ACI ACI Subgrade & Base Preparation for Slabs 1 — Questions and Answers
Question 1: What is the primary reason ACI requires proper subgrade preparation before placing a concrete slab?
- To provide uniform support and prevent differential settlement (Correct answer)
- To increase the compressive strength of the concrete
- To reduce the water-to-cement ratio
- To allow faster bleeding of the concrete
Correct answer: To provide uniform support and prevent differential settlement
A well-prepared subgrade distributes loads uniformly, preventing the differential settlement that causes cracking.
Question 2: ACI 302.1R recommends that subgrade fill be compacted to a minimum of what percentage of maximum dry density?
- 95% (Correct answer)
- 80%
- 70%
- 90%
Correct answer: 95%
ACI 302.1R specifies 95% of maximum dry density (per ASTM D698) to ensure adequate bearing capacity and limit future settlement.
Question 3: When a soft spot is discovered in the subgrade during slab preparation, the correct action is to:
- Remove the unsuitable material and replace it with properly compacted fill (Correct answer)
- Place extra concrete thickness over the soft area
- Add reinforcing steel over the soft spot only
- Increase the cement content of the concrete mix
Correct answer: Remove the unsuitable material and replace it with properly compacted fill
Soft spots must be excavated and replaced with compacted granular fill to restore uniform bearing capacity.
Question 4: Just before concrete is placed, the subgrade surface condition should be:
- Uniformly damp but not saturated or muddy (Correct answer)
- Completely dry to prevent excess water in the mix
- Flooded with water to prevent moisture absorption
- Covered with plastic sheeting to retain moisture
Correct answer: Uniformly damp but not saturated or muddy
A damp subgrade prevents it from absorbing water from the concrete mix, but standing water or mud would increase the water-cement ratio and reduce strength.
Question 5: The main purpose of a granular subbase layer beneath a concrete slab-on-grade is to:
- Provide a stable, uniform bearing surface and improve drainage (Correct answer)
- Increase the compressive strength of the slab
- Act as a vapor retarder under the slab
- Prevent the use of reinforcing steel
Correct answer: Provide a stable, uniform bearing surface and improve drainage
A granular subbase levels out subgrade irregularities, distributes loads, and facilitates drainage away from the slab bottom.
Question 6: When compacting fill beneath a concrete slab, ACI recommends placing fill in lifts not exceeding:
- 6 inches of loose material per lift (Correct answer)
- 12 inches of loose material per lift
- 24 inches of loose material per lift
- 3 inches of loose material per lift
Correct answer: 6 inches of loose material per lift
Compacting in lifts of 6 inches or less ensures the compaction energy reaches the full depth of each layer uniformly.
ACI ACI Subgrade & Base Preparation for Slabs 1
What is the primary reason ACI requires proper subgrade preparation before placing a concrete slab?