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RE: Concrete Crack Width

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Roger,

I think that I understand the question now.  Specifically I believe that you are looking at ACI 318 section 10.6.4 and the commentary.  It says that if you have 40 grade rebar, don't even bother with the calcs.  For ACI 318 ("Building Code Requirements") that is enough.  Admittedly 10.6.4 is an oversimplification, but it is good enough for buildings.  The inference is that if you use 40 grade rebar, your service load stresses will be lower and you don't need to check cracking.

That section was written as a result of the move to strength design and 60 grade bar.  In the old ACI 318-63 and ACI 350 days when working stress design and 40 grade rebar was the norm in concrete tank design, cracking was not generally a problem.  It was generally not even calculated.  

As ACI moved to "ultimate strength design", water tank designers followed suit and ran into the problem of excessive cracks.  That gave rise to the Z calculations in ACI 350.  ACI 318 simplified the calculations and basically said don't worry that much if your structure is a building.  ACI 318-63 had basically the same provisions as the current ACI 318.

ACI 318 section 10.6.5 also states that if you have aggressive exposure or need to be water tight, section 10.6.4 is not sufficient.  This is why water containing structures have to go to ACI 350 for Z calculations.

What I tried to illustrate in my calculations and commentary in the last post was when you increase cover, the "d" dimension decreases, the stresses go up, the crack size for a given section increases in size, and you need more rebar to limit the crack size.  I think that you understand this, and the only issue is the credibility of the simplification contained in ACI 318 section 10.6.4.  

I believe that if you look at ACI 350, it will clear things up much better than my humble explanation.

Regards,
Harold Sprague, PE
Krawinkler, Luth & Assoc.

-----Original Message-----
From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
Sent:	Tuesday, July 28, 1998 3:11 PM
To:	seaint(--nospam--at)seaint.org
Subject:	RE: Concrete Crack Width

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.
. > 

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