Evaluation of a Low-Activity Nuclear Density Gauge for Acceptance Testing of Asphalt Pavements for Percent-Within-Limit and Averaging Specifications

Compaction of asphalt concrete (AC) to a specific density is required to achieve the designed engineering properties and performance characteristics of a pavement layer. The finished AC layers do not exhibit uniform density because of variability in production and construction processes. Nuclear density gauges (NDG) are used for density acceptance testing because of their favorable measurement properties, that is, sensitivity, precision, depth of measurement or investigation, and accuracy. This study evaluated a recently developed direct transmission-mode-based method for low-activity nuclear density gauges (LNDG-TM), which notably, are exempt from US nuclear regulatory restrictions. The measurement precision of the LNDG-TM method was determined from a six gauge–operator pair inter-laboratory study. The repeatability and reproducibility standard deviations were 0.7 lb/ft 3 (11 kg/m 3 ) and 1.0 lb/ft 3 (16 kg/m 3 ), respectively. The materials and construction density variability ( δ MC ) for an interstate surface course were determined by analyzing control-strip density data. The δ MC at the 95% confidence level, as determined with the core extraction (CE) method, was 2.3 lb/ft 3 (37 kg/m 3 ), whereas those determined with two NDGs were 3.0 lb/ft 3 (48 kg/m 3 ) and 2.2 lb/ft 3 (35 kg/m 3 ), respectively. The LNDG-TM method was evaluated for acceptance testing, wherein the provided density specifications were the percent-within-limit (PWL) and averaging. Two surface courses were tested: one with the PWL specification wherein the CE method was the agency-approved method, and the other with the averaging specification, wherein the agency-approved method was the thin-lift NDG method. For both projects, results from using the LNDG-TM method agreed well with those obtained using the agency-approved methods. Thus, the LNDG-TM method is suitable for density acceptance testing of asphalt pavements.