Circular Cracking Pattern in Floor

Hotline Problem

We moved into a 60,000-square-foot distribution center 6 months ago. The building is 2 years old but we're the first user. Walls of the building are precast concrete panels and the floor slab was placed after the building was dried in. 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, nine circular or football-shaped cracks have appeared in the floor. They are randomly located around the perimeter of the building and the diameter of the circles ranges from 9 feet to 16 feet in diameter. 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 are at the intersection of two 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 could be done to determine the cause of these cracks?

Troubleshooting Response

The cracking pattern you describe doesn't sound like shrinkage cracking. It's more likely to be caused by a concentrated load. When a concentrated load is placed 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.

To verify that the cracks are caused by a concentrated load, you might try coring the floor at several of the radial and circular cracks and noting whether the crack is wider and the top or bottom. When you core, also check for evidence of subgrade settlement and measure core length to verify that the slab is 5 inches thick. The cracking might have occurred because the floor was thin in spots, because of an overload, or because of subgrade settlement combined with normal loading.

You might also flood the floor surface with water near each of the circular cracked areas to find out if the area bounded by the cracks has settled or risen.

Try to find out whether a crane or heavy forklift was used on the floor during construction or during move-in. That might be the source of the overloading.

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