Twisty And Hard To See

by Don Friedman on January 25, 2017


That photo is from a recent project in a 1910s loft building, looking up at the underside of a slab in a mechanical room. I’ve highlighted the interesting information here:



The slab’s reinforcing is not rebar as we know it today, but rather steel straps twisted into helices. That’s not the strangest thing I’ve seen embedded in concrete, but it’s strange enough that I had to poke around the old records a bit. It appears to be a system imported from France and patented in 1899 by Alphonse De Man. I’m at a loss as to what benefit this particular system brought the designers or builders: the twists would help the straps bond with the concrete, but there were plenty of other ways to achieve that goal, while getting the flexible straps to sit properly in the forms must have been an enormous pain. The fact that two segments are visible where there were minor spalls suggests that the straps are generally too close to the bottom of the slab.

The benefit of the system in general was, of course, to M. De Man, whenever he could convince someone to use it.

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