Impacts of environmental factors on flexible pavements

by 1973- Zuo, Gang

Mechanistic-empirical pavement design methods for flexible pavements are based on the assumption that the pavement life is inversely proportional to the magnitude of the traffic-induced pavement strains. These strains vary with the stiffness of the asphalt layer and underlying base layer and subgrade. Environmental factors, such as the temperature in the asphalt concrete layer and the water content in the base layer and the subgrade, have a significant impact on the stiffness of relevant layers in pavement systems, and consequently the estimated life of flexible pavements. A comprehensive instrumentation system was installed at four sites across the state of Tennessee to monitor long-term seasonal changes in flexible pavement response. Thermistors were used to measure the temperature at different depth of the pavement systems. Diurnal temperature variations in the asphalt concrete layer were as large as the annual variation. Multi-segment TDR probes were used to measure the volumetric water content. Because of the difference in signal strength along the probe, all segments do not provide the same level of accuracy. A series of laboratory testing were performed to study the sources of measurement error and the temperature dependence of the measurements in some segments. Water content measurements were recalibrated according to findings of this laboratory study and the measured seasonal variations in subgrade and base water content were small. Using environmental data from instrumented pavement sites in Tennessee, the effects of asphalt concrete (AC) temperature and base and subgrade water iv content variation were evaluated for three pavement profiles using the finite element method. The effect of AC temperature profile was found important to the critical strain in AC layer. Because the relationship between temperature and asphalt concrete stiffness is nonlinear, the additional pavement life consumed at higher-than-average temperatures is not offset by savings at lower-than-average temperatures. As a result, whenever average pavement temperatures are used to determine the asphalt stiffness, pavement life is overestimated. Furthermore, temperature and water content are neither completely dependent nor completely independent. Hence, the combined effects of temperature and water content variations were accounted for in the estimation of pavement life. The results of the parametric study showed that the temperature averaging period and the timing and duration of wet subgrade conditions are critical to estimated pavement life. v