Increasing Asphalt Pavement Density Through Mixture Design: A Field Project
Modifying the asphalt mixture design appears to be a viable method for obtaining improvements in pavement durability. This paper reports a demonstration project focused on achieving an optimal 95% in-place density through a modified mixture design. The project tasks included milling and overlaying an existing pavement in Indiana. Control and test mixtures were used to evaluate the effect of mixture design modifications on the asphalt pavement construction performance. The control mixture was designed in the laboratory at 4% air voids using the conventional Superpave volumetric mixture design method. In contrast, the test mixture was prepared at 5% air voids using a modified laboratory mixture design. Quality control and quality assurance data analysis reported a 93.3% and 95.3% inplace density average for the control mixture and test mixture, respectively. Additionally, new probabilistic and analytical metrics are proposed to compare the construction performance of both mixtures. The findings of this field study validate the potential benefits of using a modified mixture design to construct asphalt pavements with increased densities. Implementation of this methodology can be easily accomplished using conventional laboratory and construction equipment.
Laboratory and Full Scale Evaluation of Recycled and Warm Mix Asphalts
This study evaluated the cracking characteristics of asphalt materials containing RAP/RAS and prepared with WMA technology. Tests were performed in the laboratory and at a full-scale testing facility. Ten test lanes were built at FHWA’s Accelerated Loading Facility (ALF). The experimental design included three RAP percentages up to 44% by weight (40% recycled binder ratio, RBR), two WMA technologies (water foaming and chemical additive), one RAS percentage with 20% RBR, and two different virgin binders (PG 58-28 and PG 64- 22). Specimens prepared from loose mixes sampled from the construction and field cores sampled at different times were evaluated using the direct tension monotonic test. Performance grade and cracking resistance of asphalt binders recovered from tested loose mix and field cores were determined. The laboratory monotonic testing results were compared to and statistically correlated to the ALF field cracking performance. Experimental results from both laboratory mix and binder tests capture the oxidative aging that occurs with time in the top lift. Aging observed in the bottom lift of the asphalt pavement was considerably less. One of the mechanical parameters developed from the mix monotonic test closely correlated with the binder tolerance strain obtained from the binder DENT test. Long- term oven aging aged the mix significantly more severely than was observed in threeyear old ALF sections. Testing of ALF materials shows that the mixtures with 40% RAP RBR or 20% RAS RBR and stiff binder exhibited the worst cracking performance. A softer PG grade was found to be effective at improving the performance for 40% RAP RBR mixes but ineffective at improving the performance of 20% RAS RBR mix. No difference in field performance was observed between the HMA and WMA mixtures having the same mix design. Statistical analysis indicated a strong correlation between the direct tension monotonic mix test and ALF field testing in terms of evaluating the cracking
This article presents the results of the LE2AP Life+ project. LE2AP is acronym for Low Emission2 Asphalt Pavement, where the 2 indicates that emission of pollutants and noise are considered. LE2AP concentrates on a novel way of circular asphalt recycling. Key issues in LE2AP are that reclaimed asphalt is first decomposed into its components. Reclaimed aggregates hardly containing bitumen and bitumen rich mortar sand, which is a mixture of bitumen, filler and sand, are obtained. The mortar sand is used as the main ingredient for quality LE2AP mortar. During production LE2AP mortar is heated without meeting a flame or superheated air and is homogenized and treated with rejuvenator and soft bitumen. LE2AP mortar is then foamed and fed into the mixer where it is mixed with reclaimed stone at 100-110°C to obtain an asphalt mixture of high quality, containing a high percentage of reclaimed material and produced at low temperature. The LE2AP project is partially funded by a LIFE+ grant and it is believed that LE2AP may contribute to much needed circular asphalt recycling at lowered temperature and low emissions. LE2AP came to provisory conclusions in October/November 2016 with the installation of two two-layer PA, porous asphalt, test sections with a combined length of 2.3 km. Key properties of these test sections are: production temperature: 110-125°C, noise reduction: 8.3-8.4 dB(A), re-use: 82%-93%, CO2 reduction: 51%. The performance of the test sections is being monitored and until now (September 2017) the sections perform well.
Effects of Modifiers and Binder Properties on the Performance of Asphalt Mixtures in the Hamburg Wheel-Tracking Device Test
The Hamburg Wheel Tracking Device (HWTD) test has been widely used in practice and reported to be successful in identifying hot mix asphalt (HMA) mixes that are prone to rutting and/or susceptible to moisture damage. This paper presents a comprehensive study aiming to offer informative references for pavement engineers to select modified asphalt materials with good moisture and rutting resistance. First, the impacts of these modifiers on the HWTD test results were investigated. Based on the degree of their improvement in the HWTD results, additives were classified into the following three grades: (1) the first grade including branched styrene–butadiene–styrene (SBS) and Gilsonite; (2) the second grade including linear SBS, high-density polyethylene, and polyphosphoric acid; and (3) the third grade including asphalt rubber (AR) and terminal blend (TB) asphalt rubber. In addition, the effects of modifier content on the Hamburg performance of the asphalt mixes were studied. It was found that higher modifier dosages do not necessarily result in the improvement of Hamburg performance. The results show that an optimal content existed for most additives, whereas a poor-performance dosage range (10.0 % - 18.0 %) existed for crumb rubber content in the AR. Finally, based on the results of the various test materials, the roles of different properties of modified asphalt binders on the Hamburg performance were also investigated. The healing, adhesive, viscosity, and elastic properties (HAVE) of modified binders were found to play different roles: the healing property of modified asphalt binder is a necessary factor; the adhesive property is a fundamental factor; the viscosity has a maximum limitation (3.5 Paâˆ™s at 135°C); and the elastic property due to the modification is the determining factor in achieving good Hamburg performance of modified asphalt mixtures.
Joint Sealant Performance on Asphalt Mixture Longitudinal Joints
Due to the nature of construction, it is common for longitudinal joints in asphalt pavements to have lower densities and higher permeabilities than the main portions of the pavement lanes. To address this concern, many states employ joint treatments such as fog seals or void reducing asphalt membranes (VRAM). Qualitative evidence in Indiana appears to indicate that longitudinal joint lives have been improved using joint treatments, but the specific materials and application rates used to treat longitudinal joints in Indiana has not been quantitatively investigated. This research aims to investigate the fog seal materials and application rates specified for use on longitudinal joints in Indiana and to compare the performance of fog seal and VRAM treatments to provide guidelines for future joint treatments. These objectives were accomplished by employing laboratory testing of both laboratory prepared specimens and field samples. The research performed on the laboratory specimens found the application of fog seals can improve the performance of the longitudinal joints with respect to permeability. While the permeability of the asphalt specimens was reduced by the presence of a fog seal treatment, the benefits were irrespective of the fog seal material. The results also indicate that the fog seal should be reapplied at five to seven year intervals. The testing of the field samples indicated that both the SS-1h fog seal and VRAM treatments were effective in reducing the permeability of the asphalt mixtures. The VRAM samples had statistically higher permeability coefficients than the SS-1h fog seal samples, which may be attributed to potential construction or material issues. While the SS-1h fog seal treatment appears to have better performance than the VRAM, the effectiveness of the treatments over time is not known. Additional research is recommended to verify and support these results and recommendations and to further compare and understand the performance of SS-1h and VRAM treatments over time.
Long-Term Field Performance of Flexible Pavements Using Warm Mix Asphalt Technologies
An increased awareness of sustainability in the pavement industry has encouraged the use of warm mix asphalt (WMA) technologies. Compared to conventional hot mix asphalt (HMA) that requires a high production temperature, WMA has several benefits, such as saving fuel and energy, reducing greenhouse gas emissions, and improving the work environment. However, systematic analysis of long-term field performance for pavements containing WMA mixtures has been scarce. Therefore, the objectives of this paper are to evaluate the field performance of flexible pavements using WMA technologies (referred as WMA pavements in this paper) and compare the general trend of the longer-term performance between WMA and HMA pavements across the United States. Specifically, 28 WMA pavement projects along with their companion HMA pavements were evaluated for an extended performance period in terms of transverse cracking, longitudinal cracking, rutting, and moisture damage. A companion HMA pavement refers to the an pavement that shares similar pavement structure, climate, traffic conditions and mixture design with the WMA pavement; the main differences are the usage of WMA technologies in the surface layer and the reduced production temperature for WMA mixtures. The selected projects include five projects constructed during the course of the study, and 23 in-service projects covering different service ages, traffic volumes, pavement structures, WMA technologies, and four climatic zones across the United States. It was found that pavements containing various WMA technologies exhibited comparable long-term field performance as compared to that of the companion HMA pavement in terms of transverse cracking, wheel path longitudinal cracking, and rutting. No moisture-related distress was found in the field for either HMA or WMA pavements. Overall, cracking and rutting performance show a clear pattern of climate influence. Cracking distress appears to be more of a concern within wet climatic zones while less typical in dry climatic zones, which suggests that moisture should be considered in evaluating the cracking potential of asphalt mixtures. Results presented herein were part of NCHRP Project 9-49A on the Performance of WMA Technologies: Stage II—Long-Term Field Performance.
A System for Real-Time Measurement of Moisture in Aggregate Mixes During Asphalt Production
In asphalt plants, moisture fluctuations in aggregate stockpiles present a problem for process control. To improve the production process, real-time methods of monitoring aggregate moisture in stockpiles, cold feed bins, and on conveyor belts have been used with varying levels of success. This paper presents a field-evaluation of the accuracy of a real-time measurement system for measuring moisture content of virgin aggregate mixes moving on a conveyor belt for asphalt production. This system uses nuclear techniques, with no components in contact with the material flow or the belt, to determine the moisture content. Due to its use of low activity radioisotopic sources and a safe design, the system is categorized as a ‘generally licensed’ device by the US Nuclear Regulatory Agency imposing minimal regulatory burden on US plant owners. This measurement system was evaluated during the 2015 and 2016 production seasons at a continuous-type asphalt plant in Wisconsin, USA. Based on the study, the moisture measurement system showed a good sensitivity to moisture in aggregate mixes with 0.9% wt. response to a 1.0% wt. change in moisture. It also showed a good measurement precision of 0.1% wt. at one standard deviation for a 20 second averaging. Over one thousand samples, with limestone as the main aggregate type, were collected from the belt and were used for comparing the moisture measurements from the new system to that by the oven drying method. For the two methods, the correlation of determination was 0.69 and the difference in moisture content was 0.10 ± 0.35% wt. at one standard deviation. The new method did not show dependence on the nominal maximum aggregate size of the mixes.
Nondestructive Full Coverage Asphalt Stripping Detection Using 3D Radar
Since subsurface stripping extent is important for maintaining the road network, Minnesota Department of Transportation (MnDOT) uses forensic measures such as coring. The destructive nature and lack of coverage provided by this accepted method makes finding nondestructive methods for detecting stripping of in situ asphalt pavements an important need for MnDOT and many other agencies. Stripping between hot mix asphalt layers can cause premature failure of pavements. Knowledge of the lateral location, depth and extent of stripping can affect the timing and type of pavement preservation, maintenance, or rehabilitation. This paper reports on use of a DX1821 antenna to collect data at a frequency step of 20 MHz ranging from 50 MHz to 3050 MHz with a dwell time of 7.52 μs using a GeoScope Mk IV control unit on a full-scale asphalt pavement constructed at the National Center for Asphalt Technology (NCAT) test track with built-in stripping. Signals from the known stripped and non-stripped locations were used to evaluate statistical and energy based approaches. It is shown that a maximum energy ratio method, an approach that has been successfully applied to acoustic emission applications in the past, can improve signal clarity for stripping detection using ground penetrating radar (GPR). These results are confirmed using highway speed measurements at the Minnesota Road Research Facility with core validation.
Assessment of Permanent Deformation Properties of In-Place Recycled Pavement Mixtures
Cold in-place recycling (CIR), cold central plant recycling (CCPR), and full depth reclamation (FDR) are techniques that can be used to rehabilitate deteriorated pavements or used in new pavement construction projects. Use of these techniques has been found to be both fiscally and environmentally beneficial. However, there is a lack of literature that examines the mechanistic behavior of these mixtures, particularly when it comes to asphalt mixture-related test methods. This paper presents a comprehensive analysis of data from repeated load permanent deformation (RLPD) testing, a laboratory test that describes a mixture’s rutting behavior, conducted on bituminously stabilized CIR, CCPR, and FDR mixtures. The mixtures were collected as a part of NCHRP 9-51, Material Properties of Cold In-Place Recycling and Full-Depth Reclamation Asphalt Concrete for Pavement Design. The permanent deformation behavior of specimens from 23 projects sampled between 12 and 24 months after construction is presented. The impact of stabilizing or recycling agent as well as the presence of chemical additives on the permanent deformation behavior was considered. Density, one of the most prevalent factors in field-based quality control and acceptance of cold recycled mixes was also examined with respect to its impact on permanent deformation behavior.
Limiting Strain Distribution Criteria for Perpetual Pavement Design Using AASHTOWare Pavement ME Design
Perpetual asphalt pavement designs have been based on controlling horizontal strains at the bottom of the asphalt layer and vertical strains on top of the subgrade to prevent bottom-up fatigue cracking and subgrade rutting, respectively. Traditionally, single strain thresholds had been used as limits where damage accumulation begins and propagates. The concept of strain distributions was introduced at the National Center for Asphalt Technology (NCAT), and it was found to better represent the wide response spectrum in asphalt pavements caused by the different loading and environmental conditions. This concept was developed based on information and data collected from the NCAT Pavement Test Track, and from this a limiting strain distribution was created to assist with perpetual pavement design. Several studies analyzed the criteria using pavement responses calculated from PerRoad predicted strains in Test Track sections and existing perpetual pavements. A limitation of the AASHTOWare Pavement ME Design software is its inability to output the pavement responses necessary for perpetual pavement design. The objective of this study was to analyze pavement response data predicted from an integration of the AASHTOWare Pavement ME Design software outputs and a JULEA based program developed at NCAT to create a limiting strain distribution for perpetual pavement design using the AASHTOWare Pavement ME Design software. Strains were calculated for Test Track sections using these tools and based on the results a limiting strain distribution was developed. This limiting strain distribution was then further evaluated using eight sections that won Perpetual Pavement Awards. Results showed that the horizontal strain distributions obtained from Test Track sections could differentiate the sections with cracking and the sections that did not crack. In the further evaluation, the horizontal strain distributions from the perpetual pavements did not exceed the limiting strain distribution developed from the Test Track sections, except for a section which surpassed it by a small margin at the 80th and higher percentiles. Vertical strains were also determined for these pavements and the current 200 microstrains at 50th percentile design criteria was met by all the sections.
Influence of Asphalt Binder Formulation and Source on the Performances of Binders with the Same Continuous PG WALAA S. MOGAWER, ALEXANDER J. AUSTERMAN,
The objectives of this research study were to determine the effects of asphalt binder formulation and source on the performance of binders with the same continuous Performance Grade (PG) using recently developed asphalt binder and mixture tests and provide recommendations, if needed, concerning the following: (1) the effect of asphalt binder formulation and source on performance, (2) the current PG specification and (3) the tests used in this study. Various asphalt binders were formulated so they would have the same continuous high and low PGs which would meet the AASHTO M 320 specifications for a PG 58-28 binder. Other binders were formulated to meet the AASHTO M 320 specifications for a PG 64-28 binder. Thus, there were two target PG’s. Three sources of straight run asphalt binders were obtained, although one source was eliminated because for most of the modifiers obtained for this study, the target PGs could not be met. This showed that whether a continuous PG can be achieved through modification is dependent on binder source and modifier type. The modifiers used in the formulations consisted of two sources of re-refined engine oil bottoms (REOB), aromatic oil, polyphosphoric acid (PPA) and an air-blown asphalt, either alone or in combination. It is believed these modifiers are becoming more commonly utilized to supply the required PG asphalt binders for today’s paving industry. Asphalt binder formulation and source had an effect on fatigue cracking, cohesion, adhestion, moisture susceptibilty and non-load associated cracking even though the binders had closely matching continuous PGs based on high and low temperature properties. They had no effect on rutting performance except when measured by the MSCR, or thermal cracking performance except when measured by the EBBR. In conclusion, not all asphalt binders with the same continuous PG will perform identically in regard to all distresses.
Assessment of Various Approaches to Determining Binder Bending Beam Rheometer Low Temperature Specification Parameters from Dynamic Shear Rheometer Test
The current asphalt binder performance grading system employs Dynamic Shear Rheometer (DSR) testing to determine high and intermediate temperature rheological properties. In recent years, the ability to measure DSR instrument compliance has allowed researchers to reliably measure low temperature binder properties as well. Low temperature characterization using DSR requires substantially smaller amount of binder as compared to the currently employed binder testing method, Bending Beam Rheometer (BBR). For these reasons and the possibility of using one piece of equipment for full characterization of asphalt binders, previous research has investigated DSR as an alternative to replace BBR testing by determining equivalent creep stiffness (S) and slope (m-value) from shear complex modulus. Different methods have been proposed to determine BBR specification parameters from DSR data and their viability has been evaluated primarily for virgin binders. The objective of this paper is to further assess the applicability of different methods to determine S and m-values from DSR data for four neat binders as well as extracted and recovered binders from eighteen different mixture samples. The variables within the mixtures include lab versus plant production, aggregate size and gradation, binder PG and source, and recycled material type and content. The methods employ different interconversion methods, ranging from exact interconversions to regression-based estimates. The shear relaxation modulus or creep stiffness and slope are correlated to S and m-value measured from BBR testing. The study also investigates the impact and differences due to use different interconversion methods. The results show that the Christensen approximate interconversion is adequately able to predict parameters from DSR results that are equivalent to S and m-value determined from BBR testing. The exact interconverted shear creep stiffness and shear relaxation modulus using generalized Maxwell model are compared to lab measured S and m values, results show that a linear relationship exists between these parameters. Finally, a simple equation is developed to enable estimation of BBR S and m-value from a single point measurement of complex shear modulus and phase angle. This contribution is expected to have a practical use by providing a platform to estimate low temperature specification parameters from a single point DSR measurement.
A Direct Shear Approach to Assess the Interface Bonding Performance
The objective of this study was to investigate the effects of various factors affecting bond strength between hot mix asphalt (HMA) overlay and underlying pavement layers in the field. The effect of interface bonding on short-term pavement performance was also evaluated. A list of candidate HMA field rehabilitation projects was identified across the United States, representing different traffic and environmental conditions. These projects included the rehabilitation of new, existing, and milled HMA pavements, and PCC pavements. Each field project involved at least one slow setting and one rapid setting non-tracking tack coat material, thereby creating one or more pairs of tack coats for comparison. The HMA overlay construction used different types of tack coat materials at various residual application rates. Specimens were cored from the evaluated test sections at different service times to determine the interface shear strength (ISS) according to AASHTO TP 114, “Standard Method of Test for Determining the Interlayer Shear Strength of Asphalt Pavement Layers.â€ Results of this study showed that ISS was largely dependent on the type of pavement surface (i.e., HMA vs. PCC) receiving tack coat, and pavement surface texture (i.e., milled vs. non-milled). In general, milled HMA surface yielded the highest ISS, followed by new HMA, existing HMA, and PCC surface types. Non-tracking rapid setting tack coats with stiff base asphalt cement exhibited the highest ISS compared to slow setting tack coats. With respect to the effect of service time, ISS increased with service time due to tack coat curing effect and densification of overlays. Laboratory measured ISS values correlated well with short-term cracking performance of field pavements. Results presented herein were part of NCHRP Project 9-40A on the “Field Implementation of the Louisiana Interface Shear Strength Test.â€
Impact of Various Crumb Rubber Modifications on Asphalt Binder and Mixture Properties
As the price of asphalt binder continues to rise, state agencies are looking for sustainable ways to reduce the cost of asphalt pavements without compromising performance. One such alternative is the use of crumb rubber, derived from waste tires, in binders of asphalt mixtures. Blending virgin asphalt binder with ambient or cryogenic ground crumb rubbers along with additional modifiers to produce sustainable asphalt mixtures was studied. The modifiers evaluated include E-rubber (free flowing rubber pellets), SBS, sulfur, and Rpolymer (reactive polymer polyolefin blend coated micronized rubber particles). Thermogravimetric analysis was used to determine the natural rubber to synthetic rubber ratio in the ground tire rubbers. Gel permeation chromatography was used to investigate the molecular structure and changes occurring in the asphalt binder on blending with rubber/modifier. Scanning electron microscopy was used to examine the physical nature of the binder blends. All of the binder blends were evaluated using the multiple stress creep recovery test. Mixtures prepared from modified binders were characterized using semicircular bend (SCB) test at intermediate temperature and Hamburg wheel-tracking (LWT) test. Correlation of physical properties of crumb rubber modified asphalt binder with apparent molecular weight of binder components was examined. Asphalt mixtures containing ambient or cryogenic ground crumb rubber additives did not exhibit improved mixture intermediate temperature cracking performance as compared to conventional mixture as measured by SCB test Jc value. Addition of elastomeric high molecular weight polymer additives improved MSCR test results of binder blends when compared to rubber blends with no additional polymer additives. Addition of E-rubber or R-polymer to the asphalt binder improved SCB intermediate temperature test results of the corresponding mixtures. It was noted that the presence of high molecular weight elastomeric species in asphalt binder blends is necessary to obtain acceptable intermediate temperature cracking performance. Presence of crumb rubber in asphalt binder contributed to the increase in percent high molecular weight species that provided an enhanced mixture rut resistance.
Fluorescence Microscopy for Determining Diffusion Coefficient Between Virgin and Aged Asphalt Binders
Diffusion between virgin and aged asphalt plays a critical role in ensuring a final uniform asphalt binder in mixtures containing recycled asphalt pavement (RAP). Understanding the diffusion phenomenon and determining the diffusion coefficient have a significant effect on asphalt mix design and asphalt mixture performance. In this study, a new method was proposed to utilize fluorescence microscopy to determine the diffusion coefficient of aged RAP binder in recycled mixtures. First, Fick’s second law was applied to quantify the diffusion process in a two-layered virgin-aged binder model and to obtain the analytical solution to the distribution of virgin and aged binders. Then, fluorescence microscopy was used in the laboratory to differentiate between virgin and aged binders and to back-calculate their concentrations from fluorescence image. The diffusion coefficient was determined by fitting the analytical solution to the laboratory concentration measurements. Comparison of the diffusion coefficient by the proposed method and that using the dynamic shear rheometer (DSR) method from the literature shows that the diffusion coefficient determined by fluorescence microscopy was of the same magnitude to, but slightly lower than, that by DSR method. The diffusion coefficient was also predicted using another theoretical method – free volume theory. Parameters describing the asphaltic model in the free volume theory were determined from analysis of the chemical structure of asphalt, viscoelastic relaxation properties, and glass transition temperature of asphalt as well as from the diffusion coefficient measured from laboratory experiments. Diffusion coefficient predicted by the free volume theory shows that diffusion coefficients of asphalt were closely dependent on temperature and asphalt type. A diffusion simulation was performed on plant mixtures using the diffusion coefficient obtained from fluorescence microscopy. The results showed that an almost complete diffusion was achieved within five minutes in a hot-mix asphalt mixture resulting in a uniform asphalt film.
Climate-, Depth-, and Time-Based Laboratory Aging Procedure for Asphalt Mixtures
Aging causes asphalt pavement materials to stiffen and embrittle, which leads to a high cracking potential. A practical and accurate laboratory conditioning procedure that can simulate long-term aging of asphalt concrete for performance testing and prediction is required in order to integrate the effects of aging in pavement performance prediction models and other mechanistic design and analysis methods. Recent studies have suggested that loose mix oven aging at 95°C is the most promising long-term aging method to simulate field aging. This study has developed a means to determine laboratory aging durations for asphalt mixtures that best reflect the time, climate, and pavement depth for a given pavement location in the United States. A rigorous kinetics model together with laboratory experimental results demonstrate that the laboratory aging duration that is needed to match a given field condition is independent of material-specific kinetics. Project-specific laboratory aging durations that match field aging at various pavement depths were determined for a broad set of materials. The project-specific aging durations were used to calibrate a kinetics-derived climatic aging index (CAI) that was then used to determine the laboratory aging duration to match field aging at any location of interest. The CAI-determined aging durations at 95°C were used to generate aging duration maps for the United States for three field ages (four years, eight years, and 16 years) to match field aging at three depths (6 mm, 20 mm, and 50 mm).
Practical Tools for Optimizing Recycled Materials Content and Recycling Agent Dosage for Improved Short- and Long-Term Performance and Rejuvenated Binder Blends and Mixtures
The use of large quantities of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures is desirable due to environmental and economic benefits. However, recycled asphalt mixtures with high recycled materials contents are usually less workable; difficult to compact in the field; and more prone to cracking, raveling, and other durability-related pavement distresses. Recycling agents can rejuvenate the aged binders in the recycled materials to different degrees depending on type and dosage, facilitating the inclusion of increased amounts of recycled materials. Recycling agents modify the ultimate performance of the corresponding rejuvenated asphalt mixtures, thus estimating an optimum recycling agent dosage is critical to maximize its benefit without compromising the short- and long-term performance of the rejuvenated asphalt mixture. This study provides tools for estimating recycling agent dosage based on a target climate with minimum laboratory efforts by considering the type, source, and amount of recycled materials, and the source and grade of the base (virgin) binder. A total of 15 different recycled binder blends (base and recycled binders) and 32 different rejuvenated binder blends (recycled binder blends with recycling agent) were considered, including materials from eight states across the United States. Blending charts for recycled binder blends were established and verified, and later used to develop relationships to estimate the optimum dosage of recycling agent. The recycling agent optimum dosages were determined to match the continuous high-temperature performance grade (PGH) of the recycled binder blend to that required by the target climate, as this dosage yielded the best performance for rejuvenated binders and mixtures. Long-term rejuvenating effectiveness of recycling agents was verified by extensive evaluation of rejuvenated binder blends and mixtures. Discussion on optimizing RAP/RAS and base binder proportions and controlling the maximum dosage of recycling agent for economic and pavement performance considerations was also provided. Finally, practice-ready guidelines for evaluation, materials selection/optimization, and design of rejuvenated asphalt mixtures with high recycled materials contents were proposed.
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.
Low Temperature Behavior of Asphalt Binders, Mortars, and Mixtures with High Recycled Materials Content
With the increased usage of recycled materials and various evaluation techniques being utilized to evaluate them, it is becoming increasingly important to be able to obtain consistency between the various techniques. Given the wide range of recycled materials as well as treatments or additive materials being used and the complex interactions resulting between them, it is becoming increasingly important to continually verify the field performance related to the findings of the various evaluation techniques. Therefore, this effort utilized binder blending, mortar procedure, and the uniaxial thermal stress and strain test (UTSST) to examine several mixtures containing recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS) along with example recycling agents. The methodologies were compared to each other to seek similarity in findings as well as preliminary comparisons to relevant field performance. The results indicated general agreement between certain parameters of each methodology, while the UTSST showed promising relationships to measured longitudinal and transverse cracking in the observed field test sections.
Laboratory Evaluation of Performance Properties of Asphalt Binders and Mixes Containing a New Rejuvenator
As higher recycled asphalt pavement (RAP) contents are utilized in asphalt mixtures, the mixes will be stiffer and more brittle, thereby losing resistance to various types of cracking. To offset these changes in stiffness and ductility, rejuvenators have been used in asphalt mixes containing high recycled contents to improve their performance properties. The objective of this project was to evaluate the effect of a new rejuvenator, a corn oil-based coproduct, on the performance of high RAP mixes. Three mixes were evaluated in this study: two 50% RAP mixes with and without the rejuvenator and a control virgin mix. Performance grade and frequency sweep tests were conducted to explore the effect of rejuvenator on the performance of the asphalt binder. Moisture damage, dynamic modulus, rutting, and cracking tests were performed to evaluate the effect of rejuvenator on the mix performance. Results of this study indicated that the new rejuvenator was effective in lowering the stiffness of RAP asphalt binder and improving the resistance to cracking and moisture damage of 50% RAP mix without imposing negative rutting effects.
Asphalt Mixtures Containing RAS and/or RAP: Relationships Amongst Binder Composition Analysis and Mixture Low Temperature Cracking Performance
As the price of asphalt binder continues to rise, state agencies are looking for sustainable ways to reduce the cost of asphalt pavements without compromising performance. One such alternative is the use of reclaimed asphalt pavement (RAP) and/or recycled asphalt shingle (RAS) to replace virgin binders and aggregates in asphalt mixtures. To better understand low-temperature cracking resistance of asphalt mixtures containing RAP and/or RAS, 11 asphalt mixtures with various combinations of RAP, RAS, and recycling agent were evaluated. Thermal stress restrained specimen test (TSRST) was performed to evaluate asphalt mixtures’ low-temperature cracking resistance. Saturate-aromatic-resin-asphaltenes (SARA), gel-permeation chromatography (GPC), and Fourier transform infrared spectroscopy (FTIR) were conducted on extracted binders from the 11 corresponding mixtures for analyzing the molecular compositions of the asphalt binders. In addition, dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were carried out to determine the rheological index (R) and difference in critical temperatures (ï„Tc), respectively. It was consistently observed from the inter-comparisons of test results that the addition of RAS, RAP, and RAs adversely affected the low-temperature properties of both the asphalt binders and mixtures studied herein. Improvement in low-temperature properties of asphalt binder and mixture was observed when a softer virgin asphalt binder (PG 58-28) was used in lieu of the control styrene-butadiene-styrene modified PG 70-22 binder. It was also found that the molecular components fractionated from GPC have better correlations with mixtures’ low-temperature cracking performance parameters compared to that from SARA and FTIR analysis. Further, it was concluded that the use of RAS and RAP in asphalt mixtures increased the larger molecular weight species in asphalt binders (i.e., asphaltenes), which contributes to stiffening of asphalt binder and mixtures at low temperatures and, in turn, impairs the low-temperature cracking resistance of asphalt binders and mixtures.
In Germany, warm mix asphalt (WMA) is not commonly used although this technology was initially introduced to the practice between the 60s and the 80s. The current guidelines are devoted to the reduction of the mixing temperature for mastic asphalt (MA) (Gussasphalt). Due to this requirement, a number of products consisting of plain or polymer modified binder premixed with additives such as wax are available on the market. However, a well-established set of regulations addressing the production process and performance parameters is not available as in the case of conventional hot mix asphalt (HMA). For this reason, a case study which aimed to address the effects of viscosity changing additives (waxes) on asphalt mixture properties was conceived. In this large experimental study, three different additives (waxes) were used. First, the impact of waxes on asphalt binder properties and the corresponding rheological response were evaluated. Then, performance tests, such as rutting resistance tests, fatigue tests, and low temperature behavior tests, were conducted on asphalt mixtures AC 11 B S and AC 16 D S prepared with plain and polymer modified binders. No significant reduction in compaction temperature was observed when waxes were incorporated in the mix design. All additives were capable of improving rutting resistance, while poorer low temperature performance was exhibited. Based on the experimental campaign, the use of waxes could be recommended only to facilitate mixing.
Developing Life-Extending Benefit Curves for Asphalt Pavements with Preservation Treatments
Over time, new pavements deteriorate due to the effect of traffic loads and the environment. If appropriate treatments are applied during the early stages of deterioration, it is possible to improve pavement conditions and extend pavement life without increasing expenditures. The National Center for Asphalt Technology (NCAT) has partnered with the Minnesota DOT’s Road Research Facility (MnROAD) to conduct a pavement preservation study that evaluates the life-extending benefit of a variety of preservation treatments, ranging from crack sealing to thin overlays. The objective of this research partnership is to develop performance curves for the treated pavements under different conditions (climate, traffic and initial condition of the pavement). In this study, full-scale test sections were treated first in a southern location (Alabama) on roadways subjected to both low and high traffic levels, starting in the summer of 2012. The experiment was extended in 2016 to include test sections in a northern location (Minnesota) to evaluate the effect of cold climate, also for low and high traffic levels. Throughout this time, cracking, roughness, rutting and macrotexture data were collected biweekly to evaluate pavement performance. The observed trends in the first years of the experiment indicate that there is not a significant variation in roughness or rutting over time, and that performance is mainly affected by the amount of cracking. Furthermore, the condition of the pavement at the time of treatment significantly affects the performance of the treated pavements, as pavements that are treated while still in good condition tend to remain in that category for a longer time. The results also demonstrate the effectiveness of applying pavement preservation treatments versus the “do nothingâ€ scenario. After 4.8 years, the amount of cracking observed is less than the amount expected if the sections were left untreated, even for sections treated with applications that are not designed to address cracking. The results shown in this paper should be considered preliminary and used with caution. Data collection efforts continue in both southern and northern locations with the objective of determining the life-extending benefits of pavement preservation treatments as a function of initial condition, climate and traffic.
Unique Aspects of Flexible Pavement Design and Asset Management for P3 Design-Build-Finance-Operate Projects
With increasing frequency, roadway corridor development and improvement projects are being procured through Design-Build-Finance-Operate (DBFO) Public Private Partnerships (P3s). In these procurement environments the nature of the pavement design and engineering requirements change in several significant ways compared with the traditional Design-Bid-Build (DBB) in that the consequence of poor pavement performance (risk) in most aspects is transferred from the owner to the DBFO partners: designer, constructor and operator/concessionaire. A performance-based design approach is required which is founded on the principle that the design and construction of an asset is completed to achieve a set of prescribed performance results. This paper discusses the design and pavement management process for DBFO P3 flexible pavements and presents a case study of a project highlighting a particularly complex distribution-based roadway condition performance specification.
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