What is slab on gradein construction?

Slab on grade, explained.

Slab on grade describes any concrete slab that bears continuously on soil or compacted fill instead of spanning between beams or walls. You will see it called out on the foundation plan and in the structural general notes, with the assembly drawn in the typical slab details on the structural sheets and the concrete specified in Division 03 (03 30 00, cast-in-place concrete). The geotechnical report governs what happens below it: subgrade preparation, base course thickness, and compaction requirements, which often hide in Division 31. Concrete subs, GCs, and foundation contractors all price it, and on most commercial jobs it is one of the largest single concrete line items. New estimators make three common mistakes. They price the concrete and forget the layers below it (base, vapor barrier, reinforcement). They miss thickened edges and interior thickenings at column lines and bearing walls, which add real cubic yards. And they skip the small items that add up: joint sawing, curing compound, and slab depressions at tile or terrazzo areas shown only on the architectural finish plans.