The rebound hardness method provides a means for rapid classification of the hardness of rock during site characterization for engineering, design, and construction purposes (see Guide D 420), geotechnical mapping of large underground openings in rock (see Guide D 4879), or reporting the physical description of rock core (see Practice D 4543). The rebound hardness number, H _r, can serve in a variety of engineering applications that require characterization of rock material. These applications include, for examples, the prediction of penetration rates for tunnel boring machines, determination of rock quality for construction purposes, and prediction of hydraulic erodibility of rock.

This test method is of limited use on very soft rock or very hard rock (unconfined compressive strengths less than approximately 1 MPa or greater than 100 MPa).

The results of this test method are not intended for conversion to strength data suitable for design.

Note 1-Several types of rebound hammers are commercially available to accommodate testing of various sizes and types of concrete construction (See Test Method C 805) and rock material.

Note 2-The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D 3740 are generally considered capable of competent and objective testing and sampling. Users of this standard are cautioned that compliance with Practice D 3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D 3740 provides a means of evaluating some of those factors.