AAPT
Abstracts for the March 8-10, 2004 Annual Meeting

Government Engineers Forum- Sunday 1:00 pm, Rita B. Leahy Presiding

EVALUATION OF MECHANICAL MIXTURE SIMULATION IN THE MEASUREMENT OF THE DYNAMIC INTERNAL ANGLE OF GYRATION

by T. Harman, G. Al-Khateeb, K. Stuart

Abstract

The Superpave gyratory compactor (SGC) fabricates laboratory specimens for volumetric and mechanical assessment under the Superpave asphalt mixture design system. The current standard practice specifying the operation of the SGC allows two different, non-equivalent methods of angle calibration: external and internal. The internal method employs the Dynamic Angle ValidatorTM (DAV), developed by the Federal Highway Administration (FHWA) in partnership with the Test Quip Inc. Pervious studies have demonstrated the need to calibrate Superpave gyratory compactors using internal angle measurement.(1) However, the DAV procedure, although effective, is labor intensive and time consuming requiring up to a day for the required angle measurements and compactor calibration.
A mechanical system has been developed to work with the DAV to simulate mixture resistance during angle measure. The Hot-Mix Simulator (HMS) greatly reduces the time required for measurement and calibration.
This study employs a commercial version of the gyratory load-plate developed by the University of Wisconsin, Madison to characterize forces and eccentricities present during mixture gyratory compaction to evaluate the effectiveness of the HMS.
The use of mechanical simulation to eliminate mixture for the measurement process has potentially significant time and monetary benefits. The HMS developed for use with the DAV appears to simulate the mixtures used in this study. One of the HMS wedge and plate assemblies provided results within 0.01 degrees of the DAV with mix.

IMPLEMENTATION STATUS, ASSESSMENT AND BENEFITS OF SUPERPAVE

by V. Tandon, I. Avelar, E. Rodzik, J. D'Angelo

Abstract

To overcome premature failure problems and improve the nation's highways, a five year 150 million dollar Strategic Highway Research Program was initiated in 1987. A major component of this research was a 50 million dollar asphalt program that led to the development of Superpave.
At the end of Strategic Highway Research Program, the Federal Highway Administration assumed responsibility for further development and validation of Superpave system. It also initiated a national program to encourage the adoption of the system by all state highway agencies. The American Association of State Highway and Transportation Officials Task Force on Strategic Highway Research Program implementation developed the concept of Lead States for uniform implementation of Superpave. Although initial implementation progress and state highway agency needs were documented by the Lead States, the implementation status and needs beyond 2000 are not known. This information is essential for future implementation planning, allocation of resources, and current costs of Superpave mixes in comparison of conventional mixes.
To identify Superpave implementation status, benefits and concerns, a review of existing literature was performed and state highway agencies were contacted. Based on the gathered information, a questionnaire was developed and state highway agencies, Superpave centers, User Producer groups, and hot mix asphalt contractors were surveyed. The literature review information and results of the survey are presented in this paper.

Workshop Session-Monday 10:15 am, Rita B. Leahy Presiding

LABORATORY EVALUATION OF SECONDARY AGGREGATES IN BITUMINOUS MIXTURES

by G. Airey, A. Collop, N. Thom, S. Zoorob, A. Shiratori

Abstract

With a greater understanding of the need for sustainable development, the use of primary aggregates in asphalt mixtures for road or airfield pavements is seen as a wasteful use of a finite natural resource. Therefore the reuse of primary aggregates and/or the use of waste (secondary) materials are seen as being of benefit to society. Of the various waste streams, the by-products of the iron and steel making industries (blast furnace and steel slags) and recycled crushed glass (cullet) can be considered sensible alternative sources of aggregate for asphalt mixture production. These secondary aggregates have similar physical properties to conventional, primary aggregate and can be processed, crushed and screened into practical sizes for easy batching into both surfacing and base asphalt materials.
This paper assesses the mechanical performance and durability of a range of both base and surfacing materials incorporating different combinations, size fractions and percentages of two primary aggregates (limestone and gritstone) and three secondary aggregates (basic oxygen steel slag, blast furnace slag and glass cullet). The mechanical properties of the asphalt mixtures have been measured using the suite of tests (stiffness modulus, resistance to permanent deformation and resistance to fatigue cracking) possible with the Nottingham Asphalt Tester (NAT). The durability of the primary and secondary mixtures has been assessed by subjecting the materials to simulative long-term laboratory ageing and moisture susceptibility conditioning using recognised testing (conditioning) procedures and protocols.
The results indicate that the use of glass cullet fine aggregate in a 28 mm continuously graded base material only marginally reduces the stiffness modulus of the secondary aggregate modified mixture. The moisture susceptibility of the material was also shown to be less than what would be expected for a smooth surface textured aggregate such as glass, with and without the use of an anti-stripping agent. The replacement of primary aggregate with glass cullet also significantly reduced the ageing susceptibility of the mixture, while the permanent deformation resistance, although inferior to that of a primary aggregate mixture, was still acceptable with the fatigue performance being comparable to the control mixture.
The use of basic oxygen steel slag and blast furnace slag secondary aggregates significantly increases the mixture density and stiffness modulus compared to primary aggregate mixtures. The moisture susceptibility of these secondary aggregate mixtures was also found to be similar to that of the control mixtures, although the slag mixtures did show an increased susceptibility to age hardening. Overall the permanent deformation resistance and fatigue performance of the slag mixtures tended to be similar to that of the limestone and gritstone base and surfacing control mixtures.

PRELIMINARY INVESTIGATION OF A TEST METHOD TO EVALUATE BOND STRENGTH OF BITUMINOUS TACK COATS

by G. Sholar, G. Page, J. Musselman, R. Upshaw, H. Moseley

Abstract

It is generally recognized that a bituminous tack coat is beneficial for improving the bonding strength between two hot-mix asphalt layers. It is also qualitatively recognized that moisture on the surface of the tack coat can impede the bonding performance of the tack coat. Furthermore, varying tack coat application rates and aggregate interaction between hot-mix asphalt layers are also considered to have an effect on the bonding performance of the tack coat. In an effort to quantify the effects of moisture, tack coat application rate and aggregate interaction on bonding performance, a test apparatus and procedure were developed. Three field projects were also constructed and evaluated at various time intervals. Results indicate that water applied to the surface of the tack coat, representing rainwater, significantly reduced the shear strength of the specimens when compared to equivalent sections without water applied. Varying tack coat application rates within the range of 0.091 to 0.362 L/m2 had less of an effect on shear strengths. The use of a tack coat to increase bonding strength was more effective for fine graded mixtures compared to coarse graded mixtures. Aggregate gradations of the mixtures being bonded together played a critical role in the magnitude of the shear strengths achieved. Fine graded mixtures achieved significantly lower shear strengths than the coarse graded mixtures. A field project containing a milled interface achieved the greatest strengths of the projects tested. The single-operator standard deviation of the test procedure was determined to be 66.2 kPa.

SUPERPAVE DESIGN COMPACTIVE EFFORT: VALIDITY OF USING DENSITY AT THE END OF SERVICE LIFE AS A PARAMETER TO DEFINE N- DESIGN

by G. Huber, R.M. Anderson

Abstract

When Superpave was introduced in 1993, the method included a new Superpave gyratory compactor and a table of design compaction (N-design) based on the premise that density in the laboratory compactor should match density at the end of service life. The experiment performed during the Strategic Highway Research Program (SHRP) was limited to pavements at least 12 years old. Post SHRP attempts to expand that database to include pavements of all ages and refine the table of design compaction have been unsuccessful.
Changes to the design compaction have been recommended by the Superpave Mixtures and Aggregates Expert Task Group based on compactability of the aggregate skeleton to changes in N design and based on changes in mixture stiffness with changes in N design.

Session I-Monday 1:30 pm, John D'Angelo Presiding

MECHANICAL CHARACTERIZATION OF COMBI-LAYER

by M. van de Ven, A. Molenaar

Abstract

Combi-layer consists of a porous asphalt concrete layer with very high voids content (25 percent), of which the voids are filled with a cement slurry. In this paper research is reported into the mechanical characterisation of combi-layer. This was necessary because combi-layer is seen by many asphalt technologists as an intermediate between asphalt concrete and cement concrete with some concrete-like properties at high temperatures. No relevant material characterisation was available from literature.
The normal testing procedures for the mechanical characterisation of asphalt concrete mixtures have been executed. From the test results master curves for the stiffness over a large temperature area, indirect tensile strength and fatigue lines at design temperatures are reported. Because of the suggested difference with the standard asphalt concrete mixes a fundamental characterisation of the material was deemed necessary. Uni-axial tension and compression tests were performed at several temperatures and strain rates. In this paper monotonic tension and compression tests with a constant strain rate are reported.
In the paper the uni-axial properties of the combi-layer material are compared with asphalt concrete and cement concrete. From the comparison it becomes clear that the combi-layer behaves similar to an asphalt mix. At high temperatures it cannot perform without confinement under loading like cement concrete. This is best shown with the temperature dependence of the material properties.

THE NEED FOR INDUCING SHEAR INSTABILITY TO OBTAIN RELEVANT PARAMETERS FOR HMA RUT-RESISTANCE

by B. Birgisson, D. Darku, R. Roque, G. Page

Abstract

Instability rutting is a major distress mode in hot mix asphalt pavements, which occurs when the structural properties of the compacted mix are inadequate to resist critical near surface stress states consisting of high shear stresses and low confinement. On the mixture level, instability rutting is manifested in a rearrangement of the aggregate structure. This paper illustrates that it may be necessary to induce instability in mixtures to determine parameters that are relevant to mixture rut resistance. The SuperpaveTM gyratory compactor with shear measurements and the option of changing the compaction angle during compaction was used in this study. Mixtures were compacted to a density that is consistent with the air voids of field pavements immediately after construction, namely 7 percent (ñ 0.5) percent air voids. The subsequent application of high shear stresses by increasing the gyratory angle from 1.25 degrees to 2.5 degrees results in the rearrangement of the aggregate structure during which parameters can be measured that define clear differences in mixtures. Based on the testing of 31 mixtures with different void structure and aggregate characteristics, three distinctive responses during were observed: a) brittle response, b) plastic response, where once the mixtures loose strength due to rearrangement, they never regain another stable rearrangement, and c) optimal response, which is bracketed by the plastic and brittle types of responses. Key parameters were identified that are relevant to the rut-resistance of mixtures. Using these new parameters, a framework for the evaluation of mixture rutting potential is developed. Based on a comparison of predicted rutting potential to APA rut depth measurements, as well as a statistical analysis of results, the validity of the proposed framework is established. The results show that the proposed framework has may have the potential for becoming an index test for evaluating the rutting potential of mixtures.
Based on the findings of this paper, the proposed approach for evaluating mixture rut- potential should be studied further and validated for an even larger variety of asphalt mixtures.

DAMAGE EVOLUTION IN TRIAXIAL COMPRESSION TESTS OF HMA AT HIGH TEMPERATURES

by L. Tashman, E. Masad, D. Little, R. Lytton

Abstract

This paper presents the results of an experiment aimed at capturing and characterizing damage evolution in hot mix asphalt (HMA) at relatively high temperatures. HMA specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. X-Ray computed tomography (CT) was used to capture the microstructure of the HMA specimens before and after they were deformed, and image analysis techniques (IAT) were used to characterize the evolution of air voids and cracks in the deformed specimens. Image analysis techniques were developed to distinguish between air void growth and crack evolution. This is extremely important as the underlying mechanisms for these two phenomena are different. The term "voids" is used throughout this paper to refer to air voids and cracks combined.
Damage in HMA is shown to initiate following a period of microstructure hardening. The damage is found to be a localized phenomenon in the sense that there exists a critical section in a specimen that is mainly responsible for failure. The growth and propagation of cracks in this critical section was significantly larger than that in the rest of the specimen. The study shows that the top part of the tested specimens exhibited significant cracking, the middle part exhibited significant dilation, and minor microstructural changes occurred in the bottom. These variations within a specimen are attributed mainly to the heterogeneity of the HMA microstructure. The results of this study are expected to have significant impacts on the current state of knowledge with regard to damage evolution due to permanent deformation at high temperatures. The results demonstrate that there is a pressing need to prepare homogenous specimens in the laboratory. Otherwise, localization due to microstructure heterogeneity will render the interpretation of laboratory testing results a very difficult task. More importantly the findings of this study will influence the approaches typically followed in the development of permanent deformation models. These models should account for the effect of damage localization on accelerating permanent deformation.

EFFECT OF BINDER AND MIXTURE VARIABLES ON GLASS TRANSITION BEHAVIOR OF ASPHALT MIXTURES

by K. Nam, H. Bahia

Abstract

The thermal coefficient of contraction of asphalt mixtures is one of the required properties in the prediction of thermal cracking of asphalt pavements. Due to the significant influence of asphalt binders on asphalt mixtures' thermal properties, asphalt mixtures are expected to show a significant glass transition behavior. In the current practice, however, the thermal coefficient of contraction of mixtures is assumed to be linear, and determined from a simplistic relationship with binders that takes into account volumetric and physical mixture properties. Although measuring thermal properties of asphalt mixtures has been a long-standing research topic for the last decade, a limited number of studies have explored the methods for the direct measurements of these properties. Using the findings of previous studies, a system was developed in this project for the direct measurement of change in linear length of mixtures as a function of change in temperature. The system was used to show the significant glass transition experienced by mixtures and to evaluate the effect of mix variables on the thermal properties.
The investigation included the thermal properties of 23 mixture specimens produced from six types of asphalt binders, two sources of aggregates, and two levels of gradation. In the test, a wide temperature range from +40 C to 90 C induced a thermal change in the specimens. In order to study the effect of cooling rates on the properties of interest, two different cooling rates were applied while the specimens' thermal contraction was continuously measured. The measured data allowed an estimation of the Glass Transition (Tg) and thermal coefficients of contractions above and below Tg for each mixture. Statistical analysis was conducted to relate the mix variables with the thermal properties.
The results indicated the developed system is practical and repeatable, offering a means for reliable evaluations of glass transition of the asphalt mixtures. The results showed the need for considering dual thermal coefficients of contraction and that such an approach would provide more accurate results in the prediction of thermal stress. The results also indicated that the asphalt binder glass transition temperature and aggregate type appeared to have important effects on the thermal properties.

THE EFFECT OF VERTICAL INHOMOGENEITY ON COMPRESSIVE PROPERTIES OF ASPHALT MIXTURES

by H. Azari, R. McCuen, K. Stuart

Abstract

Vertical inhomogeneity, which is defined as the separation of the original mix design gradation into finer and coarser gradations along the depth of the specimen, is often observed in laboratory compacted asphalt mixture specimens. This phenomenon is believed to be the result of the heavier, coarse aggregates gravitating to the bottom of the mold thus preventing the fine aggregates from sinking. Also, the kneading effort of compaction forces the larger particles to the bottom of the mold. While inhomogeneities exist, the effect of the vertical inhomogeneity on mechanical properties of asphalt mixtures is not known. To study this effect, homogeneous and vertically inhomogeneous asphalt mixtures specimens were fabricated. In order to characterize the degree of inhomogeneity, non-destructive x-ray computed tomography (CT) was used to scan cross-sectional images of the specimens. An index of inhomogeneity was developed to evaluate the scanned images. The results clearly showed that the index could distinguish between homogeneity and inhomogeneity. The compressive modulus of the mixture was evaluated at test temperatures of 21 C and 45 C by means of frequency sweep simple performance dynamic modulus test. The resistance of the material to axial permanent deformation was measured using the simple performance flow number test at 45 C. The effects of vertical inhomogeneity on the compressive properties of the specimens were investigated at the two test temperatures. Neither dynamic modulus at both test temperatures nor the permanent axial deformation changed significantly with vertical inhomogeneity. Although, the correlation between the compressive properties and the inhomogeneity index is generally very poor, the correlation is higher at the test temperature of 21 C than at the test temperature of 45 C. The results of this study indicate that simple performance tests are not sensitive to even an extreme level of vertical inhomogeneity. Regardless of the separation of the original mixture to the coarser and the finer gradations, an average property value that is comparable to the properties of homogeneous specimens is measured. This shows that predicting the performance of the material in the field based on the compressive properties of laboratory-made specimens as they are measured using Simple Performance Tests could be reliable.
KEYWORDS: asphalt; pavements; statistics; homogeneity; compressive properties; nondestructive testing; tomography; image analysis.

Session II-Tuesday 9:00 am, Gerald Huber Presiding

PERFORMANCE-BASED PAY FACTORS FOR ASPHALT CONCRETE CONSTRUCTION; COMPARISON WITH A CURRENTLY USED EXPERIENCE-BASED APPROACH

by C. Monismith, L. Popescu

Abstract

The paper briefly summarizes a procedure to quantitatively establish pay factors for asphalt concrete pavement construction using performance models for fatigue and rutting based on the analysis of accelerated pavement tests from the Caltrans Heavy Vehicle Simulator (HVS) and the WesTrack accelerated pavement performance test program. For rutting, the influence of asphalt content, air-void content, and aggregate gradation are considered. For fatigue, air-void content, asphalt content, and asphalt concrete thickness are included. Costs are established using a cost model considering only agency cost consequences of delaying or accelerating the time to the next rehabilitation. For the as-constructed mix, the relative performance (RP) -- the ratio of off-target ESALs to target ESALs -- is determined for both fatigue and rutting. The shortest RP for the combined RP's for mix and pavement characteristics considered for a specific distress mode permits determination of the pay factor from the cost model.
Pay factors determined by this methodology are compared with those that have been used by the California Department of Transportation (Caltrans), termed an experience-based approach. This set of pay factors has been in use since 1997 for Quality Control/Quality Assurance projects. Comparisons are included for approximately 80 projects constructed in the period January 1997 to June 2000. Only a limited number of these contained measured rutting and fatigue cracking data that had been incorporated in the Caltrans Pavement Management System. Comparisons of pay factors for these projects determined by the two methodologies are also included. An analysis is presented which compares pay factors calculated at the end of the project based on the total tonnage to the average of the pay factors based on the daily total tonnage.
A computer program using the performance based approach is also included.

IN-PLACE DENSITY EVALUATION OF STONE MATRIX ASPHALT (SMA) MIXES IN ALABAMA

by M. Buchanan, J. Turner, J. Barton

Abstract

The accurate measurement of the in-place density of hot mix asphalt (HMA) mixtures is essential for quality control and assurance, performance prediction, and pay factor determination. Currently the most common methods for in-place density determination through cores and nuclear density gauge testing.
An evaluation of the in-place density determination using cores and nuclear gauge testing was conducted for four stone matrix asphalt (SMA) and one Superpave HMA mix in Alabama. The use of surface filler with nuclear gauges was evaluated along with different method of nuclear gauge calibration as methods to improve the accuracy and precision of the nuclear gauges relative to the core density. Additionally, the effect of lift thickness to nominal maximum aggregate size on the achieved in-place density was investigated for each of the mixes.
The results clearly indicate that the use of surface filler results in improved accuracy and precision relative to the nuclear gauge without surface filler being used. Also, the use of a 10- point forecasting method of gauge calibration was found to yield the most accurate and precise "predicted core density" results of the methods evaluated. No relationship for any of the mixes evaluated was determined between the in-place density and the lift thickness to nominal maximum aggregate size.

DEVELOPMENT AND FIELD EVALUATION OF ENERGY-BASED CRITERIA FOR TOP-DOWN CRACKING PERFORMANCE OF HOT MIX ASPHALT

by R. Roque, B. Birgisson, C. Drakos, B. Dietrich

Abstract

It is now well-recognized that top-down cracking is a major form of distress in hot mix asphalt pavements A detailed analysis and evaluation of 22 field test sections throughout the state of Florida resulted in the development and verification of energy-based criteria for top-down cracking of hot mix asphalt. The work clearly indicated that there is no single mixture property or characteristic that can reliably predict top-down cracking performance of hot mix asphalt. A parameter termed the Energy Ratio, which was derived using the HMA Fracture Mechanics Model developed at the University of Florida, was determined to accurately distinguish between pavements that exhibited top-down cracking and those that did not, except for mixtures with excessively low or unusually high dissipated creep strain energy thresholds. The Energy Ratio, which is defined as the dissipated creep strain energy threshold of the mixture divided by the minimum dissipated creep strain energy required, is determined on the basis of tensile properties that can be obtained from a modulus, creep, and strength test performed with the Superpave IDT at a temperature of 10 C. The Energy Ratio accounts for the effects of pavement structural characteristics on top-down cracking performance. Therefore, it can be used to suitably integrate asphalt mixture properties in the pavement design process. In addition, a rational approach was developed to adjust the minimum Energy Ratio criterion for different traffic level pavements. In conclusion, two energy-based criteria were recommended to control top-down cracking of hot mix asphalt: 1) a minimum dissipated creep strain energy threshold; and 2) a minimum Energy Ratio for mixtures with a dissipated creep strain energy threshold greater than the minimum.

PERFORMANCE OF COARSE-GRADED SUPERPAVE HMA MIXTURES

by L. Mohammad, Z. Wu, A. Raghavendra, C. Abadie

Abstract

EVALUATION OF RUTTING PERFORMANCE ON THE 2000 NCAT TEST TRACK

by E.R. Brown, B. Prowell, A. Cooley, J. Zhang, R. Powell

Abstract

The primary objective of the 2000 NCAT Test Track was to provide an accelerated loading facility that could be used to rapidly test a large number of test sections simultaneously. Primarily, the test sections were comprised of materials selected by the different sponsors to answer local questions about the performance of their HMA under accelerated loadings. However, results from the Track allowed comparisons between laboratory rutting tests and field performance for a number of mixes subjected to similar loadings that were representative of actual traffic. Also, several mini experiments were also evaluated: performance of fine graded vs. coarse graded mixes, effect of asphalt binder grade on performance, effect of aggregate type on performance, and performance of several mixture types including Superpave, SMA, and Open Graded Friction Courses. The results indicated that modfied binders densified less and produced less deformation than unmodified binders. The amount of permanent deformation on the track was negligible. Comparisons between laboratory performance tests and field deformation indicated little correlation. The repeated load creep test produced the best correlation but was highly variable.

By Title Only CONCEPTUAL PERFORMANCE CRITERIA FOR ASPHALT MIXTURES

by T. Pellinen

Abstract

The empirical nature of performing mix designs has led to significant differences in the volumetric composition for an acceptable asphalt mix. A key to the successful mix design is the balance between the volumetric composition and the raw materials used (binder, aggregate, filler, and additives) at specific climatic and traffic conditions. Thus, a balanced design can be achieved only when the climatic considerations are taken into account when selecting the volumetric criteria. A delicate balance between mixture stiffness and shear strength appears to provide the key to a successful design. Based on these findings, a fundamentally-based conceptual performance criteria is presented; the criteria rely on stiffness and shear strength to explain the mechanical behavior of asphalt mixtures.

Session III-Wednesday 9:00 am, Larry Michael Presiding

SIMPLIFYING THE HOLLOW CYLINDER TENSILE TEST PROCEDURE THROUGH VOLUME-BASED STRAIN

by W. Buttlar, M. Wagoner, Z. You, S. Brovold

Abstract

EVALUATION OF THE LOW TEMPERATURE FRACTURE RESISTANCE OF ASPHALT MIXTURES USING THE SEMI-CIRCULAR BEND TEST

by X. Li, M. Marasteanu

Abstract

THE VIRGINIA SMART ROAD: THE IMPACT OF PAVEMENT INSTRUMENTATION ON UNDERSTANDING PAVEMENT PERFORMANCE

by I. Al-Qadi, A. Loulizi, M. Elseifi, S. Lahouar

Abstract

This paper presents the description, calibration procedures, installation procedures, and performance of the instrumentation used at the Virginia Smart Road to measure flexible pavement response to loading. Also presented are the measured horizontal transverse and longitudinal strains induced in the hot mix asphalt (HMA) during compaction with a steel drum compactor both with and without vibrations. In addition, this paper presents the data collected and used to determine the vertical compressive stress pulse induced by a moving truck at different locations beneath the pavement surface. This data was also used to determine the effects of temperature, speed, and tire inflation pressure on the measured vertical compressive stress and measured horizontal transverse strain, induced by a steering-axle tire of 25.8kN, under the HMA layer. The data was used make a comparison between measured pavement responses to truck loading with those calculated using linear elastic theory. It was found that HMA mixtures are subjected to very high horizontal strains during compaction especially when vibration is used. It was also found that a Haversine equation well represents the measured normalized vertical compressive stress pulse for a moving vehicle. Haversine duration times varied from 0.02s for a vehicle speed of 70km/h at a depth of 40mm to 1.0s for a vehicle speed of 10km/h at a depth of 597mm. As expected, temperature was found to significantly affect the measured vertical compressive stress and measured horizontal transverse strain under the HMA layer. Although speed was found not to affect the magnitude of the measured vertical compressive stress, it was found to affect the loading time. On the other hand, speed was found to significantly affect the measured horizontal transverse strain under the HMA layer. Variation in tire inflation pressure from 551.6kPa to 724kPa was found not to affect the measured vertical compressive stress and the measured horizontal transverse strain at the bottom of the HMA layer. A comparison between the measured responses and those calculated using a finite element model that uses linear elastic theory indicated that the elastic theory overestimates pavement responses at low temperatures but significantly underestimates these responses at high temperatures. An improved prediction of pavement responses was achieved by modifying the bonding conditions at the interfaces, and by modeling HMA as a viscoelastic material.

CHARACTERIZATION OF AGGREGATES AND ASPHALT CONCRETE USING X-RAY COMPUTERIZED TOMOGRAPHY

by L. Wang, H. Paul, T. Harman, J. D'Angelo

Abstract

X-ray computerized tomography is a viable non-destructive method to material characterization. Its capability of obtaining three-dimensional (3D) material structure enables the characterization of aggregates and asphalt mixture on a new horizon promising applications in characterization, modeling and computational simulation to optimize mix design, predict performance and conduct forensic studies. This paper reviews the fundamentals of X-ray Computerized Tomography (XCT), its capability, and recent development in applying this tool to characterize aggregates and asphalt mixtures.

USE OF GPR FOR THICKNESS MEASUREMENT AND QUALITY CONTROL OF FLEXIBLE PAVEMENTS

by I. Al-Qadi, S. Lahouar

Abstract

This paper evaluates the performance of ground penetrating radar (GPR) in estimating the layer thicknesses of flexible pavements. The findings presented are based on field data collected from the Virginia Smart Road's pavement test facility, from a newly built section of a state highway (Route 288, Virginia), and from a section of an in-service interstate highway (I-81). The GPR data collected from the Virginia Smart Road was successfully used to evaluate the physical GPR detection limitations and to evaluate GPR's accuracy for flexible pavement layer thickness determination. The data analysis was facilitated by a complete knowledge of the different structures and compositions of the various sections of the road, and by copper plates (perfect electromagnetic reflectors) that were embedded at the different layer interfaces during the construction of the pavement.
Based on the collected data, it was found that current GPR analysis approach fails to detect layer interfaces in some circumstances because of physical limitations (such as low dielectric constant contrast between the layers combined with relatively high material loss or the presence of thin layers, compared to the GPR resolution, within the surveyed pavement system). However, GPR is a feasible nondestructive tool for estimating the layer thicknesses of HMA layers, provided that the correct data analysis technique for the type of surveyed pavement is used. For newly constructed pavements, GPR can be successfully used as a quality control/quality assurance tool using simple data acquisition techniques. However, for in-service pavements or pavements composed of multiple thin layers, special data analysis techniques should be applied to achieve reasonable thickness accuracy.

By Title Only A REALLY SIMPLE PERFORMANCE TEST

by I. Oh, B. Coree

Abstract

A simple, SGC-based performance test is proposed. Initial validation against the Nottingham Asphalt Tester is positive. Field validation is in progress.

Session IV-Wednesday 1:00 pm, Frank Fee Presiding

FATIGUE EVALUATION OF ASPHALT MIXTURES USING DISSIPATED ENERGY AND VISCOELASTIC CONTINUUM DAMAGE APPROACHES

by J. Daniel, W. Bisirri, Y. Kim

Abstract

The fatigue performance of asphalt mixtures has historically been evaluated using phenomenological approaches. As the asphalt industry moves towards more mechanistic and performance-based design methods, researchers have developed more fundamental approaches to evaluate fatigue. This paper presents a comparison of the viscoelastic, continuum damage (VECD) and dissipated energy (DE) approaches using uniaxial direct tension fatigue tests performed on eight WesTrack mixtures. The two approaches are also compared to the traditional phenomenological approach relating the initial strain to the number of cycles to 50 percent reduction in initial stiffness. The number of cycles to failure calculated using the VECD and DE failure criterion are highly correlated and are very similar if a modified DE failure criterion is applied. The VECD approach successfully ranks the fatigue performance of mixtures with various air void and asphalt contents for a particular gradation, but the ranking between two gradations did not agree with the observed field performance. The DE approach shows promise for ranking the field performance of the mixtures; however limited data at different strain amplitudes made it difficult to state any conclusions with confidence.

A DISTINCTIVE FATIGUE FAILURE CRITERION

by G. Al-Khateeb, A. Shenoy

Abstract

This paper presents a new fatigue failure criterion for asphalt paving mixtures that is simple, unique, and distinctive. Bending beam fatigue testing in the controlled strain mode at a 1000-microstrain level and 19C temperature was performed on eleven asphalt mixtures that included unmodified and modified binders. Analysis of fatigue load-deformation raw data for each fatigue load cycle was conducted to determine the true point of fatigue failure.
With application of a sinusoidal strain on a sample, a sinusoidal response stress is expected even for a heterogeneous material like asphalt concrete. In such a case, a smooth traditional load- deformation (or stress-strain) hysteresis loop is anticipated. This holds true as long as there is no fatigue damage induced in the material. With repeated load applications, the sample starts to fatigue and microcracks are induced. These microcracks introduce discontinuities in the stress paths and the stress response starts to distort. This gets reflected in the load-deformation hysteresis loop, which in turn shows this distortion. Similar distortion can also be seen by observing the sinusoidal load-deformation waveform, where the stress response is no longer dependent on the strain input due to the formation of interconnected fatigue cracks. By tracking the distortion in the hysteresis loop or in the waveform, one is able to get a clear indication of when the first microcracks appeared, and how they progressed up to the point of complete fatigue failure.

TWO-STAGE WEIBULL APPROACH FOR ASPHALT CONCRETE FATIGUE PERFORMANCE PREDICTION

by B. Tsai, J. Harvey, C. Monismith

Abstract

The purpose of this paper is to present the two-stage Weibull approach, a phenomenological approach to characterize the fatigue damage process of asphalt concrete mixes at both the crack initiation and crack propagation phases. For each test, parameter estimation can be performed using the Lagrange-Newton algorithm. This algorithm can clearly identify the repetition at which crack initiation and crack propagation are separated. A set of WesTrack tests was utilized as the demonstration example. These tests consisted of three gradations: fine, fine plus, and coarse. For each gradation, 12 beam tests were conducted [three temperatures (10, 20, and 30C), two strain levels (200 and 400 microstrain), and two replicates]. After collecting all the information for the repetitions at which separation occurs for a series of tests, integrated Weibull-type regression equations can be obtained for crack initiation and crack propagation. To correct the difference between in-situ and laboratory boundary conditions, the correction factor for the two- stage Weibull approach is also discussed. Finally, with the integrated regression equations and correction factors for crack initiation and crack propagation, the fatigue performance prediction of a WesTrack section was simulated with the "stiffness ratio hardening" fatigue damage accumulation mechanism. This two-stage Weibull approach appears to be promising for fatigue performance prediction, provided that the prediction results can be verified by more comprehensive condition survey data, especially the stiffness deterioration history of field pavements.

AN INVESTIGATION OF THE APPLICABILITY OF SCHAPARY'S WORK POTENTIAL MODEL FOR CHARACTERIZATION OF ASPHALT FATIGUE BEHAVIOR

by R. Lundstrom, U. Isacsson

Abstract

This paper presents an investigation of Schapery s work potential model applied to asphalt concrete characterization. Six asphalt concrete mixtures, all consisting of one and the same aggregate and particle size distribution, manufactured using unmodified and SBS modified binders were used. An elastic-viscoelastic correspondence principle and the time-temperature superposition principle are utilized to obtain material characteristic functions, C1(S1), using uniaxial monotonic (constant-strain rate) and cyclic (fatigue) tests at 0, 10 and 20C. The results indicate that consistent and reliable results are obtained for the mixtures investigated using monotonic testing, provided that the applied strain rate is not too low at a given temperature. It was also indicated that the time-temperature superposition principle is applicable as a single material function can be obtained at the reference temperature chosen. Results similar to those obtained at monotonic tests were also obtained at cyclic testing. However, based on statistical analysis, it was noticed that cyclically established C1(S1) curves often differ slightly between different temperatures even when shifted to the reference temperature. This result is probably due to the more complicated excitation history applied during fatigue tests. However, even though it is possible to obtain characteristic material functions from both types of tests, it was not possible to show that monotonic and cyclic testing lead to similar results, i.e. more or less identical C1(S1) curves. It was also investigated, using thermocouples, whether, and if so, to what extent hysteretic self-heating influences fatigue test results. In general, a significant influence was indicated at 10 and 20C, and especially for high amplitude tests (low cycle fatigue tests). Even though several questions still are to be answered, the model based on the work potential theory remains interesting, and is possibly, the most versatile damage model for asphalt characterization available today.

By Title Only A STUDY OF CRACK-TIP DEFORMATION AND CRACK GROWTH IN ASPHALT CONCRETE USING FRACTURE MECHANICS

by Y. Seo, Y. Kim, R. Schapery, M. Witzak, R. Bonaquist

Abstract

This paper presents findings from a comprehensive experimental/analytical study of crack growth in asphalt concrete using fracture mechanics. The primary objective of this study is to provide critical information that is complementary to the viscoelastoplastic continuum damage model (1) in modeling crack growth using finite element analysis. To simulate mode I fracture, uniaxial monotonic and cyclic tension tests were conducted on prismatic specimens with symmetric double notches. Digital Image Correlation (DIC), a noncontact, full-field displacement/strain measurement technique, was utilized to investigate the size and shape of the fracture process zone (FPZ). Irrespective of the notch size and testing conditions, the FPZ was observed to be similar in size and shape for the mixture. Also, it was found that the strain at the crack tip immediately before crack initiation is a decreasing function of strain rate.
The experimental data were analyzed using several fracture mechanics theories, including the cohesive crack model and crack growth rate laws based on the stress intensity factor, KI. The cohesive crack model analysis provides the fracture energy and softening function that describe the post-peak behavior with strain localization. The analysis based on KI shows that the specimen size has no significant effect on the crack growth rate laws. The effect of temperature is pronounced in the crack growth rate law using temperature-reduced crack speed based on KI. The time-temperature superposition principle, with the shift factor from the linear viscoelastic range, was applied to these crack growth rate laws and successfully collapsed the curves at different temperatures into a single relationship.