Are there any rules of thumb for whether a concrete pad needs metal re-inforcement and if so, how much?

Question

I may be doing some concrete work soon around the house and I'm somewhat of a novice at it. One thing that I am unsure about is when I need to reinforce the concrete with re-bar or other metal embedded in the pour.

I used to think it was absolutely essential on just about any concrete job, but have noticed that some people do it and others don't and I'm not sure what aspects of the job determine the need for it.

Can anyone give me some general guidelines about when it is necessary to install metal in a concrete pad and what type/size?

Answer 1

A simple rule of thumb for whether to include rebar or not is based on a 45 degree rule. In simple terms if the foundation is more than twice as wide as it is deep then you will probably need reinforcement, if it is not then you may be able to get away without because there will be little bending, and therefore tension in the concrete.

There are of course a number of complications to this for example you might need to make sure that if the foundation does settle, it settles evenly. In this instance the foundation may need to provide some strength and so will need reinforcement. You will also need to be careful of large plastic cracks forming in the concrete at early age if you don't put reinforcement in it.

Answer 2

There are a lot of different interpretations of a concrete pad. Are you talking about small footing pads under posts, or larger concrete slabs to be used as floors? In general, footings and walls poured into forms don't use any steel rebar in most residential construction. Floors on the other hand most always use a mesh of steel wire (re-bar) through out the pour area. There really is no rule of thumb on when to use rebar, but good advice is to use it in any slab, larger walk ways and driveways. Of course outdoor projects should also employ expansion joints. If you have a specific project in mind, let us know and we can advise you.