# Mechanical-Empirical Pavement Design Guide Applied to Portuguese Pavement Structures
## 1 Introduction  
With the objective of improving pavement design from an empirical approach to a mechanistic-empirical approach, by including pavement mechanistic behavior, aiming to correlate applied loads and observed distresses [1], the American Association of State Highway and Transportation Officials (AASHTO) developed a new pavement design method called the Mechanistic Empirical Pavement Design Guide--MEPDG [2] to replace the previous pavement design method of 1993 [3]. The MEPDG is a mechanistic-empirical pavement design method in which pavement performance is predicted using the pavement's structural response in terms of the following pavement performance indicators: (1) rutting; (2) international roughness index (IRI); (3) fatigue cracking; (4) transverse cracking; and (5) alligator cracking. A software package has been developed and made available to the pavement design community that allows the calibration and application of this new pavement design method in different countries and regions subject to different traffic, climate, and pavement foundation conditions. Initially, the pavement design method requests data input related to traffic, climate, materials, and pavement structure. The pavement performance indicators mentioned above are then calculated and presented for further analysis considering threshold values which have been established, for example, in highway public-private partnership contracts. This new pavement design method has three hierarchical levels for the input variables, including climate, traffic loading, layer structure, and material properties, which define the accuracy of the data according to three levels: level 1; level 2; and level 3. In general, the level 1 input is the most accurate value that it is possible to obtain for a certain input. This usually represents site specific measurements and laboratory data. Contrary to the previous level, level 3 input is either the default value used in the AASHTOWare Pavement-ME(r)or a nominal value provided by a local agency. This requires the least amount of data, considering default values established according to the characteristics of the region where the road will be built. Level 2 input is estimated from correlations or regression equations. In recent years, several research studies, usually supported by road agencies, have been undertaken to evaluate the application of this new pavement design method. [PERSON] et al. [4] presented the results of a comprehensive test program to calibrate the MEPDG for the state of Wyoming, consisting of field data collection and laboratory soil tests. They concluded that the results of the test program facilitated the implementation of the MEPDG. There are several documents in the literature that can be used for regional or local calibration of the MEPDG [5; 6]. [PERSON] et al. [7] presented the results of the application of the MEPDG procedure to pavements of provincial highways of Ontario, Canada. A regression analysis was carried out for calibrating the rutting and IRI models by comparing the predicted distress to the observed distress. The results demonstrated that whereas the MEPDG provided a fairly unbiased prediction of the IRI value, it often over-predicted the total rutting. [PERSON] and [PERSON] [8] presented the results of a sensitivity analysis to the inputs of MEPDG distresses to identify the effect of the accuracy level of inputs based on experimental design. [PERSON] et al. [9] compared the Alberta Transportation Pavement Design (ATPD) procedure with the MEPDG procedure to explore the possibility of MEPDG implementation for pavement design in Alberta, Canada. The ATPD pavement design method is based mainly on the AASHTO 1993 guide, with minor modifications regarding the asphalt concrete mix design, the structural layer coefficients, and design reliability levels. Six different design cases were defined with three different traffic levels and two different subgrade materials.