Had a couple of questions about chairs. As in…’’what are they?!’’
No…it’s not the type we recline on at the end of the day, when the house is long finished and the day’s work is done, with a sundowner. Not the type you pull up at the table for your breakfast. Not even those portable ones you take to the beach in the hope of staying comfortable.
We are talking about chairs in construction, and that means…the supports for the rebar.
Supports for rebar? I hear you ask. Why does one need to ‘support’ rebar? Its metal. It’s solid. Strong. What support are you talking about?
Ok. I’m talking about the kind that is needed when rebar is encased in concrete, to form that RC structure.
You see, the rebar IS strong…and heavy. If you placed the rebar grid in the area to pour the floor, it would sit on the bottom, and that’s no good. Similarly, if you placed the rebar column or beam cage in the shuttering and it touched the bottom, or the side…that’s no good either.
OK… the reason is simple. The metal contains iron, and as we all know, when iron comes into contact with water, or moist soil, or the moisture in the air all around us, it will start to decay. It forms a brittle, reddish-brown crust, which will only get deeper with time, if it is untreated.
You may know it as …rust.
Note the metal rebar is good for strength and rigidity, if it is in good condition. Rust has no inherent strength, quite the opposite in fact. It has no strength at all. So to prevent the metal turning into rust, we have to keep the moisture away from it. That’s what the concrete does. But if the metal is not completely covered in the concrete, the decay will start, will eat into the metal inside the concrete and eventually, the structure will FAIL.
In order to stop this, the metal part of that reinforced concrete must be completely enclosed in concrete. When casting pre-made pieces such as floor planks, the metal is positioned in the factory, but our on site, for ring beams, floors and columns, it’s down to the building team to ensure the metal is in the right location. The chairs, or ‘rebar support’ move the rebar metal into the centre of the structure being cast, or are placed to push the metal away from the sides of the structure before the concrete is added.
You can buy plastic chairs, or, as most do (if they are clued up and even using chairs – some builders do not!), you can make them yourself. It is basically a small circle of concrete with a couple of wires set into the concrete. Once hardened, the concrete chair can be positioned and the wires used to secure it and prevent movement when the concrete is added.
A simple but very effective and important solution.