Inland Concrete Products

A rebar chair is defined in Clause 3.1 of AS/NZS 2425 as 'the embedded items within reinforced concrete elements, which are used to maintain the position and cover of steel reinforcement during construction to ensure durability, strength, and serviceability of the as-built elements, in accordance with the design."

The U.S. Department of the Army booklet, “Concrete Masonry, and Brickwork,” (1975) states that supporting rebar “can be done by means of built-in concrete blocks, metalic supports, spacer bars, wires or other devices." As innovations have progressed, plastic rebar chairs have become the most ubiquitous form of rebar spacers.

Plastic rebar chair supports enable rebar to maintain the proper space between the bottom and top reinforcements of a slab. The plastic rebar spacers prevent the collapse of the rebar when casting begins.

The equation is fairly simple: first, divide the length of your slab's perimeter beam by 1.2; this will give you the total amount of trench mesh chairs needed; next, multiply the number of waffle pods by 4 to find the total number of rebar chairs needed.

Plastic rebar chair bar supports are used in footings and slabs to reduce the chance of the concrete collapsing and touching of the upper cage and lower cage reinforcement to one another. It's used to reduce the failure of compression and tension zone in a concrete structure.

In a typical 100mm residential concrete slab, the rebar should be placed at a depth of approximately two-thirds of the slab's thickness from its surface. Placing the rebar at the bottom of the slab will provide no benefit to the concrete.

Rebar chairs are a polypropylene support system ideal for concrete applications requiring a 4 in. to 5 in. slab, including floors, sidewalks, driveways, and many others. Unique fastening system ensures accurate positioning of rebar to code during concrete placement.

Rebar is recommended for concrete that measures 5-6 inches in depth. Rebar must be placed at the center of (or slightly above the center of) the concrete slab – hence why it should be a certain thickness for best results. “Chairs” is a term used to define supports placed beneath rebar grids to prop them up.

The most common type of chair simply suspends the rebar off the ground so that it becomes fully embedded in the concrete as it is poured. They are often used on foundation footings, concrete slabs, and other flatwork. The chairs may be made of metal or plastic or other non-corrosive material.

No, you do not need rebar for a 4-inch slab of concrete on grade. A 4-inch-thick slab cast on the ground and in permanent contact with it will float and rebar is not required. Rebar is recommended on concrete measuring 5 – 6 inches thick.

Only laborers who are experienced with steel reinforcement placement should install rebar support chairs. The maximum spacing of chairs should not exceed 48 inches. Comply with applicable codes and design criteria for the minimum spacing of reinforcement from edges of forms.

Wire Dobie is used as a chair to suspend rebar or welded wire in a freshly poured concrete slab. It maintains the reinforcement at the desired height as the concrete hardens around it.

For a 4" slab the reinforcing is typically centered in the slab, this will leave 2" of cover, although not ideal (usually 2.5" is a minimum for concrete coverage over reinforcing) this is the best you can do for the situation. The thickness of the reinforcing is really minimal and you should not see any cracks appearing above it. The reason they pull the mesh up into the slab pour is to get it centered and off of the bottom. A lot of times they do not use benches/chairs/blocks to hold mesh up and this is the only way to get the mesh into the center of the slab. Mesh or any reinforcing laying on the bottom of the slab is useless – it needs to be in the slab.