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Al-Qadi I.L.,Illinois Center for Transportation | Leng Z.,University of Illinois at Urbana - Champaign | Lahouar S.,Institute Superieur Des Science Appliquees Et Of Technology Of Sousse | Baek J.,University of Illinois at Urbana - Champaign
Transportation Research Record | Year: 2010

This research proposes the innovative use of ground-penetrating radar (GPR) for effectively, continuously, and rapidly estimating in-place hotmix asphalt (HMA) density. On the basis of electromagnetic mixing theories, three candidate models were developed to determine HMA's dielectric constant, considering dielectric and volumetric properties of its three major components of HMA: air, binder, and aggregate. Laboratory tests were conducted on midsize HMA slabs (60 cm × 60 cm × 7.5 cm) to evaluate the models. After evaluating and comparing the three models, it was determined that the prediction model based on the Rayleigh mixing theory was the most accurate. The selected model was calibrated with a field core and then validated using field GPR measurements of a composite pavement with an HMA surface. The selected model provided accurate HMA density within a reasonable range.

Shangguan P.,University of Illinois at Urbana - Champaign | Al-Qadi I.,University of Illinois at Urbana - Champaign | Coenen A.,Illinois Center for Transportation | Zhao S.,University of Illinois at Urbana - Champaign
International Journal of Pavement Engineering | Year: 2014

Ground-penetrating radar (GPR) is a promising non-destructive technique to be applied on monitoring the density change during asphalt pavement compaction. The utmost challenge of this application is the unknown effect of surface moisture, sprayed by the compactor during compaction, on GPR signals. To extract density information without the effect of surface moisture, a correction algorithm based on reference scan approach was developed. To evaluate the performance of the algorithm, a full-scale test site was constructed with compaction pass number from 0 to 10, and a large amount of GPR data were collected from the pavement with different surface moisture contents. A total of 22 cores were extracted for validation purposes. After applying the algorithm, it was found that the average density prediction error was reduced significantly. By using correction algorithm together with the density model, the density of asphalt pavement was obtained with high accuracy. © 2014 Taylor & Francis

Ozer H.,Illinois Center for Transportation | Al-Qadi I.L.,Illinois Center for Transportation | Duarte C.A.,University of Illinois at Urbana - Champaign
Transportation Research Record | Year: 2011

Near-surface cracking, sometimes referred to as top-down cracking, is one of the predominant distress types in flexible pavements. The incidence of near-surface cracking has increased in recent years with the increased construction of relatively thick (hot-mix asphalt layer > 200 mm) flexible pavements. However, understanding the mechanisms of near-surface cracking and its integration into pavement design protocols remains a challenge. Analysis of this problem can be complex because of multi-axial stress states in the vicinity of tires. The near-surface response to nonuniform tire contact stresses is investigated, and the potential for crack occurrence near the surface is analyzed in a typical relatively thick flexible pavement. The generalized finite element method (GFEM) is used to analyze pavement structure. This method provides a computational framework for the arbitrary orientation of cracks in a finite element mesh that is particularly useful for mixed-mode fracture problems. A three-dimensional (3-D) model for a typical pavement structure with a thick bituminous layer is created, and 3-D and nonuniform tire-pavement contact stresses are applied to the pavement surface. Aggregate-scale cracks are inserted at various locations and orientations in the pavement. Results of this numerical study indicate that complex stress states in the presence of strong mode mixity may cause shear or tensile fracture in flexible pavements. The importance of novel computational methods such as the GFEM to the discovery and understanding of mechanisms governing the premature failure of pavements is highlighted.

Ozer H.,Illinois Center for Transportation | Solanki P.,Illinois State University | Yousefi S.,University of Illinois at Urbana - Champaign | Al-Qadi I.,University of Illinois at Urbana - Champaign
Transportation Research Record | Year: 2014

When properly installed, hot-poured crack sealants are widely accepted as a cost-effective, routine preventive maintenance practice that extends pavement service life by 3 to 5 years. However, current ASTM specifications for selection of crack sealants correlate poorly with field performance. Therefore, an improved sealant specification and selection system is urgently needed. Recently, performance-based guidelines were developed by the pooled-fund North American Consortium expert group for selecting hot-poured bituminous crack sealants. The work proposed a sealant grade system for selecting hot-poured crack sealant on the basis of environmental conditions. A special effort was made to use the equipment originally developed by SHRP, which was used to measure binder rheological behavior as part of the performance grade system. The equipment and testing procedures used for performance grading of binders were modified in accordance with crack sealant behavior. The main objective of this study was to validate the low-temperature selection thresholds for the newly developed performance-based guidelines for selecting hot-poured crack sealants. Thresholds for the crack sealant bending beam rheometer, crack sealant direct tension test, and crack sealant adhesion test were validated. Nine hot-poured crack sealants were installed in four test sites experiencing low temperatures. The field performance of crack sealants was evaluated for 2 consecutive years by detailed field surveys. The field database consisted of 40 sections containing 647 cracks. Overall, results showed good correlation between the proposed selection thresholds and sealant performance in the field.

Al-Qadi I.L.,Illinois Center for Transportation | Al-Qadi I.L.,University of Illinois at Urbana - Champaign | Wang H.,University of Illinois at Urbana - Champaign
Asphalt Paving Technology: Association of Asphalt Paving Technologists-Proceedings of the Technical Sessions | Year: 2011

This paper discusses the distribution of contact stress at the tire-pavement interface and how to quantify its impact on viscoelastic pavement responses using a decoupled modeling approach. The authors developed a tire-pavement interaction model to predict the three-dimensional (3-D) contact stresses under various loads and inflation pressures. In this model, an air-inflated radial-ply ribbed tire was loaded on a non-deformable pavement surface. The predicted contact stresses are consistent with previous measurements and validate the non-uniformity of vertical contact stresses and localized tangential contact stresses at the tire-pavement interface. The load primarily affects the vertical contact stress at the edge of the tire contact area and the longitudinal contact stress; while the inflation pressure primarily controls the vertical contact stress in the center region of the tire contact area and the transverse contact stress. Statistical models were developed to predict the 3-D contact stresses at each rib under various loads and inflation pressures. Utilizing the realistic contact stress distribution at the tire-pavement interface, a 3-D finite element (FE) model was built to analyze the critical pavement responses under moving tire loading. The FE model simulated the asphalt mixture layer as a linear viscoelastic material and considered the cross-anisotropic stress-dependent modulus for the unbound base layer. The authors concluded that when 3-D tire contact stresses are used in the analysis, the longitudinal fatigue cracking, primary rutting, and secondary rutting potential in thin asphalt pavement are increased, compared to when uniform contact stress distribution is applied. The heavy load causes increased responses in the base layer and subgrade, while high tire pressure causes increased response impact in the asphalt mixture layer, especially the shear stress at high temperature. The results of the analysis illustrate the importance of considering the realistic contact stress distribution when analyzing pavement responses under various loading and tire pressure conditions.

Al-Qadi I.,Illinois Center for Transportation | Wang H.,Urbana University | Tutumluer E.,Urbana University
Transportation Research Record | Year: 2010

A three-dimensional (3-D) finite element (FE) model was developed to investigate the dynamic responses of thin, flexible pavement under impulsive loading similar to a falling weight deflectometer test. The FE model simulated the hot-mix asphalt (HMA) surface layer as a linear viscoelastic material and considered the cross-anisotropic stress dependent modulus for the unbound base layer. Implicit dynamic analysis was used to consider the effect of inertia on pavement structural responses. Using two thinpavement structures of different HMA layer thicknesses, 76 and 127 mm, the study analyzed the effects of cross-anisotropic stress-dependent aggregate base modulus and dynamic analysis on pavement responses, including surface deflection, tensile strain at the bottom of the HMA layer, deviator stress in the base layer, and compressive strain on top of the subgrade. Results showed that use of the cross-anisotropic stress-dependent modulus for the unbound base layer resulted in greater predicted pavement responses and, hence, less estimated pavement life for rutting and fatigue cracking. It was found that as the thickness of HMA surface layer or the ratio of horizontal modulus to vertical modulus decreases, the effects of stress dependency and cross anisotropy become more significant. Analysis-predicted surface deflections were compared to field-measured values and they were in agreement when the stress dependency and cross anisotropy of the base layer and subgrade were considered.

Knuth D.,Jacobs Engineering | Fortmann J.,Illinois Center for Transportation
Green Streets and Highways 2010: An Interactive Conference on the State of the Art and How to Achieve Sustainable Outcomes - Proceedings of the Green Streets and Highways 2010 Conference | Year: 2010

In January 2010, the Illinois Department of Transportation, the American Consulting Engineers Council - Illinois Chapter, and the Illinois Road and Transportation Builders Association released a sustainability guide and rating system entitled I-LAST, Illinois - Livable and Sustainable Transportation. The document was developed by volunteers from these organizations over a two-year period. This paper reviews the reasons for undertaking this project, the contents of the resulting document, and the decisions made during its development. © 2010 ASCE.

Nega A.,Curtin University Australia | Nikraz H.,Curtin University Australia | Al-Qadi I.L.,Illinois Center for Transportation
Airfield and Highway Pavements 2015: Innovative and Cost-Effective Pavements for a Sustainable Future - Proceedings of the 2015 International Airfield and Highway Pavements Conference | Year: 2015

Full-depth asphalt concrete pavements are generally designed to control fatigue cracking and reduce potential rutting when subjected to repeated heavy traffic loads. A particular interesting question is whether a limit load exists below which excitation shakedown in the sense that the granular layer does not accumulate further deformation. Although pavement design guides give more weight to asphalt concrete layer failures, granular failure may not be ignored; especially for thin layers and/or heavy load. The behavior of granular layers used in base and, sub-base layers of flexible pavement is complicated due to its nonlinear elastoplastic response when subjected to dynamic traffic loading. The objective of this paper is to present a new simplified simulation model for the Shakedown behavior of granular layer in flexible pavement. This method is integrated with Mohr-Coulomb criterion, which is used and applied to simulate the response of unbound granular layers to dynamic loading in a numerical analysis. The results of analysis are then compared to simplify the results of modeling without considering shakedown effects and then, the conclusions are drawn. © ASCE.

Cottrill C.D.,Illinois Center for Transportation | Thakuriah P.,University of Illinois at Chicago
Accident Analysis and Prevention | Year: 2010

In this paper, we present an analysis of the relationship between pedestrian-vehicle crashes and characteristics of areas with high low-income and minority populations in the Chicago metropolitan area (also called environmental justice or EJ areas in the United States). While related research has indicated that pedestrian crashes occur more frequently in these areas than in non-EJ areas, this paper attempts to relate the incidence to environmental characteristics and behavioral factors through a better understanding of the contributing factors present in crash occurrences in EJ versus non-EJ areas. Specially constructed small-area factors from a Spatial Decision Support System (SDSS) are used to explain pedestrian-vehicle crashes. Using a Poisson model that corrects for underreporting, we find that pedestrian crash incidents in EJ areas are related to variables of exposure (including the suitability of the area for walking and transit accessibility), crime rates, transit availability, and general population demographics such as income and presence of children. Results suggest that it may be necessary to better incorporate a safety perspective or measures of safety improvements in pedestrian and transit improvements and expansion programs within EJ areas. © 2010 Elsevier Ltd. All rights reserved.

PubMed | Illinois Center for Transportation
Type: Journal Article | Journal: Journal of occupational and environmental medicine | Year: 2016

Fossil fuel transportation by health care providers contributes to the prevalence of diseases they treat. We conducted an exploratory study to understand obstacles to, and best practices for, greener commuting among health care providers.We surveyed staff of three hospital clinics as to how they commute and why, and interviewed key staff of five hospital leaders in green commuting about their programs.Factors that might change respondents commuting choices from driving alone included financial incentives, convenience, and solutions to crime and safety concerns. Successful green commuting programs offer benefits including free or reduced transit passes, shuttle buses to transit stations, and free emergency rides home.Exemplary programs throughout the country demonstrate that modifying those factors within reach can impact the amount of fossil fuel energy used for health care provider transportation.

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