ACI Concrete Field Testing Technician - Grade I Practice Exam

What is the minimum frequency of the vibrator used in the slump test?

1. 5000 vibrations per minute

2. 6000 vibrations per minute

3. 7000 vibrations per minute

4. 8000 vibrations per minute

The correct answer is: 7000 vibrations per minute

The minimum frequency of the vibrator used in the slump test is 7000 vibrations per minute. This specific frequency is critical for ensuring that the concrete is consolidated effectively during the test. Proper consolidation allows air pockets to escape, leading to a more accurate representation of the concrete's workability and consistency. If the vibrator operates below this frequency, it may not adequately consolidate the concrete, which can result in an inaccurate slump measurement. The 7000 vibrations per minute standard provides a balance between effective consolidation and the prevention of overworking the concrete, which could also adversely affect the slump results. This standard is outlined in testing procedures and protocols to ensure that all tests are conducted reliably and can be compared across different samples and time periods.