ACI ACI Subgrade & Base Preparation for Slabs 2

Question 1

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

Accepted Answer

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

Answer

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

Question 2

ACI 302.1R recommends placing the vapor retarder:

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

0.01 perms

Answer

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

Question 4

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

Accepted Answer

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

Answer

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

Question 5

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

Accepted Answer

6 inches and sealed with tape

Answer

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

(ACI) American Concrete Institute Concrete Finisher Practice Test

(ACI) American Concrete Institute Concrete Finisher Practice Test

ACI ACI Subgrade & Base Preparation for Slabs 2