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RE: Concrete Crack Width

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Harold,

I understand the theory behind the Gergely-Lutz equation.  The Gergely-Lutz 
(crack control) formula is based on a certain width of crack at the surface 
of the concrete, being larger for interior exposure and smaller for exterior 
exposure.  

What I can't understand is, if a crack at the surface is limited to a certain 
size, as represented by z in the ACI code, why adding more cover to the 
reinforcing makes you put more reinforcing in, thereby reducing the stress in 
the reinforcing, which makes the crack smaller, when with fewer Grade 40 bars 
at a higher stress level, crack control and corrosion is not a concern.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Harold Sprague wrote:

. > Roger,
. > 
. > The object is to limit the crack width at the surface.  After a crack 
. > reaches a certain size (which is the implied limit of "Z") it is 
. > considered not acceptable.  The maximum allowable crack width considered 
. > is in the elastic range of the section whether it is designed with 40 
. > grade, 60 grade, ultimate or working strength.
. >