Concrete Assumptions, Myths and Legacy Errors that Persist - Part 3

Clarifications: "State of the Art Report" for concrete

For all the good and useful information this report has given (outlined in Parts 1 & 2), there are MANY assumptions and non-conclusions. That is one of my main complaints regarding way too many studies, where the same subject and techniques are repeated over and over again, sometimes for decades with little to no resolution, and worse, with many being more confused by the data than helped.

Freezing Water and the role of alkaline salts

In the report, freezing of the internal moisture was discussed and an area that was pointed out was the alkali salts, as the water froze, continued to get more concentrated and further reduced the freezing point of the alkali/water solution.

This discovery, made back in the 1940's, likely led to the assumption that the freezing point of water continues to decrease as the alkalinity increases. In this research, it was stated that it would take a lowering of the temperature to approximately -78oC (-108oF) to freeze all the evaporable water.

NOTE: In the field, alkaline salts can become very concentrated, and in those instances, unlike what can be ASSUMED from the report, is that the freezing point of water reverses and begins to freeze at much warmer temperatures, where a 40% concentration of sodium hydroxide will create a water solution that freezes at 59oF. In these conditions, the water no longer behaves in a predictable manner. If the sodium hydroxide should become concentrated to 80%, the freezing point of that water solution would be greater than 100oF...that is NOT a misprint!

Curing Recommendations

Curing becomes increasingly important with low water to cement ratio concrete, and even more so when SCM's and pozzolans are added to the concrete.

One such example was given to concrete placed with silica fume where in the 1980's it was referred to as "over-curing" where a much longer duration of curing was recommended for silica fume concrete, otherwise the desired properties would be greatly compromised.

Accelerated curing was also discussed where it was recognized that these methods compromise the long term durability of concrete, particularly with elevated temperatures. Although it is mentioned that such concrete suffers increased moisture loss, that subject (yet again) was not explored, nor explained.

That being said, this section also gives us invaluable clues as to why we are now experiencing more problems with "modern concrete". In the study: "In this case, the additional heat is beneficial in enhancing the pozzolanic reaction of the silica fume. However, the heat greatly increases the moisture loss from exposed surfaces, which tends to cause more shrinkage problems" (emphasis added).

Also in the report: "Certainly, the curing temperature and the kinetics of the hydration and pozzolanic reactions of the particular cementitious materials will affect the required duration of curing to attain a certain level of maturity. With respect to silica-fume concrete, some researchers have advocated prolonged curing to realize the full benefits of this material (Holland 1989; Ayers and Khan 1994). However, such recommendations are based more on professional judgement rather than data."

"The gel resulting from the hydration of portland cement and subsequent adsorption of water belongs to the “limited-swelling” classification, which means it will only take up a limited amount of water with a corresponding limited amount of swelling. Likewise, it will shrink when evaporable water is lost during drying. Some of the shrinkage that occurs during initial drying is irreversible, that is, the paste will experience some permanent shrinkage deformation upon rewetting. This permanent shrinkage results from plastic flow of the solid phase of the paste due to the stresses induced by the effects of shrinkage. This plastic deformation causes a permanent change in the structure of the cement paste."

"It is important to realize that in applications where concrete is exposed to frequently changing environmental conditions, the evaporable water will be in a state of flux. Therefore, the concrete will be subjected to continuous moisture movements, and the accompanying volume changes, throughout its life."

How this all relates to "Modern concrete"

Only within the past 20 years or so, has it been discovered that much, possibly ALL concrete placed in warm to hot environments are experiencing self-desiccation in the top surface of the concrete.

In discussions I have had with Joe Shetterly (Specification Products), I expressed that the E5 colloidal silica was indeed adding moisture that would otherwise be lost and that the internal curing properties were essentially eliminating the environment that is causing global surface self-desiccation of concrete.

A series of concrete placements by those who use the E5 product have experienced essentially no surface cracking or curling, when it was expected in the dry, windy conditions of the environment in which this concrete was consistently being placed.

One such project that really impressed me was a concrete surface, still damp after 2 hours of direct sunlight and windy conditions. Under normal circumstances, a dry surface with at least some visible cracking would be expected.

In the report, which corresponds with the theories I had with the "modern concrete", was a bit of a surprise since I had not been aware this subject was more widespread than I had thought: "To prevent self-desiccation during the curing period, water that is consumed by hydration needs to be replaced by the ingress of external moisture. Early-age, continuous curing may be critical for providing the necessary water. As hydration proceeds, the capillaries become discontinuous, thus effectively preventing the ingress of additional water into the concrete. When this state is reached, additional curing may be of little, or no, benefit, because the water may not be able to penetrate to the interior quickly enough to maintain saturation of the capillaries. This leads to early cessation of the hydration reactions within the paste. Current curing requirements, based on research on conventional concrete, do not consider these factors. Thus it may be necessary to develop new curing requirements for this new, modern class of concrete."

Quality Concrete for Flooring Projects are no longer a reasonable expectation using Standard Procedures

When SCM's, pozzolans and other methods of increasing concrete density are used, the concrete WILL dry much more slowly. If using the wrong type of moisture measurements, particularly with devices that measure only relative humidity, the user may extrapolate incorrect information declaring the concrete to contain moisture levels too excessive for a successful flooring installation, even when such information is likely incorrect and simply targeting the "correct" form of moisture. With a denser concrete, the capillaries tend to discontinue, and with an internally cured concrete containing E5, the moisture levels CANNOT become excessive. The drying rate is a one way street.

I gave the opinion that this product should be tested using a Tramex CMEX Concrete Meter and I felt the moisture content would reduce rapidly since this normally "excess" moisture was being incorporated with the additional cement formation.

This opinion proved to be correct where even after a rain, one of the dry spots was measured after the concrete had been placed for 2 weeks, and the moisture content was already less than 5% (4.7%). In many instances, a concrete placed with E5 could be floor ready in as few as 14-21 days for a majority of the adhesives now available.

Conclusion

It is vital to remember this State of the Art Report on concrete was put together in 1999. The issues that have been greatly amplified by the now more alkaline CKD cement have even more issues by at least an order of magnitude.

Many of the myths, errors and assumptions are based on data that is simply taken out of context, and with many others, improperly interpreted. NOTE: The PCA Brewer Report from 1965 has often been cited as the justification of using under slab barriers to prevent moisture-related flooring failures, when in fact, that conclusion is absent from that report.

One consistency that links most of the studies used within the State of the Art report was how dynamic the internal conditions of concrete can be when placed in field conditions, yet too many are relying on the static conditions within laboratory environments and using ONLY the lab results as "proof".

The data and proof ARE out there and if this data and proof are applied, many of the problems experienced would NOT exist! With the currently available technology that is now available, there should be NO reason for "moisture-related" flooring claims.

If properly designed, tested and monitored, the now 3 billion per years moisture-related flooring claims will cease to exist...allowing that money go back into a positive area of business rather than to satisfy claims that should not have existed in the first place.