ACI Concrete Strength Testing Technician Practice Test

Hardened concrete cylinders should be capped with which of the following materials?

1. Plaster of Paris

2. High-strength gypsum paste, neat cement paste, or sulfur mortar

3. Clay or resin

4. Regular cement or sand

The correct answer is: High-strength gypsum paste, neat cement paste, or sulfur mortar

Hardened concrete cylinders should be capped with high-strength gypsum paste, neat cement paste, or sulfur mortar to ensure accurate strength testing. These materials create a smooth, level surface on the ends of the concrete samples, which is critical for achieving a uniform load distribution during testing. Proper capping reduces the risk of stress concentrations that might occur from imperfections or irregularities at the ends of the concrete cylinder, potentially leading to erroneous test results. The selection of high-strength gypsum paste or sulfur mortar, in particular, helps achieve a strong bond and maintain the integrity of the cylinder under load. This is crucial because effective load transfer during compression testing is required for reliable strength measurements. In addition, these materials are designed to have low shrinkage, which avoids any dimensional changes that could affect the dimensions of the test sample post-capping. Other materials like clay or resin are not suited for this application as they may not provide the necessary strength and stability required during testing. Regular cement or sand may also not yield the desired uniformity and strength for effective load transfer. Thus, using the appropriate high-strength capping materials directly influences the accuracy of concrete strength assessments.