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Plan the project: For structural concrete construction, an engineer
and architect will usually do the technical design work and provide
specific information regarding the sizes, configuration, and placement of
rebar in the associated concrete work. Planning the actual fabrication and
placement, as well as the schedule of the work is your first task.
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2
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Purchase the rebar: For simple projects like typical building
foundations and slab reinforcement, you can most likely buy the necessary
rebar from a building supply center or home improvement warehouse. For
complicated applications such as grade beams, foundation walls, tanks, and
other projects, you will need to have specific shapes formed by a rebar
fabrication specialist. Here are some examples:
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Special
shapes like these stirrups are usually purchased from a rebar supplier,
rather than a builder's supply company.
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a)
Stirrups: These are shaped rebar that hold the
lateral reinforcement in a certain configuration, often called a cage.
They create a framework that keeps these larger bars in position, and may be
round, square, rectangular, or even complex combinations of shapes.
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Here
are stirrups ready for installation in a grade beam.
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b)
Dowels: These are usually L shapes,
or straight lengths of rebar with a ninety degree bend on one end.
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Here
is a stack of masonry dowels to be installed in the building footing.
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c)
Corner bars: These are also L shapes,
with each side of the ell the same length.
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d)
Offset bends: These range from a simple Z shape
to complex angles, used in reinforcing concrete walkway steps and steps
(changes in elevation) in concrete footings.
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e)
Hairpins: These are U shaped rebar
that are often used to interlock two or more individual mats of
rebar to give lateral strength to the concrete casting.
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f)
Candycanes. As the name implies, these are
straight lengths of rebar with a U shaped bend on one or both ends,
again to interlock two or more parallel reinforcing mats.
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A bar with a hook, or candy
cane on one end, a dowel bend on the other.
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3
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Consult your reinforcing placement drawings/plan: If you
purchase your rebar from a fabricator, the supplier will usually review your
structural engineer's or architect's plans and produce a shop drawing with
details and identifying tags for each type of rebar used in the project. For
simpler projects, your building plans should provide spacing requirements and
bar sizes. Use these documents to determine where and what rebar is needed in
individual locations.
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Choose the method you will use to tie the rebar.Most times,
rebar is tied with annealed steel wire, either bought in four pound bulk
rolls, or if using a bag tiespinner, in bundles of precut wire pieces
with loops formed on both ends. The latter are easier for novices to use, but
somewhat more expensive, the former is often the choice of experience rebar
tyers (rodbusters).
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A
bundle of Snap Ties, also known as bag ties.
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5
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Prepare the area where the concrete is to be placed. The ground
should be graded and compacted after any needed subgrading, excavations,
and underground rough ins for plumbing and electrical utilities is finished.
Lay out the actual perimeter or form lines for the concrete placement after
the grading and compaction and associated testing is done.
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Laying out the building
footings, here, grades are shotwith a builder's level.
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6
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Decide whether the concrete forms will be installed prior to placing
your rebar. For large footings where heavy rebar is to be used, the
formwork usually is done first, for concrete walls and grade beams, one side
of the form may be built prior to tying the rebar, but the rebar will need to
be tied in place before the formwork is completed so bars can be positioned
and tied in place. For concrete slabs, the subgrade (ground underneath the
slab) is often pre-treated for termites, and a moisture barrier or damp proofing
is installed before the mat is tied.
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Here,
the rebar cage for a grade beam is tied before the forms are set. Tying these
bars inside a plywood form would be difficult.
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Shake out the rebar: This involves removing individual
bars, stirrups, and dowels from their respective bundles acco rding to the
placement drawing counts.
(Image
on right: These stacks of rebar are positioned near where they will be used.)
An example would be a slab measuring 12 feet by 12
feet with rebars at 8 inch centers in one direction, and 12 inch centers the
other. Determine the size of bars required in each direction, mark two or
three bars with the appropriate layout measurements in each direction, and
count the marks to determine how many rebar are required for each direction.
Often, the placement drawings are specific, such as "18 (number 5)
rebar, 11 foot 6 inches long, one half each way". This gives the
following information: You need the given quantity, 18, rebar, size5(5/8 inch
diameter), with 9 bars laying in each direction, the top rows perpendicular
to the bottom ones.
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Tie your rebar: This is the primary focus of this article. Tying
the bars so that they remain in their correct respective positions is
critical to achieve the desired strength of the completed concrete structure.
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Tying
a figure 8 tie with a wrap on a concrete anchor bolt pedestal.
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9
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Place each rebar in its respective position according to the layout
described in the previous steps. The layout bars ( or mark bars)
can be marked with a soapstone marker, a paint pen, a piece of lumber crayon,
or with spray paint.
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The bright pink paint marks
where the stirrups are placed in this beam cage.
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10
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Select the appropriate type of tie you will use. For the bag
ties (Snap Ties, not to be confused with the snap ties described later).
For ordinary slab mats, where the force of the concrete interacting with the
rebar during its placement is minimal, and movement of the mats is unlikely,
using a simple, single twist of wire around each rebar intersection, twisted
together tightly, will suffice. This tie is known as a snap tie, and can
be made with the Snap Tie precut ties and a spinner, noted earlier. It
can also be done easily with a pair of 9 inch lineman's pliers and bulk wire
held on the rodbuster's workbelt in a wire reel. For other applications
where the force of the concrete placement may displace the rebars, or where
more strength is needed to hold bars in the proper configuration, more
complicated ties may be used. Here are some of them, with a simplified
description of how they are made:
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A simple saddle tie on
a pedestal band.
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Here,
annealed steel wire held on a spool is used for tying a stirrup before
installing it in the beam.
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Figure 8 ties. These are made by pulling the wire around the rear
(from the rodbuster) bar, diagonally across the front bar, back around the
rear bar, diagonally in the opposite direction across the front bar, and then
twisting back around the beginning wire. You then cut the wire feeding off
the reel, and bend the cut ends back towards the tie so no sharp ends project
from the tie. These ties will help hold perpendicular bars tightly together
while helping to prevent them from racking, or moving diagonally.
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Saddle ties. Similar to the figure 8 tie, you begin by passing the
wire feeding from your reel behind the rear bar, then across the front bar
staying parallel to the bar. You then pass it behind the rear bar again, back
around the front bar on the opposite side. You now twist the ends together,
cut the feed wire, and bend the cut ends back. This tie is often used when
tying rebar for walls or other vertical application where the rodbuster will
actually climb on the rebar framework to access higher portions of the wall.
The figure 8 and saddle tie can often be interchanged, however, technically
speaking, there are advantages to each one in certain circumstances.
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Combinations of figure 8 and saddle ties with additional wraps around
vertical rebars can be used to increase the hold of the tie so bars cannot
slip downward when weight is applied to them or the plastic concrete is
dropped into the form.
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Use your pliers for tying these ties efficiently: For all the
above mentioned ties, you pull the feeding end from the wire reel with your
non-dominant (hereafter regarded as left, plea se reverse for right handed
persons) hand. Grip the end of the wire with your pliers in your right hand,
and poke, or push it behind the rebar described in the first step of
your chosen tie. Bend or angle the end toward the place you will be grabbing
the end in the next step of the tie, then reach from that side, grip it again
with the pliers, pull it toward the next place you will route it to, pulling
enough slack wire to complete the tie. Hold resistance on the wire with your
left hand, so the wire bends snugly against the bar you are wrapping in each
stage of the tie. Release the wire so that the pliers can be used to grip it,
and do so, pulling the end around the bar and twist the two ends of the wire
together. Pull or tug the wire with the pliers so the tie is tight.
(Image : Feeding the
wire around the rebar to catch the end with the pliers.)
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12
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Tie all the bars required in their correct positions.Check your plans
to make sure each component of the reinforcement is in place. Often, in
structural concrete reinforcement, you will find several elements that
interface together in addition to the basic rebar mat discussed so far. Here
are a few to note:
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Bright
paint marks the stirrup locations in this grade beam.
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Block dowels. When placing a concrete foundation which will have
concrete masonry units (block) erected on it, you will usually find the plans
require installing block dowels, or vertic al rebar to reinforce cells at a
required spacing to give the subsequent block wall sufficient strength to
withstand conditions to which it will be exposed, or to help it support loads
it will carry as an overall part of the structure you are building.
(Image on right: A row of
masonry dowels will add strength to the block wall to be built here.)
These
bars are tied to the foundation rebar (footing bars) in a location that will
place them in the center of individual block cells. For them to be placed
correctly, you will need to establish the wall line, then determine the
spacing of these cells. If your layout begins at a corner, using 8X16 inch
regular block, you can place the first dowel 4 inches inside the outside wall
line, 4 inches from the corner, then space additional bars at their required
distances in multiples of 8 inches. For example, at 16, 24, or 32 inch
centers. This is known as blockwork spacing.
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Bulkhead dowels. In instances where a footing will not be completed
in a single concrete placement, you will need to dowel out of the
bulkhead form so the next placement will be structurally tied to the latter
one. Make sure the dowels extend far enough that the lateral reinforcement
will overlap enough to maintain the strength of the rods used. Typically,
rebar lap is calculated in bar diameters. An example would be
the number 5 rebar mentioned earlier. It has a diameter of 5/8 of an inch,
and the required lap might be 40 bar diameters. Mulitplying the diameter 5/8
by 40, you will get 200/8, or 25 inches.
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Note that in structural concrete, other types of imbeds and inserts
may be required. Place rebar in such a manner as to allow installation of
anchor bolts, sleeves, embedded weld plates, inserts, or other items in their
repective correct locations without interference. In general terms, these
items require more precise positioning, so offsetting one or two rebars may
be required.
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Here
you can see the rebar will be in the way of a column base plate that is to
set on these anchor bolts. The rebar, and the grade beam itself will be
notched to allow the steel to be set in place.
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13
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Chair or support your rebar. Once the ma t or cage is assembled,
you must hold it in position so the the concrete will cover it completely. Rebar
chairs orconcrete brick are often used for this purpose. Place
these positioners at a spacing that will not allow the rebar to bend or deflect
enough to reduce the coverage you wish to obtain with the concrete you place
in you forms. For a 12 inch thick footing, the rebar mat is usually placed
about 4 inches from the bottom of the concrete, and side clearances range
from 2 to 4 inches.
(Image: Molded plastic chairs support
the rebar in this footing.)
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14
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Observe the rebar configuration while the concrete is placed. If
shifting occurs, support the rebars with a handled tool like a shovel wedged
so that you can achieve sufficient leverage to hold its position, or alter
the direction of flowing concrete so force is applied in the opposite
direction.
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The rebar in this footing could be shoved by the concrete as it's
placed, so the concrete crew must be careful when placing it.
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15
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Cap or otherwise protect any exposed bars  while working near them. Rebar
that is sheared, or mechanically cut has very sharp surfaces at the
location of these cuts. Construction workers have suffered serious injuries
and have also been killed when they have fallen on projecting rebar dowels.
Special rod caps made of high impact plastic with a metal plate embedded
in them are required by the Occupational Safety and Health Administration
(OSHA), in the United States.
( Image : These bright orange rod
caps can literally save lives.)
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