Methods

The goal of structural design is to determine the number, material composition and thickness of the different layers within a pavement structure required to accommodate a given loading regime.  This includes the surface course as well as any underlying base or subbase layers.  This section is focused on the structural design of new pavement.  Structural design for rehabilitation is covered in Maintenance & Rehabilitation.

Figure 1: 1993 AASHTO Empirical Equation

Structural design is mainly concerned with determining appropriate layer thickness and composition.  Calculations are chiefly concerned with traffic loading stresses; other environmentally related stresses (such as temperature) are accounted for in mix design asphalt binder selection.  The principal methods of structural design in use today are (from simplest to most complex) catalogs, empirical and mechanistic-empirical. 

Design Catalogs

The simplest approach to HMA pavement structural design involves selecting a predetermined design from a catalog.  Typically, design catalogs contain a listing of common loading, environmental and service regimes and the corresponding recommended pavement structures.  State and local agencies often include them in their specifications.

The pavement designs within these catalogs can be based on a number of different design methods ranging from mechanistic-empirical to historical experience.  When using a design catalog, it is important to be aware of the author's assumptions and design procedure.  Often assumptions and design procedures are based on extremely local conditions, which may not be transferable.  For instance, basic Snohomish County structural designs may not be appropriate for Spokane County and vice versa. 

Often the required level of design does not warrant the use of advanced equations or models.  For instance, a local residential road subject to only a few heavy loads per week (i.e., school bus, garbage truck) does not warrant the expense and time of a mechanistic-empirical design approach.  However, many cities, counties and private owners do not have specified standard pavement structural designs. 

Empirical Design

Many pavement structural design procedures use an empirical approach.  This means that the relationships between design inputs (e.g., loads, materials, layer configurations and environment) and pavement failure were determined using experience, experimentation or a combination of both.  Although the scientific basis for these relationships is not firmly established, they can be used with confidence as long as the limitations with such an approach are recognized.  Specifically, it is not prudent to use an empirically derived relationship to describe phenomena that occur outside the range of the original data used to develop the relationship. 

The most common empirical design method is the equation contained in the 1993 AASHTO Guide for Design of Pavement Structures (see Figure 1).  This equation was developed from experimental data at the AASHO Road Test, a $27 million (1960 dollars) road experiment conducted in Ottawa, IL from 1956 - 1961.  The AASHO Road Test was a complex study of the performance of highway pavement structures of known thickness under moving loads of known magnitude and frequency (Highway Research Board, 1961).  The test studied both portland cement concrete and asphaltic concrete pavements, as well as certain types of short-span bridges.  The resultant design equation remains a popular method for pavement structural design.  

Expert knowledge is required to use the 1993 AASHTO empirical equation; a pavement design expert should be consulted if you are considering its use.

Mechanistic-Empirical Design

The most advanced pavement structural design uses a mechanistic-empirical approach.  Unlike an empirical approach, a mechanistic approach seeks to explain phenomena only by reference to physical causes.  In pavement design, the phenomena are the stresses, strains and deflections within a pavement structure, and the physical causes are the loads and material properties of the pavement structure.  The relationship between these phenomena and their physical causes is typically described using a mathematical model.  Various mathematical models can be used.

Along with this mechanistic approach, empirical elements are used when defining what values of the calculated stresses, strains and deflections result in pavement failure.  The relationship between physical phenomena and pavement failure is described by empirically derived equations that compute the number of loading cycles to failure.

The basic advantages of a mechanistic-empirical pavement design method over a purely empirical one are:

A mechanistic-empirical approach can also accurately characterize in situ material (including subgrade and existing pavement structures).  This is typically done by using a portable device (like a FWD) to make actual field deflection measurements on a pavement structure to be overlaid.  These measurements can then be input into equations to determine the existing pavement structural support and the approximate remaining pavement life. This allows for a more realistic design for the given conditions.

Again, expert knowledge is required to use mechanistic-empirical approaches; a pavement design expert should be consulted if you are considering its use.
 

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