Things You'll Need
Steel reinforcing bars, commonly referred to as "rebar," are a highly effective means of adding lateral strength to a concrete driveway and to minimize its tendency for cracking over time. The rebar is typically laid out in a grid pattern with the bars tied together with 16-gauge wire at each intersection. For any concrete driveway, particularly one supporting larger vehicles, such as motor homes or trucks, installing rebar strengthens the concrete slab. The procedure for calculating the materials needed requires only a calculator and a few basic measurements.
Measure the full width and length of the proposed driveway area.
Divide the overall driveway width dimension by the rebar grid spacing to determine the number of crossing bars required. Typical rebar grids use #3 (3/8-inch) rebar spaced between 18 and 24 inches apart. Add an additional crossing bar for each end of the driveway grid.
Multiply the number of crossing bars calculated in the previous step times the driveway's width minus 12 inches to determine the total linear quantity of rebar needed for the crossing bars. The deduction from the width will allow a standard 6-inch gap on each side between the ends of the rebar and the edge of the driveway concrete.
Divide the overall driveway length dimension by the rebar grid spacing to determine the number of crossing bars required. Add an additional bar for each side of the grid.
Multiply the number of lengthwise bars calculated in the previous step times the driveway's length minus 12 inches to determine the total linear quantity of rebar needed for the lengthwise bars. As with the crossing bars, the deduction from the length will allow a 6-inch gap on each end of the grid between the ends of the rebar and the edge of the driveway concrete.
Add the total linear quantities determined in steps 3 and 5 together, then divide the total by 20 to define the number of standard 20-foot lengths of rebar needed for your driveway project.
The standard practice of holding the rebar’s ends back from the driveway edge is to prevent the inadvertent exposing of the ends during the pouring of the concrete. Once the concrete driveway is poured and cured, an exposed end that makes contact with the adjacent lawn or soil at the driveway edges is susceptible to the spread of rusting in the rebar, which can lead to cracking at the concrete edges.
State and local building codes pertaining to concrete driveways may be applicable in your area. You should consult your local building department to insure compliance with legal and structural requirements for your project.