I admit it: I had to Google "rat slab".
For anyone else as ignorant as I:
Quote:
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A rat slab is a very thin layer of non-structural concrete poured over the dirt floor of a basement or crawlspace. As the name implies, it is intended primarily to prevent the intrusion of pests into the area below a building.
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https://www.quora.com/What-is-a-rat-slab