Finishing with rebar chairs means placing small supports, usually made of plastic, wire, or concrete, under steel reinforcing bars (rebar) to hold them at the correct height above the formwork before concrete is poured. This ensures the rebar ends up in the middle of the slab or beam, where it can best resist tensile forces and avoid corrosion.
Why Rebar Position Matters
Concrete is strong under compression but weak under tension. Steel rebar is embedded to take those tensile loads, but it only works if it is located where the tension occurs. In a typical slab, that is near the bottom, but not at the very bottom. If rebar sits directly on the ground or formwork, it can be exposed after the concrete cures, leading to rust and structural failure. Chairs are the standard solution to maintain the correct “cover” – the distance from the rebar surface to the concrete edge. Building codes, such as those from the American Concrete Institute (ACI), specify minimum cover based on exposure conditions, typically 1.5 to 3 inches for slabs on grade.
Types of Rebar Chairs
- Plastic chairs: Lightweight, corrosion-proof, and inexpensive. They come in various heights and styles, including wheel chairs for continuous support and snap-on clips for vertical bars.
- Steel wire chairs: Made from galvanized or stainless wire, these are strong and can be bent to different heights on site. They are often used in heavier applications like bridge decks.
- Concrete chairs: Precast concrete blocks are used for very heavy rebar mats or where plastic might crush under extreme weight.
- Bar supports with bases: These have a wide base to prevent sinking into soft subgrade, and are often used with a “sand plate” or “doughnut” base.
How to Choose the Right Rebar Chair
Selecting the correct chair depends on three main factors: the rebar diameter, the required cover depth, and the load the chair must bear. For a residential slab, plastic chairs with a height equal to the cover are usually sufficient. For a commercial foundation, you might need steel chairs with a larger footprint to spread the weight. Always check the manufacturer’s load rating – a chair that collapses under the weight of workers or wet concrete is useless. Also consider the environment: plastic chairs are fine for most interior work, but for marine environments, stainless steel or plastic is better to avoid galvanic corrosion.
Installation Steps and Tips
- Set the formwork and level the subgrade.
- Place the chairs on the formwork at regular intervals – typically 2 to 4 feet apart, depending on rebar size and load.
- Lay the rebar on the chairs, tying intersections with wire ties to keep the grid stable.
- Use extra chairs at splice locations and around openings where rebar is cut.
- Before pouring, walk the rebar to check for any chairs that have shifted or sunk; adjust as needed.
A common mistake is spacing chairs too far apart, causing the rebar to sag between supports. For #4 bars (1/2-inch diameter), a 3-foot spacing is typical, but for larger bars, you can go wider. Always refer to your structural drawings for exact spacing.
Common Problems and Misconceptions
One misconception is that rebar chairs are only needed for slabs on grade. In reality, they are used in walls, columns, and elevated decks too. Another issue is that workers sometimes step on the rebar, bending the chairs and reducing cover. This is why many contractors use “high chairs” with a top horizontal bar to walk on. Also, plastic chairs can break in cold weather if they are not rated for low temperatures – check the product specs. Finally, some think that a few extra chairs are harmless, but too many can create voids in the concrete, so follow the design.
Cost and Availability
Rebar chairs are inexpensive – a typical plastic chair costs between $0.10 and $0.50 each, depending on height and load rating. Steel chairs are more, but still a small fraction of the total concrete cost. They are available at any hardware store or concrete supply company. For a 10×10-foot slab with rebar every 12 inches, you might need around 100 chairs, adding only $10–$50 to the project. The cost of fixing a failed slab due to corrosion is thousands of times more, so chairs are a cheap insurance.
Alternatives to Rebar Chairs
For small projects, some people use bricks or stones, but these are not reliable because they can shift or crush. Another alternative is using rebar spacers that clip onto the bar and rest on the form – these are essentially chairs with a different shape. For precast concrete, manufacturers often use “doughnut” spacers that slide over the rebar. In some cases, fiber-reinforced concrete or welded wire mesh can reduce the need for rebar, but if you use rebar, you still need supports.
Frequently Asked Questions
Can I use rebar chairs for wire mesh?
Yes, but wire mesh is lighter, so you can use smaller plastic chairs or even specialized mesh supports. However, the same principle applies: keep the mesh at the correct height to avoid cracking.
How do I know the correct cover for my project?
Check your local building code or structural drawings. For residential slabs on grade, 1.5 inches is common; for exterior exposed to weather, 2 inches or more. If in doubt, ask a structural engineer.
Do rebar chairs need to be removed after concrete sets?
No, they are left in place. Plastic and concrete chairs are inert, and steel chairs are galvanized to resist corrosion. Removing them would leave voids.
What happens if I don’t use rebar chairs?
The rebar will sink to the bottom of the slab, reducing cover. This can lead to rusting, spalling, and structural failure over time. In a beam, incorrect rebar position can cause cracking or collapse.
Are there chairs for vertical rebar in walls?
Yes, there are “wheel” chairs that snap onto the vertical bar and hold it away from the formwork. They are used in walls and columns to maintain cover on both sides.
Related
- Forgotten Island Filipino Mythology: Lost Tales
- Filipino Mythology Goddesses: Key Deities & Lore
- Filipino Mythology Books: A Reader’s Guide
How pages are sourced and corrected: Sources and editorial policy.