It is not recommended to pour concrete over dirt, even though it is possible in some circumstances. When it comes to concrete slab construction terminology, this plain old dirt is called the subgrade. So while it is possible to pour concrete directly onto subgrade – either natural or compacted, there are many reasons not to do this. This is where gravel comes into the mix. The gravel that is placed on top of the subgrade is termed the subbase. There is another final layer that often goes on top of the subbase which is termed the base (or sometimes called the base course).
The decision of what type of gravel to put as a subbase onto the subgrade (dirt), is all about the nature of the subgrade. Things like will it drain properly, will it compact nicely, and what type of load will it take, are issues to consider.
Having a subbase gravel layer allows:
- the slab to be even over uneven subgrade,
- provides stability that will assist in stopping cracks in the slab,
- assists water to drain away from under the slab,
- Increases the load that the slab can take
So getting the subbase right allows you to forget about the problems associated with the subgrade. Once you put the right subbase on, all things become stable and good to go as far as the concrete slab is concerned.
Of course, that is not to say that you shouldn’t get the subgrade right first, it’s just that both the subgrade and subbase work together to make a correct foundation for the concrete slab.
What Type of Gravel Can Be Used As a Subbase For A Concrete Slab?
Depending on what needs to be achieved, there are a few gravel options you can choose from:
- Washed Clean Stones
- Crushed Stone
- Pea Gravel
- White Marble Chips
- Quarry Process
The stand-out here is the crushed stone. It is the most commonly used, followed by washed clean stone.
How Thick Should Gravel Be Under A Concrete Slab?
There are a few factors to consider when choosing gravel for under concrete slabs. The thickness of the gravel will depend on the weight of the slab and the type of gravel.
Thicker gravel will be more stable and less likely to move, but it will also be more expensive. If stability is not a concern, then thinner gravel may be preferable to lower the cost.
Another consideration is the type of gravel. Some types are better suited for wet areas, while others are better for dry areas. It is important to choose a type that will match the environment in which your slab will be installed.
As a starting point, a gravel subbase along with the base course needs to be at least 4 inches thick (10cm).
The thickness of, and choice of the subbase gravel depends on the bearing weight it will need to have. Crushed stone or washed clean stone allow for a higher bearing capacity than the other options.
It all comes down to what you are actually making the concrete slab for, how much weight will it be required to bear, and what deficiencies the ground underneath (subgrade) has that need to be accounted for.
What Treatment Does A Gravel Subbase Need?
Before the concrete slab is laid, the subbase needs to be compacted with a plate compactor to stabilize the system ready for the pouring of the concrete.
What About The Base (Base Course)
In most cases, concrete slabs for patios or light driveways are made using a combination of coarse gravel for the subbase and fine aggregates for the base. This creates a nicely compactable platform for the slab to be poured on.
For concrete slabs of this type, crushed stone and stone dust are generally the best foundation materials. Ultimately though, It comes down to the ground and soil you are laying the concrete slab on.
What Base Course Material Under A Concrete Slab
In order for a base course to function properly, the fill material should be compactable, easy to trim, and should be stable enough to support construction traffic.
Material with a fineness of 10 to 30 percent (passing the No. 100 sieve) without clay, silt, or organic matter is recommended. For example, crushed, recycled concrete aggregate is a good choice. Typical low tolerance floors have tolerances of +0 inches and minus 1 inch, while higher tolerance floors have tolerances of +0 inches and minus 3/4 inches.
Importance Of Uniform Concrete Slab Support
Compression is the strongest property of concrete, whereas tension is not as strong. Bending creates tension in a slab. Concrete that bends is compressed on one side and under tension on the other.
Concrete slabs may bend concavely up (like a smile) if the subgrade contains a soft section towards the center, creating tension at the bottom. If the top bends down (like a frown) at free edges or joints, it puts strain on the joints.
Therefore, if your entire concrete slab is not supported from below by a soil support system consisting of the right subbase and base course, it will bend and crack more easily.
Make Sure You Get The Concrete Mix Ration Right As Well
Now you’ve got the details sorted on what to put under the concrete slab, there is the very important issue of making sure you have the right concrete mix ratio to suit the needs of your concrete. Also, here are some hints on managing the issue of rain when you are trying to do a concrete pour.
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