What purpose is served by placing a granular base under an interior concrete slab? Why not just place concrete directly on the compacted subgrade?
If the subgrade is uniform (no soft spots) and a vapor barrier is placed on it, you probably don't need an aggregate concrete base course. A base serves two main purposes: It provides uniform support for the slab It serves as a capillary cutoff that prevents moisture from wicking up through the subgrade and collecting beneath the slab, A granular base also may be easier to fine-grade before placing the concrete. If the subgrade is difficult to grade, slab thickness may vary.