The Effect of Flexible Pavement Mechanics on the Accuracy of Axle Load Sensors in Vehicle Weigh-in-Motion Systems

Abstract

Weigh-in-Motion systems are tools to prevent road pavements from the adverse phenomena of vehicle overloading. However, the effectiveness of these systems can be significantly increased by improving weighing accuracy, which is now insufficient for direct enforcement of overloaded vehicles. Field tests show that the accuracy of Weigh-in-Motion axle load sensors installed in the flexible (asphalt) pavements depends on pavement temperature and vehicle speeds. Although this is a known phenomenon, it has not been explained yet. The aim of our study is to fill this gap in the knowledge. The explanation of this phenomena which is presented in the paper is based on pavement/sensors mechanics and the application of the multilayer elastic half-space theory. We show that differences in the distribution of vertical and horizontal stresses in the pavement structure are the cause of vehicle weight measurement errors. These studies are important in terms of Weigh-in-Motion systems for direct enforcement and will help to improve the weighing results accuracy.

Keywords: Weigh-in-Motion; axle load sensors; measuring of axle loads; overloaded vehicles; pavement response.

Figure 1

Diagram and photo of a WIM site.

Figure 2

Factors affecting the weighing accuracy with division according to the source of their occurrence (vehicle or WIM system) or degree of influence (main or other).

Figure 3

(a) Polymer sensor installed below the pavement surface; (b) Quartz sensor installed on the level of the pavement surface; (c) Bending plate sensor installed on the level of the pavement surface; (d) Capacitive sensor installed on the level of the pavement surface.

Figure 4

(a) Temperature characteristics; (b) Speed characteristics of polymer, quartz and bending plate sensors installed in the pavement.

Figure 5

Scheme of stress effects on the WIM sensor installed inside the pavement structure.

Figure 6

Model of the flexible asphalt pavement structure used for analysis.

Figure 7

(a) Relationship between the stiffness modulus and vehicle speed at a constant temperature of 20 °C; (b) Relationship between the stiffness modulus and the temperature at a constant vehicle speed of 70 km/h, for three types of asphalt mixtures usually used in pavement structures.

Figure 8

Pavement surface deflections under: (a) various temperatures and a constant wheel load and speed; (b) various speeds and a constant wheel load and temperature; (c) various wheel loads and a constant temperature and speed.

Figure 9

Distribution of normal stresses on the pavement structure depth under wheel load p = 35 kN, contact stress q = 850 kPa: (a,b) at a constant speed of 70 km/h and various temperatures; (c,d) at a constant temperature of 20 °C and various vehicle speeds.

Figure 10

Example of theoretical analysis of relative error occurrence on the WIM station with quartz sensors (a) ratio p between the vertical stresses σ_zz on the surface level and on the level of the sensor fulcrum (b) relative error δ caused by the change of pavement temperature and vehicle speed in relation to conditions during calibration of the WIM station (T = 20 °C and v = 70 km/h).

Figure 11

Comparison of relative errors for the quartz sensors delivered from field observations and from the theoretical model for the range of temperatures from 5 °C to 40 °C.