What Should I Consider When Choosing Rebar?

Reinforcing bar, commonly called rebar, is a steel rod that is placed in the wet concrete mix. Rebar makes the finished concrete stronger, so that it can withstand more stress and load. Rebar also helps to control cracking.

When choosing rebar, match the size of the rebar to the job requirements. Concrete rebar can be purchased in several sizes. For residential work, #3, 3/8-inch (1 cm) diameter, rebar is commonly used in flatwork, while #4, 1/2-inch (1.3 cm) diameter, and #5, 5/8-inch (1.6 cm) diameter, rebar is used in walls.

Rebar should be laid out in a consistent pattern. The type of pattern and the distance between the bars depends on the project. An 18-inch-square (45.7 cm) grid is typical for a driveway, while two parallel bars about eight inches (about 20 cm) apart is typical for footings.

Standard rebar sections are 20 feet (6 m) long, so more than one bar is usually needed to span the distance. When choosing rebar for a project, remember that two or more sections of rebar can be laid end-to-end, with the ends overlapped and wired together. The amount of overlap is determined by the size of the rebar. The bar should overlap at least 30 times its diameter. For example, following this rule, two #4 bars should overlap at least 15 inches (38 cm). Joints should be located away from the corners of the project, an important point to consider when choosing rebar layouts.

Rebar can be cut to the required lengths. There are several ways to cut rebar. A hacksaw or a reciprocating saw with a metal-cutting blade can be used to cut a little more than halfway through the bar. The pieces are then snapped apart. A circular saw with a cutoff blade can be used as well. So can a cutting torch. A special tool called a rebar cutter/bender is the most efficient way to cut rebar.

Rebar will only strengthen finished concrete if it lies midway through the thickness of the concrete, so consider positioning when choosing rebar. The rebar should be elevated on small blocks, called chairs, that position it correctly. Stone or waste concrete are suitable choices for chairs. Bricks should not be used; they are so porous that they will draw water from the concrete mix, resulting in a weakness in the finished product.

In projects that require less reinforcement, other types of reinforcement can be used. Welded wire fabric (WWF), or wire mesh, is commonly used as a reinforcing material, especially for flat concrete. It helps to control cracking in the finished concrete.

Wire mesh used for residential buildings usually has six-inch (15.2 cm) squares. It can be purchased in five feet by eight feet (1.5 to 2.4 m) sheets or in 50-foot (15.2 m) rolls. The sheets are easier to handle, because they lie flat, but the rolls are easier to transport. Consider which characteristics will make the job easier.

When laying out the wire mesh, overlap sections by at least six inches (15.2 cm). Leave an open seam of about four inches (10.2 cm) where control joints will be located in the finished concrete.

Choosing rebar also involves choosing a method to position it. Like rebar, wire mesh must be positioned properly, halfway through the thickness of the concrete. Wire mesh can be elevated on chairs, just like rebar. An alternate method is to lay the wire mesh on the ground inside the forms. As the concrete is poured and raked into place, hook the wire with the rake and raise it to the center of the wet concrete, using a shaking motion. Repeat this every few feet. A third method is to pour and level half of the concrete, place the wire mesh, and pour and level the rest of the concrete.