Unbound Aggregate Research Articles

Geosynthetic stabilization of road pavements, railroads, and airfields

Geosynthetics provide sustainable alternatives for enhanced performance, durability and cost-effectiveness of road pavements, railways, and airfields. Even when the same geosynthetic products are used for constructing these different transportation facilities, an optimal approach is needed to match the properties of soils or unbound aggregate geomaterials and the geosynthetic products to establish an effective mechanical stabilization. This Proctor lecture keynote paper presents the state-of-the-practice on the transportation applications of geosynthetics, design methods, and the recent research findings on geosynthetics used in road and airfield pavements and ballasted railway tracks. The paper introduces first the transportation applications of commonly used geosynthetic products, i.e., geogrids, geotextiles, and geocells, with special emphases to subgrade restraint and unbound aggregate stabilization applications and discusses in detail the related unpaved and paved design procedures. Next, the focus is directed towards establishing a better understanding of geosynthetic mechanisms governing stabilization applications. To summarize, this keynote paper reports on two decades of developments and research findings at the University of Illinois and elsewhere on the topic of geosynthetic stabilization using the latest technologies to quantify the stiffness enhancement in the vicinity of a geosynthetic material via the bearing capacity improvement and lateral restraint mechanisms. Recent research at the University of Illinois focused on the successful use of bender element shear wave transducer technology is discussed in detail with several examples given related to the laboratory and field efforts of unbound aggregate base and railroad ballast geosynthetic stabilization including full-scale instrumentation of stabilization geosynthetics used in actual road and airfield pavements constructed in the United States.

Evaluating different geogrid products for modulus improvement with bender element sensor technology

Geogrids extend pavement lifespans by stiffening the unbound aggregate base/subbase through geogrid-aggregate interlocking and restraining of the lateral movements of aggregates. Different geogrids may provide different mechanisms of stabilization based on aggregate size and shape distributions, geogrid properties, and the stabilization mechanism. This study proposed the experimental results of evaluating five different geogrid products: three extruded, one woven, and one welded, with different physical properties tested using a dense graded crushed limestone base course aggregate material. The direct and quantitative stiffness measurements were performed using shear wave transducers (bender elements (BEs)). Small strain shear modulus was measured using three BE sensor pairs instrumented on 300 mm height and 150 mm diameter cylindrical specimens with geogrids installed at specimen midheight. The aggregate material was used based on its as-received gradation. Geogrid-stabilized and unstabilized control tests were conducted in repeated triaxial loading at the 15 stress states following the AASHTO T 307 resilient modulus test protocol. Although the resilient modulus tests could not differentiate the effects of geogrid stabilized specimens, local measurements of small strain shear modulus using the BE sensor pairs demonstrated certain increases in shear moduli owing to the incorporation geogrids. Thus, BE sensor technology was demonstrated as an effective tool for quantifying stiffening/stabilization of different geogrids through increased small strain shear modulus.

A Laboratory-Scale Evaluation of Smart Pebble Sensors Embedded in Geomaterials.

This paper introduces a novel approach to measure deformations in geomaterials using the recently developed 'Smart Pebble' sensors. Smart Pebbles were included in triaxial test specimens of unbound aggregates stabilized with geogrids. The sensors are equipped with an aggregate particle/position tracking algorithm that can manage uncertainty arising due to signal noise and random walk effects. Two Smart Pebbles were placed in each test specimen, one at specimen's mid-height, where a geogrid was installed in the mechanically stabilized specimen, and one towards the top of the specimen. Even with simple raw data processing, the trends on linear vertical acceleration indicated the ability of Smart Pebbles to assess the geomaterial configuration and applied stress states. Employing a Kalman filter-based algorithm, the Smart Pebble position coordinates were tracked during testing. The specimen's resilient deformations were simultaneously recorded. bender element shear wave transducer pairs were also installed on the specimens to further validate the Smart Pebble small-strain responses. The results indicate a close agreement between the BE sensors and Smart Pebbles estimates towards local stiffness enhancement quantification in the geogrid specimen. The study findings confirm the viability of using the Smart Pebbles in describing the resilient behavior of an aggregate material under repeated loading.

Thermal contraction coordination behavior between unbound aggregate layer and asphalt mixture overlay based on the finite difference and discrete element coupling method

AbstractThe constraint action of the unbound aggregate layer underneath plays an important role in affecting the temperature strains in the top asphalt layer. The focus of the present paper is to investigate the interactive thermal contraction mechanisms between the asphalt mixture and granular base layers to offer a new perspective in promoting the understanding of the thermal cracking disease. In this paper, both experiments and simulations were conducted for the thermal contraction tests. A novel numerical modeling of virtual composite structures was proposed using wall‐zone coupling operation based on the finite difference and discrete element coupling method. The experimental composite structures containing three types of asphalt mixture overlays and five types of unbound aggregate base layers were developed for verification. The thermal contraction and restraint mechanism were revealed from both macro and microscopic scales. The proposed numerical modeling shows a 94% high accuracy. The particle displacement vector and contact force chain could explain the thermal contraction behavior of composite structures. Smaller particle motion displacement or stronger contact force chain result in a higher restraint strain of the asphalt overlay. The thermal contraction behavior can be coordinated through the compaction and loosening of unbound aggregates. The material parameters and cooling temperature differences in the asphalt overlay have slight effects on the constraint action of a base layer, while the gradation and mechanical parameters of the unbound aggregate layer show significant impaction. The parameters, cohesion C and friction angle φ, show a quadratic function with the restraint coefficient. This work has significant guidance on the selection of pavement structure and materials to improve the thermal cracking problem and lay the basis for the mechanical theoretical calculation to predict thermal cracking.

Effects of heavy truck braking on inverted asphalt pavement considering vehicle dynamics

Inverted asphalt pavement is an innovative pavement structure where the unbound aggregate base (UAB) is sandwiched between the asphalt concrete (AC) layer and cement-treated base (CTB). This structure has the potential to improve rutting resistance of pavement and prevent propagation of reflective cracks. However, its implementation has been limited due to the lack of experience. In addition, the braking condition of the vehicle applied on the pavement surface plays a significant role on the pavement responses, thereby affecting its service life. This study conducted a simulation using finite layer method to investigate the effects of heavy truck braking on inverted asphalt pavement considering vehicle dynamics. The results indicated that truck braking would generate a significant horizontal load and change the magnitude of the vertical load at each axle. Furthermore, the impact of vertical load on mechanical responses would gradually enhance and eventually dominate with increasing pavement depth. It was also observed that when the truck was treated as a whole, vehicle braking would result in a 3.2% to 10.2% increase in critical responses, but cause a 0.023% and 31.7% decrease in predicted rutting and life respectively. Among three truck axles, the effect of driving axle was close to that of the entire truck, but the effect of other axles was greater or lower than that of the entire truck. A comparison of three inverted pavements found that a thick AC layer could effectively reduce the critical responses and extend the pavement life. Furthermore, a thick CTB layer could also achieve most of the above purposes due to higher integral stiffness of pavement.

Bender element sensor technology to quantify local stiffness enhancement of geogrid-stabilized aggregate specimens

Geogrids improve pavement performance and extend lifespan by making the unbound aggregate base/subbase course stiffer. Extruded (i.e., punched and drawn) geogrids are widely used in pavement applications. This paper presents an experimental evaluation of multi-axial extruded geogrids with various aperture shapes for pavement applications. The study quantified modulus enhancement ratio and identified the extent of the geogrid-stiffened zone. Two types of aggregates (i.e., crushed limestone and partially crushed gravel), commonly used in Illinois for base or subbase course, were utilized. Repeated load triaxial tests were conducted on 6-in. diameter and 12-in. height cylindrical specimens instrumented with embedded shear wave transducers in the form of Bender Element (BE) sensors. The BE pairs were horizontally placed at three different heights above the geogrid. Resilient moduli of the aggregate samples were determined following the AASHTO T 307 standard test procedure. Concurrently, the localized small-strain moduli at different heights above the midheight of the specimen were determined from the shear wave velocities computed according to the first arrival times from the BE sensors recorded at the end of the application of each stress state. The results reveal that resilient modulus tests do not capture the geogrid effects, while BE sensors can quantify the local stiffness enhancement due to a geogrid, and the extent of the geogrid-stiffened zone. Results show that the multi-axial geogrids provided a 45% increase in small strain modulus for both aggregate types in the vicinity of the geogrid.

Local Stiffness Assessment of Geogrid-Stabilized Unbound Aggregates in a Large-Scale Testbed

This paper integrates and extends an earlier article presented at the 20th International Conference on Soil Mechanics and Geotechnical Engineering. The generation of a stiffened zone in the proximity of a geogrid is one of the primary mechanisms of mechanical stabilization of pavement unbound aggregate layers using geogrids. This paper focuses on the quantification of the stiffened zone through a local stiffness assessment using bender element (BE) sensors. Unbound aggregate base layers were constructed in a large-scale laboratory testbed. Geogrid-stabilized layers had geogrids with different-sized triangular apertures contributing to the geogrid-stiffened zone. Shear wave velocities were measured at three different heights using BE sensors, and the vertical stiffness profiles of the mechanically stabilized aggregate layers were evaluated. In addition, the conversion method between small-strain stiffness and large-strain stiffness was established from the repeated load triaxial tests with BE pairs to transform the vertical stiffness profile into that of the resilient modulus. Furthermore, dynamic cone penetration (DCP) and light-weight deflectometer (LWD) tests were performed at multiple locations into the stabilized and unstabilized unbound aggregates. From the large-scale experimental study, the local stiffness improvement owing to the geogrid enhancement was up to 16.2% in the vicinity of the geogrid location, and the extent of the local stiffened zone evaluated through various test methods was between 15.2 cm (6 in.) and 25.4 cm (10 in.) above the geogrid.

Resilient Modulus of Compacted Granular and Lateritic Materials Used as Base Layers for Flexible Pavements in West Africa

The objective of the study is to synthesize resilient modulus data available in Senegal and to propose linear and nonlinear elasticity parameters, which can be used directly for the design of flexible pavements in west Africa. The materials tested are unbound aggregates of basalt and quartzite and gravel lateritic soil from Senegal and Burkina Faso. The Repeated Load Triaxial device is used according to French and American procedures to measure resilient modulus. Different models of the resilient modulus are used to determine the parameters of the nonlinear models and the summary and/or characteristic resilient modulus used in the elastic linear calculation. For the nonlinear calculation, the parameters of the models of Seed et al. (1967), NCHRP (2004) and Boyce (1980) are determined. For the linear elasticity calculation, at the optimum water content and at the dry density of 95% of its maximum value, a maximum modulus of between 300 and 350 MPa is proposed for laterites in the base layer. Values ​​of 220 MPa and 150 MPa are proposed for basalt and quartzite respectively. The moduli obtained are not correlated to the CBR values of the materials. A strong variability in the modulus of laterites is also noted, contrary to the modulus of unbound aggregates. Laterites sometimes have higher modulus than unbound aggregates due to cohesion, but they are more sensitive to water. We recommend that unbound materials (untreated gravels and laterites) be compacted in situ to 97% of the maximum dry density and, for laterite, to the optimal water content -1.5 but also to ensure a good waterproofing and good drainage of the structure. Keywords: Resilient modulus; unbound aggregates; gravel lateritic soil; West Africa.

An Automated Classification of Recycled Aggregates for the Evaluation of Product Standard Compliance

Nowadays, recycling of construction and demolition waste (C&DW) is a challenging opportunity for the management of such end-of-life (EOL) materials through alternative methods to environmentally unsustainable methods (i.e., landfilling). In order to make recycling processes more effective, quality control systems are needed. In this work, the possibility of developing a sensor-based procedure to recognize different demolition waste materials from a recycling perspective was explored. An automatic recognition of different predefined constituent classes of recyclables (i.e., concrete, mortar, natural stones, unbound aggregates, clay masonry units, bituminous materials) and contaminants (i.e., glass, metals, wood, cardboard, and gypsum plaster), as established by an European standard, was carried out using hyperspectral imaging (HSI) working in the short-wave infrared (SWIR) range (1000–2500 nm). The implemented classification strategies, starting from the collected hyperspectral images of the analyzed constituents, allowed for the identification of the different material categories. Two main models were built for identifying contaminants in recyclable materials and categorizing material groups based on technical specifications. The results showed accurate category identification with Sensitivity and Specificity values over 0.9 in all models. The possibility of performing a full detection of C&DW recycling products can dramatically contribute to increasing the quality of the final marketable products and their commercial value, at the same time reducing the amount of waste and the consumption of primary raw materials.

Sequential backward feature selection for optimizing permanent strain model of unbound aggregates

This study proposes a novel framework for identifying the optimal feature set required to predict the permanent strain of unbound aggregates. An experimental database consisting of 16 input features is preprocessed and the performance of 10 machine learning models is evaluated. The best-performing model is then paired with a sequential backward selection algorithm to determine the optimal feature set for predicting the permanent strain. Finally, the selected features are used to predict the permanent strain, and the performance is compared with those obtained from the principal components analysis. Six features are selected as the optimal feature set. Furthermore, the selected features accurately predict permanent strain with a root mean square error value of 0.014, which is smaller than those obtained from principal components analysis. Thus, the feature selection approach for machine learning models effectively predicts the permanent strain of unbound aggregates using a limited set of input features.

Investigating real-time monitoring indices of compaction quality from particle movement characteristics of distinctly-graded unbound aggregate materials subjected to vibratory compaction

The compaction quality greatly affects the in-service performance of unbound aggregate materials (UAMs); however, there still lack real-time, non-destructive, and effective indices for compaction quality assessment. This paper aimed to reveal meso-scale mechanisms of macroscopic vibratory compaction behavior and evaluate compaction quality from particle movement characteristics. A series of newly-developed laboratory vibratory plate compaction tests were conducted on UAM specimens of different gradations. The innovative self-powered, wireless sensors resembling realistic particles (termed as “SR sensors”) were placed inside UAM specimens to monitor three-dimensional (3D) particle accelerations and Euler angles during vibratory compaction. The effects of gradation and vibratory frequency on both macroscopic compaction behavior and meso-scale particle motion characteristics were analyzed. The results show that the entire vibratory compaction process can be divided into three distinct stages according to discernibly-staged particle movement patterns. The degree of compaction (DOC) of UAMs is significantly correlated with two novel particle-motion-based compaction quality indicators.

Predicting permanent strain accumulation of unbound aggregates using machine learning algorithms

The objective of this study is to propose a machine learning-based framework for predicting the permanent strain accumulation of unbound aggregates. To develop the machine learning-based framework, material properties of 15 crushed aggregates determined at two different gradations are combined with the applied stress states on aggregate specimens during repeated load triaxial testing by considering the aggregate shear strength characteristics. Three single machine learning algorithms (K-nearest neighbor, neural network, and decision tree) and two ensemble learning algorithms (random forest and extreme gradient boosting) are used in this study. Based on the dataset obtained from 90 permanent deformation tests, hyperparameters, which are used to control the learning process, are optimized. The numerical results demonstrate that all five of the tuned machine learning models exhibit good generalization capacity and performance with the coefficient of determination exceeding 0.99. Among the five models, the extreme gradient boosting model outperforms the others, accurately predicting permanent strain accumulation even for load cycles of 10,000. The contributions of individual input features to the performance of each learning model depend on the dataset. The strength characteristics and the applied deviatoric stress are the two most important features. Therefore, the machine learning-based framework introduced in this study may be effectively used for predicting the permanent strain accumulation of unbound aggregate layer.

Preliminary optimization design of inverted asphalt pavement structure using response surface method

Inverted asphalt pavement (IAP) has the advantage of maintaining good performance under the condition of thin asphalt concrete (AC) layer compared with conventional flexible asphalt pavement. The stress-dependent property of unbound aggregate base (UAB) plays an important role in mechanistic responses of IAP. The focus of this paper is to quantitatively analyze the function of UAB’s stress-dependent property on the structure combinations design of IAP using response surface method (RSM). In this paper, an improved UMAT subroutine was developed to characterize the stress-dependent stiffness of UAB. The three-dimensional FE pavement model was established and validated against KENLAYER calculation and field FWD test deflection data. The critical mechanical response, maximum principal strain and bulk stress, were selected to evaluate the fatigue performance of AC layer and stress-dependent property of UAB. The significance analysis was conducted to describe the relationship between input variables (thickness and stiffness of structural layers) and the critical mechanical response variables. An optimization design method to balance the fatigue performance and stress-dependent property was proposed to obtain the optimal structure combinations of IAP. Research results demonstrate that the thickness and stiffness of the AC and UAB layers present a clearly significant impact on the maximum principal strain at the bottom of AC layer and the bulk stress in UAB, particularly during the summer season, whereas the thickness and stiffness of CTB and stiffness of subgrade have a minor impact. The responses of bulk stress and maximum principal strain can be empirically predicted by 2FI and Quadratic models. Optimal structure combinations (13.6 cm AC layer and 10 cm UAB) were recommended after quantifying the stress-dependent properties of UAB. The recommended structural combination has good fatigue resistance and can also efficiently minimize AC and UAB layer thickness.

Impact of Fines on Various Base Material Properties

The performance of the unbound aggregate base is affected by moisture content and fines (materials finer than No. 200 sieve) content. Proper moisture content and the proper amount of fines in a base material contribute to long-lasting pavements. This study evaluated an unbound aggregate base under different moisture contents and fines contents. The evaluation included the measurements of the electrical and electromagnetic properties with moisture content and fines content. The dielectric constants were measured with a percometer, and the resistivity values were measured using a Wenner probe. These measurements were correlated with the moisture content for all fines contents used in the study. Strength and modulus tests were also performed on the specimens using the unconfined compressive strength (UCS) tests and a lightweight deflectometer (LWD). The variations in these parameters with moisture content are also presented here for all fines content. A normalization scheme was used to allow for the data to be compiled and analyzed more easily. The dielectric constant increased as moisture was added with some dispersion, but no clear trend was seen among different fines contents. The resistivity decreased consistently as the moisture content increased. The peak UCS normally occurred at moisture contents of about 1% dry of optimum moisture content (OMC); specimens prepared with 1% less than optimum had higher UCS. LWD modulus tended to peak when prepared drier than OMC. The optimum strength and modulus were obtained with 5% and 10% fines contents for the unbound granular base used.

Evaluation of Open-Graded Aggregates Stabilized with a Multi-Axial Geogrid Using a Large-Scale Triaxial Test Set-Up

Geogrids are commonly used in paved and unpaved roadways to stabilize base and subbase layers by laterally restraining movements of unbound aggregates under applied wheel loading, often referred to as interlocking. The geogrid-aggregate interlock is a function of the match between the geogrid aperture size and geometry and the grain size and shape characteristics of the stabilized aggregates. Open-graded coarse aggregates (OGA) are commonly used as an alternative to dense-graded materials in permeable base/subbase applications as well as in railway ballast to facilitate drainage. This paper describes an experimental study recently conducted to investigate the repeated load-deformation behavior of a geogrid-stabilized OGA material in large-scale triaxial testing. Two multi-axial geogrids with the same aperture patterns but different aperture sizes were tested. The aperture patterns include different sizes of hexagonal, trapezoidal, and triangular openings. The multi-axial geogrid with the larger apertures better matched the OGA material properties to provide a reduction in permanent deformation in the test specimen. The improved interlocking and the related local stiffness enhancement were successfully quantified using embedded shear wave transducers termed bender elements. To stabilize aggregates with relatively uniform grain sizes, for example, AASHTO No. 57 stone studied here and railroad ballast, the ratio between the aperture size and a representative particle size (i.e., D90) served as a good indicator of the effectiveness of the geogrid. The findings of this experimental study therefore provided valuable information on the governing properties and mechanisms that play a role in geogrid-aggregate interaction with multi-axial geogrids.

Evaluation of Airport Pavement Base Layer Stiffness Characteristics via Embedded Field Sensors

Real-time stiffness monitoring of unbound aggregate layers using embedded field sensors is crucial for advanced airport pavement design and management. This paper describes research findings on monitoring base layer stiffness characteristics for airport pavements using an embedded bender element (BE) field sensor, inductive coil sensors, and pressure cells. A full-scale pavement test section was constructed at the Federal Aviation Administration (FAA)’s National Airport Pavement Test Facility (NAPTF). A BE field sensor, inductive coil sensor pairs, and pressure cells were installed in the unbound aggregate base to evaluate the layer stiffness, applied load stress, and deformation characteristics. A triple dual tandem gear loading module applied 58,000 lb per wheel to the tested section with a wander pattern consisting of 66 passes arranged in nine lateral wander positions. Dynamic responses of coil sensors and pressure cells were collected during the traffic test, whereas BE signals were collected after completing each wander pattern. Laboratory repeated load triaxial tests with BE instrumentation were also conducted to establish a correlation for converting BE field sensor readings to layer modulus properties. The in situ modulus characteristics of the aggregate base layer were evaluated during full-scale testing using two methods: shear wave velocities from a BE field sensor, and strain and stress measurements from coil sensors and pressure cells. The moduli estimated using both approaches fell into typical ranges of crushed aggregate bases and were highly comparable. The moduli from the dynamic sensor measurements clearly showed the effect of vehicle wander on aggregate base layer measured responses.

Using Bender Elements to Investigate the Effect of a Multi-axial Geogrid on Aggregate Layer Stiffness

Geogrid inclusion in low-volume road applications has been shown to improve rutting performance. Although rutting performance and traditional instrumentation response data provide meaningful insights into pavement behavior, they do not provide a direct measurement of material behavior near the geogrid. Recently, the University of Illinois at Urbana-Champaign has developed field-deployable shear-wave transducers, that is, bender elements to measure the localized stiffness enhancement attributed to geogrid inclusion in unbound aggregate layers, which have been verified in laboratory and limited field applications. Bender element sensors were installed in a full-scale accelerated pavement test experiment at the U.S. Army Engineer Research and Development Center that included a recently developed multi-axial hexagonal geogrid. The sensors were installed directly on the geogrid and 10 cm above the geogrid in the unbound aggregate layer to measure the geogrid zone of influence. The test item was trafficked with a dual-wheel truck gear, and bender element measurements were made at select intervals throughout traffic application. To estimate the unbound aggregate moduli, the data were analyzed using two techniques: first arrival time and peak-to-peak arrival time. The bender element sensors were found to be sufficiently robust to survive installation, construction, and trafficking. Calculated modulus values 10 cm above the geogrid were found to be higher than those directly on the geogrid, which was an unexpected finding. However, a forensic investigation revealed moisture and fines migration into the bottom of the unbound aggregate layer, which could have dampened the shear-wave signal, resulting in lower calculated modulus values.

Advances in the Use of Geosynthetics for Stabilization of Unbound Aggregate Layers

The governing mechanism associated with the stabilization of unbound aggregate layers in pavements is lateral restraint. Reproducing this mechanism in the laboratory maybe challenging because, while the original loading source is cyclic (traffic), lateral restraint develops through interlocking and interfacial friction between the geosynthetic and the aggregate to restrain the development of permanent lateral strains. Considering the relevance of lateral restraint in the quantification of the benefits of geosynthetics embedded within (or adjacent to) unbound aggregate layers, this study focuses on two experimental approaches to quantify this mechanism. The first experimental approach aims at defining a design parameter, identified as the Stiffness of the Soil-geosynthetic Composite (KSGC), which is obtained from Soil-Geosynthetic interaction (SGI) tests and is practical for use in specifications and design. The second experimental approach that quantifies the lateral restraint mechanism involves one-third scale accelerated pavement tests (APTs), which were performed on pavement test sections stabilized with various geosynthetics, diverse in terms of geometry and materials. The rutting from these sections was compared to that in the non-stabilized (control) section to evaluate the Traffic Benefit Ratio (TBR) at failure rut depth for each geosynthetic. The TBR obtained showed a strong linear correlation to the KSGC of the corresponding geosynthetic determined by SGI tests. Overall, the KSGC parameter was found to represent a suitable indicator of the performance of pavements with unbound aggregate layers stabilized using geosynthetics.

Using Falling Weight Deflectometer (FWD) and Ground Penetrating Radar (GPR) to monitor the effects of seasonal moisture variation on the structural capacity of pavements

Moisture fluctuations in pavement foundation due to environmental conditions (e.g., heavy precipitation, freeze–thaw cycles, groundwater table variation, etc.) can significantly affect pavements’ short- and long-term performance. Moisture variation in the pavement foundation is often monitored as a way to anticipate the structural capacity of pavements. Thus, the ability to monitor moisture variation through a proven non-destructive technology (NDT) such as Ground Penetrating Radar (GPR) would be beneficial for asset management by transportation agencies. This paper summarizes the Minnesota Department of Transportation’s (MnDOT) efforts at validating the use of GPR to monitor moisture in the pavement foundation through comparison with Falling-Weight Deflectometer (FWD) parameters that represent the structural condition of the pavement. The results in this paper are based on GPR, FWD and in-place moisture sensor data collected over 17 months on MnROAD instrumented test sections. The pavement test sections considered in this study included unbound aggregate bases (UAB) with virgin and recycled materials (i.e., RCA and RAP) covering a broad range of geotechnical behavior. Previous efforts established a strong correlation between GPR-based moisture measurements and sensor-based moisture measurements. This correlation is further validated by investigating the relationship between GPR-based moisture measurements and FWD-based indices for structural capacity. The observed correlation is reasonable and in good agreement with the expected correlation between volumetric moisture content (VMC) of the base layer and the structural capacity of a pavement. This agreement validates the use of GPR to monitor moisture in pavement foundation. GPR is not a replacement for FWD, but it can be used in asset management efforts in addition to FWD to assess moisture fluctuation in pavement foundation during and after extreme environmental events (e.g., flooding, freeze-thawing) to determine when and where FWD testing may be necessary.

Evaluating the performance of inverted pavement structure using the accelerated pavement test (APT)

• Full-scale inverted pavement was investigated. • Accelerated pavement test was applied to the inverted pavement. • Rutting performance between inverted and conventional pavements was compared. • Inverted pavement structure outperformed the conventional flexible pavement. Inverted pavement is considered an unconventional type of flexible pavement structure. In this pavement structure, an unbound aggregate base (UAB) with a low initial modulus is sandwiched (layered) between two stiffer layers, a thinner asphalt concrete layer (AC) and a cement-treated base layer (CTB). This paper presents a comparison study of the rutting performance (permanent deformation) between conventional and inverted pavement structures. In addition, the inverted pavements with different thicknesses of UAB and CTB were studied. The accelerated pavement test (APT) method was conducted on a full-scale pavement consisting of two inverted pavement lanes and one typical three-layer conventional flexible pavement lane as the control section for performance comparison. Significant permanent deformation was observed in all three pavement lanes after the completion of 100,000 passes of APT. Comparisons of the measured total deformations showed that the inverted pavement structure outperformed the conventional pavement. Furthermore, the inverted pavement with a thicker CTB layer exhibited less deformation compared with the inverted pavement with a thicker UAB layer. The stiffness of the UAB layer of the inverted pavement increased with the loading passes. A stiffer UAB could provide a cushion for the thin AC layer, resulting in less deformation in the AC surface compared with that of the conventional pavement. Furthermore, the loading on the pavement surface had a larger affected area in the inverted pavement structure. Based on the overall pavement conditions, the inverted pavement can be regarded as a promising alternative to the conventional flexible pavement, and the asphalt mixture layer thickness can be reduced during the construction.