How To Reinforce A Concrete Bench? Would Steel Expand And Crack The Concrete?

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Steel reinforced concrete has been an industry standard for over a century. Steel - when it rusts - will expand and crack the concrete. To avoid this:

  1. Don’t use rusty steel, and
  2. Place the steel in the mold so it’s at least an three quarters of an inch from the surface on any side.

I sometimes use GFRC., but I’m not a completely trusting soul just yet. Although I have to admit I’ve seen some pretty impressive demonstrations on YouTube. They “may” win me over yet!

Concrete is very strong in compressive force, but very weak in tensive force. You need to look at your item and figure out where the differing pressure will be and then add the reinforcement accordingly. On a bench where the seat is supported at both ends, the compression will be at the top, and the tension at the bottom. Place the rebar in the mold so that it will be near the bottom of the seat when the bench is assembled. Reverse this in the case of an overhang.

To give yourself a visual idea of what’s happening, get yourself a large piece of sponge foam and apply pressure in different ways. Wherever the sponge crushes will be where the compression is being placed. And the air bubbles will stretch when there is tension.

So... what do you think? Please leave me a comment.

3 Comments:

  • Vale: Very helpful! I didn’t know that about rusty steel, there is a house in my neighborhood where the owners must have run out of money because all that’s there is a rusty steel frame. I wonder what they will do when they or someone else has the money to finish the house! (Houses here are made from some type of concrete that is good for earthquakes)
  • BackYardBuilder: It depends on how badly rusted it is. If it’s not too bad, they can possibly sand blast it. But then, given the cost of rebar, it’s probably not worth saving. Especially if it’s really rusty. If it’s an "engineered" foundation or pad, the inspector will likely reject it if he sees too much rust. But then again, some don’t really care and will let it pass anyway.
  • Vale: Interesting... I live in Guatemala, I’m guess it will pass :l
    It’s interesting how they put up the houses in my neighborhood. They use metal molds around the steel frames and then poor the concrete. Every house is exactly the same (although some have the layout reversed) and they all have the same problems because of it...

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