Concrete is commonly used in sidewalk construction because of its strength and durability. The concrete is built on top of a gravel base, and is often strengthened with concrete reinforcing bars. These bars, widely known in the construction industry as rebar, are made from steel and are used to carry the tension loads borne by the concrete because the concrete is incapable of sustaining heavy tensile loads. For a sidewalk, you want to use 3/8-inch rebar.
Plan the layout of the rebar by sketching out the sidewalk and the rebar on a piece of paper. You need to lay out the rebar needs in a grid, spaced evenly apart. The spacing depends on the weight carried by the concrete. You can space rebar in a driveway 18 inches apart, but in a deck or sidewalk, you can place it 24 inches apart.
Measure and cut the rebar to fit the dimensions of the sidewalk. Use a saw with a metal-cutting blade.
Lay the rebar into the trench where you are pouring the concrete for the sidewalk. Tie the different pieces of rebar together where they intersect with rebar tie wire.
Raise the rebar off of the ground so that it will sit in the center of the concrete. You can use small rocks or bricks, or rebar supports.
The first thing most people notice when they pick up a piece of rebar is that ridges run up and down the length of the metal bar. These ridges are designed to help the bar secure itself into the concrete better. When the concrete is poured it flows under and around the ridges so that the bar in essence grips on to the concrete when it dries. This forms a much more solid reinforcement than a smooth bar would provide.
Concrete is strong in many ways. However, it does not have great tensile strength. Steel, on the other hand does have a great deal of tensile strength. Rebar placed into concrete helps to increase the overall strength of the structure, because the tensile strength of the rebar helps to hold the structure together where concrete alone would not be strong enough.
Rebar is made from carbon steel. This is the most common type of steel manufactured. Steel is made using many different elements to create alloys, but in carbon steel, no other alloy material is added to the material beyond the carbon. Carbon steel is very strong and may contain as much as 2 percent carbon in the material.
Rebar can be worked with in a number of different ways. It can be heated and bent, a technique often used to create a hook at the end of the bar to reinforce an anchor position. The material is also suitable for welding. In situations where even the small reduction in strength that might occur from welding is unacceptable, rebar is usually wired to other bars instead.
Clean rebar coated with oil or grease with a solvent designed for that purpose. Oil and grease can prevent a good bond between rebar and concrete.
Brush rusted rebar with a wire brush to remove excessive rust.
Weigh and measure the rebar after you have removed heavy rust. You need to confirm that the rebar has not dropped below minimum material standards as designated in "Building Code Requirements for Structural Concrete."
Cut or bend the rebar at the job site according to the area where you will pour concrete. Create hooks for ends near the concrete edges.
Lay the rebar in a crisscross, cage, ladder or other pattern suggested by your concrete supplier. You can tie overlapping areas with wire to prevent the rebar from moving as you pour the concrete over it.
Pour concrete into form over your tied rebar pattern. Allow concrete to cure according to building code specifications for the depth of your project.
Lay the rebar in a grid pattern across the length and width of your cement project. Position the rebar 18 to 24 inches apart, so they form perfect grid squares. Keep the last piece of rebar, horizontal with that edge, about 15 inches away from the edge.
Connect overlapping rebar by tying them together with a rebar tie wire. Wrap the wire around the two rebars to form an "X." Then twist the ends around themselves. Use pliers to bend the wire, if necessary. Repeat at each point where rebar overlaps.
Position rebar supports underneath the rebar so it does not sag. To install a rebar support, gently lift the rebar, slide the support underneath so the rebar rests in the support's groove. The number of supports you need and the distance between supports depends on the rebar's weight and the size of the project.