We built a slab on grade in a building with rectangular concrete columns. We isolated the columns from the slab by wrapping them with a compressible material. Roughly circular floor cracks developed around some of the columns. Can shrinkage cause this kind of cracking, or is there another more likely cause?
Circular cracks in floors are often caused by concentrated loads. At some radial distance from the load, the top of the slab is in tension, and this causes the circular cracking pattern. Such loads can be caused by settlement of column footings if the columns aren't free to move independently of the floor slab. For a thorough diagnosis, a consulting engineer should examine the floor, column isolation joints, and cracking pattern.