What is the purpose of a vapor retarder under slabs-on-grade?

• A. Prevents expansion of concrete.
• B. Controls moisture migration from soil into the slab.
• C. Provides insulation.
• D. Accelerates curing.

Answer: (B)

Moisture control under slabs-on-grade is what this is testing. A vapor retarder is used to limit the amount of moisture moving from the soil into the concrete slab. By reducing moisture migration, it protects flooring materials and adhesives from dampness, helps prevent problems like efflorescence and mold, and contributes to more consistent curing conditions. It’s usually a thin plastic sheet laid on the subgrade before pouring and sealed at joints. It doesn’t serve as insulation, doesn’t prevent concrete expansion, and doesn’t accelerate curing.