Unbound Aggregate Base Research Articles

A mechanistic–empirical model for base-reinforced flexible pavements

A mechanistic–empirical model for geosynthetic base-reinforced flexible pavements is proposed. The model uses traditional components of an existing unreinforced mechanistic–empirical model developed in the USA through NCHRP Project 1-37A. These components include a finite element response model, material models for the asphalt concrete, unbound aggregate base and subgrade and damage models for asphalt concrete fatigue cracking and permanent deformation in the pavement cross-section layers. New components for the reinforcement are introduced and include structural elements for the reinforcement, a material model for the reinforcement, a model for reinforcement–aggregate shear interaction, additional response modelling steps that account for the influence of the reinforcement on lateral confinement of the base aggregate during construction and subsequent traffic loading, and a modified permanent deformation damage model used for aggregate within the influence zone of the reinforcement. This paper describes the basic components of the model with a focus on the ability of the model to predict permanent deformation, which is compared to results from test sections. This comparison shows favourable agreement that is on the level seen with existing unreinforced mechanistic–empirical models and a large improvement over previously proposed models for reinforced pavements.

Micromechanics analysis of granular soils to estimate inherent anisotropy

This paper presents micromechanics models to estimate resilient properties of unbound aggregate materials. Micromechanics models account for the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles. The results demonstrate that aggregate orientation and shape influence the level of inherent anisotropy, which has a substantial effect on the pavement responses that impact pavement design. The micromechanics analysis predicted an inherent anisotropy (ratio of horizontal to vertical modulus) ranging from 1.0 to 0.4. The effect of this increased anisotropy on the performance of a pavement with an unbound aggregate base is substantial.

Accelerated Stiffness Profiling of Aggregate Bases and Subgrades for Quality Assessment of Field Compaction

As a compaction control parameter, stiffness has been investigated as an alternative to dry unit weight, because the design of pavement is based on the elastic modulus of pavement layers. A reliable and practical quality assurance system was proposed to accelerate the quality assessment of field compaction. The proposed system, which has evolved from the spectral analysis of surface waves (SASW) method, profiles the shear wave velocity of the unbound aggregate base and subgrade. The system consists of dedicated hardware for surface wave measurements and an analysis algorithm to automate the interpretation and analysis procedures. The measurement hardware consists of two sensor units and one source unit and can be operated by one person. The automation in interpretation and analysis was implemented with the peak-trough method and the frequency wave number technique. Consequently, the surface wave measurement for stiffness profiling is reduced to a 10- to 15-min task. The validity of the proposed hardware a...

Simple Methods to Estimate Inherent and Stress-Induced Anisotropy of Aggregate Base

Simple methods to estimate cross-anisotropic properties of unbound aggregate assemblies on the basis of aggregate physical properties are presented. A regression model for the cross-anisotropic material properties was developed from a database consisting of aggregates from six sources. Aggregate specimens from each source were tested with the use of different gradations and compaction moisture contents. The results demonstrate that aggregate shape and gradation influence the level of anisotropy, which has a substantial effect on the pavement responses that affect pavement design. The level of anisotropy, defined as the ratio of the horizontal modulus to the vertical modulus, was calculated from the regression model and compared with the results from a micromechanics model. This micromechanics model accounted for the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles on inherent anisotropy. Horizontal-to-vertical modulus ratios ranging from 0.4 to 1.0 were calculated with the micromechanics model. However, the experimental results and regression model show that the level of anisotropy can drop to as low as 0.15 because of the additional anisotropy induced by repeated loading. The effect of this increased anisotropy on the performance of a pavement with an unbound aggregate base is substantial.

Unbound Aggregate Rutting Models for Stress Rotations and Effects of Moving Wheel Loads

The latest research findings on stress rotations caused by moving wheel loads and their effects on permanent deformation or rut accumulation in pavement granular layers are presented. Realistic pavement stresses induced by moving wheel loads were examined in the unbound aggregate base and subbase layers, and the significant effects of rotation of principal stress axes were indicated for a proper characterization of the permanent deformation behavior. To account for the rutting performances of especially thick granular layers, a comprehensive set of repeated load triaxial tests was conducted in the laboratory. Triaxial test data were obtained and analyzed from testing aggregates under various realistic in situ stress paths caused by moving wheel loading. Permanent deformation characterization models were then developed on the basis of the experimental test data to include the static and dynamic stress states and the slope of stress path loading. The models that also considered the stress path slope variations predicted the stress path dependency of permanent deformation accumulation best. In addition, multiple stress path tests conducted to simulate the extension–compression–extension type of rotating stress states under a wheel pass gave much higher permanent strains than those of the compression-only single path tests. The findings indicated actual traffic loading simulated by the multiple path tests could cause greater permanent deformations or rutting damage, especially in the loose base or subbase, when compared with deformations measured from a dynamic plate loading or a constant confining pressure type laboratory test.

Unbound Aggregate Rutting Models for Stress Rotations and Effects of Moving Wheel Loads

The latest research findings on stress rotations caused by moving wheel loads and their effects on permanent deformation or rut accumulation in pavement granular layers are presented. Realistic pavement stresses induced by moving wheel loads were examined in the unbound aggregate base and subbase layers, and the significant effects of rotation of principal stress axes were indicated for a proper characterization of the permanent deformation behavior. To account for the rutting performances of especially thick granular layers, a comprehensive set of repeated load triaxial tests was conducted in the laboratory. Triaxial test data were obtained and analyzed from testing aggregates under various realistic in situ stress paths caused by moving wheel loading. Permanent deformation characterization models were then developed on the basis of the experimental test data to include the static and dynamic stress states and the slope of stress path loading. The models that also considered the stress path slope variati...

Validated Model for Predicting Field Performance of Aggregate Base Courses

The International Center for Aggregates Research Project 502 focused on pavement layers of unbound aggregate proper representation in mechanistic pavement models. The research team developed models for resilient and permanent deformation behavior from the results of triaxial tests conducted at the Texas Transportation Institute and the University of Illinois. The studies indicate that the unbound aggregate base (UAB) material should be modeled as nonlinear and cross-anisotropic to account for stress sensitivity and the significant differences between vertical and horizontal moduli and Poisson’s ratios. Field validation data were collected from a full-scale pavement test study conducted at Georgia Tech. The validation of the anisotropic modeling approach was accomplished by analyzing conventional flexible pavement test sections using the GT-PAVE finite element program to predict responses to load in the UAB layer and comparing these predicted responses to the measured values. Laboratory testing of the aggregate samples was conducted at the University of Illinois, and characterization models were developed for the stress-sensitive, cross-anisotropic aggregate behavior. With nonlinear anisotropic modeling of the UAB, the resilient behavior of pavement test sections was successfully predicted for a number of response variables. In addition, the stress-sensitive, cross-anisotropic representation of the base was shown to greatly reduce the horizontal tension computed in the granular base compared with a linear isotropic representation.

A simplified mechanistic approach to establish load-limit on low-volume flexible pavements

A simplified mechanistic approach is presented for predicting load limits on low-volume flexible pavements. A non-linear elasto-plastic finite element program was developed to evaluate the structural adequacy of pavements based on a bearing capacity approach using the Mohr-Coulomb failure criterion. To consider the resilient behaviors, the stress-dependent model was also incorporated into the program. The varying stress-dependent modulus and Poisson's ratio were predicted for considering the non-linear behaviors of unbound aggregate bases. A parametric study was also conducted to identify the significant resilient and plastic factors in a proposed methodology.

Construction and Performance of Fly Ash-Stabilized Cold In-Place Recycled Asphalt Pavement in Wisconsin

Cold in-place recycling (CIR) is a common rehabilitation practice used in Wisconsin to improve the ride quality and structural capacity of deteriorated asphalt pavements. In recent years, increased emphasis has been placed on incorporating stabilizers into the CIR materials to improve the structural capacity of the CIR base layer. This improvement can serve to increase the performance life of the completed pavement or to allow for a reduced hot-mix asphalt (HMA) surface thickness. The city of Mequon, Wisconsin, included asphalt emulsion and fly ash CIR stabilization over a portion of its CIR projects in 1997. Presented are the findings relating to the constructability of the fly ash–stabilized CIR pavement as well as performance trends for the CIR pavements based on distress and deflection testing results. CIR is a common rehabilitation practice used in Wisconsin to improve the ride quality and structural capacity of deteriorated asphalt pavements. In one type of CIR application, existing HMA layers are pulverized, graded, and compacted, then used as a base layer for a new HMA surface. The pulverization process is completed to provide uniformity of support to the HMA surface and to significantly reduce or eliminate the occurrence of reflection cracking of the HMA surface. In most CIR applications, pulverization is completed through the full thickness of the existing HMA layers, as well as through the top 25 to 50 mm of aggregate base. Penetration into unbound aggregate base materials aids in cooling of the bits on the pulverizer mandrel. After pulverization, graders typically are used to spread the materials to the desired width and shape. Compaction is achieved by using vibrating steel drum and pneumatic-tire rollers. The moisture content of the CIR materials is adjusted, as necessary, by surface spraying from a water tanker truck.

Evaluation of Constructing Increased Single-Lift Thicknesses of Unbound Aggregate Bases Case Study in Georgia

A study was conducted to evaluate the feasibility of compacting unbound aggregate base courses in lifts thicker than those permitted by state departments of transportation (DOTs). At present, the majority of the state DOTs allow a maximum lift thickness of 20.3 cm (8 in.) or less. Work conducted in conjunction with the widening of a state road in Georgia is described. Three test pads of the same base material were constructed and evaluated: one Target Strip and two Test Sections. The Target Strip was placed in two lifts: a 17.8-cm (7-in.) lift followed by a 15.2-cm (6-in.) lift. Each test section was constructed in a single, 33-cm (13-in.) lift. Compaction of the unbound aggregate base courses was evaluated by conventional nuclear density gauge (NDG) testing. Additionally, nondestructive seismic testing was used to evaluate stiffness profiles within the base and supporting subgrade. Spectral-analysis-of-surface-waves (SASW) tests were used for this purpose. Both the NDG and the SASW test results show that a 33-cm (13-in.)-thick layer of unbound aggregate base can be compacted as dense and as stiff as a 15-cm (6-in.)-thick layer. The seismic results also show that the upper 7.6 cm (3 in.) of all three test pads have lower stiffnesses than the bottom 25.4 cm (10 in.). This near-surface layer exhibits a lower stiffness, presumably from lower effective stresses and possibly from some disturbance caused by the compaction equipment. Also, higher moisture contents during compaction of the base yielded lower stiffnesses, even at the same in situ density.

Analysis of Granular Bases Using Discrete Deformable Blocks

A method of analysis using deformable blocks is proposed for modeling unbound granular layers used in flexible pavements. An assemblage of discrete blocks of aggregates is employed to approximate the load transfer mechanisms of the real particulate nature of granular materials. The unbound aggregate bases were modeled as particulate media composed of blocks of aggregates, which are able to transfer both shear and normal compressive loads through the interfaces. Using a finite-element approach, the discrete blocks are modeled as single quadrilateral elements connected only by surrounding interface elements at each block interface. Realistic granular layer particle characteristics including translation, sliding, and even separation are permitted for each block. An iterative procedure for equilibrium is employed in the model, and the constitutive relations used in the analysis are described for determining the behavior modes of the interface elements. The granular base discrete block representation is then used in a conventional flexible pavement section having other layers modeled as continua. Important findings are presented related to the shear resistance of the granular bases.

Investigation of Backcalculated Moduli Using Deflections Obtained at Various Locations in a Pavement Structure

The interpretation and use of deflection measurements on the prepared surfaces of the various pavement layers during construction are examined. Measurements were obtained from four asphalt concrete pavement test sections, two with unbound aggregate base and two with bituminous-treated base over an untreated aggregate base. Deflection basin measurements using a falling weight deflectometer were performed on the prepared surfaces of the subgrade, base layers, and asphalt concrete layers. The elastic moduli of each layer were computed using the EVERCALC backcalculation program. The primary finding from this investigation is that deflection measurements on the subgrade and base layers during construction can be used to control construction uniformity and provide checks on mechanistic-based pavement design assumptions. Also, subgrade uniformity has a profound impact on the entire pavement structure and subgrade variations affect total deflections and computed layer moduli of all successive layers. The backcalculated modulus is directly related to the stress state in the layer. For unbound aggregate bases, the backcalculated elastic modulus decreases with a decrease in the bulk stress, and for fine-grained subgrade soil, the backcalculated elastic modulus increased with a decrease in the deviator stress. As expected, a higher and more variable root-mean-square basin fit error value was obtained for measurements on unbound material as compared with measurements on bound material surfaces.

Assessment of In Situ Structural Properties of Lime-Stabilized Clay Subgrades

Lime-stabilized clay subgrades are used almost routinely in Texas to facilitate construction and to provide a foundation for aggregate base courses and hot mix surfaces. Research sponsored by the Texas Department of Transportation demonstrates that the in situ moduli and strength improvements afforded by lime stabilization of these layers are often significant and deserve structural consideration. A study of the range of modulus values determined from falling weight deflectometer deflection data and supported by in situ dynamic cone penetrometer data for 40 pavement subgrades indicates that the lime-stabilized subgrades provide a level of stiffness and strength that is similar to that of an unbounded aggregate base. This substantiates previous literature suggesting that properly designed and constructed lime-stabilized subgrades should be assigned AASHTO structural coefficients in the same range as unbound aggregate bases, that is, between 0.10 and 0.14.

Assessment of in Situ Structural Properties of Lime-Stabilized Clay Subgrades

Lime-stabilized clay subgrades are used almost routinely in Texas to facilitate construction and to provide a foundation for aggregate base courses and hot mix surfaces. Research sponsored by the Texas Department of Transportation demonstrates that the in situ moduli and strength improvements afforded by lime stabilization of these layers are often significant and deserve structural consideration. A study of the range of modulus values determined from falling weight deflec-tometer deflection data and supported by in situ dynamic cone penetrometer data for 40 pavement subgrades indicates that the lime-stabilized subgrades provide a level of stiffness and strength that is similar to that of an unbounded aggregate base. This substantiates previous literature suggesting that properly designed and constructed lime-stabilized subgrades should be assigned AASHTO structural coefficients in the same range as unbound aggregate bases, that is, between 0.10 and 0.14.