Slabs are used to furnish a flat and useful surface in reinforced concrete construction. It is broad, flat plate, usually horizontal, with top and bottom surfaces parallel or nearly so. It may be supported by reinforced concrete beams, by masonry or reinforced concrete walls, by structural steel members, by directly by columns or continuously by the ground.
Slabs in which the deflected surface is predominantly cylindrical is called one-way slabs spanning in the direction of curvature. This condition arises when slabs are supported on two opposite sides, and those supported on all four sides with the longer span greater than twice the shorter span and also in cantilever slabs. In many cases, rectangular slabs are of such proportions and are supported in such a way that two-way action results. when loaded, such slabs bend into a dished surface i.e., an any point the slab is curved in both principal directions, and since bending moments are proportional to curvature, moments also in both directions.
Concrete slabs may in some cases be carried directly by columns, without the use of beams or girders. Such slabs are called flat plates. In some cases, to reduce the stresses due to shear and negative bending around columns, a thickened slab region in the vicinity of column and flared column tops are incorporated. Such construction is called flat- slab construction.