Sulfur Cap Failure on High-Strength Cylinders
A laboratory is testing 4x8 in. high-strength concrete cylinders specified at f'c = 8000 psi. The technician prepares sulfur mortar caps per ASTM C617. After the caps cure, several show visible shrinkage cracks and one cap is 0.20 in. thick at one edge and 0.05 in. at the opposite edge. The lab plans to test at a loading rate of 50 psi/s. During the first test, the cylinder fractures at the cap interface at 6200 psi, well below the expected 8000 psi. The lab manager wants to know whether the caps or the loading rate caused the premature failure.
1. Evaluate the cap preparation and loading rate against ASTM requirements.
Read the worked answer
Sulfur caps must be parallel to the cylinder end within 0.002 in. and the cap thickness should not exceed 0.125 in. (1/8 in.) for a 4-in. cylinder, with a minimum of about 0.04 in. The described cap is 0.20 in. thick at one edge, exceeding the maximum, and is highly non-parallel (0.15 in. difference across the diameter). This creates eccentric loading and stress concentration at the cap interface, which explains the premature fracture at 6200 psi. Shrinkage cracks indicate the sulfur was poured too hot or cooled improperly. The loading rate of 50 psi/s is at the upper limit of the ASTM C39 range (20-50 psi/s) but is acceptable; however, for high-strength concrete the rate should be controlled to maintain a uniform stress increase, and 50 psi/s is not the primary cause. The caps are the controlling defect.
2. What corrective action and retest plan should the lab adopt?
Read the worked answer
Reject the non-compliant caps: remove them, re-cap the remaining cylinders ensuring parallelism within 0.002 in., thickness between 0.04 and 0.125 in., and no shrinkage cracks. Use a capping plate and verify with a straightedge. Allow proper cooling. Retest the remaining cylinders at a controlled rate within 20-50 psi/s, ideally near 35 psi/s for consistency. If the retest still yields low values, consider grinding the ends per ASTM C39 (grinding is permitted for high-strength concrete) instead of sulfur caps. Document the cap thickness and parallelism for each cylinder. If the original 6200 psi result is used for acceptance, note that the failure mode (cap interface) invalidates the test per ASTM C39, so the result should be discarded and the cylinder retested or replaced.