Moisture Testing of Concrete - How badly do RH Probes "Miss the Mark"?

In my prior article, I took on the intentional and unintentional misrepresentations that several distributors of RH Probes present to both the concrete and flooring industries.

This article will focus on the importance of measuring the top 1 inch depth of the concrete and what it means in obtaining meaningful data to avoid flooring and coating failures.

It's the Surface Stupid

Back in 2022, I gave a seminar to a Building Sciences Conference where I submitted 10 pages of citations (later increased to 14 pages in subsequent presentations) that verified the information I presented aligns with what actually causes the majority of flooring and coating failures.

To be provocative and adding humor, one of the organizers suggested this as a sub-title since it truly IS the surface that creates the vast majority of flooring failures, and that most of what we've ALL been taught about the underside of concrete creating flooring failures or how the underside moisture can be used as a predictor of future moisture failures...(which may be possible in desert and environments where the underside of concrete can become warmer than the interior of the building) is NOT a primary cause of flooring and coating failures; no more than the earlier ill-conceived, yet universally accepted "scientific-sounding" garbage that hydrostatic pressure and vapor pressures caused on-grade and above grade flooring failures. RH as the primary cause of flooring and coating failures has no more scientific legitimacy than hydrostatic or vapor pressure.

In the VAST majority of cases, the interior is consistently warmer than the underside of the concrete which means the warmer interior will cause moisture to move from the warmer air, to the cooler concrete surface, which then moves towards the even cooler interior...basically the exact OPPOSITE of what we are taught in moisture testing classes, lectures and even certification schools.

Concrete Has Gradients and is NOT Uniform in Density

The illustration used comes from a company that specializes in RH Probes, inadvertently proving what the facts actually indicate, which is the surface of the concrete, even in carefully controlled environments will suffer from a reduced cement formation due to inadequate RH levels within the concrete.

This illustration originates from an earlier document where the RH fell to 74% during the testing. Cement formation ceases once the RH falls below 80%, creating irreversible and permanent cement retardation = weaker, more permeable concrete surface.

In the field, this can be much more severe and has proven to be the "missing link" in why concrete is less durable and the surface remains disproportionately permeable compared to the remainder of the concrete, yet way too many are led to believe the concrete is not just monolithic, we are led to believe it is uniform in density and permeability...that is NOT true and is essentially an unachievable goal for standard, high performance and ultra high performance concrete. NOTE: The surface issues have become so common that both the ACI and the ASCC have stated that curling and cracking are to be expected, not a result of workmanship or materials.

Several studies that have used a combination of embedded humidity measurements and thermocouples show that as surface temperatures increase, there is a reciprocal increase of autogenous self-desiccation. Basically, the concrete surface dehydrates and lacks sufficient moisture for cement development.

This process has been shown that this self desiccation appears MUCH earlier than anyone could have guessed or predicted as this self desiccation comes at one of the most critical times for the durability of the concrete as this desiccation happens within the first 2-3 weeks after concrete placement, with the RH falling to as low as 50-60% in the top inch of the concrete! This damage is irreversible and unrecoverable.

This lowered RH in the surface will create a much more permeable and weaker concrete than is currently estimated.

In yet another lab study, even with a 7 day water cure, the top one inch of the concrete had a 20% lower compressive strength than the remainder of the concrete.

It's The Surface Stupid - Again!

It is a simple matter of outlining what is known and what the facts lead us to:

1. Cement formation ceases when RH is less than 80%. NOTE: It truly bothers me that this information showed up in the first evaluations conducted in the U.S., yet those who are supposedly concrete experts saw no issue with a RH that would not allow the formation of cement, yet endorsed this unhealthy RH target!
2. Moisture moves from warm to cool: This is a portion of the second law of thermodynamics that is routinely ignored in seminars and classes designed to educate us on how to properly conduct moisture tests. Hence my sharp criticisms of many certification bodies and schools.
3. Humidity does NOT cause flooring failures. I have yet to see ANY empirical data that can link humidity to a flooring failure, yet using technical "sleight of hand", this proof is assumed using truncated data and logic. Humidity can LEAD to coating and flooring failures IF the concrete surface is within 5oC (10oF) of dew point. There are elements within concrete that actively absorb moisture from the ambient conditions and can condense moisture well below atmospheric dew point. This is a chemically created condensation that has been referred to as "ionic dew point". This is easily demonstrated by the common sodium chloride (table salt) solution used to routinely calibrate moisture measurement devices. At a RH higher than 75% and much lower than 100%, sodium chloride will draw moisture from the air and cause condensation (75%+RH is the ionic dew point of sodium chloride). So the damage isn't from RH, it is from the condensation of water in whatever environment it is in that can allow/promote ionic dew point.
4. Diffusion can create conditions where water can move solids that appear contrary to the second law of thermodynamics as diffusion becomes the primary culprit where older, "drier" concrete can become more problematic than newer concrete. Elevated temperatures increase the rate of diffusion, whereas most chemicals, except for the calcium hydroxide in concrete will become increasingly active and soluble with increasing temperatures. Since moisture moves from warm to cool, accumulated salts in the warmer concrete surface, immediately beneath a curing compound, existing coating or flooring material remains in a somewhat inert form. Diffusion allows transport of dissolved material even as the water itself does not move.

However, if a surface material is removed the moisture from the ambient conditions can become readily absorbed; much faster and more efficiently than anyone could have guessed.

This happened in a teaching series last year: I informed a classroom of installers the reason they find more issues with older concrete than is conveyed is due to the salts that have accumulated over the years is now exposed to a new "moisture source", the room interior. They wanted to believe this information, but since it ran contrary to what they've been taught, the information wasn't truly embraced until 3 months later.

This same group was learning how to use a Tramex CMEX5 Concrete Meter. The area chosen was an area of the warehouse that was estimated to be at least 40 years old.

The initial moisture measurement was 1.9% over the existing curing compound. The curing compound was removed and the surface re-tested and the moisture reading was again 1.9%.

The revelation (to me as well) came when they returned to the area after 10-15 minutes and the moisture measurement increased sharply to 4.3%. They tested again....same result.

They got me on the phone and shared this information with me. I was as shocked as they were, which also pointed out a vital bit of information that flies under the radar of virtually every project where an existing flooring material, or even an existing polished concrete surface can experience, the accumulated salts behave almost like a calcium chloride test in reverse where the salts in the concrete draw moisture from the air rather than calcium chloride sealed over a concrete surface draw moisture from the concrete.

In my view, the most important aspect is that most tests are conducted a full day later after the concrete surface has been prepped.

Like many others, I was led to believe the 24 hour period would allow the concrete to vent, releasing potentially “stored” moisture...instead, just the opposite can occur.

The lesson learned is that an immediate moisture test and a slightly delayed moisture test (likely 45-60 minutes later) should be conducted on ANY concrete surface where a surface material has been removed to establish an accurate moisture baseline; and to establish if the concrete surface is rapidly/actively absorbing ambient moisture.

The MOST IMPORTANT point is that moisture can be introduced into a concrete prior to and DURING an installation. Testing with the concrete meter with continuous ambient readings doesn’t add cost to an installation, but has been instrumental in identifying changing environmental conditions that can introduce conditions leading to moisture-related failure. Installers, this procedure should be a required part of EVERY installation.

If those testing that warehouse floor had waited, they would have been led to believe the moisture baseline would have been 4.3% (likely higher) rather than the 1.9% ACTUAL baseline, leading to an improper conclusion that the moisture was originating from the concrete, NOT the ambient conditions.

Concrete Studies PROVE it is the Surface We NEED to Focus on

Concrete field studies indicate there is a consistent gradient of the concrete surface that ranges from 19-25mm (0.75-1.0 inch). THIS is the area where weathering, wear and environmental issues begin to damage the concrete.

Even ASR (alkali aggregate damage) begins from the surface and works its way into the concrete.

The alkaline salts accumulate in the surface, where I originally theorized this in 1999-2000 and was met with not just skepticism, but quite a few derisive comments since "everyone" knew that ASR damage was "time-dependent" and wouldn't form until many years had passed; until a couple years later, as researchers began to note the surface damage was consistent with ASR and appearing in newly placed concrete.

Connecting the DOTS

It takes quite a lot of alkalinity to stimulate ASR. Likewise, it takes FAR less alkalinity to reduce measurable RH.

The partial explanation of cement development creating conditions of autogenous desiccation doesn't satisfy the significant drop in RH. Alkalinity however, DOES satisfy this "missing link".

1. In elevated temperatures, calcium hydroxide loses solubility. In an alkaline environment, calcium hydroxide loses solubility.
2. Calcium hydroxide is only moderately soluble and unlike the remaining alkaline components in concrete, calcium hydroxide becomes more soluble with REDUCED temperatures.
3. The combination of embedded RH devices and thermocouples confirms that with elevated temperatures, the autogenous desiccation is more pronounced.
4. Alkalinity has a reciprocal reduction in RH with concentration. CKD now added to all domestic cement has introduced higher alkalinity to the cement and subsequently, the concrete itself.
5. Alkalinity concentration of 20% will produce a reciprocal RH of 78% and an alkalinity concentration of 30% will produce a reciprocal RH of 63%. These numbers match the field results noted, but not correlated by the researchers, simply because they are NOT looking at the alkalinity as the rowing oar in all this!
6. The specifications calling for a lower RH in newer concrete is antithetical to quality concrete...basically the manufacturers are unintentionally demanding garbage concrete, destined to have moisture and alkalinity issues, when they SHOULD be targeting concrete with no less than 80% RH, rather than a maximum 80% RH.

As stated in my prior article, moisture testing for concrete is upside down!

Caveat Emptor - Concrete Curling and Cracking - multi-industry "Hot Potato"

Rather than editorialize just how much the surface of the concrete influences moisture testing, the surface has a profound effect on cracking and curling (which deserve an article in and of themselves).

I will let the ACI (American Concrete Institute) and ASCC (American Society for Concrete Construction) inform those who wonder or are involved with issues related to cracking, curling and flatness:

"In general, when it comes to slabs, both ACI 302.1R-15 and ACI 360R-10 state in their prefaces that it’s unrealistic to expect crack-free and curl-free floors regardless of the best design and construction practices. Also, those documents state that the designer and contractor need to advise every owner that “it is normal to expect some amount of cracking and curling on every project.” ASCC Position Statement #33 provides similar guidance, stating that: “ASCC concrete contractors will meet with the design team, construction manager, and general contractor to discuss crack expectations for the project. Concrete contractors want to ensure awareness by all parties that cracking will occur when the structure is built in accordance with the Contract Documents.”

Here's the kicker; I have seen many concrete designs where cracking and curling are nearly non-existent and such concrete designs aren't prohibitive in costs, particularly when considering the alternatives are ALWAYS more costly and time consuming.

Manufacturers and installers, insist on the designs that don’t crack or curl…you get what you don’t require, so require it. Most don’t know they have these options, nor do most specifiers.