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RE: Concrete curing

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> On a side note, many 'old timers' I spoke to in Australia seem to mention a common theme wrt cracking, in that it was occurring much earlier
> than in years gone by. More often than not, shrinkage cracking was occuring within the first 2 days in recent times whereas these older guys
> recall that shrinkage cracking wasn't a problem until about a week after the pour. One suspect is the cement manufactures are grinding the
> cement to much finer particles, thus increasing the water demand for the same wc ratio. Any thoughts/comments on this. 
> derek

Hi Derek,
Cracking in the first day or two is sometimes "plastic shrinkage cracking", which 
refers to cracking that occurs while the concrete is still pretty wet (hence "plastic"), 
and which is usually the result of very rapid drying.  Theoretically, it can be reduced 
by doing a better job curing in the early stages.  The key is that it's about moisture 
and curing.  After that, the cracking due to early thermal contraction of hardened 
concrete can be the result of too much heat generation and too rapid cooling.  It's 
more about temperature, and can be addressed (with a little luck) through 
manipulation of the heat and cooling conditions (such as by providing insulation -- 
I'm sure your contractors will be thrilled at the suggestion of insulating their slabs 
during curing).
The cracks theoretically look different, too.  Plastic shrinkage cracks are typically 
short horizontal cracks that are sometimes parallel to each other (especially when 
they occur over rebar in slabs) and sometimes more random, but usually short.  
They seem to be fairly evenly distributed.  Thermal cracks are often fewer, longer, 
and tend to occur at the edges and at areas restrained from movement.  
I highly recommend the book "Concrete: Structure, Properties And Materials" by P. 
Kumar Mehta and Paolo J. M. Monteiro (incidentally both of them excellent 
professors at UC Berkeley).  I was a teaching assistant for Professor Mehta's class 
back in the early 90s (wow -- can I be that...seasoned?), and I still use the book 
regularly for reference.
So the real question may be...have the nights gotten cooler down under?  :)
Jon Brody, SE

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