Gamma-ray Spectrometric Method for Testing the Quality of Longitudinal Joints in Asphalt Pavements

Abstract: Lower density near a longitudinal joint, when compared to the specified density of asphalt pavement, can lead to premature failure of the pavement. When following best practices in construction of joints, the density near the joints can be improved. Therefore, to construct durable pavements, monitoring density during construction is crucial to make corrective action to eliminate causes of deviations to proper construction practices. Nuclear density gauges (NDG) based on the gamma-ray method are widely used for process control, quality control, and density acceptance in constructing pavement mats and joints. NDGs have been used for decades due to their effective measurement properties, including full depth density measurement, sensitivity, precision, and accuracy. As a rapid and nondestructive method, NDGs have an advantage over the core extraction method testing many locations along a joint for better joint quality estimation.
The purpose of this research was to evaluate a method for the low activity nuclear density gauge (LNDG), a recent advancement in NDG technology, for testing density near the longitudinal joints. The LNDGs are exempt from licensing requirements in the USA, eliminating the regulatory burdens associated with NDGs. The unique feature of this gauge is its use of an energy-selective gamma-ray detector with the possibility of testing the vertical density gradient of a layer, which is a measure of segregation. For this application, operating the LNDG in the 0.5-inch (12.5-mm) direct transmission mode (LNDG-TM), in addition to density, the vertical density gradient of a layer was used to determine the quality of longitudinal joints.
This paper presents the LNDG-TM measurement method and the key measurement properties such as precision, depth of measurement, and accuracy as determined from laboratory and field studies. Also, presents the sensitivity of the density measurement method for three longitudinal joint geometries. Furthermore, presents findings from two field studies of testing longitudinal joints in asphalt surface courses in North Carolina and Virginia comparing LNDG-TM measurements with those taken using core extraction method and thin-lift NDG method.