Vapor Barrier Under Concrete Driveway: Does It Increase Life?
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Is Vapor Barrier Under Concrete Driveway Necessary?
Installation of vapor barrier under concrete driveway is not necessary according to building code. However, the placement of vapor barrier under concrete driveway will help increase its life.
Vapor barrier is a film with low permeability. It prevents the migration of moisture from an area of high concentration to an area of low concentration. When you place a vapor barrier under concrete driveway, you prevent the ingress of moisture into the concrete slab from the grade below.
The moisture content of grade can increase sharply in case of high precipitation levels. Ground water tables can rise due to heavy rains, snowfall or even flooding in neighboring water bodies such as rivers and lakes.
Concrete is very porous. Moisture in the grade is sucked up by the concrete slab by capillary action. This is highly detrimental to the structural integrity and longevity of concrete. The use of an effective vapor barrier between the base and the concrete is the only possible deterrent.
How Does Concrete Deteriorate from Water?
“The most destructive agent of concrete structures and components is probably water”.
Quote from Wikipedia
Water, which is essential for concrete curing, becomes a mortal enemy after the concrete has cured. Water ingress into concrete will hurt it in every way it can.
Acidic Chemical Corrosion
The earth below the concrete driveway has several chemicals such as sulfates, chlorides and carbonates, which react with water to form weak acids.
The acid may be weak but over time corrodes both the aggregates and the steel reinforcement. The concrete loses its strength.
Alkali Silica Reaction (ASR)
The earth below the concrete driveway also has several alkaline chemicals such sodium and potassium oxides. In the presence of water they form alkalis such as sodium hydroxide and potassium hydroxide.
Alkalis that permeate into the concrete react with silica in the aggregates to form a hygroscopic gel. The gel expands, stressing the aggregates and even cracking them.
Freeze Thaw Cycle
Ground water sucked into the concrete by capillary action will freeze when night temperatures drop below freezing. Ice has a higher volume than water and hence exerts an outward pressure on the concrete structure.
In the day, when temperatures rise, the ice melts and the pressure is released. This freeze thaw cycle creates a cyclical stress on concrete. Even relatively low levels of stress can ultimately lead to failure over a period of time.
The role of the vapor barrier is to prevent ground water from penetrating the concrete driveway and weakening it.
What Is the Code for Vapor Barrier Under Concrete Driveway Slab?
Section 1907 of the 2021 International Building Code (IBC) lays down the “Minimum Slab Provisions”. The section requires that a polyethylene vapor retarder (the correct name for vapor barrier) be installed between the base or the subgrade (as the case may be) and the concrete slab.
However, the section also has a list of exceptions which includes driveways.
Reading between the lines, I believe that the list of exceptions are for slabs that will have negligible occupancy. The intent of the code is to keep the occupants safe. It does not concern itself with the performance and longevity of the concrete slab itself.
Even though the code does not require it, using a vapor barrier under concrete driveway will prevent concrete degradation.
What Is the Minimum Thickness of Vapor Barriers?
The code specifies a 6 mil vapor retarder. Of course this is the bare minimum thickness. The effectiveness of a vapor barrier is measured in “perms”.
In the USA, a perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. Lower the perm rating, the more effective the vapor barrier. So it is about the perm rating rather than the thickness of the vapor barrier.
A 10 mil vapor barrier is a good option for a concrete driveway as it has a low enough perm rating and is also strong enough not to puncture easily. A good example is STEGO® WRAP CLASS A VAPOR RETARDER (10-MIL) with physical properties as under:
- Water Vapor Permeance: 0.0254 perms
- Puncture Resistance: 3,006 grams
- Tensile Strength: 50.6 lbf/in
- Thickness: 10-mil
Where Should a Vapor Barrier Be Installed?
Quite often a vapor barrier may be used but it is not effective because it has been placed incorrectly.
The simple rule of correct placement of a vapor barrier must be such that it is in direct contact with the concrete slab. In other words the vapor barrier should be laid on top of the base and the concrete should be poured directly on it.
When the vapor barrier is installed correctly it plays a crucial role in increasing the concrete driveway lifespan!
RECOMMENDED PRODUCT
The Best Vapor Barrier for Under Concrete Slab such as Driveways, Garage Floors & Patios
A concrete vapor barrier is a material that prevents moisture from entering a concrete slab from the grade below. This helps in controlling the concrete curing conditions in the initial stages. Subsequently it prevents the wicking of moisture into the concrete slab from the damp earth.
Here is the Vapor Barrier I recommend.
Farm Plastic Supply-Concrete Vapor Barrier (10 mil)
Heavy duty durable plastic sheeting engineered from low-density polyolefin. Primary application is as a vapor barrier underneath concrete.
You should consider reading my earlier blog post Best Base for Concrete Driveway: Comprehensive Guide for more information before you commence on building your concrete driveway.
Thank you very much for reading this post. I do hope you found it informative and helpful.
