Slipform

How does Slipform work, step by step?

1. 1

   Step 1: Confirm the element suits a continuous rise

   Slipform wants height, constant or gently tapering section, and as few interruptions as possible. Openings, embedded plates, beam pockets and changes of wall thickness all have to be formed while the form is moving, and each one is a complication rather than a detail. The designer, the contractor and the slipform specialist review the element together and simplify what they can - aligning openings, standardising thicknesses, moving connections to positions that suit the climb. A project that cannot be simplified enough is usually better jump-formed, and that decision is best made early rather than discovered halfway up.

2. 2

   Step 2: Design the platform, yokes and jacking system

   The temporary works designer designs the whole assembly as one machine: the form panels and their slight taper, the yokes that carry the form, the jacks and their climbing rods, the main working deck with its access and edge protection, and the hanging scaffold below from which the emerging concrete is finished. The design covers the loading of every deck, the arrangement and control of the jacks, the intended rate of rise and how it will be adjusted, the survey and verticality control system, and the procedure for a stop. It also covers the power and hydraulic supply, including redundancy, because a hydraulic failure at height on a moving form is not an inconvenience.

3. 3

   Step 3: Assemble at the base and prove the system

   The form, yokes, decks and jacks are assembled around the base of the element, usually at ground level. Everything is commissioned before the pour starts: jacks stroked and synchronised, hydraulics tested, survey targets and control established, communications proved between the deck, the batching plant and the ground, and the crew briefed on the shift pattern and on who can stop the operation. A trial climb over a short height lets the crew find the rhythm and the supervision confirm the rate before the point of no return. This is the last comfortable moment on the project.

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   Step 4: Start the pour and establish the rate of rise

   Concrete is placed in the form in shallow layers and compacted, and once enough depth has been built up the jacking starts. The rate of rise is then found and held: fast enough that the concrete is not gripping the form, slow enough that what leaves the bottom stands. The concrete supplied has to be consistent from load to load, because a change in its behaviour changes the rate the form can safely run at. Placing, compaction and jacking settle into a continuous rhythm, and the person supervising the rate is watching the concrete emerging below as much as the gauge on the deck.

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   Step 5: Fix reinforcement and form openings on the move

   Reinforcement is fixed continuously ahead of the rising form, delivered to the deck and lapped as the wall grows. Openings are formed with boxes fixed in position and left behind as the form passes over them. Embedded plates, couplers, service sleeves and any connection details for the floors are placed to the drawings as the relevant level goes by. There is no going back for anything missed, so a checking regime runs on the deck against a marked-up drawing, level by level, in real time. This is the part of the operation most vulnerable to fatigue on a night shift, and it is checked accordingly.

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   Step 6: Control verticality continuously

   A slipform will wander if it is allowed to. Verticality and twist are monitored continuously by survey and by the system's own control, and corrections are applied gradually through the jacks. Small, early, gentle corrections work. Large late ones do not, and can distort the form. On tall elements the survey control is run as a discipline in its own right, with results logged every shift and reviewed against the tolerance the designer set, because the position error at the top is the sum of everything that was left uncorrected below.

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   Step 7: Finish the emerging concrete from the hanging deck

   Below the form face, on the suspended deck, the crew work on the concrete as it appears: rubbing up the surface, making good minor blemishes and curing the fresh face. This has to keep pace with the climb, because the window in which the surface can be worked passes with the form. The hanging deck is part of the temporary works design, is loaded only to what it was designed for, and has its own access and edge protection. Working there means working directly above the previously cast wall with the operation continuing overhead, so the deck arrangement and the exclusion below matter.

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   Step 8: Manage stops, then top out and strike down

   Every slipform has a written procedure for stopping, planned or unplanned, covering how the form is freed, how the joint is prepared and how the restart is authorised. Standby concrete supply, standby power and hydraulics, spares and cover crews exist so the procedure is rarely used. At the top of the element the form is stopped deliberately at a designed position, the last concrete is finished, and the assembly is dismantled from height and craned down in pieces - a substantial operation planned with the same care as the erection. Records of rate, survey, cast-in items and concrete supply are compiled as the as-built evidence.