| Major Topics on this Page | ||
| 2.1 | Stress | |
| 2.2 | Deflection | |
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 calculate or at least account for these stresses in design. The empirical approach uses the AASHO Road Test results to correlate measurable parameters (such as subgrade resilient modulus) and derived indices (such as the structural number and pavement serviceability index) to pavement performance. The mechanistic-empirical approach relates calculated pavement stresses to empirically derived failure conditions.
The stresses that occur in a flexible pavement under load are quite complex. Although rigid pavement stresses have been routinely calculated since the 1920s, routine calculation of flexible pavement stresses is a more recent development. First, two-dimensional layered elastic programs offered desktop computers the ability to calculate these stresses. More recently, three-dimensional finite element programs have allowed more exact and detailed calculations.
Using a two-dimensional layered elastic model, the basic relationships between layer stiffness and stress for a two-layer flexible pavement structure is shown in Figure 6.1. In reality, stress distributions are more complex, however the basic relationships hold true. This additional complexity is further elaborated on in Section 2.2.2, Three-Dimensional Finite Elements Modeling below.
Figure 6.1: Typical Two-Layer Flexible Pavement Stresses as Calculated by a Two-Dimensional Linear Elastic Model. Click the yellow boxes to view different stresses. Note that "E" refers to a layer's stiffness.
Because of the complex nature, a finite elements model is needed to get a good approximation of how a flexible pavement responds to loading. The complex stress and strains for a two-layer flexible pavement structure are shown graphically in Figure 6.2 using a three-dimensional finite elements model.
Figure 6.2: Typical Two-Layer Flexible Pavement Stresses as Calculated by EverFlex (Wu, 2001), a Three-Dimensional Finite Elements Program. Click the yellow boxes to view different stresses. Note that "E" refers to a layer's stiffness.
HMA pavements are often described as "flexible" because they deflect under load. Figure 6.3 shows schematically how pavements deflect under load. FWDs can be used to accurately determine deflection characteristics of in-service pavements.
Figure 6.3: Schematic Showing Deflections for Different Pavement Thicknesses. The same HMA material characteristics are assumed for each graph - only the thickness varies.