Published On Jan 26, 2024
Plastic under a concrete slab acts as a vapor barrier which is a way to control the amount of moisture the slab is exposed to from the ground surrounding it. Without this type of barrier, the concrete acts like a sponge and leaches moisture from the environment around it. It also absorbs the minerals from the ground moisture which can cause a host of problems like efflorescence or staining.
A concrete slab that uses plastic as a vapor barrier is exposed to less water than if plastic was not used under it. Concrete exposed to excessive amounts of water will erode more quickly than concrete that is more protected from water permeation and exposure. When water permeates the concrete, it slowly dissolves the cement component and washes it away with the water. This leaves only the aggregates of the concrete without any of the binding agent to hold it together making the concrete weak and prone to failure.
Using a plastic barrier to stop excess moisture from permeating the concrete slab protects any steel used within the concrete as reinforcement. If the embedded steel were continually exposed to excess moisture, it would rust and expand as it rusted causing failure of the concrete from within.
A concrete slab without plastic under it will still last quite a while but it will have a far longer service life when a plastic vapor barrier is used. It is a simple and inexpensive step when pouring a concrete slab that will go a long way to improve the quality and lifespan of the finished concrete slab.
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