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Below is a stack of reinforced concrete slabs with hollows (camera is positioned along the hollows' axis). Such slabs are used for floors - you build walls of the next floor, then lay such slabs such that they rest onto walls with the end shown on the picture and the opposite end. The dark round dots are steel rebars covered in rust.

stack of slabs with hollows

I've seen slabs with round hollows and with oval hollows but this shape is new to me. It's something like a rounded hexagon with an extra cavity on the bottom.

What's the name of this shape and why is it used for hollows in a reinforced concrete slab?

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It looks like a typical precast concrete "hollow-core" or "voided" slab. A quick Google image search reveals the variety of void shapes employed by manufacturers.

Voided Slab Examples

The image on the left (from Oldcastle Precast, incidentally) nicely shows the variation of void shape with increasing slab thickness.

As noted in the other answers, we care about the concrete profile more than the void profile. The voids are simply introduced to minimize concrete area where it is not structurally necessary.

In theory, a rectangular void would minimize concrete. However, the corners are better off rounded to reduce the stress concentrations (and therefore minimize cracking). Hence, the oblong voids. In thinner slabs, the voids may indeed approach a circular shape. One possible explanation for the irregular void shape in your posted question, is that the irregular void can set proper concrete cover for the rebar.

Thoughts on Void Shape

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The solid bits between the holes are probably more informative. They remind me of the shape of steel rails use for railroads.

The solid bits are buttressed at their bases, with the buttresses encroaching into the holes, resulting in the holes having an unusual shape. The tops of the holes look like they're arched. Notice also, that the reinforcing steel rods have been placed in the middle of the buttressed walls between the holes.

This all suggests such slabs would be used in high stress situations and the holes are present to reduce the mass of the slabs.

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  • $\begingroup$ Great, but why not just use round holes? I've seen a lot of slabs with round holes - they are used exactly the same way. $\endgroup$ – sharptooth Sep 11 '17 at 15:32
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    $\begingroup$ @sharptooth because the shape of the hole is not important. The shape material, the actual concrete, is what is important. $\endgroup$ – ratchet freak Sep 11 '17 at 15:43
  • $\begingroup$ @ratchetfreak The hole shape dictates the concrete shape, doesn't it? Oval holes are used (instead of round holes) to have vertically oriented concrete "inserts" more frequently across the slab. Why would this specific shape be used instead? $\endgroup$ – sharptooth Sep 11 '17 at 16:02
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    $\begingroup$ @sharptooth but the engineer doesn't start from the hole shape but instead of the material shape $\endgroup$ – ratchet freak Sep 11 '17 at 16:34

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