Why We NEED to eliminate the 28 Day Concrete Compressive Strength Test as a "Stand-alone" Requirement

One of the most frustrating aspects of my consulting, is when everyone agrees high temperatures aren't good for concrete, but is desired because it quickens the construction schedule.

Elevated Temperatures PERMANENTLY Reduce Concrete Durability

Studies and reports from the 1950's and through today continue to prove that elevated temperatures WILL permanently reduce cement formation, particularly in the critical gradient of the concrete which is the top one inch of and into the surface.

Even though elevated temperatures damage concrete, each and every time concrete is prepared with elevated temperatures, history re\ars its head to remind us of just how bad an idea that was.

Why Isn't Elevated Temperature Damage Being Explored from a Chemistry Perspective?

Known to reduce the long-term durability, the effects of temperature are known but poorly conveyed, if at all to those who are dependent upon sound information in making important, if not critical decisions in design and construction, yet here we are....

Fundamentals - Know These First

Several years ago, I was part of an Educational Event where presenters from all the different disciplines were featured.

One presentation I was particularly interested in was being given by a Professor who was teaching at a prominent University, giving a course on the fundamentals of concrete.

At first I was enjoying the presentation since the Professor had an engaging style and the beginning of the seminar was technically sound. As his lecture went on, some of the graphics and statements began a slow departure from facts and fundamentals.

Two of the most disturbing to me was the continuous mix and match of high alkalinity and high pH, as well as permeability and porosity.

The confusion of permeability and porosity was exemplified by a graphic where a water to cement ratio curve was presented, and as a resultant effect, another graphic was shown stated to be a curve indicating how the changes in water to cement ratio affected the concrete permeability. NOTE: The graphic SHOULD have depicted porosity increase, NOT permeability increase.

Likewise, another example was given where he would talk about alkalinity then mix it up with high pH. He used the terms as though they were interchangeable.

I sat there thinking just how pervasive misinformation truly is when someone so well respected and qualified could make such fundamental errors.

I refrained from saying anything during the Q and A since I did not want to embarrass or distract from what was actually an otherwise useful presentation, given without a biased intent. If that had been a sales presentation, I would have torn into him.

After the meeting I explained that his graphic depicting the permeability curve wasn't accurate. He was a bit upset when I first brought this up until I explained how I have used a vitrified porous stone and a non-fired, non-glazed Saltillo tile to demonstrate the differences between permeability and porosity.

I would place both on the desktop, pour a small amount of water onto the porous stone. The water would bead up on the surface and resist penetration unless agitated (scrubbed). The Saltillo tile on the other hand appeared to be dense and solid with no discernible porosity, yet when water was applied to the surface, the water would soak in and disappear almost as quickly as it was applied.

I also brought up that high alkalinity and high pH were completely different circumstances since a higher alkalinity can have a lower pH than a high pH solution that has lower alkalinity.

As an example I brought up the Wool Safe Study conducted back in the mid 1990's. The study came about some time after I gave a presentation and asked about the damaging effects of a high pH on fabrics and other materials.

I stated the pH wasn't nearly the concern high alkalinity was. After getting a bunch of puzzled looks, I explained that high alkalinity pertains to concentration of an alkaline chemical and/or pertained to its resistance to pH changes when mixed with an acid.

I Had to Unlearn to Learn

Since then I have comes across MANY facts that weren't facts at all, but accepted because the person before said it was a fact and the person before them said it was a fact, and so on....

How deep the unlearning needs to go is when I gave a presentation to a large epoxy manufacturer and to a group of professors at an elite University.

BOTH groups were surprised by a statement I had made how many studies over the decades were fundamentally incorrect since the studies would simply lump the "alkalinity" of concrete into one big neat pile. The problem with that is the two main alkaline components in concrete tend to be diametrically opposed when it comes to solubility in changing temperatures.

Sodium hydroxide tends to increase in solubility as temperatures increase, as is the case with most chemicals.

This is where concrete throws us a curveball, the hydration product created during initial cement formation; calcium hydroxide becomes increasingly soluble with DECREASING temperatures as it becomes essentially insoluble with increasing temperatures....see where I'm going with this?

This clue of increasing temperatures causing the secondary hydration by-product of cement formation to become increasing insoluble is further exacerbated and creates a critical point of no return if external temperatures are present since the formation of calcium hydroxide is exothermic (produces heat).

In cooler temperatures, as the calcium hydroxide cools down, it can now react with pozzolans such as fly ash and silica fume as well as react with SCM materials as an admixture.

HOWEVER......if the temperatures remain elevated, the pozzolans, SCM and/or any other type of supplementary cement CANNOT react with the heated calcium hydroxide. NOTE: In heated water, calcium hydroxide will not dissolve.

Even worse, this becomes a "cascading effect" where the initial consumption of water to create the initial cement reaction is reduced in quantity, with the other particles, including the pozzolans, SCM, aggregate, etc. will now vie for the left over moisture unopposed by the calcium hydroxide. This in turn creates and even more hostile environment for additional cement formation since "the other" alkaline component, sodium hydroxide becomes more concentrated. One of the constant when sodium hydroxide becomes more concentrated, is the internal RH has a reciprocal lowering. Once the RH is lowered to 80% or less, cement formation ceases. It has been shown in several studies the internal RH of the top one inch (concrete gradient) can be lowered to 50-60% within 2-3 weeks AFTER concrete placement.

NOW the permanent loss of long term compressive strength starts making sense!

The Necessity of Self-Curing/Internally Cured Concrete

In an ironic twist, the absorptive aggregate, once vilified by the concrete and flooring industries for "adding" water to the concrete mixture, ASSUMING this moisture increased the water to cement ratio has been shown to be beneficial in adding moisture to the area where the self desiccation is at its worst.

This has prompted studies with SAP (Super Absorbent Polymers) and other techniques to provide replacement moisture for the moisture lost in this now common scenario.

It isn't getting much press since most researchers have incomplete answers. There are few things more panic-inducing than a serious problem without a definitive solution.

This is what prompted my renewed interest in nano colloidal silica.

Unlike the alkaline silicates, colloidal silica is a passive reaction much in the same manner as the pozzolans and SCM materials.

The most significant departure from pozzolans and SCM is that a colloidal silica contains its own water and the particles are extremely small.

The small size allows for a systemic and very thorough dispersion throughout the concrete, essentially leaving no unoccupied areas in the concrete. This is the achilles heel of the absorptive aggregate and SAP's, those cannot occupy as much area and in turn are less efficient in providing a consistent moisture source for continued cement development.

Just as importantly, the water supply and extreme surface to mass ratio means that the moisture being given off by the nano colloidal silica covers a huge area within the concrete. Not only providing a uniform moisture content, but providing an evaporative cooling effect, leaving the calcium hydroxide in a consistent reactable condition.

Caveat Emptor

Due to the increased interest in the nano colloidal silicas, there are plenty who are hopping on board to introduce their latest "cure" for what ails concrete.

Not-so-fast! Nano colloidal silicas can be VERY tricky and finicky in practice, with not all colloidal silicas being suitable for concrete and some, even though technically suitable, are too inconsistent in the field.

There are a LOT of reasons for the inconsistency that I am not willing to go into simply because the parrots comes out and inject this type of information into their literature and then appear to understand the nuances.

MAKE THEM PROVE IT!

Do NOT use a product or methodology that cannot prove its claims or efficacy.

There are products that claim to have "conformed" or "complied" with certain ASTM and other standards, when in reality, the stated tests were never performed on the product, have no application within the claims or that the claims of conforming or complying to a certain standard isn't based on compliance or conformance since it is a comparative and not an absolute!

Additionally there are alkaline silicates that claim to be nano colloidal silica, when in reality, they aren't. A colloidal silica is pH responsive, and if a product has a pH greater than 10.5, it is likely an alkaline silicate, NOT a colloidal silica. To be very specific, if I take a modified colloidal silica idealized for concrete with a pH of 6-9.5 and added an alkaline component to raise the pH higher than 10.7, it is no longer a colloidal silica, it has been converted to an alkaline silicate!

That's the way the chemistry works.

Also, the pH range of a colloidal silica is only an indicator of applicability, NOT an absolute. Many colloidal silica types can become unstable for a variety of reasons, which is why I beat up those who approach me with claims of a market ready product. If I am interested in something, I am particularly brutal when it comes to veracity of their claims. If I'm not interested, I will spend very little time, if any for something that I already know can't function as claimed.

The 28 Day Curse

Look closely at the graphic depicted in the beginning of this article.

What should jump out is that each of these concrete samples were cured in the elevated temperatures for 28 days. Afterwards, each sample was kept at a temperature of 73oF and 100% humidity, for the remainder of the 365 days.

Damage from the initial heat exposure was proven to be permanent and largely unrecoverable.

If we keep doing the same thing over and over, we can't expect a different result..are you listening specifiers?

Suppliers supply what you ask for....we need to ask for something different.