[RE-wrenches] Fwd: rebar v/s fiber

[Michael Welch]

From: jfh at greenhousepc.com
Subject: [RE-wrenches] rebar v/s fiber
Date: Mon, 15 Mar 2010 06:26:13 -0700

Travis Creswell writes:
> At least for me, the need for and where to put rebar was easier to make
> sense of when it was explained to me by a very wise basement contractor that
> concrete's strength is compression and weakness is tension.
> 
> That's where the steel rebar comes in as it has excellent resistance to
> stretching.
> 
> For example; engineered prints on basement walls will actually specify the
> rebar placed closer to the living space side of the wall. Imagine the fill
> trying to bow the wall like a banana because the footer is holding the
> bottom of the wall in place while floor of the house is holding the top in
> place. The fill side of the wall is being compressed and open side is being
> stretched.
> 
> Hopefully that will help others as much as it did me.

You can't rely solely on the strength of concrete in compression when deciding whether or not rebar is needed.  When many materials are compressed, they also develop tensile forces in the direction(s) perpendicular to the compression.
The amount of tension is related to Poisson's Ratio, see

http://en.wikipedia.org/wiki/Poisson%27s_ratio

Because concrete has very little strength in tension ("None" is a good value to use -- the real value is about 400-450 psi, compared to values closer to 60,000 psi for structural steels), compression can also cause failure as the concrete is "stretched" perpendicular to the load.  In those situations, the rebar acts to resist "barreling", see

http://en.wikipedia.org/wiki/Compression_strength

Image your concrete is Play-Doh.  If your load can cause the Play-Doh to smoosh out from under the load, you need rebar.
--
Julie Haugh
Senior Design Engineer
greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on
Skype