Uneven interior slab-on-grade

[sapperDAK]

The situation is inside a 10-year old addition to a municipal building in central PA. A large conference room slab-on-grade consisting of nine 20'x20' "quadrants" separated by 2 construction joints and 2 saw-cut joints. No expansion material was observed anywhere within the placement area and no dowels, reinforcing steel or WWF was found during initial investigation. The center 3 quadrants have apparently "heaved" shortly after placement, because some flash-patch material was used along the construction joints prior to the original flooring installation. Subsequently, additional "heaving" of the center sections has created a 1/4 to 1/2 inch difference in elevation between sections. All 3 center sections have raised the same amount, and the 6 outside sections are all at the same relative elevation. The 4 to 5 inch thick slab is on a vapor barrier of plastic above minimum 6-inches of #57 stone on compacted clayey subgrade. Investigation of in-situ subgrade conditions is on-going but... what could have cause the center sections to be "jacked up" above the outside sections?

 

[hokie66]

What leads you to you think the center has gone up rather than the sides going down?

 

[sapperDAK]

We have checked elevations with adjacent slabs outside the room in question and the side slabs are the same elevation throughout - the center slab is consistently higher throughout.

 

[ishvaaag]

You need a geotechnical investigation. Elastic rebound, chemical expansion both can be the cause. To discern the likelihood of any cause the more the data the better. Even if settlement was the cause, the geotechncal study would be warranted, so press ahead.