Deficiencies in Moisture Testing - Fact, Fiction or Simple Misinterpretation.

I was going through some past information in my library, trying to organize the data from print to digital and found myself side-tracked yet again by some of the absolute rubbish involving moisture testing and claimed "deficiencies".

It bears repeating that most current test methods are NOT "deficient". What is assumed to be a "deficiency" is actually the person(s) claiming the procedure(s) to be deficient, are in fact lacking an understanding of what the various methods actually measure and what, why and how composition of the concrete and site conditions, as well as preparatory methods can and WILL affect the collected information.

Concrete Composition - How this Affects Moisture Measurements

Concrete is an amalgam of various components, with most of these components having varying levels of hydrophilic, hygroscopic and even deliquescent properties. No two concrete substrates are alike, no matter how closely controlled the concrete materials, placement and quality control is done.

These materials within the concrete will determine the measurable moisture in either liquid or vapor form of the concrete. The concrete substrate itself responds differently from surface to depth. In a 1974 study by the Army Corps of Engineers, to gauge concrete response to elevated temperatures of 150-160oF (65.5oC-71.1oC) on concrete, it was discovered that throughout the test, the interior moisture content remained constant.

There are several reasons for this, which have even confused those conducting university studies. Moisture migration is from warm to cool, disrupting this tendency are the hygroscopic and hydrophilic materials within concrete that have a strong affinity for moisture and WILL reduce free evaporation of water and its migration from warm to cool. This affinity for moisture, particularly with alkaline salts consisting of sodium and potassium tends to increase with temperature, and as moisture is slowly evaporated away, these salts continue to reduce the free evaporation which equates to a lower measurable RH.

This lower RH is an indication that the affinity for moisture is such that the continued lowering of evaporation increases the affinity for this moisture, which can be confirmed by the "critical humidity threshold" of potassium and sodium hydroxide, which is less than 9% RH.

This effectively "blocks" further removal and the more moisture withdrawn with decreasing humidity, the more the "rebound" effect occurs once the drying process stops (this has been discovered consistently with restorative drying of flooded concrete materials).

This "rebound" is the concrete attempting to reach equilibrium for the "uncorrected" ambient environment once the drying efforts have ceased.

The chemistry involved is CONSISTENTLY ignored in virtually every research study I've read, and I have no idea why this critical aspect is consistently omitted.

Without at least SOME attention given to the chemistry, an accurate conclusion is nearly impossible.

Here is where it gets really tricky and why this becomes so important; when sodium and potassium hydroxide becomes concentrated, the freezing point of the water within the salt solution changes dramatically with concentration, and NOT in a linear fashion.

A State of the Art study on concrete was released in 1999. Even then it was recognized that alkalinity created complexities, but stopped well short of just how complicated this was by stating alkalinity can lower the freezing point of the water in solution. This statement is accurate in low concentration levels and assumes concentration levels will not exceed 20%.

At 20%, sodium hydroxide will reduce the freezing point of water to -14oF (-25.5oC). However, as sodium hydroxide becomes more concentrated, the freezing point of water heads in the opposite direction and increases.

At 40% concentration, the chemically induced freezing point of water is now at 59oF (15oC) and at a concentration of 60% the chemically induced freezing point of water is now at 121oF (49.4oC) and at a concentration of 77%, the chemically induced freezing point is 160oF (71.1oC).

This single influence easily explains WHY the moisture within the concrete remained constant in the Army Corps of Engineer's Study. If the alkalinity became highly concentrated, it behaves more like a solid (ice) than a liquid, resisting removal, all the way to where a steam expansion can occur, damaging the concrete.

Hygroscopic materials, particularly alkalinity can dramatically alter the "normal" characteristics of moisture. Since these elements change the characteristics of moisture, this also changes the moisture that can be detected by the various moisture testing methods.

Moisture Testing - Competing Forces

Calcium chloride is often used as an effective desiccant to reduce RH as it actively absorbs moisture. Calcium chloride is used in at least two different ASTM procedures (ASTM F 1869 and one of two methods used in ASTM E 96).

Calcium chloride has a critical humidity threshold of approximately 28%RH, meaning that moisture in a humidity of 28% or higher will be absorbed by the calcium chloride and moisture in humidity lower than 28% will NOT be absorbed by the calcium chloride. Here is where some of the assumptions by "experts" citing "deficiencies" in the ASTM F 1869 reveal a lack of understanding of the hygroscopic elements, particularly the alkalinity within concrete.

Most of the alkaline components of concrete have a lower critical humidity threshold than calcium chloride, which effectively limits the volume of moisture that can be extracted by calcium chloride within the maximum 72 hour window of the ASTM F 1869 procedure.

Even hygroscopic aggregate will resist free evaporation, not stopping, but slowing the rate of moisture that can be absorbed by the calcium chloride within the window of time required by the ASTM F 1869 procedure.

This is not a deficiency as was stated by an overly exuberant proponent of RH Probes, but an accurate assessment of the "extractable" moisture from the concrete surface.

Interestingly, the same "deficiency" cited by the RH Probe proponent is even more pronounced when using RH sensors.

RH Sensors, unlike calcium chloride, are "passive", in that the probes do not attempt to attract/absorb moisture, these simply measure the available water vapor within whatever open space is available to the sensor.

In a very real sense, RH Probes or any other RH device does not and cannot measure the moisture content of concrete, but rather measure moisture present in any void space within the concrete, leaving any liquid moisture in capillaries, etc. as undetectable moisture.

In a rather surprising result indicated in a Finnish Study Conducted by VTT in 2010, even the calcium carbide method was found to be affected by site conditions where the concrete chemistry changed dramatically as compared to the laboratory conditions, and even within 14 weeks, the calcium carbide method could no longer correlate with the gravimetric method used as a control.

However, if one includes the effect of alkalinity, the amount of "free water" needed for the calcium carbide test can be altered as the concrete remains exposed to ambient conditions, this difference makes sense.

Moisture Testing - Interpolation of Results

If moisture content is the goal, so far only the Tramex Concrete Meter has proven to be precise in its measurement of concrete moisture content.

However, when assessing a site condition where a flooring or coating material has failed or is experiencing problems that are "moisture-related", I find that using a RH Probe, along with the Tramex Concrete Meter and Determinator Probe to be essential to give some insight as to the condition of the concrete.

As an example (particularly in the desert), I have found combinations of a very low RH reading and a high Tramex reading to be indicative of a problematically alkaline surface. This is especially true with older concrete where a former coating/flooring was removed, only to find consistent problems with the replacement coating/flooring. NOTE: Considering it is well established and even cited by several humidity probe studies that the humidity in the top one inch of concrete can be considerably lower than the current 2 inch depth required within F 2170; it would be VERY beneficial for the one inch depth to be included in these assessments.

Second Law of Thermodynamics and Moisture Testing

Moisture migration is taught by virtually every school and Certification body that moisture migrates from the bottom of the slab to the surface of the slab. In the majority of instances, this violates the second law of thermodynamics since the bottom of a slab is typically cooler than the surface of the slab.

Moisture will instead migrate from the warmer surface towards the cooler interior of the concrete, NOT the other way around. This has been in evidence with older concrete that has been coated or covered with a flooring material.

Diffusion, Absorption and Failures

What is commonly referred to as moisture migration is actually diffusion and absorption within the top surface of concrete since the soluble chemicals attempt to reach equilibrium and the higher average temperatures at the surface increases the solubility and chemical activity of most chemicals (calcium hydroxide being a notable exception).

Once the coating/flooring is removed, the concrete surface is exposed to a new moisture source, the ambient conditions. There was a rather dramatic example of this earlier this year where a demonstration was being conducted on the proper used of the Tramex Concrete Meter on a 40+ year old concrete that apparently had a clear coating/curing compound.

The initial reading over the coating showed a very low 1.9% moisture content. Immediately after removing the coating, the reading remained at 1.9%.

However, as the demonstration continued and they returned to this area estimated to be between 10-15 minutes later, the moisture content had increased dramatically to 4.3%.

It is customary and even recommended that moisture tests be conducted on a surface where a flooring or coating has been removed several hours later (to allow the surface to vent....a myth I was taught and previously subscribed to) and THEN test for moisture.

If that procedure had been used, the true baseline moisture level would NOT have been established and likely blamed on moisture "originating" from the concrete.

In reality, the moisture simply followed the second law of thermodynamics, assisted by the presence of hygroscopic materials in the concrete surface, where the concrete surface was acting similarly to calcium chloride, absorbing the available moisture, but this time, in reverse!

What is it we WANT to Measure?

Generally speaking, moisture alone isn't much of a problem once a coating or adhesive is set and cured.

Moisture can be a problem, if moisture is on or immediately beneath the surface of concrete intended for installation a flooring or coating material.

Paying attention to the second law of thermodynamics, we have a right to be cautious, but the "training" we've been getting not only ignores these facts, but distracts us from properly measuring and assessing any given site condition.

I do not disagree with what the flooring and coating manufacturers intend, but I do disagree with past and present procedures and execution.

That being said, irrespective of WHAT moisture test method is used, installers will NOT reduce moisture-related claims until that installer uses "At Time of Installation" testing.

NOTE: Most tests and procedures state testing is only good at time of testing.

With Moisture Testing - Timing is EVERYTHING!

Simply look at that example of the 40+ year old concrete. Surface condensation can be swift and extreme. I have seen conditions change dramatically DURING a flooring installation where they had to bring in floor fans to evaporate the ambient moisture, changing what could have been a "moisture-related failure" into a very successful installation, simply by monitoring the site conditions during the flooring installation.

If you want to end your moisture claims...start testing and monitoring your site conditions. Virtually ALL of these tools are a one-time purchase, with no repeating costs per project. It makes no sense to NOT test and monitor your installation.