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RE: Concrete curing

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Curing is only relevant to drying shrinkage cracks as opposed to plastic shrinkage cracks. I presume then that the focus is on drying shrinkage cracking.

Curing is often ignored. It is a regional and seasonal issue. In the summer in the Phoenix area they will place concrete in the early morning hours so that they will have the initial set and be able to put the plastic on the slab as the morning sun rises. In Denver, they have the added issue of dry climates and winter freezing to cure without freezing the concrete. Alaska has similar issues, but high humidities. In the Midwest, contractors can be lulled into complacency in the fall, but the humidity bottom drops out and the concrete cracks. All of it is predictable.

You can get lower humidities in Florida with a bit of a breeze, then you will get cracks. I like to require a wet cure. Not curing is like Russian Roulette. One of these days it will go off. I don't care to hear how many times a contractor got away with not curing. It only tells me that he is overdue.

The ACI 305-99 does a good job explaining it all. One of the best articles on curing was by Ytterberg of Kalman http://www.kalmanfloor.com/. It is a bit dated, but it shows what causes cracks in concrete slabs on grade. They will send you a copy if requested.

I prefer a wet cure. If a spray on membrane is used (not one of my favorites), they need to apply it in 2 coats with the applicator spraying one coat 90 degrees to the other.

The finer grinds of cement do not make much of a difference unless it increases the heat of hydration. Then the issue is more plastic shrinkage as opposed to drying shrinkage.

Related to this is my tendency to use polycarbonate superplasticizers and a uniform gradation on the coarse aggregates. This is to minimize cement and minimize water. The less paste in a mix, the better. The aggregates do not shrink. Reactive aggregates are another story.

Regards,
Harold Sprague


From: "Craig & April" <csmleko(--nospam--at)bellsouth.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Concrete curing
Date: Wed, 30 Jun 2004 19:00:31 -0400

Anyone with thoughts regarding concrete curing, or lack thereof in today's
building industry? ACI specifies 7 days in "moist" condition unless high
early strength concrete or accelerated curing is used. In S. Florida I find
this requirement to be often if not mostly ignored by both engineers and
contractors alike with respect to residential SOGs and even elevated
structural slabs...unless 90% outdoor humidity qualifies as a "moist
condition." Curing compounds are often avoided because they interfere with
tile mortar bond and it seems that plastic, burlap or sprinklers are too
much trouble. What are the current practices in other parts of the country?
I'm guessing that dryer climates make this much more of an issue.

Craig


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