangement and the intended rate of rise to size the yokes, jacks, rods, decks and hanging scaffolds, and to set out how the load from the whole assembly gets back into the concrete. In parallel the project plans the thing that actually decides whether slipform works here: continuous supply. Shift patterns, gang sizes, concrete delivery rates, access routes, standby plant and lighting are all fixed in advance, and the contingency for a supply failure is agreed and written down before anyone starts. The decision to slipform is taken early because it drives the crane strategy, the site logistics and the whole programme.

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   Step 2: Build the base and assemble the form

   The core is started conventionally. A kicker or a starter lift is cast off the raft or pile cap and surveyed, because everything above will be referenced to it and a base out of position never gets better. The shutters, yokes, jacks, working deck and the hanging platforms below are then assembled around the core in position, the hydraulic ring main and the power pack are connected and tested, and the jacking rods are set. Assembly is a substantial operation in its own right and is usually the largest single craneage demand of the whole core.

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   Step 3: Start the slip and establish the rate

   The first concrete is placed in the form and the operation settles into its rhythm. The rate of rise is not a fixed number - it is found on the day and then held, because it has to match how quickly the concrete gains enough stiffness to stand as it emerges below the shutter, which moves with temperature and with the mix arriving. The specialist contractor and the temporary works designer set and adjust that rate together, and the concrete emerging from the form is watched continuously as the check that the rate is still right. Going too fast and going too slow are both serious, in different ways.

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   Step 4: Place, fix and jack in a continuous cycle

   Concrete is placed in layers around the whole perimeter of the form so the shutter is loaded evenly, and it is compacted only in the fresh upper zone so that the stiffening concrete below is not disturbed. Reinforcement is fixed ahead of the form, in short lengths that can be handled on a moving deck, with laps arranged so that fixing never has to stop the slip. The jacks lift in small increments between placing rounds. Everything on the deck - concrete supply, steel, boxouts, inserts and people - is sequenced so that no single activity can hold up the lift.

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   Step 5: Form openings, pockets and embedded items on the move

   Door openings, riser and service penetrations, lift guide fixings, floor connection pockets and embedded plates are all set into the form at the right level as it passes. Boxouts are made to be placed, held and then released as the shutter climbs off them. Because the form never returns to a level, the setting-out for every insert has to be checked before it goes in, and a marked-up drawing of what is coming in the next few metres is kept live on the deck. Missing an insert means cutting and drilling into a finished core later, which is slow, disruptive and never as good.

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   Step 6: Control verticality and geometry continuously

   The form is surveyed continuously against control that does not move with it. Plumbing, laser or optical control and the level of the deck itself are all monitored, and drift or twist is corrected by small, progressive adjustments to individual jacks rather than by a single large pull. The tolerances, the frequency of checks and the point at which a deviation has to be reported to the designer all come from the temporary works design and the specification. This is the quality theme that runs through every core method, but slipform is where it is hardest, because a small error grows with every hour of rise.

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   Step 7: Finish the surface as it leaves the shutter

   The wall face is finished from the hanging platforms immediately below the form, while the concrete is still green. Working platforms are suspended from the assembly and travel with it, so the finishers work a few metres below the placing deck for the whole of the slip. Curing is applied at the same stage. There is no second chance from a scaffold later, so the finishing gang is part of the continuous shift pattern like everyone else.

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   Step 8: Top out, strip down and hand over the core

   At the top of the core the slip is brought to a controlled finish, the top lift is completed and the assembly is dismantled and craned down in pieces. The core is then surveyed as built and the results issued, because the frame, the lift installation and the facade all set out from it. Any pockets, plates and fixings for the floors are checked against the record before the frame follows. On most projects the core has led the frame by several floors throughout, and it is that lead which sets the programme for everything behind it.