ACI Concrete Strength Testing Technician Practice Test

What does the term "unbonded cap" refer to in concrete testing?

1. A rigid cap used for compression tests

2. A combination of metal retainer and elastomeric pad

3. A specific type of concrete mix

4. A measuring device for concrete strength

The correct answer is: A combination of metal retainer and elastomeric pad

The term "unbonded cap" refers specifically to a combination of a metal retainer and an elastomeric pad used in compression testing of concrete specimens, such as cylinders. The purpose of this configuration is to provide a way to apply uniform pressure to the concrete sample while preventing the transmission of stresses from the retainer to the specimen itself. This unbonded approach helps in accurately measuring the compressive strength of the concrete since it reduces the friction that can occur between the metal retainer and the concrete surface, which could otherwise lead to misleading results. The elastomeric pad acts as a cushion, allowing for the correct transfer of force during the testing process. Other choices do not accurately describe the term "unbonded cap." For instance, a rigid cap would imply a direct contact that could affect the test results by introducing additional stress concentrations. The specific type of concrete mix does not relate to the mechanical aspect of testing and a measuring device for concrete strength does not accurately capture the unique characteristics of an unbonded cap setup. Thus, the definition involving the combination of metal retainer and elastomeric pad is key to understanding its role in concrete strength testing.