New Method for Quality Assurance of Asphalt Pavements Using a Low-Activity Nuclear Density Gauge

Abstract

A method was developed for measuring the density of finished asphalt pavement layers using a low-activity nuclear density gauge (LNDG) that is exempt from licensing and other nuclear regulatory burdens in the United States. The gauge using this method, LNDG/TM, requires operating in transmission mode—inserting the gamma-ray source at 0.5 in. (12.5 mm) into the asphalt layer. This method is fast and less destructive than the industry-accepted core-extraction (CE) method—CE followed by laboratory density measurement. This paper presents the measurement principles, method, and properties of this gauge. For evaluating the method, several laboratory and field studies were conducted in North Carolina, USA. For surface course measurements, the repeatability standard deviation of the measurement was 0.3 lb/ft3 (5 kg/m3), and the reproducibility standard deviation was 0.5 lb/ft3 (8 kg/m3). The measurement sensitivity is similar to that of the CE method. The depth of measurement of the gauge is about 2 in. (50 mm). For layers with mean texture depth from 0.5 to 1 mm, the density error is less than 2 lb/ft3 (32 kg/m3). The LNDG/TM requires a density correlation method to provide accurate measurements. Following this method, LNDG/TM density measurements showed a good correlation (correlation coefficient = 0.9) and agreement (root mean square error = 1.3 lb/ft3 [21 kg/m3]) to that using the CE method. These findings indicate that LNDG/TM is a promising alternative to the CE method and has the advantage of testing more locations for acceptance testing.