Experimental investigation on skid resistance of asphalt pavement under various slippery conditions

The skid resistant performance of slippery pavement is one of the most important pavement surface characteristics, as it is associated with both pavement serviceability and traffic safety. Through simulating different pavement conditions in the freezing laboratory, skid resistance of asphalt pavement under various slippery conditions is measured with pendulum friction coefficient tester. Then, the effects of pavement temperature on skid resistance of dry, wet, icing and snowy pavements are quantitatively analysed. Furthermore, factors exerting effects on test results are taken into account, such as thicknesses of ice and water film on pavement. Through quantitative analysis, empirical evaluation model of pavement friction coefficient (PFC) under different conditions is established. To facilitate practical engineering application, reference standard values of PFC are recommended. Finally, the PFC is classified into seven levels, which illustrates the corresponding relationships of friction rank, skid resistance assessment, PFC range and pavement conditions.     

Characterisation of asphalt pavement surfaces using torque measurements

Continuous increase in traffic volume in the recent years has resulted in greater surface friction loss of asphalt pavements. While the literature presents several ways of aggregate and asphalt mix screening to ensure that surface characteristics are at an acceptable level of friction to control skid-related accidents, the recommended polishing tests using different accelerated polishing devices are time-consuming and to some extent labour-intensive. Therefore, it is imperative that existing polishing devices be improved to shorten the test duration to a level desired by the industry as well as state and federal agencies. This paper aims at addressing this improvement that uses power unit (or motor) to generate energy enough to rotate the polishing disk at constant rotational speed while being pressed against the specimen surface, then read the power needed from the display screen for the following steps. Multiple verification techniques including comparison studies and statistical analyses were used to examine the validity of this improvement. It was found that data collected by the power unit was repeatable and able to precisely detect surface deterioration history for different asphalt mixes with different polish susceptibility in a manner similar to conventional friction and texture-measuring devices. Hence, aggregates and mixes were classified based on their surface frictional properties. Additionally, results from this study correlated well with results from other studies using conventional methods. Most importantly, it was found to be possible to cut down the polishing test time using torque values in lieu of conventional surface quantification methods.     

Optimisation of hot mix asphalt performance based on aggregate selection

Researchers over the last four decades have identified and demonstrated the effects of aggregate morphological properties (particularly shape, size distribution, angularity and texture) on the mechanical properties of hot mix asphalt (HMA). Rare studies, however, have clearly established the relationships between the aggregate properties and pavement performance. Therefore, they have not provided methods to optimise aggregate properties at the design stage to improve that performance. This study focuses on understanding the effects of aggregate gradation and type on moisture damage resistance of HMA and on pavement performance as indicated by stiffness and rutting. Results show that basalt aggregate achieves higher moisture susceptibility resistance and stiffness than limestone aggregate. Coarser gradation has the highest permanent deformation, while open gradation 2C provides the lowest moisture damage resistance. Furthermore, dense gradation 4C provides the lowest rutting and the highest stripping resistance. It is indicated that suitable selection of aggregate type and gradation can improve pavement performance and reduce the moisture damage problem of HMA.     

Investigation into material optimization and development for improved ravelling resistant porous asphalt concrete

Ravelling, the loss of aggregate from the pavement surface, is the dominant defect of noise reducing porous asphalt wearing courses. Meso-mechanical simulations of porous asphalt concrete (PAC) under a moving tyre passage were performed to get insight into the in-mixture stresses. The simulation results showed that ravelling developed over a wide range of temperatures and that particularly low or high temperatures were critical. Ravelling resistance at high temperatures strongly depends on the confining stresses that follow from the pavement deflection. However, the tensile strains induced by the combined effect of pavement deflection and thermal contraction are the main cause for ravelling at low temperatures. Material optimization by changing mortar or bitumen properties can result in a significant improvement on ravelling resistance. A flexible bituminous binder with ample relaxation behaviour showed to give an optimal performance for ravelling resistance. Adhesive failure and cohesive failure are the failure mechanisms within the stone contact and the weak link is responsible for ravelling. Adhesive failure is predominant at low temperatures, while cohesive failure is the main cause at high temperatures. Aging mainly enhances the high-temperature ravelling performance, but dramatically degrades low-temperature ravelling performance.     

Correction for the asphalt overlay thickness of flexible pavements considering pavement conditions

Pavement overlays represent a common technique used for pavement rehabilitation and maintenance and to increase the structural support of the pavements. In the Department of Defense, the methodology for the design of flexible pavement overlays is contained in the Unified Facilities Criteria 03-260-02 criteria and involves the use of an empirically derived formulation. The overlay design of flexible pavements is based on the thicknesses of the existing asphalt, base and subbase layers and the required minimum thickness for the asphalt layer. However, this formulation does not take into account the quality or the structural condition of the existing surface layers. The current formulation considers the materials to have full structural strength and no deterioration. This study proposes an improved methodology for calculating the required flexible overlay thickness of a flexible pavement by taking into account the structural condition of the existing asphalt layer. An asphalt thickness correction factor is introduced to quantify the amount of the existing asphalt layer thickness that can still offer structural support, and therefore influence the overlay thickness. The asphalt correction factor is based on the existing load-related distresses affecting the asphalt surface. The implementation of this new approach showed that an asphalt layer in poor condition requires up to 60% more in thickness than an asphalt layer in good condition. The proposed methodology aims to standardise the design and evaluation of flexible pavements overlaid with asphalt layers and account for existing structural conditions. Moreover, allocation of maintenance funding can be optimised, thus limiting pavement overdesign.     

4.75 mm SMA Performance and Cost-Effectiveness for Asphalt Thin Overlays

This paper presents the design of stone matrix asphalt (SMA) with 4.75-mm nominal maximum aggregate size (NMAS) and alternative surfacing cross-section for an asphalt wearing course that may improve pavement performances while controlling costs by using locally available aggregates. The 4.75 -mm NMAS dense-graded mix was previously used in several states to reduce layer thickness and cost. Because of its poor friction and limited performance, however, it was generally used as levelling binder and placed on low-volume roads. The 4.75-mm SMA is proposed for thinner asphalt overlays in this study to improve its performance as well as surface texture. The ultimate goal of this study is to develop the 4.75-mm SMA and evaluate its performance and engineering benefits as a wearing course under laboratory and field conditions. Test pavement sections were constructed in Northern Illinois and on-site performance tests were conducted to evaluate its performance under actual traffic loading. The engineering cost-benefit analysis with respect to performances from both laboratory and field studies suggests the proper application of 4.75-mm SMA as an efficient and cost-effective wearing course for asphalt thin overlays.     

Compact tension test for fracture characterization of thin bonded asphalt overlay systems at low temperature

Asphalt overlays provide an economical means for treating deteriorated pavements. Thin bonded overlay (TBO) systems have become popular options for pavement rehabilitation. In addition to functional improvements, these systems ensure a high degree of waterproofing benefits. Conventional asphalt concrete fracture tests were developed for pavements with homogeneous asphalt concrete mixtures, and typically their thicknesses exceed 50?mm (2?inch). The use of spray paver technology for construction of TBO leads to continuously varying asphalt binder content, up to approximately one-third of the layer thickness. Commonly utilized fracture test geometries for asphalt concrete include the single-edge notched beam, SEN[B], the disk-shaped compact tension, DC[T], and the semi-circular bend, SC[B]. The SEN[B] test geometry is not preferable for use in pavement systems due to difficulties in procuring beam samples from the field. Applications of the other established test geometries, the DC[T] and SC[B] tests, are limited because of the material nonhomogeneity caused by nonuniform distribution of asphalt binder and smaller as-constructed thicknesses of TBO, which are usually less than 25?mm (1?inch) for gap-graded and 50?mm (2?inch) for dense-graded hot mix asphalt (HMA) mixtures. Both the DC[T] and SC[B] tests simulate movement of the crack fronts in transverse or longitudinal directions in the pavement. Use of these tests on field-procured samples of TBO yields a crack front that encounters nonhomogeneous material through the specimen thickness. The crack moves perpendicular to the axis of material nonhomogeneity, which makes data interpretation and fundamental material fracture characterization challenging. In addition, the crack in the specimen is correlated to a crack channeling across the pavement width rather than a bottom-up or top-down direction, which is more desirable from the standpoint of coupling experimental results with currently available simulation models. This paper proposes a test procedure for fracture characterization of graded asphalt pavement systems that have significant material property gradients through their thicknesses. Suitable specimen geometry and testing procedures were developed using ASTM E399 and ASTM D7313-07b as a starting point. Laboratory tests were performed using an optimized compact tension, or C[T], test geometry for field cores as well as laboratory-fabricated composite specimens. Laboratory testing using the proposed procedure clearly showed distinction in the fracture characteristics for specimens prepared with varying material compositions. The capability of distinguishing different materials combined with stable crack growth makes the proposed testing procedure ideal for fracture characterization of thin and graded pavement systems. Statistical analysis of test data revealed that the proposed C[T] test procedure is capable of detecting differences in fracture energy results across a wide range of pavement systems and yields a low test variability. Finite element simulations of the test procedure further indicate the suitability of the test procedure as well as demonstrating a procedure for extraction of fundamental material properties.     

Estimating the moisture damage of asphalt mixture modified with nano zinc oxide

One of the main distresses of hot mix asphalt (HMA) is moisture damage. The most common method for decreasing this type of distress is using antistrip additives. In this study, the effect of nanoparticles was evaluated as an antistrip agent on the moisture damage of HMA. Two types of aggregates were evaluated in this study with different sensitivities against moisture damage (limestone and granite aggregate) and the asphalt binder with 60/70 penetration grade and nano zinc oxide (ZnO) in two different percentages by weight of the asphalt binder. The tests employed to evaluate the effects of modifying asphalt binder by nanomaterials on the moisture damage of asphalt mixture were surface free energy (SFE) and AASHTO T283. The results showed that the ratio of wet/dry values of indirect tensile strength for the mixtures containing nano ZnO for two types of aggregate were higher than the control mixtures. In addition, the results of the SFE method showed that adding nano ZnO increased the total SFE of the asphalt binder, which led to better coating of the aggregate with asphalt binder. Nano ZnO decreased the acid to base ratio of SFE of asphalt binder, while it led to improving adhesion between the asphalt binder and acidic aggregate that are prone to moisture damage.     

Effect of aggregate nature on the fatigue-cracking behavior of asphalt mixes

Fatigue cracking in asphalt mixes is one of the most common road pavement distresses. When mixes are designed in the laboratory, it is very important to select the most suitable materials to ensure a good performance against this phenomenon. The types of binder and mortar strongly influence mix cohesion by providing tensile and shear strength. The mineral skeleton supplies it with compressive strength and bearing capacity. In this sense, the grading curve and shape of the aggregate affect crack growth (because of the internal friction), whereas the nature of the aggregate plays an important role in mix behavior (because of its adhesion to the bitumen and its resistance to fragmentation). This paper analyzes the impact of the nature of coarse aggregate on the fatigue-cracking behavior of asphalt mixes, an aspect that until now has received little attention. The UGR–FACT test was used to evaluate the cracking behavior of two mixes by varying the load amplitude, frequency, and test temperature. The results obtained showed that the nature of the coarse aggregate has an important effect in the fatigue-cracking behavior of asphalt mixes, and could be as important as bitumen type or mineral skeleton in mix design.     

Laboratory characterization of directional dependence of permeability for porous asphalt mixtures

Water permeability is an important property for porous asphalt mixtures. Previous numerical modeling showed that the permeability of the porous asphalt mixtures varies in different directions and a single permeability cannot accurately evaluate the mixture’s directional permeability. To investigate the direction-dependent water permeability of the porous asphalt mixtures, a unidirectional permeameter was used to measure the permeability in twelve directions in the vertical plane (parallel to compaction direction) and twelve directions in the horizontal plane (perpendicular to the compaction direction) on two open graded friction course (OGFC) mixtures with different nominal maximum aggregate sizes, i.e., OGFC-13 and OGFC-10. Furthermore, a new multidirectional permeameter was designed which can control the rainfall intensity and adjust transverse slope to simulate the actual water flow process in pavement. The multidirectional permeability and void saturation of eight porous asphalt mixtures were determined by the multidirectional permeameter. Results show that the porous asphalt mixtures demonstrate direction-dependent permeability properties in both vertical and horizontal planes, whereas the dependence is less in the horizontal plane than that in the vertical plane. In the vertical plane, the minimum permeability occurs in the vertical direction and the maximum value occurs in the horizontal direction. In the horizontal plane, the permeability differs in different directions, but has no obvious relationship with directions. Increasing the air void content and the nominal maximum aggregate size of the mixtures can reduce the directional difference of the permeability. The void inside porous mixture cannot be entirely occupied by water when surface runoff occurs. Increasing the air void content and aggregate particle size can lead to an increase of the permeability and the void saturation in the porous asphalt mixtures.     

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.     

Experimental investigation of basic oxygen furnace slag used as aggregate in asphalt mixture

Chinese researchers have commenced a great deal of researches on the development of application fields of basic oxygen steel making furnace slag (BOF slag) for many years. Lots of new applications and properties have been found, but few of them in asphalt mixture of road construction engineering. This paper discussed the feasibility of BOF steel slag used as aggregate in asphalt pavement by two points of view including BOF steel slag's physical and micro-properties as well as steel slag asphalt materials and pavement performances. For the former part, this paper mainly concerned the mechanochemistry and physical changes of the steel slag and studied it by performing XRD, SEM, TG and mercury porosimeter analysis and testing method. In the second part, this paper intended to use BOF steel slag as raw material, and design steel slag SMA mixture. By using traditional rutting test, soak wheel track and modified Lottman test, the high temperature stability and water resistance ability were tested. Single axes compression test and indirect tensile test were performed to evaluate the low temperature crack resistance performance and fatigue characteristic. Simultaneously, by observing steel slag SMA pavement which was paved successfully. A follow-up study to evaluate the performance of the experimental pavement confirmed that the experimental pavement was comparable with conventional asphalt pavement, even superior to the later in some aspects. All of above test results and analysis had only one main purpose that this paper validated the opinion that using BOF slag in asphalt concrete is feasible. So this paper suggested that treated and tested steel slag should be used in a more extensive range, especially in asphalt mixture paving projects in such an abundant steel slag resource region.     

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.     

Generation of urban road dust from anti-skid and asphalt concrete aggregates

Road dust forms an important component of airborne particulate matter in urban areas. In many winter cities the use of anti-skid aggregates and studded tires enhance the generation of mineral particles. The abrasion particles dominate the PM10 during springtime when the material deposited in snow is resuspended. This paper summarizes the results from three test series performed in a test facility to assess the factors that affect the generation of abrasion components of road dust. Concentrations, mass size distribution and composition of the particles were studied. Over 90% of the particles were aluminosilicates from either anti-skid or asphalt concrete aggregates. Mineral particles were observed mainly in the PM10 fraction, the fine fraction being 12% and submicron size being 6% of PM10 mass. The PM10 concentrations increased as a function of the amount of anti-skid aggregate dispersed. The use of anti-skid aggregate increased substantially the amount of PM10 originated from the asphalt concrete. It was concluded that anti-skid aggregate grains contribute to pavement wear. The particle size distribution of the anti-skid aggregates had great impact on PM10 emissions which were additionally enhanced by studded tires, modal composition, and texture of anti-skid aggregates. The results emphasize the interaction of tires, anti-skid aggregate, and asphalt concrete pavement in the production of dust emissions. They all must be taken into account when measures to reduce road dust are considered. The winter maintenance and springtime cleaning must be performed properly with methods which are efficient in reducing PM10 dust.     

Analysis of inverted base pavements with thin-asphalt layers

Inverted base pavements are flexible pavement structures built by placing a top quality compacted granular aggregate base between a rigid cement-treated base and a thin-asphalt surface layer. The proximity of the granular base to the load makes its behaviour critical to the pavement response. Three-dimensional finite-element simulations are conducted to assess the mechanical performance of different inverted base pavement structures, with emphasis placed on pavements that feature thin-asphalt surface layers. A nonlinear constitutive model captures the anisotropic stress-dependent stiffness of the granular base. Results show that the stress distribution within inverted base pavements is markedly different from that of conventional pavements due to the stiffness contrast between successive layers. Thin-asphalt layers deform more uniformly and experience lower tension than thick layers. However, in the presence of combined shear and vertical contact loads, the benefits of a membrane response in thin asphalt concrete layers may be overwhelmed by the increased tensile strain at the load edge. The transition from beam to membrane asphalt response depends on the relative stiffness between the asphalt layer and the aggregate base. In most cases, the transition takes place at an asphalt layer thickness between 25 mm and 50 mm.     

Micro hardness of interface between cement asphalt emulsion mastics and aggregates

Characteristics of interface between cement asphalt emulsion mastics and aggregate is different from that of interface between cement and aggregate or asphalt and aggregate. A novel method was put forward to evaluate interface character of mixtures in the paper. MH-5 micro hardness apparatus was adopted to study influences of fillers’ fineness, aggregate lithology and different type of cement on micro hardness of the interface between the mastics and aggregate; and some micro test apparatuses, such as SEM, XRD and EPMA, were used to analyze its mechanism. Results indicate that the increase of cement and mineral filler fineness is favorable to improve micro hardness of the interface, but over fine mineral filler is disadvantageous. In contrast to granite and granite filler, limestone and limestone filler are more helpful to increase the hardness value. Lithology changes of mineral filler have greater influences on the hardness than those of aggregate. The micro hardness value first increases, then decreases with the increase of mineral filler/asphalt emulsion (MF/AE) ratios. There exists an optimal ratio, which is 1.1. Adopting the high strength cement is an effective way to improve the micro hardness.     

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.     

Investigation into three-layered HMA mixtures

Hot-mix asphalt (HMA) mixtures consist of three phases: aggregate, asphalt binder (mastic) and air voids, of which the first two (aggregate and asphalt binder) provide the structure that withstands various kinds of loading.

Due to the nature of high inhomogeneity between aggregate and asphalt binder, significant stress and strain concentration occurs at the interface between the two phases, which causes adverse effect to HMA mixtures and potentially contributes to pavement distresses/failure.

This paper presents a novel idea to mitigate the stress and strain concentration by introducing an intermediate layer between aggregate and asphalt binder in HMA mixture. Microstructural analyses of layered system indicated that the three-layered composite HMA mixture would greatly improve the performance of asphalt mixture. The composite mixture showed more than 10% reduction in internal stress and strain and consequently its performance could be potentially improved. To validate the theoretical analyses, a laboratory experiment was conducted to compare the performance of a conventional mixture to that of a conceptual three-layered composite HMA mixture, which was formed by incorporating a stiff natural asphalt (gilsonite) as the intermediate layer. The results of the limited laboratory experiment confirmed the findings from the theoretical analyses.     

Failure mechanism analysis of asphalt–aggregate systems subjected to direct shear loading

In order to investigate the mechanical behavior of asphalt–aggregate systems subjected to direct shear loading and reveal the shear failure mechanism, four groups of direct shear tests were conducted on composite specimens under different experimental conditions with a self-manufactured direct shear test apparatus at 25 °C. Comparative studies were conducted to evaluate the effects of stone surface treatment, asphalt film thickness and loading rate on the shear mechanical behavior of asphalt–aggregate specimens. Results showed that two kinds of the complete stress–displacement curves, including the general single-peak curve and the first-known double-peak curve, were clearly observed for each condition. In addition, the internal failure mechanisms were analyzed based on qualitative and quantitative methods. It can be concluded that the potential failure modes of the direct shear test include adhesive failure at the asphalt–aggregate interface and cohesive failure within the asphalt film. The research results enhance understanding of the shear mechanical behavior and failure mechanism of asphalt mixture, and also provide a reference for the interfacial failure.     

粗骨料对沥青混凝土开裂阻力影响分析

沥青混凝土是典型的非均匀材料,在进行力学分析时通常是将其视为均匀、各向同性体,但是理论分析结果很难与实际相符合。同时均质化假设也很难解释含大粒径骨料的沥青碎石作为防裂层的抗裂性能优于小粒径沥青混凝土的现象。笔者通过在均质的沥青混凝土引入一个粗骨料,应用断裂力学平面有限元程序系统地分析了粗骨料对沥青混凝土抗裂性能的影响,分析结果可以较好地解释大粒径沥青混凝土的抗裂性能优于小粒径沥青混凝土的机理。因此,使用大粒径骨料沥青混泥土能够改善路面的使用寿命。