Current Superpave Research Articles

Characterization of Fatigue Resistance of Warm Mix Binders Using Linear Amplitude Sweep Test

Fatigue failure in asphalt pavements occur due to the accumulation of damage under repeated traffic loading. Fatigue cracks generally occur within the binder phase of the asphalt mixture. Hence the specifications for properties related to fatigue resistance of asphalt binder become an important screening tool to select binders that are not inherently susceptible to fatigue under given environmental and traffic conditions. The Current Superpave Performance Grading (PG) specifications use parameter |G*|.·sinδ of asphalt binder to assess the fatigue susceptibility of asphalt mixes. These specifications do not accurately characterize the fatigue resistance of asphalt binders because |G*|.·sinδ is determined by applying a cyclic loading in the Dynamic Shear Rheometer (DSR) at a very small strain amplitude (1%). In this study, Linear Amplitude Sweep (LAS) test was considered to characterize the fatigue resistance of asphalt binders with warm mix additives. Warm mix asphalt (WMA) is a developing technology that can allow asphalt concrete to be produced at lower temperatures. This paper presents result of a study carried out to investigate the fatigue characteristics of warm-mix asphalt binders using a linear amplitude sweep test. Three warm-mix additives; Sasobit®, Evotherm™ and Rediset® are considered. Viscosity grade (VG 30), Polymer modified binder (PMB 40) and Crumb rubber modified binder (CRMB 60) were selected as base binders for this investigation. Physical and rheological tests were carried out for the characterization of these binders. The LAS test was carried out at 25 °C in a DSR, and the fatigue parameters were calculated from stress–strain plots. Test results showed that the changing trend in fatigue life of mixes is more related to the fatigue parameter Nf from LAS test than to |G*|. sinδ values of the binders. It was also observed that most of the warm mix binders have improved fatigue resistance at 25 °C in comparison to the control binders. The effect of Rediset® additive was found to be insignificant in the case of VG 30 grade binder only.

Modern, Recycled SMA Mixtures on the Illinois Tollway and Preliminary Performance-Based Mix Design Approach

As more recycled materials are utilized in modern asphalt mixtures, the overall phenomenological behavior of this important infrastructure material becomes more complex than ever, accelerating the need to supplement volumetric-based mix testing procedures with mixture mechanical tests. This paper introduces a mixture performance test based design approach, as a supplement to the current Superpave volumetric mix design approach, which is being implemented by several agencies near Chicago, Illinois, USA. For research purposes, the practical design suite, which considers the high and low-temperature performance of asphalt mixtures through Hamburg wheel tracking and disk-shaped compact tension fracture testing, has been supplemented by additional testing and analysis including creep compliance, performance-space diagram construction, and thermal cracking modeling using the software program ILLI-TC. Field cores from seven Illinois Tollway sites in Chicagoland with welltracked pavement performance were tested and analyzed using the aforementioned methods. These modern, high-performance stone matrix asphalt (SMA) recycled mixtures contained recycled asphalt pavement (RAP), reclaimed asphalt shingles (RAS), and ground tire rubber (GTR), and used a 3.5% design void target to promote mix durability. Rutting and cracking test results, and thermal cracking modeling predictions, were found to be consistent with field observations indicating near-zero rutting and cracking levels after up to eight years of heavy tollway traffic and severe mid-continental environmental conditions. In addition, the Hamburg-DC(T) performance space diagram was used to further analyze the results, demonstrating how this plotting tool can be used to adjust future mix designs to yield even longer life on the Tollway system at little-to-no extra cost.

Engineering Properties of Asphalt Binders from Different Sources and Their Influence on Stiffness of Asphalt Concrete Mixtures

The quality and quantity of asphalt binder are crucial for proper adhesion, cohesion, and performance of asphalt concrete (AC) mixtures. The current Superpave grading system for asphalt binders provides engineers with the high and low temperature ranges at which the asphalt binder should perform satisfactorily. The objectives of this study are to document the differences in the performance of different asphalt binders with the same nominal performance grade (PG) acquired from different refineries and to investigate the impact that binder properties may have on the stiffness of the AC mixes. To that end, five PG 64-22 and five PG 70-22 binders were studied. Each binder was graded twice, in the original state and extracted from the mix conditions. The conventional performance grading tests such as the bending beam rheometer, dynamic shear rheometer, pressure aging vessel, and rolling thin-film oven were conducted on all asphalt binders. Alternative binder parameters (e.g., parameter ΔTc, viscosity) that can potentially supplement the current PG system were measured and documented. The stiffness properties of the ten mixes as measured with the dynamic modulus tests were correlated with the measured binder properties. The asphalt binders with the same performance grades yielded different cracking, rutting, and stiffness properties, which may explain the differences in their performance when used to design AC mixes.

Influence of Production Considerations on Balanced Mixture Designs

Measuring or predicting mixture performance prior to placement has become essential for many state transportation agencies due to incorporation of more recycled materials into mixtures, utilization of binders formulated with various modifiers, and utilization of new and unproven technologies. Mixtures designed incorporating these elements in accordance with the current Superpave mixture design method have unknown performance as Superpave was never designed to compensate for them. Thus, agencies have interest in implementing the balanced mixture concept that requires testing for acceptable mixture performance during the design phase. The effect of production considerations on a balanced mixture design is not known. It is unknown if a mixture remains performance balanced during production due to factors like asphalt content, gradation and binder source variations. This study was undertaken to understand the influence of these production considerations on a balanced mixture design. Two Superpave mixtures, a 9.5 mm and 12.5 mm, were developed using the balanced mixture design approach using volumetric properties, one rutting test and one cracking test. These mixtures were then modified to represent production variations in asphalt binder content, gradation passing the No. 200 sieve, and asphalt binder source. All mixtures remained performance balanced when examining the production considerations in terms of rutting and cracking using the selected tests. Examination of other performance results, like moisture susceptibility, suggest it is possible to effectively unbalance the mixture design during production. This underscores the importance of proper selection of performance tests to address the performance needs of the agency.

Mixture-property-independent asphalt film thickness model

The durability of asphalt mixtures and hence the service life of asphalt pavement surface layers depends to a large extent on the asphalt film thickness. The current Superpave Voids in Mineral Aggregate (VMA) criterion relates mixture durability with VMA. The need to modify the Superpave criterion was supported by a previous study [1]. As the Superpave VMA criterion is based on the minimum asphalt content in the asphalt mixture and not on the asphaltfilm thickness, this minimum requirement does not ensure mixture durability in many cases. Additionally, coarse asphalt mixtures that tend to have enough asphalt film thicknesses normally have difficulty fulfilling the Superpave minimum VMA criteria. From this contention, the need for simple and reasonably accurate models to estimate the asphalt binder film thicknesses in asphalt mixtures becomes essential.In this study, a model for estimating the asphalt film thickness (FTb) has been developed using only parameters/properties of the two mixture constituents (aggregate and asphalt binder) and without the inclusion of any mixture property. The derivation of the model is based on physics, and the determined model coefficients (constants) have been obtained through statistical regression analysis. Superpave aggregate gradations of three nominal maximum aggregate sizes (NMAS), 9.5, 12.5, and 19.0 mm with aggregate gradations passing above, below, crossover, hump through, and pass through restricted zone were used. Superpave Gyratory Compactor (SGC) test data of 100 compacted asphalt mixtures were used in developing the model, and SGC data of 31 mixtures were used for verification of the model. MS Excel program solver was used for regression analysis. The final outcome is a physics-based statistical regression model, with a high enough coefficient of determination (R2) value of 0.9 for FTb, which is easy to use and likely to predict the film thickness with a reasonable degree of accuracy.

Development of a Simple Fatigue Cracking Test for Asphalt Binders

Current Superpave® PG specification uses parameter | G*|sin(δ) to quantify asphalt binder fatigue resistance. The parameter’s effectiveness has been debated for a long time. AASHTO recently adopted the linear amplitude sweep test as a provisional standard, AASHTO TP 101-12. The authors evaluated the sensitivity of this standard to different aging conditions: unaged original binders, rolling thin-film oven-aged binders, and 20- to 80-h pressure aging vessel–aged binders. Test results showed, in many cases, longer predicted fatigue lives for more-aged binders. Thus this study developed a simple fatigue cracking test for asphalt binders. In this new test, the pure linear amplitude sweep (PLAS) test, peak shear strain was increased linearly from 0% to 30% over a course of 3,000 oscillatory cycles. A new fatigue parameter, the fatigue resistance energy index (FREI), was derived with fracture mechanics. The PLAS test and FREI parameter were sensitive to both binder aging conditions and rejuvenator type and dosage. Four laboratory mixtures were employed to evaluate the correlation between this new binder fatigue test and the two mixture cracking tests: the Texas overlay test and the Illinois flexibility index test. The results showed that the PLAS and FREI correlated well with the mixture cracking tests. Additionally, the proposed method was preliminarily verified with the FHWA accelerated loading facility test, and a fair relationship with the full-scale fatigue test data was observed. It is obvious that the PLAS and associated FREI need further validation through more field test sections.

Consistent distribution of air voids and asphalt and random orientation of aggregates by flipping specimens during gyratory compaction process

The current Superpave mix design determines the optimum asphalt content that would produce asphalt specimens with total air voids of 4.0% without considering their sizes and distribution within the specimens. In addition, the current Superpave gyratory compaction equipment may produce inconsistent specimens with higher air voids on the top layer than the middle and bottom layers of the specimen. To produce more consistent gyratory specimens, a new approach of compacting both ends of the specimen, rather than the one side, is presented. Based on the automated image analysis of twenty-four specimens, the difference in air voids between top and bottom layers decreased when specimens were compacted on both ends. In addition, the standard deviations of both air voids and asphalt contents throughout the specimen decreased when the specimens were compacted on both ends. Therefore, it can be concluded that the air voids and asphalt contents were more uniformly distributed throughout the specimen when the gyratory specimens were compacted on both ends. Based on the angles between major axes of aggregates and horizontal plane, the aggregates were more randomly oriented when compacted on the both ends.

Critical Considerations toward Better Implementation of the Multiple Stress Creep and Recovery Test

AbstractIn recent years, many studies have dealt with the inadequacy of the current Superpave performance grading (PG) specification for accurately capturing the performance properties of modified binders. An alternative binder testing mode that has gained much attention in recent years is the repeated creep and recovery (RCR) test, and more specifically, the multiple stress creep and recovery (MSCR) test. As with any new test procedure, a number of concerns and questions remain with regards to interpretation of results in terms of performance and sufficiency of the number of cycles and stress levels used, as well as representative temperature and stress levels to be used. This paper investigates the aforementioned issues using a set of modified and unmodified binders. The results show that the standard 10 cycles per stress level are not sufficient to reach a stable steady-state creep behavior for modified binders, thus the addition of 20 conditioning cycles are recommended, for a total of 30 cycles for e...

Conceptualizing the asphalt film thickness to investigate the Superpave VMA criteria

Asphalt binder film thickness (FTb) is the key factor that is responsible for durability of asphalt mixtures. Mixtures with coarse aggregate gradations have difficulty meeting the Superpave minimum voids in mineral aggregate (VMA) criteria even though they tend to have thick asphalt films. In this study, the concept of asphalt binder film thickness (FTb) was used to investigate the Superpave VMA criteria. Superpave aggregate gradations of three nominal maximum aggregate sizes (NMAS): 9.5, 12.5 and 19.0 mm, were used. Aggregate gradations passing above, below, crossover through, humped, and through restricted zone were all considered. Superpave Gyratory Compactor test data of 126 compacted asphalt mixtures were used in the study. The current Superpave VMA criteria relate the mixture durability with VMA and set the same VMA value for mixtures with the same NMAS regardless of other parameters. However, a poor relationship was found between VMA and FTb (a durability measure) with an R2 ≅ 0.01, and yet a relatively good relationship (R2 ≅ 0.38) was found between voids filled with asphalt (VFA) and FTb although the VFA volumetric phase is part of the VMA volumetric phase in the mixture. This result was justified by the poor relationship between VFA and VMA (R2 ≅ 0.13). Despite that the effective binder content (Pbe) as a volumetric phase represents the VFA in the mixture, the relationship between FTb and Pbe was found to be more significant (higher R2) than the relationship between FTb and VFA. In this study, asphalt mixtures that did fail the Superpave VMA criteria in some cases had adequate asphalt FTb, and mixtures that passed the criteria did not necessarily have adequate FTb. In conclusion, although the current Superpave VMA criteria are significant, findings of this study support the tendency to modify the current criteria.

High-Temperature Properties of Asphalt Binders: Comparison of Multiple Stress Creep Recovery and Performance Grading Systems

The Superpave® high-temperature test protocol has been shown to be inadequate for characterizing the high-temperature behavior of asphalt binders, particularly those that are polymer modified. Recently, a specification based on the multiple stress creep recovery (MSCR) test was proposed to address the shortcomings of the Superpave high-temperature binder specifications. This study aimed to investigate the merits of implementing the current MSCR test protocol as a replacement for the performance-graded high-temperature test. A statistical analysis was conducted on a data set from the Indiana Department of Transportation to see how MSCR and performance grading procedures differ in grading various binders. In addition, an experimental study was conducted with 16 modified and unmodified binders. The results indicated that the MSCR test was a suitable replacement for the performance grade test because it provided a better tool to rank modified asphalt binders as well as unmodified ones. That is, creep compliance from the MSCR test more fundamentally represented binder behavior at high temperatures compared with the performance grade rutting parameter. In addition, the simplified approach—known as grade-bumping—used in the current Superpave mix design system to account for high traffic levels and low speed limits can be eliminated when the MSCR test is used.

Effect of FT Paraffin Wax Contents on Performance Properties of Crumb Rubber–Modified Asphalt Binders

AbstractThe performance properties of rubberized asphalt binders at high, intermediate, and low temperatures were evaluated as a function of Fischer–Tropsch (FT)–paraffin wax content. The rubberized binders were blended using five different wax content levels (0, 1.5, 2.5, 3.5, and 4.5% by binder weight) to produce warm rubberized binders. They were then artificially aged over the short term and long term through the rolling thin film oven (RTFO) and pressure aging vessel (PAV) procedures. Superpave binder tests were performed with the rotational viscometer, the dynamic shear rheometer (DSR), and the bending beam rheometer (BBR). In general, the results of this study suggested that (1) the viscosity of rubberized binders at 135°C can be reduced by the addition of a wax additive, as expected; (2) the rubberized binders with higher wax content can result in binder performance that meets current Superpave binder requirements, especially for cracking resistance parameters (i.e., G*sinδ and stiffness); and (3)...

Field Validation of a Thermal Cracking Resistance Specification Framework for Modified Asphalt

Current Superpave® performance grade (PG) binder specifications, such as those used in Wisconsin and many other states, were developed primarily for unmodified binders and are valid only within the linear viscoelastic range of properties. Further, performance benefits from modification are not limited to continuum rheological properties because modified binders often demonstrate superior damage resistance not captured in the current PG system. This paper discusses the results of a recently completed research project sponsored by the Wisconsin Department of Transportation in Madison; the objective was to identify promising procedures through a comparison of test results with field performance and the development of appropriate modified binder specification criteria for thermal cracking resistance. A number of procedures have been introduced in recent years to address the need for damage characterization test procedures for proper assessment and selection of modified binders. In this study, the single-edge notched bending (SENB) procedure was found to correlate well with the observed field thermal cracking when tests were conducted at the project location low-temperature PG specification temperature. The device used was a modified bending beam rheometer (BBR), which could perform displacement-controlled loading with higher load cell capacity, and accommodated notched BBR beams. On the basis of the results of this study, a procedure was introduced for thermal cracking resistance potential of modified binders with the BBR-SENB test at an average annual minimum pavement temperature of +10°C. A preliminary failure limit and acceptance criterion were defined to qualify binder results in terms of the failure properties measured.

Mechanical properties of filler-asphalt mastics

There has been an increasing awareness of the addition of ordinary Portland cement (OPC) rather than limestone filler into asphalt pavements in Taiwan. Filler interpaticle interaction and physicochemical interaction between cement filler particle and asphalt improved engineering properties of asphalt mixtures. Current Superpave parameters of G*/sinδ and G*sinδ were used to link the rheological properties of asphalt binders to the pavement performance of rutting and fatigue. However, the characteristics of asphalt binders were determined in the linear viscoelastic domain by performing frequency sweeps. The asphalt binder film might perform in the non-linear range due to the considerable difference between the stiffness modulus of aggregates and the modulus of binders. In addition, the filler-asphalt interaction might not be reflected by means of the Superpave parameters. Mechanical properties of stiffness, deformation and fatigue for filler-asphalt mastics may be more appropriate in terms of establishing a correlation with asphalt pavement performance. Oscillatory and creep tests using a dynamic shear rheometer (DSR) were performed to quantify the mechanical properties of cement and limestone filler-asphalt mastics at the same levels of filler contents by weight. The test results showed that the higher complex modulus and stiffening effect of the cement filler-asphalt mastics than those of the limestone filler-asphalt mastics within a linear viscoelastic domain. In terms of creep behavior, the addition of cement filler to base bitumen increases the viscosity at a steady state. The longer fatigue lives for the cement filler-asphalt mastics indicated the addition of active filler may provide a better pavement durability.

Toward Development of a New Thermal Cracking Test Using the Dynamic Shear Rheometer

Abstract Thermal cracking is one of the primary distresses in asphalt pavements. Asphalt binder, an important component of asphaltic concrete, plays a significant role in the thermal cracking performance of asphalt pavement and should be characterized accurately for proper binder selection. The current Superpave binder specifications for low-temperature cracking, based on creep stiffness and m-values, are not applicable to modified binders. Fracture-based tests are most promising for the prediction of low-temperature cracking of binders, especially modified binders. This study develops a new performance indicator for low-temperature cracking using properties from monotonic testing at a semi-low temperature, 5°C, based on the dynamic shear rheometer (DSR). It is found that the complex shear modulus of binders does not correlate with field thermal cracking performance. Fracture energy shows a moderate correlation with field thermal cracking performance. The correlation is improved when the shear rate is increased. However, failure strain at 5°C correlates very well with field thermal cracking. In addition, failure strain is not sensitive to the shear rate at 5°C. This new test could significantly reduce the costs associated with the purchase of various types of equipment, such as that needed to conduct bending beam rheometer (BBR) or direct tension tests, and could also reduce the amount of testing time. Further study is needed to validate the findings in this study.

Influence of Warm Mix Additives on PMA Mixture Properties

The objective of the study in this paper was to investigate the influence of warm mix additives on polymer-modified asphalt (PMA) mixtures in regards to engineering properties. Twelve PMA mixtures (four control PMA mixtures and eight warm PMA mixtures) were prepared by using two additives (Aspha-min and Sasobit), two binder sources, and two aggregate sources. An evaluation of all PMA mixtures included the following testing procedures: tensile strength ratio, asphalt pavement analyzer, resilient modulus, and indirect tensile strength after longterm oven aging. The results from this research generally indicated that (1) the PMA mixtures containing the additives can satisfy the current Superpave mixture requirements (including moisture susceptibility and rutting resistance) and (2) no statistical differences existed between the control and the warm PMA mixtures for the properties that the authors evaluated in this study, suggesting using of WMA technologies in PMA mixes has no negative effect on the mixture’s engineering properties.

Performance Evaluation of Recycled PMA Binders Containing Warm Mix Asphalt Additives

Abstract The performance properties of recycled polymer modified asphalt (PMA) binders containing warm mix asphalt (WMA) additives at high, intermediate, and low temperatures were evaluated. First, each combination of two PMA binders (performance grade (PG) 76-22 from two different sources) and two WMA additives (i.e., Aspha-min® and Sasobit®) were blended to produce warm PMA binders. They were then artificially short-term and long-term aged through the rolling thin film oven and pressure aging vessel procedures. Second, long-term aged warm PMA binders were recycled using two virgin binders (PG 76-22 and PG 70-22), and the recycled binders were artificially aged again. Lastly, Superpave binder tests were performed through the rotational viscometer, the dynamic shear rheometer (DSR), and the bending beam rheometer (BBR). In general, the results of this study suggested that (1) the WMA additives and the aged PMA binders were found to have a significant influence on the performance properties of recycled binders (measured by the DSR and the BBR); (2) the use of PG 70-22 as a virgin binder was effective in offsetting the increase of binder stiffness due to the additives and the aged binders; and (3) the recycled warm PMA binders can result in satisfactory binder performance that meets current Superpave binder requirements.

Analysis of Aggregate Gradation and Packing for Easy Estimation of Hot-Mix-Asphalt Voids in Mineral Aggregate

The voids in mineral aggregate (VMA) design parameter is one of the most important in the current Superpave mix design procedure that links hot-mix-asphalt (HMA) mix properties to field performance. The success of achieving the appropriate VMA at an early stage of the mix-design process not only significantly impacts the design time and effort but also can produce an economical and reasonable design with good field performance. Although several current mix and gradation design methods have provided some guidance on adjusting gradation to achieve target VMA, these approaches still have to rely on initial experimental results with some degree of trial and error, and provide no direct correlation between aggregate gradation and VMA. This paper presents an analysis of aggregate packing and discrete-element modeling (DEM) simulation to link the aggregate gradation property to the VMA of HMA. A new definition is proposed for characterizing traditional dense-graded aggregate gradations into three types: coarse-graded, medium-graded, and fine-graded. A gradation weighing factor, fv, defined as the percentage of voids change by volume caused by unit additional aggregate, is developed to theoretically evaluate the packing of an aggregate structure. A method of VMA estimation based on the generated fv values is thus provided. Eleven mix designs from different states are used for verifying the accuracy of the prediction. Most predictions are within 1% difference of the design VMA. Using one nominal maximum aggregate size (NMAS) of 12.5 mm as an example, this study provided a promising method for a fast and accurate VMA estimation. It can be used directly in the HMA mix design to reduce the design time and effort, and contribute to the estimation of the HMA mixture performance at an early stage of the design period.

Multiple Stress Creep and Recovery and Time Sweep Fatigue Tests

Crumb rubber modification (CRM) is an effective means of improving neat asphalt performance. Numerous studies have investigated the effects of CRM using the current Superpave® laboratory tests. Recent studies have shown that the assumption of linear viscoelastic behavior or employing insufficient stress or strain levels makes current tests inadequate for properly assessing the performance of binders, especially when modified. This problem has resulted in the development of improved test methods such as the time sweep binder fatigue test and the multiple stress creep and recovery (MSCR) test for permanent deformation. The present study assesses the performance of binders modified with a wide range of crumb rubber content using these newly developed test methods. Additional binder and mixture performance tests were used to compare and validate the results. The results showed that the new tests performed well in predicting performance. Current Superpave criteria also showed good conformance with mixture test results for CRM binders. Tests results suggest that the MSCR can complement current rutting prediction testing. Also, the time sweep test produces results similar in accuracy to current Superpave tests, but it requires longer testing time and is thus not recommended for performance grading.

Characterization of Recycled Aged RAP Binders Containing Crumb Rubber Modifier Using Gel Permeation Chromatography

The recycling of hot-mix asphalt pavements containing a crumb rubber modifier (CRM) has been an area of interest since CRM was first used in asphalt paving materials over 40 years ago. Studies on the recycling of rubberized asphalt concrete materials have focused mainly on the mixture properties at the location of in-field paving. However, for the long-term performance characterizations, there is a need to investigate the aging properties of rubberized binders and the subsequent recycling properties of these aged rubberized binders. To address this issue, the previous research was conducted to identify performance properties of recycled aged CRM binders using Superpave binder test methods. The research results indicated that the recycled aged CRM binders can result in satisfactory binder performance meeting current Superpave binder requirements. This paper presents a limited study that uses gel permeation chromatography (GPC) to characterize the aging process of recycled aged CRM binders as a function of ...

The Relationship of the MSCR Test to Rutting

ABSTRACT The Multi-stress Creep and Recovery (MSCR) Test is currently being considered as a replacement for the Superpave high temperature binder criteria G* sinδ. The MSCR test can distinguish between the rutting properties of both neat binders and polymer modified binders. The test is run on the same dynamic shear rheometer test equipment currently used for the current Superpave binder testing and is easy to run. The validation of the MSCR compliance value Jnr to rutting was done through extensive mix testing using laboratory rut testers, large Accelerated Loading Facilities and actual roadway sections. This paper covers the validation of the MSCR test binder compliance value Jnr to mixture rutting for modified and neat binders. Multiple binders and mix types are included in the validation and it is demonstrated that the MSCR test provides a much better correlation to mixture rutting than the existing Superpave binder criteria.