Types of Rebar Chairs and Other Supports
Rebar, or reinforcing bar, must be encased in concrete to the proper depth (called cover) to strengthen the concrete structure as designed. Rebar chairs and similar supports are the standard devices used to prop up the rebar or separate it from the concrete form so that the rebar is surrounded by concrete to ensure the specified cover. There are many types of chairs and other supports available for different applications. The right support for any project can depend on several factors, such as the type of surface underneath the concrete, the type of concrete formwork, and the design specifications.
Standard Rebar Chairs
The most common type of chair raises the rebar off the ground. These 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 prevent the chair tips from puncturing the vapor barrier under slabs and provide a more stable surface. 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 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 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 like chairs can when the rebar is moved. 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 chair can be placed at multiple heights, allowing you to install a bottom layer, an intermediate layer, and a top layer, maintaining 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 for custom support fabrication.
Tip or Round Cap Rebar 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.
Rebar Chair Problems
Rebar chairs and other supports perform as designed only when they are properly handled and installed. One common problem is chairs getting tipped over. 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 be tipped over when workers step on the rebar. Another problem is supports being spaced too far apart, allowing the rebar to bend in the middle or leading to chairs breaking due to the weight of the rebar.
Steel chairs with plastic feet can have their feet come off during installation, creating the potential for rust reaching the concrete surface.
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