Slab on Grade

What is Slab on Grade ?

Definition of Slab on Grade in Construction

The concrete slab that is directly supported by the grade, meaning the bottom of the structure which is normally earth. The slab on grade can be installed on the virgin ground, a layer of stone or porous material, etc. A slab on grade does not have any voids below it and can be as thin as 4 inches. The design of the slab on grade is engineered by the structural engineer. If there are concerns regarding the ability of the earth to support the slab on grade, the engineer may decide to increase the thickness and install reinforcing steel within the slab itself.

In some instances, the sub grade support is so inadequate that the slab is designed to support itself , similar to an elevated slab. If this is the case, then the design of the slab on grade is an engineered structural slab, that will span any settlement that may occur within the sub grade. Another special slab on grade situation, is the presence of water under the slab. This will constitute the need for the engineer to design a pressure slab, to withstand any uplift that the buildup of hydrostatic pressure could create. Engineers will in most cases, require the installation of at least 6 inches of porous fill under the slab on grade, with a vapor barrier over the porous fill. The vapor barrier will prevent any of the moisture from entering the slab and presenting itself on the surface of the slab on grade.

The normal slab on grade is reinforced with WWF or fiber strands to reduce cracking due to shrinkage and or expansion. The perimeter of the slab will have an expansion joint installed to separate the exterior walls, or foundation wall from the actual slab. In many instances the engineer will design expansion joints to be cut in the concrete slab on grade, to minimize the effects of expansion and contraction. If there are columns located within the slab on grade, the columns will be boxed out and in-filled at a later date. A synthetic sealant is normally required to be applied to the top surface of the slab on grade to minimize dusting of the slab, and to offer some protection from salts and other corrosive material, that may fall on the slab during its lifetime.