72 Transportation Research Record: Journal of the Transportation Research Board, No. 2372, Transportation Research Board of the National Academies, Washington, D.C., 2013, pp. 72–82. DOI: 10.3141/2372-09 M. Shakiba, R. K. Abu Al-Rub, T. You, E. A. Masad, and D. N. Little, Zachry Depart- ment of Civil Engineering, and M. K. Darabi, Texas A&M Transportation Institute, Texas A&M University System, College Station, TX 77843. Alternate affiliation for E. A. Masad, Mechanical Engineering Program, Texas A&M University at Qatar, Doha, Qatar. Corresponding author: M. K. Darabi, masouddrb@neo.tamu.edu. crete materials (4–6). The mathematical modeling of the physical– mechanical processes related to moisture damage has been investi- gated via two main approaches. The first is micromechanical modeling of asphalt concrete, which considers degradation of mastic and the aggregate–mastic interface by using cohesive elements (7, 8). The sec- ond approach, macromechanical modeling, considers the mixture as a continuum without modeling its individual constituents (9–11). Caro et al. used cohesive elements at the aggregate–mastic interface (8). They captured the effect of moisture through degrading the cohesive stiffness and strength. Kringos, Scarpas, and their collaborators studied asphalt concrete at the micro scale to predict the infiltration of mois- ture and proposed a simple moisture damage model as a function of moisture content (9, 10, 12). However, their model is time independent and allows for full moisture damage recovery (or reduction) on drying, which is a controversial assumption. Recently, Graham proposed a time-dependent continuum damage model for predicting the adhesive and cohesive moisture damage in asphalt concrete (11). However, this model does not account for moisture damage history, and it has not been verified against experimental measurements. It is, then, impera- tive to develop a robust and comprehensive physically based moisture damage model that can effectively predict the performance of asphalt pavements affected by moisture intrusion. The moisture susceptibility of asphalt concrete can be easily char- acterized at the macro scale by performing experimental testing. However, moisture susceptibility of asphalt concrete may signifi- cantly change as a result of variations in the mineral composition of the aggregate portion, proportioning of the components (mix design), the physical–chemical properties of the constituents, and micro- structural features (e.g., air void content and aggregate size, dis- tribution, and shape). Considering the effects of such variations is a challenging and crucial task that demands the development of constitutive models and numerical techniques that can be used effectively to simulate the micromechanical behavior of asphalt concrete. However, because of the high complexity and expensive computational cost of such an approach, few attempts have been made to model the three-dimensional microstructure of asphalt concrete. Recently, You et al. and Abu Al-Rub et al. used two-dimensional (2-D) X-ray computed tomography (CT) images to create 2-D and three-dimensional (3-D) finite element representations of the micro- structure under dry conditions (13, 14). They used a thermomechanical constitutive approach to model the mechanical response of asphalt concrete under several loading conditions (15). Kringos et al. and Graham modeled the effect of moisture on mechanical response by using an idealized 2-D representation of asphalt concrete micro- structure (9, 11). Caro et al. presented 2-D micromechanical simula- tions of moisture-induced damage in asphalt concrete (8). However, no attempt has been recorded in the literature, to the best knowledge of Continuum Coupled Moisture–Mechanical Damage Model for Asphalt Concrete Maryam Shakiba, Rashid K. Abu Al-Rub, Masoud K. Darabi, Taesun You, Eyad A. Masad, and Dallas N. Little Despite the detrimental effects of moisture damage in asphalt pavements, few macroscale models are capable of modeling this important phenom- enon. Existing models have limitations in accounting for the irrevers- ibility and time dependency of moisture-induced damage. This study presents a moisture damage model based on continuum damage mechan- ics. Adhesive and cohesive moisture damage phenomena are modeled independently; this procedure allows for the introduction of fundamental mechanical properties for each process and for modeling the transition between adhesive and cohesive damage. Two- and three-dimensional sim- ulations are performed, and the results of the simulations are presented to demonstrate the applicability and utility of these micromechanical com- putational models. It is shown that the proposed moisture damage model can simulate the effect of moisture damage on the mechanical response of asphalt concrete subjected to different loading conditions. The model also provides useful insight into the effect of mixture design and material properties on resistance to moisture damage. Asphalt concrete pavements are constantly exposed to environ- mental conditions such as oxygen and moisture. The combined effects of dynamic traffic loading and environmental conditions gradually degrade the mechanical properties of asphalt concrete pavements. Moisture is among the important environmental factors that cause the most concern in maintaining asphalt concrete pave- ments. Moisture damage affects both safety and serviceability over the performance life of asphalt pavements. Moisture at the surface of asphalt pavements in the form of water or vapor disperses into the mixture, fully or partially fills the air voids, and diffuses through the components (binder, mastic, and aggregate). The infiltrated mois- ture aggravates the time-dependent stiffness and strength of asphalt concrete as part of chemical, physical, and mechanical processes. This detrimental effect is referred to as moisture damage and is one of the main causes of early failure. Investigations of the detrimental effects of moisture from mechan- ical, physical, chemical, and thermodynamic processes have been ongoing since 1932 (1–3). Moisture damage affects both mechani- cal response and surface bonding characteristics, leading to the degradation of the adhesive–cohesive bond strength in asphalt con-