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RE: Reasonable Crack Width in a Concrete Slab

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Larry,
 
Wetting and drying can be from any source.   In your particular application, it can be from weather or in a tidal zone.  It can even be in an arid area where the marine layer that often forms in the evening can carry minerals  and moisture to concrete from the adjacent sea water.   California and Florida have many of these areas where severe exposures exist even though the structure is not directly in the tidal area. 
 
That said, most of what is contained in ACI 224 is primarily intended for protection of embedded reinforcing steel.  If you have a topping on a slab on grade, the risk due to the corrosion of steel is minimal since there is little if any reinforcing steel in a slab on grade, and that steel is not structural in nature.  Thus the question is, what are the implications of a crack in a topping slab.  Further, you need to evaluate the time that it took the cracks to grow to the size they currently are and predict the maximum size of the cracks in the future.  There is an article, Shrinkage, Cracking and Deflection-the Serviceability of Concrete Structures by R.I. Gilbert that accounts for the time versus the percentage of shrink potential for concrete.  Mark Fintel's "Handbook of Concrete Engineering" also contains a graph of time versus concrete shrinkage potential that is a good reference.  
 
Regards, Harold Sprague
 

Date: Wed, 17 Aug 2011 17:05:03 -0600
Subject: Reasonable Crack Width in a Concrete Slab
From: larryr31(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org

Our office is involved in a project in which shrinkage cracks, varying in width from 0.003” to 0.009,” have developed in a new 3” exposed-aggregate topping slab.  The topping slab was placed over an existing, exterior structural slab.

This project is located in a mid-Atlantic state and the slab is adjacent to a seawall.  This seawall forms the edge of plaza area that is adjacent to a pool of brackish water.   The plaza is not subject to overtopping from the brackish water.

We are trying to determine which exposure condition from ACI 224, Table 4.1 is applicable to our situation, so we can determine which cracked sections need repair/replacement.  We have narrowed it down to the following:

1)      Humidity, moist air, soil - reasonable crack width 0.012”

2)      Seawater and seawater spray, wetting and drying – reasonable crack width 0.006”

I was not able to find any information in ACI 224 or from other sources that would provide additional insight on how the exposure conditions are defined.  For example, does wetting and drying mean wetting and drying in the tidal zone, or wetting and drying due to weather?  Or both?

We having a robust debate here in the office as to which exposure condition applies.  My personal opinion is that our project falls somewhere between the two exposure conditions described above.

Does anyone have any insight to offer?

Thanks,

Larry Reynolds, P.E.