What methods of capping are recommended for compression specimens?

Q. What methods of capping are recommended for compression specimens?

A. ASTM standards that control capping, C39, C617, and C1231, have all been revised to better address end preparation, especially for high-strength concretes. Capping with sulfur mortar is widely used. The revisions establish a number of additional requirements for use of sulfur capping materials to test concrete with strengths over 50 MPa (7250 psi):

- If the capping material does not have cube strengths at least as high as the concrete tested, a qualification test program by the manufacturer must demonstrate performance;

- The maximum cap thickness is reduced from 6 to 3 mm (0.25 to 0.125 in)

- The caps must be aged a minimum of 16 h rather than 2 h.

Another frequently used capping system is the ASTM C1231 practice for unbonded caps that uses an elastomeric pad in a metal retaining ring. This practice permits use of unbonded caps up to 50 MPa (7,250 psi) without requiring qualification tests. At concrete strengths from 50 to 80 MPa (7,250 to 11,500 psi), the number of reuses is limited to 50 tests instead of 100, and the user or manufacturer must perform a qualification test to demonstrate that strengths are not reduced more than 2%. Unbonded caps are not permitted for strengths exceeding 80 MPa (11,500 psi), because there are insufficient data on their performance with concrete stronger than 80 MPa (11,500 psi).

High-strength gypsum cement and neat portland cement capping materials are generally acceptable as capping materials, but rarely used. Grinding to produce plane ends has been the preferred method by some laboratories for testing high-strength concrete; however, recent research shows acceptable comparisons with ground ends at concrete strengths up to 117 MPa (17,000 psi) if the sulfur caps are less than 3 mm thick and the cylinders are capped seven days before testing.

References: SP-1(02); ASTM C39; ASTM C617; ASTM C1231

Topics in Concrete: Testing of Concrete