The long-term performance of pavement depends on the complex geomechanical properties of the unbound materials used in the construction of the pavement foundation. When the pavement is subjected to cyclic stresses, the stress is transmitted downward through the aggregates that compose the different layers (i.e., base, subbase, and subgrade). The properties of soil, including gradation, density, plasticity index, moisture content, aggregate shape, stiffness (resilient modulus (MR)), and hydraulic properties, are crucial for effective drainage, stress dissipation, and protecting pavement from distresses such as cracking and rutting. For instance, a subgrade layer composed of expansive clay undergoes significant volume changes in response to variations in moisture content. Consequently, it exerts significant pressures on the pavement structure, leading to uplift during wet periods and settlement during dry periods. The base and subbase layers protect the subgrade from excessive traffic loads while facilitating pavement drainage. Ideally, natural aggregate is used in the construction of pavement foundations. However, due to the high cost, environmental impact, and scarcity of natural aggregates, recycled concrete aggregate (hereafter referred to as RCA followed by their corresponding number) have been used as an alternative. The crushed properties of RCAs provide superior mechanical benefits, such as high stiffness, compared to natural aggregate (hereafter referred to as GM followed by their corresponding number). However, the presence of unhydrated cement and cement mortar in RCA can affect the long-term performance of pavement and drainage properties, potentially causing significant distress. While RCA is stiffer and a more sustainable option, its properties are not fully understood. Additionally, there is still a lack of consensus on the effect of geomaterial index properties on the geomechanical properties of both RCA and natural aggregates used in the construction of pavement foundation layers. To address these issues, several base (RCAs and GMs) and subgrade unbound materials with different index properties were collected from various roadway sections under construction in Michigan. An extensive evaluation was conducted to understand how their index properties affect: 1) the stress-strain response of subgrade (sand and clay) and base (RCA and GM) unbound materials; 2) the hydraulic properties (hydraulic conductivity, water content and matric suction relationship); and 3) the time required to drain 50% of a saturated base layer. The stress-strain response of sandy and fine unbound subgrade soils was evaluated using the NCHRP and Shakedown concept. Depending on their gradation and plasticity index, the materials showed stress-hardening, stress-hardening followed by stress-softening, and stress-softening. Further analysis was conducted to understand the effect of confining pressure and stress dependency of these materials. To study the effect of index properties on RCA and GM, Principal Component Analysis (PCA) was employed for dimensionality reduction and to identify patterns within the dataset. Based on the PCA results, six materials were selected, and a model was developed to estimate laboratory resilient modulus results using falling weight deflectometer (FWD) field tests. Additionally, the hydraulic properties and time-to-drain properties of the base materials were evaluated to further understand the impact of material properties on base layer performance and their unsaturated properties. The findings led to several recommendations for materials used in designing sustainable and long-life pavements. Detailed discussions of the results are provided in the following chapters.
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