Using Concrete in Renovations - Formwork
August 26th 2008 02:10
Formwork is the temporary structure which moulds concrete into the desired shape, and holds it in the correct position until it has hardened sufficiently and/or is able to support the loads imposed on it.
Formwork should have sufficient strength to resist the pressure of the fluid concrete without damage or excessive deflection. It should be rigid enough to prevent bows/bulges outside what is specified for the work. Formwork should be contructed accurately to ensure correct surface levels, dimensions and shape. It should be watertight to prevent the loss of cement paste and/or matrix from the concrete (this can cause ragged edges, hydration staining and honeycombing, which in turn can affect strength, durability and result in poor off-form finishes). Formwork will also be used repeatedly, so it needs to be robust.
Formwork should be coated with a form release agent to improve concrete finishes, aid in the removal of the formwork and thus minimise possible damage to the
concrete face from the stripping operation. There are two basic types of release agents: barrier products which simply separate the concrete from the formwork, and the more common reactive products which react with the concrete constituents to form soap-like products that prevent the concrete from adhering to the formwork. A release agent suitable for the type of formwork and finish required should be selected.
Formwork is left in place to protect the edges of the slab from mechanical damage during site works (while the concrete is hardening), and to avoid damage from the premature removal of the formwork itself. Formwork is also an effective means
of curing when it is left in place. if the formwork is stripped before 3 days have elapsed, it will need further curing.
Basic ground forms are suitable for paths and driveways on firm ground, up to 150 mm thick.
Formwork up to 300 mm in height is often required for slabs where the ground level varies. Where the height of formwork exceeds 150 mm, it should be braced appropriately. Deep edge forms can be constructed from clean-faced ply. If solid timber forms (minimum 38 mm) are used, the bracing and formwork will be different. When depths exceed 200 mm, solid timber forms should be a minimum of 50 mm thick.
Use this checklist to ensure safety and propriety when working with formwork and concrete.
1. Has the formwork system been properly designed?
A competent formwork designer, manufacturer or supplier should design the site formwork system. The formwork contractor should have erection design drawings and specifications for the particular formwork system to be constructed. Ensure a copy of the design drawings and loading calculations are available on site. Make sure the building's design engineer specifies when the formwork can be dismantled - concrete cure requirement.
2. Has the formwork been properly constructed?
All modular or framed formwork components, support timbers and structural ply, need to be in a serviceable condition. Check that the constructed formwork is on firm foundations (suitable soleplates, hardness of ground or adequacy of support structure). Make sure the formwork system is the same type and capacity as specified in the design drawing and is erected in accordance with the design. Ensure any adjustable building props are tied to each other or to the shoring frames so they cannot collapse when released.
3. Is the formwork deck being laid safely?
The work method used to lay out and secure form ply must protect the workers from falling. When required to work from the formwork itself, make sure they have a full deck of scaffold planks and safe access. When laying additional sheets from the formwork deck, workers should stay clear of the leading edge, pushing out the sheets as they go. Perimeter edge protection (temporary guardrails or scaffolding) needs to be provided. Ensure workers have safe and secure access and egress to and from all the formwork areas, including deck
4. Is steel fixing being done safely?
Make sure plastic protective caps are always placed on the ends of starter bars to safeguard workers. When fixing steel for concrete walls and columns, steel fixers will need properly constructed scaffolds. Steel fixers need protective glasses when using bolt cutters to stop steel fragments from wounding their eyes.
5. Is the formwork structurally adequate?
Before pouring concrete, use an experienced structural engineer to inspect the erected formwork system. This inspection should also include any supporting structure the formwork is constructed upon; for adequacy and the ability to take the loads of the new suspended concrete floor or beam. The engineer should supply an inspection certificate to verify the structural integrity of the support structure and formwork system
6. Are wall and column shutters safely lifted and properly secured?
Formwork shutters need to be securely slung and controlled with a tagline when they are being crane-lifted. Do not allow large shutters to be lifted in strong winds. Where possible, push-pull angled props should be fixed to cast-in anchors. Workers installing she-bolts need to work from properly constructed scaffolds or other safe temporary work platforms
7. Are workers prevented from accessing the area underneath the concrete pour?
Ensure that no worker is allowed to access the immediate area beneath the section of formwork where the concrete is being poured. If an observer is to be positioned at a lower level during the pouring operation, they must be located in a position that will safe guard them from injury if the formwork fails during concrete placement. Generally, neither the observer nor any other worker should be permitted to access the area below the pour once concrete placement has commenced, even to rectify problems.
8. Are concrete pumps being used safely?
Concrete pumps must be well maintained, fully serviceable and should comply with the requirements of the Industry Standard for Concrete Pumping. The operator of a truck mounted concrete placing boom must hold a WorkSafe certificate of competency (Class PB). Ensure mobile boom-type units are set up correctly and fully comply with the NO-GO-ZONE rules for overhead power lines. Concrete pumping lines need cleaning out after each use.
9. Are kibbles being used safely?
Crane-lifted concrete kibbles normally require a person with a WorkSafe dogging or rigging certificate to operate them and direct their movement. Make sure the dogman understands the need to release the concrete gradually from the kibble so as not to overload the formwork and risk structural failure. The sudden release of concrete from the kibble can also make the crane boom whip upwards, causing the kibble to bounce dangerously. Never allow workers to "ride the load" by standing on a kibble while it is being lifted.
10. Are concrete vibrators being used safely?
Check that vibrators are well maintained and fully serviceable. Residual Current Devices (RCDs) must be fitted for the protection of all electrical power leads and electric vibrators. Do not use petrol-driven vibrators in cellars or other poorly ventilated areas
11. Are the concreters working safely?
Make sure there are no open sides or penetrations where a worker could fall. Where required, provide temporary guardrails or a heavy duty perimeter scaffold
12. Is formwork being dismantled safely?
Do not allow formwork to be removed prior to the concrete reaching its required strength. When stripping the underside of a suspended floor slab, barricade the area off from other workers. Make sure people dismantling the formwork are working from properly constructed scaffolds or properly planked shoring frames. Never allow "drop stripping" of form ply and falsework.
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