ACI Concrete Strength Testing Technician Practice Test

What material combination defines an unbonded cap?

1. Plastic and metal retainer

2. Concrete and elastomeric pad

3. Metal retainer and elastomeric pad

4. Composite material and rubber

The correct answer is: Metal retainer and elastomeric pad

An unbonded cap typically consists of a metal retainer combined with an elastomeric pad. In this construction, the metal retainer provides the necessary structural support and positions the elastomeric pad, which serves to isolate the strain measuring device or sensor from the concrete. This isolation is crucial as it prevents the transmission of stresses directly from the concrete, allowing for accurate measurement responses without interference from the concrete's rigid characteristics. The combination of the metal and elastomeric materials is specifically chosen to ensure durability and flexibility, which are essential for the accurate assessment of concrete strength. The elastomeric pad absorbs any dynamic forces and helps to accommodate any movement or deformation in the concrete, ensuring that the testing equipment remains correctly aligned and that measurement data are reliable. Other combinations, like plastic and metal or composite materials paired with rubber, do not provide the same level of structural integrity or performance needed for effective concrete testing and would not meet the requirements for an unbonded cap.