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Interesting Test Result

#11
(05-06-2011, 05:29 PM)Ernesto Wrote:  
(05-06-2011, 04:41 PM)CC Solutions Wrote:  Hey, when have you ever seen a slab wrapped in plastic? Tongue

Well, about a year or so ago when ...believe it or not...somone was pouring a slab for the local city zoo. I was stunned. Plastic barrier in place ready to recieve mix. Then it had six mil plastic on top of the newly poured one as well! Not many of those around. Certainly not in the residential field.

Ok, that makes sense. All slabs should be constructed like this. They need a vapor retarder under them, then on top they should be wet cured for three days. After that the plastic is removed and the slab can begin drying.

This is how every slab I work on starts its life.
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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#12
It's not really unlike a lightweight concrete poured into a pan.Big Grin
Stephen Perrera dba
Top Floor Installation Co.
http://www.tucsonazflooring.com
http://www.floorsavior.com
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#13
You're so funny.... LOL

The similarity between a pan and a vapor retarder is they restrict drying to one direction only. All the moisture is coming out the top, so we drill our RH probes in to 40% of the depth of the slab.

Lightweight is a whole new beast, and if you ever run across it, plan on having high moisture for a LONG time..... Sad
JD Grafton
Concrete Answers for Flooring Problems
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#14
[quote='eaadams' pid='858' dateline='1304440304']
Just had a testing firm do a slab for us.

CaCl went from 3.7-4.7
rH went from 95.3-99.4 (not rapidrh probes, unfortunately)


The recent projects that I have worked on have ended up with similar results. CaCl Tests in the lower range (2.5 to 4.5) and then rH readings in the high range (85% to 99%). No correlation between the test methods of course and no legitiamate reasoning for the high rH values. Yes they are rapidRH probes.
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#15
CaCl tests are easily fooled by ambient conditions which may be a factor on many test results where the permanent HVAC is not on.

And a slab where the surface is sealed with either an applied sealer or a steel trowel during finishing will have a low CaCl reading while remaining wet for a long time.

And one last thought, the surface of a slab can dry well before the interior of the slab, especially if aggressive drying methods are used. CaCl tests only measure the top 1/4" to 1/2" of the concrete and do not give a realistic indication of the amount of moisture in the concrete.
JD Grafton
Concrete Answers for Flooring Problems
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#16
True. The results I'm speaking of had permanent HVAC on for the last 4 months and any sealer was removed by mechanically grinding prior to testing. The concrete surface was steel troweled during finishing which could provide reasoning.



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#17
Some sealers do a pretty good job of soaking into the surface of the concrete and a light grind will not remove them. Look for the newest revision of the ASTM to mandate some pretty aggressive grinding.

Even a heavy finishing machine with steel pans can create a dense layer on the surface that is 1/8" thick which will impede vapor movement. We have discussed this issue on other forums referring to a study conducted on a warehouse floor than remains open but very wet after 2 years of drying.

But we are getting away from your initial comment Mwoody, that the CaCl reads low and the rH reads high and there is "no legitimate reasoning for the high rH values."

I believe we can find several reasons for the high rH and we also touched on several reasons why the CaCl can read so low. My job is to help figure out what rH readings tell us about slabs, being sure to substantiate the readings we have with field evidence (because any reading is suspect until it makes sense), and help my clients determine slab condition.

If you suspect a problem with the readings you are getting, let's dissect this slab and see what we can learn! This is the fun part!
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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