Preliminary Analysis of AASHO Road Test Rigid Pavement Data Using Modern Regression Techniques

The normality assumptions with random errors and constant variance were often violated while analyzing multilevel pavement performance data using conventional regression techniques. Because of its hierarchical data structure, multilevel data are often analyzed using Linear Mixed-Effects (LME) models. The exploratory analysis, statistical modeling, and the examination of model-fit of LME models are more complicated than those of standard multiple regressions. A systematic modeling approach using visual-graphical techniques and LME models was proposed and demonstrated using the original AASHO road test rigid pavement data. The basic modeling approach includes: selecting a preliminary mean structure, selecting a random structure, selecting a residual covariance structure, model reduction, and examining the model fit. A goodness of fit plot indicates that the preliminary LME model provides better explanation to the data.

Impact of Changing Traffic Characteristics and Environmental Conditions on Flexible Pavements

Although the trend for the next generation of pavement design methods is shifting to mechanistic design, the use of design methods based on the AASHO road test results is still the current design practice in Texas and some other states. Critical to these design methods are the AASHTO load equivalency factors (LEFs), which are used to convert the mixed traffic axle loads into standard 18-kip (80.1-kN) equivalent single-axle loads. Several studies have been conducted on the subject of load equivalency for pavement design and analysis. However, there remain uncertainties related to various issues of load equivalency. Over the years, the composition and characteristics of traffic using Texas highways have been changing. The North American Free Trade Agreement has accelerated such changes in that more trucks, primarily moving among mid-western states, Texas, and Mexico, are traveling on Texas highways. In addition, the original AASHO road test was conducted at a site with environmental conditions significantly different from the environmental conditions in Texas. It is therefore critical to understand fully the impact of such changing traffic characteristics and environmental conditions on pavements in Texas. Presented is the methodology used to analyze the impact of these factors on the AASHTO LEFs.