ACI Concrete Field Testing Technician - Grade I Practice Exam 2025 - Free Concrete Field Testing Practice Questions and Study Guide

The correct choice for the minimum frequency of the vibrator used in the slump test is 9000 vibrations per minute. This specific frequency is outlined in the standards set by the American Society for Testing and Materials (ASTM) for the slump test, which assesses the workability or consistency of fresh concrete.

Using a vibrator that operates at this frequency ensures effective compaction of the concrete within the slump cone, enabling accurate measurement of slump. A high enough vibration frequency is crucial to overcoming resistance from the concrete mixture and ensuring that air pockets are minimized, resulting in a true representation of the concrete's consistency.

While the other choices represent frequencies that are typically lower than the prescribed standard, maintaining a frequency of 9000 vibrations per minute allows for proper performance of the test and more reliable and valid results, which is essential for quality control in concrete placement. Thus, adhering to this minimum frequency is necessary to obtain dependable slump test measurements, reflecting the properties of the concrete being tested.