Abstract
Studies have shown that end conditions of concrete cylinders tested in compression can have a significant effect on the measured strength of the cylinders, especially when high-strength concrete is used. The ASTM standard for bonded caps has requirements for minimum cube strength of the capping material and maximum cap thickness. However, a study by researchers at the National Ready Mixed Concrete Association (NRMCA) showed that the 50 mm cube strength may not be very useful in determining whether the capping material will perform adequately when testing high-strength concrete. In the study reported in this paper, the dynamic modulus of elasticity and modified cube strength (ASTM C 116) of various capping materials were evaluated as a function of age. The results showed that each capping material has a unique relationship between dynamic elastic modulus and cube strength. The elastic modulus of different capping materials can vary greatly at a given cube strength. For example, at a modified cube strength of 80 MPa, the elastic modulus of neat cement paste, at 30 GPa, was twice the elastic modulus of one sulfur mortar, at only 15 GPa. The elastic modulus of the capping materials was correlated with previously reported cylinder strengths. In cases where the cylinder strength was affected by the capping material, there is evidence that the cylinder strength was related to the modulus of elasticity and not to the cube strength of the capping material.
