Recycling aged asphalt pavements into new asphalt pavements is a common practice in the pavement industry. The process involves adding the reclaimed asphalt pavement (RAP) into a superheated asphalt mixing drum, where the binder contained within the RAP is expected to melt and be incorporated into the new hot-mix asphalt. Since the RAP is added part way through the mixing process, the amount of time spent inside the mixing drum is short and possibly insufficient for the RAP binder to melt and mix with the new asphalt binder. To address this issue, a numerical model is developed using the thermal properties of asphalt materials to investigate the melting potential of the RAP inside an asphalt mixing drum. Through a dimensionless analysis, the proposed model allows for the practice-ready calculation of the minimum time needed for any spherical particle to heat to a desired temperature given the initial temperature, ambient temperature, thermal diffusivity, and particle radius. Using the resulting equation, it is shown that there may be cases in which the RAP is not sufficiently heated inside an asphalt mixing drum. © 2014 Taylor & Francis.

Cracking is a major distress mechanism in asphalt pavements. Thermal cracking is especially prevalent in Northern Minnesota and other areas with cold climates. Developing asphalt mix designs that are more resistant to cracking distresses is necessary to reduce maintenance and rehabilitation expenditures. The present study involves analysis of over 32,000 asphalt mixes and approximately 12,000 field sections available from Minnesota Department of Transportation (MnDOT). The main objective of this work is to identify the effects of asphalt binder content and binder grade on the actual field cracking performance. A comprehensive database has been developed that includes mix design information (design traffic level, mix size, binder type, wear versus non-wear course), mix volumetrics and gradation (air voids, voids in mineral aggregates, voids filled with asphalt, adjusted asphalt film thickness, percent passing on control sieves, recycled fractions), and actual field performance data from MnDOT's pavement management system. This database has made it possible to quantify the effects of binder content and grade on the actual field performance. A series of statistical tests were conducted to determine if significant relationships exist between the binder content and grade, and the field cracking performance. The results show that both binder content and grade have a significant effect on the transverse cracking of pavements and for Minnesota the PG XX-34 grade may be better suited than the PG XX-28 binder grade. © 2014 Taylor & Francis Group, London.

This paper describes graduate student projects that were conducted through cooperation between University of Minnesota Duluth and the City of Duluth. While graduates students at the University of Minnesota Duluth can complete a traditional thesis-based MS, they also have the option of completing additional course work and a MS project. The graduate projects are designed to be realistic engineering problems that allow students gain and apply higher level civil engineering analysis and design knowledge. This paper describes two of these projects resulting in three MS projects. The first project determined a method of using fine dredge material from the harbor as engineered fill using locally available additives. The second project focused on transportation and structural issues in a neighborhood revitalization. These projects were evaluated using recently developed graduate student learning outcomes. The MS projects from this partnership were successful in meeting the graduate student learning outcomes when compared to MS projects from other sources. © American Society for Engineering Education, 2014.

Thin-bonded bituminous overlays are becoming an increasingly popular pavement maintenance treatment, which can be used to restore smoothness, seal and renew the pavement surface and increase skid resistance. Thin-bonded overlays (TBOs) are constructed using a specialised type of paving equipment called a spray paver. A spray paver combines the operation of applying a tack coat and laying down asphalt concrete in a single pass. This allows for the application of a high rate of polymer-modified asphalt emulsion tack coat. Due to reduced thickness, cracking distress is more of a concern in this type of system. This paper describes a new approach for the evaluation of the cracking performance of TBO systems through fracture mechanics-based testing of laboratory and field specimens. Computer simulations and early field performance data are also used in the evaluation. This study is conducted in the context of three field projects which encompass seven different pavement test sections. The test sections allowed a number of variables to be studied, including type and application rate of tack coat emulsion and type of hot-mix asphalt gradation structure (gap graded vs. dense graded). Comparisons are also made between overlays constructed using spray paver and conventional paving process. All seven sections were computationally simulated to evaluate their performance in the context of thermal and reflective cracking potential. Fairly good agreement is observed between laboratory tests, computer simulations and field performance data. The results indicate that good thermal and reflective cracking resistance are expected from TBOs. Furthermore, it was observed that the cracking performance of TBOs depends on the type of gradation for the overlay mixture and the tack coat emulsion type and its application rate. © 2013 Taylor and Francis Group, LLC.

The University of Minnesota Duluth's Department of Civil Engineering accepted its first students in 2008, graduated its first class in 2012, and first offered a capstone design course in the spring semester, 2012. Groups of five to six students designed a building on a local site. Students organized their teams based on interest in a particular branch of civil engineering, allowing individual students to focus their efforts on a particular subject. Based on feedback from faculty, practicing engineers, and students, several changes were implemented prior to the fall 2012 semester. These changes included making the group size smaller, modifying the graded submissions, and changing the project location. Most significantly, the course was reorganized to prevent students from working the entire semester in one area of civil engineering while doing little to no work in other areas. This paper compares the different capstone design experiences. Results from the analysis are part of a larger comparison between narrow, in-depth and broad, general approaches to design experiences for undergraduate civil engineering students. ©American Society for Engineering education, 2013.

The availability of mineral aggregates for pavement construction is continuously depleting. The aggregate manufacturing process requires significant amounts of energy, which ranges from 10 to 30 MJ/ton. The process also produces 5 kg/ton of carbon dioxide (CO₂), which causes significant amounts of greenhouse gas emissions. With the annual consumption of approximately 1.2 billion tons of aggregates in the United States, the environmental impact is significant. More than 125 million tons of fine-grained, crushed siliceous material is generated annually through iron ore mining in northern Minnesota. Typically, this material is referred to as "taconite tailings" and usually ends up as landfill near mining operations. This paper describes a moisture damage evaluation of asphalt mixes that contained significant fractions of aggregate as taconite tailings. The evaluation was conducted with the use of the conventional AASHTO T 283 test procedure as well as an approach with a fracture energy basis. The paper presents comparative results for two mixes: one made with taconite tailings and the other with conventional granite aggregates. The results indicated that a mix that contained taconite had acceptable moisture-damage resistance. The results also pointed out the limitations of the AASHTO T 283 procedure, especially the process of moisture conditioning. The fracture energy results indicated that, although mixes underwent reduced tensile strength, the overall capability of mix to strain without cracking significantly increased after the AASHTO-recommended moisture conditioning process. The study also included a set of samples that were field-conditioned over the winter and spring months. The mechanical behavior of field-conditioned samples was quite different from the behavior of samples conditioned in the laboratory with the AASHTO procedure.

Low temperature cracking (LTC) is a major distress and cause of failure for asphalt pavements located in regions with cold climate; however, most pavement design methods do not directly address LTC. The Thermal Cracking Model (TCModel) utilized by the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) relies heavily on the phenomenological Paris law for crack propagation. The TCModel predictions are primarily based on tensile strength of the asphalt mixture and do not account for quasi-brittle behavior of asphalt concrete. Furthermore, TCModel uses a simplified one-dimensional viscoelastic solution for determination of thermally induced stresses. This paper describes a newly developed comprehensive software system for low temperature cracking prediction in asphalt pavements. The software system called "IlliTC" utilizes a user-friendly graphical interface with a stand-alone finite-element based simulation program. The system includes a preanalyzer and data input generator module that develops a two-dimensional finite element pavement model for the user and which identifies critical events for thermal cracking using an efficient viscoelastic pavement stress simulation algorithm. Cooling events that are identified as critical are rigorously simulated using a viscoelastic finite element analysis engine cou pled with a fracture-energy based cohesive zone fracture model. This paper presents a comprehensive summary of the components of the IlliTC system. Model verifications, field calibration and preliminary validation results are also presented. © 2013 Taylor & Francis.

The finite-element (FE) method is used for modeling geotechnical and pavement structures exhibiting significant non-homogeneity. Property gradients generated due to non-homogeneous distributions of moisture is one such example for geotechnical materials. Aging and temperature-induced property gradients are common sources of non-homogeneity for asphalt pavements. Investigation of time-dependent behavior combined with functionally graded property gradation can be accomplished by means of the non-homogeneous viscoelastic analysis procedure. This paper describes the development of a generalized isoparametric FE formulation to capture property gradients within elements, and a recursive formulation for solution of hereditary integral equations. The formulation is verified by comparison with analytical and numerical solutions. Two application examples are presented: the first describes stationary crack-tip fields for viscoelastic functionally graded materials, and the second example demonstrates the application of the proposed procedures for efficient and accurate simulations of interfaces between layers of flexible pavement. © 2011 John Wiley & Sons, Ltd.

Asphalt overlays provide an economical means for treating deteriorated pavements. Thin bonded overlay (TBO) systems have become popular options for pavement rehabilitation. In addition to functional improvements, these systems ensure a high degree of waterproofing benefits. Conventional asphalt concrete fracture tests were developed for pavements with homogeneous asphalt concrete mixtures, and typically their thicknesses exceed 50 mm (2 inch). The use of spray paver technology for construction of TBOleads to continuously varying asphalt binder content, up to approximately one-third of the layer thickness. Commonly utilized fracture test geometries for asphalt concrete include the single-edge notched beam, tension, or C[T], test geometry for field cores as well as laboratory-fabricated composite specimens. Laboratory testing using the proposed procedure clearly showed distinction in the fracture characteristics for specimens prepared with varying material compositions. The capability of distinguishing different materials combined with stable crack growth makes the proposed testing procedure ideal for fracture characterization of thin and graded pavement systems. Statistical analysis of test data revealed that the proposed C[T] test procedure is capable of detecting differences in fracture energy results across a wide range of pavement systems and yields a low test variability. Finite element simulations of the test procedure further indicate the suitability of the test procedure aswell as demonstrating a procedure for extraction of fundamental material properties. © RILEM 2012.

Significant increases in the cost of asphalt paving and increased awareness of the need for sustainable infrastructure in recent years have in turn increased the use of recycled asphalt pavement (RAP) in the manufacture of hot-mix asphalt (HMA). The use of RAP reduces the overall cost of HMA and provides significant environmental benefits. Experience has shown, however, that the addition of RAP to HMA can have a negative effect on the low-temperature fracture characteristics of the pavement. The purpose of this study was to determine the effects of RAP amounts on the low-temperature cracking performance of asphalt mixtures. Different percentages of RAP material, ranging from 0% to 50%, were studied. The embrittlement temperature of mixtures was determined with the use of an acoustic emissions technique. The disk-shaped compact tension [DC(T)] test was used to determine the fracture energy of asphalt mixtures. DC(T) fracture tests were conducted on two control mixtures with no RAP and mixtures that contained 10%, 20%, 30%, 40%, and 50% RAP. Both control and RAP mixtures were manufactured with PG 64-22 and PG 58-28 as the virgin binders, which brought the total number of mixtures tested to 12. In addition to DC(T) fracture testing, indirect tensile testing was conducted on HMA specimens that contained 20% and 40% RAP. Test results clearly indicated the effects of the presence of RAP materials on the low-temperature performance of mixtures. This study demonstrates the benefit of performing fracture tests before RAP is added to the asphalt mixture, and it demonstrates the use of an acoustic emissions-based testing procedure to screen mixtures susceptible to cracking at low temperatures.

Low temperature or thermal cracking in asphalt concrete pavements is a major cause of pavement deterioration. Fundamental fracture evaluation of asphaltic materials is necessary in order to design pavements that are resistant to thermally induced cracking. At present, stress-strain response of asphalt binders within the linear material behavior range is commonly utilized in criteria for material acceptance. The fracturing of a material is a highly complicated phenomenon, and evaluation of the material beyond the linear response range will help close the gap between experimental results and actual field performance. In recent years it has been well established that at low temperatures asphalt mixtures behave in a quasi-brittle manner. For complete evaluation of asphalt mixture cracking performance, it is necessary to consider mixture response past the peak strength. In the present work, a set of nine mixtures, encompassing a variety of variables, including type of binder modification, presence of recycled asphalt pavement (RAP), and low temperature binder grade, are studied to assess their fracture behavior in light of two new fracture testing techniques. Testing was conducted in two stages; fracture energy measurements were obtained using the disk-shaped compact tension (ASTM D7313) test. Fracture energies were determined at two temperatures, two air void levels and two levels of age conditioning. Fracture energy testing was followed by acoustic emissions (AE) evaluation to characterize low temperature cracking behavior of the asphalt mixtures. The AE testing procedure, which has been demonstrated to successfully capture low temperature behavior of asphalt binders, was extended to include low temperature characterization of asphalt mixtures. Testing results also quantify the effects of RAP on low temperature fracture behavior and provide new insights on the effects of commonly used binder modifiers on mixture fracture behavior.

Thermally induced cracking in asphalt pavements remains to be one of the prominent distress mechanisms in regions with cooler climates. At present, the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) is the most widely deployed pavement analysis and design procedure. For thermal cracking predictions, MEPDG utilizes a simplified one-dimensional stress evaluation model with a simple Paris-law (i.e. linear elastic fracture mechanics) based crack propagation procedure. The user-friendly graphical interface for MEPDG makes it an attractive design procedure of choice, however, the over simplicity of the model and lack of a physics-based representation to accurately capture the nonlinear fracture behavior of rate-dependent asphalt concrete reduce(s) the reliability of predictions. This study presents an interactive thermal cracking prediction model that utilizes a nonlinear finite element based thermal cracking analysis engine which can be easily employed using a user-friendly graphical interface. The analysis engine is comprised of (1) the cohesive zone fracture model for accurate simulation of crack initiation and propagation due to thermal loading and (2) the viscoelastic material model for time and temperature dependent bulk material behavior. The graphical user interface (GUI) is designed to be highly interactive and user-friendly in nature, and features screen layouts similar to those used in the AASHTO MEPDG, thus minimizing transition time for the user. This paper describes the individual components of the low temperature cracking prediction software (called LTC Model) including details on the graphical user interface, viscoelastic finite element analysis, cohesive zone fracture model, and integration of various software components for thermal cracking predictions. © 2011 ASCE.

The indirect tension test (IDT) is frequently used in civil engineering because of its benefits over direct tension testing. In the mid-1990s, an IDT protocol was developed for evaluating tensile strength and creep properties of asphalt concrete mixtures, as specified by the American Association of State Highway Transportation Officials (AASHTO) in AASHTOT322. However, with the increased use of finer aggregate gradations and polymer modified asphalt binders in asphalt concrete mixtures, the validity of IDT strength results can be questioned in instances where significant crushing occurs under the narrow loading heads. Therefore, a new specimen configuration is proposed for indirect tension testing of asphalt concrete. In place of the standard loading heads, the specimen was trimmed to produce flat planes with parallel faces, creating a "flattened IDT." A viscoelastic finite element analysis of the flattened configuration was performed to evaluate the optimal trimming width. In addition, the numerically determined geometry was verified by means of laboratory testing of three asphalt concrete mixtures in two flattened configurations. This integrated modeling and testing study showed that when using fine aggregate gradations and compliant asphalt binders, crushing is significantly reduced while maintaining tensile stresses near the center of the specimen. Furthermore, creep compliances were evaluated using the flattened IDT and compared with those obtained followingAASHTOT322. Some variation was observed between the creep properties evaluated from the different geometries, particularly for higher compliance values. As a preliminary assessment, the flattened IDT seems to be a suitable geometry for the evaluation of indirect tensile strength of asphalt concrete. Further testing and analysis should be performed on the flattened IDT arrangement for evaluation of the creep compliance. This study provides an initial step towards a possible revision of the current AASHTO standard for IDT testing of asphalt concrete mixtures . Copyright © 1996-2011 ASTM.

Capability to effectively discretize a problem domain makes the finite-element method an attractive simulation technique for modeling complicated boundary value problems such as asphalt concrete pavements with material non-homogeneities. Specialized "graded elements" have been shown to provide an efficient and accurate tool for the simulation of functionally graded materials. Most of the previous research on numerical simulation of functionally graded materials has been limited to elastic material behavior. Thus, the current work focuses on finite-element analysis of functionally graded viscoelastic materials. The analysis is performed using the elastic-viscoelastic correspondence principle, and viscoelastic material gradation is accounted for within the elements by means of the generalized iso-parametric formulation. This paper emphasizes viscoelastic behavior of asphalt concrete pavements and several examples, ranging from verification problems to field scale applications, are presented to demonstrate the features of the present approach. © 2011 ASCE.

Asphalt concrete pavements are inherently graded viscoelastic structures. Oxidative aging of asphalt binder and temperature cycling due to climatic conditions are the major cause of such graded non-homogeneity. Current pavement analysis and simulation procedures either ignore or use a layered approach to account for non-homogeneities. For instance, the recently developed Mechanistic-Empirical Design Guide (MEPDG) [1], which was recently approved by the American Association of State Highway and Transportation Officials (AASHTO), employs a layered analysis approach to simulate the effects of material aging gradients through the depth of the pavement as a function of pavement age. In the current work, a graded viscoelastic model has been implemented within a numerical framework for the simulation of asphalt pavement responses under various loading conditions. A functionally graded generalized Maxwell model has been used in the development of a constitutive model for asphalt concrete to account for aging and temperature induced property gradients. The associated finite element implementation of the constitutive model incorporates the generalized iso-parametric formulation (GIF) proposed by Kim and Paulino [2], which leads to the graded viscoelastic elements proposed in this work. A solution, based on the correspondence principle, has been implemented in conjunction with the collocation method, which leads to an efficient inverse numerical transform procedure. This work is the first of a two-part paper and focuses on the development, implementation and verification of the aforementioned analysis approach for functionally graded viscoelastic systems. The follow-up paper focuses on the application of this approach. © (2010) Trans Tech Publications.

This paper describes a comprehensive investigation of strain tolerant type reflective crack relief interlayer systems through fundamental laboratory testing, computer aided design, and accelerated pavement testing. One of the widely used methods to control the reflection of cracks from underlying, cracked pavement into a new asphalt overlay involve the use of conventionally paved 'interlayers' that tolerate the very high tensile and shear strain that exists above cracks and joints in the underlying pavement. While these systems often slow down the rate of reflective cracking relative to untreated control sections in the field, when cracks do appear they are often offset from the location of the underlying discontinuity. A recently completed study sponsored by the National Science Foundation led to the development of a new fracture test (ASTM D7313-07b - the Disk-Shaped Compact Tension Test for Asphalt Concrete) and new techniques for finite element modeling of fracture in asphalt overlay systems. After successful validation of these tools on three field projects, it was decided to conduct further validation using the Advanced Transportation Loading System or ATLAS device and to experiment with new overlay configurations. A large experimental matrix was used to select promising interlayer materials and pavement layer and joint configuration details using finit e element analysis. A 500 ft(165 m) test pavement was constructed, instrumented, and tested in the cold of winter in 2008. This paper describes this comprehensive investigation, the new test sections developed, the types of distress observed under accelerated loading, and how the results were used to validate a new mechanistic analysis and design tool. Moreover, significant new insights towards the mechanisms and prevention of reflective cracking were obtained and have been summarized.

Reflective cracking of asphalt concrete (AC) overlays is one of the most extensive pavement distress and damage mechanisms in composite pavement structures. Numerous studies have been performed to evaluate the reflective cracking potential of AC overlays under different loading scenarios. Most of these studies have focused on reflective cracking due to tyre loading. A very limited amount of work has been performed to evaluate non-load-associated thermal reflective cracking of overlays. Thermal reflective cracking mechanisms have been studied and are described in this paper using recently developed hot-mix asphalt mixture tests and fracture models. A series of finite-element-based pavement simulations were performed in an effort to better understand thermal reflective cracking mechanisms as a function of several key material and pavement structure variables. The enhanced integrated climatic model was used to estimate pavement temperature gradients as a function of position and time. A fracture mechanics-based cohesive fracture model was used for the simulation of damage and cracking, which was tailored for use with quasi-brittle materials such as AC. The pavement simulation model utilises creep and fracture properties from American Association of State Highway and Transportation Officials and American Society for Testing and Materials-specified tests and analysis procedures. Three asphalt mixtures manufactured with Superpave low-temperature performance grades of -22, -28 and -34 were studied in pavement structures with three distinct overlay thicknesses. Simulations were conducted with three Portland cement concrete (PCC) slab conditions to study the effects of joint spacing and rubblisation on thermal reflective cracking. The simulation results provide a new insight towards the mechanisms underlying the development of thermal reflective cracking. The curling of PCC slabs due to temperature differential and joint opening caused by pavement cooling was found to be critical in the initiation of thermal reflective cracking. This effect is greatly minimised or eliminated in the case of pavement rubblisation. © 2010 Taylor & Francis.

This is the second article in a series of two papers describing simulation of functionally graded viscoelastic properties in asphalt concrete pavements. The techniques developed are applicable to other viscoelastic material systems with continuous, spatial grading of material properties. A full-depth asphalt concrete pavement has been simulated to demonstrate the applicability and importance of the graded viscoelastic analysis method. Based on the graded finite elements developed by Kim and Paulino[1], Buttlar et al. [2] used graded finite elements to determine typical responses to tire loading for an aged asphalt concrete pavement. In the current study, a similar pavement section is studied using the viscoelastic graded analysis (rather than elastic). Graded, layered and homogeneous material variations were used for a series of simulations, and the results from different approaches were compared. © (2010) Trans Tech Publications.

The use of recycled asphalt pavement (RAP) in hot-mix asphalt leads to significant environmental and economical benefits. The major benefits are realized through reduced demand for new or virgin aggregates and asphalt binders. A limited amount of research has been performed to investigate the effects of RAP on the low temperature fracture characteristics of asphalt mixtures. However, this is an important subject, since the inclusion of RAP in Hot-Mix Asphalt (HMA) has the potential to increase mixture stiffness and brittleness unless properly compensated for. Most design procedures for mixtures consisting of RAP recommend an asphalt binder grade adjustment to compensate for the aged binder through the use of a softer virgin binder. This study evaluates the validity of this grade adjustment procedure in the context of four RAP materials using mixture fracture testing. The RAP materials used in this study were chosen from a larger database of materials to represent a wide variety of binder aging levels. Various fracture tests for asphalt concrete were identified through literature review. The disk-shaped compact tension (DC(T)) test, as standardized in ASTM D7313-07b, was used in this study to characterize mixture fracture energy. DC(T) fracture tests were conducted on various asphalt mixtures, namely: a control mixture with no RAP, manufactured with PG 64-22 asphalt binder; four mixtures containing 30% RAP from different sources, with an adjusted virgin binder grade of PG 58-28, and; a reference mixture containing no RAP, manufactured using PG 58-28 binder. The test results indicate significant reduction in the fracture energy of asphalt mixtures with 30% RAP and no virgin binder grade compensation. The reduction in fracture energy was surprisingly consistent between the four RAP sources irrespective of the significantly different RAP binder stiffnesses that were determined to be present in these mixtures through binder recovery and testing. However, when the RAP mixtures were produced using a softer virgin binder (PG 58-28), the measured fracture energy was higher than the control mixture. The study provides some assurance that adequate cracking resistance can be maintained in mixtures containing 30% RAP when designed properly, and demonstrates how fracture testing with ASTM D7313-07b can be used in the design and control of RAP mixtures, particularly where cracking is of concern. © Chinese Society of Pavement Engineering.