Melbourne’s Leading Concrete Pouring Specialists
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Concrete weighs roughly 2.4 tonnes per cubic metre. Before it hardens, something has to carry that weight, keep it in place and hold the slab to the right dimensions.
That's the job of slab formwork.
For a ground-floor slab, the setup might be little more than well-braced edge boards. A suspended slab several storeys up needs a complete temporary support structure beneath it. Both require accurate set-out, but the loads, installation methods and stripping process are very different.
Slab formwork is the temporary mould and supporting arrangement used to shape and contain freshly placed concrete until it can support itself.
The setup may include plywood sheets, timber or steel framing, edge boards, adjustable props and bracing.
For suspended slabs, it helps to distinguish two parts:
These systems work together during placement and curing. The concrete cannot take over its structural role until it has developed sufficient strength.
A ground-bearing slab is supported by the prepared ground beneath it. The formwork mainly contains the edges and any steps, recesses or thickened sections.
A suspended slab has no continuous ground support underneath. Until the concrete can carry its own weight, the temporary works must support the entire wet slab, reinforcement, workers and construction equipment.
This distinction also affects labour and equipment. Edge formwork can often be erected quickly on an accessible site. Suspended deck formwork requires more materials, lifting, access provision and coordination between trades.
The system depends on slab geometry, height, available equipment and whether the same arrangement will be repeated across several floors.
Timber bearers and joists support formply sheets, creating the surface against which concrete is cast.
This arrangement is adaptable, particularly around irregular slab edges, penetrations and changes in level. It also involves more cutting, measuring and individual assembly than some modular systems.
Modular systems use reusable panels, frames and engineered connections.
They are useful on projects with repeated floor layouts. Once the crew becomes familiar with the system, panels can be installed and moved between levels with less site fabrication.
The system still needs to be erected within its design limits. Mixing incompatible components or changing support spacing without approval can compromise the arrangement.
Table forms are large sections of deck formwork assembled into units that can be moved between pours.
They suit multi-storey buildings with repetitive floor plates and suitable lifting access.
The main advantage is reduced dismantling and reassembly between levels. However, crane availability, perimeter access and safe movement of the tables must be planned from the start.
For suspended slabs, installation follows the temporary works design and the planned sequence of construction.
The exact method varies between systems, but the main stages are similar.
The crew works from the structural drawings and formwork design to establish the slab perimeter, finished levels, beams, penetrations and supporting structure.
This is where errors become expensive.
A misplaced set-down can affect a bathroom floor. An incorrect penetration can interfere with services. A deck installed at the wrong height can leave the completed slab outside its specified tolerances.
Checking the set-out before installation is far easier than correcting hardened concrete.
Props and frames need a reliable surface beneath them.
On ground, the supporting material must be suitable for the imposed loads. On an existing concrete floor, the structure must be assessed for the loads transferred through the props.
Where required, soleplates or other designed bearing arrangements distribute those loads.
A support system may look stable while unloaded but behave very differently once wet concrete is placed above it.
Temporary supports are assembled in the positions and spacing specified by the formwork designer.
Props must be properly secured and braced. Connections, pins and adjustable components need to be compatible with the system.
The crew also needs safe access while the structure is being erected.
Bearers and joists carry the plywood or modular deck panels.
Once installed, the deck is checked for alignment, levels and continuity.
Joints should be properly fitted to prevent excessive grout leakage during placement.
Perimeter boards establish the slab edges, while additional formwork creates beams, steps, rebates and openings for stairs or services.
Penetrations need particular attention. Their positions must match the approved drawings, and openings require suitable protection against falls.
A pre-pour inspection checks the installed system against the design.
The inspection includes support positions, bracing, connections, deck condition, edge protection and the intended loading arrangement.
Any defects need to be resolved before concrete placement.
Urban Pour's article on formwork design rules covers the stability and load checks that should be addressed before a pour.
A suspended slab formwork system carries several loads at once.
These include the weight of fresh concrete, reinforcement, workers, equipment and forces generated during placement.
A 150 mm thick slab of normal-weight concrete, for example, contains approximately 0.36 tonnes of concrete per square metre. That is around 360 kg before reinforcement, formwork and construction loads are included.
It is easy to see why support spacing cannot be guessed.
The temporary works design determines the capacity and arrangement of props, frames, bearers and bracing.
Concrete placement also needs to follow the planned sequence. Concentrating excessive concrete or equipment in one area can overload part of the system.
WorkSafe Victoria's guidance on preventing formwork failures highlights the need for proper design, inspection, loading control and monitoring during concrete placement.
Slab formwork can be removed when the concrete has developed sufficient strength for the loads it will carry, and removal is authorised under the applicable design and stripping procedure.
There is no universal number of days that applies to every slab.
The timing depends on the concrete mix, curing conditions, slab span, structural design and construction loads.
Ground slab edge forms may be removed relatively early once the concrete can retain its shape without damage.
Suspended slab formwork is different. The deck and supports may need to remain in place considerably longer.
Before stripping begins, the responsible competent person must establish that the concrete can safely carry the loads expected at that stage.
Strength testing, specified curing periods and engineering approval may all form part of that decision.
A suspended slab that looks hard on the surface is not necessarily ready to carry its own weight over the full span.
Stripping needs a planned sequence, especially beneath suspended floors.
The crew generally works through the following process:
Dropping formwork components without control creates a serious risk to workers and can damage both the materials and newly cast concrete.
The Safe Work Australia guide to formwork and falsework provides further guidance on dismantling and temporary support requirements.
Backpropping is the installation or retention of temporary supports beneath a concrete slab to help manage loads during construction.
It is common in multi-storey projects where newly poured slabs cannot yet carry all the loads imposed by construction activities above.
For example, a contractor may be preparing formwork for the third floor while the second-floor slab is still gaining strength.
Without an appropriate temporary support arrangement, the lower floor could be overloaded.
Backpropping distributes construction loads through the supported floors according to an engineered design.
The number of levels requiring backpropping and the support arrangement depend on the structure, concrete strength and planned construction sequence.
Backprops should not be moved or removed simply because the formwork panels have already been stripped.
Formwork problems often become visible at the worst possible time: when the concrete trucks have arrived.
A well-planned system also needs room for reinforcement crews, pump hoses, concrete placement and inspections.
If those activities have not been considered during formwork planning, the deck may need alterations immediately before the pour.
Slab area matters, but it rarely tells the whole story.
Two slabs with the same floor area can have very different formwork costs.
A simple rectangular deck with repeated bays is generally easier to form than one with deep beams, multiple set-downs, curved edges and numerous service penetrations.
The main cost drivers are:
The most reliable pricing comes from structural drawings, formwork requirements and a clear construction programme.
Most cast-in-place slabs require some formwork to contain the concrete or form edges, beams and openings. Ground slabs may need only edge forms, while suspended slabs usually require extensive temporary support.
Common materials include formply, timber, steel and proprietary modular panels. Adjustable steel props, frames and bracing are frequently used to support suspended decks.
Yes. Timber, plywood and modular systems can often be reused if they remain in suitable condition. Damaged components must be assessed and replaced where necessary.
Seven days is not a universal stripping period. Removal depends on concrete strength, structural requirements and the approved stripping sequence. Suspended slab supports may need to remain longer.
Formwork shapes and contains the concrete. Falsework provides temporary structural support for the formwork and construction loads until the permanent structure can support itself.
A slab pour runs more smoothly when the formwork, reinforcement and concrete placement have been planned together.
Urban Pour works on residential and commercial concrete construction across Melbourne, including concrete slab construction and structural concrete works.
For projects involving ground slabs, suspended decks or complex concrete structures, contact Urban Pour to discuss the drawings, site conditions and construction requirements.
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