This test method describes the procedure for determining the in-place density of asphalt mixtures by use of nuclear gauge. The density of the material is determined by the backscatter/air-gap ratio method. Density??The total density of asphalt mixtures is determined by the attenuation of gamma radiation where the source and detector(s) remain on the surface (backscatter method). The density in mass per unit volume of the material under test is determined by comparing the detected rate of gamma radiation with previously established calibration data. SI Units??the values stated in SI units are to be regarded as the standard. The inch-pound equivalents may be approximate. It is common practice in the engineering profession to concurrently use pounds to represent both a unit of mass (lbm) and of force (lbf). This implicitly combines two systems of units@ that is@ the absolute system and the gravitational system. In the U.S. Customary units system@ the pound (lbf) represents a unit of force (weight). However@ the use of balances or scales recording pounds of mass (lbm) or recording density (lbm/ft3) should not be regarded as nonconformance with this standard. 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. See Section 6@ Hazards.