AAPT
Abstracts for the March 10-12, 2003 Annual Meeting

ESTIMATING THE RUTTING POTENTIAL OF ASPHALT MIXTURES USING SUPERPAVE GYRATORY COMPACTION PROPERTIES AND INDIRECT TENSILE STRENGTH

by R. Michael Anderson, Donald W. Christensen, and Ramon Bonaquist

Abstract

The Strategic Highway Research Program (SHRP) ended with the introduction of the Superpave mix design and analysis system. The Superpave mix design process incorporated volumetric mix design followed by mechanical property testing and performance prediction analyses. Unfortunately, problems with the performance prediction models and the cost and complexity of the mechanical property tests required to provide input to these models reduced the Superpave mix design process to a volumetric mix design with no simple test for assuring adequate performance.
Several projects were initiated by the National Cooperative Highway Research Program (NCHRP) to study the issue of a simple performance test for asphalt mixtures. One of these projects, NCHRP Project 9-16, examined the use of properties available from the Superpave gyratory compactor (SGC) during the compaction process. The research was based on the hypothesis that some property of the asphalt mixture available during compaction in the SGC was related to the rutting resistance of the mix. The research found that properties during SGC compaction, such as compaction slope, appeared to be somewhat related to aggregate characteristics (gradation, particle shape/texture, and angularity) but were insensitive to asphalt binder characteristics such as asphalt binder content and grade (stiffness).
In a separate study of simple performance testing, the Pennsylvania Transportation Institute (PTI) and Advanced Asphalt Technologies (AAT) discovered that the indirect tensile strength of an asphalt mixture was related to mixture cohesion in the Mohr-Coulomb failure theory, and thus, was related to rutting potential. Unlike the SGC properties, the indirect tensile strength was sensitive to asphalt binder stiffness.
This research expands on the NCHRP 9-16 project and the research conducted by PTI and AAT to examine the possibility of using indirect tensile strength and SGC properties to predict the rutting potential of an asphalt mixture following the Mohr-Coulomb failure theory. The data indicates that a good, simple, rational model (R2 = 0.82, unadjusted) was identified that can predict laboratory rutting potential using indirect tensile strength, compaction slope, and VMA.

TERNARY PROPERTY MAPS FOR ASPHALT CONCRETE

by Donald W. Christensen, Jr., and Ramon F. Bonaquist

Abstract

Asphalt concrete can be considered a three-phase particulate composite. As such, its composition can be effectively represented using a triangular chart, in which the three axes are voids in mineral aggregate (VMA), effective binder content (Vbe), and air voids in total mix (VTM). Using triangular charts and mathematical models relating performance-related properties to volumetric composition, ternary property maps can be constructed. These charts graphically illustrate the complex relationships among volumetric composition and such properties as modulus, rut resistance, fatigue resistance, and permeability. Although construction of a simple triangular chart showing the composition of one or more asphalt concrete mixtures is straightforward, development of ternary property maps is a more complex procedure. On the basis of several ternary property maps presented in this paper, a few general observations can be made concerning the relationships between volumetric composition and the potential performance of asphalt concrete mixtures. Rut resistance tends to increase with decreasing VMA; fatigue resistance appears to increase with increasing Vbe and/or VFA. Permeability tends to decrease with increasing VFA and increasing aggregate fineness. Both minimum and maximum VMA values should be established for Superpave mixtures. Consideration should also be given to establishing minimum Vbe values linked to design traffic levels.

COMPARISON OF THE SATURATED SURFACE-DRY AND VACUUM SEALING METHODS FOR DETERMINING THE BULK SPECIFIC GRAVITY OF COMPACTED HMA

by L. Allen Cooley, Jr., Brian D. Prowell, and Mohd Rosli Hainin

Abstract

Typically, the bulk specific gravity of compacted hot mix asphalt (HMA) is determined using the saturated surface dry condition. Procedures for determining bulk specific gravity by this method are outlined within AASHTO T166 and ASTM 2726. However, within the HMA community there has been an increasing concern about determining the bulk specific gravity of compacted HMA by this method. This is especially true for mixes having coarse gradations like many Superpave designed mixes and stone matrix asphalt.
The concern with the saturated surface dry condition method is that these coarse-graded mixes contain a large proportion of interconnected air voids, especially in field compacted HMA. Large interconnected air voids that are connected to the surface of a sample potentially allows water to exit the sample during the patting of the sample to reach the saturated surface dry condition. This loss of the water from the sample causes the potential errors with the saturated surface dry method. Recently, a new method of determining the bulk specific gravity of compacted HMA has been developed. This method uses a vacuum-sealing device to tightly conform a plastic bag around compacted samples. A number of recent studies have shown that this method provides a good estimation of bulk specific gravity, even when samples have low density (i.e., high air voids). However, most of the work has focused on laboratory compacted samples (e.g., Superpave gyratory compactor samples). Also, guidance on when this new test method should be used instead of the saturated surface dry method has not been provided.

HIRSCH MODEL FOR ESTIMATING THE MODULUS OF ASPHALT CONCRETE

by Donald W. Christensen, Jr., Terhi Pellien, and Ramon F. Bonaquist

Abstract

The purpose of this paper is to present a new, rational and effective model for estimating the modulus of asphalt concrete using binder modulus and volumetric composition. The model is based upon an existing version of the law of mixtures, called the Hirsch model, which combines series and parallel elements of phases. In applying the Hirsch model to asphalt concrete, the relative proportion of material in parallel arrangement, called the contact volume, is not constant but varies with time and temperature. Several versions of the Hirsch model were evaluated, included ones using mastic as the binder, and one in which the effect of film thickness on asphalt binder modulus was incorporated into the equation. The most effective model was the simplest, in which the modulus of the asphalt concrete is directly estimated from binder modulus, VMA, and VFA. Models are presented for both dynamic complex shear modulus (|G*|), and dynamic complex extensional modulus (|E*|). Semi-empirical equations are also presented for estimating phase angle in shear loading and in extensional loading. The proposed model was verified by comparing predicted modulus and phase angles to values reported in the literature for a range of mixtures.

COMPARATIVE ANALYSIS OF AXIAL AND SHEAR VISCOELASTIC PROPERTIES OF ASPHALT MIXES

by Shadi Saadeh, Eyad Masad, Kevin Stuart, Ala Abbas, Thomas Papagainnakis, Ghazi Al-Khateeb

Abstract

The dynamic axial and shear moduli of asphalt mixes have been under consideration for many years by the pavement community as measures of the mix stiffness and its resistance to permanent deformation. However, previous research has shown that there are discrepancies between the results of the axial and shear tests. Limited work has been done in the past to explain these discrepancies. This paper presents detailed comparative analysis of the axial and shear tests in an effort to explain the factors influencing their results.
The comparative analysis is performed in this paper based on the results of testing four mixes with the same aggregate source and four, modified and unmodified, binders. The shear and axial tests were performed at two temperatures of 46 C and 58 C, a range of frequencies, and different stress (or strain) values.
In general, the results show that the modulus decreases and the phase angle increases with an increase in strain level in the shear test. However, these viscoelastic properties remain almost constant with an increase in stress level in the axial test. In addition, the Poisson's ratio calculated from Hooke's law, relating the axial and shear moduli, is significantly higher than that measured using the axial test.
The principles of shakedown theory, used in the past to describe the behavior of unbound aggregates, are modified in this study to explain the deformation and energy dissipation in asphalt mixes under dynamic loading. Consequently, these principles are used to highlight the factors that influence the results of the axial and shear tests. This paper shows that the differences between the axial and shear tests in terms of the mode of loading (controlled stress versus controlled strain), direction and reversal of principal stresses, and stress and strain distribution within a test specimen are the main factors causing the discrepancies between the shear and axial testing results.

RUGGEDNESS EVALUATION OF THE SHEAR FREQUENCY SWEEP TEST FOR DETERMINING THE SHEAR MODULUS OF ASPHALT MIXTURES

by R. Michael Anderson and Robert B. McGennis

Abstract

At the conclusion of the Strategic Highway Research Program (SHRP), the asphalt industry was introduced to the Superpave mix design and analysis system. This mix design system was developed by the SHRP researchers to include volumetric mix design, mechanical property tests, and performance prediction analyses for evaluating asphalt mixtures, particularly for pavements exposed to high traffic or high stress. The Superpave mechanical property test procedures conducted using the Superpave Shear Tester (SST) were adopted by the American Association of State Highway and Transportation Officials (AASHTO) as a provisional standard (AASHTO TP7).
As part of the implementation of the SHRP products, the Federal Highway Administration (FHWA) was charged with evaluating the "ruggedness" of the AASHTO TP7 method. A ruggedness experiment evaluates a proposed test procedure so that potential sources of variability can be identified and corrected before precision and bias testing is conducted.
This research discusses ruggedness testing that was conducted on the shear frequency sweep test at constant height (FSCH) described in AASHTO TP7. Although the SST tests were never fully implemented as envisioned by SHRP, there still are enough research agencies using shear tests to measure mechanical properties of asphalt mixtures to warrant an investigation into the variability inherent in the test procedures.
The ruggedness evaluation of the shear frequency sweep test indicated that test temperature had a significant effect on the shear modulus (stiffness) at 10 Hz loading frequency (G*10Hz). However, the temperature tolerance specified in AASHTO TP7 (+- 0.5 C) appeared reasonable, resulting in an expected difference of approximately 10% in G*10Hz. The percentage of air voids in the specimen also affected the value of G*10Hz, but not significantly according to the ruggedness evaluation. A tolerance of +- 0.5% in the specimen air voids should likewise be expected to change the G*10Hz value by no more than 10%. All other main factors were considered non- significant.
Although the NCHRP 9-19 research has indicated that axial tests are recommended for mechanical property testing of asphalt mixtures, the concept of ruggedness testing and effects of main factors should remain somewhat unchanged. In other words, the sensitivity of shear frequency sweep results to test temperature, percentage of air voids, or applied strain should be similar to the sensitivity of dynamic modulus results.

STRESS DEPENDENT BEHAVIOUR OF ASPHALT MIXTURES AT HIGH TEMPERATURES

bu Paul W. Antes, Arthur E. van Dommelen, Lambert J.M. Houben, Andr‚ A.A. Molenaar, and Umesh Parajuli

Abstract

In most of the commonly used design procedures the behavior of asphalt mixtures is considered to be linear. Either linear elastic or linear visco-elastic behavior is assumed. It is however a well known fact that most of the road engineering materials have a stress dependent behavior, their behavior is especially influenced by the magnitude of the confining stresses. This fact has not received the amount of attention as it should have received and the research findings in this field have hardly penetrated the common pavement-engineering field. This is surprising be-cause not taking into account the stress dependent nature of asphalt mixtures at elevated temperatures might result in significant errors in the prediction of stresses and strains in the bituminous layers and so in pavement predictions. This is especially the case if predictions on surface cracking and, permanent deformation have to be made and to some extent it might also affect the quality of our attempts to model raveling of surface mixtures.
The research that is described in this paper deals with the determination and the modeling of the stress dependent behavior of asphalt mixtures. To this end 5 different types of asphalt mixtures were subjected to a triaxial testing program in which the specimen were tested at various stress conditions, temperatures and loading frequencies.
The results of the testing program were used to model the resilient modulus of the mixtures as a function of the applied stresses. It appeared that a simple model could be developed that showed the dependency of the resilient modulus on the confining stress. It was determined that the effects are not negligible since the Mr values obtained from the experimental program were significantly different from the values determined using well known nomographs and equations to predict the Mr, or stiffness, value. It is believed that the results can be and should be used in design analyses based on "elastic" behavior.
In the second part of the program ample attention was paid to the stress dependency of the parameters of the Burger's model which is often used to describe the visco-elastic behavior of asphalt mixtures. In order to do so the axial strains calculated by means of the Burger's model were fitted to the measured axial strains. When the best fit was obtained, the values for the Burger's parameters that were used to obtain that fit, were taken as the true values for those parameters. The analyses showed that most of the Burger's parameters were stress dependent and also some unexpected frequency dependency was observed.
The results of the experimental program are now used to predict the permanent deformation of some accelerated pavement test sections which were already tested and which were made of the same asphalt mixtures.

SIMPLISTIC MIXTURE DESIGN USING THE SGC AND THE DSR

by Hussain Bahia, Eyad Masad, Anthony Stakston, Samer Dessouky, and Fouad Bayomy

Abstract

During the last decade major advances in methods for evaluation of asphalt mixtures and binders have been made. There is no doubt that the best mixture design should include not only volumetric properties, but also pre-failure (stiffness) properties and possibly damage related criteria. This however requires multiple testing machines, advanced analysis, and significant time. It is therefore clear that asphalt practitioners remain skeptical about the practicality of some of the tests being proposed and continue to look for a simplistic approach for initial selection of mixture components and a practical test or tests for quality control or consistency of mixtures. Most practitioners are aware of the importance of mechanical properties and damage characteristics, but they rightfully seek a simplistic test that can eliminate some of the possible combinations and reduce the required time.
Recently, several researchers have considered the Superpave Gyratory Compactor as one of the best starting points to capture performance related properties of mixtures or aggregate blends (1-7). It appears that there are merits for using the gyratory, with or without modification, to evaluate the potential of an aggregate blend to resist traffic loading as well as the potential for resisting compaction during construction.
This approach allows advancing the level of the Superpave Volumetric Mixture Design to a new level in which volumetric properties play only a partial role in selecting components and adds mechanical properties to the selection of the design blend and asphalt content. This is achieved without the need for a different testing machine and without significant addition of testing time. If the concept is proved valid, it could be done non-destructively in the sense that the sample generated can be used for other volumetric, durability (moisture damage), or mechanical properties. This leaves the question of how to estimate binder effects on critical failures such as rutting and fatigue cracking. Using recent advances in binder testing, binder contribution to rutting and fatigue can be measured at realistic pavement temperatures and traffic conditions.
This study is an attempt to summarize the concepts introduced to utilize the Superpave gyratory compactor to measure the mixture resistance to densification and distortion. These measurements are used to estimate quality of aggregate structure. A set of samples tested for the Wisconsin DOT is used which includes 4 different aggregate sources, various levels of aggregate angularity, different asphalt contents, and two different gradations. The study also covers the subject of using the DSR results to compliment the mixture design procedure by using the Creep and Recovery binder testing and the binder fatigue testing results as recommended by the NCHRP 9-10 project.

USE OF RECLAIMED ASPHALT PAVEMENT (RAP) UNDER SUPERPAVE SPECIFICATIONS

by Rebecca S. McDaniel and Ayesha Shah

Abstract

This project was conducted to investigate the performance of Superpave asphalt mixtures incorporating RAP. This study was closely coordinated with a national study on the same topic (NCHRP 9-12, Incorporation of Reclaimed Asphalt Pavement in the Superpave System). Specifically, this regional project looked at typical materials from the North Central United States to determine if the findings of NCHRP 9-12 were valid for Midwestern materials and to expand the NCHRP findings to include higher RAP contents.
Three RAP materials from Indiana, Michigan and Missouri were evaluated. Mixtures were designed and tested in the laboratory with each RAP, virgin binder and virgin aggregate at RAP contents up to 50 percent. The laboratory mixtures were compared to plant produced mixtures with the same materials at the medium RAP content of 15-25 percent. Binder and mixture tests were performed.
Briefly, the results showed that mixtures with up to 50 percent RAP could be designed under Superpave, provided the RAP gradation and aggregate quality were sufficient. In some cases, the RAP aggregates limited the amount of RAP that could be included in a new mix design to meet the Superpave gradation requirements. Linear binder blending charts were found to be appropriate in most cases. In general, increasing the RAP content of a mixture increased its stiffness and decreased its shear strain, indicating increased resistance to rutting if the virgin binder grade was unchanged. To limit cracking in RAP mixtures, especially with high RAP contents, decreasing the virgin binder grade is recommended. It is important to consider the RAP aggregate gradation and quality in the mix design, since a poor aggregate structure could reduce mixture stiffness and ultimately performance. Provided the RAP properties are properly accounted for in the material selection and mix design process, Superpave mixtures with RAP can perform very well.

EVALUATION OF THE INTERNAL ANGLE OF GYRATION OF SUPERPAVE GYRATORY COMPACTORS IN ALABAMA

by Brian D. Prowell, E. Ray Brown and Michael H. Huner

Abstract

The application of a compaction effort that will produce similar densities from one SGC to another is crucial to the proper design and production control and acceptance of HMA mixes. The angle of gyration is believed to be an important factor affecting the compaction effort. This study evaluated the relationship between the internal angle of gyration measured by the Dynamic Angle Validation Kit (DAVK) versus the resulting sample density for a wide range of SGC's. The DAVK device was used to verify that all of the SGCs used in the state of Alabama; by the contractors, mix designers, and the Alabama Department of Transportation (ALDOT); are compacting with an angle of gyration, under load, that is within the specification values (1.23 to 1.27 degrees) provided in AASHTO T312. Following the measurements with the DAVK, three replicate samples of a standard mix were compacted to evaluate the sample density produced by the compactor. This procedure was conducted on 116 SGC's throughout the state of Alabama.
Based on the data collected, there is an evident trend between internal angle of gyration and SGC type (Brand and Model). There is an evident trend between internal angle of gyration and sample density. On average there is a strong trend. The trend does not seem to hold true for the Interlaken and Rainhart compactors. However, there is a trend for the Interlaken compactors. Based on the averages by brand and model of SGC (excluding the Interlaken and Rainhart SGCs) a change in 0.1 degrees of internal angle will result in a change of 0.015 Gmb units or a difference in air voids of 0.6 percent. Though there is a trend between external angle of gyration and sample density for individual compactors, there is no trend between external angle of gyration and sample density for different brands of SGCs.

PERFORMANCE OF STONE MATRIX ASPHALT PAVEMENTS IN MARYLAND

by L. Michael, G. Burke, and C.W. Schwartz

Abstract

The Maryland State Highway Administration (MSHA) has constructed over 85 Stone Matrix Asphalt (SMA) projects totaling over 1300 lane miles since 1992. The SMA mixes have been placed exclusively on high volume, high speed highway segments. Analyses of nearly 1000 sets of construction quality control test data and approximately 300 sets of pavement performance measurements for up to 10 years of service life conclusively demonstrate the superior performance of SMA mixes in Maryland. Cumulative total rut depth for Maryland SMA projects averaged 0.14 inches for 12.5 mm mixes and 0.13 inches for 19 mm projects. Cumulative IRI values averaged 75.7 inches/mile for 12.5 mm mixes and 97.1 inches/mile for 19 mm projects. Average friction numbers at the last survey were 49.1 for 12.5 mm mixes and 46.5 for 19 mm projects. Mean annual changes in measured performance were less than 0.035 inches/year for rut depth, 1.8 inches/mile-year for IRI, and 1.3/year for friction number. Statistical analyses of Maryland SMA mix volumetric and gradation properties and pavement performance confirm that the Maryland SMA mixes have been well controlled with uniformly excellent performance.

CHARACTERIZATION OF ASPHALT CONCRETE IN UNIAXIAL TENSION USING A VISCOELASTOPLASTIC MODEL

by Ghassan R. Chehaba, Y. Richard Kimb, Richard A. Schaperyc, Matthew W. Witczakd and Ramon Bonaquiste

Abstract

The objective of the research presented herein is to develop an accurate and advanced material characterization procedure to be incorporated in the Superpave performance models system. The procedure includes the theoretical models and its supporting experimental testing protocols necessary for predicting responses of asphalt mixtures subjected to uniaxial tension loading. The model encompasses the elastic, viscoelastic, plastic and viscoplastic components of asphalt concrete behavior. Addressed are some of the major factors affecting asphalt concrete response, such as rate of loading, temperature and damage. The modeling strategy is based on developing separate models for strain components and then integrating those models to form a viscoelastoplastic model. The developed model accurately predicts responses up to localization when microcracks start to coalesce and grow. After that, fracture process zone strains detected using digital image correlation are used to extend the model's ability in predicting responses in the post-localization stage. However, once major macrocracks develop and propagate, the currently developed model ceases to predict responses accurately. At that state, fracture mechanics needs to be integrated with the current continuum damage-based model to predict the response.

FIELD VALIDATION AND PARAMETRIC STUDY OF A THERMAL CRACK SPACING MODEL

by David H. Timm and Vaughan R. Voller

Abstract

Thermal cracking of asphalt pavements is a common mode of distress leading to premature deterioration and failure of the pavement system. As such, much research has been devoted to the thermal cracking phenomenon. Recently, a mechanistic model that predicts the spacing between cracks has been developed. This model, that accounts for both the asphalt concrete surface layer and supporting granular medium, is briefly described. Further, a field validation study using live pavement test sections is presented. In addition, to better understand the model behavior, a parametric study is undertaken. It was found that model predictions compared favorably with field section observations. Also the crack spacing derived by the model was most influenced by the stiffness of the asphalt concrete and supporting layer, the thermal coefficient of contraction of the asphalt concrete, and the frictional properties of the unbound supporting layer. In the future, the model could be used to predict crack spacing as a function of environmental conditions and material types and help plan mitigation strategies.

DEVELOPMENT OF AN OVERLAY DESIGN MODEL FOR REFLECTIVE CRACKING WITH AND WITHOUT STEEL REINFORCING NETTINGS

by Imad L. Al-Qadi, Mostafa Elseifi, and Didier Leonard

Abstract

Hot-mix asphalt (HMA) overlays are typically used to rehabilitate cracked pavements. Cracks in the existing pavement move continuously due to thermal expansion and traffic loadings, and then propagate upward to the new pavement surface causing reflective cracking. Steel reinforcing nettings have been used successfully in Europe for the past two decades to improve HMA resistance to reflective cracking. By summer 2002, steel reinforcements were used in at least 15 projects in the US. To investigate the potential of steel reinforcing nettings to mitigate the reflection of cracks, a theoretical approach is presented based on Three-Dimensional (3D) finite element modeling. The 3D finite element model accurately simulates steel reinforcement as a non-homogeneous interlayer with openings. Both the crack initiation and propagation phases for the reinforced and non-reinforced cases were considered. Results of this analysis were then used to develop design equations that may be used to predict the overlay service life against reflective cracking with and without steel reinforcement. In general, based on finite element analysis (FEA), steel reinforcement was found to improve the overlay service life by a factor ranging from 50 to 90 percent depending on the overlay thickness and the pavement structural capacity.

EVALUATION OF WATER DAMAGE USING HOT MIX ASPHALT FRACTURE MECHANICS

by Bjorn Birgisson, Reynaldo Roque, and Gale C. Page

Abstract

Moisture damage in hot-mix asphalt (HMA) mixtures occurs when water can infiltrate the pavement system. Pore water in mixtures can cause premature failure of hot-mix asphalt pavements, primarily through loss of adhesion between the asphalt binder and the aggregates or the loss of cohesion in the asphalt binder. Loss of adhesion can lead to stripping and raveling. Loss of cohesion can lead to a weakened pavement that is susceptible to pore pressure damage and premature cracking. Depending on materials, loading, and environment, it may be that one or all of the mechanisms of water damage are present and dominant in an actual pavement. However, for a proper evaluation of any given mixture and testing procedure, it is necessary to isolate and quantify the effects of each of the predominant mechanisms contributing to moisture damage.
In this paper, two sets of mixtures were prepared. The first group involved fine-grained (above restricted the zone) and coarse-grained (below the restricted zone) limestone mixtures commonly used by the FDOT that were produced with multiple void structure and permeability configurations by varying the gradations and proportions for a common set of aggregates and asphalt cement. The second set of mixtures consisted of three granite-based mixtures commonly used by the FDOT. Nondestructive testing was used to identify and isolate the effects of pore water in mixtures after conditioning. Based on the results from the nondestructive testing, SuperPave IDT creep, resilient modulus, and strength tests were performed on conditioned and unconditioned mixtures without the complicating effects of the presence of pore water. The results illustrate the effects of moisture damage on the fracture properties of mixtures and the influence of aggregate type and gradation characteristics on moisture damage susceptibility. Water damage in mixtures is complicated by aggregate structure and aggregate type, so that each mixture property is affected differently and to different degrees by water damage from one mixture to another.
Based on the results in this paper, the use of a single parameter to evaluate moisture damage must be questioned. Rather, a single unified framework that accounts for changes in key mixture properties is needed to consistently evaluate the effects of water damage in mixtures. In this paper, the HMA fracture mechanics framework, developed at the University of Florida, is used to integrate the varying effects of water damage on key mixture properties into a single number (ratio of the number of cycles to failure after and before conditioning) that reflects the change in the cracking performance of the mixture due to water conditioning. The results show that HMA fracture mechanics provides a rational framework for the evaluation of moisture damage in mixtures that accounts for changes in multiple parameters, not just a single parameter. The approach presented can be used to evaluate the effects of water damage, independent of the conditioning procedure. Using a consistent framework for evaluating the detrimental effects of water damage, the effects of various different conditioning procedures can also be evaluated more thoroughly.

INVESTIGATIONS ON BITUMEN REJUVENATOR DIFFUSION AND STRUCTURAL STABILITY

by Robert Karlsson and Ulf Isacsson

Abstract

This paper compiles work carried out to enlighten the process of mixing old and new (rejuvenator) binders by diffusion during asphalt recycling, as well as studies of structural stability of recycled binders.
FTIR-ATR (Fourier Transform Infrared Spectroscopy by Attenuated Total Reflectance) was employed to monitor rejuvenator diffusion. Generally, diffusion rates are influenced by temperature, size and shape of the diffusing molecules and the viscosity of the medium in which the diffusion takes place. Since bitumen ageing normally leads to a considerable increase in viscosity, it was decided to investigate its influence on the diffusion process. The results presented in this study imply that it is the maltene phase viscosity that governs the diffusion rate, rather than the viscosity of the bitumen as a whole, and that bitumen ageing does not have a significant effect on the diffusion process. Diffusion was also monitored using rheological detection, which correlated fairly well with the rates of diffusion observed chemically using FTIR-ATR.
Structural stability of mixtures of old and new bituminous binders were studied using three-dimensional turbidimetric titration, where the three dimensions are a visualisation of the Hansen solubility parameters having dispersive, polar and hydrogen bonding components. The study was initiated to investigate the possible negative consequences of mixing old and new binders during asphalt recycling. The results regarding mixtures of old and new binders indicated that structural stability is not a limiting factor in asphalt recycling, provided that the old and new binders are compatible.

ROLE OF ADHESION AND THIN FILM TACKINESS OF ASPHALT BINDERS IN MOISTURE DAMAGE OF HMA

by Kunnawee Kanitpong and Hussain U. Bahia

Abstract

Moisture damage is well known to be an aggregate- asphalt problem affected by many factors among which asphalt chemistry, asphalt rheology, aggregate surface chemistry and physical properties play important roles. For many years the focus of moisture damage research has been focused on testing the complete mixture as used in the field. This approach, although realistic and logical in terms of simulating the total HMA system, is too complex to allow differentiating between the role of asphalt binders and the role of aggregates in damage. Thus, this approach is too difficult to use to determine fundamental bonding properties of asphalt binders and how they change with chemistry or with modification.
Although the asphalt research community accepts asphalts as adhesives, and although we keep referring to stickness of asphalts, we have not done enough to treat asphalts as adhesives and measure the stickness of different asphalts. No doubt there are many difficulties in differentiating between adhesion and cohesion of asphalts, but we can only start understanding these two mechanisms by acknowledging these as two possibly different properties and by focusing on developing systems to measure each.
This study is an attempt to treat adhesion separately from cohesion and to propose two systems to measure them independently. To measure adhesion characteristics of selected asphalts to various mineral surfaces, using a modified pull-off test (PATTI) is proposed. To measure cohesion, the Dynamic Shear Rheometer was used to develop a test protocol to measure thin film tackiness of asphalts. The Thin Film Tackiness is proposed as a relevant measure of cohesion under thin film conditions typically seen in asphalt mixtures.
The results offer evidence that we need to differentiate between adhesive failures and thin film cohesive failures. The results show that additives, including anti-strip and polymer additives, can affect adhesion differently than thin film cohesion. The study suggests that, although there is no replacement to testing the total HMA mixture, designing binders with improved adhesion properties and/or improved cohesive properties could be an essential step in improving resistance of HMA to moisture damage.

DIRECT TENSION TESTS - A USEFUL TOOL IN EVALUATION OF THE LOW-TEMPERATURE IMPROVEMENT IN AIR BLOWN ASPHALT

by Susanna Ho, Ludo Zanzotto, and Daryl MacLeod

Abstract

The low-temperature performance grade of asphalt binder can be graded according to Superpave MP1 using the bending beam rheometer technique (BBR) or the MP1a using a combination of BBR thermal stress curve and direct tension test (DTT) failure stress values to predict the critical cracking temperature (Tcritical).
It is well known that air blown asphalt demonstrates better low-temperature characteristics than the straight run material. In this study, we found that using the Superpave MP1 specification on the air blown or straight run asphalt from a crude known to produce good quality asphalt, the air blown asphalt definitely had better low temperature properties. When the MP1a specification was applied, this difference was even bigger.
However, according to Superpave MP1a, the Tcritical shows that there was an 11C low-temperature improvement in the air blown asphalt compared to the straight run asphalt produced from a waxy crude oil source. This improvement was not detected by the Superpave MP1 specification using the BBR technique since the low-temperature parameter was limited by BBR creep rate (m-value) failure. The DTT secant modulus and failure energy results show that even the low-temperature improvement indicated by the Tcritical was conservative since the air blown asphalt had lower stiffness and higher failure energy at Tcritical than the straight run asphalt from the same crude oil. The DTT technique thus allows us to appreciate the low-temperature improvement of air blown asphalts from waxy crude oils. This improvement would otherwise go undetected by the BBR technique.

INVESTIGATION OF THE USE OF RECYCLED POLYMER MODIFIED ASPHALT BINDER IN ASPHALT CONCRETE PAVEMENTS

by Louay N. Mohammad, Ioan I. Negulescu, Zhong Wu, Codrin Daranga, William H. Daly, and Christopher Abadie

Abstract

Since polymer modified asphalt cement (PMAC) has been employed for a decade, the lifetime and wear on of some of these pavements is reaching a stage where resurfacing will be necessary. This paper considers the potential problems associated with recycling PMAC's; in particular blending aged PMAC with tank PMAC. A styrene-butadiene-styrene PMAC was selected and characterized using typical asphalt binder qualification techniques, i.e., the Superpave PG protocol. Procedures were developed to separate the PMAC into its asphalt resin and polymer additive components as well as to characterize the relative concentrations of each component. Infrared and thermogravimetric spectrographic techniques were used to identify changes in the components as a result of aging. The impact of the extraction and recovery process on binder properties has been ascertained and found to be minimal.
An eight year old polymer modified asphalt binder was recovered from a wearing course mixture located in US61 Hwy, Livingston Parish, Louisiana. The field aged binder was characterized with respect to its composition and rheological properties. No residual polymer was detected and extensive oxidative age hardening of the binder had occurred. The binder was quite brittle at low temperature as demonstrated by both force ductility and bending beam tests. Extensive age hardening both chemically and rheologically and all changes in its properties due to aging were noted. In addition, blends of the US61 recovered binder with fresh PMAC were prepared and analyzed. The resultant blends showed much stiffer than anticipated under both low and high temperatures.
A 19 mm Superpave mixture containing blends of fresh PMAC with various percentages of US61 binder and original aggregates was evaluated by a suite of fundamental engineering tests including beam fatigue, indirect tensile strength and strain (ITS), indirect tensile creep, Asphalt Pavement Analyzer rut (APA), and repeated shear at constant height (RSCH). Test results demonstrate that as the increase of the percentage of US61 binder in mixtures, the rutting resistance increased, whereas the fatigue resistance decreased. Both the rutting factor of G*/sin(d) at high temperature and the fatigue parameter of G* sin(d) at intermediate temperature of Superpave binder were found to correlate fairly well with the results of mixture performance tests.