ACI Subgrade & Base Preparation for Slabs Flashcards
6 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 ACI Subgrade & Base Preparation for Slabs flashcards as text
What is the primary reason ACI requires proper subgrade preparation before placing a concrete slab?
Answer: To provide uniform support and prevent differential settlement
A well-prepared subgrade distributes loads uniformly, preventing the differential settlement that causes cracking.
ACI 302.1R recommends that subgrade fill be compacted to a minimum of what percentage of maximum dry density?
Answer: 95%
ACI 302.1R specifies 95% of maximum dry density (per ASTM D698) to ensure adequate bearing capacity and limit future settlement.
When a soft spot is discovered in the subgrade during slab preparation, the correct action is to:
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.
Just before concrete is placed, the subgrade surface condition should be:
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.
The main purpose of a granular subbase layer beneath a concrete slab-on-grade is to:
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.
When compacting fill beneath a concrete slab, ACI recommends placing fill in lifts not exceeding:
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.