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What is a Subbase/Subgrade?

What lies below your concrete slab is critical to a successful job. This is no different than the foundation for a building. A slab on ground (or slab on grade) by definition is not intended to be self supporting. The "soil support system" beneath it is there to support the slab.

The key for the soil support system is UNIFORM support rather than strong support. Sure, it has to be able to support the slab, but on most ground that's not a big problem, at least across the middle of the slab, since the load is spread across so much area. Good strong support at the edges and at any joints can be a different matter—to prevent cracking and joint spalling we need to support the slab at those locations where it can behave like a cantilever and bend into the subbase. But with a good subbase that's really not a big issue either.

The terminology used for soil support systems, unfortunately, is not completely consistent, so let's follow the American Concrete Institute's definitions, starting from the bottom:

  • Subgrade--this is the native soil (or improved soil), usually compacted
  • Subbase—this is a layer of gravel on top of the subgrade
  • Base (or base course)—this is the layer of material on top of the subbase and directly under the slab

The only one of these layers that is absolutely required is the subgrade—you have to have ground to place a slab on ground on top of. If the natural soil is relatively clean and compactable, then you can put a slab right on top of it without any extra layers. The problems with that are that the soil may not drain well, it can be muddy during construction if it gets wet, it may not compact well, and it can be difficult to get it flat and to the proper grade. Typically, the top of the subgrade should be graded to within plus or minus 1.5 inches of the specified elevation.

A subbase and base course, or both, provide several good things. The thicker the subbase, the more load the slab can support, so if there are going to be heavy loads on the slab—like trucks or fork lifts—the designer will probably specify a thick subbase. A subbase can also act as a capillary break, preventing water from wicking up from the groundwater table and into the slab. The subbase material is usually a reasonably low cost gravel without a lot of fines.

A base course on top of the subbase makes it easier to get to the proper grade and to get it flat. If you use some sort of a choker course of finer material on the top of the subbase, it will support your people and equipment during concrete placement. It will also keep your slab thickness uniform, which will save money on concrete—the most expensive part of the system. And a flat base course will allow the slab to slide easily as it shrinks, reducing restraint and the risk of cracks as the concrete contracts after placement (drying shrinkage).

The entire subbase and base system should be at least 4 inches thick—thicker if the engineer feels it is needed for proper support. The base course material, according to ACI 302, "Concrete Floor and Slab Construction," should be "compactible, easy to trim, granular fill that will remain stable and support construction traffic." ACI 302 recommends material with 10 to 30% fines (passing the No. 100 sieve) with no clay, silt, or organic materials. Manufactured aggregate works well—crushed recycled concrete aggregate can also work well. Tolerances on the base course are +0 inches and minus 1 inch for Class 1 through 3 floors (typical low tolerance floors) or +0 inches and minus ¾ inches for higher tolerance floors.

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