Silo repairs show real fibre

The clean coal storage silos at Rio
Tinto’s Mount Thorley Wark­
worth mine, located near Single­
ton, are a crucial to the productivity of
the whole operation.

The sizable structures, each with
circumferences of 48 m, are designed
to store up to 3000 tonnes of coal before
it is transported by rail to the
Port of
Newcastle.
The structures were constructed in
1984 with a single 250 mm layer of
reinforced concrete.
However, less than 12 months after
construction was completed, cracks
became visible on the external faces of
the silos.
The mine decided to apply 154 post-
tensioned steel cables horizontally
around the silos in order to restrain the
walls against the coal loading and prevent
further cracking.
While the cables proved effective
for some time, they eventually began
to corrode and fail.
This caused the silos’ capacity to
decrease steadily every year, so the mine
sought out Newcastle-based engineer­
ing firm Izzat Consulting to propose a
more long-term solution.


Remedial building contractor Build­
corp Asset Solutions was commissioned
in 2008 to carry out the repairs.
By this stage, the silos could only
store 25% of their total capacity.
The company’s construction manager,
Brendan Walsh, oversaw the $3.2 million
project.
“Izzat developed a plan to remove
the cables, repair the concrete and then
apply layers of carbon fibre to rein­
force the structure,” he told
Australian
Mining
.
“The problems arose because the
silos were under-reinforced from the
start.
“In some places we actually had to
apply up to four layers of carbon fibre
in continuous 300 mm wide strips.”
According to Walsh, the Izzat engi­
neers chose carbon fibre for its high
tensile strength and resistance to corro­
sion.
We could have simply replaced
the cables, but they would have
inevitably failed again,” he said.
Because the silos were still opera­
tional while the project was underway,
it was difficult to gain access to the
structures.


“We were only allowed to work
on two metre high horizontal sections
of the silo at any one time to ensure
we did not weaken the structure in any
way,” he said.
“So basically, the work would alter­
nate between two metre areas at the
top and bottom of the silo.”
The original tender documents
recommended the use of scaffolding.
However, the repair team quickly
realised this would not be satisfactory.
“To install full-height perimeter
scaffolding all over both silos would
have been a bit of a headache, because
in some cases it would actually restrict
the working space,” Walsh said.
“It was also quite expensive to put
that much on structures so large.
“So we decided to use a specially-
designed mast climber system, which
reduced the cost by around $300,000
and actually gave us a much better
working method.”
The access system consisted of six
vertical mast climbing units fixed at
equal intervals around each silo’s circum­
ference, as well as separate decks fitted
between each mast.
“This meant the silo was divided
into six separate working areas and
we could use any one of the decks to
travel up and down the structure,”
Walsh said.
“The decks could also be locked
together to provide access to the complete
circumference.
“Each section was basically set up
as a climbing workshop.”
 
Safety tensions
The team had to devise a method to
safely restrain and remove the cables,
which had tensile forces of 20 kN.
“We fitted about 20 steel plates to
prevent the cables from flying away
from the structure when we cut them,”
Walsh said.


After cutting the cables, the workers
removed the damaged concrete with
hydro-demolition, using low volumes
of water at pressures up to 20,000 psi.
The concrete was then replaced
with shotcrete to provide a stable surface
for the carbon fibre.
In all, the project involved 566 m
of external crack repairs, 1633 m of
internal crack repairs, 14.42 km of
carbon fibre wraps and two kilome­
tres of carbon fibre laminates.
 
Withstanding the deluge
The project hit a major stumbling block
when heavy rainfall and flooding hit
the Hunter region mid-way through
work on the first silo, forcing the site
to be evacuated.
“The silo actually filled up with
water because the roof had been removed
and one of the post-tensioned cables
actually snapped under the strain of
this extra load,” Walsh said.
“This forced us to revise our methods
and make sure the cables were secured
before we restarted work.”
Since the project was completed,
the silos have been running at full capac­
ity and are expected to last another 15
years before further maintenance will
be required.
“Carbon fibre is completely resist­
ant to temperature variations, rain and
hail so it only requires minimal main­
tenance,” Walsh said.
“Ultraviolet radiation will cause
deterioration, but that can be prevented
by applying a resistant coating to the
surface.
“If this is done regularly, the carbon
fibre could have a limitless lifespan.”

 

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