ACI ACI Subgrade & Base Preparation for Slabs 2 — Questions and Answers
Question 1: According to ACI 302.1R, the primary purpose of a vapor retarder beneath a concrete slab is to:
- Prevent moisture from migrating upward through the slab and damaging floor coverings (Correct answer)
- Increase the bond between the concrete and subbase
- Reduce cracking due to plastic shrinkage
- Eliminate the need for curing after placement
Correct 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.
Question 2: ACI 302.1R recommends placing the vapor retarder:
- Directly beneath the concrete slab with no granular blotter layer between them (Correct answer)
- At least 4 inches below the slab on top of the subgrade
- On top of the granular subbase, separated by 2 inches of sand
- Immediately below the reinforcing steel
Correct 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.
Question 3: For Class A applications (most moisture-sensitive floor coverings), ACI recommends a vapor retarder with a permeance not exceeding:
- 0.01 perms (Correct answer)
- 0.10 perms
- 0.50 perms
- 1.00 perm
Correct 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.
Question 4: When is the use of a vapor retarder most critical under a concrete slab?
- When moisture-sensitive flooring, coatings, or adhesives will be applied to the slab surface (Correct answer)
- When the concrete will be exposed to outdoor weather
- When the slab will receive a broom finish
- When the concrete mix contains fly ash
Correct 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.
Question 5: Overlapping joints in vapor retarder sheets should be lapped a minimum of:
- 6 inches and sealed with tape (Correct answer)
- 1 inch with no tape required
- 12 inches without taping
- 3 inches and stapled together
Correct 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.
Question 6: Punctures or tears in a vapor retarder discovered before concrete placement should be:
- Repaired immediately with compatible tape before any concrete is placed (Correct answer)
- Left unrepaired if they are smaller than 1 inch in diameter
- Covered with an additional sheet of plastic without taping
- Reported to the engineer but concreting may proceed
Correct 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.
ACI ACI Subgrade & Base Preparation for Slabs 2
According to ACI 302.1R, the primary purpose of a vapor retarder beneath a concrete slab is to: