ACI Certification Practice Exam

What is an acceptable method for capping test cylinders?

1. With a thin layer of wax

2. With a rubber cap

3. With a thin layer of portland cement paste

4. With a metal cap

The correct answer is: With a thin layer of portland cement paste

Capping test cylinders is a critical step in ensuring accurate compressive strength testing results for concrete. The correct method of using a thin layer of portland cement paste is acceptable because it closely replicates the concrete material in terms of properties and behavior. When a thin layer of cement paste is applied, it ensures that there is a proper bond between the cylinder and the capping material, allowing for even distribution of stress during testing. This method also minimizes any potential discrepancies caused by different materials, which could affect the test outcomes. In contrast to this method, using a thin layer of wax lacks the structural integrity needed for accurate testing, as wax does not provide a rigid support and might deform under load. A rubber cap might be too flexible and can absorb some of the applied load, which could lead to an inaccurate reading. Similarly, while a metal cap offers a sturdy option, it does not provide the same level of compatibility with concrete as a cement-based solution, potentially leading to issues like stress concentrations or slip. Therefore, the use of a thin layer of portland cement paste ensures that the conditions of the test reflect the actual performance of the material under load, which is essential for reliable results.