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RE: More Concrete

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Farzin--knowing that you are placing pile caps, grade beams, and slab
(13 -14 in thick!) as a monolithic placement, just a few cents more of
my jaded opinions----

1. You'll have a potential for shrinkage cracking since the deeper
sections (pile caps, grade beams) will serve as "keys" that may restrain
shrinkage, especially for cement-rich mixes (5000 psi, pump mix?).
Curing is the answer......but be prepared for some level of cracking
that the reinforcing may not be able to hold closed. I am guessing the
prime places are in  the slabs, at mid-span between whatever is serving
as a "key"....
2. Harold is right on--controlling elevation, flatness and levelness is
going to be tough without something like a "copperhead"--it is tough to
strike off to a good level and flatness when the flatwork is that thick
and varying in thickness (again, the caps and GB's)---we have used grade
stakes, screed rails, wet pads, copperheads with lasers, and regardless
of method it is tricky. Getting Ff/Fl is an art for this type of slab.
3. Cool=good for delivery without short-batching; but again, that volume
means a lot of timing that must be controlled for batching, transit,
delivery, trimming loads, and pumping....
4. Rain? We would call that a "wet cure" (yes, we are being sarcastic)
and at that thickness, it may take a while to bleed out the concrete to
be ready to finish. If you can trowel it out to close the surface tight
before the rain, that is great but if you get rain prior, you may have a
finish issue. When this happens in our work, we try to cover it even if
it is green and at least be ready so when the rain passes, we can
squeegee the material off and finish it as best we can to the point
where we can come back later and grind/polish the floor to provide a
reasonable facsimile to a trowelled finish. That is costly too. Here is
a trick that has worked well for us on thick placements--have a few 5
gallon pails ready and if it does look like rain or it starts to rain,
dig out some plastic concrete if possible and set the pails in the pour;
if the reinforcing is in the way, cut tie wire and shove it around to
get at least 6 inches into the pour, trim it off flush with or just
below the slab with a utility knife, smooth out the concrete around the
pail and presto, you have an instant sump to which you can squeegee or
broom the water even if the surface is not fully set, and from there you
can pump it with a small sump pump. Beats trying to chase the water 60
feet with a broom to get it to the edge......

Again, best of luck to you in this endeavor!! I do not post often but I
love this kind of stuff.

Richard W. Stone, P.E.
Madison Concrete Construction


To: seaint(--nospam--at)seaint.org
Subject: Re: More Concrete
Date: Mon=2C 20 Dec 2010 12:27:39 -0500
From: fsrahbar(--nospam--at)aol.com


Rick:
=20
You must have done this a lot. Thank you for the advice.
=20
One thing that slightly work to our advantage is the fact that the area
of =
pour is much less than 20=2C000 sq. ft. we are pouring the slab=2C the
grad=
e beams=2C pile caps. The slab is an average of 13" to 14" thick. They
are =
planning to pour this in the first week of January. The weather is going
to=
 be cool=2C however=2C we do have a concern with rain!

=20
Thank you=2C

Farzin S. Rahbar=2C SE
Vice President
David C. Weiss Structural Engineer & Associates=2C Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041



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