Constructing Concrete Forms and Reinforcement The National Association of Certified Home Inspectors.

Constructing Concrete Forms and Reinforcement

The National Association ofCertified 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.– $ 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 – 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– 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 foundationsLoad-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.

A. Stake G.Tie, Snap

B. Brace H.Spreader

C. Tie, 1" x 4" I.Tie, Wire

D. Sheathing J.Footing

E. Wale K.Key

F. Holder L. Marker Nail

Constructed Wooden Forms

Custom made and used for certain custom foundation work.

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' LengthsSizes

 Number 2 3 4 5 6 7

8 9* 10* 11*Inches 1/4 3/8 2 5/8 3/4 7/8 1 1

1 1/8 1 1/4 

 * Equivalent to Square Cross Section

Area 

Tensile Strength70,000 - 90,000 PSI

 InstallationLap at least 24 X the diameterNot less than 12 inches

Reinforce For ConcreteWire Fabric Mesh5' X 150' Rolls 6" X 6" 10, 8, 6 & 4 Gage

  4" X 4" 13, 10 & 4 Gage 

T Tensile Strength Installation60,000 - 70,00 PSI 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.