ACI Concrete Field Testing Technician - Grade I Practice Exam

What is the minimum frequency of an internal vibrator used in concrete testing procedures?

1. 6000 vibrations per minute [100 Hz]

2. 7500 vibrations per minute [125 Hz]

3. 9000 vibrations per minute [150 Hz]

4. 12000 vibrations per minute [200 Hz]

The correct answer is: 9000 vibrations per minute [150 Hz]

The correct response regarding the minimum frequency of an internal vibrator used in concrete testing procedures aligns with industry standards that emphasize the efficiency of consolidation when working with freshly mixed concrete. An internal vibrator with a frequency of 9000 vibrations per minute, which translates to 150 Hz, is often recommended for effectively removing air pockets and ensuring uniform distribution of materials within the concrete mix. This frequency helps achieve optimal results in achieving the desired workability without compromising the concrete’s integrity. Utilizing a vibrator at this level ensures that the concrete is cohesive and free from segregation. Frequencies below this threshold may not be effective enough for proper consolidation, which could lead to voids or weak spots in the poured concrete. Higher frequencies, while they may seem effective, could introduce excessive vibration that may lead to segregation or other issues in the concrete mix. Thus, a frequency of 9000 vibrations per minute is recognized as the correct standard for effective internal vibration in concrete testing.