Comparison of Load Restriction Timings Determined Using FHwA Guidelines and Frost Tubes

Abstract

The Federal Highway Administration (FHwA) method and frost-tube measurements are used in Wisconsin to determine the timing and duration of spring load restrictions. In both methods, depth of freeze or thaw is used to determine when to apply load restrictions and to determine their duration. Average daily air temperatures are used in the FHwA method to predict the start and end of freeze and/or thaw. Frost tubes are used to directly determine depth of freeze and/or thaw. An analysis is conducted to compare these two methods using air-temperature data (for the FHwA method) and frost-tube data obtained in Wisconsin. The analysis shows that differences exist between predicted (FHwA method) and measured (frost tubes) freeze and thaw conditions. The FHwA method is early (unconservative) in predicting the start of freeze and late (unconservative) in predicting the start of thaw. However, the method is late (conservative) when used to predict the end of thaw. A case history is also presented to compare the two methods.