Concrete Assumptions, Myths and Legacy Errors that Persist - Part 2

A "shot" across the bow of misinterpretation in misperception

A common misperception by WAY too many is that moisture in concrete is primarily in vapor form. The majority of moisture in concrete is in a liquid form; which WAS recognized by the referenced Report: "Water of constitution is the water that is bound chemically in the hardened paste and is an integral part of the solid hydration products (chemically bound water).

Water contained in the gel pores of the paste that is bound physically by the surface forces of adsorption (gel water).

Water that is held within the capillary pores of the paste (capillary water). "

Interestingly, there is an allusion to gravimetric, even though it isn't specifically identified, which is the measurement of moisture that is NOT chemically bound, or as identified within the report (and I love the term) as; "Water of Constitution". GREAT designation since it avoids dynamics involved with what constitutes bound versus unbound water.

Self-Desiccation, the Common Problem that is "Commonly" never mentioned!

In the Study, self desiccation was well identified and well described: "When the capillary pores in the paste become partially emptied due to the consumption of water during hydration, self-desiccation may occur (self-desiccation is an important consideration in the curing of high-performance concrete and will be discussed in more detail later in this chapter). Studies have shown that, for low water-cement ratios, even specimens cured properly in moist surroundings are not able to adsorb enough water to avoid some level of self-desiccation." NOTE: I recently spoke to a representative of one of the largest cement producers who simply dismissed self-desiccation as a concern. To say I was irritated is a gross understatement; this issue is the most important when it some to flooring and coating of concrete. The surface of nearly all concrete being placed today now suffers from self-desiccation (as noted in several global studies) with the advent of the added CKD int the cement production process.

What was True then, is no longer True

One of the clearly and accurately identified aspects of cement hydration is just how important the need is for full hydration: "The significance of the degree of hydration in the proper curing of any type of concrete, particularly high-performance concrete, cannot be over emphasized. Hydration produces the solid binding material that gives concrete its most basic engineering properties - strength and durability. The rate of hydration is greatest immediately after final setting and decreases gradually with age. Under certain conditions,hydration can continue for years. The key feature for continued hydration is the presence of water-filled capillaries. When water-filled capillaries cease to exist, hydration stops and the mechanical properties cease to develop."

Why the last statement is no longer true; with the addition of CKD (Cement Kiln Dust), the increased alkalinity can create conditions where the internal RH is reduced to lower than 80% even in the presence of water-filled capillaries. In the presence of salts, particularly alkaline salts in concrete, it is the concentration of salt, rather than volume of moisture that dictates the humidity level(s).

The Pivot Point and is CRITICAL to Understand the concrete of today

"Self-desiccation can influence the maximum degree of hydration that is achievable in a given mixture. If there is not a sufficient supply of water, it will not be possible to attain the potential maximum degree of hydration. Proper curing practices may help to offset the tendency toward self-desiccation, which can be a problem in low water-cement ratio concretes.

"However, for a low water-cement ratio concrete, the moisture transport properties of the outer layer become very low with continued hydration, effectively preventing the passage of an external supply of moisture to the self-desiccating interior region. Thus it is not clear whether self-desiccation in low water-cement ratio concrete can be prevented. "

Read that last paragraph, study it and KNOW it! THIS is why I keep telling people that todays concrete CANNOT be properly cured using current concrete mix designs or techniques...todays concrete MUST be designed to be self and INTERNALLY cured. We have already been told this, shown this, proven this, but too many refuse to believe it!

Self-Desiccation and moisture testing

With the reality of higher alkaline cements, the self desiccation of the top 19-25mm (0.075-1.0 inch) of concrete is nearly unavoidable UNLESS the concrete contains an effective method to internally cure.

The irony in this is that any emphasis towards a higher quality concrete can yield negative benefits when this reality is ignored or not considered.

The Compounding of Negative Effects through Misunderstood Test Methodology

This is a biggie......when the concrete self-desiccates, it can actually fool an inspector or anyone else who is trying to measure the moisture content if that person is using any form of humidity measurement (ASTM F 2420, ASTM F 2170) or a desiccant method (ASTM F 1869, ASTM E 96).

As self-desiccation progresses, the alkalinity has a reciprocal increase since there are more solids/salts in the remaining water, creating an ever-increasing concentration, which in turn has a reciprocal lowering of the internal RH.

A lowering of measurable RH happens when there are forces involved that reduce the free evaporation of moisture. The ability to either extract the moisture using calcium chloride or other desiccants is severely compromised and as a result reduces the measurable humidity and/or moisture uptake/vapor emission, giving "low" moisture (water vapor/humidity) measurements; with contained liquid moisture remaining undetectable! NOTE: Several studies indicated that the RH within the top inch of concrete in the first 2-3 weeks of the critical initial cure can have an RH in the range of 50-60%. Cement formation ceases once the humidity drops below 80%. Note: I and nearly everyone else has been taught that internal humidity is uniformly high all throughout the concrete as the concrete cures, particularly if it is covered with water for the first 7 days, that is a FALSE precept and is but one area where accuracy in analysis can be undermined.

NOTE: The theory that concrete will equalize to a steady and uniform RH in field conditions would be laughable if it weren't for so many people believing that fairy tale. READ this portion again: "....the moisture transport properties of the outer layer become very low with continued hydration, effectively preventing the passage of an external supply of moisture to the self-desiccating interior region. Thus it is not clear whether self-desiccation in low water-cement ratio concrete can be prevented. "

What this means: Concrete has a series of gradients that have different responses to the environment, with the most important gradient being the top one inch of the concrete, yet we are led to believe the interior and underside moisture is "the issue" which by and large is an absolute crock. Second Law of thermodynamics has moisture movement from warm to cool...so considering the underside of concrete is typically cooler than the top of the concrete and the ambient conditions over the concrete surface are typically warmer than the top surface of the concrete, we are supposed to forget facts and believe that moisture within concrete magically ignores the second law of thermodynamics.

In Part Three, I will cover the contradictions within the study versus fact (even from within the composite report itself).