Reconsideration of the fatigue tests for asphalt mixtures and binders containing high percentage RAP

When applying reclaimed asphalt technology in a flexible pavement project, most performance concerns are related to low temperature and fatigue cracking since the stiffness of the HMA mixture could dramatically increase through adding a high percentage of reclaimed asphalt pavement (RAP) material. The purpose of this study is to evaluate asphalt mixtures with high RAP contents, prepared using two RAP addition methods, for their performance based on fatigue-cracking resistance rather than relying on volumetric properties. Asphalt mixture samples were prepared with three RAP binder content replacement percentages (30, 40 and 50%) using two preparation methods: the as-is RAP gradation (traditional method) and the splitting of the RAP gradation into coarse and fine fractions (fractionated method). Asphalt mixture beam fatigue and binder fatigue time-sweep tests were performed. Beam fatigue samples also underwent freeze–thaw cycling for freeze–thaw damage evaluation. Rather than basing the performance based solely on S–N_f curves to illustrate the fatigue performance, the beam fatigue test data was analysed through a dissipated energy approach. Faster fatigue degradation was observed for the 40% RAP binder and beam mixture when subjected to repeated loading. From a morphology aspect, this can be explained by the binder’s phase separation and physical hardening effects.     

Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixtures

This paper focuses on the microstructure and fracture surface morphology of neat and carbon nanofibers (CNF) modified asphalts and hot mix asphalt (HMA) mixtures using scanning electron microscopy (SEM). Asphalt binder was modified with 1.5 % of CNF by weight of binder. The modified asphalt was used to construct HMA mixtures at various CNF dosages, mixed with aggregate, using the Superpave Gyratory compactor. Small rectangular specimens extracted from the center of large HMA samples were tested under direct tension and the fracture surface was examined under SEM. The SEM analysis developed a fundamental understanding of the role that the CNF modification plays in the performance enhancement of asphalt and HMA mixtures. It was found that CNF not only possess good adhesion characteristics but also exhibits high connectivity and were evenly distribution throughout the binder. The fracture surface morphology also revealed that CNF exhibited crack bridging at micro/nano scale which may enhance the resistance to cracking due to repeated traffic loads.     

Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling

Fatigue cracking is a major form of distress in asphalt pavements. Asphalt binder is the weakest asphalt concrete constituent and, thus, plays a critical role in determining the fatigue resistance of pavements. Therefore, the ability to characterize and model the inherent fatigue performance of an asphalt binder is a necessary first step to design mixtures and pavements that are not susceptible to premature fatigue failure. The simplified viscoelastic continuum damage (S-VECD) model has been used successfully by researchers to predict the damage evolution in asphalt mixtures for various traffic and climatic conditions using limited uniaxial test data. In this study, the S-VECD model, developed for asphalt mixtures, is adapted for asphalt binders tested under cyclic torsion in a dynamic shear rheometer. Derivation of the model framework is presented. The model is verified by producing damage characteristic curves that are both temperature- and loading history-independent based on time sweep tests, given that the effects of plasticity and adhesion loss on the material behavior are minimal. The applicability of the S-VECD model to the accelerated loading that is inherent of the linear amplitude sweep test is demonstrated, which reveals reasonable performance predictions, but with some loss in accuracy compared to time sweep tests due to the confounding effects of nonlinearity imposed by the high strain amplitudes included in the test. The asphalt binder S-VECD model is validated through comparisons to asphalt mixture S-VECD model results derived from cyclic direct tension tests and Accelerated Loading Facility performance tests. The results demonstrate good agreement between the asphalt binder and mixture test results and pavement performance, indicating that the developed model framework is able to capture the asphalt binder’s contribution to mixture fatigue and pavement fatigue cracking performance.     

Fatigue life and endurance limit prediction of asphalt mixtures using energy-based failure criterion

Fatigue cracking is one of the major types of distress in asphalt mixtures and is caused by the accumulation of damage in pavement sections under repeated load applications. The fatigue endurance limit (EL) concept assumes a specific strain level, below which the damage in hot mix asphalt (HMA) is not cumulative. In other words, if the asphalt layer depth is controlled in a way that keeps the critical HMA flexural strain level below the EL, the fatigue life of the mixture can be extended significantly. This paper uses two common failure criteria, the traditional beam fatigue criterion and the simplified viscoelastic continuum damage model energy-based failure criterion (the so-called G^R method), to evaluate the effect of different parameters, such as reclaimed asphalt pavement (RAP) content, binder content, binder modification and warm mix asphalt (WMA) additives, on the EL value. In addition, both failure criteria are employed to investigate the impacts of these parameters in terms of the fatigue life of the study mixtures. According to the findings, unlike an increase in RAP content, which has a negative effect on the mixtures’ fatigue resistance, a higher binder content and/or binder modification can significantly increase the EL value and extend the fatigue life as was proved before by other researchers, whereas WMA additives do not significantly affect the mixtures’ fatigue behaviour. A comparison of the model simulation results with the field observations indicates that the G^R method predicts the field performance more accurately than the traditional method.     

Effect of warm mix additives on the interfacial bonding characteristics of asphalt binders

The quality of the interfacial bonding between asphalt binder and aggregates plays a significant role in determining the durability of asphalt mixtures. Warm mix asphalt (WMA) modifiers have been used extensively in the last decade primarily to reduce production and compaction temperatures as well as to improve workability of asphalt mixtures. This study aimed to provide better understanding of the effects of these WMA modifiers on the interfacial bonding between asphalt binders and aggregates. The evaluation focused on measuring surface energy of binders in unaged and aged states and aggregates and then calculating energy parameters that describe the potential of a given asphalt-aggregate combination to resist fatigue cracking and moisture damage. Results show that the combination of asphalt-WMA additive, as well as the content applied of WMA additive has a significant impact on the fatigue cracking and moisture damage resistance. The results suggest that it is poor practice to use a given type and percentage of WMA modifier without regard for binder type. Instead, test methods are recommended to evaluate the compatibility of asphalt binder, WMA additive type/content, and aggregates for improved performance at different conditions.     

Aging effect on combined mode fracture resistance of bitumen

Crack growth behaviour of aged and unaged 30/40 asphalt binder at subzero temperature was studied both experimentally and theoretically using a test sample called inclined notched bend beam specimen. A series of fracture tests were conducted on this specimen at two subzero temperatures and different combinations of modes I and II. The fracture toughness of tested asphalt binder was significantly dependent on the aging condition such that the load‐carrying capacity of aged bitumen was up to 60% less than the corresponding fracture resistance of unaged bitumen. Moreover, the effective fracture resistance value of tested binder was reduced by increasing the contribution of mode II and also decreasing the test temperature. The obtained experimental results for both binder types were also evaluated theoretically using a stress base crack growth theory.     

Fatigue resistance investigation of warm‐mix recycled asphalt binder,mastic, and fine aggregate matrix

Fatigue cracking is one of the primary distresses in warm‐mix recycled asphalt pavements. This paper evaluates the fatigue resistance evolution of warm‐mix recycled asphalt materials in different scales during the service period. The strain sweep test and time sweep test were performed, respectively, by dynamic shear rheometer to determine the linear viscoelastic limits and to characterize the fatigue behavior of warm‐mix recycled asphalt binder, mastic, and fine aggregate matrix with different ageing levels and recycling plans. The dissipated energy method was used to define the failure criterion and to construct the fatigue model. Effects of ageing levels and recycling plans on stiffness and fatigue resistance were investigated. Performance correlations among warm‐mix recycled asphalt binder, mastic, and fine aggregate matrix were developed, respectively, by the statistical method to determine the critical material scale for stiffness and fatigue resistance.     

Influence of commercial waxes and polyphosphoric acid on bitumen and asphalt concrete performance at low and medium temperatures

The influence of adding four types of commercial wax and one polyphosphoric acid to a non-waxy bitumen was investigated with regard to binder and asphalt concrete mixture performance. Binder properties were determined using dynamic shear rheometer (DSR), bending beam rheometer, force ductilometer and various conventional test methods. Asphalt concrete performance was investigated using tensile stress restrained specimen test (TSRST), creep test at −25,^˚C, dynamic creep test at 40,^˚C and complex modulus test at 0, 10 and 20,^˚C. Totally eleven binders and eight asphalt mixtures were investigated. TSRST fracture temperatures of the asphalt mixtures were marginally influenced by the addition of any of the additives, and significant physical hardening of the binders, observed by BBR testing, could not be established using TSRST. Also in creep testing of asphalt mixtures at −25,^˚C, recorded effects were less pronounced compared to binder testing. In dynamic creep testing, the smallest permanent strains were recorded for the asphalt mixtures containing FT-paraffin or montan wax, indicating better resistance to rutting. Adding polyethylene wax or polyphosphoric acid to the non-waxy bitumen used, showed considerable positive stiffening effects on the binder at medium and higher temperatures. However, this increase in stiffness could not be shown in dynamic creep testing (at 40,^˚C) of asphalt concrete mixtures containing these additives.     

Impact of nanosized additives on the fatigue damage behaviour of asphalt mixtures

The study presented in this paper aimed at evaluating the impact of different nanosized additives, including an organophilic nanoclay and multiwall carbon nanotubes, on the fatigue properties of dense‐graded asphalt mixtures. Cyclic direct tension fatigue tests were carried out, and the corresponding results were interpreted by means of a simplified version of the visco‐elastic continuum damage model. The experimental investigation also included linear viscoelastic characterization of the considered materials. Results derived from tests carried out on the mixtures containing nanosized additives were compared with those obtained for a reference standard mixture. It was found that the use of the abovementioned additives can give a substantial contribution to the enhancement of the fatigue damage resistance of asphalt mixtures. Moreover, when comparing the two types of additives, it was observed that organophilic nanoclays can outperform multiwall carbon nanotubes.     

Heterogeneity effects on mixed‐mode I/II stress intensity factors and fracture path of laboratory asphalt mixtures in the shape of SCB specimen

Edge cracked semi‐circular shape specimen subjected to three point bend loading is a favourite test specimen for determining fracture toughness of asphalt mixtures. However, in the vast majority of previous experimental works, the homogeneous medium assumption has been considered for determining the stress intensity factor and geometry factors of asphalt mixtures tested with this test configuration. As a more realistic model and in order to consider the effects of heterogeneity on corresponding values of stress intensity factors, the asphalt mixture was modelled as a two‐phase aggregate/mastic heterogeneous mixture and its fracture behaviour was investigated using numerical models of asymmetric semi‐circular bend (ASCB) specimens. The generation and packing algorithm was employed to randomly distribute the aggregates with different shapes and sizes inside the mastic part. The effect of the mechanical properties of asphalt mixture (elastic modulus and the Poisson's ratios of aggregates and mastic), coarse aggregates distribution and crack length were studied on modes I and II geometry factors by means of extensive two‐dimensional finite element analyses. Moreover, the effect of the elastic modulus of asphalt mixture components was evaluated on the fracture path using the maximum tangential stress criterion. It was shown that crack tip location, elastic modulus of aggregates and mastic are the most important affecting parameters on the magnitude of modes I and II geometry factors. It was also shown that the geometry factors are not sensitive to the Poisson's ratios of aggregates and mastic. In addition, fracture cracking path is affected by the elastic modulus of the asphalt mixture components such that, depending on the difference between the stiffness of stiffer coarse aggregates and softer mastic part, the crack may propagate either through the aggregates, mastic or interface of aggregate/mastic.     

R-curves characterisation analysis for asphalt concrete

Fracture tests, especially at lower testing temperatures, have become quite popular in quantifying low-temperature cracking. However, current fracture testing analysis methods often use a single number, such as fracture energy or fracture toughness, to quantify cracking resistance. These tests do not capture both the initiation and propagation of the crack. The Resistance Curve, or R-curve, is widely applied in many fields, such as metal, polymer and composites. The R-curve considers cracking resistance as a function of crack extension, which includes initiation and propagation. In this research, three asphalt concrete mixtures, including hot mix, hot mix with reclaimed asphalt pavement (RAP) and warm mix with RAP were tested at two temperatures, three levels of ageing and two levels of moisture condition by the Semi-Circular Bend fracture test. R-curves were constructed using the data from the fracture test, and digital images were utilised to capture the crack extension. In addition to capturing the traditional fracture energy, two new parameters were explored using the R-curves: the cohesive energy and the propagation parameter energy rate. It was found that cohesive energy was always in a narrow range (approximately 500–1000 J/m²) compared to the fracture energy range (approximately 500–1700 J/m²) over all combinations of ageing and moisture conditions, which indicates that the crack initiation may not be as sensitive to temperature, ageing and moisture as fracture energy. The results of energy rate indicated that moisture and short-term ageing impact the crack propagation by reducing the resistance of crack growth. These results proved that R-curves are a potentially useful tool to quantify the cracking resistance of asphalt concrete in both crack initiation and propagation.     

Experimental assessment of flue gas desulfurization residues and basic oxygen furnace slag on fatigue and moisture resistance of HMA

Basic oxygen furnace (BOF) slag and flue gas desulfurization (FGD) residues both are industrial wastes. Research on using BOF slag as a novel aggregate and FGD residues as a filler in road construction has benefits both in environment and economics. The main objective of this research was to evaluate the effect of FGD residues and BOF slag on the fatigue performance and moisture resistance of asphalt mixtures. The fatigue performance of asphalt mixture was conducted by means of indirect tensile fatigue test. Stress loading control mode, with four stress levels (300, 400, 500 and 600 kPa), was used in this research. Statistic t‐test was adopted, and it had approved the positive effect of BOF slag and FGD residues on the fatigue lives of asphalt mixture. Moisture resistance of asphalt mixture was investigated by retained Marshall stability test and tensile strength ratio test. Research results indicate that BOF slag and FGD residues can improve the fatigue and moisture resistance, when the BOF slag and FGD residues based asphalt mixture was designed properly.     

Effect of load control mode on the fatigue performance of asphalt binder

Fatigue cracking is one of the major distresses found in asphalt pavements. The ability of asphalt binder to resist the accumulation of fatigue damage can have a profound effect on the service life of asphalt pavements. However, different from other materials, the fatigue characteristics of asphalt binder depend heavily on the load control mode. That is the fatigue characteristics of asphalt binder under controlled-stress mode differ from those under controlled-strain mode. This discrepancy affects the analysis results of asphalt binder fatigue performance. Therefore, this study is aimed at analyzing the effects of the load control mode on the fatigue performance of asphalt binder through a comparison of the fatigue characteristics under two different load control modes. The effect range of the load control mode was confirmed, and the method to differentiate the region affected by load control modes from the region not affected by load control modes was put forward. The relationship between the fatigue performances of asphalt binder under different control modes was analysed. Based on the results, it is concluded that the fatigue process can be divided into two regions using the curve of normalized dynamic modulus versus normalized phase angle. And, if the controlled stress and the controlled strain are equivalent levels, the fatigue characteristics of asphalt binder are independent of load control modes in the first region. The accumulated dissipated energy at the division point of the two regions is independent of both control modes and load levels, and serves as the effect threshold of the load control mode. Besides, the curve of accumulated dissipated energy versus number of cycles under controlled-stress mode is symmetrical to that curve under controlled-strain mode.     

Cohesive Modeling of Fracture in Asphalt Mixtures at Low Temperatures

Low temperature cracking is the major distress observed in asphalt pavements in the northern US and Canada. In the past years fracture mechanics concepts were introduced to investigate the fracture properties of asphalt mixtures at low temperatures. In this paper the cohesive zone model (CZM) is used to describe the fracture behavior of asphalt mixtures at low temperatures and the interface element is used to numerically simulate the material response under monotonic loading. The simulation is calibrated with the experimental results from a newly proposed semi circular bend (SCB) test. A parametric analysis of the input material properties indicates that the tensile strength has a significant effect on the peak load in the SCB configuration, the modulus has a strong effect on the calculated stiffness of the SCB specimen, and the fracture energy influences the post-peak behavior of the asphalt mixtures. The calibrated numerical model was applied to simulate the low temperature cracking in a simplified asphalt pavement and to study the influence of these material parameters on the performance of asphalt pavements.     

Fracture toughness prediction using Weibull statistical method for asphalt mixtures containing different air void contents

Brittle fracture of asphalt mixtures at low temperatures is one of the main deterioration modes of pavements. Hence as an important design parameter, it is required that a reliable value for fracture toughness of asphalt mixtures is known. However, because of natural inhomogeneity of asphalt mixtures and inherent sources of scatters such as random distribution of ingredients and preparation process, the use of statistical analyses might provide better estimations for the crack growth resistance of asphalt mixtures. In this paper by conducting several low temperature fracture toughness experiments on three types of asphalt mixtures with different air void contents, the effects of air void percentage on mode I fracture toughness are studied statistically. Fifty six edge cracked semi-circular bend specimens containing 4, 5 and 7% air voids were tested, and the corresponding two and three-Weibull distribution parameters were determined for each set of data. It was shown that the Weibull model can be used successfully for predicting the statistical nature of tensile cracking phenomenon in asphalt mixtures. The mean fracture toughness values and the Weibull parameters were reduced by increasing the air void content. Furthermore, the distribution parameters obtained experimentally for the mixtures containing 4% and 5% voids were also predicted quite well in terms of the Weibull parameters of a reference mixture containing 7% air void.     

Fatigue and healing performance assessment of asphalt binder from rheological and chemical characteristics

The fatigue and healing performance of asphalt binder affect the durability of asphalt concrete and by extension, asphalt pavements. The objectives of this paper are to (1) estimate the fatigue and healing characteristics of asphalt binder by newly developed linear amplitude sweep (LAS) and LAS-based Healing (LASH) protocols, and (2) investigate the relationship between chemical composition of asphalt and engineering performance. Three neat asphalt binders (Pen-30, Pen-50 and Pen-70) and one SBS modified binder are selected for this study. Experimental results indicate that the SBS binder has advanced fatigue resistance among all tested binders and the softer neat binder with a higher penetration grade generally displays better fatigue performance. The fatigue failure occurrence is a significant threshold for healing potential comparison. The rate of healing (H^R) results suggest that the best healing potential is with Pen-70 binder in pre-failure conditions followed by the SBS binder, Pen-50 and Pen-30 binders. However, the SBS binder presents better healing performance than Pen-70 binder in post-failure condition. Further solvency fractionation, into saturates, aromatics, resins and asphaltenes, indicates that the asphaltene content is negatively proportional to the quantified binder fatigue life whereas the H^R index is found to be well correlated to the weight percents of saturates and ratio of saturates to aromatics (S/Ar). The combined use of LAS and LASH tests is recommended for effectively distinguishing and designing the fatigue-healing performance of neat and modified asphalt binders. Limiting the contents of asphaltenes would be of help to improve the binder fatigue resistance and either saturates percent or S/Ar parameter should be considered to assure the self-healing potential of asphalt binder.     

Low-temperature cracking of polymer-modified asphalt

A laboratory investigation was conducted to study different types of polymer-modified asphalt with regard to low temperature behaviour. The thermal stress restrained specimen test (TSRST) was used to assess the disposition of asphalt mixtures to cracking at low temperature. Five polymer modified bitumens and three mixture types (dense graded, stone mastic and porous asphalt) were investigated. In addition, three different “gussasphalt” mixtures were analysed. Based on the results obtained, it was concluded, among other things, that (1) the polymer type, mixture type and degree of ageing influence the low temperature properties of asphalt mixtures; (2) the increase in fracture temperature during ageing is dependent on mixture and polymer type; (3) the use of modified binders may improve the low temperature properties of ‘gussasphalt” mixtures.     

Laboratory investigation of utilizing high percentage of RAP in rubberized asphalt mixture

The utilization of crumb rubber and reclaimed asphalt pavement (RAP) has proven to be economical, environmentally sound and effective in increasing the performance properties of the asphalt mixtures. The objective of this research was to investigate the laboratory engineering behavior characteristics of the rubberized asphalt binders and mixtures made with PG 64-22 and a softer binder (PG 52-28) containing a high percentage of RAP (30%). Some of the testing used for this research included viscosity, dynamic shear rheometer (DSR), bending beam rheometer (BBR), indirect tensile strength (ITS), resilient modulus, and fatigue life evaluations. The experimental design included the use of two aggregate and RAP sources, two virgin binder grades (PG 64-22 and PG 52-28), two types of crumb rubber (ambient and cryogenic), and four rubber contents (0%, 5%, 10%, and 15%). The results indicated that: (1) the crumb rubber improved the aging resistance of the aged binder and prolonged the fatigue life of the mixtures containing 0% RAP, in addition, results indicated a decrease of ITS and resilient modulus values was found as the rubber content increased, regardless of rubber type; (2) the utilization of softer binder decreased the influence of aged binder and decreased the resilient modulus values of the mixtures. In most cases, regardless of rubber types, the rubberized mixtures containing 30% RAP made with PG 52-28 binder did not show a significant increase in fatigue life with those made with PG 64-22 binder.     

玄武岩纤维对老化沥青混合料路用性能的影响

为研究玄武岩纤维对老化沥青混合料路用性能的影响,通过车辙试验、低温弯曲小梁试验、浸水马歇尔试验和冻融劈裂试验研究了玄武岩纤维对沥青混合料抗老化性能的作用。试验结果表明:虽然玄武岩纤维沥青混合料的动稳定度随老化时间增加,但相对于普通沥青混合料而言,其增加的幅度减缓,提出采用相对变形率作为老化性能评价指标;玄武岩纤维延缓了沥青混合料老化性能的衰变,使得老化后的混合料低温抗裂性改善;经短期老化和长期老化后玄武岩纤维沥青混合料水稳定性能均优于普通沥青混合料,且玄武岩纤维显著降低了长期老化试件的未冻融劈裂强度,因此在应用中应适当增加碾压次数。     

Introducing a new specimen shape to assess the fatigue performance of asphalt mastic by dynamic shear rheometer testing

Early failure of asphalt pavements is a common issue all around the world. Damages are caused by various reasons like insufficient binder or aggregate quality, an inadequate mix design or improper handling in the production/construction process. The effects of binder, aggregates and mix design have been widely studied and state-of-the-art testing methods are available for both, hot-mix asphalt (HMA) and for each component. An important part in HMA belongs to the asphalt mastic, where no standardized method is available to allow a quality control. Asphalt mastic is the mix of bitumen with aggregates smaller than 63 (125) µm and covers the coarse aggregates as the actual binding component in the mix. This research aims at developing a testing method for asphalt mastic based on fatigue tests. The dynamic shear rheometer (DSR) was found as a suitable device for this purpose. The DSR fatigue test consists of the 8 mm parallel-plate geometry widely used for binder performance grading with a sample height of 3 mm. Instead of a cylindrical specimen shape, a hyperboloid of one sheet is applied. This shape predetermines the point of failure and prevents adhesion/interface failures between the mastic specimen and the upper or lower DSR stainless steel plate. The specimens are prepared directly in the DSR employing a silicone mould to ensure an exact specimen shape. This test can be applied to all DSR devices without costly changes or additional equipment as long as sufficient cooling capacity and torque can be provided from the DSR. This fatigue test makes it possible to assess the fatigue performance of binders and mastic samples.