The Civil Engineering Contractor March 2019 | Page 25

ON SITE
permits the overlap to take place,”
he explains. The precast sections are
manufactured with holes through
which cables, with a maximum of
5mm play, are threaded and later
post-tensioned to keep the wall panels
tightly in place, using Corestruc’s
unique methodology.
While conventional construction
methods use reinforcing and post-
tensioning to control applied forces,
Corestruc’s method uses tensioning
vertically and horizontally to resist
applied forces. About 6.6km of post-
tensioning ducts and cables were
installed by hand between the joints of
the wall panels, before the grout was
pumped around the circumference of
the reservoir. This is a vital component
of the construction project and the
team has to pay meticulous attention
to detail to avoid potential future
leakages that have been so common
in the past using less-sophisticated
precast-concrete technologies.
The voids between the panels are
sealed using a specially designed grout
that is pumped between the precast-
concrete wall slabs. This grout has
been designed to reach a compressive
strength of 100Mpa within four days
— stronger than the concrete of
70Mpa — and to further react when
the medium comes into contact with
water when the reservoir is being
filled. It is blended and packaged on
behalf of Corestruc by Epoxerite, a
South African construction chemicals
specialist. Self-healing concrete is
used for the grouting, which enables
the concrete over time to grow
tentacles into surrounding concrete
surfaces. This is Corestruc’s own
design, explains De Jager, which leaves
‘pure’ un-hydrated cement inside the
mix into which the tentacles can
expand. The company also designed
its own post-tension grout as part of
its seamless methodology.
“It is an extremely high-flow grout
to flow through all the post-tensioning
ducts while being pumped from a
single position.” The success is achieved
in the installation, whereby the open
time is extended by cooling the grout
www.civilsonline.co.za
down to 7 ° C — or the grout would set
in 30 minutes. This method — which is
similarly applied to the roof — forms a
single monolithic unit.
What could go wrong
but doesn’t
The trick lies in careful installation,
explains De Jager, as each panel
weighs between 8t and 11.5t, which
means safety is a priority. The process
is constantly monitored using a Total
Station that enables each panel to
be placed with miniscule error. A
‘prism’ using GPS coordinates is
used to place each slab by crane.
If it is out of kilter by even 5mm,
the installation will not work, as
a bolt in the steel plate grouted in
place on top of the ring beam in the
structure has to align with a hole in
the slab to 5mm accuracy, while steel
plates at the top have to similarly
line up. “The crane is equipped
with a load-moment indicator, but
requires expert human rigging to
ensure accurate and safe installation.
If it doesn’t fit, you know you have
a problem. If the gaps between each
panel are even slightly too large, then
the last panel won’t fit.” However,
says De Jager, “It never happens.”
Bezuidenhout confirms this, lauding
the precision of the process and noting
that the final panel fitted perfectly.
“Over the years, we’ve been
through the school of hard knocks,”
says De Jager. “Through experience
we have been able to refine our
systems and quality control to the
extent that we predict problems
before they occur. For instance, the
3D prints didn’t work well on a
The view from Mathysenloop is hilly and scenic, with numerous reservoirs visible on surrounding
hills. This project is but one of a series of 10Mℓ reservoirs being constructed in the area.
CEC March 2019 | 23