Yuanjie Xiao

Gradation Effects Influencing Mechanical Properties of Aggregate Base–Granular Subbase Materials in Minnesota

Aggregate gradation effects on strength and modulus characteristics of aggregate base–granular subbase materials used in Minnesota are described. The importance of specifying proper aggregate grading or particle size distribution has long been recognized for achieving satisfactory performance in pavement applications. In the construction of dense-graded unbound aggregate base–subbase layers, well-graded gradation bands were often established years ago on the basis of the experience of the state transportation agency and may not have a direct link to mechanical performance. To improve specifications for superior performance targeted in the mechanistic–empirical pavement analysis and design framework, there is a need to understand how differences in aggregate gradations may affect unbound aggregate base–subbase behavior for site-specific design conditions. Aggregates with different gradations and material properties were compiled in a statewide database established from a variety of sources in Minnesota. Analyses showed nonunique modulus and strength relationships for most aggregate base and especially subbase materials. Laboratory resilient modulus and shear strength results were analyzed for critical gradation parameters by common gradation characterization methods. The most significant correlations were between a gravel-to-sand ratio (proposed based on ASTM D2487-11) and aggregate shear strength properties. Aggregate compaction (AASHTO T99) and resilient modulus characteristics could also be linked to the gravel-to-sand ratio and verified with other databases in the literature. The gravel-to-sand ratio can be used to optimize aggregate gradations for improved base–subbase performances primarily influenced by shear strength.

Mechanistic–Empirical Evaluation of Aggregate Base and Granular Subbase Quality Affecting Flexible Pavement Performance in Minnesota

Since high-quality aggregate materials are becoming increasingly scarce and expensive, optimizing the use of locally available materials for aggregate 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 thicknesses 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 program 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 (MR) models from a statewide laboratory-tested aggregate MR 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 program 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.

Resilient Modulus Behavior Estimated from Aggregate Source Properties

Resilient modulus (MR) is a key mechanistic pavement analysis input for designing conventional flexible pavements with unbound aggregate base and granular subbase layers. For satisfactory pavement design and performance, it is often challenging to determine unbound aggregate layer modulus inputs when only limited aggregate source property data are available. This paper presents established correlations between aggregate physical properties and stress-dependent MR characterization model parameters by utilizing the Minnesota Department of Transportation (Mn/DOT) aggregate property databases. In addition to gradation, percent passing No. 200 sieve (or fines content), moisture content and dry density, aggregate particle shape properties quantified as Flat and Elongated (F&E) ratio, Angularity Index (AI) and Surface Texture (ST) index by the University of Illinois Aggregate Image Analyzer (UIAIA) were also included as predictor variables for developing correlations. A subsequent Monte Carlo type simulation was performed via the software @RISK to investigate sensitivities of MR to the various aggregate source properties. It was found that the inclusion of aggregate shape properties significantly improved the established correlations. On the basis of Monte Carlo simulation results, the design reliability of the current MnPAVE program Fall input moduli for aggregate base/granular subbase materials was demonstrated to be greater than the current estimate of 85%.

Gradation and Packing Characteristics Affecting Stability of Granular Materials: Aggregate Imaging-Based Discrete Element Modeling Approach

AbstractUnbound aggregate base and subbase in layered pavement structures provide an essential function of wheel load distribution through proper angular or crushed particle interlocking of coarse aggregate particles in the granular matrix. This paper used a validated model utilizing a particle image-aided discrete element method (DEM) to evaluate aggregate gradation and shape/morphological properties on pavement granular layer packing characteristics and load-carrying performance. The DEM packing simulations for different granular material gradations and particle shape categories were first performed to investigate the influence of the No. 4 (4.75-mm) sieve, separator of sand-sized and gravel-sized particles according to the Unified Soil Classification System (USCS), for typical dense-graded aggregates specified by the Minnesota Department of Transportation (MnDOT). Aggregate particles were modeled in the DEM as three-dimensional (3D) polyhedral discrete elements with both low and high angularity categor...

Performance Evaluations of Unbound Aggregate Permanent Deformation Models for Various Aggregate Physical Properties

Permanent deformation or rutting is the main performance indicator of unbound aggregate layers used in flexible pavements. This paper evaluates the prediction abilities of unbound aggregate base or subbase permanent deformation models in use or proposed for use in the Mechanistic–Empirical Pavement Design Guide (MEPDG) approach. Repeated load triaxial-type permanent deformation tests were conducted on three unbound aggregate materials—limestone, dolomite, and uncrushed gravel—commonly used for pavement base and subbase and subgrade replacement applications in Illinois. The test matrix was designed to evaluate effects of aggregate physical properties, including moisture content, gradation, types and amounts of fines, aggregate mineralogy, and particle shape, texture, and angularity. The laboratory-measured permanent deformations were compared with those predicted by four rutting models evaluated in this study. The permanent deformations predicted by the original 1989 Tseng–Lytton model and the 2006 El-Badawy model were generally in good agreement with the measured values. The current MEPDG rutting model and its enhanced version proposed in 2013 by Hashem and Zapata tended to overpredict permanent deformations and have a low sensitivity to changes in aggregate physical properties. In addition to enhancements recommended for the four evaluated models, a unified rutting model was developed; it used a shear stress ratio concept and imaging-based aggregate morphological indexes. With a single set of calibrated model parameters, the unified rutting model produced reasonably accurate permanent strain predictions for all unbound aggregate materials used in this study.

DEM approach for engineering aggregate gradation and shape properties influencing mechanical behavior of unbound aggregate materials

It has been well established that the mechanical behavior of unbound aggregate materials is strongly influenced by the shape of the particles and the complete grain size distribution (i.e., gradation). This paper presents the use of an image-aided discrete element method (DEM) approach to realistically model the micromechanical interactions of aggregate particles including size and shape effects. A realistic unbound aggregate model is developed from previous studies to simulate triaxial compression tests based on the DEM approach and the innovative use of membrane elements surrounding the cylindrical aggregate specimen in the DEM model. To calibrate the DEM model, six different types of aggregates were used to prepare samples of twenty-one unbound aggregate blends at the same gradation and voids content but different crushed percentages. The shape properties of flatness and elongation, surface texture and angularity were quantified by imaging then correlated to strength and permanent deformation characteristics obtained from triaxial testing. The experimental test results are utilized to determine required parameters for the discrete element model by minimizing the differences between DEM-simulated triaxial shear strength results and experimental ones. It is found that the developed DEM model is not only capable of reproducing the typical shear strength behavior of different unbound aggregate materials with varying shape properties, but also has the potential for optimizing the selection of size and shape properties of various unbound aggregates to achieve desired shear strength (or rutting resistance) and hydraulic behavior for open-graded permeable aggregate base.

Framework to Improve the Pavement ME Design Unbound Aggregate Rutting Model by Using Field Data

The AASHTOWare Pavement ME Design and the Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures provide a methodology for analyzing response and predicting rutting performance of unbound granular pavement layers. However, this methodology has been reported to have low sensitivity to the properties of the base and subbase and the subgrade. The primary objective of this paper is to provide a framework for improving the prediction ability of the Pavement ME Design rutting model for unbound aggregate pavement layers. This goal was achieved by using accelerated testing data from full-scale pavement test sections constructed with different aggregate materials. These test sections were loaded to failure by using the accelerated transportation loading assembly at the University of Illinois. Nonlinear finite element solutions of the pavement test sections were studied so that stress-dependent modulus characterization models could be used for the anisotropic unbound aggregate base and isotropic subgrade. This approach enabled prediction of realistic stress states and accurate pavement responses. Realistic stress states and the use of a shear stress ratio concept were incorporated into the proposed framework needed to improve the rutting predictions of the Pavement ME Design damage model. Finally, a definite correlation appeared to exist between shear stress ratio values and the shift factors that were proposed for the adjustment of the model’s predicted rut depths so they would match the field-measured performance trends. The framework studied in this paper is applicable to the three-parameter Tseng–Lytton rutting model, the original version of the Pavement ME Design damage model, for the modifications needed to generate improved granular layer rut predictions throughout pavement design service life.

In-situ Hydraulic Properties of Unbound Aggregate Layers Measured Using Gas Permeameter Test (GPT) Device

Unbound aggregate layers are widely used in pavement base/subbase applications for both conventional flexible and rigid pavements. Hydraulic properties, i.e., hydraulic conductivity and moisture-suction characteristics, of these layers dictate their effectiveness in providing adequate drainage, which is essential for the long-term pavement performance. Ideally, determining in-situ hydraulic conductivity of an unbound aggregate layer would be the most desirable for reliable mechanistic based pavement design and construction practices. This paper presents findings from a recent field study undertaken at the University of Illinois utilizing an innovative Gas Permeameter Test (GPT) device to measure in-situ hydraulic properties of unsurfaced pavement test sections. The test pavements were constructed using both crushed and uncrushed granular layers on a weak subgrade of controlled strength to study effects of various unbound aggregate material properties (or qualities) on subgrade rutting performance. The repeatability of GPT measurements was quite satisfactory and saturated hydraulic conductivity (Ksat) values obtained were closely linked to field measured moisture and density properties. The GPT-measured Ksat values were statistically correlated to fines contents in the aggregate matrix, void ratios, and imaging based quantifiable aggregate shape properties (flat and elongated ratio and angularity index). The different unbound aggregate Ksat values estimated from grain-size distributions using empirical Hazen’s and Chapuis’ models were also compared quite favorably with the in-situ GPT measurements to verify their applicability and potential for predicting field permeability properties of pavement base/subbase materials.

Laboratory and Field Measured Moduli of Unsurfaced Pavements on Weak Subgrade

This paper presents findings on the effectiveness of field modulus determinations and modulus based assessment of constructed pavement foundation geomaterials, i.e. subgrade soil and base/subbase unbound aggregate material. Full-scale unsurfaced pavement test sections were constructed on controlled strength weak subgrades and tested to failure for studying effects of aggregate quality on pavement performance. Differences in aggregate quality were assessed by changes in aggregate properties such as particle angularity, fines content and plasticity of fines. In-situ moduli of the constructed layers were measured using both light weight deflectometer (LWD) and GeoGauge™ type field devices. Tests were conducted on the engineered subgrade as well as on the finished aggregate layer surface with data collected consistently from the same locations using the two devices. Both the GeoGauge™ and the LWD were successful in identifying anomalies in construction conditions, i.e., increasing or decreasing trends in moduli. However, field achieved dry densities and aggregate qualities could not be linked to the field modulus values, which also differed from the laboratory-measured resilient modulus properties.

Evaluating Characteristics of Cyclic Plastic Strain Accumulation of Unbound Granular Materials Subject to Moving Wheel Loads

Cyclic plastic strain behavior of unbound granular materials (UGMs) exhibits significant stress path dependency. Using a customized triaxial apparatus capable of applying stress path loading, a series of laboratory repeated load triaxial (RLT) tests were conducted on two typical UGMs through simultaneously varying the axial stress and the radial stress. Effects of realistic in-situ stress paths due to a passing wheel on cyclic plastic strain behavior of unbound granular base and subbase materials were investigated and quantified. The analysis of experimental results revealed that the accumulated plastic strain responses of both UGMs subjected to different stress path loadings can be described by the shakedown approach. The shakedown ranges of different stress path loads were classified for both materials. Finally, the significance of the findings made in pavement design practices was highlighted to evaluate permanent deformation resistance of UGMs and their suitability for use in pavement foundation layers.