or proper pavement performance. From a mechanistic–empirical pavement design perspective, modulus and deformation character- istics linked to aggregate quality and properties and layer thickness are the primary factors influencing the mechanistic response behav- ior and pavement performance of unbound aggregate. Higher material quality reduces the layer thickness requirement, and vice versa; however, in many regions throughout the United States where high- quality aggregates are scarce and need to be imported, the high cost of material purchasing and hauling would probably trade off the cost savings from reduced layer thicknesses. “Quality” here refers exclu- sively to structural support, not to other important aspects of quality such as freeze–thaw or wet–dry durability, and so on. In Minnesota, aggregate base materials are becoming increas- ingly expensive in many places because gravel mines and rock quar- ries are being lost to other land uses. Under such circumstances, the selection and utilization of locally available materials to construct strong and durable bases and subbases are a major concern. This concern is mainly due to sustainability issues, which emphasize economical and environmental advantages, primarily the lower cost, optimized material use, and reduced waste. An awareness of optimizing aggregate type and quality and layer thicknesses dur- ing mechanistic-based pavement design according to performance requirements of locally available materials would be useful for road design specialists to better utilize construction dollars. From the perspective of mechanistic–empirical pavement design, it may be challenging to determine how to best utilize different qual- ities of locally available aggregate materials in road bases and sub- bases. For example, Lukanen (1) found early on that certain Minnesota Department of Transportation (DOT) Class 3 aggregates were even stronger than Class 6 aggregates when placed in pavement granular layers. This finding was a surprising field evaluation considering the fact that as Minnesota DOT aggregate classes increase, usually bet- ter materials, such as a Class 6 high quality, are designated. During the Minnesota Road Research Project (MnROAD) study, similar contradictory trends were also observed in backcalculated base layer moduli from falling weight deflectometer (FWD) testing of flexible pavements (2). For both thin (<15 cm) and thick (>15 cm) asphalt concrete surfacing, the backcalculated base moduli of Class 3sp materials were often found to be greater than those of higher ma- terial classes, such as classes 4sp, 5sp, and 6sp (2). In the light of these findings, several issues may need to be addressed, such as how to specify material properties on the basis of their end use perfor- mance; where in pavements to place locally available materials of marginal quality (either natural or recycled); what type of pavement Mechanistic–Empirical Evaluation of Aggregate Base and Granular Subbase Quality Affecting Flexible Pavement Performance in Minnesota Yuanjie Xiao, Erol Tutumluer, and John Siekmeier 97 Since high-quality aggregate materials are becoming increasingly scarce and expensive, optimizing the use of locally available materials for aggre- gate bases and granular subbases on the basis of cost and mechanistic properties linked to pavement performance has become an economically viable alternative. This study investigated the effect of quality of unbound aggregate material on conventional flexible pavement performance in Minnesota through a mechanistic–empirical pavement design approach. A comprehensive matrix of conventional flexible pavement layer thick- nesses and mechanistic design moduli was carefully designed to conduct mechanistic analyses for the Minnesota Department of Transportation flexible pavement design program (MnPAVE) with the MnPAVE pro- gram for pavement sections in two climatic regions in Minnesota. The type and the quality classes of unbound aggregate materials, identified as high, medium, and low, were characterized with stress-dependent resilient modulus (M R ) models from a statewide laboratory-tested aggre- gate M R database. Despite conventional wisdom to the contrary, in some cases the granular subbase material had much higher moduli than the aggregate base. The typical high, medium, and low modulus values for the aggregate base and granular subbase layers, determined from the modulus distributions predicted by the nonlinear finite element pro- gram GT-PAVE, were subsequently input during MnPAVE analyses to calculate fatigue and rutting life expectancies for the comprehensive matrix of pavement structures studied. From the results, use of locally available and somewhat marginal materials may be quite cost-effective for low-volume roads, provided that the 20-year design traffic level does not exceed 1.5 million equivalent single-axle loads. A high-quality, stiff subbase was also found to exhibit a bridging effect that better protected the subgrade and offset the detrimental effects of low base stiffness on rutting performance. Unbound aggregate base and granular subbase layers commonly used in low- to medium-volume conventional flexible pavements serve as major structural components for distributing wheel loads and providing adequate protection of the subgrade to ensure longevity Y. Xiao and E. Tutumluer, Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, 205 North Mathews Avenue, Urbana, IL 61801. J. Siekmeier, Office of Materials and Road Research, Minnesota Depart- ment of Transportation, 1400 Gervais Avenue, Maplewood, MN 55109-2044. Corresponding author: E. Tutumluer, tutumlue@illinois.edu. Transportation Research Record: Journal of the Transportation Research Board, No. 2227, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 97–106. DOI: 10.3141/2227-11