Constructing Concrete Forms and Reinforcement

Report
Constructing Concrete
Forms and Reinforcement
The National Association of
Certified Home Inspectors
Definitions of terms associated
with form construction:
• Footer form (footing) - a continuous slab of concrete that
provides a solid, level foundation for block and other masonry.
• Construction joint - place where one pouring of concrete stops
and another starts. Also called a ‘cold joint’.
• Control joint - planned break which permits concrete to expand
and contract without cracking.
• Reinforced concrete - concrete slabs or structures that are
strengthened with embedded steel rods or wire mesh.
• Wale - a heavy plank extending along the sides of wooden
concrete forms for reinforcement of the 2 x4" studs.
Selecting materials for concrete
forms.
• Metal forms
• Synthetic materials
• Wood (most commonly used). It should be straight,
sound lumber, free of knots, decay and other defects.
Commonly, forms are steel framed plywood.
Well constructed concrete forms
should be:
• Substantial enough to retain their correct shape when filled.
Freshly mixed concrete exerts great pressure since concrete
weighs from 130 to 150 lbs. per cubic foot.
• Tight to prevent the escape of the water-cement paste, because
the loss of water will change the strength of the remaining
mixture.
• Constructed so they can be easily filled from a truck or
wheelbarrow.
• Easily removed after the concrete has hardened.
Construction of forms.
•
Use soft, clean, straight lumber.
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–
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•
•
•
•
•
$ Use spruce 2" x 4" or 2" x 6" for form because it will not warp as bad as yellow pine.
$ Use green lumber when possible because it does remove the water from the fresh
concrete
like kiln dried lumber will.
Sharpen stakes evenly so they can be driven in straight.
Place stakes about 30 inches apart along the outside of the form for 4 inch thick
concrete. Place the stakes close when the concrete is more than 4 inches thick.
Use a transit or level to adjust the height of forms for the desired slope or fall of
the slab.
Drive nails through the form and into, but not through, the stakes.
Be sure the stakes do not stick up above the top of the form. If they do, saw
them off so they are level with or tapered down from the form.
For smooth walls use plywood panels.
Form Layout
Types of points.
• Isolation joints
– Used to separate floors from points of abutment with walls
columns, or building footings.
• Control joints
– May be properly called crack control joint.
• To prevent random cracking, predetermine the crack location by
making a crack control joint or by sawing into the floor to make a
weakened plane so that the crack will occur where you want it.
• Construction joints
– Created where concrete stopped and later was started again.
– Most construction joints are actually a combination of a control
joint and a construction joint.
Corner Layout
Forming the Outside Wall
Forming the Inside Wall
Types of reinforcement
•
Steel bars
– Steel reinforcing bars have ridges that increase the bond between the
concrete and the steel.
– Bars may be ordered by number or by diameter. It is available in
diameters from 1/4 to 1 inch and over. They may be purchased in 20,
40, or 60 foot lengths.
– The size bars needed for the job depends on the amount of tensile
strength needed in the concrete.
– Rebar should be lapped 24 times its diameter.
– Rods may be placed in concrete slabs in cross-sectional pattern and
wired together.
– Reinforcing bars should be free of rust, dirt, oil or other materials that
will reduce adhesion by the concrete.
Types of reinforcement
•
Welded wire fabric
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Generally available in a 6 by 6 inch pattern and consist of number 6, 8, or 10
gauge wire.
Used for jobs requiring relatively light reinforcement.
Used to help reduce the cracking due to changes in temperature and moisture in
the concrete.
Reinforcement wire fabric needs to be placed so it is protected by an adequate
coverage of concrete.
Lap welded wire a minimum of 13 inches or at least one full spacing plus two
additional inches.
Fiberglass fibers
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Fibers may be mixed in concrete mixtures for increased tensile strength.
By using the fibers in the mixture the fibers are mixed completely from the top to
the bottom of the slab.
Cost is comparable to the price of welded wire fabric.
Form Panel Details
Form Panel Sizes
Form Sizes
4’
2 bar
4’
3 bar
5’
3 bar
6’
3 bar
6’
4 bar
6’8”
5 bar
8’6”
5 bar
9’
5 bar
9’
6 bar
10’
7 bar
Footings and foundations
• No substitute for an adequate foundation, which is the key part
of every building.
• Adequate footing provides a stable base and directly affects
both the life and performance of the building.
• Protects against rats, mice termites, water and the elements.
• Foundation consists of:
– Its bed, with the earth giving support
– Its footing, the widened part of the structure resting upon the bed
– Its wall, the structural part resting upon the footing.
• The size of the footing depends on the load-carrying capacity of
the soil and the weight of the building and its contents.
Footings and foundations
Load-carrying capacities of Soils
Type of soil
Tons per sq. ft.
Soft clay
1
Firm clay or fine sand
2
Compact fine or loose coarse sand
3
Loose gravel or compact coarse
sand
4
Compacted sand-gravel mixture
6
Footings and foundations
• Approximate ratio of foundations size to the wall it supports.
– Footing thickness the same as foundation wall thickness.
– Footing width is equal to twice the thickness of the foundation
wall.
• Foundation should be reinforced with rebar to increase the
strength of the concrete.
Care of concrete forms
• Forms should be coated with used motor oil.
• Use paraffin oil diluted with kerosene or benzene if the concrete
is to be painted or stuccoed.
• Form must be braced to prevent bulging.
• Form should not be removed until the concrete is strong enough
to stand alone (time varies with weather, mix and admixtures).
• Do not saw into the top of the form.
• Do not allow concrete trucks to bump or run over the forms.
• Be careful in removing forms as to not damage concrete.
Constructed
Wooden Forms
Custom made and
used for certain custom
foundation work.
A.
B.
C.
D.
E.
F.
Stake
Brace
Tie, 1" x 4"
Sheathing
Wale
Holder
G.
H.
I.
J.
K.
L.
Tie, Snap
Spreader
Tie, Wire
Footing
Key
Marker Nail
Joints Commonly
Used in Concrete Construction
Control Joints
Reinforcing Affects: Strength of Concrete
Structures: Reinforce Concrete to Increase Tensile Strength
Reinforcing For Concrete
Steel
Reinforcing Bars (Re-Rod)
20' Lengths
Sizes
Number 2 3
4
5
Inches
1/4 3/8 2 5/8
6
7 8
3/4 7/8 1
9*
1
10*
11*
1 1/8 1 1/4
* Equivalent to Square Cross Section Area
Tensile Strength
70,000 - 90,000 PSI
Installation
Lap at least 24 X the diameter
Not less than 12 inches
Reinforce For Concrete
Wire Fabric
5' X 150' Rolls
T
Mesh
6" X 6"
4" X 4"
Tensile Strength
60,000 - 70,00 PSI
10, 8, 6 & 4 Gage
13, 10 & 4 Gage
Installation
Overlap 1 mesh plus
2
inches
Concrete is strengthened greatly by the addition of
steel rods or wire mesh
Footing rebar must be laid in
such a manner as to allow for
concrete movement.
Ends are tied, not continuous
or welded.
Interior and exterior rebar
should exchange at corners
Intersections should extend to
the exterior
The footer should be as thick as the wall resting on it is wide.
The footer should also be at least twice as wide as the wall.
Concrete footers are placed below the frost line to provide a
solid base for masonry walls.

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