Concrete is strong in compression but weak in tension. Rebar provides the tensile strength that concrete lacks. The placement, spacing, size, and coverage of rebar are governed by ACI 318, and getting any of these details wrong can result in cracking, structural deficiency, or failed inspections.
This guide covers the ACI 318 rules that concrete contractors and structural engineers encounter most frequently: minimum reinforcement for temperature and shrinkage, maximum spacing, clear cover requirements, lap splice lengths, and development length. It is written as a practical reference, not a code commentary. For code interpretation questions, consult the ACI 318 commentary or a licensed structural engineer.
Temperature and Shrinkage Reinforcement
All concrete members that are not designed for flexural reinforcement still need temperature and shrinkage steel to control cracking. ACI 318 Section 24.4 specifies the minimum reinforcement ratio based on the type of steel:
Grade 40 or 50 deformed bars: As = 0.0020 x Ag (where Ag is the gross cross-sectional area per foot of width).
Grade 60 deformed bars or welded wire reinforcement: As = 0.0018 x Ag.
For a 6-inch slab with Grade 60 rebar: As = 0.0018 x 6 x 12 = 0.130 in2/ft. This is satisfied by #4 bars at 18-inch spacing (As = 0.133 in2/ft) or #5 bars at 28-inch spacing (As = 0.133 in2/ft). Most contractors default to #4 at 18 inches because it is easy to lay out and slightly exceeds the minimum.
Temperature and shrinkage steel must be placed in both directions. For one-way slabs, the primary reinforcement handles the structural direction and the temperature steel handles the perpendicular direction. For slabs on grade that are not designed for structural loads, temperature steel in both directions is the only reinforcement needed.
Grade 60: As = 0.0018 × Ag
Grade 40/50: As = 0.0020 × Ag
6-inch slab, Grade 60:
As = 0.0018 × 72 = 0.130 in²/ft
Use #4 @ 18" (0.133 in²/ft) or #5 @ 28" (0.133 in²/ft)
Concrete Rebar Spacing Calculator
Calculate rebar count, spacing, lap splice length, and development length per ACI 318. Supports #3 through #11 bars with clear cover requirements.
Maximum Bar Spacing
ACI 318 limits bar spacing to prevent wide cracks between bars. The maximum spacing depends on the type of member and the purpose of the reinforcement:
Temperature and shrinkage steel: 5 times the slab thickness or 18 inches, whichever is less (ACI 318-24.4.3.3). For a 6-inch slab: 5 x 6 = 30 inches, but capped at 18 inches. So maximum spacing is 18 inches.
Flexural reinforcement in slabs: 3 times the slab thickness or 18 inches, whichever is less (ACI 318-7.7.2.3). For a 6-inch slab: 3 x 6 = 18 inches.
Walls: Primary reinforcement spaced at 3 times the wall thickness or 18 inches, whichever is less. Secondary reinforcement spaced at 5 times the wall thickness or 18 inches.
Beams and grade beams: Stirrup spacing governed by shear design, typically d/2 for minimum shear reinforcement and d/4 for heavy shear regions, where d is the effective depth of the beam.
The practical takeaway: 18 inches is the most common maximum spacing for slab reinforcement. If your spacing calculation produces a value greater than 18 inches, reduce it to 18 inches and use a smaller bar if needed to meet the minimum steel area.
Clear Cover Requirements
Clear cover is the distance from the nearest concrete surface to the nearest surface of the rebar. It protects the steel from corrosion and fire, and provides bond development between the concrete and the bar. ACI 318 Table 20.6.1.3.1 specifies minimum cover:
Concrete cast against and permanently exposed to earth: 3 inches.
Concrete exposed to earth or weather, #6 through #18 bars: 2 inches.
Concrete exposed to earth or weather, #5 bars and smaller: 1.5 inches.
Concrete not exposed to weather or earth, #11 bar and smaller: 3/4 inch for slabs, walls, and joists. 1.5 inches for beams and columns.
These are minimums. Many projects specify greater cover for durability in corrosive environments, de-icing salt exposure, or marine conditions. Epoxy-coated rebar allows the use of standard cover in harsh environments but adds cost.
In the field, cover is maintained using chairs (supports), bolsters, and spacers. Rebar that sags to the bottom of the form reduces cover and compromises the structural design. Inspectors measure cover before concrete placement and will reject work that does not meet the specified minimum.
Lap Splice Length
When rebar lengths are not long enough to span the full member, bars are lapped (overlapped) to transfer force from one bar to the next through bond with the concrete. ACI 318 specifies minimum lap splice lengths based on bar size, concrete strength, cover, and bar spacing.
For most conditions, the minimum Class B tension lap splice is 1.3 times the development length (ld). Development length depends on bar size, yield strength, concrete strength, cover, and bar spacing. For #4 Grade 60 bars in 3000 PSI concrete with standard cover and spacing, ld is approximately 18 inches and the Class B lap splice is approximately 24 inches.
Common lap splice lengths (Grade 60, fc = 3000 PSI, standard conditions): #3: 16 inches. #4: 24 inches. #5: 30 inches. #6: 36 inches. #7: 42 inches. #8: 48 inches. These are approximate values for typical conditions. Actual lengths depend on the specific cover and spacing in your detail.
Lap splices should be staggered so that not all bars are spliced at the same location. ACI 318 requires that when more than 50 percent of the bars are spliced at one location, a Class B splice is required (1.3 x ld). When 50 percent or fewer are spliced, a Class A splice (1.0 x ld) may be used.
Concrete Rebar Spacing Calculator
Calculate rebar count, spacing, lap splice length, and development length per ACI 318. Supports #3 through #11 bars with clear cover requirements.
Rebar Size Reference
Standard rebar sizes in the US follow ASTM A615 and are designated by bar number. The bar number approximately equals the diameter in eighths of an inch: #3 = 3/8 inch, #4 = 4/8 (1/2) inch, #5 = 5/8 inch, and so on through #11 (1.375 inch). The #14 and #18 bars do not follow this naming convention.
Key properties by bar size: #3 (0.375 inch, 0.11 in2, 0.376 lb/ft). #4 (0.500 inch, 0.20 in2, 0.668 lb/ft). #5 (0.625 inch, 0.31 in2, 1.043 lb/ft). #6 (0.750 inch, 0.44 in2, 1.502 lb/ft). #7 (0.875 inch, 0.60 in2, 2.044 lb/ft). #8 (1.000 inch, 0.79 in2, 2.670 lb/ft).
For residential and light commercial work, #4 and #5 bars cover the majority of applications. #3 bars are used for stirrups and ties. #6 and #7 bars are common in footings and grade beams. #8 and larger are used in heavy structural members, retaining walls, and columns.
Grade 60 (yield strength 60,000 PSI) is the standard grade for structural reinforcement. Grade 40 is occasionally used for stirrups and secondary reinforcement. Grade 80 and higher grades are used in seismic design and heavy structural applications.
Common Rebar Placement Mistakes
The most common field mistakes that lead to structural deficiency or failed inspections:
Insufficient cover: Using 1.5-inch chairs for slabs on grade that require 3-inch cover. This is the single most common reason for rebar inspection failures. Verify chair height matches the required cover before placing any concrete.
Rebar on the ground: In slabs on grade, the rebar must be elevated off the subgrade to the specified cover position. Rebar lying on the ground provides zero tensile reinforcement because it is not in the tension zone. Use adequate chairs spaced close enough to prevent sagging (typically 3-4 foot spacing for #4 bars).
Missing continuity: Forgetting to extend footing reinforcement into the wall or column above, or failing to provide dowels at construction joints. Reinforcement must be continuous through the load path or properly lapped at connections.
Wrong bar size or spacing: Substituting a different bar size or spacing without checking that the resulting steel area meets the design requirement. If the plans call for #5 at 12 inches (0.31 in2/ft) and you use #4 at 12 inches (0.20 in2/ft), you are 35 percent under the required steel area.
No inspection before pour: The rebar must be inspected and approved before any concrete is placed. Once the concrete is poured, the reinforcement is invisible. Never pour concrete on unapproved reinforcement.