Concrete Surfaces to Receive Flooring Materials - Necessary Changes

In the confusion of who should be accountable for the various moisture-related failures over a concrete surface; there has essentially been zero progress in what TYPE of concrete needs to be placed, what EFFECTIVE moisture testing NEEDS to be done, and WHY are those, who are trained to install flooring materials being held accountable for a material/substrate where they have no input, nor have any idea what is needed or how to get what is needed.

Concrete - The REAL background versus what is Conveyed or What is Believed

Concrete specifications, even with ACI 302, which does classify different concrete for different end-uses are NOT giving an adequate product to ensure coatings and flooring materials will not have problems with moisture-related issues. Most of this is based on legacy beliefs that although proven to be invalid, still persist and remain essentially unchanged for the past 100 years or so.

Concrete is believed to be somewhat homogenous and consistent throughout and is evaluated by compressive testing. Standard compression testing does not and cannot differentiate a weakened or permeable surface that is common to field concrete and to a lesser extent, laboratory prepared samples.

Concrete surfaces, particularly in the top 19 mm - 25 mm (0.75 inches - 1.0 inches) presents the most drastic gradient in essentially all placed concrete. In studies conducted globally and for the past 20+ years, with improved technology and applied testing that wasn't a matured technology in years past, it has been found that the gradient portion of the concrete is subject to self-desiccation, with even THAT revelation not being very well explored.

In several studies where embedded humidity sensors have been placed with thermocouples, it has been found that the concrete surface gradient (as stated earlier) tends to self-desiccate.

Like many others, I was NOT aware of this, nor had any idea how pervasive this has been historically. The self-desiccation prevents the most susceptible portion of the concrete to adequately endure environmental issues and ambient conditions to become the "weak-link" in the chain. Concrete surfaces that are designed to last decades are instead having significant issues within 15-20 years. The concrete surface gradient not only self-desiccates, it does so very early on after placement. In only 2-3 weeks after placement, concrete surface gradients have been shown to have a sharp reduction in RH, as low as 50-60%!

To initiate and continue cement formation, the RH MUST be higher than 80%, or cement formation ceases. Even in a lab setting, Dr. Zollinger with the Texas Transportation Institute conducted a study to compare the effects of an air-cured, versus 7 day water-cured concrete.

The surprise was not that the 7 day water cure produced a stronger concrete surface, but the true surprise was that in the compressive values, both air-cured and water-cured samples achieved the "design strength" of 4,000 psi; this surprise was that when the top one inch of the concrete was sliced off and evaluated for compressive strength, the 7 day water cured concrete surface was still a full 20% weaker than the remainder of the concrete.

Site Influences exacerbate a more porous and weaker concrete surface

There are activities within concrete that have not been evaluated, which has baffled me for decades, these activities create a concrete surface that is very vulnerable to temperature and humidity immediately and for several weeks after placement.

To optimize the durability, lower permeability and lower porosity of a concrete surface, the internal RH should remain at 90% or higher. The presence of water alone does NOT guarantee this!

There are many intricacies and interconnecting influences that it would be difficult to explain these adequately in a single article. Instead, I will simply focus on temperature, cement formation and alkalinity.

The "Conspiracy of Elevated Temperature, Cement formation and alkalinity" Inextricably intertwined.

Studies dating back to the 1950's, even the 1940's have indicated that elevated temperatures can create higher early (28 day) compression values, even as they decrease the long term (365 day) compression values. This is but one area that has been neglected for proper evaluation.

Later, it has been found that as water-cement ratios have been lowered to produce a "higher quality concrete", there were many inexplicable situations where the concrete was no where near as durable as the lab studies indicated.

As this was more thoroughly explored, it was discovered that in the cement formation process, the lowered moisture content would create a self-desiccation, particularly in the concrete surface gradient. This led to cracking, curling and warping of the concrete.

Different approaches were and continue to be used to try and reduce or compensate for the surface damage, with very little success. This was even to the point where the NIST State of the Art Report on Concrete (published 1999) indicated concrete will crack and curl and it was unrealistic to believe otherwise.

Later, ACI and ASCC both adopted a measuring standard that required the flatness testing to be conducted within 72 hours of placement, with the acknowledgment that concrete will continue to crack, curl and warp with time.

The most frustrating point of this, if concrete were made to reduce or eliminate the surface desiccation, these accepted inevitabilities would NOT be an issue!

Surface temperature has a profound effect on one of the main cement hydration products, calcium hydroxide. Calcium hydroxide, unlike most other alkaline materials within concrete, does NOT become more soluble with increasing temperatures, but rather becomes LESS soluble. This effectively neutralizes secondary cementitious admixtures, rendering them not only ineffective, but likely a CONTRIBUTOR to the self-desiccation since these particles must be dampened in order to react and essentially compete for water that is needed for the primary cement!

As less moisture is made available, this concentrates the alkalinity of the remaining moisture. With increased alkalinity, there is a reciprocal reduction in RH!

If sodium hydroxide, and/or potassium hydroxide concentrate, the formed alkaline solution will reduce the RH. At a concentration of only 20%, the RH will be reduced to 78%, which prevents cement formation. Further adding to this unwanted combination is that the presence of sodium and/or potassium hydroxide inhibits the solubility of calcium hydroxide, even when absent of elevated temperatures! Adding the elevated temperatures, to the alkalinity and reduced moisture, the concrete surface gradient consistently self-desiccates.

Standard Concrete Practices and Standard Concrete Curing Practices are Inadequate to produce a quality concrete surface.

With some of the inexplicable emphasis toward internal RH measurements as a method of measuring moisture in concrete, these trends actually move us away from the area of concern, which is the concrete surface gradient.

Even more mind-numbing to me, is that RH probes, used in compliance with ASTM F 2170, by-pass the problem area entirely!

Add that RH probes were evaluated ONLY to measure the initial drying rate of concrete, and still to this day, I have yet to ready ANY empirical data that can justify the use of RH probes for concrete that has been in service. Even the Lund University study used to qualify RH probes for drying rates has cautioned not only are there different drying curves with concrete that has been in service or has has been rewetted, the study also pointed out that alkalinity LOWERS the measurable humidity even if the moisture content remains unchanged...and this was in a lab setting!

The coupe de gras; Even when the announcement was made for RH Probes were to be marketed for moisture testing of concrete, the effective range given for the RH Probes in that announcement was 10-90% RH...NOT above or below those numbers. NOTE: This limitation has been confirmed by several studies that indicate RH measurements were unreliable above 90%, yet RH numbers above 90% are being presented as absolutes, even when these absolutes are NOT confirmable nor reliable; AND The ASTM F 2170 does NOT list concrete moisture testing, but lists humidity testing as the Standard undermines ALL claims to the contrary.

In other words, any claim of RH probes measuring concrete moisture is false...and we can add that humidity is measurement of water vapor, which is what would be measured in any given open space; i.e. crack, pores, rock pockets, etc., but NOT the concrete itself. RH measurements do not and cannot measure moisture outside of a vapor form.

How this method gained any traction is a testament to marketing rather than any resemblance to science.

Making Durable Concrete - IT HAS to be internally/self curing

This technology not only exists, but the track record of the verifiable products has been nothing short of amazing.

It is ironic that some of the absorptive aggregates that were vilified in the past, have actually shown to be a good source of internal moisture that helps to replace the moisture consumed by cement formation. This type of aggregate is very beneficial, but should NOT be considered a stand alone by any measure. For internal curing to be effective, it needs to be well dispersed and even throughout the concrete, otherwise internal strains could occur, reducing the optimization of the internal/self curing concrete.

Next: How to effectively specify and produce internally/self curing concrete