ABSTRACT
One of many factors connected with the quality of concrete is its hardness. Efforts to measure the surface hardness of a mass of concrete were first recorded in the 1930s; tests were based on impacting the concrete surface with a specified mass activated by a standard amount of energy. Early methods involved measurements of the size of indentation caused by a steel ball either fixed to a pendulum or spring hammer, or fired from a standardized testing pistol. Later, however, the height of rebound of the mass from the surface was measured. Although it is difficult to justify a theoretical relationship between the measured values from any of these methods and the strength of a concrete, their value lies in the ability to establish empirical relationships between test results and quality of the surface layer. Unfortunately these are subject to many specific restrictions including concrete and member details, as well as equipment reliability and operator technique. Indentation testing has received attention in Germany and in former states
of the USSR as well as the United Kingdom, but has never become very popular. Pin penetration tests have, however, received attention in the USA and Japan (see Section 4.1.2). The rebound principle, on the other hand, is more widely accepted: the most popular equipment, the Schmidt Rebound Hammer, has been in use worldwide for many years. Recommendations for the use of the rebound method are given in BS EN 12504-2 (58) and ASTM C805 (59).
