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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 Trucks Most 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 lbs. per cubic foot
Slump Stronger concrete requires less slumpThe 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 cementSecond 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 done
Hand Mixing Add sand and then cement and mix thoroughlyThen add and mix gravel
Hand Mixing Last thing to add is water and mix well
Machine Mixing Uses a concrete or mortar mixer
Machine Mixing First, add about 10% of water needed so that ingredients will not stick to mixer
Demonstrate Concrete Skills
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, ” 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
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% = 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
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