Evaluation of deformation properties of asphalt mixture using aggregate slip test

Asphalt mixture is a multiphase particulate material composed of aggregate, asphalt and filler. The deformation property of asphalt mixture is an external reflection of aggregate slip behaviour. To evaluate the high-temperature deformation properties of asphalt mixture, an aggregate slip device was developed and aggregate slip tests were conducted on five asphalt mixtures for different gradations under different test conditions. Four evaluation parameters, the slip failure load (F_s), the slip failure deformation (D_s), slip modulus parameter (M) and slip energy index (SEI), were obtained according to the load–displacement curves. The relationship between these parameters and rut depth (RD) was analysed. The effects of test temperature and asphalt content on slip resistance of asphalt mixture are studied in this research. The results indicate that the parameter F_s has limitations for large nominal maximum particle-size mixture, and SEI is an effective parameter to evaluate the aggregate slip properties for different nominal maximum particle-size asphalt mixtures. SEI has the strongest relationship to RD, which is the best parameter to evaluate the slip deformation behaviour of asphalt mixture. With the increase in asphalt content, SEI has a peak value and a valley value. When the optimum asphalt content is used in asphalt mixture, aggregate skeleton effect and asphalt cohesive force can both reach a high level, and asphalt mixture has the best deformation resistance.     

Effects of high friction aggregate and PG Plus binder on rutting resistance of hot mix asphalt mixtures

The rutting resistance of hot mix asphalt (HMA) Superpave? mixes in surface course materials was investigated using asphalt material characterisation tests and a digital imaging processing (DIP) technique. The effects of the type of aggregate, the type of binder and the binder content on rutting resistance were quantified. Two types of aggregate were examined: Superpave? SP12.5 and high friction SP12.5 FC2. Both a modified (PG Plus) and an unmodified binders were considered at the optimum binder content and the optimum content plus an additional 0.5%. To accurately identify the effect of each variable, the shear upheave of these mixes was also quantified. The DIP technique involved estimating the number of aggregate contacts, the total contact length and internal structure index of two-dimensional images of the experimentally tested samples. The results showed that both the rutting resistance and stiffness of HMA surface mixes were sensitive to aggregate type, binder type and binder content. A high friction aggregate provided a better internal structure characteristic, as well as superior rutting resistance and stiffness for HMA mixes. The use of PG Plus and the addition of 0.5% to the optimum binder content negatively affected HMA stiffness and rutting resistance. However, the levels of rutting resistance for all mixes were acceptable (rut depth < 12.5 mm), even when the shear upheave was considered. Internal structure indices measured by DIP were effective for capturing changes in the internal HMA structure with respect to aggregate type and asphalt cement content.     

Improved method of considering air void and asphalt content changes on long-term performance of asphalt concrete pavements

Mixture properties (aggregate gradation and volumetric quantities), rate of loading and environmental conditions are the most important factors that affect the |E*| values. The main objective of this study was to develop a rational approach to investigate and model the effect of air voids and asphalt content on the |E*| master curves and consequently predict pavement performance. In this study, |E*| tests were conducted on three asphalt concrete mixtures with the same aggregate gradation, but different binder grades. For each of these mixtures, the air void and asphalt contents were varied at three levels. It is found that the developed method provides a more accurate estimate of the effects of volumetric changes in hot mix asphalt. The application of the proposed approach would be most beneficial for quality control/quality assurance purposes, performance-related specifications and for estimating contractors' incentives and penalties, where |E*| is utilised to predict the pavement 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.     

Sensitivity analysis of dynamic response and fatigue behaviour of various asphalt concrete mixtures

This paper presents the dynamic response (|E*|) and fatigue behaviour of various asphalt concrete mixtures subjected to sinusoidal compressive loading. Eight different wearing and base mixtures including Superpave, Asphalt Institute, British Standard dense bituminous macadam and Pakistan's National Highway Authority gradations were selected, and gyratory compacted specimens were fabricated. Laboratory investigations of |E*| at various temperatures (4.4 to 54.4 °C) and loading frequencies (0.1 to 25 Hz) were used to construct stress‐dependent master curves separately, for wearing and base course mixtures. The indicators of dynamic response and viscous (or elastic) properties of the mixtures were used to derive fatigue parameter to estimate the resistance to fatigue, and results revealed that Superpave wearing and NHA‐B base course had better resistance to fatigue for evaluated mixtures. Also, the sensitivity of the dynamic modulus to the variation in hot mix asphalt mix properties using different aggregate gradation, diverse loading frequencies and temperature were evaluated.     

Investigating the effect of nanoparticles on the rutting behaviour of hot-mix asphalt

Rutting is considered as one of the major damages in asphalt mixtures. In this study, different types of nanoparticles such as TiO₂, Al₂O₃, Fe₂O₃ and ZnO in different percentages were added to the base asphalt binder in order to decrease the rutting potential of hot-mix asphalt (HMA). In the first step, asphalt binder tests for characteristics such as penetration grade, ductility, softening point and viscosity were performed on the asphalt binder modified by the nanoparticles. Then, after preparing HMA samples, the static creep test was done at two stress levels at a specific temperature. Results of this study showed that using the nanoparticles improved the behavioural properties of the asphalt binder and decreased rutting in asphalt mix samples. Furthermore, scanning electron microscope images taken from the asphalt binder samples modified by the nanoparticles demonstrated that these nanoparticles were properly distributed in the asphalt binder space and had a positive effect on the rutting performance of the asphalt mixes.     

Evaluation of asphalt–aggregate interaction based on the rheological properties

The silicon dioxide (SiO₂) and calcium oxide (CaO) analytical reagents are selected to prepare asphalt mastics and the effects of aggregate chemical composition on asphalt–aggregate interactions (AAI) are evaluated based on the complex modulus and phase angle. It is found that the oxide analytical reagents significantly affect the rheological properties such as complex shear modulus and phase angle, and the effects of CaO are greater than SiO₂ due to the stronger interaction between asphalt binder and CaO analytical reagents. Both the modulus stiffening ratio and the phase angle-based K. Ziegel-B coefficient could be used to evaluate the AAI, and the latter is the better index. Results show that the indexes increase with the test temperature, but decrease with the loading frequency, and tend to be constant. The higher adhesive strength between asphalt binder and limestone than basalt is likely attributed to the higher content of CaO in limestone aggregate and the stronger asphalt–CaO interaction.     

An artificial neural network model for virtual Superpave asphalt mixture design

This study presents an artificial neural network (ANN) model to predict the asphalt mixture volumetrics at Superpave gyration levels. The input data-set needed by the algorithm is composed of gradation of the mix, bulk specific gravity of aggregates, low- and high-performance grade of the binder, binder content of the mix and the target number of gyrations (i.e. N_ini, N_des and N_max). The proposed ANN model uses a three-layer scaled conjugate gradient back-propagation (feed-forward) network. The ANN was trained using data obtained from numerous roads with a total of 1817 different mix designs. Results revealed that the ANN was able to predict V_a within V_a (measured) ± 1.0% range 85–93% of the time and within V_a (measured) ± 0.5% range 60–70% of the time. Currently with the developed ANN model, Superpave mix design can take approximately between 1.5 and 4.5 days, which corresponds to 3–6 days of savings.     

Pavement smoothness of asphalt concrete overlays

Nine asphalt concrete overlays were prepared using three different mix designs: Type C, 12.5 mm Superpave, and Coarse Matrix High Binder Hot-Mix (CMHB-C) mixtures and three different coarse aggregate: siliceous gravel, quartzite and sandstone. Pavement overlays were placed on field sections constructed along IH-20 in Harrison County, Texas. Field sections included each of the nine different surface mixture types. The base course was the same for all surface mixtures and was designed with 90% limestone and 10% local field sand. For all mixtures including the base course, PG 76-22 binder was used. Three pavement condition surveys were conducted on the outside lanes of eastbound and westbound field sections; immediately after, after two years, and after three years of the construction of asphalt concrete pavement system. IRI values were estimated from the left and right wheel paths. Each data-set was analysed separately to compare the pavement smoothness of different asphalt concrete overlays. Section 3 (Superpave quartzite mix) in the westbound lanes and Section 4 (CMHB-C gravel mix) in the eastbound lanes showed the best ride qualities. A paired t-test was conducted for each section in order to assess changes in IRI values with time under real traffic conditions. Test results and statistical analyses indicated that except for the IH-20 westbound left lanes of field Section 3 (i.e. Superpave quartz mix) the IRI values of the asphalt concrete overlays were time stable over the research period of three years.     

The low temperature properties of conventional and modified asphalt binders evaluated by the failure energy and secant modulus from direct tension tests

In our previous direct tension test (DTT) studies, SBS polymer modification was found to increase the failure stress values with increasing polymer levels. The predicted critical cracking temperatures (T_critical) in Superpave MP la specification were found to be 3 to 6°C lower than the bending beam rheometer (BBR) low temperature parameter according to Superpave MP1 specification. In this study, the DTT results were analyzed and compared in terms of the DTT failure energy and secant modulus instead of failure stress or failure strain values. As expected, the DTT secant modulus was found to increase with increasing hardness of the non-modified asphalt binder materials; however, the secant modulus decreases upon Styrene Butadiene Styrene (SBS) polymer modification. If the secant modulus were used to evaluate the stiffness of asphalt binder materials at low temperature, the SBS modified asphalt binders would not only have better low-temperature properties than predicted by the BBR low-temperature parameter in Superpave MP1 but also better than predicted by the T_critical in Superpave MP1a specification. The failure energy of SBS modified asphalt binder at T_critical was found to be invariably higher than the T_critical failure energy of non-modified asphalt binders, even though the T_critical of PMA was already 3–6°C lower than the T_critical of the non-modified asphalt binder. The elastic polymer network in the PMA probably contributes to higher DTT failure stress, failure strain and failure energy values.     

The effect of aggregate gradation on creep and moisture susceptibility performance of warm mix asphalt

It is clear that the purpose of mixture design is to select optimum asphalt content for a desired aggregate structure to meet the prescribed criteria. Aggregate makes up high proportion of volume and mass of mixtures; hence, it is considered as an important constituent of asphalt concrete. This study postulates that the gradation is an important characteristic of the aggregate in adoption of the optimum mixture. One aggregate source, three gradations and different percentages of Sasobit^® was used to manufacture hot mix asphalt and warm mix asphalt. The test results indicated that the aggregate gradation affects the rutting resistance and especially the moisture susceptibility of the introduced mixtures, differently. Rutting resistance was evaluated using the flow number parameter, and in order to determine the moisture sensitivity mechanism, a mechanical and visual inspection tests were carried out. At the end, it is concluded that the optimum aggregate gradation for these two types of mixtures is different.     

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.     

Field performance of hand-mixed and machine-mixed asphalt in labour-based asphalt maintenance works

Asphalt premix for labour-based maintenance works can be produced manually (hand-mixed) or by mechanised asphalt plants – the conventional method. Hand-mixed asphalt (HDMA), common especially in most developing countries, is claimed to equal machine-mixed asphalt (MMA) in terms of durability. Recently, however, there have been concerns about reduced durability of labour-based asphalt (LBA) works. The current study compared durability of HDMA and MMA with a view to establish suitable scope and appropriate traffic conditions for LBA maintenance works. The study was based on laboratory tests – on binder, aggregate and site premix – and field experiments which involved asphalt production and placement of small and big patches of HDMA and MMA on a heavily trafficked Likuni Road and a lightly trafficked Tsiranana Avenue in Lilongwe, Malawi. The study monitored the patch durability for two months after the maintenance work. Effects of the study variables – production method (HDMA and MMA), scope of works (small and big patches) and road category (heavily trafficked and lightly trafficked roads) – on patch durability were statistically analysed. The study has shown that production method affects durability of LBA the most. HDMA had 67% less durability than MMA. Scope of works and road class affected LBA durability by 27% and 23%, respectively. At a level of significance (p-value) of 0.05, the effect of production method was significant, while that of patch size and work scope was not statistically significant. It is imperative of asphalt producers to adhere to proportions of premix components if quality of HDMA is to improve. Apart from improving the HDMA production process, the quality of LBA works could be improved by increasing contractors’ access to MMA. The study recommends contractors’ cooperation and adoption of production techniques (cost-cutting techniques) such as use of recycled asphalt pavement and warm mixture asphalts in order to improve contractors’ access to MMA.     

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.     

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.     

The effects of chemical composition on asphalt microstructure and their association to pavement performance

The primary objective of this paper was to investigate the impact of asphalt chemical composition on the microstructure and performance characteristics of asphalt binder. The methods implemented in this study include adsorption–desorption chromatography analysis and a range of atomic force microscopy (AFM) and chemical force microscopy techniques. It was revealed through the study that certain asphalt chemical parameters have a consistent and measureable effect on the asphalt microstructure that is observed with AFM. Particular microstructures that emerged via chemical doping were then discovered to have unique chemical polarity, which explicitly impact durability and performance of asphalt. In fact, a surprising correlation was found between the saturates chemical parameter and the effects of oxidative aging on asphalt behaviour. The findings from this research directly contribute to the improvement of modelling capability while also creating new prospects for enhancing the performance characteristics and durability of asphalt binder.     

Effect of compaction conditions on aggregate packing using 2-dimensional image analysis and the relation to performance of HMA

Characterization of the asphalt mixture microstructure using two dimensional (i.e., 2-D) imaging techniques could be an economically efficient approach. However, the features that have been captured and quantified using 2-D imaging techniques in most published research are limited to simplistic analyses of aggregate structure. This paper focuses on introducing a more elaborate method for characterization of the internal structure of aggregates to define performance related parameters that could be used as quality indicators of mixes. These indicators are proposed as important properties that complement the volumetric properties so wide relied on for acceptance of mixture designs. The results of the study show that aggregate structure can be characterized using a combination of newly developed image analysis indices namely: number of aggregate-to-aggregate proximity zones, total proximity zone length, and proximity zone plane orientation. A software developed in a previous study and significantly modified for this study, is used to process digital images of a set of asphalt mixtures with different gradations, binder contents, types of modification, compaction efforts, compaction temperatures, and methods. The results demonstrate that the internal structure indices correlate well with rutting performance, as well as with low temperature thermal contraction of asphalt mixtures. Additionally, the indices can be successfully used to show the effects of compaction effort, compaction method and temperature, gradation of aggregates, and binder modification on the mixture internal structure. The results indicate potential for using this method for quality control of mixtures during production.     

Mechanical and structural assessment of laboratory- and field-compacted asphalt mixtures

Compaction forms an integral part in the formation of the aggregate orientation and structure of an asphalt mixture and therefore has a profound influence on its final volumetric and mechanical performance. This article describes the influence of various forms of laboratory (gyratory, vibratory and slab-roller) and field compaction on the internal structure of asphalt specimens and subsequently on their mechanical properties, particularly stiffness and permanent deformation. A 2D image capturing and image analysis system has been used together with alternative specimen sizes and orientations to quantify the internal aggregate structure (orientation and segregation) for a range of typically used continuously graded asphalt mixtures. The results show that in terms of aggregate orientation, slab-compacted specimens tend to mimic field compaction better than gyratory and vibratory compaction. The mechanical properties of slab-compacted specimens also tend to be closer to that of field cores. However, the results also show that through careful selection of specimen size, specimen orientation and compaction variables, even mould-based compaction methods can be utilised with particular asphalt mixtures to represent field-compacted asphalt mixtures.     

基于原子力显微镜技术的沥青与矿料表面粗糙度及黏附特性

采用原子力显微镜技术(AFM)的轻敲和力曲线模式,在微观尺度下对沥青与矿料表面的二维图像和三维形貌信息进行采集和分析,并测试和计算表征材料物理黏结特性的表面能。测试和分析结果显示,不同油源的沥青具有明显不同的化学组分和表面粗糙度,具有蜂形结构的沥青表面粗糙度更大。老化将明显减少沥青的表面粗糙度,降低其表面能,进而对沥青与矿料的黏附产生不利影响。结合宏观的黏附拉拔力学试验,对沥青和矿料表面粗糙度、表面能与宏观拉拔强度间的关联进行统计分析。结果表明,对于含蜂形结构的沥青,沥青和矿料表面粗糙度对黏附力学性能较为重要;而对于不含蜂形结构的沥青,其与矿料的黏附性更决定于所选材料的表面能,即两种材料间的物理黏结作用。     

Investigation of fatigue and fracture properties of asphalt mixtures modified with carbon nanotubes

Load‐induced cracking is one of the primary forms of distress in asphalt pavements at intermediate temperatures. Binder modification is a good alternative to promote the cracking resistance of asphalt mixtures. In the current research study, the effects of carbon nanotubes as a binder modifier on the fatigue and fracture performance of asphalt mixtures are investigated. The carbon nanotubes are added at five different percentages ranging from 0.2% to 1.5% to the base binder to study their effects on the fracture resistance and fatigue life of the asphalt mixtures. Using the cracked semi‐circular bend specimen, the critical value of J‐integral (Jc) was obtained for the investigated modified asphalt mixtures. Also, the fatigue behaviour of asphalt mixtures was studied using flexural beam fatigue test specimen. By employing the ratio of dissipated energy change approach, the plateau value of tested mixtures was determined as a measure of fatigue performance. Results showed that the carbon nanotubes can enhance both fracture resistance and fatigue performance of tested asphalt mixtures especially at higher percentages of the carbon nanotube.