There are a number of factors that need to be considered when selecting the most appropriate suspended-slab system for a building project.
This was a major focus of CoreSlab’s presentation at an event that was recently hosted by the South African Institute of Architects (SAIA), an industry body that is committed to maintaining the highest standards of professionalism, integrity and competence in the local architectural industry.
CoreSlab has worked with a number of the association’s reputable members on various developments as a leading manufacturer of rib-and-block – otherwise known as “plank-and-block”, or “beam-and-block” systems – in addition to hollow-core slabs (HCS).
Jaco de Bruin, managing director of CoreSlab, says that hollow-core slabs are favoured when construction needs to be fast-tracked and when there is very little room for error, considering that the installation of the system is undertaken by a specialist sub-contractor.
“These reinforced or pre-stressed concrete slabs, which contain cores, vary in thickness of between 150 mm and 320 mm and span up to 12 m, depending on loading. They are cut to length at the factory according to the building dimensions once the concrete has reached its compressive strength and then dispatched to site,” De Bruin says.
The preparation and placement of the between 10 mm and 15 mm-thick mortar bed on the load-bearing walls by the main contractor is the only work associated with the construction of the floor slab that is undertaken on site.
This is done shortly before the HCS arrive on site ready to be installed by a competent CoreSlab team, also comprising a seasoned project manager, rigger and crane operator.
Single leaf, collar jointed, cavity and diaphragm walls and those with fins are all suited to this method of slab construction, and the main contractor ensures that they have been built to an acceptable standard, in terms of plumb, line and level.
As part of the final phases of the installation process, CoreSlab also ensures that the structural topping, comprising a 40 mm-thick levelling screed of river sand and cement, is satisfactory.
This manner of constructing concrete floor slabs has also fast become a preferred approach to building multi-storey residential projects in-and-around Polokwane, the location of the company’s main factory.
Many of these projects are undertaken by smaller contractors that are working to tight deadlines.
By outsourcing the construction of the floors to a specialist, they are reassured of a high quality installation the first time round.
Work can commence almost immediately at the next level after the HCS have been installed and, as there is very seldom a need for propping, the trade teams also have instant access below.
They also benefit from significant cost-savings in construction consumables and materials.
HCS have long been deployed on large property developments where suspended floors and roofs are required.
This is considering their weight savings of up to a third or more of high-strength concrete, while pre-stressing has enabled HCS to achieve larger spans than would be possible using traditional cast-in-place methods.
In addition to servicing the residential market, CoreSlab has built suspended floors and roofs for malls and university accommodation blocks. This is in addition to manufacturing and installing HCS on civil-engineering projects, such as reservoirs, booster pump stations and water-treatment plants.
While demand for HCS remains high, De Bruin says that CoreSlab also continues to supply a vibrant market for “rib-and-block” systems.
“One of the benefits of this system is the flexibility it provides for the construction of floors with irregular shapes. However, it is a much slower means of constructing suspended floor slabs. Bear in mind that propping needs to be installed and the contractor has to wait for the poured concrete to cure before work can proceed above and below,” he says.
CoreSlabs’ rib-and-block system is manufactured to the same high standard as its other precast-concrete systems.
It generally comprises rectangular-shaped reinforced or pre-stressed ribs that support rebated filler blocks. The latter are placed between two ribs and concrete is then poured between and over the blocks to form a monolithic slab with depths that vary between 170 mm and 255 mm and spans of up to 6 m.
Considering their smaller spans, the beams are lighter than HCS and can be installed by hand by the main contracting team without the need for a crane.
However, it is important to also consider the additional costs in consumables and materials used in their installation.
Errors made in their installation may also lead to costly delays and waste.
Delegates at the SAIA event also had an opportunity to engage with representatives of CoreSlab after the presentation at its stand as one of the main of exhibitors.