Concrete cover
Concrete cover is minimum distance from concrete surface to reinforcement face. AS 3600 sets by exposure class. Insufficient cover causes rust staining.
Ask Chalkline about this →Concrete cover is the minimum distance from the surface of cured concrete to the nearest face of reinforcement (rebar, mesh, or post-tensioned strand). It is the protective layer that keeps moisture and oxygen away from the steel, preventing corrosion that would otherwise cause expansive rust to crack and spall the concrete.
AS 3600:2018 (Concrete structures) sets cover requirements by exposure class, the most stringent in coastal, splash, and chemically-aggressive environments. AS 2870 applies similar logic for residential slab and footing reinforcement.
Typical residential cover values:
| Element | Exposure | Minimum cover |
|---|---|---|
| Internal slab, dry | A1 (mild) | 20 mm |
| External slab, away from coast | A2 (mild-moderate) | 25 mm |
| External slab, coastal | B1 to B2 (moderate-severe) | 30 to 40 mm |
| Slab on ground (top) | A1 | 20 mm |
| Slab on ground (bottom, where in contact with soil) | A2 | 25 to 30 mm |
| External walls, exposed concrete | B1 to B2 | 30 to 50 mm |
| Marine splash zone | C1 to C2 | 50 to 75 mm |
The standard’s table is more granular; the engineer’s drawings specify the exact cover for each element.
Why cover matters. Two failure modes from insufficient cover:
- Carbonation-driven corrosion: atmospheric CO₂ neutralises the alkaline pore water in concrete from the surface inward. When the carbonation front reaches the steel, the steel’s passivating layer is destroyed and corrosion begins. More cover = longer time before carbonation reaches the steel = longer service life. A 25 mm cover typically delays carbonation-corrosion for 50+ years; a 15 mm cover for 15-25 years.
- Chloride-driven corrosion: in marine and de-iced environments, chloride ions penetrate concrete and trigger corrosion at lower carbonation depths. Higher cover required.
Once corrosion starts, expansive rust multiplies the steel’s volume by 4-8x, cracking the surrounding concrete from the inside. The visible symptom is rust staining running down the concrete face, followed by spalling of the cover layer over months to years.
How cover is achieved on site:
- Bar chairs (plastic, steel, or fibre-reinforced cement) elevate the reinforcement off the formwork at the spec’d cover height. The most common method.
- Spacers (plastic clips or pre-cast concrete cubes) maintain side cover.
- Continuous reinforcement supports at top steel locations.
The concretor sets the chairs and spacers before the pour. The certifier checks at pre-pour inspection.
Common defects:
- Chairs displaced during pour: workers walking on top steel can push it down. Visible as wavy reinforcement line in the cured concrete.
- Chairs too low for the spec’d cover. Often comes from using “yard chairs” instead of the engineer-spec’d height.
- No side spacers: reinforcement drifts to one side of the form, reducing cover on that face.
- Cover loss at slab edges: reinforcement runs out beyond the edge form. Compounded by chamfer in the edge profile.
For builders.
- Check chair height and spacing at pre-pour inspection, not after. Once concrete is in, cover is set.
- Don’t let trades walk on top steel during the pour. If walkway boards are needed, use them.
- Photograph the reinforcement layout before pour. Years later when someone asks about cover, this is the only evidence.
Also known as: cover to reo, concrete cover to reinforcement, rebar cover.
Category: Materials / concrete / reinforcement.
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Last updated: 2026-05-14. Verified: 2026-05-14.