Design
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Flexible Structural Design

An introduction to concepts and empirical equations used in the structural design of flexible pavements.

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Flexible Pavement Empirical Design Example

Design a new flexible pavement for a major interstate highway using the following conditions (four lanes each direction):   Pavement location: Urban Pavement functional classification: Interstate Number of lanes: 3 lanes in each direction Traffic distribution: Assume 80% of the loading occurs in the design lane Annual growth rate: Assume 2% Design period: Investigate 20, … Read more »

Figure 1. Limiting horizontal strain criterion for HMA fatigue cracking.

Flexible M-E Failure Criteria

The main empirical portions of the mechanistic-empirical design process are the equations used to compute the number of loading cycles to failure.  These equations are derived by observing the performance of pavements and relating the type and extent of observed failure to an initial strain under various loads.  Currently, two types of failure criteria are … Read more »

"Typical" Pavement Sections

Flexible Pavement Response

How a pavement responds to applied stresses determines how it will behave structurally. Stresses and the resultant pavement response are the combined result of loading, environment, subgrade and pavement material characteristics. This section presents the typical stresses and stress characteristics experienced by a flexible pavement structure under load. There are a variety of ways to … Read more »

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1993 AASHTO Flexible Pavement Structural Design

Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance).  This article presents the 1993 AASHTO Guide basic design equation for flexible pavements.  This empirical equation is widely used and has the following form: (these variables will be further explained in the Inputs section) where: W18 = predicted number … Read more »

Figure 3. EverFlex 3-D drawing (adapted from Wu, 2001). The drawing shows the discrete elements, wheel loads (tire patch loads),  a modeled crack and a slip interface (where on layer can slip - move independently - from another).

Flexible Pavement Mechanistic Models

Mechanistic models are used to mathematically model pavement physics.  There are a number of different types of models available today (e.g., dynamic, viscoelastic models) but this section will present two, the layered elastic model and the finite elements model (FEM), as examples of the types of models typically used.  Both of these models can easily … Read more »

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A Mechanistic Computer Program

The Washington State DOT has developed a layered elastic-based software package called the Everseries Pavement Analysis Programs (Sivaneswaran, Pierce and Mahoney, 2001[1]).  Everseries (for short) contains three independent programs: Layered elastic analysis (Everstress) FWD pavement modulus backcalculation (Evercalc) Flexible pavement overlay design (Everpave) To install the Everseries programs on your computer, click the install icon … Read more »

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Perpetual Pavements

“Perpetual Pavement” is a term used to describe a long-lasting structural design, construction and maintenance concept for HMA pavements. If properly maintained and rehabilitated, a perpetual pavement can be designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses … Read more »

Figure 1: Design chart of example pavement (150 mm (6 in.) base).

Failure Criteria Example

Results from a mechanistic pavement analysis are compared to empirically derived failure models to determine pavement life (in terms of loads to failure) and the controlling failure mechanism. In this example, various thicknesses of HMA were plotted against the computed number of repetitions to failure for 150 mm (6 in.), 250 mm (10 in.), and … Read more »

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