Concrete

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Concrete•Cement

•Sand

•Gravel

•Water

Portland Cement• A dry powder made from

limestone and is usually packaged in bags containing 1 cubic foot and weighs 94 pounds

Portland Cement•When mixed with water, it forms the “glue” to hold the aggregate together

Fine Aggregate•Sand

•It should be clean of clay, silt, and organic matter

Course Aggregate•Gravel and rocks over ¼” in diameter

Water•Should be clean

Concrete TrucksMost concrete is delivered by trucks to the construction site as ready-mixed concrete

Mortar•Does not contain coarse aggregates and is used to join bricks or blocks

Cement• Although many people

call concrete “cement”, cement is the powdered material and concrete is the mixture of ingredients

Weight•150 -160 lbs. per cubic foot

Slump•Stronger concrete requires less slump

•The consistency or wetness of the mixture

Slump• If concrete has too much

slump (is too wet), adding aggregate such as sand and gravel will correct the problem

Uniformity• Is thoroughly mixing all

ingredients so that the “glue” which is wet cement will be around all sand and gravel particles (aggregates)

Workability•Refers to the ease with which concrete can be placed and shaped

Ratio•Expressed as a three digit number

•Example: 1,2,3

Ratio• The first number is parts

of cement

• Second number is parts of fine aggregate (sand)

Ratio• The third number is parts

of large aggregate (gravel)

• Parts may be shovels full, pounds, cubic feet, etc.

Ratio• Less sand, gravel, and

water are used for the same amount of water to make concrete stronger

Hand Mixing•May be done in a wheelbarrow or mixing box (mortar box) by using a shovel or hoe

Hand Mixing• Add sand and then

cement and mix thoroughly

• Then add and mix gravel

Hand Mixing•Last thing to add is water and mix well

Machine MixingUses a concrete or mortar mixer

Machine MixingFirst, add about 10% of water needed so that ingredients will not stick to mixer

Demonstrate Concrete Skills

6831.16

Form•A wood or metal structure that confines concrete to the desired shape until it hardens

Form•Trenches are dug in the ground for footings of buildings, and the soil walls are the forms

Form•Wood or metal can be used

Form• When wood is used,

2” X 4”, 2” X 6”, 2” X 8” and other 2” size lumber is used because forms must not bend or warp and must be braced to prevent movement

Form• In warm weather, forms

should stay in place 2 to 3 days and twice that long if weather is cold

Pour or Place• Concrete should be

poured or placed as evenly as possible in the forms to prevent having to move it

Finishing•Bringing the surface of concrete to the proper grade and smoothness

Finishing• Screeding is using a

straightedge such as a 2” X 4” board to level concrete with top of form

Finishing•Bull float is used to impact the concrete at the surface and to make it easier to finish

Finishing•Texture of the finishing tool will determine smoothness of concrete surface

Finishing• Wood float, stiff broom,

or burlap will make a gritty, non-slip surface for floors, driveways, or walkways

Finishing•For a very smooth surface, a steel cement finishing trowel should be used

Reinforcement• Needed because concrete

has low tensile strength and high compressive strength

Reinforcement•This means it has low stretch and high compaction

Reinforcement•Helps equalize and prevent cracking or breaking

Reinforcement• Clean steel rods or bars

called rebar and clean welded wire are used in concrete for reinforcement

Estimating Materials• Cubic yards equal length in

feet multiplied by width in feet multiplied by thickness in feet divided by 27, which is the number of cubic feet in a cubic yard

Estimating Materials• Note: thickness of

concrete is usually in inches and will need to be changed to a fraction of a foot

Estimating Materials

•Examples: 3’ = ¼ foot, 4” = 1/3 foot, 6” = ½ foot

Estimating Materials• Example: How many

yards of concrete are needed to pour a driveway 100’ long, 8 feet wide, and 3” thick?

Estimating Materials•Formula:Cu. yds. = 100X8X1/4= 200= 7.41 27 27

Estimating Materials• Example:If concrete is purchased in only

whole yards and 10% should be allowed for waste and uneven ground level, how much ready mixed concrete should be ordered?

Estimating Materials

•7.41 X 10% = 8.15 To get the right amount, 9 cubic yards must be purchased

Treat With Oil• Wood and metal forms

should be treated with oil and wet down with water before concrete is poured

Treat With Oil•Oil makes the forms easier to remove and water will prevent swelling

Bleeding• Water rising to the top

surface of concrete poured in tall or deep forms is called bleeding

Bleeding• This problem can be

corrected or reduced by pouring concrete slower and by using a stiffer or less watery mix