Does pouring concrete over grass work? The answer is no, for most situations, and yes sometimes. It depends.
Generally, pouring concrete works best as a well-thought-out and planned event. Concrete can quickly get expensive, especially if you purchase a quality product, and most people want a structure to last as long as possible.
Pouring concrete over grass is not likely to provide the long-lasting strength most people want. So, let’s discuss how you should pour concrete and go over how to lay it on grass, if necessary.
What Happens if You Pour Concrete Over Grass?
For most purposes, pouring concrete over grass will not result in long-lasting concrete. There are a few reasons for this:
- Subsidence
- Moisture ingress
- Air pockets
- Uneven Surface
- Cracking and grass growing through concrete
The result will probably be low-quality concrete that will not last long before it breaks apart or nature tears it apart one crack at a time.
When Is It Okay to Pour Concrete Over Grass?
Pouring concrete over grass should be done seldomly and only if the concrete is temporary or if repeated repairs will not cause problems, or if it only needs to support something light and non-critical, like a mailbox.
If any structure, or purpose, that uses concrete needs to last a lifetime, pouring over grass, instead of properly cleared, compacted, and leveled land, will never produce this result.
On the other hand, there are times when people only need a temporary structure that lasts a couple of years, and pouring concrete over grass may work in such a case.
Steps to Prepare Concrete
Since the steps to prepare to pour concrete do not differ much whether you are going to lay it on a prepared surface or grass, we can go over choosing the right concrete and preparing it first.
Different jobs require different mixes of concrete, and here we are talking about using outdoor concrete, so make sure not to buy indoor concrete. Choose a concrete that dries fast, within 24 hours, and is simple to prepare.
Simple to prepare means that, ideally, you only need to add water to start mixing the concrete.
When you shop for the concrete, prepare to pay a little more for a quality brand. Quality means the concrete contains sand, stones, concrete paste, and eventually water.
Aim for about 15% concrete and a high percentage of rocks to get the best results.
When it comes time to mix concrete, each brand will have instructions you should follow printed on the bag.
How Do You Lay Concrete Over Grass?
Even though you intend to pour concrete directly on grass, there is still some prep work that you should do to make the area ready for concrete.
Remember, after laying concrete, you are stuck with the result, so have a plan.
Till the Existing Grass to Prepare for Concrete
Tilling destroys the roots beneath the grass and should prevent the grass from growing back. Make sure to thoroughly till all the grass covering the project area. Be careful to shred all the roots.
Lay the Concrete
Set up some sturdy boards around the area where you intend to pour the concrete. These boards should not move when the concrete’s weight pushes on them.
Installing wooden formwork allows the concrete to settle and dry in the shape you want without surprisingly running off in a different direction.
Follow the bag’s instructions and mix the concrete in a large plastic tub.
Pour in the concrete and make sure to completely cover the area with at least six inches of concrete to prevent grass from finding a way to grow through the slab.
Be sure that pouring at least six inches does not cause any overflow over the boards.
Also, the concrete needs to be thick enough to support any weight you place on it. Factor into your plans variables like light foot traffic or a heavy piece of machinery.
Lastly, do not move or remove the boards while the concrete sets. Wait plenty of time before removing the formwork, so the final product keeps the right shape.
If you carefully take all these steps, pouring concrete over grass can be done. However, remember the procedure above is still not the recommended method for laying concrete you want to withstand the test of time.
How Do You Prepare the Ground Before Pouring Concrete?
What if you need strong concrete that can hold the weight of multiple vehicles in the driveway?
The above method for laying on grass will not last for decades because the ground needs proper preparation for heavy, extended use.
The Preparation
Use a shovel to dig out all the grass, making sure to remove any root systems and rocks in the process.
Smooth out, level, and compact the dirt. Then lay sand or gravel to make a bed for the concrete to sit on. This bed provides stability because sand and gravel do not shrink and expand in water.
Dirt does, and you do not want a concrete slab on an unstable bed.
Lay down a layer of polyurethane plastic over the bed to prevent water from wicking up into the concrete. Over time, water that sits in contact with concrete will soak in, get absorbed, and weaken the concrete.
Lay boards around the concrete in the desired shape and repeat the rest of the method laid out in the previous section. However, for extra strength, lay the concrete over some rebar.
Add one extra step called moist curing, which adds extra strength to the concrete.
Every 12 to 16 hours, spray a little water over the top of the setting concrete to prevent water from evaporating too quickly and weakening the final product.
Final Thoughts
While it is generally not recommended to pour concrete on grass, it is acceptable for some situations and purposes.
Primarily, concrete on grass should not support anything important, so if grass pushes through and begins to crack the concrete apart, a dangerous accident does not occur.
For projects where strength and reliability are paramount, prepare grassy land properly to ensure concrete stays sturdy and lasts for many years.
