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Pumps
and Slipform System Prove a Formidable Duo in Texas Silo
Build
The
one-two punch of a well-designed slipform system and solid
concrete pump performance has helped a quartet of cement
storage silos rise at an impressive clip. One of the final
phases in cement producer Holnam, Inc.'s massive Midlothian,
TX plant expansion, each silo measures 60 feet in diameter
by 216 feet high, and will eventually be used to store 14,500
metric tons of cement.
Sixteen
Silos, No Waiting
Expansion of Holnam's Midlothian site actually began nearly
three years ago with construction that essentially doubled
the processing capabilities of the plant to one million
metric tons per year, according to Glen Stewart, superintendent
for Borton, L.C., general contractor for the project.
"This
phase is to create storage capacities more in line with
what the plant is able to produce now. At the start of construction
back in 1998, Holnam had nine silos onsite; we added three
a while back and these four will bring their total to sixteen
silos. The four new silos alone will afford the company
an additional 58,272 metric tons of storage.
Going
Deep
The massive weight of each silo when full, dictated a heartier-than-normal
slab construction. In this case, the slab is actually four
circular mats, each measuring roughly 15 feet wide (60-feet
to silo center) and 6 1/2-foot thick. That, however, is
only the tip of this proverbial iceberg.
"The
circular slabs sit on 100 pilings each measuring four feet
in diameter and reaching down to a depth of 35 feet,"
says Stewart. "The mats alone took about 2,700 yards
of concrete. Because it is difficult to get such huge volumes
out to this location at one time, Central Concrete Pumping
(Ft. Worth, TX), using 47-meter and 36-meter pumps, did
that in two pours which were completed in 12 hours. From
there we started the vertical rise of the silos which, by
project's end, will have taken an additional 14,000 yards."
Giving
Them the Slip
As is the case with virtually all silo projects, steady
upward progress is ensured by use of a slipform system.
The system in use at the Midlothian expansion, designed
and built by Bow, NH-based Scanada International, Inc.,
features a series of 104 hydraulic jacks, 16 of which were
rated for three tons, 84 of which had six-ton capacities;
and 4 jacks with 22-ton capacities.
"The
full lifting capacity of the system in place at Midlothian
is 640 tons," says Klaus Hieronymus, Scanada's general
manager. "To some that might seem exceptionally high.
However, when you consider that, as it rises, the slipform
system will be carrying a work crew - in this case about
120 workers, equipment, walkways, tons of rebar and the
weight of the forming system itself, suddenly 640 tons doesn't
seem excessive at all."
Using
a Schwing BPL 4000 line pump, concrete was pumped in 8-inch
increments, with the slipform system set to raise the form
at seven minute per inch moves. That rate, however, was
adjustable, says Stewart.
"The
forms themselves are four feet tall and we like to keep
our concrete wet down to about 32 inches. So if the mix
stars setting faster than we like, we can adjust the system
timer to rise slightly faster. Conversely, if concrete isn't
setting up sufficiently, we can slow the rate down. The
slipform system performed just as we had anticipated allowing
us to maintain a steady rate of 10 inches of rise per hour.
With a round-the clock operation we were doing 20 feet of
silo each day and had the silos poured in 14 days."
Hot
on the Bottom
To accommodate both the downward weight loads and a metal
loadout hopper, wall thicknesses from the base of each silo
up to 87 feet were a robust 36-inches. At that 87-foot point,
the walls tapered to a thickness of 14-inches and maintained
that thickness to the top of the silo. The difference in
wall thicknesses made some modification to the standard
4000 psi mix necessary as well.
"The
thick walls at the base of each silo had a tendency to generate
a good deal of heat," says Stewart. "The temperatures
coming off that mix - even with a cool 40°F air temp
- were about 95°F. To offset those high temps we used
a Master Builder Pozzolith 300R set retarder. Obviously,
as we got to the higher parts of the walls, where the thicknesses
come down to 14 inches, the heat was no longer present so
we had to move to an accelerator, in this case, an Pozzolith
NC 534 accelerator, again from Master Builder."
All
concrete was supplied by Cleburne, TX-based Transit Mix,
Inc. which, according to Central CP's Carl Walker, really
rose to the occasion for this project.
"We
just can't say enough about what Transit Mix did, both in
terms of logistics and quality of service. On the slab pour,
they had 18 trucks running between the site and Transit
Mix's plant in Cleburne about 20 miles away and never missed
a beat. For the slipform portion of the job, they set up
a two batch plants onsite - one as a backup - and again,
the marathon pour went off without any major problems at
all."
Friction
in the Line
In addition to the temperatures generated by lower wall
densities, Borton had to also deal with loss of moisture
in the mix from other causes.
"We
used a fairly sizeable amount of 5 1/2-inch pipe on this
project," says Stewart. "Taking into consideration
the pipe that runs from the line pump to the base of the
silos, then up the total height of the silo, then to the
various reaches atop each silo, you are looking at as much
as 230 feet of vertical pipe and another 250 feet of horizontal
pipe. And anytime you pump concrete through that amount
of pipe, friction is going to take its toll. For that reason,
we opted to keep the mix a bit on the wetter side - with
a slump of 5 to 5 1/2 inches. That wetter mix also seemed
to be better at getting through the rebar and into the forms."
Rebar
was definitely in ample supply in all facets of the project.
Stewart estimates that the mat alone contains about 300
tons of # 11 bar and the silos have another two million
lbs. of steel running through them.
Placement
atop each of the four silos was made possible through use
of a "swing-boom" system in which concrete is
pumped into four swiveling delivery towers. Using gates,
hoses to three of the towers are kept closed while the fourth
accepts concrete. The process is repeated until all areas
of all four silos have been poured.
Making
it Happen
Despite the abundance of rebar, a number of varied mix designs
and a fairly decent height to which to pump, Stewart says
Central Concrete Pumping and its Schwing pumps ensured that
getting concrete down was never a problem.
"There
were areas in which the mats overlap each other and the
bar got very dense, but the combination of a good mix and
solid pump performance allowed us to pour those areas -
and others - without a hitch. I was really pleased with
the level of service we got from Central Concrete Pumping.
They went the extra mile to make certain all possibilities
were covered, going so far as to keeping a spare pump on
hand in the event one of theirs failed. A failure never
did occur, but its nice to have that peace of mind in knowing
a breakdown won't spell disaster. On a project such as this,
continuous delivery is the real key to success and we definitely
got that."
The
Holnam Midlothian expansion is slated for total completion
by June 1st of this year.
# #
#
General
Contractor: Borton L.C., Hutchinson, KS
Pumping Contractor: Central Concrete Pumping, Ft. Worth,
TX
Ready Mix Supplier: Transit Mix, Cleburne, TX
Pumps: Schwing 47-meter and 62-meter boom pumps, Schwing
BPL 4000 line pump.
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