Yes, rebar should be off the ground. Reinforcing steel has to be held on chairs, bolsters or precast concrete blocks at the height shown on the drawings, so it is fully surrounded by concrete with the specified cover and sits where the designer assumed it would. Bars lying on the base have no cover underneath, are exposed to moisture from the ground, and are rarely in the part of the slab or footing where they do any good. The supports stay in the concrete permanently, and purpose-made ones do not weaken it.
This post answers the common questions about getting rebar off the ground. For chair types and buying, see our guide to buying rebar chairs in Canada. For the right height in a slab, see where rebar should sit in a concrete slab.
Why rebar has to be off the ground
Rebar only works if three things are true: it is bonded to concrete all the way around, it is protected by enough concrete cover, and it is at the depth in the member that the designer used in the calculations. Bars on the ground fail all three.
- Bond. Concrete grips a bar through its deformations. A bar pressed into the base has concrete on top only, and the bond is reduced.
- Cover. Concrete cover protects steel from moisture and corrosion. CSA A23.1 sets 75 mm for concrete cast against and permanently exposed to earth, and 20 mm or more elsewhere depending on exposure. A bar on the ground has none on its underside. See our concrete cover guide.
- Position. In a footing, the steel works near the bottom but must still have its cover. In a slab on grade, the designer may place the mat near mid-depth or in the upper part of the slab, where it helps keep shrinkage cracks tight. A mat on the base is not where either design put it.
The cover rule is strict. The RSIC Manual of Standard Practice notes that, regardless of placing tolerances, concrete cover should not be reduced by more than one third of the specified cover.
What happens when rebar is left on the ground
The usual story on a residential pour: the bars are laid on the base, and someone plans to pull them up into the concrete with a hook as it is placed. It does not work.
- The bars end up at random heights. Some come up partway, some not at all, and they drop back under the weight of the concrete and the finishers.
- Corrosion starts from below. With no cover on the underside, moisture from the ground reaches the steel. Rust takes up more room than the steel it comes from, which can crack and spall the concrete over time.
- Cracks open wider. A slab whose steel is at the bottom has little reinforcement where cracks show at the surface.
- Inspections fail. On permit work, bars on the ground are an obvious defect and the pour can be held up.
Do rebar chairs stay in the concrete?
Yes. Chairs, bolsters, precast blocks and spacers are cast in and stay there for the life of the concrete. Nobody removes them, and there is no need to.
That is why the material matters:
| Situation | Support to use |
|---|---|
| Slab or footing on soil | Precast concrete blocks (dobies) or chairs with sand plates |
| Slab on vapour barrier or insulation | Plastic chairs with wide bases |
| Suspended slab on formwork, soffit hidden | Plain wire chairs and bolsters |
| Soffit exposed or architectural concrete | Plastic or plastic-tipped supports, so no rust spots show |
| Epoxy-coated or GFRP bar | Coated or all-plastic supports |
| Wall and column sides | Wheel or clip-on side spacers |
The RSIC Manual of Standard Practice sorts bar supports into classes, from bare wire for use where surface staining can be tolerated, through plastic-protected wire, to precast concrete blocks intended for contact with soil and footings, and all-plastic supports.
Do rebar chairs weaken concrete?
Purpose-made bar supports used as intended are standard practice, not a defect. CSA A23.1 requires reinforcement to be supported and secured against displacement, and the RSIC manual covers bar support types, sizes and use in detail. Plastic chairs are moulded with open shapes so concrete flows around them, and precast concrete blocks are concrete themselves.
Where supports do cause trouble, it is because the wrong one was used:
- Bare wire chairs at an exposed surface. The tips sit at the surface of a soffit or a slab edge and can rust and stain. Use plastic-protected or plastic supports there.
- Chairs that sink. Narrow legs on soft soil push in under the weight of the bars and the crew, and cover is lost. Use blocks or sand plates.
- Improvised supports. Bricks, stones, wood blocks and chunks of broken concrete are not bar supports. Wood can rot and leave a void. Clay brick and random rubble absorb water and can break. All of them shift.
- Supports overloaded by traffic. The RSIC manual is clear that bar supports are designed to carry the reinforcing steel, not concrete buggies or runways. Use planks and walk boards where pump hoses and crews travel.
Rebar dobies and supports on soil
On footings and slabs on ground, the most dependable support is the precast concrete block, often called a dobie. Most have a tie wire cast in, so the bar can be tied to the block and cannot roll off. Blocks come in several heights, so pick the one that matches the specified cover.
Chairs with sand plates are the other option on soil: the plate spreads the load so the leg does not sink.
Whichever you use:
- Match the height to the cover. The nominal height of a support is measured from the bottom of the leg or plate to the underside of the bar. The RSIC manual cites a tolerance of plus 3 mm on stated height.
- Space them closely enough that bars do not sag between them. Small bars need more supports than large ones. See rebar chair spacing and placement.
- Tie the bars to them, at least around the edges and wherever the mat will be walked on.
Rebar without chairs: what is acceptable
Sometimes the question is whether rebar can go in without chairs at all. The answer is that it always needs some approved support, but not always a manufactured chair:
- Support bars shown on the structural drawings, such as bars that carry a top mat, are acceptable because the designer has accounted for them.
- Precast concrete blocks replace chairs on soil.
- Placing the concrete in two lifts and laying the mat on the first lift is sometimes proposed on small slabs. It is not a substitute for supports unless the designer accepts it, because the mat can still be pushed down and the two lifts have to bond.
What is not acceptable is laying the steel on the base and lifting it during the pour, or using rubble to prop it up.
For chair heights in common slab thicknesses, see rebar chair height for 4 and 6 inch slabs.
Get supports matched to your rebar
Chairs and blocks only work if their height matches the cover and bar layers on your drawings. We detail, cut and bend 10M to 55M rebar and supply GFRP bar, delivered across the GTA and Ontario. We don't supply chairs or accessories, so source those from a building supply yard.
Send your drawings for a free take-off and quote, or see our rebar products.
Frequently asked questions
Should rebar be off the ground?
Yes. Rebar has to sit on supports at the height shown on the drawings so it has concrete all around it and is in the position the designer assumed. Bars lying on the soil or base have no cover underneath and do little of the work they were put there for.
Do rebar chairs stay in the concrete?
Yes. Chairs, bolsters and precast blocks are left in place and become part of the slab or footing. They are designed for that, which is why the material matters where the surface will be exposed.
Do rebar chairs weaken concrete?
Purpose-made supports used as intended do not create a structural weak point; supporting bars on chairs is standard practice in CSA A23.1 and the RSIC Manual of Standard Practice. Problems come from the wrong support: bare wire rusting at an exposed surface, chairs sinking into soft ground, or bricks, stones and wood used in place of chairs.
What are rebar dobies?
Dobies are small precast concrete blocks used to support rebar, often with a tie wire cast in so the bar can be tied down. They are the usual choice for footings and slabs on soil, because they bear on the ground without sinking.
Can you pour concrete over rebar without chairs?
Not properly. Lifting bars with a hook during the pour leaves them at random heights, often back on the ground. The only acceptable alternatives to chairs are other approved supports, such as precast blocks or support bars shown on the drawings.
