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CONSTRUCTION WORLD
SEPTEMBER
2016
PRESTRESSED SLABS
specified for security wall
PROJECTS AND CONTRACTS
Designed by Delta Built Envi-
ronment Consultants in close
collaboration with leading
prestressed hollow-core concrete
slab manufacturer, Topfloor, the wall is being
erected by KP Construction, the project’s
main contractor.
According to Topfloor director, Wessel
Prinsloo, the slabs are being precast at
Topfloor’s Cape Town factory where they are
prestressed with steel reinforcement before
being delivered on flatbed trucks
to Saldanha.
“Prestressing provides hollow-core slabs
with additional strength and the slabs being
used for the Saldanha wall boast a strength
rating of 50 MPa plus. As a result, nothing
short of mechanised demolition equipment
or high explosives would make any sort of
destructive impression on the wall. This is
why prestressed slabs are being used on an
increasing basis to safeguard property of
strategic importance or high value in other
parts of the country,” advises Prinsloo.
The wall is being built to a simple yet
effective design which uses galvanised
steel H sections to support the six metre-
long slabs. The H sections are bolted onto
six threaded bars cast into the reinforced
concrete foundations. These extra heavy duty
foundations were specified by Delta to ensure
that the wall can withstand the frequent and
sometimes gale-force winds prevalent on this
stretch of coastline.
The wall follows the natural contours
of the undulating fynbos-strewn land and
rises 3,2 and 4,2 m above ground level; a
section of approximately 400 mm is buried
below ground.
Construction, which began in December
2015 and is due for completion by the end
of 2016, is effected by hoisting the slabs
using a truck-mounted crane. A special grab
mechanism had to be designed by Topfloor
to lift and install the slabs. The crane has
sufficient reach to service three wall bays
from one location. Once installed, the slabs
are caulked into the H sections with a plaster
sand and cement mix.
Part of the contract involves the planting
of a line of various types of trees on the outer
perimeter of the wall to add a green and
aesthetic element to what would otherwise
have been a stark monolith.
Additional aesthetic appeal is achieved
by mounting the panels so that the slabs’
smooth soffit sides are positioned on the
outer (public) side of the wall. Moreover, the
slabs have been cast with bevelled edges
which form V-shaped channels where the
slabs meet to further enhance the appear-
ance of the walls.
Prinsloo says there are several advan-
tages to this type of walling, speed of
construction and the superior strength of the
wall being major considerations.
Some 3 400 prestressed
hollow-core concrete
slabs are being used to
construct a five kilometre
security wall for the
Department of Public
Works at South Africa’s
naval base in Saldanha.
“Eight to 10 bays or 48 to 60 linear metres are
completed daily (eight hours). A conventional
masonry wall, which is not nearly as strong
or durable, would have taken two to three
times as long to build.
“The cost of constructing a security wall
using prestressed hollow-core slabs is consid-
erably less than an in-situ wall offering the
same properties. Moreover, precast walling
requires no shuttering or propping, onsite
curing, formwork or grouting.
“As with other walls built with
prestressed hollow-core panelling, the
Saldanha wall will have a very long life
span. It is maintenance free, and other than
occasional cleaning, no other servicing is
required. The slabs can also be dismantled
and re-used elsewhere,” adds Prinsloo.
>
Topfloor contracts manager, Norlando Nomdoe,
stands on one of the concrete foundations.
A section of completed security wall built with Topfloor prestressed hollow-core slabs at the Saldanha
Naval Base.
An inner section in which the attractive
V-shaped indentations between the slabs are
clearly visible.
A Topfloor wall slab is offloaded prior to
installation between the steel H sections.