When should you drill to 20% instead of 40% of the concrete slab?

ASTM F2170 calls for drilling to 20% of the slab depth when the slab is drying from both sides, rather than the 40% used for slabs drying from one side.

Drying from both sides means consistent air movement on both faces. The slab dries at the top surface, is wettest in the middle, and dries from the bottom as well, because there’s evaporation from that side too.

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