Concrete cover is the distance from the surface to the nearest bar, and it is the primary defence against reinforcement corrosion. Eurocode 2 makes the designer put a nominal cover on the drawing — cnom = cmin + Δcdev, where cmin is the greater of the bond and durability requirements for the exposure class, and Δcdev is an allowance for site deviation (typically up to 10 mm, depending on quality control). But the drawing does not deliver cover — the spacer does. A plastic chair costing a few cents is the only component physically holding the steel off the formwork, and if it is the wrong height, spaced too far apart so the bars sag, crushed underfoot, or made of the wrong material, the built cover is not the designed cover and the durability calculation quietly becomes fiction.
1. Why cover is the whole durability story
Reinforced concrete works because concrete is alkaline, and that alkalinity keeps a passive film on the steel. Destroy the alkalinity — by carbonation creeping in from the surface, or by chlorides arriving from de-icing salt or seawater — and the passive film goes, the steel starts to corrode, the corrosion product expands, and the cover concrete spalls off.
The cover is the distance that carbonation and chloride have to travel before they get there. That is the entire mechanism. Everything else in the durability specification — the concrete grade, the binder, the exposure class — exists to slow that journey down. The cover decides how long the journey is.
The durability chain. Every link is engineered and specified — except the one in red, which is usually bought on price and installed by whoever is free.
2. The cover equation, taken apart
Eurocode 2 does not let the designer just pick a number. Nominal cover — the number that actually appears on the drawing — is built up like this:
cmin — what the structure needs
The greatest of three things:
• c_min,b — the bond requirement, tied to bar diameter;
• c_min,dur — the durability requirement, read from the table for your exposure class and structural class (with permitted adjustments);
• an absolute floor of 10 mm.
Δcdev — what the site will get wrong
An allowance for deviation on site — the gap between the drawing and reality. Typically 0 to 10 mm, and the value depends on the quality-control regime (see the National Annex).
In other words: the worse your site control, the more concrete you must pour.
Two things follow that most people never connect:
- Exposure class drives the cover. A dry internal element and a chloride-exposed marine element are the same structure with completely different covers, because the environment is what is attacking the steel.
- Δcdev is a tax on poor control — and it is refundable. A project with rigorous measurement and inspection of the placed reinforcement can justify a smaller deviation allowance. That directly reduces cnom, which reduces section depth and concrete volume. Better spacer discipline literally buys back concrete.
3. Designed cover vs built cover — the gap nobody measures
Here is the uncomfortable truth of this product family. The designer produces a number. The spacer produces the cover. They are not the same thing, and only one of them protects the building.
Designed cover
A number on a drawing, derived from the exposure class, the structural class, the bar diameter and the deviation allowance. Rigorously calculated. Checked. Signed.
It has never touched a reinforcing bar.
Built cover
Whatever distance the bar actually ends up from the surface — determined entirely by the height of the spacer, whether the bar sagged between spacers, whether a spacer was crushed underfoot, and whether the mat got walked out of position before the pour.
This is the one that corrodes, or doesn't.
4. Spacer types — and what actually decides the choice
The choice is not a preference. It is decided by three questions: which face are you protecting, how much load must the spacer carry, and what is it standing on?
| Type | What it does | Use it when | Watch out for |
|---|---|---|---|
| Chair / bar support | Lifts a mat to a set height on a horizontal face | Slabs, foundations, precast beds — the workhorse | Point load punching into soft blinding or a membrane |
| Clip-on wheel / clip spacer | Holds a single bar off a face, clipping to the bar itself | Walls, columns, beams — vertical and curved faces | Clips that pop off when the mat is jostled |
| Continuous bar support | Spreads the load along a line instead of a point | Soft substrates, membranes, blinding that a point would punch | Nothing — this is what you use when a chair sinks |
| Plastic shim / packer | Levelling and small adjustment | Setting elements level, taking up tolerance | Stacking shims to make up a big gap |
5. Spacing: design it to the sag, not to the habit
There is no universal spacing number, and anyone who gives you one without asking about your bars is guessing. The rule is a deflection rule:
Which means the spacing depends on:
- Bar diameter and stiffness — a stiff bar spans further between supports than a light one before it sags.
- Mat weight — the top mat also has to carry itself.
- Site traffic — if people walk the mat (and they will), the spacing has to survive it.
- Placement method — concrete being dropped and poked around is a load, not a gentle filling.
And it means the check is visual, at the mat, before the pour: sight along the bars. If you can see a sag between the spacers, the cover at midspan is not the cover on the drawing, and no amount of correctly-specified spacer height will fix it.
6. The spacer is inside the cover — so it can become the corrosion path
This is the subtlety that turns a commodity into an engineering component. A spacer sits inside the cover zone. It touches the bar at one end and reaches the concrete surface at the other. Geometrically, it is a bridge across the exact barrier you built the cover to be.
Which is why the material and geometry are specified, not assumed:
| Requirement | Why |
|---|---|
| Minimal contact area at the concrete surface | The smaller the footprint at the face, the smaller the pathway for water and chloride to travel inwards. |
| No continuous conductive route to the steel | A steel or wire spacer touching the bar and reaching the face is a corrosion bridge — it rusts, and the rust reaches your reinforcement. |
| Adequate load capacity | A spacer that crumples under a boot delivers zero cover exactly where it was crushed — and nobody will ever see it after the pour. |
| Stable, non-degrading material | It stays in the concrete for the life of the structure. It must not shrink away, soften, or rot and leave a void. |
| Correct, repeatable height | The spacer is the cover. A batch with sloppy height tolerance is a batch of random covers. |
7. The pre-pour check that actually catches it
Once the concrete is in, the cover is whatever it is — forever, and invisibly. So the entire quality system for cover is a five-minute walk over the mat before the pour:
- Right spacer height? Check against the drawing's nominal cover — not against "what we usually use".
- Right type for the face and the substrate? Chairs on firm blinding; continuous supports where a point would sink; clips on vertical faces.
- Right spacing? Sight along the bars. Any visible sag between supports means the midspan cover is wrong.
- Any crushed or displaced spacers? Walk the mat and look. A crushed spacer is a guaranteed cover defect, and it is only visible now.
- Any improvised spacers? Rebar offcuts, stones, packers. Remove them.
- Record it. After the pour, a cover meter can verify on a sampling basis. And the records are exactly what let a project justify a smaller Δcdev next time — the inspection pays for itself.
8. Frequently asked questions
What is concrete cover and why does it matter?
Concrete cover is the distance between the concrete surface and the nearest reinforcing steel. It is the primary defence against reinforcement corrosion: the cover concrete is what carbonation and chlorides have to penetrate before they reach the steel and start it rusting. Cover also provides bond for the bar and fire resistance for the section. Too little cover is not a cosmetic defect — it is a durability failure that shows up as spalling and rust staining years later.
How is nominal cover calculated in Eurocode 2?
The designer specifies nominal cover on the drawing as cnom = cmin + Δcdev. The minimum cover cmin is the greatest of the bond requirement (cmin,b, related to bar diameter), the durability requirement (cmin,dur, taken from the table for the relevant exposure class and structural class, with permitted adjustments), and an absolute floor of 10 mm. Δcdev is the allowance for deviation on site — typically between 0 and 10 mm depending on the quality control regime, per the National Annex.
What is Δc_dev and why can it be reduced?
Δcdev is the margin the designer adds to the minimum cover to account for the fact that site work deviates from drawings. Its value depends on the quality-control regime: a project with rigorous measurement and inspection of the placed reinforcement can justify a smaller allowance than one without. That is worth understanding commercially — better spacer discipline and inspection literally reduce the concrete you must pour, because the nominal cover shrinks with the deviation allowance.
What actually delivers the cover — the drawing or the spacer?
The spacer. The drawing specifies cover; the spacer is the only physical component that holds the reinforcement away from the formwork or blinding so that the cover exists. Everything else in the durability chain — concrete grade, exposure class, cover calculation — is undone if a spacer of the wrong height is used, if spacers are too far apart and the bars sag between them, or if they are crushed or displaced before the pour.
How far apart should rebar spacers be placed?
Close enough that the reinforcement cannot sag or deflect between them under its own weight and under the loads it will see before and during the pour — including operatives walking on the mat and the pressure of placed concrete. There is no single universal number: the spacing depends on bar diameter and stiffness, mat weight, and site traffic. Design the spacing to the deflection, and check it by eye at the mat before the pour, because a sagging bar between correct spacers has the correct cover only at the spacers.
What types of spacer are there and when do you use each?
Chairs and bar supports lift a top or bottom mat to a set height, and are the workhorse for slabs. Clip-on wheel and clip spacers hold a single bar off a vertical or curved face and are used for walls, columns and beams. Continuous bar supports spread the load over a line rather than a point, which is what you want on soft blinding or membranes. Plastic shims and packers handle levelling and small adjustments. The choice is driven by the face you are protecting, the load the spacer must carry, and whether point loads will punch into the substrate.
Can a spacer itself cause corrosion?
Yes, and this is why the material matters. A spacer sits inside the cover zone, touching the bar at one end and reaching the surface at the other — so it is a potential path straight through your durability barrier. Use spacers designed for the purpose, with minimal contact area at the surface and no continuous conductive route to the reinforcement. Improvised spacers — a rebar offcut, a stone, a bolt — create exactly the corrosion bridge the cover was there to prevent.
Does the spacer need to carry load?
Yes. The spacer holds the reinforcement mat up while operatives walk on it and while concrete is placed and vibrated. A spacer that crumples under foot traffic delivers zero cover exactly where it was crushed, and nobody will ever see it once the pour is in. Specify a spacer whose load capacity suits the mat weight and site traffic, and on soft substrates use continuous or wide-footed supports that spread the load instead of punching through.
How do I check that the built cover matches the designed cover?
Before the pour, verify spacer type, height and spacing across the mat, and look for sag between spacers, crushed spacers, and bars pushed out of position by traffic. After the pour, cover can be verified non-destructively with a cover meter on a sampling basis. Recording those checks is what allows a project to justify a smaller deviation allowance in the cover calculation — the inspection pays for itself.
References
- EN 1992-1-1 (Eurocode 2), Section 4 — Durability and cover to reinforcement, the source of cnom = cmin + Δcdev, the exposure classes and the minimum-cover tables.
- Worked calculation of nominal cover to reinforcement — a step-by-step of the same equation, useful for checking your own numbers.
Need spacers whose height tolerance you can actually trust?
20+ years of export experience. Plastic rebar chairs, clip spacers, continuous supports and shims — with the height tolerance and load capacity that make the built cover match the designed cover. Send us the cover spec and the bar schedule and we'll quote the right ones.
