Concrete Assumptions, Myths and Legacy Errors that Persist

I saw an "older" document by the NIST, printed in 1999 which was entitled: "Curing of High-Performance Concrete: Report of the State of the Art".

The information as presented was riddled with assumptions and inaccuracies, and the more I read through it, the more appalled I was...NO WONDER problems are worsening and not improving, particularly as we add elements to the equation that in many cases are knee jerk and/or additional assumptions that simply add to the downward spiral of projects that will suffer increased maintenance and repair issues, even as these are presented as methods to "improve" these qualities.

In the past, I would read publications and studies without a critical eye, accepting what was stated as factual and continuing on, even as many things never quite seemed to add up to any sort of a tangible conclusion. I now question EVERYTHING!

In that vein, I will be breaking this into segments since this subject appears to be a deep dive, but when fully realized, is actually only scratching the surface.

Setting the Table - The Goal of High Performance Concrete

It is important to note this report is specific to "High-Performance Concrete" and curing practices.

High Performance concrete at the time of publication was stated to be approximately 10% of total annual concrete production and proposed as a method to increase the durability and life expectancy of concrete (the current buzzword being "sustainability").

Unfortunately, until very specific details, legacy errors and assumptions are addressed, concrete technology will continue to stumble along where "happy mistakes" become the true area of desired progress rather the result of a well-planned blueprint and/or directive.

NOTE: It cannot be overstated is the intent of this publication; optimized concrete should have a discontinuous capillary pore structure. This point was highlighted in the introduction of silica fume where reduction in permeability was likely a more important benefit than strength improvement.

A secondary area of importance after it was stated that silica fume concrete tended to suffer from cracking was the use of fly ash to help reduce heat of hydration when used as a substitution for Portland Cement.

Workability of the concrete is emphasized as well, particularly with slag, which has comparable heat of hydration reduction to fly ash. The densification is believed to be a function of reaction with calcium hydroxide and other alkalies.

Self Desiccation

NOTE: We have "modern problems" and issues that did not exist at time of this publication. Self desiccation of the concrete surface has reached critical proportions and is NOT being adequately addressed, if at all by way too many organizations and agencies.

From the Report: "One of the potentially detrimental side effects from the use of the low water-cement ratio concretes is self-desiccation, Self-desiccation refers to the process by which concrete dries itself from the inside. Internal moisture is consumed from within the paste by thehydration reactions, and the internal relative humidity continues to decrease to the point atwhich there is not enough water to sustain the hydration process. The result is that the hydration and maturity of the concrete will terminate at an early age if additional moisture is not provided. Therefore, self-desiccation effects are important considerations in the performance of high-performance concrete, particularly in the curing practices that involve “sealing” the concrete.

Self-desiccation may be especially harmful to the durability properties of high-performance concrete since the microstructure of the paste is adversely affected. Without adequate hydration, the near-surface regions become more susceptible to the penetration of deleterious materials from the surrounding environment. For this reason, proper curing of low water-cement ratio concrete at early ages is essential if the concrete is to attain its potential properties. The detrimental effects of self-desiccation can be largely controlled by careful attention to curing, especially during the initial 7 d after placement. Acceptable curing practices will be discussed in greater detail later in this report." NOTE: With the introduction of CKD between the years of 2002-2018, the "adequate curing" as outlined in early publications are, in my opinion not only inadequate, but there is a critical need to completely rethink and modify our entire approach to what is "adequate curing", particularly for high-performance concrete.

I will take the errors (and severely outdated information that can lead to errors) in the order as presented in this publication.

"Evaporable" versus "Non-evaporable" Water

The Study cites the following: "Powers and Brownyard defined evaporable and non-evaporable water in terms of an initially saturated, hardened paste according to its volatility. Evaporable water was defined as the portion of the water that escaped from a sample of saturated cement paste after it had been dried at 23°C (73.4°F) to constant mass in a vacuum desiccator containing a strong drying agent (magnesium perchlorate). The remaining non-evaporable water is essentially the chemically combined water plus some portion of the water in the gel pores, whereas the evaporable water comes from the capillary pores and some of the gel pores. These types of water will be discussed further in the section to follow on water fixation."

This information appears to be the genesis of what determines/determined what is considered "chemically bound" versus "free water". Although a step in the right direction, it was much later where it was discovered that the desiccant used (magnesium perchlorate) has what is termed a higher "critical humidity threshold" than calcium chloride (which was subsequently misjudged to be "over-drying" the concrete, giving exaggerated moisture content volume).

Magnesium Perchlorate begins to absorb moisture in RH exceeding 40%, whereas calcium chloride begins to absorb moisture at a RH of less than 22%. However; the two alkali components normal to concrete have critical humidity thresholds MUCH lower than calcium chloride (calcium hydroxide, a RH of less than 13% and sodium hydroxide a RH of less than 9%). This in turn gave us a cascade of errant assumptions and misinformation since it was assumed that calcium chloride was capable of over-drying, when in fact, the alkaline components within concrete are stronger desiccants than calcium chloride! NOTE: these critical humidity thresholds can be greatly influenced by temperature; where calcium hydroxide becomes increasingly soluble as temperatures decrease, sodium hydroxide tends towards higher solubility as temperatures increase. This complicates and undermines moisture content measurements using any form of RH (ASTM 2170) or desiccation method (ASTM F 1869, ASTM E 96). Such RH and desiccation methods CANNOT measure concrete moisture content.

NEXT: Assumptions and errors within this "State of the Art Report"