Work done by several researchers reveals that unbound aggregate materials show nonlinear cross-anisotropic behavior. The incorporation of cross-anisotropic properties significantly improves the predictions of stress distribution by reducing tensile stresses computed within granular layers. Existing pavement analysis and design approaches, however, generally assume the pavement structure to be linear isotropic layered system. This assumption is motivated by the difficulties in determining cross-anisotropic resilient material properties from laboratory experiments and lack of pavement anisotropic analysis programs. Recently, the International Center for Aggregates Research (ICAR) developed a methodology to characterize unbound aggregate layers by considering stress-sensitivity and nonlinear cross-anisotropy. The ICAR model requires nine coefficients to account for stress-sensitivity and anisotropy of vertical, horizontal, and shear moduli. Unfortunately, ICAR testing protocol is time-consuming and expensive to perform and certainly do not lend themselves to routine testing. Since it is important to be able to consider the stress-sensitive and anisotropic nature of unbound granular materials, a simple procedure was proposed by accounting for the effects of aggregate gradation and shape properties in predicting the cross-anisotropic modular ratio of unbound granular materials. Variable confining pressure type repeated load triaxial tests were performed on six aggregate sources with three different gradations and three different moisture contents. The experimental results were analyzed within the framework of nonlinear cross-anisotropic elastic model in order to determine the model coefficients. Image analysis techniques were utilized to measure aggregate shape properties. The gradation and shape properties were fitted using a cumulative distribution function and nonlinear regression analysis, which is capable of capturing the complete distribution of these properties. The experimental and analytical results indicate that the vertical resilient modulus is greater than the horizontal resilient modulus and that aggregate physical properties significantly affect the anisotropic resilient behavior. Based on finite element analysis, the anisotropic resilient behavior has substantial effect on the critical pavement responses. Thus, it is extremely valuable to approximate the degree of cross-anisotropy in unbound aggregates and to use it as input in the pavement analysis programs to adequately model unbound aggregate bases for pavement design and analysis.

An inverted base pavement is a new pavement structure that consists of an unbound aggregate base between a stiff cement-treated foundation layer and a thin asphalt cover. Unlike conventional pavements which rely on upper stiff layers to bear and spread traffic loads, the unbound aggregate inter-layer in an inverted base pavement plays a major role in the mechanical response of the pavement structure. Traditional empirical pavement design methods rely on rules developed through long-term experience with conventional flexible or rigid pavement structures. The boundaries imposed on the unbound aggregate base in an inverted pavement structure change radically from those in conventional pavements. Therefore, current empirically derived design methods are unsuitable for the analysis of inverted base pavements. The present work documents a comprehensive experimental study on a full-scale inverted pavement test section built near LaGrange, Georgia. A detailed description of the mechanical behavior of the test section before, during and after construction provides critically needed understanding of the internal behavior and macro-scale performance of this pavement structure. Given the critical role of the unbound aggregate base and its proximity to the surface, a new field test was developed to characterize the stress-dependent stiffness of the as-built layer. A complementary numerical study that incorporates state-of-the-art concepts in constitutive modeling of unbound aggregates is used to analyze experimental results and to develop preliminary guidelines for inverted base pavement design. Simulation results show that an inverted pavement can deliver superior rutting resistance compared to a conventional flexible pavement structure with the same fatigue life. Furthermore, results show that an inverted base pavement structure can exceed the structural capacity of conventional flexible pavement designs for three typical road types both in rutting and fatigue while saving up to 40% of the initial construction costs.

The purpose of this study was to improve flexible base course performance within the state of Texas while reducing TxDOT’s testing burden. The focus of this study was to revise the current specification with the intent of providing a “performance related” specification while optimizing sample sizes and testing frequencies based on material variability. A literature review yielded information on base course variability within and outside the state of Texas, and on what tests other states, and Canada, are currently using to characterize flexible base performance. A sampling and testing program was conducted at Texas A&M University to define current variability information, and to conduct performance related tests including resilient modulus and permanent deformation. In addition to these data being more current, they are more representative of short-term variability than data obtained from the literature. This “short-term” variability is considered more realistic for what typically occurs during construction operations. A statistical sensitivity analysis (based on the 80th percentile standard deviation) of these data was conducted to determine minimum sample sizes for contractors to qualify for the proposed quality monitoring program (QMP). The required sample sizes for contractors to qualify for the QMP are 20 for gradation, compressive strength, and moisture-density tests, 15 for Atterberg Limits, and 10 for Web Ball Mill. These sample sizes are based on a minimum 25,000 ton stockpile, or “lot”. After qualifying for the program, if contractors can prove their variability is better than the 80th percentile, they can reduce their testing frequencies. The sample size for TxDOT’s verification testing is 5 samples per lot and will remain at that number regardless of reduced variability. Once qualified for the QMP, a contractor may continue to send material to TxDOT projects until a failing sample disqualifies the contractor from the program. TxDOT does not currently require washed gradations for flexible base. Dry and washed sieve analyses were performed during this study to investigate the need for washed gradations. Statistical comparisons of these data yielded strong evidence that TxDOT should always use a washed method. Significant differences between the washed and dry method were determined for the percentage of material passing the No. 40 and No. 200 sieves. Since TxDOT already specifies limits on the fraction of material passing the No. 40 sieve, and since this study yielded evidence of that size fraction having a relationship with resilient modulus (performance), it would be beneficial to use a washed sieve analysis and therefore obtain a more accurate reading for that specification. Furthermore, it is suggested the TxDOT requires contractors to have “target” test values, and to place 90 percent within limits (90PWL) bands around those target values to control material variability.