Sofia Martinez
Plastic under outdoor concrete is a topic that has been widely discussed, with varying opinions. Some people argue that a vapor barrier is essential to prevent moisture damage, while others claim it is unnecessary and a waste of time and money. Concrete is porous and can absorb rainwater from the ground, leading to potential structural damage. Vapor barriers, such as plastic sheets, are meant to protect the concrete from moisture, but they may also prolong the time needed to get rid of bleed water. The decision to use plastic under outdoor concrete depends on various factors, including the climate, the type of concrete used, and the specific requirements of the project.
| Characteristics | Values |
|---|---|
| Purpose | Prevent moisture from getting into the concrete and causing damage |
| Use cases | Concrete exposed to the outdoors, concrete covered with an impenetrable finish, concrete shielded and heated by a fixed structure, concrete driveways, concrete outbuildings, concrete carports |
| Benefits | Extends lifespan of concrete, prevents cracks, reduces moisture in the concrete leading to a stronger bond |
| Drawbacks | In areas subject to freeze/thaw, plastic could prolong the time needed to get rid of bleed water |
| Recommendations | Use a vapor barrier with a permeance of 0.1 Perms IMP or less, use thicker poly for a better barrier and more puncture resistance |
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What You'll Learn
Plastic under outdoor concrete can prevent moisture damage
Concrete is porous, allowing rainwater from the ground to permeate the surface. This can cause issues such as mould growth, slab cracking, and structural damage. To prevent this, a vapour barrier can be installed underneath the concrete.
Plastic sheets are a great option for a vapour barrier. They prevent moisture from the soil from permeating through the slab, reducing the amount of water that wet concrete sucks up from the ground. This leads to a stronger bond of concrete and a reduction in moisture-related issues.
The American Concrete Institute (ACI) recommends using a vapour barrier with a permeance of 0.1 Perms IMP or less. For residential concrete, the ACI suggests using 10-mil poly or plastic with a 0.03 Perms IMP score as the lowest barrier. Thicker poly barriers, such as 15-mil and 20-mil, are often more absorbent and durable.
In addition to preventing moisture damage, plastic under outdoor concrete can also act as a deterrent against termites and protect against radon gas infiltration in regions prone to radon.
However, it is important to note that plastic under outdoor concrete may not be necessary in all cases. For example, if the concrete is in an area subject to freeze-thaw, it is recommended to use air-entrained concrete instead. Additionally, if the concrete is exposed and not covered by an impenetrable finish, a vapour barrier may not be required.
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Plastic vapour barriers are not always necessary
For example, if you are constructing an exposed outdoor slab of concrete, such as a driveway or an open pad of concrete, a plastic vapour barrier is generally not required. In these cases, any moisture on the surface of the exterior concrete slabs or slabs in unheated structures is not typically a cause for concern. Additionally, if the concrete is not covered with an impenetrable finish or shielded by a heated structure, a plastic vapour barrier may be unnecessary.
It's worth noting that some sources suggest that a plastic vapour barrier can help extend the lifespan of a concrete driveway by preventing moisture from expanding and deteriorating the concrete. However, this is not a mandatory requirement and can be omitted if desired.
In certain regions that experience freeze-thaw cycles, using plastic under outdoor concrete may even be detrimental. In these areas, air-entrained concrete is recommended, and the use of plastic could prolong the time needed to get rid of bleed water, potentially leading to surface de-lamination.
Ultimately, the decision to use a plastic vapour barrier under outdoor concrete depends on various factors, including the specific use case, local climate conditions, and the type of finish or covering that will be applied to the concrete. While plastic vapour barriers can provide protection from moisture damage, they are not always necessary, and alternative methods, such as sealing the concrete or using admixtures, can also be considered.
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Plastic can prolong the time needed to get rid of bleed water
Concrete is a mixture of stone, sand, cement, and water. As the concrete settles, the heavier particles like cement and stone sink, pushing the lighter water upwards, which is known as "bleeding". This water collects on the surface of the concrete and is called "bleed water".
Bleed water can have a detrimental effect on the quality of the concrete. If the finisher begins to finish the concrete with the bleed water present, the water will be forced into the top layer and will weaken the concrete. The water can also collect under reinforcing steel, creating air voids and weak points. Excessive bleeding can also increase permeability and the water-cement ratio, undermining the concrete's resistance to freeze-thaw cycles.
In some cases, it is desirable to use plastic sheeting to cover concrete. This is particularly useful in cold and/or wet conditions, as it helps to retain the heat generated during cement hydration and prevents the surface from drying out. However, applying plastic too early, when finishing and late bleed water are still present, can cause issues. The moisture coagulates with the plastic, and if there are areas where the plastic is not in full contact, air pockets will form. This results in a mottled finish, with distinct shading variations.
In some parts of the world, using plastic under an outside slab may be a bad idea. In areas subject to freeze-thaw cycles, air-entrained concrete is necessary. The use of plastic could prolong the time needed to get rid of bleed water. If the concrete is finished too quickly before the bleed water has dissipated, surface de-lamination can occur.
Therefore, while plastic can be useful in certain situations, it is important to be aware of the potential drawbacks. In the case of outdoor concrete, it may prolong the time needed to get rid of bleed water, which can have negative consequences on the final product.
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Plastic under concrete can extend the lifespan of a driveway
Concrete is a porous material that can absorb rainwater from the ground. This can lead to dampness, rust, discolouration, bubbling, and peeling. Therefore, it is essential to install a vapour barrier to prevent or slow down the entry of moisture.
A vapour barrier is a layer of plastic that prevents water vapour from passing through a floor and causing issues for floor coverings or the environment inside a building. The American Concrete Institute (ACI) recommends installing a vapour barrier for concrete flooring covered with an impenetrable finish or shielded and heated by a fixed structure.
While some people argue that installing a vapour barrier under outdoor concrete is unnecessary, others claim that it can help extend the lifespan of a driveway by preventing moisture-related issues. One argument in favour of installing a vapour barrier is that it can help prevent concrete from absorbing water and curing incorrectly. Plastic under the concrete can also create a uniform, flat surface, making it easier to pour the concrete and resulting in a stronger final product.
However, it is important to note that the effectiveness of a vapour barrier depends on the climate and the type of concrete used. In areas subject to freeze-thaw cycles, using air-entrained concrete is essential, and installing plastic under the concrete may prolong the time needed to get rid of bleed water. Therefore, it is crucial to consult with local building experts before making a decision.
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Plastic under concrete can help achieve a nice, strong pour
Concrete is a porous material, which means that rainwater from the ground can permeate the surface. This can cause damage to the concrete slab and any flooring on top of it. To prevent this, a vapor barrier can be installed underneath the concrete. This vapor barrier can be made of plastic.
Additionally, plastic under concrete can help to extend the lifespan of the concrete by preventing moisture-related issues. Moisture can cause the concrete to swell, expand, cup, and degrade, eventually leading to failure. Plastic under the concrete creates a barrier that prevents this moisture movement and the associated issues.
It is important to note that the decision to use plastic under outdoor concrete may depend on the specific climate and conditions. In areas subject to freeze-thaw cycles, for example, the use of air-entrained concrete is recommended, and plastic may not be necessary or even advisable. However, in general, plastic under concrete can be a cheap and easy way to protect the concrete from moisture damage and achieve a strong pour.
When using plastic under concrete, it is recommended to choose a durable and impermeable material. The American Concrete Institute (ACI) suggests using a 10-mil poly or plastic with a 0.03 Perms IMP score as the lowest barrier for residential concrete. Thicker poly barriers, such as 15-mil or 20-mil, are often more absorbent and durable. It is also important to consider the climate and specific requirements of the project to choose the most appropriate vapor barrier.
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Frequently asked questions
No, it is not necessary to put plastic under outdoor concrete. However, if you are planning to seal the surface with an impermeable membrane or layer, it is recommended to use a vapour barrier.
A vapour barrier is a layer that prevents or slows the entry of moisture vapour. It is usually made of polyethylene and must be at least 10-mil thick or thicker to be considered effective.
When a concrete slab is sealed with an impermeable layer, ground moisture moving upwards will not be able to escape the concrete surface and will be trapped inside. This can cause the concrete to swell, expand, cup, and degrade, leading to damage.
Plastic under outdoor concrete can protect it from moisture damage. Moisture from the soil can penetrate concrete, causing it to crack over time and potentially leading to structural damage. Plastic helps to prevent this by reducing the moisture in the concrete slab, leading to a stronger bond.
In areas subject to freeze-thaw cycles, using plastic under outdoor concrete is not recommended. The plastic can prolong the time needed for bleed water to evaporate, and if the concrete finishes setting before the bleed water has dissipated, it can cause surface de-lamination.
