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.
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?