The Dangers of Laboratory Testing Using and/or Relying on Truncated Data - Part 2

Non-Analysis of Chemical Reactions

This is another area of testing and study that has and continues to confound me, particularly in instances where there is a precipitous and unexpected drop of internal RH during the curing process.

The Noted RH Drop in Concrete Studies

In several studies, the use of thermocouples and RH measurements in concrete have revealed unanticipated reductions in the RH within the concrete, particularly within the gradient portion of the concrete which consistently ranges from 19-25mm (0.75-1.0 inches) into the exposed surface.

In every technical article and lab study I had seen (prior to using the combination of thermocouples and RH measurements) has asserted that a well cured concrete surface will develop its full strength and that most, if not all the cement will hydrate if the concrete surface remains moistened.

As it turns out, this was an ASSUMPTION. I have scoured research papers and studies and have yet to read any empirical data that confirms a dampened concrete surface will fully hydrate the cement clinker.

Recent studies over the past 15-20 years have indicated that standard concrete mix designs, and even more so the HPC (high performance concrete) and UHPC (ultra high performance concrete) WILL self desiccate.

The only portion of this self desiccation that has been clearly identified is the cement formation consumes the available water and with lower water-cement ratios, there is insufficient water to hydrate the remainder of the concrete.

Truncated Assumptions

The lack of sufficient moisture was assumed and others ran with it, not asking enough questions.

The uptake of moisture by cement alone does NOT adequately explain the precipitous drop of internal RH. It takes VERY little moisture to create an RH of 100%, even in elevated temperatures. There were other influences either ignored or simply not being identified.

In further research after discovering that a dampened surface will NOT produce a concrete surface with the assumed full cement formation, I began to add up what is known about concrete/cement chemistry and seeing if the chemistry may lead somewhere useful.

I interacted with a gentleman on Linked In by the name of Robert Rodden who was at the time, working with a company MEGASLAB? . He posted a graphic by a researcher with the Texas Transportation Institute Dan Zollinger that REALLY piqued my interest.

Dr. Zollinger cured concrete by air curing and a 7 day water cure (which is considered by many to be a field optimized method of concrete curing).

The concrete specimens BOTH had a stated compressive strength of 4,000 psi. But Dr. Zollinger did something quite different from other researchers; he took the top one inch of the concrete and measured it independently for compressive value.

His results confirmed to me that self-desiccation can occur even in carefully controlled lab conditions. The top one inch of the 7 day water cured concrete was a full 20% lower in compressive value than the value given to the fully intact concrete specimen. Note: I later contrasted this to a sample where the concrete was internally/self cured where the surface of the concrete was actually denser than the remainder of the concrete, quite a contrast!

Connecting The Dots

Once again, I had to ignore decades of what I thought was factual information being disseminated from concrete research and began to establish a logic to the sometimes dramatic differences between what concrete does in a laboratory setting versus field conditions and what the chemistry in cement and concrete may contribute to a concrete surface not being fully, or in some cases with poor and incomplete cement development.

These Negative Influences towards cement development were considered:

Elevated temperature - alkalinity - availability of moisture/water - interference of legacy Issues and Assumptions

Elevated temperature: greater temperatures have an initial increase in cement development, then a drop off, with the severity of this drop off increasing with the increase of temperatures. This has largely been incompletely explained and remains essentially unexplored. NOTE: Unexpectedly, one of the best resources for the negative effects of elevated temperatures with concrete came from steam cured pre-cast concrete data. The steam cured concrete ALWAYS has a very permeable surface, but the strength gain is very quick and the internal portions of the precast are very strong. NOTE: If the second law of thermodynamics is placed here, that makes perfect sense; the heat will increase the impetus of moisture to move from warm to cool, creating a potential "super-saturation" of the concrete interior, virtually guarantying a full hydration of any available cement. So the end result is an extremely permeable surface and a very low-perm interior.

Alkalinity: alkalinity, particularly with regards to sodium hydroxide have multiple negative influence when regarding cement development. I will take a deeper dive immediately after describing the negative influences.

Availability of moisture/water: This physical presence of moisture and water are assumed to be adequate, they are not. MANY studies have claimed that poor curing is due to lack of moisture in the capillaries, but the supporting data is at best questionable. The steam curing where the internal portion of the concrete is cooler and at 100% RH saturation does not appear to be dependent upon moisture volume, but the RH remaining well above the minimum RH of 80% to assure cement formation. The availability of water/moisture appears to be directly tied into temperature and alkalinity.

Interference of Legacy Issues: This is a biggie. I coasted along for over 30 years not questioning "facts" I was given because everyone "knew" what the facts were. It was very sobering to realize many of these facts were legacy issues, guided mostly by assumption or the non-recognition of critically important changes that occurred with cement chemistry and concrete practices. The legacy issues did not keep pace with the changes.

Alkalinity and It's role in the development or NON development of cement

Pick up any inspection book or read about pH testing and the range given for newly placed and healthy concrete is a pH range of 12-13. When the pH is measurable at 13 or higher, that is NOT the natural alkalinity of cement hydration, yet this pH range is blithely mentioned with no cause for differentiation nor influence, yet this may be the single most important, as well as neglected portion that could explain most, if not ALL the "mysteries" surrounding the puzzle of optimizing cement formation and in turn, good quality, durable concrete!

I read a 1999 NIST State of the ART Report on Concrete and read yet another example of truncated data that can steer researchers in the wrong direction, or to simply ignore potential influences of alkalinity, specifically sodium hydroxide.

The Report stated that sodium hydroxide acts as an anti-freeze, then unceremoniously dismissed and rarely discussed as a factor in durability except for potential damage from ASR and the damage to coatings and adhesives.

The Report uses the left side range as shown in the title graphic. Up to a 35% concentration, sodium hydroxide DOES act as an antifreeze, even though the variability changes quite dramatically with concentration.

At higher concentrations however, sodium hydroxide can become something of a freeze initiator...in other words, it changes course and does the OPPOSITE of what is described in the report!

At 40% concentration, this will cause a chemical "freezing" of the solution (sodium hydroxide and water) at 15oC (59oF) rather than the EXPECTED 0oC (32oF), a dramatic departure from being an "anti-freeze".

This is where too many assumptions originate: truncated data can lead one to believe that chemical reactions are linear, exponential or even predictable. This is created by too many reactions reviewed as a singular reaction or as a singular event....we HAVE TO STOP THIS!

Chemical reactions, particularly in concrete are almost never a "stand-alone" reaction and be influenced by other reactions that can increase, decrease or simply change the anticipated reaction where these influences are not noted nor included.

Liquid water can and will go through changes with changes in sodium hydroxide concentration.

Sodium hydroxide WILL inhibit the solubility of Portlandite (calcium hydroxide). By inhibiting calcium hydroxide solubility, the calcium hydroxide changes from a slightly soluble compound to a water-resistant compound and wherever present as it originates from the unhydrated clinker that is designed to eventually produce cement, this now insoluble Portlandite inhibits moisture contact to the unhydrated clinker surface.

As the initial cement formation begins, there is a heat of hydration that creates a dormant period in cement formation, the initial cement formation which reduces the available water now produces a higher concentration (increased alkalinity). Sodium hydroxide WILL reduce measurable RH with the concentration levels appearing to coincide with the unexpected reduction of internal RH in the top one inch of the concrete, where in the first 2-3 weeks, the internal RH can decrease to levels measured as low as 50-60%. Cement hydration ceases at RH levels lower than 80%.

The concrete surface, exposed to elevated temperatures, particularly with direct sunlight and the increased alkalinity not only inhibit initial cement formation, this essentially renders any SCM or pozzolanic material ineffective and actually deleterious since SCM's and pozzolans are very fine particles and MUST be dampened to be effective, which in turn competes for the same moisture needed to hydrate the clinker.

The lowering of the RH and increased alkalinity also creates "barriers" within the concrete.

Moisture wants to migrate from warm to cool, even as alkalinity becomes more chemically reactive with increasing temperatures, producing something of a "stalemate" within the concrete where moisture simply stops moving, caught between competing forces. NOTE: This HAS been noted in at least two studies, but once again, frustratingly NOT explored.

Being noted, but not explored, leaves researchers puzzled when an area of 90% RH is immediately adjacent to an area of 60% RH, with little to no change over indeterminant periods of time.

So as the concentrating alkalinity causes the sodium hydroxide/water solution to increase its freeze point; this creates a force attempting to absorb more moisture and creating a subsequent lowering of RH and lowered evaporation, which allows the concrete to heat up much faster than concrete with an unrestricted surface evaporation that helps to cool the surface (evaporative cooling effect)...little wonder concrete surfaces self-desiccate! Too much emphasis is placed on retaining moisture, NOT retaining its availability.

Self-Curing/internally cured Concrete and the Need for monitoring effectiveness during and immediately after placement

In the past, when EVERYONE "knew" that moisture migrated from the bottom of a slab to the top of the slab, absorbent aggregates were referred to in VERY derogatory terms, with the ASSUMPTION (there's that word again) such aggregate would increase the water-cement ratio.

Those assumptions were incorrect (in the past, I had bought into this assumption), yet repeated often enough to become "fact".

It has been found that absorptive aggregate actually helps to replenish some of the lost moisture that occurs during the initial hydration process.

This has created a reversal of emphasis where products that can be added to concrete and not interfere with the initial water-cement ratio while giving the concrete a non-disruptive moisture "reservoir" are being evaluated at an increasing rate.

The problems with some of these is poor distribution within the concrete. This can create isolated areas of restraint, whose effects remain largely unknown as to whether this produces other down-the-line issues/concerns (this needs to be explored).

There exists technology that can actually monitor and either confirm the effectiveness of a given concrete mix design and whether or not the attempt(s) and/or materials are sufficiently effective for improving the quality of concrete.

The technology consists of as yet matched up devices that can measure compressive value, density, internal RH and moisture content within the concrete and scalable to deeper and shallower depths.

Because of legacy Issues, the top one inch of the concrete has all but been ignored, even though it has been proven the top inch is the most susceptible portion of the concrete in water-tightness, weather ability and durability. The monitoring should be able to give information within hours or days rather than months or years as we are currently stuck with.

It should take only a couple years to confirm and reaffirm the initial readings with long-term durability. Then this affirmation can be used to eliminate the inevitable waiting period that comes with "guessing" what happened or may have happened after the concrete was placed and cured.

Reboot

Concrete technology needs a serious reboot so we can start shedding legacy myths that are contributing to problems that should have corrected decades ago.

If you want green and sustainable concrete that lowers the carbon footprint, look no further than increasing the durability of the materials at hand rather than constantly trying to reinvent the wheel.

Novel and exotic SOUND good....but as what was experienced with the super-strong concrete debacle in the 1990's and the recent issues with Type IL cement; what sounds good can be steering us in the wrong direction.

There is no good reason we can't take existing materials, technology and provide concrete that will last many times longer than anything produced since the Roman times.