For most interior slabs, the vapor barrier should be placed on top of the subbase before placing the concrete.

So we've finally got the subgrade compacted and the subbase and base course placed and compacted. But what happens if there is a delay at this point before the concrete is placed? If the subbase gets rained on or frozen prior to concrete placement, it can go from being ready to being too soft.

The best way to know if the subbase is properly compacted and ready for the slab is by proof-rolling, which is running a heavily loaded truck (such as a fully loaded concrete truck) across the subbase immediately before placing the concrete to see if any areas sink more than others. This should be done on some sort of grid pattern and the tires should not sink into the surface more than ½ inch. If there is any rutting or pumping of water in any part of the subbase or subgrade, then that area needs more compaction or addition of granular materials—or simply to be allowed to dry out. In the worst cases, trenches or sumps can be cut and the water pumped out.

Stego Industries in San Clemente, CA

Just prior to placing the concrete, you may also want to place a moisture barrier. For interior floors, the best location is usually between the base course and the concrete. For more on this see Vapor Barriers for Concrete Slabs.

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