CSLB General Building (B)Site Work, Foundations and ConcreteMedium
A concrete finisher is placing a footing pour and uses a mechanical vibrator to consolidate the concrete around the rebar cage. What is the primary purpose of vibrating the concrete during placement?
- ATo increase the water-cement ratio for better workability
- BTo eliminate the need for proper curing afterward
- CTo remove entrapped air voids and ensure the concrete flows completely around reinforcement
- DTo accelerate the initial set time of the concrete
Show answer & explanationAnswer & explanation
Correct answer: C. To remove entrapped air voids and ensure the concrete flows completely around reinforcement
Vibration consolidates freshly placed concrete by releasing trapped air bubbles and helping the mix flow tightly around reinforcing steel and into corners of the formwork, preventing honeycombing and voids that weaken the structure.
Why the other options are wrong
- A. Vibration does not alter the water-cement ratio of the mix.
- B. Curing is still required after placement regardless of vibration.
- D. Vibration does not chemically accelerate the concrete's set time.
Concrete Vibration (Consolidation)
Mechanical vibration during placement removes trapped air and ensures concrete fully encases reinforcement, preventing honeycombing and voids.
- Removes entrapped air bubbles
- Helps concrete flow around rebar and into corners
- Over-vibration can cause aggregate segregation
Memory trick: Shake out the bubbles so the mix hugs every bar.