ACI Concrete Field Testing Technician - Grade I Practice Exam

When using internal vibration for a standard flexural strength test specimen, what is the maximum distance between vibrator insertions along the beam center line?

1. 4 in. [100 mm]

2. 5 in. [125 mm]

3. 6 in. [150 mm]

4. 8 in. [200 mm]

The correct answer is: 6 in. [150 mm]

The maximum distance between vibrator insertions along the beam center line during internal vibration is crucial for achieving uniform consolidation of the concrete. It ensures that air pockets are minimized and that the concrete mix achieves proper density and strength. The correct choice of 6 inches [150 mm] aligns with standard practices that aim not to exceed this distance, as it is a balance between effective consolidation and the physical constraints of handling the equipment and the mix itself. Using this specified distance helps technicians maintain an optimal flow of vibrations throughout the beam, particularly in areas that might otherwise be prone to segregation or incomplete consolidation. It reflects guidelines designed to produce reliable and consistent results in flexural strength testing, vital for assessing the material's performance in structural applications. The other distances provided may not sufficiently ensure this level of effectiveness in consolidation.