6.2 Limiting Strain Criteria
While research continues in this area, there is one “special
case” that easily lends itself to mechanistic structural evaluation,
albeit without robust regard to reliability. Experts have defined
limiting strain
criteria
with some confidence. 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 strain criteria find their full potential in
the
perpetual pavement
design. 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 strain criteria presuppose
that quality materials are used and that proper construction procedures
are followed. The limiting strain criteria
reported
by experts in the field (such as Nunn and Monismith) are:- 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 21to roughly estimate an initial thickness design, then use an analysis program that calculates the primary responses to traffic loading for the heaviest anticipated wheel loads for each season of the year at the critical locations, or
- the designer speculates at the sufficiency of a desired structure and then evaluates it at the critical locations using a mechanistic analysis program to see what performance life is predicted.
- In either case, the design would then be modified to achieve the desired or optimized performance.