Consider the piece of spalled concrete shown in the attached image. It has spalled due to the corrosion of the reinforcement bar. There is a small circular piece of metal embedded within the spalled concrete. What is it?

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Additional image of circular objects still in-situ in reinforced concrete air tower structures:

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Additional close up image of concrete plug containing metalic horseshoe like object from another part of the wall.

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  • $\begingroup$ A picture of the object that is in focus would certainly help. $\endgroup$
    – Fred
    Jan 4, 2021 at 20:19
  • $\begingroup$ Is there an image of the other side? - i.e. where the spalled part came from. $\endgroup$
    – D Duck
    Jan 4, 2021 at 20:25
  • $\begingroup$ @Fred I've added another photo to the question. $\endgroup$
    – egg
    Jan 5, 2021 at 12:58
  • $\begingroup$ I think we need better photos. I note that there isn't any staining around the circular thing in the 'what is this' photo, so why do you say it's metallic? The location in the second photo, if that concrete element is relatively narrow into the page, is possibly the location of through-ties for the shutter when it was cast. It might be a connector or end piece associated with them. However, at the moment, the photos aren't any more help than saying 'what's round and in concrete?' $\endgroup$
    – achrn
    Jan 5, 2021 at 13:35
  • $\begingroup$ @achrn I've added another close up photo of the plug of concrete that comes easily away from the wall. The walls are 12" thick so it could easily be a metalic cone piece associated with the formwork tie rods. See the plastic cone here concrete.org.uk/images/fingertips/tiebolt.jpg $\endgroup$
    – egg
    Jan 5, 2021 at 14:21

1 Answer 1


It looks like a prestressed cable anchor.

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  • $\begingroup$ If it's a prestress cable anchor, where's the prestress cable gone? This seems incredibly unlikely to me. You'd see the prestress cable sticking out from where the concrete came from, or if it has pulled out of the concrete sticking out from the anchor. If it let go suddenly you'd expect a lot of damage to the concrete. $\endgroup$
    – achrn
    Jan 5, 2021 at 13:31
  • $\begingroup$ As far as I am aware the reinforced concrete structure in question is not prestressed. $\endgroup$
    – egg
    Jan 5, 2021 at 14:22
  • $\begingroup$ @achrn: I'm not convinced it's a prestressed anchor either, but it seems to be the most plausible explanation to me. It may have been a temporary aspect of the column (construction support for the beams?) which was later removed and the duct was then filled with grout or something. I'm not wholly convinced because it'd be cheaper to just leave that prestressed support after its actual period of use, but it's the best idea I have. I've designed many columns with prestressed supports at the top, but those were where the beams would rest, so not exactly applicable to this case, but still... $\endgroup$
    – Wasabi
    Jan 5, 2021 at 15:52
  • $\begingroup$ The other option is a lifting anchor. There are a lot of different types. They are usually recessed and patched over after erection. Most have some sort of mushroom head that is captured by a slotted lifting eye. With pretentioned cables, the ends are torched off flush so you can pull the piece out of the form. $\endgroup$
    – Phil Sweet
    Jan 5, 2021 at 23:51

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