Presentation on theme: "Constructing Concrete Forms and Reinforcement"— Presentation transcript:
1Constructing Concrete Forms and Reinforcement The National Association ofCertified Home Inspectors
2Definitions 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.
3Selecting materials for concrete forms. Metal formsSynthetic materialsWood (most commonly used). It should be straight, sound lumber, free of knots, decay and other defects.Commonly, forms are steel framed plywood.
4Well 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.
5Construction 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.
7Types of points. Isolation joints Control joints Used to separate floors from points of abutment with walls columns, or building footings.Control jointsMay 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 jointsCreated where concrete stopped and later was started again.Most construction joints are actually a combination of a control joint and a construction joint.
11Types of reinforcement Steel barsSteel 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, , 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.
12Types of reinforcement Welded wire fabricGenerally available in a 6 by 6 inch pattern and consist of number 6, 8, or 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 fibersFibers 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.
14Form Panel Sizes Form Sizes 4’ 2 bar 4’ 3 bar 5’ 3 bar 6’ 3 bar
15Footings 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 supportIts footing, the widened part of the structure resting upon the bedIts 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.
16Footings and foundations Load-carrying capacities of Soils Type of soilTons per sq. ft. Soft clay1Firm clay or fine sand2Compact fine or loose coarse sand3Loose gravel or compact coarse sand4Compacted sand-gravel mixture6
17Footings 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.
18Care 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.
19Constructed Wooden Forms Custom made and used for certain custom foundation work.A. Stake G. Tie, SnapB. Brace H. SpreaderC. Tie, 1" x 4" I. Tie, WireD. Sheathing J. FootingE. Wale K. KeyF. Holder L. Marker Nail
22Reinforcing Affects: Strength of Concrete Structures: Reinforce Concrete to Increase Tensile Strength
23Reinforcing For Concrete SteelReinforcing Bars (Re-Rod)20' LengthsSizesNumber * 10* *Inches 1/4 3/ /8 3/4 7/ / /4* Equivalent to Square Cross Section AreaTensile Strength70, ,000 PSIInstallationLap at least 24 X the diameterNot less than 12 inches
24Reinforce For Concrete Wire Fabric Mesh5' X 150' Rolls 6" X 6" 10, 8, 6 & 4 Gage " X 4" 13, 10 & 4 GageT Tensile Strength Installation60, ,00 PSI Overlap 1 mesh plus inches
25Concrete is strengthened greatly by the addition of steel rods or wire mesh
26Footing 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
27The footer should be as thick as the wall resting on it is wide 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.