ACI Certification Practice Exam

What must be done to strength test specimens molded from concrete with a slump less than 25mm unless specified otherwise?

1. Require no special treatment

2. Be consolidated by vibration

3. Be consolidated by hand

4. Be allowed to air dry

The correct answer is: Be consolidated by vibration

When testing the strength of concrete specimens molded from mixes with a slump less than 25mm, it's crucial to ensure proper consolidation. This type of concrete is usually more stiff than those with higher slumps, making it less likely to be adequately compacted through standard methods. Therefore, vibration is employed to enhance consolidation by removing air pockets and ensuring that the concrete reaches a uniform density throughout the specimen. This process is essential for obtaining accurate and reliable strength test results. Using vibration helps to achieve a homogenous mix and proper filling of the molds, particularly in coarse-grained mixtures or those with additives that might lead to segregation. Relying solely on other methods of consolidation, such as hand consolidation, may not be sufficient for stiffer mixes and could result in inconsistent test outcomes. As such, the requirement of consolidating by vibration is a critical step in ensuring that the concrete specimens adequately replicate the conditions under which the concrete would perform in actual use.