ACI ACI Subgrade & Base Preparation for Slabs 1

Question 1

What is the primary reason ACI requires proper subgrade preparation before placing a concrete slab?

Accepted Answer

To provide uniform support and prevent differential settlement

Answer

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?

Accepted Answer

95%

Answer

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:

Accepted Answer

Remove the unsuitable material and replace it with properly compacted fill

Answer

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:

Accepted Answer

Uniformly damp but not saturated or muddy

Answer

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:

Accepted Answer

Provide a stable, uniform bearing surface and improve drainage

Answer

A granular subbase levels out subgrade irregularities, distributes loads, and facilitates drainage away from the slab bottom.

(ACI) American Concrete Institute Concrete Finisher Practice Test

(ACI) American Concrete Institute Concrete Finisher Practice Test

ACI ACI Subgrade & Base Preparation for Slabs 1