ACI Flatwork Placement & Leveling 3 — Questions and Answers
Question 1: What is the recommended maximum free-fall height when depositing concrete to minimize segregation?
- 6 feet
- 5 feet
- 3 feet (Correct answer)
- 10 feet
Correct answer: 3 feet
ACI guidelines recommend limiting free-fall to no more than 3 feet to prevent coarse aggregate from separating from the mortar.
Question 2: When screedng concrete between forms, the screed should be moved with what motion?
- Straight push only, no side-to-side motion
- A sawing side-to-side motion while advancing forward (Correct answer)
- Circular motion to distribute aggregate
- Pulling only, never pushing
Correct answer: A sawing side-to-side motion while advancing forward
A sawing back-and-forth lateral motion while advancing forward fills voids and levels the surface without leaving ridges.
Question 3: What is 'subgrade drag' and why does it matter during flatwork placement?
- The weight of screeding equipment on the subgrade
- Friction between the subgrade and concrete slab that can cause cracking as the slab shrinks (Correct answer)
- Contamination of concrete by subgrade soil
- Settling of the subgrade under load
Correct answer: Friction between the subgrade and concrete slab that can cause cracking as the slab shrinks
Subgrade drag is the restraint force between the slab and subgrade that resists shrinkage movement, potentially causing cracking if not relieved by joints or proper subgrade preparation.
Question 4: Which type of screed is best suited for large flatwork areas requiring high flatness tolerances (high FF numbers)?
- Manual rod screed
- Laser-guided or GPS-controlled vibratory screed (Correct answer)
- Hand-held jitterbug tamper
- Rubber float
Correct answer: Laser-guided or GPS-controlled vibratory screed
Laser-guided or GPS-controlled vibratory screeds provide the precision and consistency needed to achieve high FF/FL tolerances on large slab areas.
Question 5: When pumping concrete for flatwork, the discharge hose should be positioned to deposit concrete:
- Directly against the forms to fill corners first
- As close to final position as possible to minimize movement (Correct answer)
- At the center of the pour and rake outward
- From a height of at least 5 feet for proper flow
Correct answer: As close to final position as possible to minimize movement
Depositing near the final position minimizes the need to rake or move concrete, reducing the risk of segregation and maintaining mix uniformity.
Question 6: What is the purpose of 'strike-off' in concrete flatwork?
- Removing excess concrete to bring the surface to the level of the forms or screed guides (Correct answer)
- Applying a release agent to the forms
- Removing the top layer of bleed water
- Marking control joint locations
Correct answer: Removing excess concrete to bring the surface to the level of the forms or screed guides
Strike-off (screeding) removes excess concrete and brings the surface to the required grade established by the form tops or screed guides.
Question 7: In cold-weather concreting, what minimum concrete temperature (as delivered) does ACI 306 recommend for sections 12 inches or less?
- 35°F (2°C)
- 45°F (7°C)
- 55°F (13°C) (Correct answer)
- 65°F (18°C)
Correct answer: 55°F (13°C)
ACI 306 requires a minimum delivered concrete temperature of 55°F (13°C) for thin sections up to 12 inches to ensure adequate hydration and strength gain.
ACI Flatwork Placement & Leveling 3
What is the recommended maximum free-fall height when depositing concrete to minimize segregation?