Subgrades and Subbases for Concrete Slabs

What is a subbase/subgrade?

What about the soil?

How can we improve the subgrade?

Compaction equipment

How does the subgrade/base affect slab design?

Placing the concrete

Related Information:

How to Build High Quality Slabs on Grade

Preventing Cracks in Concrete

Calculating the Amount of Base Fill

Plate Compactor Video: Concrete pouring and finishing tool video

Other Resources:

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People ask me why a concrete slab needs a subbase underneath it—isn't a concrete slab strong enough to support itself and the loads placed on top of it? The short answer is "no." The long answer is "yes, but..."

Concrete is very strong in compression and not so strong in tension. In a slab, tension is often created by bending. When a piece of concrete bends, it is in compression on one side and tension on the other side. A concrete slab may bend concave up (like a smile) if the subgrade has a soft spot in the middle, putting the bottom in tension. It may bend down (like a frown) at free edges or at joints, putting the top in tension. So if your entire concrete slab isn't being supported from below, by the "soil support system," it's going to bend more easily and is probably going to crack.

The other question I often get is why the subgrade and subbase allows the concrete to move at all—shouldn't it be completely rigid? The fact is that any soil or gravel base course is going to compress if the load is high enough, unless the slab is placed on solid rock. And in some ways that's good, because slabs curl and if the base can deflect a little, it can continue to provide support for the slab even when it curls. But if it doesn't provide uniform support, if the slab has to bridge over soft spots, the slab will probably crack. There doesn't even need to be much of a load on the slab--its own weight is usually enough since a slab on grade is not typically designed to even carry the dead load. And when it does crack, that crack is going to go all the way through the slab. If the under-slab support is bad enough, you can then get differential settlement across the crack that leaves a very unfortunate bump and a very unhappy owner.