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structures, including durability, quality, and speed of construction, appropriate finishes and cost-effectiveness. “Concrete, both in-situ and precast, is one of the world’s most durable materials. Well-designed, well-produced concrete offers exceptional performance and long life, and is a first line of defence against most forms of erosion.” Precast production facilities can either be situated in an established factory or on site. Because precast concrete is produced in a controlled environment (typically referred to as a precast yard), it is possible to monitor all stages of production including curing, compaction, the quality and depth of cover concrete, and strength requirements. Precast concrete is generally cast at
A cross-sectional view of the Isibonelo Colliery bunker under construction, showing cast in-situ vertical columns, sloping precast concrete beams, precast panelling, and a cast in-situ bunker floor, approximately one metre thick.
ground level which assists with safety and productivity and offers greater control
Case study: Isibonelo Coal Bunker
caused by the loading and off-loading of coal
of quality and workmanship, unlike the
An 8 000 ton coal bunker at Anglo Coal’s
in the bunker could be resisted.
exposed and awkward access presented
Isibonelo Colliery illustrates the advantages
T he panels wer e cast horizontally
by in-situ casting. In addition, the precast
and flexibility of hybrid concrete technology.
under ideal conditions and after seven
route often results in better durability with
Designed by Lyonell Fliss & Associates and
years of continual usage, the concrete
consequent savings in maintenance costs,
constructed by Murray and Roberts in
shows no visible sign of deterioration. By
time, materials and energy.
2005, the foundations and columns were
contrast, in-situ constructed coal bunkers
constructed with in-situ concrete.
usually require 50mm thick steel plates
Moreover, the forms used in precast concrete production may be re-used
The horizontal bottom slab was cast in-
or rail liners for added wear-protection.
hundreds if not thousands of times be-
situ on precast beams which in turn rested
This is because in-situ casting in sloping
fore being replaced. This ensures that
on in-situ columns. The remainder of the
formwork often results in honeycombing
the costs of formwork per unit are lower
bunker, including inclined and vertical wall
and inadequate compaction, leaving a
than that of in-situ construction, where
panelling, as well as sloping beams, were
30MPa concrete vulnerable to the abrasive
formwork can cost between 35 - 60 % of
all constructed with precast concrete
nature of sliding coal.
the cast elements.
elements and lifted into position by crane.
Controlling quality standards is much
The use of both in-situ and precast
easier in a precast yard and concrete
concrete meant that connecting the
elements which don’t meet the quality
various concrete elements to each other
requirements can be rejected before instal-
was considerably simpler.
lation. It is far more difficult to assess and
A considerable amount of time was
maintain quality in an in-situ constructed
saved by casting the precast elements in
bunker, where climbers are required to
concert with the excavation work and the
assess and repair poor quality concrete
in-situ construction of the foundations,
in awkward dangerous positions/heights.
columns and the horizontal floor-slab. This
In short the advantages of constructing
meant that the precast elements were
the concrete bunker walls and floor in
ready to be lifted into position soon after
precast concrete as opposed to in-situ
the in-situ concrete columns had been
concrete are as follows:
completed.
• Cost savings on steel plate or rail liners
A no ther bene f i t o f using pr ecast
A precast panel is lifted into position prior to installation on the sloping wall of Isibonelo Colliery.
PRECAST VOLUME 7 • ISSUE 1 • 2013
• Better concrete durability
concrete for the bunker was the quality and
• Easier jointing
finish of the concrete panels used to build
• Speed of construction
the inclined and vertical bunker walls. A 60
• Cost savings due to less labour and
MPa concrete with a surface hardener was
specified to produce a smooth, durable and
• Superior quality resulting in less
hard-wearing surface so that the abrasion
maintenance.
material required P