Capping and Compressive Strength Testing of Cylindrical Specimens
Capping
Capped ends must be flat, perpendicular, and parallel so the load is applied uniformly. Use sulfur mortar or another approved capping material, and follow its curing requirements. Keep the cap thin; a thick cap can crush or flow and change the result. The capped end should not depart from perpendicularity by more than about 0.5 degrees. Use a flat capping plate and check the cap with a straightedge.
Compression testing
Measure the cylinder diameter and calculate the cross-sectional area. Center the specimen in the testing machine and apply load at 35 ± 7 psi/s (0.25 ± 0.05 MPa/s) for standard cylinders. Compressive strength = maximum load ÷ cross-sectional area. Report the average of the required specimens and note any unusual fracture pattern.
Traps and worked example
Traps include a cap that is too thick, a tilted cap, an eccentric specimen, and loading too fast. Another trap is using the nominal diameter without checking the actual diameter, or testing a damaged or dry specimen.
Rule of thumb: flat, thin, perpendicular cap; slow, steady load. Worked example: a 6 in. diameter cylinder has an area of π × 3² = 28.27 in². If the maximum load is 85,000 lb, strength = 85,000 ÷ 28.27 = 3,007 psi, reported as about 3,010 psi.