7 Day Water Curing of Concrete - It isn't WORKING!

Again I read a series of studies that concentrate ONLY on concrete compressive value limited to the 28 day strength.

Interestingly, there are discrepancies that are noted but not investigated, even as the discrepancies threaten to undo the data that was apparently the original objective.

Texas Transportation Institute Study

There are many international studies that closely correlate with the study conducted in 2006 by Dr, Dan Zollinger (Texas Transportation Institute), a very well respected concrete researcher, so rather than pile on all the corroborating data, I will focus on some profound findings that are lab results, which if considered closely, would be an optimal condition for field concrete that seldom even comes close to the well controlled laboratory conditions that eliminate many environmental factors that would otherwise influence the outcomes.

The 7 Day Water Cure Results Versus Air Cure

It is interesting that many of the other studies concluded that air cure, water cure and curing agents were nearly interchangeable in their results to where a 7 day water cure sometimes had a negligible to minor beneficial effect on the compressive value of concrete at 28 days.

In the TTI Study, air cured and the 7 day water cured concrete samples were measured for internal RH at the concrete surface and various depths of 0.5 inches, 1 inch and 3 inches.

NOTE: Cement formation ceases when the internal RH is less than 80%.

The air cured concrete: the top 0.5 depth, the RH fell below 80% at 13 days, the top one inch reached 84% RH at 30 days and remained above 80% for the 30 test duration, at 3.0 depth there was a significant RH drop off on the first day where the RH was recorded at 83%, the RH gradually increased to approximately 93% at 5 days.

The 7 day water cured concrete: the top 0.5 depth fell below 80% at 14.5 days, the top one inch fell to 80% at 30 days and the three inch depth fell below 80% on the first day, but gradually increased to 93% and remained between 93 and 92% for the 30 day test duration.

Compressive strength calculations were also made at these depths, with the air cured concrete having a 0.70 at the surface, 0.85 at 0.5 depth and 1.00 past the one inch depth.

The 7 day water cured concrete having a 0.80 at the surface, 0.90 at 0.50 depth and 1.00 past the one inch depth.

What The?!?!?! Take-Aways

First off, 80% RH is the MINIMUM humidity needed for cement formation. Cement will form, but not very quickly. The higher the RH, the more robust the cement formation. NOTE: If for any reason, the RH falls below 80% during the initial cure, cement formation will be lessened, permanently; how much depends on the chemistry of the concrete and environmental factors.

What appears as an anomaly, where concrete covered with a layer of water would STILL have a reduction in RH makes no sense, nor would it, in a open, unobstructed environment. NOTE: This is an argument that I have to constantly educate people on since they do not understand the complexities of concrete; the straight forward air pressure differentials have little relevance when concrete chemistry complexities are introduced.

Concrete ISN'T an open, unobstructed environment. As cement is formed, the internal moisture is reduced, creating an increase in alkalinity. With the depth and surface activity, even covered with water, the water is blocked by the alkaline solution which can be saturated, but also lower in RH. This doesn't and won't happen in an open environment, which people mistakenly believe applies to concrete...it DOESN'T, yet we can't get researchers off center to start dealing with the reality rather than a disproven concept that water retention alone will maintain a RH suitable for cement formation.

A water covered concrete surface cannot experience surface evaporation, so water loss from the surface is now eliminated as a cause of "moisture loss".

With a reduction in RH, particularly after only 14 days, there is definitely something else going on!

The other significant take-away here is that the concrete surface IS weaker, whether air cured or water cured, yet when surface tests are conducted with rebound hammers (ASTM C 803 and ASTM C 805), these are dismissed as "inaccurate" since these consistently give lower compressive values than the standard concrete core compression test.

This is more of an indictment of the compression procedure unable to discern a weaker surface, since these weaker surfaces tends to crush rather than break, and it is only at break that the concrete compressive strength is determined.

I feel that rebound hammers are an invaluable tool and criminally underrated and undeservedly dismissed. If we could walk it backwards, I would wager that the rebound hammers have been extremely accurate, yet due to the industry believing the surface of concrete is as strong as the remainder of the concrete, another assumption has cost us valuable information over several decades.

This is essentially confirmed by this study where the top one inch of concrete was sliced off and exhibited a lower compressive value than the full depth would indicate.

This study and others from the field that correlate quite closely, other than field conditions have a much more severe issue since those studies indicate the RH within the top one inch of the concrete can fall to levels as low as 60%-70% within the first two weeks of placement, which ALSO correlates with the lab samples in the TTI Study where the drop offs are happening within the first two weeks, in a highly controlled environment!

Why hasn't anyone Taught This?

It was only by comparing the various studies where I got an "aha" moment where the dots began to connect and the information we have ASSUMED was accurate where a water cured concrete will be uniform and homogenous is a false legacy assumption rather than a fact.

Self Curing Concrete To The Rescue

I have been a party to and have observed different concrete types that are partially and some fully self/internally cured.

Even absorptive aggregate has been found to be very beneficial, though inadequate to fully compensate for autogenous self-desiccation. The irony is that absorptive aggregate was uniformly derided as damaging to the water-cement ratio and quality of the concrete; instead, absorptive aggregate has been shown to improve the quality of concrete, yet that assumption delayed beneficial discoveries for DECADES!

I used to buy into that hogwash as well, so I've been trying to "unlearn and then relearn" what is fact versus the fiction I've been brainwashed with. I now challenge anything and everything until I can find out first hand what is root cause.

Many I looked up to in my formative years were wrong, because the ones THEY learned from were also wrong...so we have to "un-wrong" a LOT of information to start dealing with facts rather than legacy myths.

RH Testing - Useful AND Misleading

I admittedly have been unrelentingly savage when dealing with some of the RH Probe peddlers who have been misusing and misrepresenting an extremely useful tool.

RH measuring devices of all types have been extremely helpful and even indispensable when analyzing root cause damage and gauging the initial curing rate of concrete. That is where these belong.

It is an unforgivable disservice that a cabal of influential people have managed to con several industries into believing the emperor has new clothes, claiming RH Probes or ANY RH measuring device for that matter can actually measure the moisture content of concrete.

They DON'T and they CAN'T. RH does not measure liquid water...period...only water in its gaseous form, which exists in open spaces, not the concrete itself.

There are MANY reasons that building science prioritizes moisture control and drying in a very specific order: 1. Bulk water: any other form of moisture is essentially irrelevant until bulk water is either removed or not present, 2. Capillary water: any interstitial moisture can be a challenge to correct/remove, depending upon the chemical make-up of the building material. Any mineral-based material that contains capillaries have competing forces where the attraction of the water to the solid may be close to or greater than the attraction of the water to itself...this can present a LOT of challenges, particularly with concrete, where removal of bulk water and then capillary water becomes increasingly difficult. Alkaline salts in concrete concentrate as bulk and then capillary water is removed/reduced. As alkalinity concentrates, the resistance to removal increases as the reciprocal internal RH is reduced. 3. Water vapor: water vapor is WAY down the pecking order and is by several orders of magnitude less important than bulk or capillary water.

NOTE: One of the ironies with the data used by RH Probes to "prove" their usefulness for moisture testing of concrete is that Lund University, released a study on RH Probes that was specific to mix water, and even though the study cautioned regarding alkalinity and concrete that has been rewetted, this study is the one the F2170 is largely based upon which is to ONLY measure relative humidity, NOT moisture content (it doesn't say that anywhere in the ASTM Standard, so why is it being promoted as such?) and is NOT qualified nor tested for concrete that has been in service....then the irony goes further, this same university produced a study, predating the RH Probe (Hedenblad study) that states in no uncertain terms: "measurement of capillary water is more accurate that the measurement of water vapor when testing for concrete moisture content". Didn't the RH promoters know of the capillary test?

How does RH Testing Tie Into Concrete Curing?

There is technology where embedded humidity devices have been used, which , coupled with thermocouples, have DISCOVERED that field concrete tends to self desiccate WAY earlier than ANY of us could have imagined. Until these devices proved otherwise, everyone ASSUMED that current curing techniques were sufficient, even considered optimized.

We can take this same technology, coupled with an electrical impedance concrete moisture meter, thermocouples and actually measure and monitor in real time, the quality of concrete being placed!

This process could also be used to validate products and/or mix designs that can reduce or even eliminate concrete self desiccation.

Such products DO exist, have been functioning incredibly well, but lost in the land of confusion and snake oil salesmen. Rather than simply accept a good line of bull...how about we instead, set out to prove this and in turn, improve concrete to where it should be, not as it is.

We have the tools and the technology and now we have methods that can validate rather than ASSUME.