Concrete appears strong from the outside, but it can be fragile when poured out. Rebar, also known as reinforced steel, supports the weight of the concrete and anything on top of the concrete. Various thicknesses of rebar are available for use horizontally and vertically. The amount of rebar to use will depend on what you are building and your location.
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Rebar is a shortened name for reinforced steel. Numbers on the pieces of rebar indicate the thickness of the bar and the strength. The smaller of the two numbers will indicate the width of the bars in eighths of an inch. For instance, if the thickness is 4, the rebar will be 4/8, or 1/2, inch thick. The larger of the two numbers is the grade indicating the amount of weight in kilopounds the rebar can support per square inch. This is usually 40, 60 or 75. Take the number and multiply it by 1,000 to find the pounds per square inch the rebar supports. For instance, 60 grade rebar, which is usually required in earthquake zones, can support 60 x 1,000 = 60,000 pounds per square inch.
Why Use Rebar
Rebar must be used because concrete cannot support weight in a single area that would cause the concrete to bend. Concrete does not bend, it breaks. Without rebar, a concrete walkway or foundation would crack under the daily use of people walking on top of it. Rebar increases the amount of force concrete can withstand applied to its longest side, such as the top of a sidewalk or the face of a wall.
Horizontal Rebar Amounts
Horizontally, rebar is used in driveways, walkways or foundations. The grade of rebar for home construction is typically 40, but this varies based on the area you are building. Some places require stronger rebar because concrete structures must support the excess weight of snow in the winter or earthquake stresses. The rebar should be at least three inches below the top of the concrete, according to "Craftsman's Construction Installation Encyclopedia" by Stephen Diller, et al. If you overlap rebar pieces at the ends, overlap the pieces 30 times the diameter of the bar. For instance, if you have 1/2-inch-thick rebar, pieces should overlap 15 inches. This depth means for a typical 4-inch-thick foundation slab or driveway, you will need to support the rebar on 1-inch blocks before pouring the concrete. Tie pieces of rebar together to form a grid with 6-inch squares before pouring the concrete.
Vertical Applications of Rebar
To add a concrete block wall over a foundation, insert vertical 1/2-inch-thick pieces of rebar into the footer every 24 inches while the concrete in the footer is still wet. The holes in the concrete blocks will fit over the pieces of rebar. Use l-shaped rebar for the first set of vertical bars. Tie additional bars to overlap the first bars as you increase the height of the wall. Concrete is later poured into the block holes with pieces of vertical rebar in them to strengthen the rebar.