Search online for DIY instructions on pouring your own concrete slab and you are likely to find plenty of recommendations to use either sand or gravel between the concrete and soil. The question is whether or not you can pour concrete directly on top of dirt.
Pros and other experts will generally recommend sand, gravel, or some other aggregate as a base for the slab. A limited number of pros will tell you that, while they wouldn’t do so, it is possible to pour a slab and get a good finished product without the added material. They recommend sand or gravel for a very good reason though: it allows for proper drainage.
Why Concrete Slabs Sink
Concrete slabs can sink whether they are installed by professionals or DIYers. Almost all sinking problems are in some way related to underlying soil giving way. For example, you might have a property that was not properly leveled and landscaped when the home that sits on it was first built. Thus, you could have consistently poor drainage.
Excess water in the soil over a long period of time can lead to erosion. When erosion occurs, the weight of a heavy concrete slab pushes down on the soil. That is when sinking occurs. This sort of sinking is pretty common in areas where the soil is primarily clay or sand.
Choosing not to use sand or gravel underneath a slab does not necessarily guarantee it will sink. But it does make drainage more difficult. Water can get trapped under the slab and, over time, cause the same sort of erosion. Both sand and gravel act as a filtering system that prevents water accumulating directly under the slab. As such, direct erosion is less likely.
It Is All About Compaction
The Concrete Raising Company is a Salt Lake City company that specializes in foundation and concrete slab repair. They say that there is a bigger issue than whether or not to use sand or gravel. That issue is compaction. Poorly compacted soil naturally settles over time. The less compacted it is, the more settling occurs.
The technicians at The Concrete Raising Company have repaired their fair share of concrete slabs found sinking due to poor soil compaction. So whether you use filler material or not, compaction is key to your success. The best way to prevent slab sinking is to compact the soil as tightly as possible.
By Hand or Machine
Many DIY guides recommend compacting soil with a hand tamper. This is nothing more than a heavy tool with a flat face that you continually drop on the soil to compact it. Hand compaction is fine for small slabs of no more than a couple of feet in length and width. If you are pouring a full concrete slab to replace the patio, for example, hand compaction is probably not the way to go.
Hand compaction is subject to significant variations over larger areas. So the larger your concrete slab, the less likely that the soil underneath will be uniformly compacted throughout. That says nothing of the fatigue involved in hand compacting a 20 x 8 slab.
For larger slabs, it is best to use a machine. You can rent soil compactors from your home improvement store or a local equipment provider. Your project will cost slightly more than you had planned, but the extra expense will be worth it in the end.
Yes, it is technically possible to pour concrete directly on top of dirt. Most experts do not recommend it, but it can be done if you compact the soil properly.