Q.: We moved into a 60,000-square-foot tilt-up concrete building 6 months ago. The building is 2 years old but we're the first tenant. The floor is 5 inches thick and contains 6x6--W2.9x2.9 welded wire mesh. Control joints are 20 to 25 feet apart. Before we bought the building, we had an environmental audit done. As part of the testing, three cores were taken from the floor and tested in compression. The average core strength was more than 9000 psi.
Since we occupied the building, we've noticed nine circular or football-shaped cracks in the floor. They're randomly located but are generally near the building perimeter. Diameter of the circles ranges from 9 feet to 16 feet. In some of the circles there are also radial cracks. Some of the circular cracks are in the middle of a bay, away from any joints, and some intersect sawed joints.
Because the building isn't very old and hasn't been in use long, we're concerned that the cracking will get worse. Are we experiencing subgrade settlement, shrinkage cracking, or something else? What testing can be done to determine the cause of these cracks?
A.: The cracks don't look like shrinkage cracks and are most likely to have been caused by a concentrated load. When there's a concentrated load on a slab, the bottom of the slab beneath the load is in tension and radial cracking results. At some radial distance from the load, the top of the slab is in tension and this causes a circular cracking pattern.
Since the structure is a tilt-up building, overloading may have occurred under the crane outriggers when panels were being lifted. A similar pattern could show up, however, if the floor is thin in spots and is subjected to heavy concentrated loads under service conditions. Taking cores at some of the cracked areas would allow you to check floor thickness.
Some circular floor cracks occurred at midpanel and some intersected joints. Cracks are painted yellow. Overloading is a likely cause of the cracking.