Does this make sense? When Limestone is used to make concrete and can be formed easily?
Look at those Rai stones and the center hole is perfect while many of them are irregular shaped disks. If they were experts at carving stone then why would they make them so imperfect?
It's because they poured the concrete around a pole and it dried.
The stones themselves are the evidence. Concrete is made from Limestone and they quarried the limestone from the islands 400 miles away.
Tell me these don't look like poured concrete slabs:
Building roads and office building take money and, as you can see, Yap was in short supply of currency, so they invented deflation instead.
Are you sure they don't look like concrete?