A Rare Failure

That’s the underside of a large, built-up steel girder encased in concrete. (You can see the edges and grain of the wood boards that were used as formwork when the concrete was placed.) You can see the steel because the concrete is broken on the bottom, maybe because it failed spontaneously and maybe because someone broke it.

Pre-WWII concrete tends to have problems from a lack of proper vibration during placing. The most common is segregation, where the fines (sand and cement) drifted up towards the top of the form while the coarse aggregate drifted towards the bottom. This appears to be a genuine void in the concrete. That can happen because the concrete didn’t flow around the beam in the space between the steel and the form, which in turn can happen because the space wasn’t big enough, the mix wasn’t wet enough, or there was no vibration at all.

In any case, it’s not a defect we see very often, and therefore it’s worthy of a photo.