Testing Moisture in Concrete - Deconstructing what Moisture Measurements Represent

Although it is easy to make light of this subject, the history of moisture testing prior to the installation of coatings and flooring materials is not much more sophisticated in its historical approach than what is depicted in this cartoon.

History of Moisture Testing for on-grade Concrete - A series of False Precepts

After being involved with waterproofing concrete structures above and below-grade, in the mid 1980's I was made aware of an increased occurrence of flooring failures due to "moisture".

When I began to investigate the problems, I was appalled at the complete lack of checks and balances as well as those who were blindly following the "scientific-sounding" causes that simply were not only a "non-issue", but in many cases the suggested (and in some opinions, proof) of "cause(s)" violated the laws of physics.

In no specific order; the blame: "Hydrostatic Pressure", "Water Vapor Pressure", "Vapor Emission", Water moving through concrete from sub-base to flooring surface in concrete less than 2 years in place.

The fact that ANY of these causes were accepted and cited by testing, inspection, manufacturers an installers alike is a modern example of "The Emperor's New Clothes".

I will ask a simple question: How can you test for moisture and properly interpret the data when the person testing doesn't understand what the moisture measurements actually represent?

Hydrostatic and Vapor Pressure

I will address the two most common "causes" that were cited in the 1980's through to the early 2000's and STILL somehow manage to persist even when the data indicates just how easy it is to disprove either one of these influences as a causality.

Hydrostatic is a branch of physics that deals with the principles of water at rest. Water at rest means it isn't moving...which is the opposite of dynamic.

If the water isn't moving, how can hydrostatic pressure exist? It exists due to a singular force: gravity. Water is only one of two naturally occurring materials that are essentially incompressible. Meaning that no matter how much force is applied, the volume of water remains constant as is the pressure applied from gravity.

Hydrostatic pressure is governed by the downward pressure of gravity and how MUCH hydrostatic pressure is exerted is dependent on the depth of water. This is how water pressure is calculated when designing submersibles for underwater exploration and calculating how much pressure is needed for a fire hose when dealing with high rise fires.

Compared to other forms of pressure, hydrostatic is a very "weak" force that is of no concern for any on-grade or above grade project.

Water Vapor Pressure

It was alarming just how many experts in the mid to late 1990's, were touting water vapor pressure as a major concern when addressing coating and flooring failures. Vapor pressure in any habitable environment is even less impressive than hydrostatic pressure. Vapor pressure of moisture is governed by humidity and temperature. Higher humidity tends to move towards lower humidity and higher temperature tends to move towards lower temperature. In any livable environment, it would be difficult for vapor pressure to reach, much less exceed 1.0 psi.

Unfortunately, this is where most are left to figure out why issues still occur when such a simple concept/constant exists.

When Moisture Doesn't Follow the Script

Moisture will respond to external influences, particularly when the external influences attract or repel moisture. For the purposes of "moisture-related" issues, we will simply focus on what ATTRACTS moisture.

There are many solids and semi-solids that have a range of moisture attraction. Some will adsorb moisture (where moisture is enticed to cling to a solid surface), whereas others can attract moisture to the point of combining with the moisture to greater and lesser extents, and some materials will actively absorb moisture, and resist free-evaporation. This is where the information discconect really comes into play. Most elements within concrete can be classified as hydrophilic, hygroscopic and even deliquescent. These are all moisture attracting forces that are capable of not "following the rules" when it comes to where moisture will move from, to or accumulate.

The best explanation I have ever read was by a gentleman by the name of Joseph Lstiburek of Building Science Corporation. He pointed out that moisture, unlike what we are taught, comes not in three, but in four forms.

The first three are the commonly accepted; water vapor (gas), water (liquid) and ice (solid).

The fourth form is ABSORBED moisture. Absorbed moisture can and usually WILL behave differently than moisture in any other form. This is a massive information disconnect that is WAY overdue to be taught and understood.

Many times diffusion models are used on in-situ concrete, with the expected absorption/desorption no longer following predicted models. This is where the theory of concrete hysteresis came into play, with very little still known on the exact mechanisms that create hysteresis. The ONLY agreement is that hysteresis is dependent upon the history of THAT specific concrete. With concrete, the internal chemistry is forever in a state of change. The only realistic method to minimize these changes is to create concrete that is dense enough to the point where these long term changes do not interrupt the original design requirements of the concrete. THIS is the key to minimize, if not, eliminate moisture movement in, out and through concrete.

Misconceptions and False Precepts

One of the most common misconceptions, with MANY of our approaches to construction practices and moisture testing based on the free transport of moisture moving from the sub grade through the concrete to the evaporation surface. It has been assumed that this is the primary cause of coating and flooring failures. Except for the fact that many suspended concrete surfaces can have higher "moisture measurements" than an on-grade concrete foundation in the exact same project. I have even witnessed to puzzlement of some "flooring inspectors" that have tested over concrete, gotten high moisture readings, grind into the concrete and get lower moisture readings, then core through the concrete and note the under slab soil is dry, yet STILL they insist the moisture is "originating" from the sub-base.

Another "favorite" of mine was an assignment I had with a producer of a waterproof concrete. There was ZERO free water in the concrete, with transport from sub-base to the evaporation surface an impossibility, yet, when moisture tests were taken using a RH probe, the concrete was designated as having "excess" (95%) internal humidity.

No amount of logic could be applied to get the specifier off that mental block that somehow 95% RH equated to excess moisture volume.

It was agreed that if the concrete could get to an internal RH of 85%, that would be an "acceptable limit".

I had the concrete manufacturer add a very small amount of a relatively inert salt to the concrete mix design. Nothing else was changed and the salt/standard concrete samples were compared side-by-side. Even though the moisture content was identical in each sample, the concrete with the salt added brought the RH numbers down to a maximum of 85%, whereas the "standard" concrete remained at 95% RH for several months.

Was there any less moisture or anything else about the concrete that was different? Nope...simply adding an additional hygroscopic element to the concrete was able to trick what has proven to be one of the most nonsensical "moisture tests" ever required by the continuously manipulated flooring, concrete and coating industries.

Moisture Testing - Measure What Matters!

If one stops to think, and I mean REALLY think about how misrepresented RH probes have been for the flooring trade, there should be a LOT of VERY angry people.

RH probes measure water vapor in an air space...period. These do not and CANNOT measure concrete moisture content. RH Probes cannot measure water in a liquid and much less, in an adsorbed and/or absorbed form within concrete.

Water vapor (RH) doesn't cause flooring failures, water liquid is what causes flooring failures. So why is everyone insisting on testing what doesn't cause flooring failures (RH) rather than what actually causes flooring failures (concrete moisture content)?

Water carries and suspends salts and solids, water vapor does not. Humidity WILL NOT cause a failure UNTIL the moisture is able to condense on a surface and then it is no longer water vapor and it is no longer measurable using any sort of humidity measurement,

Predicting or diagnosing moisture-related floor failures using RH probes is no more scientific than what is represented in that comic strip.