LABORATORY EVALUATION OF COMPACTED DENSITIES OF BITUMINOUS MIXTURES

Abstract

The study was carried out to apply a non-destructive laboratory technique for density measurement of cylindrical specimens by adaptation of a twin-probe field gamma-ray gage into a laboratory device. The device can achieve an accuracy of of ±0.01 g/cm3 at the 95% confidence for a useful range of density from 0.8 to 2.7 g/cm3. The technique uses a 5 milliCurie (mCi) Caesium 137 source with a scintillation detector and counter assembly to record the radiation transmitted through the specimen. This is converted to a density value by calibration to an empirical radiation attenuation law. The application of the device to laboratory density measurement is examined with respect to the influence of count variation, material type, specimen position between source and detector, source-to-detector spacing, specimen diameter, and curvature effect of specimens. From the radiation attenuation law, a diameter ratio to count ratio relationship is established. In the accuracy assessment, it is found that a narrow range of source-to-detector spacings for unbiased density estimation exists and a spacing of 280 mm is recommended for the device studied. Density measurements of cylindrical specimens of dry sand and gravels were made and tested. Results indicate that specimens of uniform densities and uniform void distributions must be used for calibration. Major considerations related to the operational features of the gage, the characteristics of the composite structure and compaction of bituminous mixture are highlighted. Three different bituminous mix types, namely sand asphalt, dense-graded mix, and open-graded mix, were included in the study. Better precision and accuracy were achieved with specimens having more uniform density distribution within the mass, and with specimens which were less porous or prepared with higher compactive effort. Recommendations concerning the use of gamma-ray probes for laboratory density measurements of bituminous paving mixtures are made. An illustration showing the ability of the twin-probe gage to determine the density profile of a tall specimen is presented. Two different procedures of non-destructive density measurements of cylindrical specimens contained within a mold by using the twin-probe gamma-ray gage were established. Based on the radiation attenuation law, a theoretical basis for the linear calibration in the two cases are found. Comparison of the theoretical slopes of the calibration lines with the experimental slopes showed excellent agreement, thus verifying the validity of the calibration procedures.