Do these tiles have to be put over a concrete base? I am thinking of using under a hot tub. The soil is firm with gravel.

You can install our Designer Grip-Loc Tiles over crushed gravel under a hot tub. We typically recommend installing these tiles over concrete or asphalt, but any hard flat surface also works. If there is a major flooding event, these tiles may become unstable and may float away, but under normal conditions, these tiles will perform just fine.