PREDICTING THE EFFECTS OF FREEZING AND THAWING ON PAVEMENT SUPPORT

The objective of this study was to develop a method to predict the timing of weight limits on secondary highways in Wisconsin. Two types of weight limits are of interest: overloads when the pavement is frozen and weight restrictions during thawing and post-thawing recovery period. To meet this objective, three sections of secondary highways with flexible pavements were instrumented and monitored to determine how freezing, thawing, and post-thaw recovery affect pavement stiffness. Data collected from these sites were used to develop a computer model (UWFrost) that can be used to predict seasonal changes in the support capacity of pavements. The computer model is operated from a Microsoft Excel spreadsheet. Freezing and thawing were found to have a significant effect on the stiffness and capacity of flexible pavements. The pavement stiffens during winter when the base freezes, and softens substantially when the base and subgrade thaw. A gradual recovery process occurs after thawing is complete during which the pavement stiffens. This recovery process extends through a non-dimensional modulus ratio function that varies with time. This function is an intrinsic component of UWFrost. UWFrost simulates how freezing and thawing affect the stiffness and capacity of pavements. The user enters meteorological data, pavement location, pavement layer geometry, and timing criteria. A one-dimensional heat transfer module uses the input to define the moduli of the pavement layers, which are used in a layered elastic analysis. The elastic analysis is used iteratively to determine the load that induces no more damage than a design load applied during late summer. This load is referred to as the equivalent damage load (EDL). EDLs are defined for fatigue and rutting damage. UWFrost computes EDLs for each day during the prediction period selected by the user. EDLs are as much as three times higher than design loads when the base and subgrade are frozen. At the end of thaw, when the pavement reaches its softest condition, the EDL can be less than one-half the design load.