Table of Contents
Rebar Basics
Rebar (reinforcing bar) is steel bar used to reinforce concrete and increase its tensile strength. Concrete is strong in compression but weak in tension; rebar provides the tensile strength needed to prevent cracking under load.
Rebar is typically placed in a grid pattern within concrete slabs, with bars running in both directions. The spacing, size, and placement depth depend on the structural requirements specified by the engineer or building code.
Calculation Formulas
Rebar Size Reference
| Bar Size | Diameter | Weight (lb/ft) | Weight (kg/m) | Common Use |
|---|---|---|---|---|
| #3 | 3/8" (0.375") | 0.376 | 0.560 | Slabs, light footings |
| #4 | 1/2" (0.500") | 0.668 | 0.994 | Slabs, driveways, walls |
| #5 | 5/8" (0.625") | 1.043 | 1.552 | Footings, foundations |
| #6 | 3/4" (0.750") | 1.502 | 2.235 | Columns, beams |
| #7 | 7/8" (0.875") | 2.044 | 3.042 | Heavy structural |
| #8 | 1" (1.000") | 2.670 | 3.973 | Heavy structural |
Placement Guidelines
- Concrete cover: Minimum 3 inches for concrete cast against earth, 1.5 inches for slabs not exposed to weather.
- Lap splice length: Minimum 40 bar diameters for #3-#5 bars, 48 bar diameters for #6-#8 bars (for Grade 60 rebar).
- Slab placement: Rebar is typically placed at mid-depth for 4-inch slabs, or at one-third depth from the bottom for thicker slabs.
- Chair supports: Use rebar chairs every 3-4 feet to maintain proper height during the concrete pour.
- Tie wire: Use 16-gauge tie wire at every other intersection for slab grids.
Frequently Asked Questions
How much rebar do I need for a 20x20 slab?
For a 20x20 ft slab with #4 rebar at 12" spacing in both directions: You need 21 bars each direction (42 total pieces, each 20 ft long), totaling 840 linear feet. At 0.668 lb/ft, the total weight is about 561 lbs. You would need 42 standard 20-foot bars.
What size rebar for a 4-inch slab?
#3 or #4 rebar is standard for 4-inch residential slabs. #3 at 18" spacing is common for light-duty slabs (patios, walkways). #4 at 12" spacing is used for driveways and garage floors that support vehicle traffic.
How much overlap do rebar splices need?
The minimum lap splice is typically 40 times the bar diameter. For #4 rebar (0.5" diameter), the minimum lap is 40 x 0.5 = 20 inches. For #5 rebar (0.625"), the lap is 25 inches. Always add 10% to total rebar quantities to account for splice overlaps and waste.