Aging of asphalt mixtures occurs during production and construction and continues throughout the service life of the pavement. Although this topic has been studied extensively, recent changes in asphalt mixture components, production parameters, and plant design have raised a need for a comprehensive evaluation that considers the impacts of climate, aggregate type, recycled materials, WMA technology, plant type, and production temperature. In this study, field cores were acquired from seven field projects at construction and several months afterwards, and raw materials were also collected for fabricating laboratory specimens that were long-term oven aged (LTOA) in accordance with selected protocols. The resilient modulus and Hamburg wheel tracking tests were conducted on both specimen types to evaluate the evolution of mixture stiffness and rutting resistance with aging. The concepts of cumulative degree-days and mixture property ratio were proposed to quantify field aging and its effect on mixture properties. Test results indicated that the LTOA protocols of two weeks at 140°F (60°C) and five days at 185°F (85°C) produced mixtures with equivalent in-service field aging of 7–12 months and 12–23 months, respectively, depending on climate. Finally, among the factors investigated in the study, WMA technology, recycled materials, and aggregate absorption exhibited a significant effect on the long-term aging characteristics of asphalt mixtures, while production temperature and plant type had no effect.
Evaluation of Asphalt Mixture Laboratory Long-Term Aging Methods for Performance Testing and Prediction
Aging has long been recognized as a major distress mechanism for asphalt concrete and, by extension, asphalt pavements. Aging causes the material to stiffen and embrittle, which leads to a high potential for cracking. Although a significant amount of effort has been placed on understanding the aging process of asphalt binder, less effort has been put forth to develop laboratory aging procedures for producing aged mixture specimens for performance testing. An optimal laboratory conditioning procedure to simulate long-term aging for performance testing and prediction is required in order to integrate the effects of long-term aging in pavement prediction models and other mechanistic design and analysis methods. In this study, oven aging and pressure aging vessel aging are applied to both loose mix and compacted specimens in order to evaluate and select an aging method to simulate long-term aging for performance testing and prediction. The selected method must be able to maintain specimen integrity in order to be used for performance testing and prediction. Efficiency, practicality, and versatility also are considered in evaluating the aging methods. The results demonstrate that loose mix aging in an oven is the most promising aging method to produce mixture specimens for performance testing in terms of efficiency, specimen integrity, versatility, and cost.
Selection and Preliminary Evaluation of Laboratory Cracking Tests for Routine Asphalt Mix Designs
Cracking has become a primary mode of distress in recent years that frequently drives the need for rehabilitation of asphalt pavements. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers, and multiple warm mix asphalt technologies. Thus, there is an urgent need to identify reliable cracking tests that can be used for routine mix design to eliminate brittle mixes. This paper includes a critical review of cracking mechanisms and laboratory tests. A total of twelve cracking tests were discussed at a cracking test workshop held as part of the National Cooperative Highway Research Program (NCHRP) Project 9-57. Seven cracking tests were selected for further laboratory evaluation and field validation. Four of the simpler cracking tests from the seven were evaluated in this paper, these being Texas Overlay Test (OT), Disk-shaped Compact Tension (DCT) test, Semi-Circular Bend test from the Louisiana Transportation Research Center (SCB-LTRC), and SCB test at room temperature from Illinois (SCB-IL). A laboratory sensitivity study was performed, and the results showed that all four cracking tests were generally sensitive to asphalt mix components. However, there were some concerns with the DCT, SCB-LTRC, and SCB-IL. Both the DCT and SCB-IL were found to be not sensitive to asphalt binder content; and both the DCT and SCB-LTRC showed an unexpected increase in cracking resistance when adding RAS to the mix. Additionally, two sets of field test sections were used for preliminary validation of these four cracking tests. It was found that the OT, DCT, and SCB-IL provided rankings which matched the measured field performance for the two sections on US62, Texas; and the OT and SCB-LTRC were valid for six APT test sections. Further validation with different mixes, traffic and climate is needed.
Uniaxial Shear Tester—Test Method to Determine Shear Properties of Asphalt Mixtures
The main focus of the paper is to present the concept of a newly developed Uniaxial Shear Tester (UST) and to investigate the correlation between results from the UST and the Superpave Shear Tester (SST), a tool broadly recognized for asphalt mix design and rutting susceptibility evaluation. In this study, the UST testing principles, finite element analysis of stresses, and comparison of measured data are presented. The correlation was assessed on the basis of two tests, the repeated shear test and the small amplitude oscillation test also referred as the shear frequency sweep test. It was shown that the material characteristics determined from UST and SST are highly correlated. The dependencies are discussed in the sense of linear correlation and correlation coefficients. Test variability is discussed in the paper.
Correlating Field Performance to Laboratory Dynamic Modulus from Indirect Tension and Torsion Bar
Dynamic modulus has several useful functions in flexible pavements, including stress/strain characterization, rutting and cracking characterization, an input into several analytical and numerical models, and a primary input into PavementME Design. While the traditional dynamic modulus test is run in the uniaxial configuration, this is not possible for field cores. Therefore, the Indirect Tension dynamic modulus (IDT |E*|) and torsion bar shear modulus (torsion bar |G*|) have been developed. However, there has been limited research looking at analyzing the data from field cores for these two geometries, comparing modulus data from the two geometries, examining in-service aging of dynamic modulus, and quantifying pavement conditions using dynamic modulus. This research examines ten field sections in Arkansas, comprised of four “goodâ€ performing sections, two “mediumâ€ performing sections, and four “poorâ€ performing sections in an attempt to address these four questions. First, this research found that using AASHTO T 342 and AASHTO R 62 can lead to irrational coefficients but provide rational results. Second, while the IDT |E*| and torsion bar |G*| values were similar at high modulus values, the IDT |E*| values began to increase as the modulus decreased compared to the torsion bar |G*| values, increasing to over a decade of difference. Third, a noticeable difference was observed between the modulus values of the bottom surface layer and top surface layer, with the bottom surface layer showing higher modulus values in all cases. While the upper surface layer showed higher oxidation, other weathering effects such as moisture and traffic appear to have overwhelmed the oxidation effect and pavement deterioration has reduced the integrity of the mix. Finally, both the IDT |E*| and torsion bar |G*| were not able to quantify a noticeable difference between poor and medium performing sections, and medium and good performing sections, but were able to quantify a difference between the poor and good behaving sections. Overall, the IDT |E*| and torsion bar |G*| tests were able to produce consistent master curves, correlate to each other, identify differences between surface course lifts, and quantify differences in field performance.
Laboratory Performance of Re-Refined Engine Oil Bottoms (REOB) Modified Asphalt
Re-refined engine oil bottoms (REOB) are one of several products obtained in the refining of recovered engine oil and have been used in the asphalt industry since the 1980’s. Generally, REOB is used to help soften the base asphalt binder and is commonly used from 3 to 10% by weight in order to achieve desired low temperature asphalt binder properties. Recently, poor cracking performance in a number of Canadian and northern United States pavement sections have been blamed on the use of REOB to modify the asphalt binder. This has prompted state agencies in the northeast United States to ban its use, without necessarily understanding how REOB affects asphalt binder and mixture performance. A research effort was conducted to evaluate the laboratory performance of asphalt binders and mixtures modified with REOB. Two different sources of REOB were blended with different base asphalt grades at varying dosage rates in the study to achieve “softerâ€ asphalt binders—similar to the current practice of REOB modification in the asphalt industry. Performance grading, master stiffness curves, double edged notch tension test, and Black Space analysis were conducted on the asphalt binders at different levels of laboratory aging. Additionally, the asphalt binders were used to produce asphalt mixtures for stiffness, permanent deformation, fatigue cracking and low temperature cracking performance. The research study showed that while being able to achieve softer asphalt binder grades in accordance with AASHTO R 29, the addition of REOB accelerates the age hardening effects in the asphalt binder, with higher levels of age hardening occurring at higher REOB dosage rates. The study also indicated that while the stiffness properties at low temperatures are not impacted by the REOB, the relaxation properties, as measured using m-slope of the Bending Beam Rheometer (BBR), are highly affected. Both the Black Space analysis, using the Glover-Rowe approach, and the DENT test show promise at identifying the age hardening effects and correlated well to mixture fatigue cracking in the Overlay Tester. Differences were found between the various asphalt mixture fatigue cracking tests in their respective ranking of fatigue cracking performance. Asphalt mixture permanent deformation and dynamic modulus of REOB modified asphalt mixtures showed similar performance to the neat asphalt binders when compared to the REOB modified asphalt binders of the same PG grade.
Performance Evaluation of REOB Modified Asphalt Binders and Mixtures
This study evaluated a limited number, but well-controlled group of asphalt binders of the same PG grade made with a wide range of REOB contents; 0%, 2.5%, 6% and 15%. The most practical indicator of the possible presence of a considerable quantity of REOB with measureable changes to the rheological and performance characteristics was the difference between the BBR m-value temperature grade and the BBR S stiffness temperature grade, DTcritical. When a binder exhibited a large DTcritical it was associated with larger differences in performance losses depending on the binder tests and to a lesser extent the mixture test. The DTcritical performance disruption was made worse by oxidative aging by means of double PAV conditioning as well as holding the binder at extended low temperatures before testing. The impact of REOB on moisture damage resistance showed higher moisture sensitivity with increasing REOB content, but it did not interfere with liquid anti-strip additives. Mixture cracking tests results were mixed. Low temperature relaxation, strength and fracture measured with TSRST showed the fracture strength had slight increases or decreases with 2.5% and 6% REOB and could be interpreted as unaffected. Aging also improved the average strength of these mixes. However, the strength of the highest 15% REOB mix was measurably decreased and made worse by aging. The impact of REOB on intermediate temperature fatigue cracking performance depended on the aging condition and whether stress-control or strain-control performance was considered. To minimize risks a best practice needs to be developed which includes a maximum use level, taking into account both the variability of REOB and the effects on asphalt binders from different sources. Asphalt binder specifications might be refined by placing lower limits to BBR stiffness or a maximum allowable DTcritical. More focus should be placed on intermediate REOB levels such as near 10% to the 6% used in this study to better identify performance pitfalls and performance benefits from REOB and the corresponding DTcritical.
Using Binder and Mixture Space Diagrams to Evaluate the Effect of REOB on Binders and Mixtures after Aging
This study was conducted to address some of the New England state transportation agencies’ concerns associated with the use of Re-refined Engine Oil Bottoms (REOB) in asphalt binders and mixtures. The effects of REOB on the physical and rheological properties of an asphalt binder were investigated. This evaluation was completed after AASHTOspecified short-term and long-term aging and after extended long-term aging. Also, the effect of REOB modified binders on the performance of asphalt mixtures after short- and long-term aging was evaluated in terms of moisture damage, rutting, and cracking. Two straight run binders (PG 58-28 and PG 64-22), a typical PG 64-28, two sources of REOB, an aromatic oil, and PolyPhosphoric Acid (PPA) were utilized. The PG 64-22 was modified with each source of REOB separately to attain a PG 58-28. These modified PG 58-28 binders were compared to the straight run PG 58-28. The modified and the straight run PG 58-28 binders were further modified with PPA to attain a PG 64-28 which is commonly specified in the Northeast. This modified PG 64-28 was compared to a typical PG 64-28 that has been used in the past without REOB. This same binder modification to attain a PG 58-28 and PG 64-28 was repeated with the aromatic oil for comparison purposes. Rheological results plotted on the Black Space and wo—R-value Space diagrams showed that the addition of REOB to attain the PG 58-28 caused the binders to age more relative to the straight run binder. The results also indicated that the use of higher dosages of REOB can cause increased binder aging. Mixture moisture damage tests showed that REOB could result in mixture premature failure. The rutting tests indicated that generally the REOB did not cause the mixtures to fail. Cracking tests conducted at intermediate temperature on REOB-modified mixtures generally indicated a reduction in fracture energy, as compared to the straight run or typical binder counterpart. A flexibility index (FI) calculated from the same cracking test showed that the aromatic oil modified mixtures generally had a higher value than the REOB modified mixtures suggesting less susceptibility to cracking. Interaction plots developed using the FI and Hamburg test results revealed significant differences among mixes with various binder formulations and the effects of long-term aging on the interaction plots were evident. Low temperature cracking evaluations detected minor effects on low-temperature fracture properties associated with various combinations of REOB tested. Performance space diagrams indicated that the REOB modified mixtures remained within the passing zone in a Hamburg Wheel Tracking-Disc Shaped Compact Tension DC(T) tests diagram for low to medium traffic level. Overall, the particular mixture tests used in this study did not provide evidence that using REOB decreases performance. The performance data collected by using the two REOB sources varied by the particular test, and sometimes by the type of REOB or by the PG of the binder. The use of aromatic oil did not show this variability and the results generally indicated good performance. The variability of the data when REOB was used underscores the importance of thoroughly testing REOB modified binder and mixture for performance. Finally, other important issues like dosage and consistency of REOB are still currently being investigated.
Effect of Rejuvenator on Performance Characteristics of High RAP Mixture
As more reclaimed asphalt pavement (RAP) is utilized in asphalt mixtures, there are increasing concerns about the potential negative effect of the aged RAP binder on the field performance, especially cracking resistance, of high RAP mixtures. To address the concerns, there has been increasing interest in utilizing rejuvenators to improve the cracking performance of high RAP mixtures. The objective of this study was to determine the benefits of using a new rejuvenator made from renewable sources in asphalt mixtures with high RAP contents. The study was conducted by determining and comparing the laboratory performance properties of three mixtures and the binders extracted from the mixes. The three mixtures evaluated in this study included two 50% RAP mixtures (RAP binder ratio = 0.55) with and without the rejuvenator and a comparable virgin mix. Results of this study suggested that the new rejuvenator was effective in improving both the intermediate and low temperature cracking performance characteristics of the 50% RAP mix close to those of the virgin mix at the short-term laboratory aging condition without affecting its rutting and stripping resistance. It is recommended that a field study of this rejuvenator be conducted to further evaluate its effect on the long-term field performance of high RAP mixes.
Asphalt Mixtures Containing RAS and/or RAP: Relationships amongst Binder Composition Analysis and Mixture Intermediate Temperature Cracking Performance
The use of recycled asphalt shingles (RAS) as a partial replacement for petroleum-based virgin asphalt binder has received considerable attention in recent years. The objective of this study is to correlate the molecular structure and corresponding compositional analysis of asphalt binders of conventional asphalt mixtures as well as of mixtures containing recycled asphalt shingles (RAS) and/or reclaimed asphalt pavement (RAP) with their cracking potential at intermediate temperature. Laboratory testing evaluated the molecular composition of asphalt binders obtained from asphalt mixtures evaluated in this study using thin layer chromatograph/FID (Iatroscan) and gel permeation chromatography (GPC). Fracture resistance of laboratory-produced mixtures was assessed using the semicircular bend (SCB) test at intermediate temperature. Molecular fractionation through GPC of RAS samples confirmed the presence of associated asphaltenes in greater concentrations than recycled asphalt pavement (RAP) samples. High concentrations of high molecular weight asphaltenes decrease the fracture resistance of the asphalt mixtures. The use of rejuvenating agents, Cyclogen-L, Hydrogreen, asphalt flux and re-refined engine oil bottoms (REOB), did not reduce the concentration of the highly associated asphaltenes; further they failed to improve the cracking resistance of the asphalt mixtures evaluated in this study.
Size Effect in Asphalt Mixture at Low Temperature: Type I and Type II
Low temperature cracking is a serious distress for asphalt pavement built in cold regions, such as the northern U.S. and northern Europe. Therefore, accurate assessment of the strength and fracture properties of asphalt mixtures is fundamental for ensuring the long term integrity of the entire pavement structure. It has been shown that asphalt mixtures behave in a quasibrittle manner at low temperatures and, consequently, their nominal strength strongly depends on the structure size. Most of the research performed in the past has experimentally addressed this phenomenon either on unnotched or deep-notched specimens corresponding to Type I and Type II size effects, respectively. However, the evolution of the pavement conditions during the service life can lead to the formation and propagation of cracks which strongly affect the response of the material and, eventually, determine a complex size effect. In this paper a comprehensive experimental study is performed to evaluate the effect of different notch depths on the scaling law for quasibrittle fracture. Three-point bending tests are performed on plain and notched asphalt mixture beams of different sizes at low temperature. The results on unnotched specimens are analyzed through the Type I energetic-statistical size effect law (SEL) in combination with the weakest link model, while, in the case of deep notched specimens, the Type II SEL is used. A significant variation in the evolution of the SEL is observed with a dramatic decrease in strength over size and notch length. In addition, an empirical relation between the initial fracture energy, Gf, and the total fracture energy, GF, is found, and the characteristic length, cf, associated to the length of fracture process zone, is determined.
Fundamental Evaluation of Moisture Damage in Warm-Mix Asphalts
Warm-mix asphalt (WMA) technologies have been used extensively in the last decade. The benefits of WMA have motivated contractors and agencies to expedite the implementation of this technology. However, some research studies have raised concerns regarding WMA laboratory performance in terms of resistance to moisture damage, while WMA has demonstrated good performance in the field. These experiences motivated further research to understand the fundamental characteristics of WMA. This study comprised a comprehensive evaluation of WMA prepared using different aggregate sources, asphalt binders, and WMA additives. A dynamic mechanical analyzer was used to test the mastic phase of conventional hot-mix asphalt and WMA. The test specimens were evaluated at different conditions—dry and wet—and at different aging stages—unaged and 3-month aged—in a controlled environmental room (i.e. 60°C). A fracture mechanics approach was used to analyze the test results. This approach incorporated fundamental material properties including adhesive bond energy between aggregates and asphalt binder. The results show that WMA performance improved with aging and the overall performance of the WMA can be improved if the selection of materials (i.e., aggregate source, asphalt binder, WMA technology) is optimized based on the compatibility of their surface energy. In addition, surface energy results were able to explain some findings from the mechanical testing related to moisture susceptibility of WMA.
Investigation of Binder Aging and Mixture Performance of In-Service RAP Mixtures
In 2007, the Virginia Department of Transportation piloted a specification allowing up to 30% reclaimed asphalt pavement (RAP) in certain dense-graded asphalt surface mixtures while changing virgin binder grade requirements. The change affected only mixtures requiring an end binder grade of either PG 64-22 or PG 70-22. For mixtures specifying PG 64-22 binder, the virgin binder grade at RAP contents of 30% or less was no longer required to change. For mixtures specifying PG 70-22 binder, the virgin binder grade at RAP contents of 21-30% was no longer required to change from PG 64-22 to PG 64-28. Prior to this, both types of surface mixtures were allowed to contain only up to 20% RAP before binder grade adjustments were required. An initial laboratory study of mixtures produced under the pilot specification indicated that there were no significant differences for fatigue, rutting, and susceptibility to moisture between the higher content (21-30%) RAP mixtures and control mixtures (having 20% RAP or less). The current study evaluated the inservice performance of these mixtures after approximately 7 years and encompassed field visits and a laboratory investigation of a sample of 23 in-service pavement sites used in the initial laboratory evaluation. Cores were collected from each site and used to evaluate the binder and mixture properties. Binder data were compared to data from the original construction when available to assess the changes in properties over time. Overall study results revealed no systematic effect on field and laboratory performance with increasing RAP contents up to 30%. Test results from roadway cores showed no conclusive trends in performance with RAP content. Testing of extracted binder indicated that RAP content appears to have an influence on the rate of aging of virgin binder–RAP blends; initial grades were lower for blends having lower RAP contents, although after 7-8 years of service, all blends aged to similar grades. Binder analysis also revealed that depth within the surface layer (in this case, the top half versus the bottom half) significantly affects binder properties, with stiffness decreasing with depth. However, increasing RAP contents appeared to mitigate the difference in failure temperature before and after aging, possibly attributable to the preexisting aged composition of the RAP and its influence on the virgin binder properties.
Use of Performance Based Testing for High RAP Mix Design and Production Monitoring
In 2014 the Wisconsin Department of Transportation (WisDOT) and industry developed a pilot program for hot mix asphalt (HMA) with higher recycled asphalt content that required use of performance tests during mix design and production. Following the balanced mix design concept mixture tests were selected to address rutting resistance after short-term aging and durability after long-term aging. The test selected were the Hamburg Wheel Tracking (HWT) test, the semi-circular bend (SCB) test at intermediate temperature and the disc-shaped compact tension (DC(T)) test at low pavement temperatures. Asphalt binder extraction and grading from aged mix was also required. The focus of this paper is to summarize the mixture performance test and recovered binder data gathered during the pilot project on STH 77 in Ashland County, Wisconsin; suggest modifications to the SCB test procedure; and present accelerated aging protocols for continued use of performance testing in practice. Semi-circular bend test results collected during the project at 25°C did not relate well to values published in the literature or show adequate sensitivity to changes in mix properties. The effects of test temperature and an alternative analysis method are presented. Based on the results recommendations include use of a climate based approach for test temperature selection and inclusion of post peak analysis to better discriminate between mix composition and aging conditions. Accelerated long-term aging protocols involving loose mix aging at 135°C for 12 and 24 hours are compared to AASHTO R 30 compacted mix aging using recovered binder and mixture fracture properties. Results found that 12 hour loose mix aging produced similar recovered binder grading to AASHTO R 30, whereas the effect of aging on mixture fracture tests was inconclusive. The relationship between laboratory and field aging is investigated through comparison of field cores to laboratory aged plant produced mix from a project constructed in southeast Minnesota in 2006. Lastly, the laboratory performance of the high recycled and conventional mix designs are compared on the basis of mixture cracking resistance and recovered asphalt binder properties after extended aging. The high recycle mix exhibited equal or better performance relative to the conventional mix across all selected performance tests. This comparative analysis also provides an example of how the inclusion of performance testing can influence the materials selection process and produce test results indicative of improved overall performance of the mix.
Forensic Laboratory Tests to Evaluate Long-Term Performance of Reclaimed Asphalt Pavements: Connecticut Case Study
This paper presents the most significant findings from the forensic evaluation of the long-term cracking performance of asphalt mix designs including Marshall and Superpave mixes with various performance grades of binders and RAP content of 25% total weight of aggregates. The experiment targeted comparison of permeability, stiffness, low-temperature behavior, and oxidation susceptibility of the mixes and correlation of those properties with deflection and cracking data from the six LTPP SPS-9A sections on Route 2 in Connecticut. The mechanical testing in the laboratory included measuring hydraulic conductivity by a Flexible Wall Permeameter, dynamic complex modulus by the Asphalt Mixture Performance Tester (AMPT), creep compliance and tensile strength by Indirect Tension Test, and fracture properties by Semi-Circular Beam (SCB) test. The evaluation of field performance included analysis of deflection basins and back-calculated elastic moduli from Falling Weight Deflectometer data as well as visual evaluation of surface distresses, such as cracking and weathering. The forensic laboratory testing revealed reasonable correlations between some laboratory test results and field performance. For instance, the dynamic modulus values measured by AMPT at 20 C at the highest and lowest frequency were found to be similar to the backcalculated asphalt layer moduli. The extent of transverse cracking appeared to be highly associated with the Young moduli estimated from SCB fracture energy and toughness. The amount of longitudinal wheelpath cracking correlated better with SCB fracture energy. On the other hand, neither fracture properties nor tensile strength was found to be correlated with the extent of longitudinal joint cracking observed. The laboratory testing revealed overall higher stiffness and oxidation in RAP-containing mixes. The use of those stiffer mixes, however, did not affect much load-related performance of the experimental pavement sections. On the other hand, a very fast deterioration of longitudinal joints occurred in all pavement sections, which was found most likely related to creating cold joints during paving. This phenomenon has been reduced in current practice with the introduction of wedge joints by the CTDOT.
Performance-Space Diagram for the Evaluation of High and Low Temperature Asphalt Mixture Performance
This paper presents a simple, yet powerful method for simultaneously evaluating the high and low temperature performance of asphalt paving mixtures for the purpose of mixture design, evaluation, and forensic investigation. A performance-space diagram approach is described, with an emphasis on Hamburg-DC(T) plots presented in this paper. Specifically, a plot of Hamburg wheel tracking results, plotted in reverse order on the y-axis using an arithmetic scale, along with DC(T) fracture energy results, plotted on the x-axis, constitutes the Hamburg-DC(T) plot. Plotting candidate mixture designs, research results, etc., yields a surprising amount of insight towards mixture variables that affect overall performance. For instance, substitution of one straight-run binder grade for another results in a clear, predictable trade-off in the Hamburg-DC(T) performance space. Polymer modified grades, on the other hand, provide a more beneficial shift in the Hamburg-DC(T) space. The benefits of using this approach in the design of mixtures containing recycled asphalt mixture and recycled asphalt shingles are also presented. Effects of rejuvenators and the benefits of stone mastic asphalt designs are also demonstrated. Finally, a broad look at a large database of mixtures recently designed in Illinois is presented.
A Novel Back-Calculation Approach for Determining the Rheological Properties of RAP Binder
The determination of the rheological properties of the aged binder in reclaimed asphalt pavement (RAP) materials is a challenging problem. Conventionally, extraction and recovery are used to obtain the RAP binder for further experimental characterization; however, this procedure is not entirely reliable and accurate. Alternative and more precise approaches are based on asphalt mixture tests in combination with complex and sophisticated back-calculation methods which are costly and time consuming. In this paper a new and simple approach to estimate the rheological properties of RAP binder at intermediate temperature is proposed. This is based on Dynamic Shear Rheometer (DSR) tests performed on mortars, composed of a selected fine fraction of RAP and virgin binder, together with a new back-calculation solution. The properties of the bituminous blend of virgin and RAP binders are obtained through the manipulation of the Nielsen model equation to take into account the effects of frequency and temperature on mortar stiffness. The Voigt model is then used to estimate the complex modulus and the phase angle of the RAP binder from the complex modulus and the phase angle of the back-calculated binder blend.
Relaxation Spectra of Asphalt Binders and the Christensen-Anderson Rheological Model
The development of the well-known Christensen-Anderson (CA) rheological model grew out of attempts to model the relaxation spectra of asphalt binders using a skewed logistic distribution function. For this reason, there are very strong relationships between the CA model parameters and the characteristics of relaxation spectra for asphalt binders. This paper presents a recently developed equation that allows direct and accurate calculation of the relaxation spectra from CA model parameters, demonstrating the nature of this relationship. Of the CA model/spectrum parameters, the most important in terms of describing overall behavior and potential performance is the R-value, which describes the shape and skewness of the spectrum. This parameter and other similar rheological parameters have been linked to various important aspects of asphalt binder behavior, including fatigue resistance, chemical composition and degree of oxidative aging. This makes the parameter R a potentially useful parameter for inclusion in asphalt binder specifications; although care must be taken in how it is determined to ensure that it is accurate, repeatable and reflects the performance characteristics of interest.
A Mixture-Based Black Space Parameter for Low Temperature Performance of Hot Mix Asphalt
Low temperature cracking is a critical distress form and is heavily influenced by the relaxation and strength capabilities of the material. These properties are related to the stiffness through principles of viscoelasticity. Recently, there is elevated pressure on decisions made by pavement and materials engineers to produce the longest lasting, most resourceful pavement systems possible to optimize monetary and non-renewable resource usage. The primary objectives of this study are to: 1) assess the value of a parameter which can describe low temperature cracking resistance by using dynamic modulus (|E*|) and phase angle () of the mixture and laboratory-measured performance; 2) present shape parameters of a mixture master curve that are directly related to the relaxation spectra, which is expected to play a pivotal role in low temperature distress resistance with aging; 3) define failure lines in Black Space which correspond with laboratory-measured performance and operate under a well-understood basis reinforced by the literature; and 4) provide agencies with a tool to aid in the movement towards a performance-based mixture design, acceptance, or rehabilitation decision-making system. An analysis of the mixture master curve is done to establish parameters which describe the relaxation spectra and aging potential of materials. A mixture-based Black Space parameter is presented based on results from the |E*| master curve construction and the thermal stress restrained specimen test. This approach holds promise, but must be calibrated with a robust database before serious implementation considerations are made. Future work will look to determine a common stiffness condition to better define the failure threshold and to identify possible alternatives to the modified Glover- Rowe function used in this study. Further evaluation is also needed to optimize the temperature-frequency combination of the Black Space parameter itself and ensure a condition is specified that can be captured by test equipment an owner agency or contractor may possess as part of a performance-based specification framework.
Effect of Asphalt Binder Modification Type on Low Temperature Performance Determined Using Asphalt Concrete Cracking Device (ACCD)
For current asphalt paving practice, many unconventional asphalt binders and additives have been introduced. Through physical and chemical interactions with base binders, these modifications usually affect the binder and mix strength and fracture properties as well as stiffness. However, the currently used binder grading system is based on the low temperature stiffness and relaxation behavior as measured using the Bending Beam Rheometer (BBR). The absence of strength and fracture properties in the asphalt binder grading process may result in inaccurate prediction of field performance. Five PG grade asphalt binders including unmodified, polyphosphoric acid (PPA) modified and styrene-butadiene-styrene (SBS) modified binders were tested using the BBR and the Asphalt Binder Cracking Device (ABCD). Asphalt mixes prepared with these five binders were also tested with a revised Asphalt Concrete Cracking Device (ACCD) test procedure. The test results showed improved strength and fracture resistance characteristics for SBS modified asphalt binders in the ABCD test and mixtures in the ACCD test, resulting in better performance than that predicted by the BBR low temperature PG grade. However, the PPA modified asphalt binder and the corresponding mixture exhibited worse performance in the ABCD and ACCD tests, respectively, than that predicted by the BBR low temperature PG grade, which may be attributed to the poor fracture resistance.
Evaluation of the DC(T) Test in Discerning the Variations in Cracking Properties of Asphalt Mixtures
As the temperature of an asphalt pavement drops to below freezing point, the asphalt material starts to lose its ductility, and consequently, the asphalt mixture becomes more susceptible to cracking. The disk-shaped compact tension [DC(T)] fracture test has been used to quantify the fracture properties of asphalt concrete at subzero temperatures for more than a decade. The Asphalt Institute laboratory has successfully utilized the DC(T) test in several research studies. As a result of these studies, a valuable database has been gathered which represents the sensitivity of the DC(T) test, and exhibits how the test is capable of capturing the effects of various factors on the performance of asphalt mixtures at low temperatures. This paper briefly presents the findings from some of these studies. The paper presents the response of the DC(T) test to the variations in several factors relative to asphalt pavements including the crude source of the asphalt binder, aging of the asphalt mixture, deficiency in the in-place density, pavement temperature, using a warm-mix agent, using RAP and RAS materials, and chip sealing as a preservation method. The test proved to be well capable of capturing the variations in these factors; therefore, it can be utilized by pavement managers to assess changes in specimens from asphalt pavements over time to help determine if any maintenance or rehabilitation action is required. Furthermore, the test can be used to evaluate laboratory-made mixtures and ascertain if they would perform satisfactorily in their designated environmental conditions.
Evaluation of Cracking Performance for Polymer-Modified Asphalt Mixtures with High RAP Content
Fourteen reclaimed asphalt pavement (RAP) mixtures designed with different combinations of RAP sources, contents (up to 40%) and mixture conditioning levels were evaluated to determine the maximum allowable amount of RAP material in surface courses without jeopardizing pavement cracking performance. Extracted RAP binder was blended with virgin polymer-modified asphalt (PMA) binder at various RAP binder replacement ratios. All blends behaved effectively as PMA binder as they met the multiple stress creep recovery (MSCR) % recovery requirement, and in addition they had satisfactory binder fracture energy density (FED) values. RAP gradation was found to significantly affect the fracture properties of RAP mixtures as it controls the distribution of RAP binder and potentially the degree of blending between virgin and RAP binder. Increased RAP content resulted in stronger (i.e., higher tensile strength) but more brittle (i.e., lower failure strain and lower mixture fracture energy) mixtures. However, after long-term oven aging (LTOA) plus cyclic pore pressure conditioning (CPPC) which was used to simulate long-term field aging conditioning, all RAP mixtures still exhibited dissipated creep strain energy to failure (DCSEf) values above 0.75 kJ/m3 and energy ratio (ER) values well above 1.0, indicating acceptable cracking performance. It must be emphasized that all RAP mixtures had good gradation characteristics as they all met Superpave design criteria and dominant aggregate size range and the interstitial component (DASR-IC) requirements. Therefore, satisfactory inclusion of up to 40% RAP was acceptable for well-designed PMA mixtures
Glass and Carbon Geogrid Reinforcement of Asphalt Mixtures
Application of geogrids for asphalt layers in road and airport pavements has been studied since early 1980’s. This type of geosynthetics is feasible to mitigate reflective cracking and to prolong fatigue life, i.e. to reinforce asphalt layers within a pavement structure. Up to now, many laboratory and field studies have demonstrated significant benefits of using geogrids but still several aspects of their working mechanism within adjacent asphalt layers remain not fully understood and need to be further explored. This paper presents an initial part of a long-term study initiated in Poland in order to fully assess the benefits of using geogrids within asphalt layers. The first part of this paper presents a state-of-the-art review on geogrid applications in asphalt layers. The next part demonstrates and discusses the results obtained on geogrid reinforced beams under monotonic and cyclic loading conditions. Three types of large beams were prepared under laboratory conditions. One group comprises reference beams without any reinforcement and the other two were reinforced, respectively, with glass and carbon geogrids. Specimens were evaluated in a custom-made 3PB/4PB fixture at 13ºC in order to observe their behavior under force-controlled fatigue test and displacement-controlled fracture test. Results demonstrated a beneficial impact of fiber reinforcement on the development of beam deflections in the fatigue tests and significantly higher fracture energy in the post-peak softening region. These results will be used to prepare a new experimental campaign to closely investigate the reinforcement mechanism of geogrids within asphalt layers. The paper concludes with short- and long-term plans necessary to develop an official design procedure for the pavements with geogrid reinforced asphalt layers in Poland.
Development of Oxidation Kinetics Models for Rheological and Damage Properties Based on “In-Serviceâ€ Asphalt Binders
Oxidative aging can lead to the susceptibility of asphalt pavement to cracking. Most studies on oxidative aging of asphalt binders are related to the prediction of the aging properties of asphalt binders based on oxidation kinetics. Development of kinetics models often involves short-term and long-term aging in the laboratory of original binder at different temperatures and for different times, and experiments of original and aged binder. However, for many rehabilitation projects or forensic studies, the original materials of pavements that were constructed years ago are not readily available to develop kinetic models. The development of a kinetic model based on asphalt binder in an in-service pavement is greatly needed to predict the properties of asphalt binder subjected to further aging for the purpose of overlay design, or to backcalculate the original properties of binder before aging for the purpose of forensic studies. This study develops a methodology to develop Arrhenius kinetics aging models using asphalt binders that already have been aged, instead of using original binders. Five asphalt binders (one neat and four modified) were included in this study and were subjected to three stage aging. The carbonyl areas of the aged binders were determined using Fourier transform infrared spectroscopy, and the rheological properties and damage properties were determined using a dynamic shear rheometer (DSR). First, the relationships among the model parameters of the oxidation kinetics models for the crossover modulus, DSR function, shear strength, and critical strain energy density of the asphalt binders were developed based on aging of original materials. Based on the relationships between model parameters and experiment results of asphalt binders that have been aged separately, Arrhenius kinetics aging models were developed. The effectiveness of these models which are developed based on aged binder were verified. The developed kinetic models for crossover modulus, DSR function, shear strength, and critical strain energy density of the asphalt binders can be used to predict the properties of asphalt binders subjected to further aging in the field for the design of overlays in the Mechanistic-Empirical Pavement Design Guide or Pavement ME, or to determine the properties of original asphalt binders before aging for the purposes of forensic studies, based on the cumulative aging effects of in-service pavement temperatures.
Asphalt Mix Cracking State of the Art: Materials, Design, and Testing
Mix testing that can properly characterize cracking of asphalt pavements is one of the most relevant present day issues for the industry. The 91st Annual Meeting (AM) of the Association of Asphalt Paving Technologists (AAPT) provides evidence to this effect as, in the assessment of the authors, cracking was far and away the meeting’s prevailing theme. The 91st AAPT AM began with a Leading Edge Workshop (LEW) that focused exclusively on cracking and associated test methods, and a key factor in most of the remaining content presented at the meeting was cracking. This paper stemmed from the 91st AM LEW in response to cracking’s perceived prevalence in the minds of so many AAPT members. The primary objective of this paper is to provide the state of the art in asphalt mix cracking from the perspective of the authors and to use this perspective to explain what has been accomplished in the cracking arena and what needs to be accomplished. First, a historical perspective beginning in the mid-1950s and ending in present day is presented that is generally divided into pre and post Superpave mix design. With this historical perspective as a reference, emphasis is shifted to the AM and LEW as they pertained to cracking. A set of prevailing themes was established and used as a segue into discussion of cracking test methods (first what has been accomplished and then what needs to be accomplished). Six areas were identified by the authors where advancement needs to be made with regard to mitigation of cracking: 1) closing the gap between the state of the art and the state of practice; 2) evaluating rapid mix test methods usable during production; 3) emphasizing education; 4) improving reliance on accelerated pavement testing; 5) improving cracking models; and 6) performing large test programs where parallel testing occurs on comparable mixes by way of several test methods.
Bitumen: Current Situation and Future Challenges
Optimized Mix Design Approach— Contractor’s Perspective
Mix durability related performance issues have been observed in recent years with a commonly reported contributing factor being a deficiency in mix asphalt binder content. In many instances, these performance issues are occurring with mixes that have met all governing mix specifications. As a result, asphalt mix specifications are changing rapidly as agencies search for solutions to improve durability. Lower design gyrations (Ndesign), lower target air voids, and increased voids in the mineral aggregate (VMA) are among the implemented specification modifications. Often, individual specification changes are implemented without evaluating their potential conflict with other specification items or at a rate that makes establishing true “cause and effectâ€ on mix performance difficult or impossible to determine. Oldcastle Materials recognizes the need for improved mix performance and has developed a framework for an Optimized Mix Design Approach (OMEGA) in which innovation and mix engineering efforts are emphasized. The ultimate design approach objective is determining the optimum mix binder content for the specific mix design application (e.g., design traffic, pavement layer location, climate, etc.). This is accomplished through an enhanced materials evaluation and selection process, along with appropriate performance testing (stability and durability/cracking) to help ensure the desired end result performance is provided by the designed mix.
AAPT Symposium: Development and Implementation of Balanced Mix Design in Texas
This paper briefly presents the development, implementation, and challenges of balanced mix design method in Texas.
Moving Forward from Performance-Based Design to True Balanced Mixture Design in New Jersey
Implementation of a Balanced Asphalt Mixture Design Procedure: Louisiana's Approach
Conventional asphalt mixture design methodologies such as Superpave, Marshall, and Hveem are used to determine the optimum asphalt binder content by means of physical and volumetric laboratory measurements. All three procedures ensure the materials proportion and quantity of the asphalt cement binder are adequate to meet stability and durability concerns. However, with the increased use of recycled materials, there is a need to develop laboratory mechanical tests in order to evaluate the quality of the asphalt cement binder to complement the Superpave volumetric mixture design procedure. An important component to successful mixture design is the balance between volumetric composition and material compatibility. Balanced asphalt mixture design offers innovation in designing mixtures for performance and evaluation of the quality of a mix design relative to the anticipated performance using a rational approach. This research documents the selection of laboratory mechanical tests, in addition to volumetric requirements, that can ascertain a mixture’s resistance to common asphalt pavement distresses. Factors in the selection of laboratory mechanical tests such as availability of standard test procedures, advantages and limitations, laboratory-to-field correlations, and sensitivity to mixture composition are reviewed. Further, an implementation framework and case histories is also discussed