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  • Overview
  • Types of Slabs
  • Design Guidelines
  • Other Slab Systems
  • Void Formers

Types of Slabs

A concrete slab, including the beams or columns that have formwork (temporary or permanent into which concrete is pored, keeping it in place until it cures). And typically designed by an Engineer; cast with this is the steel reinforcement which is necessary to provide the strength required; which too would typically be specified by the Engineer.

These types of slabs are normally used in applications where greater spans or loads or both are required, like in multi storey buildings. Also used when difficult shapes need to be cast, like staircases, or if longer cantilever projection is required.

A reinforced or pre-stressed concrete slab and designed as a ribbed slab, containing cores; generally varying in thickness from 120mm to 250mm and depending on loading, spanning up to 12m. The width is normally .900 or 1.200m Cores are typically either circular or elliptical. Hollow cores afford a reduction in self weight of 30% or more, compared with a solid slab of the same depth. For most applications, no propping is necessary during construction, but crane access is essential.

Composed of rectangular shaped (generally) precast concrete reinforced or pre-stressed ribs, supporting rebated non-structural hollow concrete fi ller blocks placed between two ribs; the most common rib spacing being 560, 600 and 650mm. The system is sometimes referred to as plank and block or beam and block. A structural concrete topping is poured between and over the blocks, with a minimum thickness of 40mm. Welded mesh reinforcement is placed in this topping to control possible shrinkage cracks.

The filler blocks are available in different heights ranging from 120mm to 350mm which produces an overall depth of slab from 170mm to 400mm, and with a clear span of up to 10m, depending on loading.

This type of slab requires temporary supports (propping) at approximately 1, 500 m centres.

Some of the advantages of this type of slab system are;
  • It provides an economical, versatile light weight monolithic slab system. Components are relatively light and no mechanical handling is necessary.
  • Slabs may be designed as either simply supported or fully continuous.
  • They are ideal for Soffit plaster but fixing of suspended ceilings is also easy and simple.
  • Electrical and plumbing services are readily catered for by omitting hollow blocks at specifi c locations or fitting thinner blocks to allow for these services.

Design Guidelines

Many in situ slabs are designed to span in two directions, with the main reinforcement running in both directions, provided suffi cient lateral support exists to carry such slabs. Typically these types of slab allow greater spans and loads to be accommodated, like parking garages.

Other alternatives to accommodate greater loads and spans include downstand beams and or upstand beams, although not usually used in residential buildings. Mushroom heads on columns reduce potential bearing problems (push through) of fl at slabs where downstand beams need to be avoided.

One way spanning slabs only require lateral support to the walls or columns that carry them, and since they do not bear on walls in the non-span direction, this type of slab design is used more in residential type applications, as it can allow for greater flexibility of layout/design of a typical double storey building.

An alternative form of suspended slab is “a waffle slab”, or more commonly referred to as a “coffer slab”, formed by in situ concrete beams integral with the slab. Coffers are formed on the soffit by formers which are later removed; (reducing self weight), and hence the use of less concrete. Flat slabs in upper floors have been mainly used where ceilings are to be formed directly on the soffit. Coffered soffits of in situ tee beams can be found mainly where suspended ceilings are used. And more rarely where the soffit is left exposed as cast.

Slabs are designed as simply supported ribbed slabs in the conventional manner. However they are more versatile than the pre-stressed slab, since longitudinal top reinforcement can be cast-in for cantilever action.

Slabs are designed as simply supported pre-tensioned ribbed slabs, in accordance with the requirements of SABS 0100-1 or the appropriate National code. The pre-stressing force opposes the tendency to downward deflection and causes an upward camber in the units under no-load conditions. Together with the high-strength concrete employed, larger span/depth ratios can be achieved than with reinforced concrete. In lightly loaded roof slabs, for instance, span/depth ratios around 50 are not uncommon.

Rib and block system slabs are designed as a series of ‘T’ sections with the in situ cast concrete providing the compression flange, and the precast beam the tension reinforcement. The beams and the composite slab are designed for specific spans and loads and are reinforced accordingly, (complying with the relevant National Code.) Two or more beams may be placed together to accommodate concentrated line loads parallel to the span. If necessary blocks may be omitted over the support to increase the shear capacity (called a stiffener rib).

Other Slab Systems

Windeck is a prestressed concrete manually-assembled, suspended fl oor system comprising prestressed concrete beams and reinforced concrete tiles. Dimensionally, Windeck is based on a 450mm module. The Windeck assembly results in an off the shutter tile top surface, off the shutter surfaces to the beams and a self finished underside to the tiles. Other finishes may be applied if desired. Windeck accepts electrical cabling between tiles before grouting and tooling, and is fire resistant.

  • Bond-Lok’s unique side lap inter-locking system provides for fast and simple construction.
  • Bond-Lok’s spanning capabilities allow for a reduction in the number of temporary supports required.
  • Bond-Lok eliminates the need for formwork and reinforcement.
  • Bond-Lok will accept most floor service systems.
  • Bond-Lok’s soffi t forms the finished ceiling without the need for plastering.
  • Bond-Lok has been fire tested by the CSIR and has qualified for a fire rating of 120 minutes.

  • Bond-Dek is a new composite steel flooring system for multi-storey steel or concrete buildings.
  • Bond-Dek’s unique side lap interlocking system provides for fast and simple construction. The speed results in major labour cost savings.
  • Bond-Dek is able to span up to 3 metres unsupported under wet concrete with a minimum depth of 65mm over the profi le.
  • Bond-Dek will accept most floor services systems.
  • Bond-Dek is available in a galvanised coated steel, pre-primed for painting on one side, and stainless steel 0,8mm; 1,0mm and 1,2mm thick.
  • Bond-Dek has been fi re tested by the CSIR and has qualified for a rating of 120 minutes. Brownbuilt also offers a complete design service if required.
  • The Bond-Dek profile displaces 0,037m³ per m² of deck.

Void Formers

Expanded polystyrene (EPS); can be used in various different ways:

To create different levels of the finished concrete level of a slab especially from internal to external thresholds, or where steps are required;

Special designs in soffits of flat slabs;

Recesses (voids) for lighting or other services required in the soffit of a slab. Sagex polystyrene is manufactured in a range of grades for a variety of applications to suit customer requirements:
  • 16D is used in building insulation applications as core material for panels, cold room insulation, packaging and flotation.
  • 24D and 32D are used in applications which require greater cross-breaking and compressive strengths, such as in floor and roof slab insulation, and where a better finish is required, such as in display applications.
  • FR Grade is treated with a flame retardant and is ideal for industrial applications where fire safety requirements must be met. As the material shrinks away from ignition sources without further burning, it is suited to exposed applications. The above are available in FR grades.
  • X Grade is a low-density material used mainly in void forming and packaging.
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