Wiss, Janney, Elstner Associates, Inc.’s Post

Thank you all for your comments and likes to the post. Please continue to share your thoughts and insights on the topic. Some of my thoughts/observations: 1. Secondary deposits do not always loosely line air voids or occur as long needle crystals featuring "opportunistic" growth in free spaces. 2. Air voids densely packed with short ettringite needles are often observed and are a good indicator that the concrete has been saturated. 3. Smaller air voids are filled before larger ones. Voids, 1 mil or smaller in diameter, when fully filled with ettringite, can be overlooked in air void analysis using lapped sections, resulting in inaccurate parameters for ASTM C457. 4. My calculations indicate that a thin layer of deposits with a thickness of 20% of the radius of an air void will account for approximately 50% of the void's volume. Put another way, air content will decrease by 50%. A filled void is essentially not a void anymore.

5. The same amount of secondary deposits (a layer of 20% of the radius thickness) would cause an increase in the spacing factor by 25% (decrease in specific surface by 36%). 6. With this alteration of the air void system/parameters, the ability of the air void system to mitigate freeze-thaw distress will likely be compromised. It is a matter of how much quantitatively. 7. A couple of references I found useful are linked below, and I will share more as I come across them. https://www.fhwa.dot.gov/pavement/pubs/hif17009.pdf (PDF) Why is the Air There? Thinking about Freeze-Thaw in Terms of Saturation (researchgate.net)

There are a lot of articles on this topic in the past, one inmediately came to my mind is by Detwiler et al. who stated “The observed large deposits of ettringite are a consequence of saturation and freeze-thaw damage, not a cause.” https://journals.sagepub.com/doi/abs/10.3141/1893-10

Before responding, I would love to know the age and exposure of this concrete. Since it is intentionally air entrained, it is likely an outdoor exposure. Your point that concrete changes is critically important. Another interesting aspect is the air entrainment chemicals are organic and subject to deterioration, particularly in an alkaline environment such as concrete. this could partially be an accumulation of materials on a now “sticky residue”. Wood resins, which are also used as air entrainment WILL develop a sticky residue as a result of decomposition.

Understanding air could hold humidity, may I ask at which percentage those mini pockets of air become dangerous for the concrete estructure?

Good topic Hugh. In a lot of our older dam and airfield concrete with wet/dry exposure I often see voids like you’ve shown that are almost totally filled with ettringite. Yet they don’t seem to exhibit any freeze thaw damage even with severe exposure. It made me think that the voids are still functional since the precipitates dont fully pack the void or perhaps when saturated many of the deposits dissolve back into the pore solution. Some in-situ monitoring with maybe microCT while varying saturation would be interesting. Thanks for broaching this topic.

Beautiful images, congrats! And a good topic for discussions. Hugh, do you have an idea about the sulfate source to produce those secondary ettringite crystals? In our studies (not about FT) we found two possible sources from lab analyses and SEM-EDS & XRD investigations on concrete cores drilled from some structures of generation projects and also on concretes cast in the laboratory with long term investigations: 1. ISA-DEF produced from high temperatures in fresh concrete due to hydration heat of cement and reduced formation of primary ettringite - later, DEF occurs; and, 2. ISA from sulphide minerals from aggregate, such as pyrrhotite and chalcopyrite. In both cases expansions had occurred leading to cracking of cement matriz and detachments in the ITZ, causing also reduction in the mechanical properties of concrete.

Could these voids becaused by organic mater or other particles that may be found in clay. With Portland Cement using roughly 20% clay, there seems to be a good possibility of impurities getting into dense packed UHPH via variances in clay. Hydralic cements do not use clay, and Portland Cements who use refuce clinker by adding fine ground limestone would also seem to have reduced anomoloies that could cause issues with freeeze/thaw and retained moisture.

It's an interesting research area! It's not limited to the compounds you mentioned. The secondary CSH has the potential to grow inside voids as well. In terms of flexibility which we're looking for, it depends very deeply to flow dynamic of water especially before the freezing cycle begins. Unstable crystals or quasi polymer chains like portlandite, may strengthen or weaken the structure of the bubbles based on location (totally inside or bridging on the surface of bubbles. On the other hand, secondary CSH structures usually fortify and stretch the bubbles while keeping some flow rates inside them (in the case of capillary connections). It's a wide-range discussion and needs to be specified based on the environment dynamics and constituent materials.

I'd be happy to read about secondary "ettringite" (with a bit of Si, so somewhere in the ettringite -thaumasite series) totally filling the porosities in old concretes. I have samples "for fun" and I always wondered about the possibility of deleterious ettringite filling such porosities.