So, let's finish-up with some real-world testing-issues and questions that we ran across weekly, where people in the field, doing relative humidity testing, very critical, and they have a lot of questions and comments. And again, we hear many of them every week.
So, let's talk about one of the major obstacles, or hurdles in construction these days, which causes, and some misperceptions, by the way, that really cause some headaches in the field.
Fast-track construction, well that's a good thing, but, the problem is, people, owners, general contractors are often in a big hurry. They don't want to believe that the slab is not ready to put some kind of a floor covering or coating down. They want to move along, get the building open, and allow the owners to start generating revenue from whatever business there going to be in that building. The problem is, what's very misunderstood, but, well documented and scientifically is that, concrete does not dry that fast.
For example, I've had calls from flooring contractors, stating to me that they had a job, they need to get a floor covering or coating down, and my, almost my first question is, "How long is that slab been down?" I've heard some, almost ridiculous answers to the tune of, "Weeks"; 6 weeks, 8 weeks, and they want to know if that slab is ready, or the owners pushing them to put something down. Just the drying rate of concrete, just the drying rate of concrete alone tells you that, that slab is not ready.
For example, a rule of thumb with concrete drying is approximately 30 days for each inch of thickness of that concrete slab, just a rule of thumb. And that's under pretty optimum drying conditions. But often, very often, in fact probably most of the time, those drying conditions in that building are not optimum. The building has been opened. It may have been opened for months. They have only had the climate controls turned on in the last two, or three weeks and they want to go ahead and begin putting the floor covering or coating down. They're getting relative humidity readings with a very rapid and very accurate Rapid RH method in the high 80's and 90's, and the reaction is from the owner, or general contractor often, or even if the installer themselves, is that, there's no way that the slab can be that wet. But the answer is, yes it can.
Because it takes a long time for a slab to dry and a lot longer when it doesn't have ideal conditions. And lightweight aggregate concrete, this concrete is often used on second and third floor, not slabs on grade. Lightweight aggregate concrete can take two, to three times longer than regular aggregate concrete to dry. So, it can take many months for that slab to dry-out. If the surface is a hard-rolled surface, it slows it down. If the water-cement ratio was pretty high when that concrete went in, that would mean the concrete would take longer to dry. There are number of factors here. And the relative humidity testing is just telling them what they should already know.
So, let's look at another field example that we get in question, and that is, "What levels of relative humidity are common?" "What levels of relative humidity are okay?" "Levels of temperature", "ambient conditions" and things in this nature.
First of all, the correct maximum relative humidity levels vary from product to product, from adhesive to adhesive, whether it's epoxy, a rubber floor or what have you. And that can range, quite a range actually anywhere from maybe 75% up into the mid 90's, depending on what it is.
Another example would be, a lot of sheet goods, vinyl, composition tile, products such as these and the associated adhesives, 75% to 80% is a very common number, but the manufacturers set that guideline. Most of the major manufacturers of resilient flooring and in the wood side and some of the other sectors know what that value is. They've set those guidelines for relative humidity.
And as we saw earlier in Howard's presentation, there is no correlation between relative humidity testing down in the slab and calcium chloride testing. That's why they have a separate spec for relative humidity. That's very important. And so, it is a range. There are some moisture control or moisture mitigation products, tappable products that go on a concrete slab that can stand very high relative humidities. But again, (it's), you need to rely on the manufacturers; it's common upon the manufacturer to set those guidelines.
Now what about ambient conditions?
They vary. We saw earlier in Howard's presentation that ambient conditions have, especially temperature, has very little effect on relative humidity readings down in the slab. It can have a huge impact on calcium chloride reading; so, they are very affected by ambient conditions. And so, those ambient conditions can range quite a bit. Now, concrete slab temperatures, typically normal temperatures will run in the mid 50's to the mid 60's. Sometimes a little higher, sometimes just a little bit lower. But, that's a pretty common range for slab temperatures when we're measuring relative humidity.
And again, talking about, so called "safe relative humidity readings", those will vary depending on the product that you're going to use, so you need to refer to the manufacturer's guidelines when you're doing this.
Let's just step back a minute, and touch on another topic related to the fast-track construction situation. Often, what I've seen is, the general contractor, or the owner, not liking what they're seeing with accurate relative humidity readings with the Rapid RH and we've already seen why it is very accurate in doing this job according to ASTM F2170. So, what will happen is, they would want to call-in a third party to do the so called independent testing to try and determine, do the readings square with other relative humidity testing that they want to have done.
Again, let's go back to a problem that you can see with sleeve methods with probes that may not be verified for calibration with not going to the correct depth. What we have often seen, you may have readings with the Rapid RH with the mid 80's to 90 range, a third party will come in, not knowing how to do, correctly do the sleeve and external probe method; they have not had their probes calibrated or verified for calibration; they may be contaminated. And often they are, and they're going to read down scale, they'll come in with very low readings, and I'll give you a good example of this, a slab that's only been down two months has not had ideal drying conditions and they may be getting readings in the mid 70's. It's not even possible. So, without doing any relative humidity testing, you know that, that concrete slab has to be in the high 80's to 90. Yet, they're getting readings that are very, very dry and appeared to be okay, but they're not okay.
Because of, again, any number of reasons, either the probes have been contaminated, they're reading down scale, the sleeves are not tight in the hole, we're getting contamination from ambient air, or just the sleeve design itself is flood, so we're getting relative humidity readings from different gradients in the slab, and that is not correct, and we've seen that earlier.
So, sloppy testing methodology or non-calibrated probes can cause this problem. But, with the Rapid RH, only reading at 40% of depth, having NIST traceable calibration, for the Rapid RH smart sensors, allows you to easily meet ASTM F2170 and not be concerned about possible under-estimation of the relative humidity in a concrete slab.