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RE: retarder in slabs on grade

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David,

The retarder delays the initial set, and the finishers can't help but get on
it too soon even if you didn't see the bleed water.  This screws up the top
½" or so to the point that you have no durability.  The top ½" will spall
off due to wear within a year.  It will also dust like crazy.  The only
thing that they should have done once the error was found is to fog the slab
until it took a set.  I would not allow a retarder for flatwork.  If it is
hot and you are trucking a long way, use ice and a polycarbonate super.

I could be wrong, but I'll bet the finish is lousy with a lot of blistering.
Especially if it is a steel troweled smooth finish.

I prefer using polycarbonate super plasticizer which is active until the
concrete stops moving.  From that point in time, the set times are cut in
half.  That means that the finishers aren't standing around fogging and
waiting for the set.  Polycarbonates are added at the plant.

You might want to consider pulling a few cores and having petrographic tests
done.  If they test out O.K., your client is lucky.  But you are probably
looking at blast tracking and topping.

Regards,
Harold Sprague


	-----Original Message-----
	From:	David Handy [SMTP:dhandy(--nospam--at)trg.ca]
	Sent:	Tuesday, July 11, 2000 4:14 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	retarder in slabs on grade

	I am interested to know what everybody thinks about the use of
retarder in concrete slabs on grade. We have recently had a situation where
the supplier added retarder to the mix. Unfortunately he added 7 units
instead of 0.7 units. The resulting slab was similar to pudding with a crust
on top. The surface was unfinishable because it crusted before you could get
on it. Then by the time the rest had set there was no a drop of bleed water
to be found. We are considering eliminating it from the master spec
altogether. In this instance there was no need for retarder as the plant was
within a half hour from the site and we have not had any really hot weather
yet this year. 
	Any opinions as to the need for this. 
	BTW the mix had a slump of 1 ½" to 2" arriving on site at which time
super-p was added to bring slump to pumpable range at 4" to 5". This was not
my spec but was the owner's. They have used it a lot with great success.
	David Handy, P.Eng.
	dhandy(--nospam--at)trg.ca <mailto:dhandy(--nospam--at)trg.ca> 
	The Thompson Rosemount Group, Cornwall, Ont. Canada Opinions
expressed are personal only.


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