Minimum Concrete Cover for Rebar (ACI 318)
Concrete cover is the distance from the surface of the concrete to the nearest reinforcing bar. It protects the steel from corrosion and fire and lets the bar bond to the concrete. These are the minimum specified covers for cast-in-place, nonprestressed concrete in ACI 318-19, which the IBC adopts by reference.
| Concrete exposure | Member | Reinforcement | Specified coverin |
|---|---|---|---|
| Cast against and permanently in contact with ground | All | All | 3 |
| Exposed to weather or in contact with ground | All | No. 6 through No. 18 bars | 2 |
| Exposed to weather or in contact with ground | All | No. 5 bar, W31 or D31 wire, and smaller | 1-1/2 |
| Not exposed to weather or in contact with ground | Slabs, joists and walls | No. 14 and No. 18 bars | 1-1/2 |
| Not exposed to weather or in contact with ground | Slabs, joists and walls | No. 11 bar and smaller | 3/4 |
| Not exposed to weather or in contact with ground | Beams, columns, pedestals and tension ties | Primary reinforcement, stirrups, ties, spirals and hoops | 1-1/2 |
Notes
- "Cast against ground" means concrete placed directly on soil with no form, such as the bottom of a footing. Formed surfaces later backfilled fall in the "exposed to weather or in contact with ground" row.
- Cover is measured to the outermost steel, which is usually the tie or stirrup, not the main bar.
- Corrosive environments (deicing salts, seawater) and fire-resistance ratings can require more cover than these minimums.
- Table number: ACI 318-19 Table 20.5.1.3.1 (ACI 318-14 Table 20.6.1.3.1). Residential work under the IRC follows IRC R404 and R506 and is similar.
Frequently asked questions
How much concrete cover does rebar in a footing need?
Concrete cast against and permanently in contact with soil needs 3 inches of cover, so the bottom bars of a footing poured directly on the ground must sit at least 3 inches above the soil.
Is cover measured to the main bar or to the tie?
To the outermost reinforcement. In a beam or column the stirrups or ties sit outside the main bars, so the cover is measured from the concrete surface to the tie.
Why does cover matter?
Too little cover lets moisture and chlorides reach the steel. The steel rusts, expands and cracks the concrete. Too much cover reduces the effective depth and strength of the member.
Sources
- ACI 318-19, Building Code Requirements for Structural Concrete, Table 20.5.1.3.1 Specified concrete cover for cast-in-place nonprestressed concrete members
- American Concrete Institute FAQ, https://www.concrete.org/frequentlyaskedquestions.aspx?faqid=903
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