Is Mechanistic-empirical pavement design approaches better than empirical ones in developing countries?

As we know, a pavement's primary function is to distribute the vehicle loads from the top of the pavement to a larger area of the subgrade without causing any damage to the subgrade. Flexible and rigid pavements are the two types of pavements available.?The combination of flexible and rigid pavements is also possible and known as composite pavement. Flexible pavements use asphalt materials in the surface layer, which flexes with load and temperatures. Rigid pavements use PCC slabs as a surface layer, which is less flexible under loads.

Different countries used different methods to design flexible or rigid pavements. Let us look at some of the countries' design methods.

• United States Pavement Design approach

The American Association of State Highway and Transportation Officials (AASHTO) 1993 pavement design guide and the AASHTOWare pavement mechanistic-empirical (ME) design guide, often known as pavement ME design, are the two types of pavement design guides used in the United States.

1.???The AASHTO 1993 Pavement Design Guide

The AASHTO Guide for Design of Pavement Structures (AASHTO, 1993) was developed based on field performance data collected from the American Association of State Highway Officials (AASHO) road test project during 1956–1960 at Ottawa, IL. The design philosophy of the AASHTO (1993) method is to limit vertical stress on subgrade within a tolerable limit. It does not consider the pavement's performance over time, such as cracking, rutting, and smoothness.

2. The AASHTOWare Pavement Mechanistic-Empirical (ME) Design Guide

Around the year 2000, a new mechanistic-empirical (ME) method was begun to develop in order to overcome the limitations of the AASHTO 1993 pavement design guide.?The AASHTOWare pavement ME design method is mechanistic-empirical. The developed stress strain in the pavement is determined using mechanics, but the amount of distress is predicted using empirical equations.

• Australia Pavement Design Approaches

The Australian pavement design methodology (called Austroads) was first implemented in 1979, improved in 1987, 1992, 2004, 2008, and 2012, and the current form was implemented in 2017 (Austroads, 2017). The current is mechanistic-empirical.

For flexible pavement, a trial pavement is assumed, and the expected load groups are assigned with the project reliability. Then, the trail pavement is analyzed using linear elastic analysis by a program called CIRCLY. The pavement surface materials are assumed homogeneous, elastic, and isotropic. The unbound materials such as base, subbase, and subgrades are assumed anisotropic.

Rigid pavement is designed using the Fatigue life of the concrete slab and the Unbound materials erosion factor. In addition, the strength of the subgrade (minimum CBR of 5%) and the flexural strength of the concrete slab is evaluated. The base layer is commonly not used in rigid pavement, rather a subbase layer is placed in between the subgrade and the concrete slab. The required minimum flexural strength of a concrete slab is 4.5 MPa, and if steel reinforcement is used, the required minimum flexural strength of a concrete slab is 5.5 MPa. To determine the slab thickness, a trial slab thickness is selected. Then the fatigue life (allowable load repetitions) is calculated using the fatigue equation. Then, the percent of fatigue damage for each axle is calculated, and the sum of fatigue damage must be less than 100% for the entire service life.

• United Kingdom Pavement Design Approaches

The pavement design procedure for both flexible and rigid pavements used in the United Kingdom was originally developed in 1970, revised in 1987, and the current form took shape in 2006 (Nunn, 2004). The design method is mostly empirical

• South African Pavement Design Approaches

It is similar to the AASHTOWare Pavement Mechanistic-Empirical (ME) Design Guide. However, the models used to determine different distresses are different and locally developed.

As we observe, the empirical method is decreasing its popularity and the mechanistic-empirical (ME) design is gaining its popularity in different countries.

A deeper understanding of the characteristics of pavement materials is required for pavement design. In the case of Mechanistic empirical pavement design, mechanics are used in estimating the produced stress-strain in the pavement, but empirical equations are used to anticipate the degree of distress. The method used by the AASHTOWare Pavement ME Design program is contingent on the availability of input factors such as material density, modulus, strength, traffic volume, traffic dispersion, axle load spectra, climate, and so on.

The design equations, methodology, reliability, and other aspects of the AASHTO 1993 design technique were established using road test data from Ottawa, Illinois. One disadvantage of AASHTO 1993 is that it is empirical, meaning that the temperature, materials, and traffic behavior in the test portion are not representative of other places in the United States or other countries. The climate varies from city to city, material behavior varies from pavement site to pavement site, traffic behavior (speed, distribution, load spectra, etc.) changes highway to highway. Furthermore, the material and traffic conditions evolve with time.

To summarize, developing countries have limited resources, particularly when it comes to infrastructure investments such as road pavement construction. Input parameters for design purposes are necessary to apply a mechanistic-empirical pavement design approach, which needs additional investments. However, if they insist on empirical ones, there will still be design concerns. In my opinion, whatever approach is adopted, the pavement design should meet the standards.