SA Affordable Housing March - April 2020 // ISSUE: 81 | Page 19

FEATURE
SUSPENDED SLAB TECHNOLOGIES – HORSES FOR COURSES
There are a number of factors that need to be considered
when selecting the most suitable suspended slab
technology for a building project. Jaco de Bruin, Managing
Director of CoreSlab, a leading South African manufacturer
of hollow-core slabs (HCS) and rib-and-block (RB)
technologies, discusses some of these considerations with
SA Affordable Housing.
“Both HCS and RB technologies provide a faster and more
cost-effective means of constructing floor slabs than
conventional cast-in-place methods. However, each system
has distinct advantages that make them better suited to
specific construction projects and teams,” says De Bruin.
HCS are reinforced or pre-stressed concrete slabs,
comprising cores that extend the full length of the elements,
which generally vary in thickness from 120mm to 250mm.
They span up to 12m depending upon loading and are
between 900mm and 1200mm wide. These precast-concrete
elements are placed on a 30mm to 45mm-thick levelling
screed on masonry walls with a minimum bearing of 100mm.
To accommodate thermal movement on roofs or exposed
balconies, bitumised soft-board or other similar materials are
used. This makes allowance for changes in camber or
deflection, particularly where light parapet walls are built on
pre-stressed HCS. Light mesh reinforcement is also placed in
the finishing screed or topping in these circumstances.
Notably, HCS are significantly lighter than a large solid
concrete floor slab of equal thickness or strength due to the use
of fewer raw materials. This reduces the cost of manufacture and
transportation of the ‘planks’ to site where they are lifted and
placed, levelled and grouted to complete the floor structure.
However, one of the biggest advantages of HCS is that they
are manufactured in a controlled factory environment where
the non-modular sections are also cut to size according to the
building dimensions immediately after the concrete has
reached the required strength.
The various elements are then transported to site where
they are lifted directly from truck trailers and placed on top of
the load-bearing walls using a mobile crane. Once they have
been installed, building contractors and trades are able to
work on the structure almost immediately to significantly
accelerate the building project.
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sustainability and impact on our infrastructure. With
non-compliance of quality and durability standards,
consumers are unaware of the risk they face.
It takes up to 28 days for cement to develop strength and
fly ash does not start developing strength before 28 days.
The durability of mortar or concrete is primarily dictated by
the amount and the strength performance of the cement that
is used.
Retailers, builders and construction companies are also at
risk of future legal action or loss of income as selling and using
these sub-standard products can have a negative effect on the
perception consumers have of their businesses and standards.
Lukela says, “The use of sub-standard cement products has
been identified as one of the main causes of building collapse
globally. PPC has decided to take a stand against sub-standard
cement products to ensure the safety of consumers and
longevity of structures.”
Ribs are placed on the masonry walls with a minimum bearing of
100mm at approximate centres.
CoreSlab is able to install an element every six minutes to
complete a floor structure in record time, and the company is
maintaining a similar track record on a large affordable
housing project in Limpopo where it is working alongside a
leading black-owned building contractor. Another major
advantage is the extremely high quality outcome that is
possible the first time round by outsourcing the manufacture
and installation of the floor structure to a specialist while the
main contractor builds the load-bearing walls and prepares
the mortar bedding.
De Bruin says that this attribute remains strong marketing
and sales points for HCS, especially in the residential
construction market where building capabilities and
capacities are often limited. “The growing popularity of HCS
in this market is largely being driven by the severe workload
in a competitive and volatile industry. These factors are being
exacerbated by widespread skills shortages in the
construction sector. Home builders and their professional
teams want peace of mind that the installation will be done
correctly the first time round, while smaller architectural and
structural-engineering firms are also increasingly specifying
HCS in the early design stages of the project to mitigate risk
and reduce construction costs,” De Bruin says.
Accuracy also reduces wastage, another notable benefit
that is complemented by the significant cost-savings in
concrete, propping and labour that is associated with the use
of both in-situ construction methods and RB systems.
However, there are still many skilled and experienced
building contractors that prefer RB suspended floor systems,
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