Technical information

4-1
CHAPTER 4 - BASIC MECHANISTIC ANALYSIS
This chapter addresses the primary study objective by establishing a relation between
the NCAT and APT experiments. The scope and approach are discussed in Section 4.1.
Section 4.2 contains the development of a basic mechanistic model for the pavement
systems considered; it is based on layered elastic theory (LET) with the necessary
material properties obtained via inverse analysis of APT results. Section 4.3 deals with
NCAT response prediction; the calculation methodology is first explained and then
applied to forecast selected responses.
4.1 SCOPE AND APPROACH
In Section 3.6 it was argued that a fundamental approach is needed to link the NCAT
and APT experiments. This is pursued hereafter focusing on load related resilient
responses using isotropic LET. The APT experiment is first analyzed. The n1 pavement
system is modeled using four isotropic layers (see Subsection 4.2.1) comprised of
HMA, base and subgrade on top of a semi-infinite concrete medium. The latter
represents the concrete floor present at the bottom of the APT pit. The unknown
material properties (layer moduli) are obtained through backcalculation by matching the
gauge readings collected during one pass of the APT carriage (see Subsection 4.2.2).
Due to the temperature and rate sensitivity of the HMA, the resulting properties
represent the environment and loading configuration in the APT only. This ‘inverse
analysis’ is performed twice, for the pavement in the initial phases of the experiment,
after 5,000 passes, and also later in the experiment after 80,000 passes. Pavement
properties in both cases (i.e., layer moduli) are presented and compared (Subsection
4.2.3), showing that the structure experienced permanent property changes under the
repetitive APT passes.
Next, the calibrated APT model is extended to apply to other loading
configurations and environments. Assuming similarly constructed pavement systems,
the methodology consists of changing the HMA modulus to reflect different loading
speeds and temperatures. This is done after additional analysis of laboratory complex
modulus results (see details in Subsection 4.3.1). The unbound material properties,