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Reverse Curl & Relaxation
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10-03-2012, 11:36 AM #1
eaadams Concrete Moisture Expert *****
Status: Offline Posts:587 Threads:83 Joined:Jul 2010
Concrete can curl as it drys. This is caused by the differential in drying rates for different parts of the panel.

Scott Tarr did a great article in Concrete Construction last month on curling http://www.concreteconstruction.net/industrial-projects/demystifying-concrete-shrinkage.aspx

I asked a question of CC via Twitter and got an amazing response.

My question was about how to predict reverse curl in a slab based on the RH of the slab. Reverse curl is what happens when a slab 'relaxes' after you put down flooring. It used to be that the flooring would come up with relaxation as a moisture issue as the sub slab moisture would come to the surface and get the slab to 'relax'. Here is the best article so far on the subject: http://www.ctlgroup.com/insights/detail/19

However, new adhesives are holding floors down. But, we still get this reverse curl and it creates what I call a 'mole line' along joints where patching compound comes back up. If you look at the CTL article you will see the result on a gym floor on page 3 Fig. 9.

So, my hope is that with good RH testing I could predict this and avoid it on wet slabs. The answer I got back from Scott Tarr was that that internal RH must be consistent (whether high or low) between the top and bottom of the slab to be assured that "reverse curling" won't occur once flooring is installed.

I am encouraging the editor from CC to print his response as it is an unknown thing in the flooring / concrete world on 'how to do it'. I don't know if anyone @ Wagner knows how to test the RH as a profile on a slab but I know you know the same people ... might we worth weighing in.

10-03-2012, 07:45 PM #2
rapidrhrep Concrete Moisture Tutor **
Status: Offline Posts:78 Threads:3 Joined:Aug 2011
EA: If I am seeing the question correctly, you want to know how to profile a floor, utilizing RH testing, correct? If this is the case, based on the science, people will typically install the sensors at varying depths in the floor, say at 1" depth variances. Of course this is done prior to flooring installation and this will allow you to see the moisture gradient in the slab.


10-04-2012, 10:21 AM #3
eaadams Concrete Moisture Expert *****
Status: Offline Posts:587 Threads:83 Joined:Jul 2010
Yea I was just giving a heads up.

10-07-2012, 07:11 PM #4
CC Solutions Concrete Moisture Evangelist *******
Status: Offline Posts:1,065 Threads:69 Joined:Dec 2009
A few other things to consider:

The top of the slab will be drier than the bottom (as long as the slab is drying and has a proper vapor retarder) until the slab reaches equilibrium with its environment.

If there is no proper vapor retarder and the slab is on grade, the slab will absorb moisture from the soil and will not remain at a constant RH.

Realistically, a slab's RH gradient will not remain consistent.

JD Grafton
Concrete Answers for Flooring Problems

10-02-2013, 12:58 PM #5
eaadams Concrete Moisture Expert *****
Status: Offline Posts:587 Threads:83 Joined:Jul 2010
Might there be a new product idea here? Could wagner make a RH profile testing stick that could be placed in a single hole and test the RH profile or gradient of the floor?

The cost of doing five tests on each slab at different depths adds a lot of cost to (for me) a 6000 sf gym with 5" slab. I already have to pay for 6 probes. To do another 5 more probes further increases the cost by more than $250. A re-usable stick in a 5" hole might be a better product.

OR must the tests be exposed to the bottom of a hole to get a good RH reading?

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