Common Misinterpretations in Concrete Moisture Testing

It never ceases to amaze me how incredibly misinformed too many professionals are when attempting to test concrete for moisture, particularly with a list of common false precepts they are taught to believe are factual.

An essential element in all of this is that concrete chemistry has changed dramatically, with most, if not all testing prior to 2019 now being relegated to a historical context, with very little that is applicable to concrete being used today.

False Precept #1: Concrete today is the same as it has always been.

Concrete has changed dramatically since the 1920's and continues to change as this article is being written.

Between the 1920's through the 1950's, cement grinding has changed the amount and techniques involved with placing concrete and proper curing. As grinding became finer, to achieve a targeted compressive strength, less and less cement was added to reach this goal. The finer grind produced more heat, and proper curing became increasingly important and was considered equal to the water-cement ratio in obtaining the targeted quality of concrete.

A "modern example" is to compare Type I cement to Type III cement. Type I is suitable for most general construction, but generally combined with Type II, which gives some sulfate resistance and reduces the heat of hydration as compared with Type I.

Type III is seldom, if ever used for concrete that is placed in any notable thickness because of the high heat generation that can create cracking problems and other issues that are not desirable for standard construction. Here is the kicker: Type I and Type III cement are identical in chemistry, with the ONLY difference being that the Type III is a finer grind.

In a 1986 ACI publication, Ev Munro pointed out that concrete of equal compressive value in the 1980's as compared to concrete of the 1950's was significantly lower in cement and substantially more permeable.

The most recent, and in my view the most substantive change is the addition of CKD (Cement Kiln Dust) into the cement production process.

This change was initiated by the EPA to reduce the flue gases emitted by the cement producers. CKD is an alkaline by-product that has increased the alkalinity to the point where the cement industry will no longer offer "low alkali cement".

This increase in alkalinity is critical to consider whenever durability or suitability for coatings and coverings are a necessity.

In my opinion, the dramatic complications introduced by this higher alkalinity is now producing a concrete with properties so different, that for most intents and purposes, it is an entirely new product, with testing and evaluation prior to the implementation of the CKD into the cement process as being of little value, other than as a historical reference...yes it IS that important!

False Precept #2: Moisture migration from the underside of the concrete is the main cause of the flooring failures.

The second law of thermodynamics state that heat flows from an object of higher temperature to an object of lower temperature. In a closed system; this will occur until equilibrium is reached. One of the first issues is that concrete is NOT a closed system, yet we are consistently taught to ignore this 2nd law of thermodynamics with the second part of this false precept that in most construction sites, it has been noted that the underside of a concrete slab is cooler than the top surface of a concrete slab and that the ambient conditions are typically warmer that the top surface of the concrete slab.

Moisture migrating from the underside is a second violation of this law, as is the ASSUMPTION that the concrete will achieve equilibrium. Now, equilibrium is possible in a lab environment provided the temperature and humidity remains unchanged, but I defy ANYONE to show me where this happens in the field!

Moisture problems are generally created by the warmer air moisture being absorbed by the cooler concrete surface, and wants to move downward if the remaining concrete is cooler, NOT the other way around!

False Precept #3: Humidity Testing measures the moisture content of concrete

There are MANY reasons this is a false precept; 1. this assumes ALL moisture in concrete is in a vapor form; 2. moisture distribution in concrete will equalize; and three; the most common form of RH testing are the probe type, which have proven to be unreliable in humidity levels below 10% and above 90%. There are even studies and reports stating that in order to get an accurate humidity reading, many times the RH sensor gets "stuck" and has to be removed, thoroughly dried, recalibrated, then reinserted. Each time the researcher did this, they noted a change in the RH measurements.

Question: Has ANYONE ever seen this caveat in the manufacturer's written instructions or anyone that understands this can happen and has gone through this process in the field?

The data used to establish RH probes is based SOLELY on the initial drying progress of freshly placed concrete mix water, NOT for concrete that has been in place. Even the standard used; F2170 VERY clearly states this method measures the relative humidity and does NOT claim to measure the moisture content of concrete. ANYONE claiming RH Probes measure the moisture content of concrete, intentionally or not is LYING TO YOU! This is damaging many people, companies and reputations even as it is false...this HAS to stop!

False Precept #4: High pH and High Alkalinity are interchangeable terms.

This area is particularly frustrating since this has become a common misconception even as it has been proven in many other professions these are two separate issues.

High pH is the relative strength of a compound such as tomato juice and lemon juice. Lemon juice has a pH of 2, whereas tomato juice has a pH of 4. This is a logarithmic scale where the lemon is 100 time more acidic than the tomato. As these are diluted with water, the pH will begin to increase towards the "neutral" pH of water, which is 7. The alkaline components in concrete are actually a bit more complex than the acids I used as a comparative. Calcium hydroxide at full concentration has a pH of 12.5, sodium carbonate at full concentration has a pH of 12 and sodium hydroxide at full concentration has a pH of 14.

Each of these alkaline materials native to concrete are buffered, meaning these change very little in pH over a relatively significant concentration range. The pH of sodium hydroxide can reach 13 on a pH scale at a low dilution and will not reach a pH of 14 until the concentration is substantially higher.

This explains some of the flooring failures at a pH of 10, whereas some floors have been successfully installed at a pH of 13. The damage is caused by concentration more so than the pH. This was proven in the Wool Safe Study where high pH cleaners were thought to damage wool fibers. I gave them the opinion that it is the concentration, NOT the pH that would damage the fibers...which was proven out in a laboratory study.

Likewise, a high pH won't cause ASR (alkali-aggregate reaction) in concrete until it reaches a more concentrated state. NOTE: This is why the added CKD is such a concern.

False Precept #5: Condensation can only occur at atmospheric dew point

With concrete, condensation can occur well below atmospheric dew point. This is NOT being taught in any of the inspection curriculum I have seen, which is in my view is completely unacceptable. Those who become certified without the essential knowledge base may be certified, but they are NOT qualified...there is a substantial difference.

I use a very simple example how condensation will occur well under atmospheric dew point. Plain table salt, if sprinkled over a hard surface and the humidity in the room is at 70%, the salt will remain in a dry, granular state.

When the humidity in the room is raised to 80%, which is a full 20% lower than atmospheric dew; the salt will liquify (condenses), this is called "ionic dew point". The alkaline salts present in concrete will liquify at humidity levels well below what the stated "safe levels" are. In fact, the lower the humidity, if the concrete is less than 5 years old, the less likely the concrete is of a suitable quality for a flooring installation. NOTE: What this also shows is that liquid water can be present even as the RH measurements are giving what many have been led to believe is a safe level. This safe level has been established by people who don't understand what they are measuring or that the concrete could be even saturated as the RH levels read at "safe" levels.

False Precept #6: Compliance with Manufacturers Moisture Testing assures protection

I often refer to the statement proudly proclaimed by one of the RH promoters from 1997: "If moisture testing is so accurate, why are we still having problems?"

Well, since that bold proclamation and with many manufacturers switching over to the RH Probes, the problems have tripled from 1 billion per year to approximately 3 billion in 2022. If nothing changes from the current approach, not only will the claims amounts continue to soar, the flooring industry will successfully chase away the quality mechanics that are tired of being sued and held accountable for issues they should NOT be responsible for. NOTE: The caveat with moisture tests clearly states: "results are accurate only at time of testing". Translation, everyone is now exposed and on their own.

With the CKD and other changes being made to cement, the protocols in ASTM F 2170 and ASTM F 1869 virtually assures compromised moisture test measurements and incorrect data gathering. Those who require such moisture testing, in the words of Bette Davis: "buckle your seatbelts, it's going to be a bumpy ride".