NASCLA Accredited Commercial General ContractorConcreteHard
A contractor is pouring a concrete wall that is 10 feet high. The concrete mix has a maximum aggregate size of 1 inch. To prevent segregation and ensure proper consolidation, what is the maximum recommended drop height for the concrete when discharging from the chute?
- A3 feet
- B5 feet
- C4 feet
- D2 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 5 feet
According to ACI 301 and ACI 304, the maximum recommended free fall for concrete to prevent segregation is generally 5 feet (1.5 meters). Exceeding this height can lead to aggregates separating from the paste, weakening the concrete.
Why the other options are wrong
- A. 3 feet is below the generally accepted maximum, potentially leading to slower placement than necessary.
- C. 4 feet is also below the generally accepted maximum for preventing segregation.
- D. 2 feet is a very conservative height, not the maximum recommended, and might be inefficient for standard pours.
Concrete Drop Height
The maximum vertical distance concrete can fall freely during placement without significant segregation.
- Critical for maintaining concrete homogeneity.
- Generally limited to 5 feet (1.5 meters) by ACI.
- Excessive drop causes aggregates to separate from paste.
Memory trick: Drop with care, Vibrate right, Lift slowly, Segregation fight.