Rolling thin-film oven (RTFO) and pressure aging vessel (PAV) aging methods do not necessarily reflect low-temperature oxidative aging of asphalt binders as it occurs in pavement structures constructed with modern asphalt materials, it has been shown. Development of an alternative laboratory aging apparatus employing a compact Attenuated Total Reflection spectrometer and airflow Aging Cell (ATRAC) equipped with a temperature controller has been targeted by this ongoing research. Asphalt mix samples are collected, concurrently, from several pavements of different ages constructed in Connecticut and Rhode Island with non-modified PG 64-28 (AC-20) and crumb rubber-modified PG 76-34 binders. A good correlation is shown by preliminary results between oxidation rates of binders aged by the ATRAC and oxidation rates of asphalt mixes collected from the existing pavements. In addition, it is indicated by comparison of ATRAC measurements on New England PG-graded binders and Strategic Highway Research Program (SHRP) core asphalts that there is a potential use for ranking performance of asphalt binders at high temperatures by oxidation rates produced by ATRAC.
The current design standard for asphalt mixtures provides guidance on selection of aggregates, asphalt binder and includes requirements for the amount of mineral filler to be included. The amount of filler that can be included is limited to a ratio in the range of 0.6-1.2 by mass of the binder. However, this range is based on experience rather than on scientific evaluation of the interaction between filler and binder. Although many researchers acknowledge the physico-chemical interaction between asphalt binder and the mineral filler, currently a procedure to quantify this interaction, and consider it in selecting favorable filler to binder ratio, is not available. In this paper, the effect of fillers on the glass transition temperature (T sub g) of the base binder was used to evaluate the physico-chemical interaction in mastics. The total reinforcement of the filler, which is measured in terms of relative viscosity of the filled binder to the unfilled binder, consists of two parts: mechanical and physico-chemical. The mechanical reinforcement part is calculated based on micromechanical models commonly used in the literature that take into account volume filling effects and particle-to-particle interactions. Physico-chemical reinforcement is estimated based on the change in the T sub g in both Williams-Landel-Ferry (WLF) and Arrhenius time-temperature shift models. The concept introduced in this study is evaluated by viscosity and dilatometric T sub g testing of three binders mixed with three different fillers, at different concentrations. Results show that the physico-chemical interaction between the mineral filler and the binder can be accurately estimated from the difference in the glass transition temperature of the mastics and the binder.
Surface free energy is a thermodynamic material property representing the work required to create new surfaces of unit area in a vacuum. Surface free energy has been used to quantify and screen both the cohesive bond energy of asphalt binders and the adhesive bond energy of asphalt binder-aggregate interfaces in wet and dry conditions. The bond energy is computed based on the surface free energies of the constituent materials. The total work of fracture is the cumulative effect of energies applied to the sample to create two new surfaces of unit area. These energies include the bond energy, calculated from surface free energy, dissipated plastic energy, and dissipated viscoelastic energy. This paper presents experimental results from a series of pull-off tests using asphalt binder-aggregate samples that demonstrate the relationship between bond energy and total work of fracture. In order to fully explore this relationship, temperature, loading rate, specimen geometry, and moisture content were varied.
Two high polymer mixtures (HPM) were placed at the 2009 National Center for Asphalt Technology (NCAT) Test Track as a field trial. These mixtures featured 7.5% polymer in contrast to the more typical 2-3% polymer contents. During construction, each mixture (one surface and one base mixture) was sampled for evaluation using laboratory performance tests. The results of the dynamic modulus test, asphalt pavement analyzer, flow number, bending beam fatigue, indirect tension creep compliance and strength test, energy ratio, and moisture susceptibility tests were compared with the results from comparable control mixtures placed in the same round of testing. The laboratory test results suggest HPM mixtures can be placed to develop more efficient (i.e., thinner) pavement cross-sections due to their enhanced fatigue and rutting resistance.
Aggregate packing concepts developed in the field of high-performance cement concretes, initially by Caquot (1937) then by contemporary researchers since the 1970s, were transposed to the field of asphalt concretes. These concepts, associated with the use of the gyratory compactor on aggregates only, enabled the development of a new laboratory design procedure of dense high-modulus asphalt concretes. These mixes are characterized by single or double gap-graded curves, great coarse aggregate interlock and no need for low penetration grade bitumens to fulfill the European EME2 specification requirements, in particular the 14,000 MPa stiffness modulus value at 15°C. In addition, the use of polymer modified binders (PMBs), at a content of about 4% up to 4.5%, combined with such an optimized aggregate packing leads to the design of the so-called high-performance asphalt concretes (HPAs) characterized by great compactability, very high stiffness modulus and high fatigue resistance in a single formulation, allowing for reduced pavement thickness and increased longevity. Moreover, the proposed mix design and the 4?4.5% binder content makes PMBs use affordable in base courses. Laboratory assessment of such materials consisted in the evaluation of compactability, moisture resistance, rutting resistance at 60°C, complex stiffness modulus at 15°C and fatigue resistance at 10°C. Apart from these results, the paper also addresses the successful application of this new material on different job sites, located mainly in France. The proposed HPA material may be potentially considered as a relevant solution for sustainable long life pavements that do not deteriorate structurally, needing only timely surface maintenance.
An aggressive adoption of new practices within the asphalt paving industry have been spurred by recent improvements in warm mix asphalt technologies. Among federal and state agencies, concerns have arisen regarding the effects on the performance of asphalt pavements of this line of products. While varying the loose mix aging time, the authors performed an investigation of the effects of lowering mixing, compaction and aging temperatures using Meadwestvaco's Evotherm 3G™ 09 chemical additive. Flow Number, Fracture Energy testing, Hamburg Wheel-Track Testing, and Dynamic Modulus testing were used to evaluate mechanistic properties of the materials. It was found that rutting related testing was influenced significantly by lowering production temperatures, and for dynamic modulus and low temperature fracture energy testing, the effect was negligible.
Commonly used to modify asphalt binder for use in hot-mix asphalt construction, ground tire rubber (GTR) is the focus of this study. For use as co-modifiers with GTR in asphalt binder modification for asphalt pavement construction, various additives have been recommended. Co-modifiers have been promoted more recently to be incorporated into hot-mix asphalt through dry addition into asphalt mixtures of GTR and additive. An improved modified asphalt paving binder is represented by these additives to be provided by incorporating a small amount of additive into GTR modified asphalt binder. This, in turn, leads to easier mixing, reduced cracking, reduced tackiness, lower life-cycle cost, longer service life, and less permanent deformation. The results of a comprehensive evaluation of trans-polyoctenamer (TOR) modified binder incorporated with GTR in modification of asphalt binder are reported in this paper. Evaluated in dense, open graded friction course (OGFC), and stone matrix asphalt (SMA) mixtures were four binders, as follows: GTR wet pre-blended; GTR dry added to mixture; performance grade (PG) 67-22; and Styrene-Butadiene-Styrene block copolymer (SBS).
A comprehensive study focusing on methodologies to improve the performance of high reclaimed asphalt pavement (RAP) content mixes is presented in this paper. One major concern that many agencies have is addressed, firstly, in this paper - that of RAP variability. In Texas, it was found that, within individual stockpiles, the RAP materials are consistent and have low variability in terms of aggregate gradation and asphalt content. In addition, when the authors evaluated the impact of RAP on optimum asphalt context (OAC), cracking resistance, and rutting/moisture resistance, they found that OAC generally increases with more RAP (and reclaimed asphalt shingles (RAS)) usage, but when the RAP content is below 20% the increase in OAC is small; and as measured in the Hamburg wheel tracking test (HWTT), increasing RAP content always improves rutting/moisture resistance. Yet, particularly when RAP content is 30% and above, and also when RAP/RAS combinations are used, it was found in the laboratory that cracking resistance always reduces with increasing RAP content. This study proposes a balanced RAP mix design approach. The authors recommend the use of the Overlay Tester (OT) to directly measure cracking resistance of RAP mixes and the HWTT for evaluating rutting/moisture resistance, recognizing the challenge of accurately estimating voids in mineral aggregates of the RAP mix. Furthermore, a maximum density of 98% is recommended to avoid over densification and potential bleeding for Texas dense-graded hot mix asphalt (HMA) mixes designed using the Texas gyratory compactor. The final asphalt content is selected after optimizing the mix density, HWTT, and OT requirements in the proposed balanced mix design approach. The mixing and compaction temperatures used for the virgin binder should be used for the RAP mix as well, as was recommended. Finally, through the construction of field test sections containing different levels of RAP, the balanced RAP mix design procedure is demonstrated and validated. The fact that cracking requirement in terms of OT cycles should vary, depending on the climate (cold vs. hot), traffic (heavy vs. light), and existing pavement condition (overlay over cracked pavements vs. new construction), is one of the interesting findings. A minimum OT requirement for overlays over severely cracked pavements of 300 cycles previously proposed was further validated with performance data from the RAP sections on Interstate Highway (IH) 40 near Amarillo, Texas. In order to develop criteria for different climatic zone and different pavement conditions, more work is needed.
A Federal Highway Administration (FHWA) funded study was conducted to investigate the influence of extraction methods on aggregate properties. The properties of the virgin aggregates were compared with those of aggregates extracted from laboratory-produced recycled asphalt pavement (RAP) from four different aggregate sources. The extracted and actual asphalt binder contents were also compared. The study investigated the influence of the extraction method on tendencies to under- or over-estimated certain mix design properties. The test results were also examined to determine the impact of the RAP aggregate properties on the voids in mineral aggregate (VMA) over different RAP percentages. Recommendations were made for the most appropriate method to estimate the RAP aggregate specific gravities based on acceptable levels of error in VMA for mixtures with varying levels of RAP.
The main focus of this study was to obtain plant produced Reclaimed Asphalt Pavement (RAP) mixtures, to document the mixture production parameters and to evaluate the degree of blending between the virgin and RAP binders. The effect of mixture production parameters on the performance (in terms of stiffness, cracking, rutting, and moisture susceptibility) and workability of the mixtures was evaluated. Eighteen plant produced mixtures were obtained from three locations in the Northeast United States. RAP contents (zero to 40%) were varied and softer binders were used. The data and analysis illustrated that the degree of blending between RAP and virgin binders is a function of production parameters. The stiffness of the mixtures increased as the percentage of RAP increased, but not when the discharge temperatures of the mixtures were inconsistent. The cracking resistance was reduced as the percentage of RAP increased. The rutting and moisture damage resistance improved as the percentage of RAP in the mixtures increased. Finally, reheating the mixtures in the laboratory caused a significant increase in the stiffness of the mixtures.
This paper presents the results of a study to evaluate the mixture properties of plant-produced asphalt mixtures containing up to 40% reclaimed asphalt pavement (RAP). Five sets of asphalt mixtures were tested to determine their dynamic moduli and low temperature tensile creep compliance and strength. The mixture moduli were analyzed to assess changes in mixture behavior over a range of temperatures (frequencies) and in combination with extracted binder properties to analyze the extent of binder blending in the mix. The low temperature test results were analyzed to predict the critical cracking temperature of the mixtures. The results suggest that, for these materials, up to 25% RAP could be added to the mix with no change in the virgin binder grade without detriment to the low temperature properties of the mix.
The effect of reduced gyration levels on mix design and performance was quantified in this study. Changes in fatigue resistance and rutting resistance when design gyrations were decreased using different approaches were captured in laboratory tests. Using four separate mixtures, a standard 75 gyration design, a reduced 65 gyration design meeting standard volumetrics using additional binder, and a reduced 65 gyration design where standard volumetrics were met by adjusting the fine aggregate gradation rather than through the addition of binder, three various were studied. Using a suite of laboratory tests, the properties of the various mix designs were evaluated. Laboratory tests included the following: fatigue resistance, rutting resistance, and dynamic modulus. Using the shear response measured during gyratory compaction, compactability was assessed. Reducing gyrations and adjusting the fine aggregate gradation increased the average modulus, as shown in laboratory tests, and the average modulus decreased at lower gyrations when binder was increased. The same trend as dynamic modulus where adjusted fine aggregate gradations increased average resistance to permanent deformation was yielded by flow number tests, and added binder decreased average resistance to permanent deformation relative to the standard, reference mix design. Nonetheless, the inherent variability of the lab tests could not be overcome by the trends in the average modulus and permanent deformation responses, and the differences were largely insignificant. Both types of reduced design gyrations improved the compactability relative to the reference mix with higher design gyration, as indicated by the Compaction Force Index computed from the gyratory shear resistance. It is implied by this study that lowering design gyrations by a moderate level can, but not always, be used to achieve more compactable and more crack resistant mixtures without jeopardizing rutting resistance in a significant way. However, it is crucial to note that performance tests need to be completed due to the fact that a general or consistent rule does not apply. The reduced gyration level negatively affected some mixtures in the study. Due to the particulars of local materials, laboratory performance tests which can be conducted with an Asphalt Mixture Performance Tester are recommended for guidance.
This study was mainly focused on identifying appropriate relationships between asphalt mixture component characteristics and asphalt mixture properties known to control cracking performance. The current lack of material property models that can accurately describe the changes in material properties over time in the field is probably the greatest deficiency in our ability to accurately predict pavement performance. Therefore, there is a need to evaluate existing material property models, and develop improved models for use in the prediction of pavement performance. Relationships able to predict initial fracture energy and creep rate, which are the properties known to govern the change in material property over time and are also required for performance model predictions, were developed in this study. In addition, conceptual relationships were identified to describe changes in these properties over time (aging) by including the effect of the non-healable permanent damage related to load and moisture. This can serve as the foundation for further development of improved models to predict mixture properties as a function of age in the field based on additional field data and laboratory studies using more advanced laboratory conditioning procedures. The verified relationships will also serve to provide reliable inputs for prediction of service life using pavement performance prediction models.
The semi-circular bend (SCB) test configuration has been favored by many researchers due to the ease of sample preparation, including cores removed from the field and the quick and simple testing procedure. It offers the potential of assessing the cracking resistance of asphalt mixes in the laboratory in the design phase as well as in QA (quality assurance) testing activities. The objective of this study was to conduct a comprehensive evaluation of the SCB test and to utilize this test to evaluate a number of asphalt mixtures against cracking failure. Results of the experimental program were used to validate a three-dimensional (3D) finite element (FE) model, which was used to interpret and to analyze the failure mechanisms in the SCB test. Results of the experimental program showed that the SCB test results successfully predicted the fracture performance of the evaluated mixes and was able to differentiate between them in terms of cracking resistance. Mixtures prepared with polymer-modified binders were the best performers in this test against fracture. Results of the SCB test were in agreement with the DCSE (Dissipated Creep Strain Energy) test and identified the mixtures with high recycled asphalt pavement (RAP) content and the one prepared with unmodified binder as possible poor cracking performers in the field. The SCB test process as well as the propagation of damage were successfully simulated using 3D FE and cohesive elements. The presented modeling approach was in good agreement with measured test results for all mixtures. Based on the results of the FE model, damage that propagates in the vicinity of the notch is mainly caused by a combination of vertical and horizontal stresses in the specimen. The effect of shear was negligible in progressing damage in the specimen.
This paper presents a unique approach to analyzing surface cracking in hot mix asphalt (HMA) pavements, based upon the concept of reduced cycles. In the first half of the paper, empirical equations are presented for predicting damage curves for any HMA mixture under a wide range of loading conditions. These equations have been developed using a large database of fatigue test results, and confirmed with data from several independently gathered data sets. The equations use as predictors modulus and other parameters calculated from the change in modulus with time and/or temperature. The second half of the paper applies these equations to fatigue experiments carried out at the Federal Highway Administrations Automated Loading Facility (ALF) in McLean, VA. This analysis involved estimating the reduced cycles at critical points in the surface of the test sections at the initial appearance of surface cracking. Statistical methods were then used to develop equations for predicting the onset of cracking. Although the approach is promising, some additional work is needed before the method should be applied to general problems in pavement design and analysis.
The Semi-Circular Bending (SCB) fracture test is commonly used to evaluate the low temperature fracture properties of asphalt mixtures. The present work investigates the presence of a size effect in SCB fracture testing of asphalt mixtures. Un-notched and notched geometrically similar SCB specimens of various sizes are tested at -24°C loaded by crack-mouth opening displacement (CMOD). The effect of specimen size on the nominal strength is investigated through the well-established size effect theories and conclusions are drawn regarding the fracture behaviour of mixtures at low temperature.
The behavior of the interface between adjacent pavement layers is one of the most important factors affecting pavement performance. Despite the importance of interface behavior between different pavement layers, there are few guidelines that can be used for construction, and the selection of tack coat type, application rate, and placement is usually based on empirical judgment. This paper presents a new fracture-energy based Interface Bond Test (IBT), which can be a practical method to evaluate the bond between pavement layers. The results of laboratory and field studies demonstrate the ability of the test to distinguish between samples produced with different tack coat application rates and modified versus unmodified tack coat material. Results from the IBT test were also compared with direct tension tests, and similar trends were found to exist.
Top-down cracking is a distress mode that is of particular concern for pavements with Open-Graded Friction Course (OGFC) because open-graded mixture has considerably lower resistance to fracture (lower fracture energy limit and lower resistance to damage) than dense-graded mixture. This particular cracking phenomenon initiates on the pavement surface and propagates downward; so because the OGFC layer is thin, cracking performance relies on the properties and characteristics of three components near the pavement surface: OGFC, underlying structural layer, and the interface between. For this reason, to increase the durability of pavements surfaced with OGFC, it is significant to ensure a quality fracture resistant bond between OGFC and the structural layer. This research investigated top-down cracking performance of OGFC with different tack coats using a newly developed composite specimen interface cracking (CSIC) test. In addition, X-ray computed tomography (CT) was employed to analyze the interface characteristics between OGFC and dense-graded HMA. HMA fracture mechanics was employed to quantify the effect of polymer modified asphalt emulsion (PMAE) on pavement top-down cracking resistance enhancement. Results clearly indicated that PMAE-created bonded interface conditions greatly increased pavement top-down cracking resistance as compared with conventional tack coat.
The performance of asphalt mixture is governed by its viscoelastic properties, especially those in tension mode. However, due to the difficulties in performing tests in direct tension mode, the test methods commonly used are in either compression or indirect tension (diametrical compression) mode. Previous studies show that loading mode has a significant effect on the viscoelastic properties of asphalt mixtures, especially at high temperatures. It is of great importance to develop a testing method to effectively and efficiently characterize the tensile properties of asphalt mixtures. In this study, a flexural tension test was proposed to utilize a loaded wheel tester (LWT) to characterize the viscoelastic properties of asphalt mixtures. In this test, beam specimens were subjected to constant or cyclic loads provided by moving wheels of a LWT. With transducers mounted on the beam, flexural bend deformations are measured. In addition to the LWT test, uniaxial tests in tension mode, tension-compression mode, and compression mode as well as indirect tension creep test were conducted on the same asphalt mixtures for comparison and validation. The results showed that the LWT test was able to characterize the viscoelastic properties of asphalt mixtures made with different aggregates and asphalt binders. The results from the LWT tests were found to be in general accordance with those from other tests. Compared to uniaxial and indirect tension tests, the LWT test could better represent the stress state in pavements and thus was more suitable for characterizing the viscoelastic properties of asphalt mixtures.
Characterization of the asphalt concrete microstructure using two-dimensional (2-D) imaging techniques is an economically efficient approach. However, the features that have been captured and quantified using 2-D imaging in most published research have been limited to simplistic analyses of aggregate structure. The present research focused on introducing a more elaborate method of characterization of internal structure, and proposing new indices to relate to and explain rutting resistance performance of asphalt mixtures. The aggregate internal structure provides the skeleton of the asphalt concrete, which plays an important role in rutting resistance. It is shown that this structure can be captured using a combination of image analysis indices developed in this research, namely: number of aggregate-on-aggregate contact points, contact length/area, and contact plane orientation. These parameters are defined for both the total aggregates and for the effective load bearing aggregate structure, referred to as the 'skeleton' in this study. Software developed in a previous study and significantly modified for this paper, is used to process digital images of a set of asphalt mixtures with different gradations, binder contents, types of modification, and compaction efforts. The results demonstrate a correlation between the internal structure indices and the mixture rutting performance. Additionally, the indices were successfully used to capture the effect of compaction effort, gradation quality, and binder modification on the mixture internal structure.