Section 5: Perpetual Pavement Design and Mechanistic Design Guidelines
Anchor: #i1004019General
Structural Design of Pavements is an evolving process. Gradually, we have moved from purely empirical design relationships based largely on the results of observations at the AASHO Road Test to the current mechanistic-empirical process incorporated in FPS-19W where material stiffness is related to performance (loss of smoothness) through use of the surface curvature index (SCI, a deflection parameter). Eventually, the goal is to nail down other material characteristics and their relationships (transfer functions) to the progression of specific forms of distress, thus having multiple performance measures on any given structure subjected to a given regional environment and regional or site specific axle loads. While research continues in this area, there is one “special case” mechanistic structural evaluation where experts have defined with some confidence limiting criteria whereby if these mechanistic benchmarks are not exceeded, then there is a very high likelihood that the pavement will not suffer traditional bottom-up fatiguing or full-depth (subgrade failure) rutting. These limiting criteria find their full potential in the perpetual pavement design, since thinner structures are generally subjected to similar maximum axle loads, but offer insufficient stiffness or thickness to stay below the limiting criteria. As with other design criteria, these limiting criteria presuppose that quality materials are used and that proper construction procedures are followed. The limiting criteria reported by experts in the field such as Nunn and Monismith are as follows:
- tensile strain at the bottom of the composite HMA layers: <70 µ-strain
- compressive strain at the top of the subgrade: <200 µ-strain.
To develop a design and determine whether these criteria are met is a two step process. Either the designer must use a program such as FPS to develop an initial thickness design, then use an analysis program that calculates the primary responses to traffic loading at the critical locations, or the designer “guesses” at a desired structure (experience, judgment), and then evaluates it at the critical locations using an analysis program.
Anchor: #i1004048Designing a Perpetual Pavement using FPS-19W
It is recommended that a pavement design Type 3 be used for this type of structure. Select a 20- or 30-yr. analysis period. This will not be critical since ultimately we are interested in meeting the limiting strain criteria specified above. However, as previously stated, you must enter the 20-yr. cumulative ESALs in the 18-kip ESAL field, regardless of the analysis period length. Use lanal distribution reduction factors when three or more lanes are planned in one direction. Because of limitations in the number of pavement layers allowed in FPS-19W, material inputs will require some judgment as to consolidating some layers and possibly disregarding those that contribute least (for example, permeable friction course surface). As an example, say the top 6.0 in. of HMA consists of one lift of SMA and one lift of 3/4 in. SuperPave (SP-B) where both mixes will use a PG 76-XX binder and the lower lifts of HMA will all use PG 64-XX binder. The top two lifts can be consolidated into one FPS-19 layer with a design modulus of 850 ksi, while the remaining HMA layers can be consolidated and designed using a modulus of 500 ksi. The third layer will be the prepared foundation (lime-stabilized subgrade, flexible base, etc.) with appropriate modulus. Follow general guidelines for all other inputs. Select the “Go!” button to run the design. Depending on the thickness ranges selected, the resulting designs may indicate a requirement for an overlay prior to the end of the analysis period. This may not appear to be consistent with “perpetual” design, but the performance module within FPS-19W is based on different criteria. Choose a solution that has a time to first overlay of greater than 15 yr. A mechanistic check using the mechanistic design check option should show “unlimited” fatigue and rutting life (>50 million ESALs).
Anchor: #i1004060Checking the Proposed Design for Compliance with Limiting Strain Criteria
FPS-19W has a built-in mechanistic analysis module (incorporates WESLEA®) that can be used to determine compliance with the recommended limiting strain criteria. Once you have run your FPS-19W design, select “check design” and then select the “stress analysis” button. Note, for full use of the features in this module, consult the Flexible Pavement Design System (FPS) 19: User’s Manual. An example of the output is depicted in Figure 5-4 below:
Figure 5-4. Analysis for limiting strain criteria using the FPS-19W Stress Analysis Module.
In this example, the proposed structure meets both of the limiting strain criteria; the maximum tensile strain at the bottom of all HMA layers is 52 µ-strain (< 70) and the maximum compressive strain at the top of the subgrade is 124 µ-strain (<200). While these calculated strains may show that this structure is somewhat conservative, some allowance should be made for the assumed material modulus (composite HMA lifts) and the expected variability inherent in subgrade support, material properties, and construction execution.
Alternatively, other analysis programs may be used to verify the limiting stain criteria (linear elastic, finite element, etc.). However, other analysis programs are not specifically supported within the department so knowledge in their use is left up to the designer.