ACI Concrete Field Testing Technician - Grade I Practice Exam

If the concrete has a slump of less than 1 in. [25 mm], how must the test specimens be consolidated?

1. Rodding

2. Vibration

3. Pressing

4. Mixing

The correct answer is: Vibration

When concrete has a slump of less than 1 inch (25 mm), it indicates a very stiff mix. In such cases, adequate consolidation of the test specimens is crucial to ensure that the concrete is uniformly compacted, which affects its overall properties and allows for accurate testing results. Vibration is the preferred method for consolidating stiff concrete, as it effectively eliminates air pockets and achieves a dense mix without damaging the concrete's structure. Vibration ensures that the concrete flows into corners and around reinforcement, which is especially important for achieving a proper sample that accurately reflects the characteristics of the mix. This method introduces energy into the mix through mechanical means, thereby loosening it enough to settle conveniently into the mold while maintaining the integrity of the specimen. In contrast, rodding may be less effective for stiff mixes since it relies on manual manipulation to push the concrete down and might not adequately consolidate the material throughout, especially in denser areas. Pressing is typically not a recognized method for concrete consolidation in laboratory settings, and mixing is generally not designed for the purpose of consolidation of specimens after the initial pour. Therefore, vibration stands out as the most effective method for consolidating concrete with a low slump.