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Re: crack control

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Roger Turk wrote: 
 
> I have a problem understanding the crack control requirements of the ACI  
code.  I understand the stated theory about the crack control requirements,  
but don't understand why additional cover will require more reinforcing.  If
 
the width of the crack at the surface is the limiting factor, then increased
 
cover should have less effect on the exposure of the reinforcing to
corrosive  
environments...Since the crack control requirements do not apply if Grade 40 
steel is used, that is the maximum amount of reinforcing that I will use in
a 
footing, regardless of whether or not Grade 60 reinforcing is used. < 
 
The crack control formulas are based on limiting crack width at the concrete 
surface.  Greater concrete cover correlates to greater crack width at the 
surface.  Providing closer spaced, smaller diameter bars helps reduce crack 
width.  More steel reduces service load stress, so reduces crack width.  
Design for Grade 40 bars was assumed to provide low enough stress to avoid 
special crack control design.  Crack control for buildings is based both on 
corrosion protection and on aesthetics of cracks (wider cracks are less 
pleasing).  
 
Based on these facts, I would agree that if you use Grade 60 steel but base 
your design on Grade 40 steel, you should not need to calculate crack 
equations.  Also, for water retaining structures, ACI 350R states that 
Z-values were established for cover less than 2-inches and when providing 
concrete cover greater than 2-inches the extra cover should not be used in
the 
Z-equation but should be "regarded as added protection" (i.e., control the 
crack width at 2-inches from the steel and provide additional by judgement).
 
Thus greater cover may provide wider looking cracks but should provide 
adequate corrosion protection.  Hope this helps you.