Volume 37, Issue 1 (January 2009)
Field Verification of Smoothness Requirements for Weigh-in-Motion Approaches
The smoothness of the pavement surface approaching a Weigh-in-Motion (WIM) scale directly affects the device’s ability to accurately estimate static loads from measured dynamic forces. Lack of smoothness creates difficulties in reducing the weighing error to an acceptable level. Efforts were made in this study to determine appropriate values of profile length and roughness of the WIM approach. Roughness data for 19 WIM sites across Texas were collected, and the corresponding calibration records from those WIM sites were employed to verify the smoothness guidelines proposed by AASHTO MP 14 and ASTM E1318. Since there are multiple lanes at each WIM site, a total number of 70 profiles were collected. Based on the calibration records from the WIM sites and field roughness measurements, it was found that the upper threshold value of Long Range Index (LRI) of AASHTO method MP 14 should be adjusted in order to screen out the locations that yield unacceptable weighing error. The lower threshold value was found to be unnecessary, because more than 90 % of WIM sites yielded acceptable weighing error even when they exceeded the lower threshold value. The LRI is preferable to the Short Range Index (SRI) as it matched well with the field WIM calibration results. Based on the 70 profiles used to determine appropriate lengths, a profile length of 25.8 m preceding the WIM is found to be sufficient. That is the length required to obtain a valid LRI, according to AASHTO MP 14. The ASTM E1318 requirement of 60 m is too conservative. It is concluded that an LRI of 1.74 m/km for the 25.8 m leading up to the WIM is the only critical factor in AASHTO MP 14 and is the limit recommended.