A System for Real-Time Measurement of Moisture in Aggregate Mixes During Asphalt Production

Abstract

In asphalt plants, moisture fluctuations in aggregate stockpiles present a problem for process control. To improve the production process, real-time methods of monitoring aggregate moisture in stockpiles, cold feed bins, and on conveyor belts have been used with varying levels of success. This paper presents a field-evaluation of the accuracy of a real-time measurement system for measuring moisture content of virgin aggregate mixes moving on a conveyor belt for asphalt production. This system uses nuclear techniques, with no components in contact with the material flow or the belt, to determine the moisture content. Due to its use of low activity radioisotopic sources and a safe design, the system is categorized as a ‘generally licensed’ device by the US Nuclear Regulatory Agency imposing minimal regulatory burden on US plant owners. This measurement system was evaluated during the 2015 and 2016 production seasons at a continuous-type asphalt plant in Wisconsin, USA. Based on the study, the moisture measurement system showed a good sensitivity to moisture in aggregate mixes with 0.9% wt. response to a 1.0% wt. change in moisture. It also showed a good measurement precision of 0.1% wt. at one standard deviation for a 20 second averaging. Over one thousand samples, with limestone as the main aggregate type, were collected from the belt and were used for comparing the moisture measurements from the new system to that by the oven drying method. For the two methods, the correlation of determination was 0.69 and the difference in moisture content was 0.10 ± 0.35% wt. at one standard deviation. The new method did not show dependence on the nominal maximum aggregate size of the mixes.