TYPES OF SLABS
IN SITU (REINFORCED CONCRETE)
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.
HOLLOW CORE SLAB (HCS)
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.
RIB AND BLOCK SLAB (RB)
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.