Having good support for your strong concrete slab on ground is critical to success!

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.

What Makes a Strong Concrete Slab?

The terms used for soil support systems is not always consistent. Here are the American Concrete Institute’s definitions:

  • 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 layer that is absolutely required is the sub-grade. You need ground to place a slab on ground on top of.

A subbase and base course offers 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.

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. A flat base course will also 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 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.

Get in touch with our team at Eakle Development for a free consultation.