Eshan V. Dave

Effects of Recycled Asphalt Pavement Amounts on Low-Temperature Cracking Performance of Asphalt Mixtures Using Acoustic Emissions

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.

Thermal reflective cracking of asphalt concrete overlays

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.

IlliTC – low-temperature cracking model for asphalt pavements

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) utilised by American Association of State Highway and Transportation Officials Mechanistic-Empirical Pavement Design Guide relies heavily on phenomenological Paris law for crack propagation. The TCModel predictions are primarily based on tensile strength of asphalt mixture and do not account for quasi-brittle behaviour of asphalt concrete. Furthermore, TCModel utilises a simplified one-dimensional viscoelastic solution for the determination of thermally induced stresses. This article describes a newly developed comprehensive software system for LTC prediction in asphalt pavements. The software system called ‘IlliTC’ utilises a user-friendly graphical interface with a stand-alone finite-element-based simulation programme. The system includes a preanalyser and data input generator module that develops a two-dimensional finite element (FE) 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 FE analysis engine coupled with a fracture-energy-based cohesive zone fracture model. This article presents a comprehensive summary of the components of the IlliTC system. Model verifications, field calibration and preliminary validation results are also presented.

Synthesis of standards and procedures for specimen preparation and in-field evaluation of cold-recycled asphalt mixtures

The use of recycled asphalt (RA) materials in pavement rehabilitation processes is continuously increasing as recycling techniques, such as cold recycling (CR), are being utilised in increasing magnitude and greater awareness for use of recycled materials and consideration of sustainable practices is becoming common in the construction industry. The focus of this paper is on developing a state of the art and state of the practice summary of processes used for classification of RA as well as the curing and specimen preparation practices for cold-recycled asphalt mixtures. A variety of topics were explored through an exhaustive literature search, these include RA production methods, definition of RA materials, stockpiling practices, industrial operations, specimen curing and preparation practices and in-field evaluation of cold-recycled rehabilitation. This paper was developed through efforts of CR task group (TG6) of RILEM Technical Committee on Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems (TC-237 SIB).

Development of a Flattened Indirect Tension Test for Asphalt Concrete

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 AASHTO T322. 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 following AASHTO T322. 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.

Low Temperature Cracking Prediction with Consideration of Temperature Dependent Bulk and Fracture Properties

ABSTRACT Thermally induced cracking due to low temperature is a prominent pavement failure mechanism in colder regions. In recent years significant advances have been made in laboratory characterization and computer simulation of asphalt pavements in the context of low temperature cracking. With the advent of fracture energy test protocols that provide fundamental material separation characteristics, it is now possible to accurately simulate crack initiation and propagation in asphaltic materials through use of fundamental fracture mechanics tools in a practical manner. A cohesive zone fracture model allows for accurate representation of the quasi-brittle nature of asphalt concrete by modelling the highly nonlinear fracture process-zone ahead of the macro crack. Several authors have previously demonstrated that the asphalt material bulk and fracture properties vary significantly with temperature. While improvements in cracking prediction accuracy has been obtained through the use of temperature dependent bulk properties, the effect of temperature dependence on fracture properties also needs to be considered to achieve even more realistic simulation results. In the current study, computer simulation models have been proposed with temperature dependent bulk and fracture properties. These include: viscoelastic relaxation moduli, coefficient of thermal expansion and contraction, and fracture energy. A series of verification cases are presented to demonstrate the veracity of the numerical models developed and implemented. Five pavement sections from the MnROAD test site are simulated using the proposed model. The results are compared with previously reported simulation results as well as field data. The proposed model yields a better match between the simulated results and the field observations.

Thermal Cracking Prediction Model and Software for Asphalt Pavements

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 physicsbased representation to accurately capture the nonlinear fracture behavior of ratedependent 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.

Asphalt Pavement Aging and Temperature Dependent Properties through a Functionally Graded Viscoelastic Model, Part-I: Development, Implementation and Verification

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.

Graded Viscoelastic Approach for Modeling Asphalt Concrete Pavements

Asphalt concrete pavements exhibit severely graded properties through their thickness due to oxidative aging effects, which are most pronounced at the surface of the pavement and decrease rapidly with depth from the surface. Most of the literature to date has focused on use of layered‐elastic models for the consideration of age stiffening. In the current work, a graded viscoelastic model has been implemented within a numerical framework for the simulation of asphalt pavement responses under thermal and mechanical loading. The graded viscoelastic work is extension of the previous work by Paulino and Jin [1], Mukherjee and Paulino [2], and Buttlar et al. [3]. A functionally graded generalized Maxwell model has been used in the development of a constitutive model for asphalt concrete considering aging and temperature gradients. The aging gradient data from laboratory test results reported by Apeagyei [4] is used for obtaining material properties for the graded viscoelastic model. Finite element implementatio...

Flexible pavement thermal cracking performance sensitivity to fracture energy variation of asphalt mixtures

Thermal cracking in asphalt pavements continues to be a significant pavement distress mechanism in cold climate regions. The formation of discontinuities due to thermal cracking causes extensive damage to the integrity of the pavement and forms pathways for intrusion of water into the base and subgrade layers. Current use of the performance-based binder specifications has not been shown to effectively lower the propensity for this distress. When this is combined with advent of newer asphalt mix manufacturing and construction technologies as well as desire for incorporation of greater amounts of recycled materials in paving mixtures, it has led to significant research and implementation efforts on asphalt mixture performance-based specifications. On the basis of various past research studies on low temperature cracking, a performance specification that utilises fracture energy of asphalt mixtures through use of disk-shaped compact tension (DCT) test has been developed. A significant number of present studies are underway to implement these specifications including two states which are in the pilot implementation stage. A major question that has been raised during the implementation of these specifications has been in the lack of information on sensitivity of pavement thermal cracking performance to the variations in fracture energy of the mixture. The present study focused on determining the effects on thermal cracking performance of pavements for variations in DCT fracture energy of asphalt mixtures. The sensitivity was determined through use of the IlliTC thermal cracking simulation system. The IlliTC system utilises asphalt mixtures fracture and viscoelastic properties to conduct finite element-based simulations using realistic pavement thermal loading conditions. Although this system has been validated in the past, the present study conducted further validation through comparison of predicted cracking performance with field-measured cracking performance. For sensitivity analysis, three types of asphalt mixtures for three climatic conditions and three pavement structures were evaluated. Apart from other things, the fracture mechanics in asphalt concrete at lower temperatures depend on the materials fracture energy as well as its tensile strength. In order to ensure that the effects of fracture energy variations were the focus, a critical tensile strength value was determined for each scenario (for each mix for each climate), which allowed researchers to conduct the simulations with varying fracture energies (six different levels). The results show that a variation of 25 J/m2 in the fracture energy could lead to significantly different pavement thermal cracking performances. This is a significant finding that will aid in continued implementation of the fracture energy-based material specifications and provides guidance to transportation agencies in development of the final version of their material specifications.