Moisture Testing Concrete for Flooring Installation - It's the Surface Stupid!

In January I will be presenting at a Mold conference about the proper methods of moisture testing, the challenges in drying of concrete and WHY tests should be conducted in a specific manner.

There is an ongoing lie that BOTH the concrete and flooring industries have bought into and when the lie has been challenged and those who begin testing and monitoring correctly, have all but eliminated moisture-related claims.

The False Precept(s)

Like so many others, I spent YEARS believing that the majority of moisture issues with concrete was created by moisture moving up from the soil, through the concrete and eventually to the surface. There were even cited studies, such as the Portland Cement Association sponsored Brewer Study, where I finally realized that most of us are "Headline Readers" and if someone we respect or trust, tells us what the report concluded, we believe them and go running off citing second-hand information that may have NO basis in fact. The Brewer Study is a perfect example.

The Brewer Study

This eponymous study was conducted back in 1965 and from everything I could gather, was the "jump-off" point that established moisture migration from soil, through the concrete as the cause of flooring failures; after all the Study Title was: "Moisture Migration - Concrete Slab-On-Ground Construction".

This study remains as an oft-cited "proof" how moisture migration adversely affects floor covering materials..only problem...the conclusions actually state it is surface moisture that causes flooring failures and doesn't even mention underside moisture migration in the conclusions!

Further, in one portion of the study, an underside vapor barrier did little to nothing to stem the moisture emanating from the concrete surface. In essence, this study concluded it is surface moisture that creates flooring issues.

When logic is applied to site conditions and the dramatic differences between the porosity and permeability of the concrete surface versus the interior section of the concrete, I say to this who INSIST the most important "moisture testing" is to measure the humidity content 40% into the concrete; It's the surface stupid!

Facts Versus Conjecture

After reviewing ALL the current cited testing for measuring humidity in the interior of concrete, I have found NO confirming data other than assumptions, conjecture, illusory correlations and circular logic for this "necessity".

Whereas when regarding the top surface of concrete, the facts are well-established, numerous, with ample history and in my view, inarguable.

Several areas of conjecture are;

1. Interior humidity is a "predictor" of future issues. In 40+ years of experience, I have yet to see a tangible connection, but it sounds good and "scientific" and easily embraceable by those who do not wish to dive any deeper.
2. Humidity measurements are the least affected by ambient conditions of any current moisture measurement. Actually the opposite is true, even a 2oF change in either direction can raise or lower humidity measurements by 5%.
3. Humidity measurements are the best method to measure the total moisture content of concrete. I challenged that nonsense early on, and thankfully, this nonsense is no longer being claimed, but the damage was done as MANY people bought into that sales-pitch, regarding this claim as fact!

Which brings us to facts

1. There are numerous studies, conducted over the past 2 decades that confirm the top surface of concrete is consistently more porous and permeable than the remainder of the concrete. One such example is the TTI (Texas Transportation Institute) study that identified the top .75 inches of the concrete surface as a "gradient". Simply stated, a gradient is an area that is affected by the environment it is exposed to (ambient conditions), even as the remainder of the concrete remains in a comparatively consistent condition. There are several other studies, conducted not only in the U/S., but worldwide that come to nearly identical conclusions.
2. Field correlation establishes fact, NOT laboratory testing! In a laboratory setting, the environment does not and CANNOT duplicate uncertainties and the unpredictabilities that are a given in field conditions. Unbeknownst to most, as these changes occur, changes within the concrete ALSO occur that do NOT exist in the carefully controlled laboratory setting(s). These differences deftly avoid categorization or even accurate identification since most studies rely on Diffusion models. As long as laboratories continue this mistake, the inevitable contradictions WILL continue. NOTE: Without taking a deep dive, when concrete is subject to environmental changes, this introduces hysteresis into the equation. As cited by MANY studies, hysteresis undermines any and all diffusion models and is confined solely to the history of THAT specific concrete.
3. Concrete can experience moisture issues whether on-grade or 15 stories up. That ALONE puts the kibosh on migration from the underside, particularly with unvented steel pan construction. One prime example is how moisture issues in non-conditioned spaces can be worse in the upper floors, rather than on-grade. This problem is VERY common and is referred to as "the stack effect" or sometimes the "chimney effect", where warm air rises and "holds" more moisture, which is absorbed by the concrete in these upper floors.
4. Another fact that has flown under the radar is how contaminants can move from the underside of concrete, into and through to the surface. It amazes me that we forget basic physics and chemistry we learned in junior high school. Contaminants can move quite readily, even as the water itself remains stationary. This is easily demonstrated with a desktop set up where two containers are separated by a tube. One container is pure water, while the other is saturated with salt, but isolated from each other. Once there is no obstruction between the two containers, the salt migration begins and continues until BOTH containers end up with basically the same concentration of salt...the water was stationary, but the contaminants moved. I have heard consultants explain that the evidence of contaminants proves that moisture has migrated up THROUGH the concrete (face palm). If liquid water is present in concrete, the moisture from underneath CANNOT travel! Water is incompressible and must vacate or move from an area before additional water can be introduced! That activity takes YEARS, even without an underside vapor barrier. REMEMBER: Water does not have to move to transport contaminants, no more that wires have to move to transport an electrical current.
5. In my opinion, this next fact is the "biggie". In recent worldwide studies, where humidity measurements and thermocouples are used to monitor the initial curing of concrete; with few exceptions, it has been discovered that the concrete surface self-desiccates, which PERMANENTLY increases the porosity and permeability of the concrete surface as compared with the remainder of the concrete! This problem increases in both damage and intensity with increasing temperatures, which brings us to our current conundrum: warmer temperatures encourage rapid initial strength gain, even as the long-term durability is compromised. As things speed up, this exacerbates the problems where in my opinion, the surface of standard concrete is now compromised, which necessitates SURFACE moisture testing, NOT in-depth testing! Most testing and studies which ALWAYS seem to be focused on the 28 day strength gain, actually ENCOURAGE the reality of a compromised concrete surface in favor of immediate access and usage.
6. The porous and/or permeable surface of concrete is not discoverable using humidity probes. In conditions where humidity is somewhat constant, a higher porosity or permeability remains "invisible" to any humidity measurement. A humidity measurement CANNOT and DOES NOT identify the amount/volume of space being measured. Unless the volume of space being measured is known, there is NO ASSIGNABLE water vapor content that can be given to ANY humidity measurement. The internal pores of a dense area versus a porous area within the same concrete can vary in volume by several orders of magnitude, yet a humidity measurement will give them equal value. For example, if an environment is at 90% RH, but the space is unknown and blindly reading one area that is the size of a tea cup and the other is the size of a 55 gallon drum, the humidity device cannot differentiate these volumes and assumes equal value! Likewise, the porosity and permeability within concrete can vary significantly, becoming increasingly variable towards the concrete surface.
7. Hygroscopic materials reduce evaporation. The type and concentration of varying hygroscopic materials can create interior environments within the concrete where the concrete can be close to saturation, but have low humidity readings. For example, the industry standard sodium chloride calibration solution used to properly calibrate moisture measurements could vary significantly in liquid volume, even as the humidity measurements remain constant. If a calibration container were 10% full, the humidity over the air space would measure 75%. Now, if the calibration container were 90% full, the humidity over the air space would STILL measure at 75%. Liquid in any and all forms is undiscoverable with humidity devices, and this issue is exacerbated by the presence of hygroscopic materials!
8. Humidity levels do not correspond with flooring damage. Early on, I wanted to see if there were any tangible levels where humidity and damage could be correlative. To try and establish a baseline, I began testing under successfully installed floors, with some floors newly installed, some for months and some for years. With few exceptions, according to most manufacturer recommendations at the time, every single one of those floors should have failed, none were less than 90% RH.

The definition of insanity is doing the same thing over and over and expecting different results. In my opinion, the most encouraging technologies are those that impart self-curing properties to concrete, and those that are most effective, should be required in ANY concrete scheduled for any form of flooring or coating. Self curing helps to alleviate, or possibly even eliminate the otherwise inevitable effects of self-desiccation. One of the ironies of a dense concrete surface is that particular care is needed to prevent surface condensation. It takes VERY little moisture to create a bond-inhibiting water layer over a dense surface.

NOTE: Many times a high quality concrete surface has been rejected as "too smooth or dense" for a proper adhesive bond. In several of these instances, where fans have been used to dry the surface, particularly when the concrete surface is cool, the bond numbers were equal to or sometimes superior to that of a standard concrete surface.