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z factor for crack control

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     Wade,
     
     You are right in that the ACI 318 "z" factors of 145 and 175 are not 
     adequate for special exposures.  You need to look at ACI 224R "Control 
     of Cracking".  ACI committee personal in the days of yore were very 
     leery about putting in actual crack width requirements for fear the 
     general public would not understand that concrete by definition will 
     crack so they came up with the non descript factor of "z".  Table 4.1 
     of ACI 224R gives crack width requirements for several different types 
     of exposures.  For "Seawater and seawater spray; wetting and drying" 
     the tolerable crack width is .006 inch which equates to a "z" factor 
     of 66.  
     
     Thomas Hunt
     Fluor Daniel-Irvine


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Subject: z factor for crack control
Author:  JENSEN@dmjm.com_at_-FDInternet at FDINET
Date:    8/8/97 6:02 PM


I have been facing a little dilemma lately regarding the crack control 
provisions in section 10.6 of the ACI code and would appreciate some 
advice...
     
I am designing grade beams and mat slab for a large under/ above ground 
parking structure in Korea.  Local water table conditions are terrible, 
the structure must be designed assuming water to the ground surface or 
even a few meters above at flood stage.  To add to the problems, the 
structure is located in a highly marine environment.
     
Looking at 10.6.5 in the code, I would venture a guess that a z value of 
145 is not sufficient for this situation (Yes, i remembered to reduce the 
value for the slab, for all you nitpickers out there!).  What sort of 
number would be appropriate for this scenario and how can I justify using 
it?  I have also tossed around the idea of simply increasing the cover and 
leaving the z at 145.
     
Any help you can give would be greatly appreciated. 
Thanks in advance.
     
Wade Jensen