A structure is composed of several connecting elements of construction like walls, beams, columns, foundation and slabs. Out of these, slab are likely the most important. A slab is used to construct horizontal flat surfaces such as floors and roofing floors and helps the other components of the building to withstand different loads. In both load-bearing and framed constructions, the slabs are built of reinforced concrete that distributes the weight. Slabs are classified into one-way and two-way slabs. The former is supported on two sides and the latter is supported on four sides.
Solid reinforced slabs poured into formwork on site was the typical way in which such suspended slabs were constructed. Pressure to reduce the costs of both labour and material, and the need to erect buildings in the shortest possible time, have led to the new methods of slab construction featuring the use of precast concrete components,
There are three types of slab used in South Africa:
- Cast in-situ reinforced concrete
- Rib and block
Cast In-situ Reinforced Concrete
In-situ slabs are shuttered on site and entails the erection of steel or/and timber formwork. This formwork system provides the reinforcement and essentially works like a mould that the concrete is poured into on site. The reinforcement is provided by straps that are installed horizontally and vertically. The horizontal straps are known as primary reinforcement bars, while the vertical straps are known as distribution bars.
This method is expensive due to the extensive use of steel, timber and concrete, and requires curing time which extends the construction timeline, but it offers more flexibility than any other method to create dramatic curves and spans. It is generally common practice in other countries where they build with columns and slabs instead of brickwork as load bearing.
Hollow Core Slabs
The voids or cores that run through these precast units give these slabs their name and it is essentially a precast system. The hollow core ribbed slabs have between four and six longitudinal cores running through them, the primary purpose of the cores being to decrease the weight, and use of material yet maintain maximal strength. To further increase the strength, the slabs are reinforced with steel strands. The cores have a sustainability advantage as it reduces the volume of material used and provides excellent fire resistance and sound insulation.
Hollow core slabs not only reduce building costs and time, but they also reduce the overall weight of the structure by up to 30%. The system is ideal for situations where quick, simple construction is required and cranes are typically used to place the slab units. The holes are also perfect for installing wiring and plumbing, which further reduces the time and cost of the construction project.
Rib and Block
For buildings with more than one floor, rib and block is one of the most trusted slab systems in Southern Africa. It is a a composite precast concrete flooring system so it is flexible, cost effective and easy to install. Different designs are easily achieved as lintels and blocks are packed to the desired shape.
The precast components of the slab comprise of a rectangular shaped precast reinforced concrete rib and hollow concrete blocks that fit between the ribs to form non-structural void-formers where the slab is redundant from a design point of view. There are two types of blocks that can be used, polystyrene blocks or concrete blocks; the ribs, or lintels, remain the same. The compressive component is provided by the in-situ concrete which is poured onto the precast components and the bonding of the two creates the composite slab.
The components of a Rib and Block system are light enough to be placed using manual labour and uses up to 40% less concrete when compared with conventional in-situ slab systems, The Rib and Block system is also easy to use so it saves time, doesn’t require costly skilled labour and is not affected by weather conditions.
For expert advice on the correct cement products to use to achieve the best possible results contact the Sephaku call centre on 0861 32 42 52 or speak to your technical representative.