Concrete Floor Slabs - Should these now be classified as "Deficient" or even "Defective"?

In some recent conversations I've had with several prominent industry experts, it appears that the concrete industry is beginning to recognize "standard construction methods" such as current recommended curing practices, are no longer adequate in producing a concrete surface that can ensure coating or flooring success.?

Succession of Change

Over the decades, concrete has undergone some profound changes, even when not noted, or simply not conveyed to those involved with concrete surfaces scheduled for installation of coatings or flooring materials. Likely the most obvious reason?these changes have gone un-noted is that the 28 compression strength requirements have been maintained. Essentially that is the ONLY requirement (other than a targeted water/cement ratio) for standard concrete; particularly non-structural concrete.?

It often comes as a surprise to many that on-grade floor slabs, unless designed for a very specific purpose, are not a "structural" component. The confusion is that although an on-grade slab is designed to support weight, that doesn't suggest it being a structural component. This is why there are several different ACI Standards that differentiate non structural (ACI 301, ACI 302) from structural concrete (ACI 318).?

Standards that have not reflected changes in concrete?

Over the decades, concrete has undergone significant changes, even as those in and around the concrete industry are generally not aware of these changes.?

One of the more significant changes, which occurred over a 20+ year period, were the changes from a comparatively coarse grind cement to a finer grind cement.?

From the 1930's and well into the 1950's cement grinding became increasingly finer, which also led to significant changes in cement content. It was stated in an article by Ev Munro (1986 ACI Journal) that a finer cement grind produced greater early strength. In this time period, there was a move to standardize a 28 day compressive strength value in order to maintain a specific and minimum load bearing capacity for an intended purpose.?

It is important to note that compressive strength is NOT an interchangeable term for concrete strength and durability; it is simply a load bearing indicator.?

With specific targeted load bearing, less cement was needed to achieve the minimum 3,000 psi value (and of these other values as well). This led to a reduction of approximately one-third less cement needed to meet the 28 day goal. With less cement, this dramatically increased the permeability of concrete. If a cured coarser grind cement concrete were compared to a cured finer grind cement concrete, the permeability increase was estimated to have increased by as much as 500% with the finer grind.?

Also noted with the finer grind was the generation of heat within the concrete. If the original cement content remained constant and not reduced, the concrete tended to crack more readily from?internal heat generation from a finer grind. NOTE: This can readily be demonstrated even today: Type I and Type III cement are chemically identical, with the only significant difference of Type III being a much finer grind than Type I. This finer grind speeds hydration as the faster reaction develops higher temperatures; Type III is often referred to as "High Early Strength".?

Type III is seldom used for thicker concrete placement due to a high generation of heat that again, can (usually) cause problematic cracking.?

Coarse to Fine Cement - NOT Noted nor adjusted to fit changing curing characteristics of a finer grind.?

Concrete curing was developed to try and maximize the early strength development of the concrete, particularly the exposed surface which is subject to the majority of ambient and climate changes. With a coarser grind cement, 3-7 days virtually ensured a high quality concrete, as well as a high quality concrete surface.?

With finer grinding, it was found that curing a minimum of 7 days would generally produce a good quality concrete and concrete surface. However, with the smaller volume of cement with the finer grind concrete, temperatures were noted to have a much more dramatic effect upon the concrete. Standards were then developed to cope with both high and low temperature concrete placement and curing. For the most part, this continued to be adequate for most concrete placement; particularly since the only "standard" was the 28 day compressive value. Having said that, a very dramatic change was incorporated that began in the planning stages in 1990, began to be enacted in 2002 and brought into full compliance in 2019.?

EPA Requirements for CO2 Emissions

In many respects, the EPA requirements for recovery of flue gases has the potential to be the most important issue ever faced by not only the concrete industry, but anything and everything associated with concrete.?

Cement plants produce the highest level of CO2 than any other man-made source. This isn't surprising since cement produces concrete.?Concrete is so ubiquitous to the worldwide human population; concrete is second ONLY to water as the most globally used material.?

The push for CO2 reduction was directed towards flue gas recovery. In this recovery, part of the process was to introduce a portion of the waste product; CKD (Cement Kiln Dust) back into the cement production process.?

In 1990, compliance was effectively delayed when one of the largest cement producers at the time stated that the introduction of CKD into the cement process would essentially destroy their business since they would no longer be able to produce "low-alkali cement".?

This delay lasted until 2002 as cement plants were finally prepared to begin bringing the plants into compliance. Full compliance was likely met at the end of 2018 which then prompted the announcement by the Concrete Industry that low-alkali cement will no?longer be available.?

That rather innocuous-sounding announcement has potentially devastating effects on not only concrete, but any products that use or are reliant on Portland cement in product production, formulation, etc.?

CKD - Cement Kiln Dust - "Know Thine Enemy"

CKD is a highly alkaline material that effectively increases the alkalinity of ALL Portland cement. CKD is non-uniform, in that much of the alkaline material introduced into the cement process can differentiate significantly from region to region, based on the available raw materials. One early estimate (that has somehow disappeared from the internet) was that the alkalinity may be increased by upwards of 400%. Here is where things get confusing: an increase in alkalinity does not necessarily have a reciprocal increase in pH. The alkalinity (NaO and CaO) in CKD tends to be buffered, meaning that it maintains a fairly consistent pH over a wide range of concentration/dilution.?

CKD is capable of introducing a series of complications that will befuddle researchers, manufacturers and users alike if they aren't informed as to how these complications can manifest.?

Problems associated with Increased Alkalinity: Ionic Dew point, concrete hysteresis, internal reduction of RH. Alkalinity inhibits solubility of Portlandite (calcium hydroxide), with a tendency to accumulate in the top 0.75-1.0 inches (19-25 mm) into the concrete surface. In site conditions where temperatures can increase, the inhibition of cement formation and Portlandite are magnified significantly, with Portlandite becoming essentially insoluble at temperatures above 100oF. In the presence of sodium hydroxide, even lower temperatures can render the Portlandite essentially insoluble. NOTE: It was interesting that even though the studies were in different areas of the globe and unaware of the others, the 0.75-1.0 inch concrete surface depth where self-desiccation was noted and very consistent irrespective of the?global location.

Growing Complications With Increased Alkalinity?

After placement, cement begins to form, and in that formation, almost half the water used in the concrete mix design will (or should) be consumed in the development of cement.?

As cement is formed, the water volume decreases. As water volume decreases, the alkalinity (concentration) increases. In ALL cases, increased alkalinity lowers measurable RH.?

Not coincidentally, has been the discovery that concrete containing embedded humidity devices along with thermocouples are indicating the top 0.75-1.0 inch (19-25 mm) of the concrete surface tends to self desiccate, with the humidity often falling well below 80%. Cement formation ceases when humidity levels drop to 80% or lower. The severity of cement retardation increases with the temperature; with some to most initial damage being unrecoverable. Some of this self-desiccation occurs 2-3 weeks after placement, which at first seems odd, until it is realized that the water content (particularly towards the surface) has reduced considerably by then, and it may take 2-3 weeks before the alkalinity is concentrated to the point of desiccation where cement formation is interrupted. The self-desiccation is always exacerbated by increased temperatures, which again, makes the isolation of this deleterious effect in the top 0.75-1.0 become understandable.?

The weaker surfaces are even showing up in laboratory samples (WHEN the researchers bother to look for it that is).?

NOTE: I have included a graphic of a lower surface strength in both cured and uncured concrete samples, under laboratory conditions.?

Connecting the Dots?

The information and facts cited in this article are well-known and well-established in physics and chemistry, but are NOT being applied to concrete for whatever reason.?

Salts, ALL salts, particularly alkaline salts reduce humidity and continue to reduce humidity with increased concentration.?

One example is sodium hydroxide common to concrete as well as CKD. Sodium hydroxide will begin to absorb moisture from the air even if the humidity level is less than 10% (which is why bulk sodium hydroxide is rarely stored or shipped as a solid).?

Alkalinity Makes Water Behave Differently

Depending upon the alkaline type and concentration, water does NOT behave as what is expected and relied upon (assumed).?

One of my favorite statements was made by Joe Lstiburek (Building Science Corporation) where he identifies moisture not in three, but in four forms: water (liquid), gas (water vapor), solid (ice) and absorbed (when a material absorbs moisture and this absorbed moisture no longer behaves in the manner as water in a pure form).?

Sodium hydroxide and water, at very low concentrations; the sodium hydroxide acts like an antifreeze where a 3-4% solution can reduce the freezing point of water down to -14oF (-25.5oC). However, if this concentration is raised to 40%, the freezing point of the water/alkali solution is raised to 59oF (15oC), that is not a misprint. This is the reason those who store and ship bulk sodium hydroxide heat the tanks and lines above 70oF (21oC), to keep the water/sodium hydroxide from freezing in the tanks and lines.?

Water/atmospheric dew point is 100% RH. Alkalinity (and any other salt or hygroscopic material for that matter) creates what is termed "ionic dew point", where condensation can occur well below 100% RH. In chemistry, this is often referred to as "Critical humidity threshold".?

The industry standard calibration solution is an excellent example of that. Sodium chloride, used in the calibration solution will ccaue condensation at 74% RH, alkaline salts can cause condensation well below that figure.?

There is irony where many of the targeted values are issued through manipulated data to the flooring manufacturers, where a maximum of 75% RH is targeted for newer concrete. The flooring manufacturers may inadvertently be demanding poor quality concrete that is very alkaline. Basically using RH probes as a definitive moisture test method will produce WAY more problems than these could ever hope to solve. RH does not equate to moisture volume, it is simply a measurement of water vapor in a given air space.?

That being said, in older concrete, usually 10+ years in-place, 75% RH would not be considered a red-flag, particularly in desert climates.?

REMINDER: Cement formation ceases when RH is less than 80%.