Disk-shaped Compact Tension Research Articles

Performance evaluation of cold in-place recycling materials through a simple semi-circular bending test

Cold in-place recycling (CIR) is a method of recycling existing Hot Mix Asphalt (HMA) pavements. In this study, a new performance-based test method, called Fracture Index Value for Energy (FIVE), is developed to capture the fracture energy of the CIR materials. Furthermore, various stabilisation agents were used including engineered emulsion (EE), high float asphalt emulsion (HFMS-2s), commodity asphalt emulsion (CSS-1) and foamed asphalt. The FIVE test was first validated using disk-shaped compact tension (DCT) test and then was implemented on the study mixtures at their optimum stabilisation agent contents. The ranking of the mixtures and reproducibility of the FIVE test was proved to be true through inter-lab comparisons of the test results between two independent testing laboratories. The statistical testing results postulate that the FIVE test is a simple test method that is capable of characterising the cracking performance of the CIR mixtures.

Performance Evaluation of Pelletized Solid Polymer Modified Asphalt Mixtures

The purpose of this research is to provide a comprehensive evaluation of the effects of a solid pelletized plastomeric polymer on asphalt mixtures’ properties and performance with respect to different distresses. Five asphalt mixtures—a control mixture (no polymer); asphalt mixtures with 2.5%, 5.0%, and 7.5% polymer content; and a mixture with 5% polymer and lower asphalt binder content—were evaluated. The laboratory testing campaign included complex modulus, direct tension cyclic fatigue, semi-circular bending, disk-shaped compact tension, and asphalt pavement analyzer tests. Advanced performance-based simulation programs—MnPAVETM, FlexPAVETM, and ILLITC—were utilized to predict mixture performance in the context of pavement structure and local traffic and climatic conditions. In addition, four field test sections were constructed for all but the mixture with the reduced binder content and falling weight deflectometer (FWD) testing was conducted on the test sections. Based on the results of laboratory testing and performance simulation it can be concluded that the study modifier significantly improved the rutting performance and slightly improved the mixture fatigue performance. The study modifier did not show a considerable positive or negative effect on the thermal cracking performance. Based on the FWD results, solid polymer is potentially a good option to increase the stiffness of the asphalt concrete (AC) layer.

Statistical Analysis Framework to Evaluate Asphalt Concrete Overlay Reflective Cracking Performance

The purpose of this paper is to provide a robust process to statistically analyze reflective cracking field performance data. There is often a lack of consistency and transparency in statistical analysis of pavement field performance data, which may not satisfy ANOVA or regression modeling assumptions. Twelve full-scale asphalt concrete (AC) overlay pavement test sections located at the MnROAD test facility are used to demonstrate the statistical framework. The percentage of cracking reported at joint locations (%RC) is used to represent reflective cracking performance, and its relationships to pre-overlay load transfer efficiency (LTE), truck traffic, overlay thickness, and common performance indices determined from laboratory tests are investigated. The three laboratory tests considered in this study are the disk-shaped compact tension (DCT), semi-circular bend (SCB) and overlay tester (OT). Logistic regression models are used for estimation. Predictive abilities of various models are compared in relation to the percentage odds (%odds) of reflective cracking. Model output is binary: it estimates not the relative amount of reflective cracking but instead the probability of a given pavement structure cracking. Varying ability to perform asphalt mixture laboratory performance testing is assumed. One such model, where no laboratory performance testing variables are included, shows that a one-unit increase (1-in.) in AC overlay thickness may result in approximately a third decrease in the %odds of reflective cracking. A logistic regression model developed that considers laboratory performance data from DCT, SCB, and OT results in the optimal model balancing best fit and best prediction properties without overfitting.

Assessment of Asphalt Mixture Disk-Shaped Compact Tension Test Indexes for Reflective Cracking Performance

Abstract Disk-shaped compact tension (DCT) testing is a commonly used low-temperature fracture test to determine the cracking resistance of asphalt mixtures. The current testing specification only considers the fracture energy (Gf) from load-crack mouth opening displacement test data. However, Gf does not directly take into consideration the behavior of the post peak region of the curve, which may indicate the mixture’s ability to resist crack propagation and provide insight into fracture processes (e.g., crack growth velocity). It is possible to have two DCT specimens with similar Gf values but dramatically different load-displacement responses. The main focus of this paper is to make a comparative evaluation of various performance indexes developed for DCT fracture testing with respect to field reflective cracking performance of 10 full-scale asphalt concrete overlay test sections. This study evaluates Gf in addition to three other indexes: fracture strain tolerance (FST), rate-dependent cracking index (RDCI) and a proposed DCTIndex from Minnesota Department of Transportation that considers the post peak load-displacement behavior. An equivalent performance index approach is adopted to make comparisons of test sections with varying overlay structures in terms of thickness and material properties. Results from this study showed there was relatively good agreement between all equivalent laboratory performance indexes in identifying the best and worst performing overlay sections, according to normalized field performance indexes after approximately three years of service. In general, the equivalent FST and RDCI laboratory indexes rank test sections similarly, while equivalent Gf and DCTIndex have comparable ranking.

Evaluation of the Effects of Engineered Crumb Rubber (ECR) on Asphalt Mixture Characteristics

Abstract This study examined the effects of modification of dense-grade asphalt mixtures with chemically engineered crumb rubber (ECR) via the dry process. The ECR mixtures were compared to unmodified and polymer-modified mixtures in various criteria. The study mainly had three objectives: (1) comparison of mixture performance, (2) investigation of warm mix equivalence, i.e., to determine if the chemical coating on the ECR resulted in benefits similar to warm mix additives, and (3) to determine the effects of ECR on mixture draindown. For the first objective, the disk-shaped compact tension test was conducted to determine resistance to thermal cracking, and the Hamburg wheel tracking test was performed to determine resistance to permanent deformation (rutting) and moisture damage. To fulfill the second objective, the mixtures were prepared with warm mix asphalt (WMA) additives wherever necessary, and for the third objective, a stone mastic asphalt (SMA) was employed instead of the dense-grade mixture that was used for the rest of the study. Findings showed that modification with ECR greatly improved resistance to rutting and cracking, indicating that ECR pavements will exhibit longer service life. In terms of warm mix equivalence, a comparison of volumetrics data showed that ECR without WMA additive compacted in a similar manner as a polymer (PG 76-22) mixture containing WMA additive. Additional mix cost savings may be realized with ECR modification because of the ability to avoid the cost of WMA additive for the goal of achieving mix workability and compaction at lower temperatures. Finally, an ECR-SMA mix was used for draindown assessment and was found to pass the draindown test without the need for fibers, nearly matching the draindown resistance of the polymer-WMA mix containing cellulose fibers, thereby presenting another cost-saving avenue for contractors. The study also includes details of three field projects that used ECR and reported excellent field performance.

Asphalt Mixture with Scrap Tire Rubber and Nylon Fiber from Waste Tires: Laboratory Performance and Preliminary M-E Design Analysis

Scrap tire rubber and nylon fiber are waste materials that could potentially be recycled and used to improve the mechanical properties of asphalt pavement. The objective of this research was to investigate the properties of scrap tire rubber and nylon fiber (R-F) modified warm mix asphalt mixture (WMA). The high-temperature performance was estimated by the Hamburg wheel-tracking testing (HWTT) device. The low-temperature cracking performance was evaluated by the disk-shaped compact tension (DCT) test and the indirect tensile strength (IDT) test. The stress and strain relationship was assessed by the dynamic modulus test at various temperatures and frequencies. The extracted asphalt binder was evaluated by the dynamic shear rheometer (DSR). Pavement distresses were predicted by pavement mechanistic-empirical (M-E) analysis. The test results showed that: (1) The R-F modified WMA had better high-temperature rutting performance. The dynamic modulus of conventional hot mix asphalt mixture (HMA) was 21.8%~103% lower than R-F modified WMA at high temperatures. The wheel passes and stripping point of R-F modified WMA were 2.17 and 5.8 times higher than those of conventional HMA, respectively. Moreover, the R-F modified warm mix asphalt had a higher rutting index than the original asphalt. (2) R-F modified WMA had better cracking resistance at a low temperature. The failure energy of the R-F modified WMA was 24.3% higher than the conventional HMA, and the fracture energy of the R-F modified WMA was 7.7% higher than the conventional HMA. (3) The pavement distress prediction results showed the same trend compared with the laboratory testing performance in that the R-F modified WMA helped to improve the IRI, AC cracking, and rutting performance compared with the conventional HMA. In summary, R-F modified WMA can be applied in pavement construction.

Developing a prediction model for low-temperature fracture energy of asphalt mixtures using machine learning approach

ABSTRACTThis paper presents an augmented full quadratic model (AFQM), artificial neural network (ANN) and an innovative machine learning technique called self-validated ensemble modelling (SVEM) approaches to predict low-temperature fracture energy of asphalt mixtures. An experimental database including 852 fracture energy values obtained from low temperature disk-shaped compact tension (DCT) testing was utilised to develop the prediction models. The fracture energy was predicted in terms of several variables that are available during the mix design process. The collected data were categorised into three groups based on the availability of the data at different points during the mix design process. A sensitivity analysis was conducted to assess the impact of the design variables on fracture energy. Based on the model development results, both ANN and SVEM methods showed higher prediction accuracy than AFQM. Prediction models based on the ANN were time-consuming and computationally expensive due to the optimum model architecture. The SVEM technique was found to be a reliable prediction method with high prediction reliability even with a limited amount of data. Based on the sensitivity analysis, design traffic level, PG low temperature (PGLT) binder grade, amount of aggregate passing 9.5 mm sieve, and the voids in mineral aggregate (VMA) are the most effective factors impacting low-temperature asphalt mixture fracture energy. A web-based prediction model platform was developed using prediction models based on the SVEM technique which can be utilised as a predesign tool to evaluate low-temperature fracture energy of asphalt mixtures when laboratory testing is not feasible.

Effect of irradiation temperature on the fracture-mechanical behaviour of tungsten irradiated to 1 dpa

We investigate the influence of neutron irradiation on the fracture-mechanical behaviour of ITER-specification conform stress relieved tungsten bar. The neutron irradiation of miniaturized Disc-shaped Compact Tension specimens with electrical discharge machined notches was performed in the Material Testing High Flux BR2 reactor of SCK CEN in Mol. Damage doses close to 1 dpa were achieved at multiple irradiation temperatures of 400, 600 and 1000°C. Suppression of thermal neutrons fluence by applying stainless steel capsules resulted in reduction of Re (Os) content down to fusion application relevant level of 2 at% (0.2 at%). Neutron irradiation strongly impacted fracture-mechanical behaviour of the material. Depending on the irradiation temperature, the DBTT increases by about 650°C or even higher as has been deduced on the basis of the load-displacement curve analysis. The degradation of the mechanical properties is attributed to the alteration of the microstructure and fracture mode. The results are compared with the fracture-mechanical data on irradiated tungsten obtained previously in the course of same neutron irradiation campaign.

At the frontline for mitigating the undesired effects of recycled asphalt: An alternative bio oil-based modification approach

Soybean oil-derived modifiers were used for the improvement of properties of asphalt materials prepared for a pavement demonstration project. The rheological properties of base, biomodified and extracted binders were measured/compared using rheometers. The binder modification resulted in a decrease of 1.2 °C and 2.3 °C in, respectively, the high-and low-temperature grades of base binder, and when the effect of RAP binder was considered, the continuous performance grade (PG) became almost identical with that of base/control binder. Due to the biomodification and the presence of RAP, the binder’s elastic recovery (R) increased by 8.0% and its non-recoverable creep compliance (Jnr) decreased by 0.13 kPa−1. The tests conducted to evaluate the mechanical performance of the mixtures proved the efficacy of the bio-modifiers used in reversing the undesired effects of reclaimed asphalt pavement (RAP) and improving the performance of asphalt pavements at different temperatures. For instance, the Hamburg wheel tracking (HWT) test results revealed that the presence of bio-modifiers resulted in the increase of stripping inflection point (SIP) by 3619 passes. The disk-shaped compact tension (DCT) test proved the effectiveness of the bio-modifiers used, as these modifiers increased the fracture energy by 113 J/m2. The master curves constructed for the asphalt binders and mixtures indicated an increased stiffness/elasticity at intermediate and high temperatures.

Laboratory Performance and Field Case Study of Asphalt Mixture with Sasobit Treated Aramid Fiber as Modifier

The use of fiber in asphalt mixtures can improve the mixture’s tensile strength and increase its cracking resistance, but the clumping of fiber in the mixture can weaken the improvement effect. The aim of this research is to assess the properties of Sasobit treated aramid fiber modified asphalt mixture and to validate the field case study. The rutting and stripping resistance of the fiber-modified mixture was identified with the Hamburg wheel tracking device. The low-temperature cracking characteristics of the asphalt mixture were quantified with the disk-shaped compact tension test. Moreover, the dynamic modulus test was adopted to reflect the response of the mixture to different loads and frequencies. The creep slope and stripping slope decreased by 67%, and the number of passes to stripping point and fail point increased by more than 250% after fiber modification. The fracture energy, peak load, and maximum crack mouth opening displacement (CMOD) of the mixture increased more than 10% after fiber modification. The dynamic modulus, rutting parameter, and fatigue parameter of the mixture were improved after fiber modification. The fiber in the mixture improved the stiffness of the mixture at high temperatures. The tensile strength improvement at low temperatures promoted the cracking resistance of the mixture. The cracking number in the fiber modified asphalt pavement was less than that in the control asphalt pavement. Thus, fiber modification could significantly restrict the propagation of cracking in the asphalt mixture. The implementation of fiber in the project can provide experience for future fiber application in asphalt pavement.

Performance Evaluation of Bioengineered Recycled Asphalt Materials

In this study, asphalt mixtures were engineered with bio-renewable soybean oil-derived modifiers, and then used for pavement demonstration projects in the U.S. states of Iowa and Minnesota in the summer of 2019. The performance grade, elastic recovery (R), and non-recoverable creep compliance (Jnr) of the binders were evaluated. The modification of asphalt binder for the Iowa project almost maintained the high and low temperature grades, and the presence of modifier in the mixture of Minnesota project resulted in a slight decrease in the low temperature grade while maintaining the high temperature grade. The Jnr and R values proved the increase of elasticity and relaxation of the asphalt binder modified for the Iowa project. The Jnr calculated for the Minnesota project revealed a considerable increase in the relaxation of the asphalt binder. Disc-shaped compact tension, Hamburg wheel tracking, and push-pull were the mechanical performance tests performed on the mixtures. Based on the results obtained from these tests, the Iowa mixture, produced with modified asphalt binder, showed a significant improvement in resistance to low-temperature cracking, rutting, moisture damage, and fatigue cracking. The Minnesota mixture, modified in the plant, showed a significant improvement in the fatigue performance and a slight improvement in low-temperature cracking resistance.

Performance grade of asphalt mixtures based on mixture performance test thresholds

At present, the way of fingerprinting a multi-binder, heterogeneous asphalt mixture systems involves the testing of extracted and recovered binder from lab specimens or field cores. However, concerns have been raised over the recovery of a truly representative binder sample from mixtures with modified binder system (additives, recycled binder, etc). On the other hand, mixture performance tests have been shown to: a) distinguish between different constituents of an asphalt mixture, and b) correlate well to the field performance. This study attempts to establish necessary groundwork to allow the development of a mixture-based, usable temperature grading system as an alternative to the existing grading system for recovered binders. A novel Mixture Performance Grading system is proposed that utilizes two mixture tests, namely, the Hamburg wheel track test (HWTT) and Disk-shaped compact tension (DC(T)) test, to compute a mixture-based performance grade. Both tests have shown excellent correlation with field performance data, and permit high and low temperature grading along with the computation of a mixture-based Usable Temperature Interval (UTI). The grading system involves conducting the tests at multiple temperatures and then using appropriate thresholds to find a failure temperature, analogous to current specification to determine failure temperatures of recovered binder. Eight mixtures were tested in this study including mixtures placed on Missouri roads (referred to as Level 1 mixes) and mixtures with various modifications (referred to as Level 2 mixes). The mixture grading approach was found to appropriately credit mixtures composed of high-quality aggregate structures, binders, additives, interactions between mixture ingredients, and those with good mixture volumetric designs. Obviously, the ability to evaluate the efficacy of mixture-based ingredients and mixture design variables is a key evolutionary step over traditional (and cumbersome) binder grading for modern, heterogeneous recycled asphalt mixtures.

Development of Warm In-Place Recycling Technique as an Eco-Friendly Asphalt Rehabilitation Method

Cold In-place Recycling (CIR) has been widely used in the world since it is easy to apply it in the field at a low cost. However, it is not normally used as a surface layer as a result of its inconsistent quality due to an excessive amount of fine aggregates pulverized during the milling process. Hot In-place Recycling (HIR) can retain the original shape of the aggregates, but it often produces a large amount of Volatile Organic Compounds (VOCs). Therefore, a third in-place recycling technique is introduced in this paper: Warm In-place Recycling (WIR). The WIR technique overcomes the limitations of both CIR and HIR techniques by lowering a heating temperature while adding a Tetraethylenepentamine (TEPA)/Soybean/SBS additive. To identify the effect of the additive on the RTFO-aged binder, viscosity and dynamic modulus values were measured at different temperatures. Based on Hamburg Wheel Tracking (HWT) and Disc-Shaped Compact Tension (DCT) tests, the additive improved the moisture susceptibility and low temperature cracking resistance. The indirect infrared heating equipment reduced the emission by lowering the pavement surface heating temperature by 20 °C from 140 to 120 °C. Compared with the heating at 140 °C, the LPG usage for heating at 140 °C was lowered by 21%. The proposed WIR equipment with an additive would revolutionize the in-place recycling practices.

Comparison and correlation of asphalt binder and mixture cracking parameters incorporating the aging effect

Cracking is one of the primary distresses in asphalt pavements, and aging can significantly affect the cracking properties of asphalt material. The primary objective of this study is to investigate comparisons and correlations between binder and mixture cracking property parameters subjected to different aging conditions, and further evaluate how these relationships would change with aging. Nine plant produced mixtures are conditioned following various conditioning protocols, and the corresponding binders are extracted and recovered from those mixtures. Mixture properties are measured from Complex Modulus (E*), Direct Tension Cyclic Fatigue (DTCF), Semi Circular Bending (SCB) and Disk-shaped Compact Tension (DCT) tests. Binder tests include Complex Shear Modulus (G*) and the Linear Amplitude Sweep (LAS) test. The binder and mixture parameters measured include thermo-rheological parameters, fatigue indices and fracture parameters. Different statistical analysis methods including the Pearson, Kendall and Hoeffding’s D analysis, and frequency curves are used to investigate the comparisons and correlations between the parameters. The results from this study indicate that while considering all the aging conditions, binder parameters evaluated generally correlate well with each other. Mixture parameters also show good correlations with the exception of the fatigue parameters. Mixture rheological and fracture parameters generally show moderate to strong correlations with all the binder parameters, while mixture fatigue parameters only show strong correlations with binder fatigue parameters. Further looking into these relationships with respect to the individual aging condition, the correlations between parameters typically are improved with increase of aging levels. In addition, the distributions of these performance parameters with aging show a close trend with change of asphalt chemical composition over time, indicating the fundamental relationship between asphalt chemistry and its engineering performance. The overall good correlations between binder and mixture parameters reinforce the concept that binders play a significant role in the cracking properties of asphalt mixtures.

Mesoscale modeling of fracture in cement and asphalt concrete

In this paper, mesoscale modeling is performed to simulate and understand fracture behavior of two concrete composites: cement and asphalt concrete using disk-shaped compact tension (DCT) tests. Mesoscale models are used as alternative to macroscale models to obtain better realistic behavior of composite and heterogeneous materials such as cement and asphalt concrete. In mesoscale models, aggregate and matrix are represented as distinct materials and each material has its characteristic properties. Disk-shaped compact tension test is used to obtain tensile strength and fracture energy of materials. This test can be used as a better alternative to other tests such as three points bending tests because it is more convenient for both field and laboratory specimens in addition to its accurate results. Comparing the numerical results of the mesoscale models of cement and asphalt concrete specimens with experimental data shows that these models can predict the behavior of these composite materials very well as seen in the curves of load-crack mouth opening displacement (CMOD). Also, the mesoscale modeling highlights the variability of crack direction where it is dependent on the random distribution of aggregate.

Development of a Performance-Related Framework for Asphalt Mixture Design for the Illinois Tollway

Various agencies have begun to research and introduce performance-related specifications (PRS) for the design of modern asphalt paving mixtures. The focus of most recent studies has been directed toward simplified cracking test development and evaluation. In some cases, development and validation of PRS has been performed, building on these new tests, often by comparison of test values to accelerated pavement test studies and/or to limited field data. This study describes the findings of a comprehensive research project conducted at Illinois Tollway, leading to a PRS for the design of mainline and shoulder asphalt mixtures. A novel approach was developed, involving the systematic establishment of specification requirements based on: 1) selection of baseline values based on minimally acceptable field performance thresholds; 2) elevation of thresholds to account for differences between short-term lab aging and expected long-term field aging; 3) further elevation of thresholds to account for variability in lab testing, plus variability in the testing of field cores; and 4) final adjustment and rounding of thresholds based on a consensus process. After a thorough evaluation of different candidate cracking tests in the course of the project, the Disk-shaped Compact Tension—DC(T)—test was chosen to be retained in the Illinois Tollway PRS and to be presented in this study for the design of crack-resistant mixtures. The DC(T) test was selected because of its high degree of correlation with field results and its excellent repeatability. Tailored Hamburg rut depth and stripping inflection point thresholds were also established for mainline and shoulder mixes.

Characterization of discontinuous crack closure behavior after the application of a single overload cycle

This paper aims to analyze the discontinuous crack closure behavior observed after a single tensile overload (OL) under otherwise constant ΔK loading. The investigation was performed on a disk-shaped compact tension DC(T) specimen made of Al6351-T6 with thickness of 2 mm, in order to simulate the case of a crack propagating under plane stress conditions. The Digital Image Correlation (DIC) technique was used in situ to obtain the full-field displacement and strain solution around the crack surfaces during the fatigue crack propagation. The experimental data obtained from DIC analysis revels that the residual hump of stretched material left in the wake of the propagating crack by the overload acts like a compliant spring, leading to a discontinuous crack closure mechanism. Moreover, this residual hump has a significant effect on the post-overload transient crack growth, even at distances much larger than the plastically deformed area created by the overload in front of the crack tip.

Relationship between Rheological Indices and Cracking Performance of Virgin, Recycled, and Rejuvenated Asphalt Binders and Mixtures

Long-term cracking performance of asphalt mixtures is heavily influenced by asphalt binder rheology, and changes in binder stiffness, ductility, and aging condition significantly affect the cracking resistance of the corresponding asphalt mixture. This study evaluated the effectiveness of several common and recently developed binder rheological indices in capturing the effects of binder performance grade (PG) and source, recycled binder content, recycling agent (rejuvenator) dose, and long-term aging. These binder rheological indices included the Superpave intermediate-temperature PG (PGI), the Glover-Rowe (G-R) parameter, the crossover temperature (Tδ = 45°), the rheological index (R-value), and ΔTc. This study also directly compared the binder rheological indices with the cracking performance of corresponding asphalt mixtures to explore possible correlations and their robustness. Asphalt mixture cracking performance was evaluated using the Illinois Flexibility Index Test (I-FIT) for intermediate-temperature cracking, and the disk-shaped compact tension (DCT) test and the uniaxial thermal stress and strain test (UTSST) for low-temperature cracking. Results indicated that all the binder rheological indices (except PGI) consistently captured the effects of binder blend composition and proportions and aging condition, with a few exceptions. Results also showed that the G-R parameter, the crossover temperature (Tδ = 45°), and ΔTc had the best correlation to asphalt mixture and field core cracking performance as compared with other rheological indices (PGI and R-value), with ΔTc demonstrating the overall best correlation to mixture cracking performance.

Effects of Laboratory Compacted Asphalt Mixtures Air-Void Variations on Fracture Properties at Low Temperatures

Abstract Asphalt mixtures contain several constituents, among which air void content has a significant effect on pavement performance. In this research, the impact of laboratory compaction air-void variations for asphalt mixtures was investigated on asphalt-mixture fracture energy and peak load, as two primary outcomes of the disk-shaped compact tension test. Two groups of asphalt mixtures with air void contents of 7 ± 1.0 % were selected, and comprehensive statistical analyses, such as Pearson’s correlation coefficient, analysis of variance (ANOVA), Welch’s test, and Tukey’s test, were conducted to explore any possible impacts of specimen-to-specimen air-void variation on fracture energy and peak load. Statistical analyses have been carried out on individual asphalt mixtures in groups and all asphalt mixtures together as a group, as well. Results showed that there is a weak-to-negligible relationship between air void, fracture energy, and peak load. In addition, results of the ANOVA at the significance level of 0.05 revealed that air void content does not have any significant effect on asphalt mixtures’ fracture energy and peak load, except for peak load data when all asphalt mixtures are grouped as single population. Finally, results of Tukey’s test demonstrated that in a range of 7 ± 1.0 %, air void does not have a significant effect on fracture energy at the significance level of 0.05. The findings of this study can help researchers and agencies extend the air-void variation tolerance to 1 % in asphalt mixtures with target of 7 % air void content to assess low temperature cracking resistance.

Laboratory and Field Evaluation of Pre-Treated Dry-Process Rubber-Modified Asphalt Binders and Dense-Graded Mixtures

Pre-treatment of ground tire rubber is emerging as a popular method to incorporate rubber particles in dense-graded asphalt mixtures. This study investigates the effects of a chemically engineered Dry-Process Ground Tire Rubber (DP-GTR) modification in asphalt binders and mixtures. The DP-GTR is comprised of rubber particles measuring 400 to 600 µm in diameter (minus #30 mesh) coated with a non-elastomeric liquid. No change in aggregate gradation is necessary in DP-GTR modification of asphalt mixtures. In this study, the effects of DP-GTR modification on binder properties were measured by dynamic shear rheometer, Multiple Stress Creep and Recovery (MSCR), and bending beam rheometer tests. Additionally, mixture properties measured by three cracking tests: Disk-shaped Compact Tension (DC[T]) test, Illinois Flexibility Index, and indirect tensile asphalt cracking test and one rutting test (Hamburg wheel track test) were evaluated. Results showed: (a) 10–12°C bump on binder high temperature performance grade with 10% DP-GTR modification by weight of binder; (b) improvement in non-recoverable compliance in MSCR test indicated higher rut resistance; (c) increase in DC(T) fracture energy at low temperatures; (d) decrease in rut depth; and (e) decrease in flexibility index and cracking test index. Field performance of the chemically treated DP-GTR sections located in different states was examined to address discrepancies observed in the cracking tests. The cracking and rutting performance of all the field sections was good-to-excellent, suggesting that some of the currently popular simple cracking tests may not be able to properly assess the cracking resistance inherent in GTR-modified asphalt mixtures.