Concrete Testing - Missing Critical Data

Having read some very useful studies, I have also found the same studies to be missing critical data, which is irritatingly consistent with virtually every other study I have read in the past 30+ years. The missing data involves most anything outside of a 28 day targeted value.

Don't get me wrong, the 28 day values are important, but can be every bit as misleading as they are helpful, particularly since virtually every specification for concrete compressive strength is based SOLELY on 28 day compressive values.

The Missing Critical Data

There are two areas that stand out where a 28 day value falls flat and can create a false sense of security:

1. Standard Compressive Strength Tests do not reveal differences between an inadequately cured versus an adequately cured concrete.
2. Standard Compressive Strength Tests do not and cannot be predictive for long-term durability, even as the industry seeks to provide a more durable/sustainable concrete.

Inadequate Curing is Nearly Inevitable

Using the current standards, no matter how tightly some may adhere to industry recommended curing process, it is a virtual certainty that any concrete placed and cured in elevated temperatures will NOT be adequately cured.

This is due to a combination of influences; the first being a more alkaline cement. More alkalinity creates a reciprocal reduction in humidity, even in the presence of liquid water applied to a concrete surface. The remaining mix water after the initial concrete bleeding and set begins to react with the clinker (unhydrated cement) to form cement. This in turn lowers the water content with a reciprocal increased alkalinity. Increased alkalinity reduces humidity of any available air space as it concurrently reduces free-evaporation of water. This effect is then exacerbated by the elevated temperature since portlandite (calcium hydroxide) becomes increasingly insoluble with increasing temperatures. This in turn reduces the needed contact between the remaining water and clinker (unhydrated cement).

Studies have shown that the RH with the top 19-25 mm (0.75-1.0 inch) is reduced below the necessary 80+% needed to form cement. This creates a permeable and porous surface within the range that leads to long term durability issues. This compromised area will NOT reflect in standard compressive value testing. The compromised area WAS revealed through a study conducted by Dr. Zollinger, who conducted compressive strength tests on concrete laboratory samples, where the top 25 mm (1.0 inch) depth of the concrete were tested for compressive strength on both 7 day water-cured samples and uncured samples.

The study indicated the uncured concrete surface was 1200 psi weaker than the remaining concrete, and even with a 7 day water cure, the surface was still 800 psi weaker than the remaining concrete, and this was with laboratory specimens, NOT field specimens which would likely be quite dramatically weaker than what would be experienced in a closely controlled laboratory environment.

28 Day Compressive Strength - WHY it is Misleading

There is a singular focus on construction schedules and meeting and/or exceeding the targeted 28 compressive values. This has lead to an unintentional reduction of durability - let me explain; In elevated temperatures, the 28 day values tend to meet and even well exceed the target values. This is regarded as an asset, when in reality, the higher the 28 day number, the more likely the concrete will have a reduced durability. It is largely due to more sophisticated monitoring have we been able to establish a reasonable causality of WHY elevated temperatures produce a less durable concrete.

28 Day vs 365 Day Compressive Strengths

Decades of studies have indicated the correlations as depicted in the graphic.

Recent studies conducted by the Texas Transportation Institute and many others have found RH have NOT remained elevated. Instead, it has been ASSUMED the RH levels would be and remain consistent during the curing period, which is incorrect and may have always been an incorrect assumption, particularly as cement grinds have been finer, producing a higher and more immediate heat of hydration.

Nearly every study has indicated that concrete cured in elevated temperatures is less durable than concrete cured in lower temperatures. However, because of the sole focus on 28 day values rather than incorporating 365 day values into consideration, specifiers are inadvertently REQUIRING what is inevitably deficient or even defective concrete, with very few aware of, much less addressing this mounting issue.

Interestingly, even the Hedenblad Study used by RH probes as an indication of veracity, inadvertently indicated the RH towards the surface didn't remain above the 80% RH needed for cement hydration. This is notable particularly since this study excluded any moisture that wasn't an original part of the concrete mix design.

Caveats

It is critical to note that some of the later studies may or may not have been affected by the EPA requirements that started in 2002, as the sourcing of concrete materials may have not have been in compliance with the "new" CKD additions.

Also ignored is how temperature alone can affect humidity: From Lund University: "An external increase of temperature by 1 °C in the air decreases RH by about 6%. An internal increase of temperature works in the opposite direction, i.e. increases RH in concrete. Since water expands more than the concrete matrix, RH in concrete will increase with the temperature, anyhow between 0 °C and 100 °C. The most extreme example of this phenomenon is the formation of water in concrete during explosive fire spalling [98]. Even in dry concrete free water is formed during accidental fire. When temperature increases also RH increases and the water moves from the concrete surface towards the inner part of the concrete. Once RH = 1 the water vapour is transformed into water and the pressure increases in the concrete pores above the external." NOTE: I disagree with the notion stated that the RH is increasing with temperature, water moves from warm to cool, which technically isn't an increase in RH, but the natural movement of moisture from warm to cool, and when an area of saturation is reached, the expansive force of the water exposed to high temperatures creates the spalling; NOT an increase in humidity. I am wondering if something was lost in translation and NOT what the author intended. Rather, the heat and spalling are consistent with increased concentration of alkalinity and reduction of free evaporation as the cause of moisture/heat spalling.

I have heard proponents of RH probes repeat the claim of humidity increasing with temperatures, and wondered where they could have extrapolated such unscientific nonsense...this does remind me of the Housing Bureau mis-printing moisture emission levels, expressing vapor emission levels for standard concrete reaching 12 gallons per day per 1,000 s.f. when the true number was likely12 pounds (slightly less than 1.5 gallons, rather than 12) and this mis-cited number was quoted as fact for MANY years. To put 12 gallons in perspective, the vapor emission levels would have to exceed 100 lbs per day per 1,000 s.f.