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Why do you have to remove grass before pouring concrete?

You should not directly pour concrete over grass. A concrete slab when poured over grass will crack over the course of time due to moisture ingress, lack of support as the vegetation beneath regresses and lack of rigidity as factors such as external weight will cause the structure to weaken.

flourishingplants.com - Can you Pour Concrete Over Grass? How its Correctly done!
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Concrete structures can be done almost anywhere once the right preparation work is done. Pouring concrete on grass is something that you have to weigh the pros and cons before considering carrying out such a project.

Can you Pour concrete over grass?

You should not directly pour concrete over grass. A concrete slab when poured over grass will crack over the course of time due to moisture ingress, lack of support as the vegetation beneath regresses and lack of rigidity as factors such as external weight will cause the structure to weaken. There are many different preparations that you should make in order to be able to successfully pour a slab of concrete over the area you want it. The initial preparations will involve removing and replacing the top layer of soil. In this article, we are going to explain why you should not pour concrete over grass as well as how to properly prepare a grassy area so you can effectively cast a concrete slab or flooring.

Related articles you may find helpful –

Cement spilt on grass

Can cement harm plants?

Why do trees cause concrete to crack

I have used this moisture barrier time and again when preparing spaces for pouring concrete. It does a great job at keeping moisture out and is also cost-effective. You can see it by clicking here. View on Amazon

If you were to pour concrete over grass

The benefits would include –

Less labor in preparation work

Less money spent on materials for preparation

Less time from start to finish.

However considering the benefits, would it really be worth it? What you really have to be wary of, is what will happen over the course of time. The initial result may be appealing and it may seem like a successful project, but really and truly, Without proper preparation and reinforcement for the concrete, it will fail and crack as time passes.

A little food for thought!

Concrete is like a human body, although it is hard and rigid, it moves! Yes, it moves so slightly the naked eyes cannot see it. What gives it this ability to move is the steel which is with its structure, as the bones in a human body. This is why there are so many high-rise buildings in our cities that stand for decades without crumbling. You can feel the swaying of the building when standing at the very top and it is the steel that has been tied into and embedded within the concrete that allows it to flex without breaking apart.

Why you should not pour concrete over grass

Subsidence

What you will have to worry about is when the grass and other organic material start to decay beneath the concrete structure. It will create what is known as subsidence. Subsidence is when the ground beneath a property sinks, pulling the property’s foundations down with it. This process can cause the walls and floors to shift, leading to cracks and potentially destabilising the construction of the property. Pouring over previously installed plants or trees, while possibly damaging them, may be possible depending on how far their root system goes. In this case, the roots will have to be removed before the concrete is poured. This will cause stresses in the concrete structure over time weakening it strength and causing it to develop unsightly cracks.

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Uneven surface and Air pockets

Without the preparation work being done, the surface beneath the concrete structure may not be even. An uneven substructure can lead to things such as air-pockets which will cause stresses on the concrete and result in cracking.

Moisture ingression

With proper planning, any concrete expert would agree that there should be a moisture barrier in place before pouring concrete. A barrier would prevent moisture in the soil beneath from moving upwards and penetrating the concrete. Over time, water will be able to “soak” into the concrete structure. The rebar within the concrete will start to rust.

Nothing holds back the force of expansion when rusting starts.

The force created on the concrete by the rusting rebars will cause massive cracks in the structure. Unfortunately, without proper planning and preparation, the moisture barrier will not be present. Moisture ingression in the concrete as a result of this would lead to a number of issues.

Cracking

The concrete will develop large cracks as smaller cracks develop allowing water to seep through them and into the concrete structure weakening it. The cracking will then progress and cause structural damage over time. Without the proper infrastructure and prep work done the concrete slab will not be able to be used for the intended purpose in the first place and would lead to a waste of time and money in the long run. Although pouring concrete over grass can be done we do recommend preparing the area before concrete is poured. This will include some preparation work. which will include the removal of the top layer of the grass.

How to Prepare a Grass Area for Pouring Concrete

The proper prep work is what makes a concrete slab last over time without cracking.

Here are the steps:

Use a shovel to remove grass from the area where you want to pour concrete Dig out any roots or rocks in the ground that might interfere with your project

Laydown plastic sheeting (polyurethane) to keep moisture from wicking up from underneath

Install the form work (fancy wording for placing a wooden box barrier to hold the concrete in the shape you want it) Lay down plastic sheeting or tarps around the formwork to protect your lawn from any spills onto surrounding grass. Install a layer of rebar so the concrete can have something to bind to, so it will have rigidity and strength. (this will prevent it from cracking in the future) Mix cement, sand, and gravel together until it has a consistency of peanut butter Pour the mixture into your desired shape on top of the dirt you just removed from the ground at least four inches thick to totally cover the rebar or steel. Wait 24 – 36 hours for the concrete to dry before walking on or driving over it so that it can set properly Note: wet the slightly with a sprinkle of water every 12 to 16 hours. Known as “moist curing,†this allows the moisture in the concrete to evaporate slowly. Moist-cured concrete can be up to 50 percent stronger than concrete that was cured without being dampened!

Remove tarp and form work from around the cured concrete.

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How does Cement Affect Grass?

Cement is composed of rock salt limestone and powdered gypsum, a type of chemical that is formed from non-metallic minerals. Cement chemically reacts with acid water in the soil and changes the physio-chemical properties of soil. Soil contains air, water, nitrogen, iron, potassium, manganese, and microbes. Soil normally has a pH of 6.4. The damages resulting from spilled cement on the grass depending on the amount of cement slipped on the specific area. Normally this happens during construction work, done in a grassy lawn, agricultural field, grassland, national park, or garden for beauty and facilitation purposes but it damages our environment including soil, flora, and microorganisms. Cement can kill lawn grass through chemical reactions of soil with limestone (CaCO3), rainwater (H2O) Carbon dioxide (CO2) in the air and makes the soil severely alkaline at the pH level of greater than 8.0. This chemical reaction turns the lush green grass into a pale one and damages it. The pH level of a soil determines the mobility of nutrients in the soil. It varies in different types of vegetation. Liquid cement creates a solid layer or dense crust at ground soil and restricts the mobility of air, water, and nutrients in lower layers. Without their mobility grassroots cannot grow and survive in the soil. The thick crust of cement limits the sunlight approach to the grass, which is important for the formation of starch in the photosynthesis process. It also stops the germination process in the plants. High alkalinity can reduce iron absorption in the soil and grass become yellowish. Pink snow mold is a type of infectious disease suffered by the lawn grass when soil pH crosses the 7.0 limit. This infection is spread and gets worse in cool, humid weather. Grass becomes brown due to this infection. Cement can reduce the yield of soil which retarded the growth of any kind of grass. I used this cheap soil improver in places on my lawn where the grass died and it worked wonders. See Price on Amazon

The Takeaway

Concrete is a less expensive option for homeowners when compared to asphalt or other types of construction. The biggest benefit to installing a concrete slab in the front yard is that it will always stay the same color, and it can be used as a driveway if desired. Concrete can be installed in a grass area such as a lawn but there is some preparation work that firstly has to be done in order for the project to last over the course of time. Concrete and cement can also affect the grass surrounding the area where the concrete is being poured and some damage control has to be done to prevent any unwanted grass damage.

flourishingplants.com - Can you Pour Concrete Over Grass? How its Correctly done!
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