ACI Subgrade & Base Preparation for Slabs Flashcards

According to ACI 302.1R, the primary purpose of a vapor retarder beneath a concrete slab is to:

Answer: Prevent moisture from migrating upward through the slab and damaging floor coverings

A vapor retarder blocks the upward movement of ground moisture that would otherwise raise the slab's moisture content and damage adhesives or coatings.

ACI 302.1R recommends placing the vapor retarder:

Answer: Directly beneath the concrete slab with no granular blotter layer between them

Current ACI guidance places the vapor retarder directly under the slab to minimize moisture migration; a sand blotter layer was previously recommended but is now discouraged.

For Class A applications (most moisture-sensitive floor coverings), ACI recommends a vapor retarder with a permeance not exceeding:

Answer: 0.01 perms

Class A vapor retarders must have a permeance of 0.01 perms or less, providing the most stringent barrier against moisture transmission.

When is the use of a vapor retarder most critical under a concrete slab?

Answer: When moisture-sensitive flooring, coatings, or adhesives will be applied to the slab surface

Vapor retarders are essential when the slab will support materials sensitive to moisture vapor transmission, such as vinyl tile, hardwood, or epoxy coatings.

Overlapping joints in vapor retarder sheets should be lapped a minimum of:

Answer: 6 inches and sealed with tape

ACI requires a minimum 6-inch overlap at seams, which must be taped to maintain the continuous barrier against vapor transmission.

Punctures or tears in a vapor retarder discovered before concrete placement should be:

Answer: Repaired immediately with compatible tape before any concrete is placed

Any breach in the vapor retarder must be repaired before placement to maintain the continuous moisture barrier.