The key to a good slab is a well-compacted subgrade and subbase.
Wacker Neuson The key to a good slab is a well-compacted subgrade and subbase.

Place a concrete floor on an unstable subgrade or subbase and you’ll probably pay for it two ways. During construction you’ll either waste concrete or end up with a floor that’s too thin in spots because the subgrade/subbase is uneven. After construction, the floor is likely to settle and crack because of poor subgrade/subbase support. Strengthening the support system once the floor is placed is costly or impossible so it makes sense to place the slab on a strong and stable base at the start.

Let’s start with a couple of definitions. The subgrade is the soil that is at the bottom of the slab system. It can be the native soil or fill brought in from elsewhere. The subbase, according to ACI 302.1, “Guide to Concrete Floor and Slab Construction,” is a layer on top of the subgrade of “compactible , easy to trim, granular fill that will remain stable and support construction traffic.” In some cases, a base layer is placed on top of the subbase. All of these layers together comprise the soil-support system for the slab.

The design of the soil-support system should be part of the specification for the slab. On most larger projects, or when there is any concern about unstable soil, a geotechnical engineer should evaluate the subgrade material to determine what needs to be done to make it stable and supportive. The specification should also indicate the thickness of the subbase, its composition, and the required percent compaction.

Robotic plate compactors work well on sand and gravel subbases.
WackerNeuson Robotic plate compactors work well on sand and gravel subbases.

The bottom layer is the soil subgrade. This obviously varies widely by geographical region. In some regions the subgrade may be supportive enough to place the slab directly onto, in others it can be so poor that the only choice is to build a structural suspended slab. The critical characteristic of a good subgrade is not so much strength but uniform support, which means being especially diligent about compacting any filled areas. “Backfilling at footings, foundations, and pipeline and utility trenches should be done with soils similar to those surrounding the trench and compacted in layers to duplicate moisture and density conditions in the adjacent soils,” states PCA’s “Concrete Floors on Ground.”

“There’s a lot of geographical variation,” says Bryan Birdwell, Senior Concrete Floor and Paving Consultant with Structural Services Inc. “Along the Gulf Coast there are many sites where the slab can be placed directly on the subgrade, although it’s very sandy so you can’t even drive a pickup on it. Contractors use plywood runners to get the laser screed and concrete into position.”

On top of the subgrade, for most industrial slabs, a subbase is placed. Although the subbase is not mandatory, it serves as a work platform for construction of the slab and a cushion for more uniform support of the slab. Most road base material approved by the local DOT is suitable for the subbase. “We use 6 to 8 inches of crusher run,” says Steve Lloyd, Lloyd Concrete Services, Forest, Va. “It includes lots of fines and we compact it to 98%. We will not use #57 stone, that’s like trying to place a slab on marbles. I’ll turn down any job that specifies #57 stone as a subbase.”

The two most frequently used types of fine grading materials are open-graded or closed-graded crushed rock. Open-graded stone materials lack small particles so water can easily pass through. Kevin MacDonald, president of Beton Consulting Engineers, says you can't compact a layer of open-graded stone but you can adjust it—shaking stone into place with a compactor. He adds that a layer of open-graded stone will stop the capillary rise of water, but not the rise of water vapor. A way to control capillary movement of water is to specify a layer of open-graded stone with a layer of closed-grade stone on top.

A vapor retarder on top of the subbase and directly in contact with the concrete serves to both prevent moisture movement into the slab and as a slip sheet to allow the concrete to shrink without cracking. Use a vapor retarder that’s a minimum of 10 mils thick—15 mils is better moisture protection and also less likely to be punctured during construction.Some designers will specify a loose sand layer on top of the subbase as a cushion layer. This practice has been shown to be detrimental to the slab (see sidebar, Reasons to Avoid Placing a Sand Layer). ACI 302.1 states that, “This type of sand will be difficult, if not impossible, to compact and maintain until concrete placement is completed.”

Every soil support system must be densified through compaction. This can be done using a variety of equipment some of which works better on different types of material.

Clay (cohesive soils) can best be compacted using a vibrating sheepsfoot roller or a rammer (also called a jumping jack). Rammers have a smaller plate that concentrates the force. Rammers are also good for backfilling trenches—keep lifts in backfilled areas no thicker than 6 inches.

With the right material, a drum roller compactor can produce a supportive subbase.
Lloyd Concrete Services With the right material, a drum roller compactor can produce a supportive subbase.

Gravel and sand compacts best with a drum roller or a vibrating plate compactor. “We use an 8-ton rubber-wheeled drum roller,” says Lloyd, “and compact to 98%--both the subgrade and the subbase. When we’re done it’s almost like concrete. When the laser screed or ready-mix trucks drive across it they won’t even leave tracks. But still, I’ll re-roll it—the more the better.”

On smaller projects, or around the edges, pipes, or columns, a plate compactor provides good compaction for sand or gravel. The larger plate spreads the force out and helps to flatten the surface. “When using a plate compactor on fill,” says Mike Murray with Concrete Cares, “we recommend placing 3-inch lifts—the plate compactor doesn’t provide enough force for a 6-inch lift.”

The maximum compaction of any granular material can only be achieved at the optimum moisture content. This is determined in a laboratory using a Proctor test. The material will be slightly damp but not wet. The testing technician will measure the compaction with a nuclear density test and can indicate when the specified density has been achieved.

Following compaction, the most effective way to assure that the soil-support system is ready for concrete is to proof roll the surface. Use a loaded dump truck or a ready mix truck and drive over the subbase in a grid to cover as much of the surface as possible. “Some contractors will try to proof roll with a loader with a full bucket,” says Birdwell. “That really doesn’t work.” As the truck moves across the subbase, the crew (or the geotechnical engineer) should be watching for any rutting or pumping. Rutting, according to ACI 302.1 is “when the surface of the subbase is wet, greater than three percentage points above optimum moisture content.” Pumping is “when the surface of the subbase is dry and the underlying soils are wet.”

Bottom line with preparing the soil-support system is that no slab can succeed without solid and uniform support beneath. “Compacting the subbase is the most important thing I do,” says Steve Lloyd. Then, once the subbase is ready, protect it until the concrete slab is placed (see sidebar, Maintaining the Slab Support System).

*Parts of this article were contributed by previous articles written by Boyd Ringo, Bruce Suprenant, Ward Malish, Ronald Lech, and Joe Nasvik.