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RE: Concrete Slabs-on-Grade

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I sometimes think concrete quality has suffered due to an obsession with
saving time. People tend to focus on concrete strength and the demands of
the schedule, and forget some of the side effects of doing things too
quickly. Everybody is in a hurry, but good concrete, like good wine or
bread, takes time. 

People specify higher strength concrete or fatten up the cement factor for
quicker strength gain and "durability", but we pay a price in a hotter mix
that suffers a lot more shrinkage. I find some contractors either don't
appreciate the importance or have the patience to do a good curing job
either. And almost nobody likes joints, although they can mitigate a lot of
the problems you mention.

Another thing I've discovered is that it's hard to get straight Type II
cement. You get Type I/II and they grind it as fine as they can get away
with. The contractors love the rapid strength gain, but you pay for it in
extra heat and cracking.

Personally I think we would be well advised to do away with building code
minimums for concrete strength based on exposure and just leave it up to the
engineer's judgment. Your r/c slabs and walls might be a little thicker, but
you will save cement and have fewer of the problems you mention. Another
thing that would help would be a tighter aggregate gradation spec than ASTM
C33. A more well graded mix would reduce the paste required, allowing less
cement and less water, which would reduce shrinkage. I think most batch
plants would resist this, but it would make a better product.

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Tuesday, July 30, 2002 10:01 AM
To: 'SEAINT'
Subject: Concrete Slabs-on-Grade


It seems today that there are more problems (curling, shrinkage cracks,
etc.) with concrete slabs-on-grade then in years past. Years ago when we
used 3000 psi and few if any additives, there were fewer problems. What has
changed?

Today, we use 4000 psi concrete, more admixtures, faster schedules, less
skilled workers, etc. I seemed to recall that we use 4000 psi in slabs for
more durability but we really don't need much durability in schools,
offices, clinics, etc. I could see using 4000 psi for industy or warehouses
and sidewalks...perhaps 3500 psi for retail. Should we reconsider where we
are using 4000 psi?

Jim K.





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