1.1 This test method covers the testing apparatus, sampling, test specimen preparation, and testing procedures for determining the rebound hardness number of rock material using a spring-driven steel hammer, referred to variously as a rebound hammer, impact test hammer, or concrete test hammer. 1.2 This test method is best suited for rock material with uniaxial compressive strengths ranging between approximately 1 and 100 MPa. Test Method D7012 provides more information on compressive strength of rock. 1.3 The portable testing apparatus may be used in the laboratory or field to provide a means of rapid assessment of rock hardness or to serve as an indicator of rock hardness. 1.4 Rebound hammers are available from their original manufacturers in several different energy ranges. For a given plunger tip diameter and radius of curvature, the impact energy of the rebound hammer determines its range of applicability. Accordingly, this limitation should be kept in mind when selecting a hammer type. Earlier recommendations for rock mechanics applications were only for hammers with an impact energy of 0.735 N·m, especially on smaller core samples and weaker rocks (see also Brown 1981 2 ). This test method applies only to hammers with an impact energy not to exceed 0.735 N·m. Hammers with energies above 0.735 N·m tend to break the rock and are not recommended. 1.5 Rocks exhibiting vesicular texture may be beyond the scope of this test. Care should be taken when testing such rocks and conglomerates as the rebound values will vary between testing a large piece of aggregate versus softer matrix of the conglomerate. 1.6 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026. 1.6.1 The procedures used to specify how data are collected/recorded or calculated, in this standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user's objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analytical methods for engineering design. 1.7 For determining the rebound number of concrete, see Test Method C805/C805M. 1.8 Units- The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.