Comparative Study on Compressive Strength of Concrete with New Sand-Cap and Neoprene Pad

The most typical capping method for concrete structures is a sulfur-mortar compound capping, provided it satisfied the standard criterion set forth by ASTM C 617, but this conventional bonded-type method has many problems. It exhibits relatively the smaller unreliable value of the strength of high-strength concrete due to the differences of elasticity and strength between the cylinder and the cap, and manifests poor serviceability such as dangerous working tasks or a waste of the working time. To prevent these problems, unbonded-type capping methods have taken the place of the conventional methods in recent years. One of the popular methods is the use of synthetic rubber like a neoprene pad. Serious problems still remain in this method, which include the consideration of its chemical characteristics in consideration of the selection, the safekeeping and the economy of the pads. Moreover, the synthetic rubber pads cannot be used in concrete cylinder with strength greater than 80 MPa according to ASTM C 1231-00. New 'sand-capping method' presented in this study, can be applicable to the compressive strength evaluation of the high strength concrete in the range of $70{\sim}100\;MPa$. This new method has better simplicity and reliability than those of existing 'sand-box', because usual materials such as standard sand and simply-devised apparatus are used for the capping system. The statistical analysis of the test results revealed that the new sand-capping method exhibited the smallest deviation and dispersion, attesting for its much better reliability than other methods specified in ASTM C 1231/1231M.