A New Method for Measuring In-Place Density of Asphalt Pavements Using a Low-Activity Nuclear Density Gauge

Abstract

In recent years, low-activity nuclear density gauges (LNDG) have become available and are used for soils and asphalt compaction testing. These gauges, which are exempt from licensing and other nuclear regulatory requirements in the United States, use gamma-ray scattering technique for measuring density. A new method was developed for using these gauges for agency acceptance testing of finished asphalt pavements. In this method, the gauge is used in transmission mode, placing the gamma-ray source at a depth of 12.5 mm (0.5 in.) in asphalt layer, using a predrilled hole with a diameter of 19 mm (0.75 in.). This paper presents the measurement method and properties of a gauge using this new method (LNDG/TM). The measurement properties were determined from laboratory and field studies conducted in North Carolina. An LNDG/TM was evaluated by side-by-side testing with a thin-lift type conventional nuclear gauge (TL-NDG) that is allowed to use for acceptance testing. These studies showed that the sensitivity and precision of measuring density by an LNDG/TM were similar to those by a TL-NDG. Based on measurements taken on a 100-mm-thick (4-in.) intermediate layer made from a 19-mm Superpave mix, the precision, repeatability standard deviation, for the LNDG/TM was 5 kg/m3 (0.3 lb/ft3). The measurement time including for site preparation was about 2.5 min. When gauge measurements were correlated to the industry-accepted core-extraction method, the agreement between LNDG/TM and a TL-NDG for measuring density was within 32 kg/m3 (2 lb/ft3).