Comprehensive evaluation of small strain viscoelastic behavior of asphalt concrete

Asphalt concrete (AC) is a heterogeneous material affected by viscoelastic and viscoplastic processes, as well as by damage and localized irregularities in the material fabric. However, under specific strain regimens, certain of these mechanisms are less active, and the constitutive relationships of the material can be modeled using simplified mechanistic principles. One simplifying assumption that is typically applied to AC for the purposes of response and performance modeling is that of linear viscoelasticity (LVE). In this paper, the behavior of AC at small strain levels, when LVE models can most accurately describe the constitutive relationship, is described. This work differs from the significant literature presented elsewhere because a more strict definition of the LVE strain regimen has been adhered to. This protocol limits the total peak-to-peak strain amplitude to 50 to 75 microstrains, the tensile strain amplitude to 37.5 microstrains, the total accumulated compressive strain to 1500 microstrains, and the total accumulated tensile strain to 150 microstrains. It is shown that when this stricter protocol is followed, AC exhibits the same fundamental characteristics whether loaded in compression, tension, or indirect tension. Evidence is also presented showing that AC, when compacted by gyratory compaction, does not show anisotropic tendencies at these strain levels. AC shows stress state and strain level dependencies that are inconsistent with rigorous LVE theory. Due to the inconsistencies between the material responses and the theory, the behavior of AC at even the small strain levels used in this research cannot be rigorously referred to as LVE. Further study is needed in order to fully assess the implications of this finding as it relates to day-to-day engineering practice and to the advanced mechanistic modeling of AC materials.