NASCLA Accredited Commercial General ContractorConcreteMedium
A general contractor is constructing a multi-story building using cast-in-place concrete. The specifications require the use of an internal vibrator during concrete placement for columns and beams. The vibrator head has a diameter of 1.5 inches. What is the generally recommended maximum spacing for insertion points of this vibrator to ensure proper consolidation?
- A18 inches
- B30 inches
- C12 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 18 inches
The effective radius of action for an internal vibrator is typically around 4-7 times its diameter. For a 1.5-inch vibrator, this means a radius of 6 to 10.5 inches. To ensure complete overlap and proper consolidation, the spacing of insertion points should generally be no more than 1.5 times the radius of action, or approximately 18 inches for a 1.5-inch vibrator. ACI 309R, 'Guide for Consolidation of Concrete,' recommends a spacing of about 1.5 times the radius of action.
Why the other options are wrong
- B. 30 inches is too far apart and would result in unconsolidated areas.
- C. 12 inches might be adequate for smaller vibrators or very stiff mixes, but 18 inches allows for better coverage with a 1.5-inch head.
- D. 6 inches would be excessively close and inefficient for a 1.5-inch vibrator.
Internal Concrete Vibration
The process of consolidating fresh concrete using a vibrating probe inserted directly into the concrete mix to remove entrapped air and achieve maximum density.
- Removes entrapped air voids.
- Improves density, strength, and durability.
- Spacing of insertions is critical for effective consolidation.
Memory trick: Vibrator's reach, consolidate each patch.