Each material layer used in the structure will have a modulus input that shall characterize the average seasonal stiffness of that material over the course of the year. The construction process, inherent material variability (initially and over time), and effects of environment (moisture and temperature) and traffic loading will typically introduce considerable variance about the average value. Modulus inputs for HMA are based on a temperature of 77°F.

Overestimating this material property can result in a structure with poor permanent deformation performance and may subject the surface to early fatigue, while underestimating can result in an uneconomical pavement.

Additionally, materials that have an average in situ modulus in one circumstance may have a different average modulus if placed in another environment. This is particularly true of unbound base material; the modulus can be significantly influenced by the confinement provided by the layers above or below, absence of paved shoulders, or by the amount of moisture infiltrating the structure if the materials are moisture susceptible.

In evaluating a design that consists of layers that were pre-existing (including the subgrade), the falling weight deflectometer (FWD) is indispensable in determining what stiffnesses (through backcalculation) these layers can contribute to the new structure. Proposed virgin and reclaimed material moduli will require knowledge by the designer (preferably through past use and subsequent evaluation), tempered by the specifics of the current project.

Studies conducted on perpetual pavements indicate that stone-on-stone designed Superpave hot-mixes and thick composite HMA structures, using any type of HMA, have much stiffer in-place moduli values than conventional thinner surfaced HMA structures. Laboratory and field testing continues on these mixes to establish stiffness-temperature curves and better define “design” stiffness values by type, if necessary.

All backcalculated material modulus values are manually entered into their respective fields (values are not read from the MODULUS backcalculation summary file). Typically, adjustments (see 4.4.1. Backcalculation Limitations and Adjustments below) to the averages produced in the backcalculation process are necessary prior to entry in the FPS program.