Dropping concrete more than five feet onto reinforcing bars commonly causes what problem?

Dropping concrete more than five feet onto reinforcing bars commonly causes what problem?

• A. No effect
• B. Reinforcing bars will move
• C. The aggregate can separate causing poor consolidation
• D. Ties will loosen

Answer: (C)

Dropping concrete from a height can cause the mix to segregate: the heavier coarse aggregate tends to separate from the cement paste as the concrete falls, and around reinforcing bars this separation prevents the paste from fully filling the space. The result is poor consolidation, with voids and a lack of proper contact between the bars and surrounding concrete. This weakens the bond between steel and concrete and can lead to cracks or reduced durability. The other possibilities—no effect, bars moving, or ties loosening—don’t accurately describe what happens when a drop is too high, since bars won’t shift and ties aren’t directly affected by the drop height. To avoid this, keep drop heights low, use vibration or other consolidation methods, and place concrete so it fully surrounds the reinforcement.

Dropping concrete from a height can cause the mix to segregate: the heavier coarse aggregate tends to separate from the cement paste as the concrete falls, and around reinforcing bars this separation prevents the paste from fully filling the space. The result is poor consolidation, with voids and a lack of proper contact between the bars and surrounding concrete. This weakens the bond between steel and concrete and can lead to cracks or reduced durability. The other possibilities—no effect, bars moving, or ties loosening—don’t accurately describe what happens when a drop is too high, since bars won’t shift and ties aren’t directly affected by the drop height. To avoid this, keep drop heights low, use vibration or other consolidation methods, and place concrete so it fully surrounds the reinforcement.