The Fallacy of Using Truncated Data in Testing and Analysis of Concrete.

Way too often we are either panicking into decisions by relying on truncated data, or lulled into a sense of security by relying on truncated data.

I will be giving an example of how easy it is to be manipulated by what isn't being shown, as much as what IS being shown.

Calcium Hydroxide (Portlandite) - By-product of cement formation

It has been established that the pH of calcium hydroxide is somewhere in the range of 12.4-12.5.

Like many others, I used this pH as a reference point in attempting to interpret testing and data....my mistake.

The pH of calcium hydroxide is "capable" of reaching a pH of 12.4-12.5, but for all practical purposes, it doesn't get there in any environment we as humans find comfortable (tolerable?).

How can calcium hydroxide get to a pH of 12.4-12.5?

Like many others and in study after study, this pH range gets thrown in as a constant....the problem is, this ISN'T a "constant".

Can calcium hydroxide get to a pH of 12.4-12.5? Sure, if the temperature is just above 0.25oC (32.5oF).

In an absolute sense, the pH can reach 12.4-12.5, but in a practical sense, the actual measurable pH is much lower than that. At a temperature range of 20oC (68oF) to a surface temperature of 50oC (122 oF), the pH range is 12-11.5. This pH range is assuming an absence of sodium hydroxide, which by its presence, reduces calcium hydroxide solubility.

The difference between a pH of 12.4 and 12 is not to be dismissed...the pH scale is logarithmic and a pH of 12.3 is approximately twice as alkaline as a pH of 12.

You can ONLY measure pH of a soluble product....until something dissolves into a solution (forming a liquid) all the humidity in the world won't allow a pH measurement..it HAS to be in a solution/liquid.

Tests that can be fooled by influences within the concrete: standard pH testing (meters, strips, etc.) and the venerable phenolphthalein test.

A pH measurement of any kind simply gives a number....and if that number is alkaline, it is usually assumed this is measuring calcium hydroxide...even as it measures other alkaline and non-alkaline materials that will typically be "hidden" by the most assertive chemical compound present.

The presence of sodium hydroxide, which is now more present since the EPA requirement of adding CKD (Cement Kiln Dust) to cement production, it is more difficult than ever to determine the presence of anything other than sodium hydroxide or sodium carbonate...which brings us to the second test that can be fooled: "Indicator test" (phenolphthalein).

The indicator test has been used for several decades to determine if concrete has carbonated or not. The simplicity is this: if phenolphthalein is applied to a concrete surface and there is no color change, the concrete is carbonated and requires additional testing and potential repair work, particularly for steel reinforced concrete.

The ASSUMPTION is that if there is a distinct color change from a bright red to a slight purple color change, the concrete has not carbonated.

In concrete, if calcium hydroxide carbonates, it becomes insoluble and will not change color using the indicator test...so far so good, except the reality is that not only is sodium hydroxide very alkaline and soluble, so is its carbonated form; sodium carbonate.

Worse, sodium carbonate is right in the "sweet spot" where a healthy amount of calcium hydroxide is present (11.5-12.0 pH). That is also the range of sodium carbonate.

Whether or not sodium carbonate provides protection against steel reinforcement corrosion has been artfully dodged. It simply isn't even mentioned in ANY of the corrosion studies, reports, etc., I have ever read.

Most reports simply have the "copy and paste" language that repeats ad nauseam: "Ideally speaking, a good concrete is supposed to provide adequate protection to the embedded steel. This is due to the protective alkaline environment (pH value as high as 12.5) provided by fresh concrete resulting in formation of protective coating on the surface of the steel, which passivates it from further corrosion."

Further complicating factors of the indicator test is that some suggest the indicator can become colorless if the pH is too high, with estimates as varied with a pH as low as 10 becoming colorless (which I've never found) to a pH of over 13 which produce no change in color.

Why this is so disturbing, if the concrete is carbonated yet "hidden" by the presence of sodium carbonate, are we inadvertently dooming many of our structures to further deterioration and putting off needed repairs ASSUMING the concrete isn't carbonated?

EVEN more disturbing to me...

1. Why has this slipped through the cracks for so many decades?
2. Where is the information on the potential passivation by sodium carbonate?
3. How did so many concrete experts miss such fundamental chemistry of and within the concrete?

Question EVERYTHING!

I grew up in the protest era of the 60's-70's to question authority. My biggest mistakes professionally and personally is when I have failed to question authority.

From my viewpoint, the collective biggest mistakes in virtually ALL industries, professions and science, is the failure to question authority.

1. In any and ALL testing, context is everything. If a study doesn't have the context, the study may not be worth the paper it is written on!
2. Truncated Data: sometimes a context will "appear" to be valid, but is using truncated data.
3. Since it is WELL-ESTABLISHED that elevated temperatures will decrease the long-term durability of concrete, why are we STILL producing study after study "proving" how elevated temperatures improve the 28 day compressive strength of concrete?

An example I am using for the upcoming Building Sciences course is comparing two solubility curves of sodium chloride.

In a compressed data example (using 10oC increments), the solubility curve of sodium chloride looks VERY impressive and capable of getting oohs and aahs from the audience; yet when presenting a less compressed (using .25oC increments), rather than a compressed temperature curve, the solubility curve of the sodium chloride is nearly flat.

It's ALL in the presentation folks!