Can You Pour Concrete Directly on Dirt? (The Correct Way)

Pouring your own concrete is a relatively simple process and a great way of saving money. But can you pour concrete directly on dirt?

As a general rule, avoid pouring concrete directly on the ground before preparing a proper concrete base. The best practice is to prepare the ground by leveling and compacting the subgrade thoroughly. Then, add a subbase of gravel for additional load support of the final slab if needed.

In this article, we’ll explain the different layers used in creating a solid concrete slab. Plus, we’ll also explain other ways you can ensure your concrete cures stay firm and can weather the elements.

Why Dirt And Concrete Don’t Mix

Concrete slabs are frequently used as a foundation or base level because they provide a sturdy platform.

They are also a trendy finish for modern DIY projects, but you must treat the area where you will pour the concrete correctly to ensure you get a smooth finish. (Here’s a detailed guide to making a concrete surface smooth.)

In cases where you’re laying a concrete slab to support other building projects, it’s also necessary that you do so correctly for the sake of protecting your other buildings. Unstable concrete can crack and give way when it’s unable to support the weight placed upon it.

Concrete is the name given to a mixture of cement and aggregate, making it a composite construction material (source).

It is a common misconception that you can add just about anything to this mixture, such as dirt.

This misconception is most prevalent when it comes to setting pavement or concrete tiles through a garden.

But here’s the thing. Any additional elements added to the concrete mixture can negatively impact how it cures and the setting’s long-term integrity.

For this reason, you must remove loose dirt or compact it to avoid interference or mixture with the concrete as it would alter the delicate ratio of cement, water, and aggregate — all of which bond together, resulting in concrete.

What to Do Before Pouring Concrete

There are many wrong ways to pour concrete, while construction experts agree that there is only one correct way to pour concrete.

Following these steps will result in a well-cured concrete slab.

1. Clear the Area

When laying concrete outdoors, it is important to be meticulous about clearing the area thoroughly. Look for natural debris like leaves, grass, and insects, and ensure no animals or children will interfere with the concrete setting (source).

It may be wise to cordon off the area where you’ll be pouring concrete if you feel as though there may be external interference. This may also require setting up an umbrella or shade awning to protect your concrete from the elements while it cures.

As you prepare the ground to become the subgrade for your concrete, look for potential obstacles that may interfere with it.

Tree roots, electrical cables, conduits, and water pipes are major concerns that may affect how you lay your concrete. Whereas, smaller obstacles like ant nests can be dealt with so that the ants relocate and do not tunnel into your concrete while it cures.

2. Prepare the Subgrade and Subbase

Preparing your subgrade and subbase is easily the most important step, as it plays the most significant role in how your concrete sets.

As mentioned earlier, concrete is made by mixing cement with aggregate. Aggregate is essentially rocky material, ranging from fine to coarse, which the cement binds to while it cures. The aggregate consists of two levels: the subgrade and the subbase.

Subgrade Concrete Preparation

This is the name given to the prepared ground, normally compacted and improved natural soil or infill (source).

Loose dirt is undesirable as it impacts the composition of the concrete. This is why the foundation level is rolled and compacted to provide a stable base on which to place the subbase.

Construction experts consider the subgrade layer to be the most important when it comes to determining the load that the concrete slab will be able to withstand. In short, concrete can only be as strong as its subgrade layer.

The subgrade preparation is the first opportunity to grade the area where you will lay concrete. Leveling your slab is critical, so ensure that, when compacting, you are leveling the ground as well so that the layers to come will also be even.

The subgrade layer can be improved by adding stabilizing components, such as Portland cement, calcium chloride, or lime. These components would be added to the soil and then compacted, creating a dry and stable environment ready to receive the subbase.

Subbase Concrete Preparation

There are various subbases to choose from, where the coarser materials are used in heavy-duty, industrial applications. As the thickness of the subbase layer increases, so does the concrete slab’s load-carrying capacity.

The subbase prevents water from moving upwards through the soil and into the concrete. This would deteriorate the concrete’s integrity in time. Also, providing a solid argument for why one shouldn’t pour concrete directly onto the dirt.

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3. Construct Your Concrete Form

Many DIY concrete layers move from the second step straight to pouring concrete. There are still two further steps that are equally important in producing a well-cured concrete slab.

First, a form would be incredibly helpful when pouring, as it helps direct the pouring, funneling the concrete as it lands. It will also keep your edges square as the concrete cures. The form’s edges are also useful when leveling and screeding the top layer of concrete (source).

Forms also fulfill another function, keeping loose dirt from the ground surrounding the slab you’re laying out. As discussed, this is important since it would change your concrete’s composition and affect its durability.

4. Add a Wire Mesh

A final measure to increase your concrete slab’s structural integrity is to add a wire mesh to it. This is essentially a “skeleton” onto which the concrete clings.

Concrete is able to bind to itself while curing, and for smaller tasks, one may get by without using a mesh. In most situations, construction experts recommend your concrete is reinforced regardless so as to keep your structure strong.

It is a common misconception that the steel bars used in the mesh are tack-welded together. However, construction experts advise against this since it reduces the steel cross-section and, therefore, offers less integrity than when they are unjoined.

For the mesh to be optimally effective, it is important to set the steel rods at a suitable height, depending on the thickness of the concrete you’re looking to lay.

This is another reason why it is important to follow these four steps instead of pouring the concrete directly onto dirt since the subgrade preparation provides a flat surface to get the mesh ready for pouring.

Speaking of pouring concrete, have you ever wondered if you can concrete over sewage pipes? Check out the guide to learn more about it.

Things to Keep in Mind When Pouring Concrete

After following these four steps, you’ll be ready to pour your concrete. There are several subtle details to consider when preparing your concrete. For instance, mixing the right amount of water into your aggregate. We cover this topic specifically in a separate article, “Can I Pour Dry Concrete?”

Safety gear is also highly recommended when mixing and pouring concrete to protect yourself from the splashback.

Spreading the concrete evenly is important right from the pouring stage, so it is recommended that you enlist help when pouring. Use shovels, rakes, and come-along rakes to assist with spreading the concrete evenly across your form.

Screeding will help you get an even finish at the top layer of concrete and will allow you to put an adequate slope on your concrete to direct rainfall. Floating the surface is also a good idea to compact the concrete further and help it set.

By the way, also check out this guide on pouring concrete during the rainy season. In that guide, we cover what can happen if you pour concrete while it rains. Also, we explain the concrete drying time required before rain.

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Final Thoughts

Laying concrete requires more steps than simply pouring concrete into the dirt. One needs to prepare the ground surface, make a subgrade and subbase, and add the correct amount of aggregate. Then, construct a form to mold the concrete into the desired shape and reinforce the slab with a wire mesh.

The composition of your concrete is important, as well as following these steps accurately, as they all play a part in the overall strength of the concrete once it sets.

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