Avoid Applying Past Solutions to Current Problems. The Solution Should Fit the Challenge

After some wonderful exchanges and listening to younger and older experts in their respective fields (I do not use the term "expert" lightly), then reading some irritatingly inconclusive Research reports and studies after returning, it amazes me how the same approach(s) that haven't worked and WON'T work are still being insisted upon using circular logic.

Concrete - Then and Now

Over the past 100 years, concrete has undergone significant changes, yet these changes have rarely been dealt with effectively.

Part of the problem is that a lot can go wrong before anything is obviously wrong. That is the benefit and detriment to concrete. If all anyone wants is an inexpensive method to provide a walking surface and capable of supporting heavy loads, even garbage concrete will likely suffice.

In the past, concrete had much coarser cement, more cement, comparatively lax construction schedules, cheap labor, quality aggregate available from multiple sources and methods of coating and painting that are illegal today.

Ev Munro pointed out that UK cement in the 1920's through the 1950's had much coarser cement particles than modern cement. This singular difference CANNOT be overstated in importance!

Concrete of equivalent compressive value of 1950's concrete as opposed to 1980's concrete had 34% more cement. The concrete of that era was also 500% less permeable and because of the coarser cement, generated heat less intensely and more gradually than cement of today.

A couple years back during a seminar I was giving, the importance of cement fineness was contested in its importance.

Rather than going back and forth, I simply pointed out the current differences between Type I and Type III cement.

Type I and Type III cement are essentially identical in composition, yet Type III is not suitable for thicker concrete sections due to its higher heat generation during the initial curing. Type III because of its more rapid strength development and generation of internal heat is often recommended for winter concrete placement.

Why is there so much more heat generated by Type III? Cement fineness...Type I and Type III, despite being essentially interchangeable from a chemistry standpoint have that dramatic difference due to the much finer grind of the cement in Type III, which is why it is designated as "high early strength" cement.

In warming temperatures and particularly if the temperatures are elevated, type III cement tends to suffer significant cracking from internal heat generation.

Why is that an important point? Because the difference between 1950's cement and 1980's is even greater than that of the difference between Type I and Type III, yet little has changed in the approach to ensuring proper curing of the concrete.

More Differences Have Been Introduced

Since Mr. Munro's article, there have been significant changes in cement AND concrete that again, are being approached with procedures and processes that have changed little from the 1920's.

Plasticizers are now very common, with the formulations changing over decades to more efficient plasticization of concrete, to where different definitions were added to differentiate these newer plasticizers from those of a lesser effect (i.e. "Plasticizers" versus "super plasticizers", "water-reducers" versus "high-range water-reducers", etc.).

Also incorporated into the cement processing are grinding aids. The evolution of grinding aids has undergone significant changes, with the most common and dramatic being the chemistry and a move away from "mechanical" grinding aids.

The EPA "effect" - CKD (Cement Kiln Dust)

One of the more recent and dramatic changes in U.S. cement production is the requirement that CKD is recovered and re-introduced back into the cement production.

This is part of the questionable efforts (in my opinion; ill-advised) to lower the carbon footprint of cement and concrete.

The implementation of this switch occurred mostly between 2002 and 2018.

So how was this switch announced? The concrete industry announced in early 2019 that low-alkali cement will no longer be available.

That is about as much of an understatement as I have ever read!

What is even more dramatic, is that prior to the addition of CKD, NIST came out with a State of the Art Report/Summary (1999) that summed up how it was unrealistic to expect concrete that doesn't crack, curl or warp.

The ACI and ASCC have recommended that flatness testing needs to be conducted within 72 hours of placement, even to the point where it is suggested that flatness testing could begin before the relief joints are cut.

Recent Discoveries of issues with self-desiccation

With the massive push to lower the water-cement ratio of concrete, all these procedures have conspired to create concrete (even the HPC - High Performance and UHP - Ultra High Performance concrete) that self desiccates within the top one inch of the concrete surface exposed to ambient conditions.

These discoveries have been remarkably consistent as they are global.

Until these discoveries, it was always assumed that concrete would be reasonably uniform in density, permeability and strength. So much so that on-site compressive testing conducted with rebound hammers (ASTM C 803 and ASTM C 805) were considered only for estimation purposes since the rebound numbers tended to be consistently lower than the actual compressive strength tests conducted in the laboratory.

Turns out, the rebound hammers may be MUCH more accurate than credited for!

Self-Desiccation of the Concrete Surface

Using thermocouples and embedded humidity measurements, it was discovered that the concrete surface within the top one inch was self-desiccating in the first 2-3 weeks of placement, where some of the RH measurements were as low as 50-60%.

This was a very unpleasant discovery/realization as these results were essentially the same, irrespective of where the site tests were conducted globally.

It was assumed (there's that word again, ASSUMED) that a topical curing method, particularly a water cure would prevent the concrete surface from self-desiccation, it didn't, does not, will not and cannot!

This was confirmed in a study conducted by the Texas Transportation Institute where Dr. Zollinger took concrete samples in a laboratory setting and cured with air cure and a seven day water cure. It is interesting to note that both air-cure and 7 day water cure both managed to conform to the 28 day goal of 4,000 psi.

What the study did that was different, is the top one inch of the concrete was removed and the compression strength test was used on the now isolated surface.

As expected, the water cure was significantly higher in compressive value than the air cured. What was notable is that even with a 7 day water-cure, in a laboratory setting, the compressive value was a full 20% lower than the remainder of the concrete.

It is my suspicion that these and the continuing bad news being reported from around the globe has stimulated considerable research into alternative measures that significantly deviate from the traditional method used for past concrete mixes and mix designs that are no longer used or relevant.

To The Rescue - Self-Curing/Internally Cured Concrete

Over the past two decades, there has been an increasing amount of study dedicated to self-curing concrete, of which I whole-heartedly agree with.

These studies have made great strides in getting us to realize what is needed, even as some of the results were rather embarrassing.

One embarrassment is the oft-berated absorptive aggregate that was routinely dismissed in the past by the concrete industry as a negative, where it was assumed (there's that word again) the moisture contained within the aggregate would produce an inferior concrete. Rather the reverse is true in that the moisture in the aggregate helps to compensate for moisture loss, allowing more cement development.

The downside is that this aggregate is comparatively large and not as uniformly dispersed as it needs to be to produce an optimized concrete.

SAPs (Super Absorbent Polymers) have also been used, which when properly dosed and due to their smaller size, tend to be more evenly dispersed than the absorptive aggregate.

There are significant downsides though; unlike the absorptive aggregate, the SAPs do not contribute to the strength of the concrete, and there is a very narrow range of effectiveness that has yet to be fully established. Too little of the SAPs and the concrete can suffer localized restraint cracking. On the other side, too high a dosage of the SAPs can significantly weaken the concrete.

Pozzolans and SCM

In laboratory studies, which is one of my chief criticisms where lab results do not match field results, these materials have improved the density and durability of the concrete.

In the real world however, these are essentially powders with a huge surface to mass ratio that require a LOT of water to fully dampen these powders. The addition of water basically undoes much of the benefit so it is typical to use a high range water reducer and/or super plasticizer to reduce the water requirement. Unfortunately this creates a condition where these secondary cements can interfere with the initial cement formation, competing for the limited moisture available in the mix design.

This is where the CKD rears its ugly head to additionally interfere with the otherwise beneficial effects of pozzolans and SCM's.

CKD is very alkaline and with a higher alkalinity, the internal humidity of the concrete surface is reduced. Higher alkalinity always produces a reciprocal reduction in measurable humidity. For cement formation, the RH HAS to be 80% or greater. Once the internal RH drops below 80%, cement formation ceases.

Without the initial cement formation, the pozzolans and SCMs are of no value and in many ways, potentially detrimental to a durable concrete since these materials are still competing for the leftover moisture, depriving the primary cement from this increasingly scarce moisture. THEN the deck becomes stacked against the cement formation even more as the lower RH and higher alkalinity retards the solubility of the reactant hydration product; calcium hydroxide (portlandite).

If the temperatures increase, the solubility of the calcium hydroxide is lessened even more! NOTE: Unlike most chemicals, calcium hydroxide becomes more soluble as temperatures decrease and becomes functionally insoluble in warmer temperatures.

Editorial: Research studies frequently identify the self-desiccation and reduction of the long term durability assigned to elevated temperatures, yet fail to "connect-the-dots" when it comes to WHY elevated temperatures decrease durability even as it is KNOWN calcium hydroxide is adversely affected by elevated temperatures, which would inhibit cement formation as calcium hydroxide accounts for upwards of 15% of the hydration by-products, partially shielding the yet-to-be-hydrated particle from full moisture contact; in essence becoming a barrier rather than a receptor for moisture.

Nano Colloidal Silica - Moisture-Compensating Pozzolan

I was first exposed to the potential of nano colloidal silica through my friend and renown petrographer Shondeep Sarkar back in March 2000.

Some of the concrete properties were so dramatically improved, it seemed as though the results had been embellished, they weren't.

That being said, colloidal silica tend to be quite vulnerable to changes in temperature, exposure to sunlight/UV, and a host of other conditions to where the colloidal silica needed to be formulated and handled in a very specific manner.

Due to the passing of Shondeep where his input and laboratory were no longer possible, I abandoned my efforts to develop a fully functional and practical colloidal silica type, optimized for concrete.

There are many industries that use colloidal silica for hundreds of application types. My interest was specific to concrete, but I found many of the other uses could give useful information that might not have been otherwise discoverable within the application(s) with concrete.

A study conducted by the Army Corps of engineers back in the 1970's used a colloidal silica in a concrete mix design. The intent was to discover if the concrete mix design could be improved with the addition of colloidal silica. The focus was very specific to dispersion, which the colloidal silica not only was very evenly dispersed throughout the mix design, the presence of colloidal silica also improved the dispersion of all the other ingredients within the concrete mix design.

This ability to be well dispersed throughout the concrete mixture is a key element in producing concrete that does not self-desiccate.

The difference in effectiveness lies not just with the dispersion, but also with the characteristic that these nano silica particles contain water. These do not compete for the limited moisture available within the concrete and actually compensate for any moisture loss by adding the moisture contained within the colloidal silica particle upon reaction with the calcium hydroxide produced by the primary cement.

This provides another clue as to why there is so little cracking, curling or warping with a properly formulated colloidal silica, the released moisture creates points of evaporation in the vulnerable concrete surface. Moisture evaporation creates a cooling effect and the additional moisture prevents increased alkalinity, both favorable to cement formation.

Summary

Traditional curing practices are NOT getting the job done. We NEED to modernize our approach if we want to produce a durable concrete. Self-curing/internal curing is the surest way to achieve this goal.

Nano colloidal silica is in my view, the most promising technology to correct the modern changes in concrete.

Caution MUST be exercised in determining if a colloidal silica product has sufficient testing and track record to warrant its inclusion for a specification.

A performance specification is the most likely route to ensure a quality concrete.