WORKSHOP 03 - CONCRETE SLAB 2

32 • CONCRETE SLABS �--+------------------------------· Site Class (Table I) A or S

Sloping site

Flat or near flat site ( 150mm max. depth of fill

Floor type

Slab on ground

Unstepped floor

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Stepped floor

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footmg slab

Infill slab

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Waffle raft

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Stiffened raft

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SOME CHOICES FOR REINFORCED CONCRETE SLABS Every site is different and needs to be checked by a structural engineer.

These sketches are to assist at the building design stage only

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CONCRETE SLABS • 32 Suspended concrete slabs Suspended slabs on fillings This method of constructing concrete slabs has all the simplicity of the floating slab method but the slab sits on top of the base walls rather than between them. The slab is reinforced to span the distance between supports. The filling is generally of poor­ quality unconsolidated fill, which will settle away from the underside of the slab with time. However, as the slab is designed to be suspended, the result is a simple-to-con­ struct concrete floor in which all the building loads are transferred to the footings.

Suspended concrete slabs on fill are only used on ground-floor slabs where the height from the ground to the underside of the slab is 600 mm or less. If the clearance under the slab exceeds 600 mm then removable form­ work is generally more economical.

Suspended slabs on formwork Reinforced concrete slabs cast on formwork are generally used where access to the underside of the slab is desirable-as in basement garages or first floors. Without doubt, suspended concrete floors on tempo­ rary or permanent formwork are expensive, and, if they are to be used by homebuilders, good design and planning are essential to gain the best value for the dollars outlaid.

The design of suspended reinforced con­ crete slabs is very definitely the province of a structural engineer, no one should attempt to design their own suspended slabs unless they are thoroughly convinced of their ability.

Although it is possible for OBs to build their own formwork, it is generally not eco­ nomical, except if permanent formwork is being used. The formwork shuttering boards and the joists and props can be used on multiple projects; therefore it is nearly always cheaper for the 0B to employ a formworking gang that supplies its own material, than it is to buy or hire (although this is not always possible) the materials for the formwork.

If OBs decide to place and tie the reinforc­ ing steel for their own suspended floor, they should ask themselves this simple question: Am I convinced that the job I will do is going to be good enough for me to sleep peace­ fully at night, on or under the concrete slab in which I placed and tied the reinforce­ ment? If the answer is not a resounding YES, then employ someone who knows what they are doing and reduce the risk.

Remember to cast knock-out holes in the slab for any penetrations needed for drainage pipes or other services. Electrical conduits should be cast into the slab.

The underside of a suspended concrete slab is the ceiling of the floor below, so if a quality strip finish is required then quality formwork will be required first.

When the concrete is placed, adequate vibration is required to make sure that the undersurface of the slab is free from defects such as 'honeycombing', which not only looks unsightly, but in extreme cases can result in structural failure.

COMMON USES OF CONCRETE IN HOUSES

Single storey cottage

Upper floor slab

Floors to wet areas, e.g. bathrooms, laundries, toilets.

(This 1s required by regulation 1n some states) Floors to patios, verandah and steps.

Upper floor terrace and balcony

Upper floors should be designed by a structural engineer.

Ground floor walls generally required to be masonry.

Weatherproofing of slabs over rooms needs special consideration.

Cantilevered slabs require engineering design.

�plywood formwork on supporting timber frame plywood formwork / formwork bearers

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props

bracing

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TEMPORARY FORM WORK

concrete

'Bonded' or similar, galvanised steel

PERMANENT FORM WORK

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plate

reinforcing steel

OBs should take great care if they erect their own formwork-remem­ ber concrete is very heavy: 2.4 tonnes per cubic metre. A concrete slab that is 125mm thick and cov­ ers an area of 80 square metres, which is a common upper floor area of a house, would weigh 24 tonnes, making a formwork failure expen­ sive and dangerous.

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