Design

When designing pavements (both mix design and structural design), there are three fundamental external design parameters to consider: the characteristics of the subgrade upon which the pavement is placed, the applied loads and the environment.
First, the subgrade upon which the pavement is placed will have a large impact on structural design. Subgrade stiffness and drainage characteristics help determine pavement layer thickness, the number of layers, seasonal load restrictions and any possible improvements to subgrade stiffness and drainage itself.
Second, the expected traffic loading is a primary design input (both in mix design and structural design). Traffic loads are used to determine pavement composition, layer type and thickness, all of which affect pavement life.
hird, the environment has a large impact on pavement material performance. Environmental factors such as temperature, moisture and ice formation can affect pavement durability, binder rheology, structural support and ultimately pavement life and failure.

Mix Design

The two key components of pavement design are mix design and structural design. This section deals with HMA and PCC mix design. The goal of mix design is to determine the optimum mixture of component materials for a given application. This includes detailed evaluations of aggregate, asphalt and portland cement as well as a determination of their optimum blending ratios. This section covers the following for HMA and PCC mix design:
● Mix design fundamentals. These are the fundamental philosophies and parameters of mix design such as (1) why it is done, (2) what basic assumptions are made and (3) the specific goals of mix design.
● Mix design methods. These sections cover the various mix design procedures used. For HMA, the Hveem, Marshall and Superpave methods are covered. For PCC, the ACI Method is covered.
● Performance Tests. These are the tests performed on laboratory designed mixes (or field samples) to characterize their performance. They can consist of basic physical property measurements (such as stiffness or strength) or laboratory simulation of field conditions (such as rutting potential or chloride penetration).
This section is only meant to provide a brief overview of mix design methods as well as their assumptions, inputs and outputs. Resources that provide a detailed description and analysis of each mix design method are listed in the beginning of each section.

Structural Design

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 module is focused on the structural design of new pavement. Structural design for rehabilitation is covered in the Rehabilitation Module.
For HMA pavements, 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. The two principal methods of HMA pavement structural design in use today, empirical and mechanistic-empirical, are covered.
For PCC pavements, structural design is mainly concerned with determining the appropriate slab thickness based on traffic loads and underlying material properties, and joint design. This is done by considering a variety of stresses which affect rigid pavement performance: curling (temperature stresses), warping (moisture stresses), wheel load and shrinkage/expansion. The two principal methods of rigid pavement structural design in use today, empirical and mechanistic-empirical, are covered.
Overall, this Module is only meant to provide a brief overview of the different structural design techniques as well as their assumptions, inputs and outputs. Detailed analysis of the design methods presented here can be found in:
● Empirical Method: The AASHTO Guide for Design of Pavement Structures.
● Mechanistic Method: The NCHRP 1-37A Design Guide, which is still under review, and other state design procedures.

Suggested Reading

● American Association of State Highway and Transportation Officials (AASHTO). (1993). AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials. Washington, D.C.
● The Asphalt Institute. (1999). Thickness Design – Asphalt, Manual Series No. 1. The Asphalt Institute. Lexington, KY.
● Asphalt Pavement Alliance (2002). Perpetual Pavements: A Synthesis. Asphalt Pavement Alliance. Lanham, MD. http://www.asphaltalliance.com. Accessed 7 August 2002.

Publications Cited

● Highway Research Board. (1961). The AASHO Road Test: Report 1, History and Description of the Project. Special Report 61A. Highway Research Board, National Academy of Sciences. Washington, D.C.
● Mahoney, J.P. (December 2001). Study of Long-Lasting Pavements in Washington State. Perpetual Bituminous Pavements. Transportation Research Circular No. 503. Transportation Research Board, National Research Council. Washington, D.C. pp. 88-95.
● Transportation Research Board (TRB). (2001). Perpetual Bituminous Pavements. Transportation Research Circular No. 503. Transportation Research Board, National Research Council. Washington, D.C. http://www.asphaltalliance.com. Accessed 7 August 2002.

Design Parameters

When designing pavements (both mix design and structural design), there are three fundamental external design parameters to consider: the characteristics of the subgrade upon which the pavement is placed, the applied loads and the environment. First, the subgrade upon which the pavement is placed will have a large impact on structural design. Subgrade stiffness and drainage characteristics help determine pavement layer thickness, the number of layers, seasonal load restrictions and any possible improvements to subgrade stiffness and drainage itself. Second, the expected traffic loading is a primary design input (both in mix design and structural design). Traffic loads are used to determine pavement composition, layer type and thickness, all of which affect pavement life. Third, the environment has a large impact on pavement material performance. Environmental factors such as temperature, moisture and ice formation can affect pavement durability, binder rheology, structural support and ultimately pavement life and failure.