Volume 21, Issue 4 (July 1993)
Density Measurement of Cylindrical Specimens Within a Mold by Gamma-Rays
This paper describes a series of tests used to establish a method of nondestructive density measurements of cylindrical specimens contained within a mold by using a twin-probe gamma-ray density gage. Cylindrical specimens within molds are common occurrence in civil engineering practice; examples are soil, concrete, or bituminous cores obtained by in situ sampling or laboratory compaction. In several applications it may be desirable to determine the core densities without removal from the mold, and this can be achieved by the use of a gamma-ray density gage which is an adaptation of a twin-probe direct transmission field gage for laboratory density measurement. It is shown that linear calibration relationships can be obtained by two different procedures. The first case is to use the count ratio between the count rate of specimen with mold to count rate of air plotted against the composite density of the specimen and mold. The composite density is computed using the irradiated volumetric proportions of the specimen and mold. The other case is to use the count ratio between the count rate of specimen and mold to the count rate of mold only plotted against the density of the specimen. Experiments were conducted with three aluminum molds of 116, 119, and 124 mm external diameters and 102 mm internal diameter, for five specimens of 100 mm diameters with density froms 0.87 to 2.70 g/cm3. Based on the radiation attenuation law, a theoretical basis for the linear calibration in the two cases are established. Comparison of the theoretical slopes of the calibration lines with the experimental slopes showed excellent agreement, thus verifying the validity of the calibration procedures.