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Nothing but high RH
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04-30-2016, 06:55 AM #1
FM4 Newbie *
Status: Offline Posts:1 Threads:1 Joined:Apr 2016
[First time poster trying to get some ideas of RH in Louisiana slabs]
We are a commercial flooring contractor and 9 out of 10 projects we RH test (using RapidRH 4.0 EX), we get high RH readings (99%). It seems not to matter if the slab is 4",5",6", 9" etc. Not to matter if the slab is 6 months old, 5 years old or 50 years old. Not to matter if it's new construction, renovation or even a tenant build-out that has been sitting empty for 1-5 years. We install hundreds of thousands of squarefeet of flooring every year and therefore have been using hundreds of RH probes. Anhydrous Calcium Chloride (ACC) used to be our primary method of testing, but because RH is being spec'ed more by design firms and many flooring manufacture's state that RH is the qualifying standard, we have swapped to RH testing being our primary test method. But we are really questioning the validity of these tests. Or how high RH% would correlate to a failure. And this is what I mean; in most cases we still perform some ACC testing. When doing so, ACC results are almost always acceptable and RH is not. (I understand that ACC & RH are measuring two different things). But just consider this discrepancy as I explain, I'm sure that in years past when ACC testing was the only test we did, high RH was likely to be present (it had to be, its present on almost every job we test now). With ACC results being within tolerance, we would install flooring. And how many jobs had issues due to moisture? maybe 5 jobs had issues over the past 5 years and all were isolated incidents with reasonable explanation (none were large scope issues, all were isolated). In addition to this, over the past 18 months, we have completed a (small) handful of projects that had high RH, where the contractor, owner, & architect waived warranty rights because mitigation could not be afforded and no issues have surfaced yet. I know research documents claim it so but does high RH indicate likely issues????

We follow the wagner installation instructions to the "T", we have spoken with wagner tech to confirm installation instructions, we've viewed the training videos that wagner has online, and we've spoken with tech reps from the flooring vendor/distributor that sells wagner (which they claim, lots of flooring contractors, at least the ones that test, are complaining about the constant high RH %) and it always seems to result in high RH.

How often do you guys see projects resulting in 85% or less RH?
Is there any common denominators that you see on jobs with 85% or less RH?
Any thoughts?

05-02-2016, 10:58 AM #2
WagnerMeters Newbie *
Status: Offline Posts:1 Threads:0 Joined:May 2016
Thank you for your post.  Every year, hundreds-of millions-of-dollars in floor failures and associated costs occur.  The industry has come to accept in-situ RH testing as a better and more accurate approach to as a means of minimizing these costs, and manufacturers understand the relationship between high RH conditions and flooring/adhesive failures.  The RH test method has become popular for these reasons, and the fact that the industry as a whole believes it is a much better predictor of the success or failure of the flooring/adhesive than calcium chloride or other test methods.

Next, we want to quickly answer your first question that you posed at the end of your post, and assure you that it is very common and routine to see RH test levels below 85%, and even lower.  There are very  valid reasons why you may not be experiencing this, and we trust  that we can provide some answers for you.

Let’s first discuss new construction (i.e., new slabs, not renovation applications).  The actual drying rate of concrete slabs is still often misunderstood.  Drying rates, and the factors that affect them have been well-studied over the years, but again, the misconceptions in this area still exist.  As a general, rule-of-thumb, we can use 30 days of drying time for each inch thickness of concrete for a slab to dry to 85% RH.  Again, this is just a rough guideline, and there are many factors that can negatively impact this drying rate.  For example, this guideline assumes that the original water/cement (w/c) ratio is normal (not high; typically below .50).  The drying rate guideline also assumes ‘good’ drying conditions early on in the process (often, this just does not happen).  Of course, both of these factors, especially the second one are often not in practice on jobs, therefore leading to slower drying rates, and the associated frustration of a project delay.

Other factors that are often in play that can negatively impact drying rates are the use of curing compounds that don’t get removed, heavy power-troweling (and associated densification) of the slabs, added water of convenience during the finishing process, or occurrences  of inconvenient rain storms on slabs.  

Additional water or rain, even after a slab has gone through the initial 30 days of hydration (curing) can have a very adverse effect on a slab’s drying time.
In short, there are many factors that can slow things down in the drying process, and therefore, rather than a ‘normal’ drying rate, we unfortunately end up with something much slower.

Lastly, let’s briefly review renovation applications.  Yes, even years-old slabs can exhibit high moisture (RH) conditions.  Causes vary, but old slabs with no vapor barrier immediately underneath, and an old slab that was covered with a flooring too soon after the pour that locked in the high moisture (RH) condition.  NOTE:  Old, VOC-laden adhesives were more resistant to failure, so a failure may not have been exhibited, but new flooring and adhesives may fail.
Without a true vapor barrier that isolates the slab from the ground, aggregate or sand, the moisture condition immediately underneath the slab is always an issue.  It is very common to see the lack of a vapor barrier or adequate vapor barrier in older slabs.  Therefore, high moisture contents in older slabs are very common.

Wagner Meters
The Moisture Measurement Leader
326 Pine Grove Road
Rogue River,Oregon 97537
Email: info@wagnermeters.com
Phone: 844-854-2463

05-02-2016, 04:23 PM #3
eaadams Concrete Moisture Expert *****
Status: Offline Posts:600 Threads:88 Joined:Jul 2010
FM4 - I'll tell you we rarely see Rapid RH tests under 85%.  But our flooring has a 98% limit.  We almost never get probes reading under 85% even on slabs that might be fully conditioned for months.  I was at an ICRI meeting last week where +/- the tone of the meeting is all the concrete moisture testing is BS. The meeting was more on how to evaluate epoxy coating products than to produce low RH concrete.  Generally people were saying that they think there are a lot of false positives.  Slabs look wet and aren't.  I even heard one person say "both the MVER and RH test are BS" 

One of the key questions came from someone who works for IFTI (a group that tests slabs professionally for the likes of wallmart and others)  and they were asking for how do flooring manufacturers determine RH limit.  They don't.  They figure out where things work from trial and error.  A lot of pushing to a RH % and seeing if they get failures.  

Personally I think it has a lot to do with differential RH.  A lot of the slides at the meeting talked about static pressure (the air pressure where RH & Temp combine) vs pressure in the slab (RH & Temp in the slab).  Perstonally I see a LOT of failures in buildings in California where we get a big variation between the two.  Buildings with swamp coolers, buildings that get up to 105F and DRY vs a very wet ground.  I personally think someday they will find that there is a relationship between the ambient air and the slab concrete.  Keeping the two in sync will be important.  We just are not there yet. 

Anyway, my answer to you is I bet it has a lot to do with where you are and what sort of use those buildings get.  Remember these RH requirements are to protect against WORST CASE scenarios.  Just count yourself as lucky you've never had that happen.

05-04-2016, 08:06 PM #4
CC Solutions Concrete Moisture Evangelist *******
Status: Offline Posts:1,067 Threads:69 Joined:Dec 2009
Hi FM4, and welcome.
I understand your frustration but can assure you many slabs are under 85%, in fact I was personally responsible for getting slabs my company built to less than 75% within 5 months after pouring. It is possible.
The moisture within a concrete slab indicates the potential a slab has for failure. The likelihood that a moisture induced problem will occur when the slab RH is 75% is nearly nil, whereas the probability of a moisture related failure happening with a slab at 99%RH is very high. As we install, test and evaluate moisture sensitive flooring on various slabs our knowledge grows and we make changes to our recommendations.

Common denominators on low RH slabs are:
Proper and functioning vapor retarder placed directly beneath the slab.
Steel trowel/closed surface is not specified. The slab is finished with combo blades at most.
No sealant applied to the slab. Water cure for three days then begin drying.
Keep water off the slab. A day of rewetting can set the slab back a month of drying.
Ambient conditions conducive to drying. Low RH, air movement and warm temps all help dry the concrete.
Proper mix design with a low W/C ratio, and a low cement content. More cement means more water.

I'm interested why you are getting so many high readings, especially since you are doing all you can to assure proper installation of the probes.

JD Grafton
Concrete Answers for Flooring Problems

05-04-2016, 08:39 PM #5
eaadams Concrete Moisture Expert *****
Status: Offline Posts:600 Threads:88 Joined:Jul 2010
CC Solutions

What are combo blades? 
- I'd also add careful aggregate selection.  Some aggregate will retain more water than others.  Many don't believe it so I use the example of an opal.  What gives an opal the colorful look is the moisture in it.  Put an opal in an oven and it looses the luster.  Why? because there isn't water in it anymore. 

I'm also interested in the slab designs that work/dont work.  

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