Choosing the Right Support Chairs When Using Rebar in Poured Concrete
Rebar Supports Ensure Proper Concrete Cover
Rebar—the common term for the metal bar used to reinforce poured concrete—must be embedded to the proper depth (known as cover) in order to provide the proper strength. Rebar chairs, or similar devices, are used to prop up the rebar, separating it from the concrete form or subbase, so that the rebar is embedded in the concrete to the specified cover depth.
There are many types of chairs and other supports available for different applications. Choosing the right support for a particular project depends on several factors, such as the type of surface beneath the concrete, the type of concrete formwork, and the design specifications of the project.
Common support devices include:
- Standard rebar chairs
- Spacer wheels
- Multi-level rebar chairs
- Tip (round-cap) spacers
Standard Rebar Chairs
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. They provide stability and are lightweight, economical, and easy to install.
Specialty versions of standard chairs include flat-bottom chairs that provide a stable surface and prevent the chair tips from puncturing the vapor barrier under slabs. Standard steel chairs may be fitted with plastic feet to prevent spot corrosion on the concrete surface. There are also stainless steel chairs, which are non-corrosive and are used when plastic is not allowed into the concrete, or when the weight of the rebar is too much for plastic chairs.
Spacer wheels are used on columns, walls, beams, pre-cast formwork, and round formwork. Made of plastic, the wheels have an internal webbing or framework that holds the rebar at the center of the wheel. And because the wheel is round, it can't tip over when the rebar is moved, in the way that chairs sometimes do. Spacer wheels are strong and light and make minimal surface contact, and they are available for different rebar sizes.
Multi-Level Rebar Chairs
Multi-level chairs are used on mats or other configurations that involve multiple layers of steel. The multi-level chairs can be placed at multiple heights, allowing you to install a bottom layer, an intermediate layer, and a top layer, maintaining proper spacing between all of the rebar layers. These chairs typically provide great stability and ensure proper concrete flow in between the spacers. This solution can save time and money by reducing or eliminating the need to fabricate custom supports.
Tip (Round-Cap) Spacer
A tip, or round cap spacer, is called for when rebar is installed vertically and requires bottom or top concrete cover. The cap is installed at the end of the rebar and has a thin, elongated tip that supports the rebar and positions it at a precise height. The tip is also designed to give you better concrete flow to prevent breakouts.
Common Problems with Rebar Chairs and Other Supports
Rebar chairs and other supports perform as designed only when they are properly handled and installed.
- One common problem is chairs getting tipped over during the pour. This occurs most often when the required cover is over 2.5 inches, as the height-to-width ratio of the chair may make it easy for the chair to tip over when workers step on the rebar. Increasing the number of chairs can help prevent this problem.
- Another problem is when the supports are spaced too far apart, allowing the rebar to bend in the middle or causing chairs to break due to the weight of the rebar. Here, too, increasing the number of supports can prevent the problem.
- Steel chairs with plastic feet can have their feet come off during installation, creating the potential for rust reaching the concrete surface. Care should be taken to make sure the chairs are intact during installation.