ACI ACI Subgrade & Base Preparation for Slabs

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

To increase the compressive strength of the concrete

Answer

To reduce the water-to-cement ratio

Answer

To allow faster bleeding of the concrete

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

80%

Answer

70%

Answer

90%

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

Place extra concrete thickness over the soft area

Answer

Add reinforcing steel over the soft spot only

Answer

Increase the cement content of the concrete mix

Question 4

Just before concrete is placed, the subgrade surface condition should be:

Accepted Answer

Uniformly damp but not saturated or muddy

Answer

Completely dry to prevent excess water in the mix

Answer

Flooded with water to prevent moisture absorption

Answer

Covered with plastic sheeting to retain moisture

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

Increase the compressive strength of the slab

Answer

Act as a vapor retarder under the slab

Answer

Prevent the use of reinforcing steel

Question 6

When compacting fill beneath a concrete slab, ACI recommends placing fill in lifts not exceeding:

Accepted Answer

6 inches of loose material per lift

Answer

12 inches of loose material per lift

Answer

24 inches of loose material per lift

Answer

3 inches of loose material per lift