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Re: Misc Comments on Concrete Discoloration

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Thanks you for another great post. 
I have a question regarding discoloration of concrete along/at cracks in SOG (say, thru-thickness cracks). Concrete along the crack edges is often discolored (usually, it is darker than the surrounding slab) - in your opinion, how long does it take for that to happen?
Thank you.
Steve Gordin    
----- Original Message -----
Sent: Friday, January 28, 2005 8:11 AM
Subject: Misc Comments on Concrete Discoloration

Surface Discoloration on Concrete Slabs
Surface discoloration can appear as light and dark blotches, or as color changes over large areas. Although discoloration is typically a cosmetic problem, if it is caused by poor finishing practices, the serviceability of the slab may be impaired.  One of the most common causes of discoloration is variation in the w/cm ratio of the concrete. A high w/cm ratio will usually produce a light-colored concrete; a low w/cm ratio will produce a darker color. The difference in color is a function of the hydration of the ferrite (iron) compounds in the cement.  Ferrite compounds become lighter as they hydrate; if the w/cm ratio is low, some of the ferrite may remain unhydrated.  <...>