Dense-graded Asphalt Mixtures Research Articles

Multi-criteria optimization of aggregate gradation based on thermal, surface and mechanical properties of the asphalt solar collector

The aggregate gradation of asphalt concrete for a road pavement is a critical element since it influences its strength and mechanical properties, and also has a major role on the thermal properties. An experimental study was set up to develop a multi-criteria optimization of AC12 dense-graded asphalt mixtures designed for asphalt solar collector layers according to the Bailey method (fine, middle or coarse gradation). Samples were tested in terms of thermal potential, surface properties and mechanical performance. It was found that a coarser gradation increased the heat concentration within the asphalt (more efficient mixture for asphalt solar collector), thanks to lower specific surface area, thicker asphalt films and fewer interconnected internal voids. The surface and mechanical performance of all asphalt systems matched the common technical prescription limits for surface road layers. Overall, the multi-criteria evaluation was effective in optimizing the characteristics of the solar collector asphalt concrete.

Exploring mechanical properties and cracking behavior of AC-13 and OGFC-16 aggregate-segregated asphalt mixtures

Aggregate segregation compromises the homogeneity of asphalt pavement, which diminishes the road performance of the asphalt mixture. The segregation behavior and damage mechanism of asphalt mixture are complex. To investigate the mechanical properties and cracking behavior of asphalt mixtures with varying degrees of aggregate segregation, the dense-graded asphalt mixture with the nominal maximum size of 13.2mm (AC-13) and Open Graded Friction Course with the nominal maximum size of 16.0mm (OGFC-16) with varying degrees of segregation were designed. Indirect tensile test and the digital image correlation (DIC) technology was employed to analyze the strain distribution and strain evolution at the crack initiation point within the asphalt mixtures. The parameters were also proposed to evaluate the resistance to cracking for the segregated asphalt mixtures at a mesoscopic scale. The results show that increased segregation of coarse aggregates in AC-13 mixtures leads to decreased indirect tensile strength, lower maximum tensile strain along the horizontal direction (E11), and reduced failure displacement. Conversely, segregation of fine aggregates in OGFC-16 mixtures results in increased tensile strength, maximum strain, and failure displacement. Higher amount of coarse aggregates in the asphalt mixture contributes to a more intricate strain distribution and a greater number of strain peaks. Dense-graded mixtures show larger strain concentration zones prior to cracking, while smaller stress concentration zones are observed in open-graded mixtures. The horizontal strain-time curve displays distinct stages of stable growth, rapid growth, and rapid decline. Aggregate segregation results in more intricate crack patterns with larger inclination angles, influenced by the random distribution and shapes of coarse aggregates. Novel indexes such as strain uniformity (P1,2) and strain density during crack development (DW) effectively evaluate mesoscopic cracking resistance of asphalt mixtures. The results offer valuable insights into cracking behavior and improve evaluation methods for the mechanical performance of aggregate-segregated asphalt mixtures.

Replacement of conventional aggregates and fillers with steel slag and palm kernel shell ash in dense-graded asphalt mixtures

Large quantities of steel slag and palm kernel shell ash (PKSA) – waste products from steel production and palm oil milling, respectively – are generated annually in several countries, and their disposal is challenging. Meanwhile, the over-reliance on conventional rock aggregates for asphalt mixture production poses increasing sustainability challenges. This study investigated the potential of entirely replacing granite aggregates with steel slag and PKSA in a dense-graded asphalt mixture. Two sets of asphalt mixtures were prepared; the control mixture contained crushed granite aggregate and hydrated lime, while the other set incorporated steel slag as coarse aggregate and PKSA as fine aggregate and filler. Both mixture types utilized AC-30 viscosity-graded asphalt binder. The properties of the waste materials met the quality standards required for aggregates in asphalt mixture production. Both mixture types were designed according to the Marshall design procedure and were evaluated for durability (Cantabro abrasion loss), fatigue cracking Resistance, rutting Resistance, and moisture damage susceptibility. The Cantabro abrasion loss test indicated that the waste-based mixture was 3% less durable than the control. However, the cracking Resistance of the waste-based mixture was approximately twice that of the control. Even though the rapid rutting test indicated that the control mixture was slightly superior in rutting Resistance, the Marshall quotient suggested otherwise. Both mixture types exhibited similar moisture damage resistance. Overall, the steel slag and PKSA samples have shown high potential to replace virgin granite aggregates and lime in asphalt mixtures fully and are, thus, recommended for field performance evaluation and possible adoption.

Assessment of Ground Tyre Rubber Impact on Stiffness Modulus and Cracking Resistance of Bituminous Mixtures

Abstract The objective of this study was to evaluate and characterize the performance of ground tire rubber (GTR) modified dense graded asphalt mixtures to recommend a suitable GTR content to produce a mix that provides superior pavement performance characteristics. The research effort encompassed investigation of stiffness and cracking resistance of six dense graded asphalt mixtures, including one conventional and five modified mixes covering 42 samples. Addition of GTR using wet process showed decrease in stiffness during loading. Cracking resistance of GTR modified mixtures increased with higher levels of dosages regardless of the base binder. An increase in the GTR dosage from 0 to 10% did not result in a significant change in fracture energy but a further increment to 25% increased the fracture energy. The laboratory Results indicated that 10% GTR was showed as the optimal content for the stiffness modulus and 25 % GTR as better content that provides better cracking resistance.

Development of Macrotexture Test Method for Dense-Graded Asphalt Mixtures

The design and construction of pavements necessitate careful consideration of longevity, cost-effectiveness, sustainability, and safety, with a focus on enhancing the durability and lifespan of dense-graded asphalt (DGA) surface mixtures. This paper discusses a research project the Federal Highway Administration (FHWA) initiated to assess the macrotexture characteristics of DGA mixture pavement—particularly concerning safety at higher speeds. The project aims to develop an efficient testing procedure for measuring macrotexture during mixture design and field-testing processes, striking a balance between longevity, cost-effectiveness, sustainability, and safety in pavement design and construction. The study began with a comprehensive review of existing practices, leading to the selection and testing of a unique laser texture scanner capable of noncontact macrotexture measurement. The subsequent phase involved validating the suggested macrotexture measurement system through data collected from State-sponsored projects under the FHWA Mobile Asphalt Technology Center program. Macrotexture measurements were collected from field cores and laboratory-compacted specimens from various State departments of transportation projects and analyzed. Preliminary analysis of the limited dataset to date revealed a positive correlation between average macrotexture measurements from gyratory specimens’ top surface and field core macrotexture measurements. The FHWA developed a draft test method for measuring macrotexture on laboratory specimens and field cores, thereby aiming to reduce procedural variability. Interlaboratory macrotexture test results demonstrated good reproducibility, with an average coefficient of variation of approximately 20%. Overall, this research project seeks to advance the understanding and measurement of macrotexture in DGA mixture pavements and thereby contribute to the development of safe pavements.

Research on Basalt Fiber Oil/Asphalt Absorption Performance and Test Methods Suitable for Asphalt Mixture with Different Structures

A basalt fiber asphalt mixture could improve the road performance of pavements and prolong the service life. The oil/asphalt absorption capacity of basalt fiber affects the road performance of asphalt mixtures to a certain extent. However, using kerosene as the medium to measure the oil absorption rate of bundle fibers by the vibration method, as the Chinese specifications recommends, is unreasonable. Therefore, the aim of this paper is to study the effect of the basalt fiber morphology on the oil absorption rate and the oil/asphalt absorption test methods suitable for asphalt mixtures with different structures (dense-graded and gap-graded), and to also explore the appropriate method to determine the oil/asphalt absorption rate of fiber to kerosene and asphalt. The results showed that the filamentous basalt fiber (FBF) was easier to disperse uniformly in asphalt than the bundled basalt fiber (BBF), and the oil absorption capacity of the FBF could more accurately characterize the actual working state of the fiber in the asphalt mixture. For the gap-graded asphalt mixture, the appropriate method to measure the fiber oil absorption rate is the combination of the vibration and centrifugation methods, while the fiber asphalt absorption rate is measured by the vibration method. For the dense-graded asphalt mixture, the combination of the extrusion and centrifugation methods are more reasonable to determine the fiber oil absorption rate, while the extrusion method is suitable for determining the fiber asphalt absorption rate. The concept of an effective fiber oil absorption rate is proposed to characterize the ability of fiber to adsorb kerosene in asphalt mixtures with different structures. A temperature of 160 °C is recommended as the test temperature to determine the fiber asphalt absorption rate. Kerosene as the asphalt absorption test medium could not directly reflect the ability of fiber to adsorb asphalt.

Coupled effects of Crumb Rubber and zeolite on the performance of dense-graded asphalt mixture: a study using a balanced mix design approach

ABSTRACT This research aims to investigate the effect of different levels of Zeolite and Crumb Rubber (CR) on the durability and mechanical performance of dense-graded asphalt mixture to specify a suitable dosage of these additives. 2%, 4%, and 6% of Zeolite as a water-based warm mix additive and 8% and 16% CR were used. Indirect Tensile Strength (ITS), Semi-Circular Bending (SCB), and Hamburg Wheel Tracking (HWT) tests were respectively used to investigate the moisture damage, fracture and rutting resistance. Moreover, performance interaction diagrams are used to determine the appropriate percentage of the additives. The results showed that modified asphalt with Zeolite and CR have better resistance to rutting and cracking, and with the increase in the amount of these additives, this improvement has been more significant. However, CR diminished the resistance against moisture damage; Zeolite had a positive effect on moisture damage and it compensates for the weakness of modified dense-graded asphalt mixtures with CR. The performance interaction diagram of dense-graded asphalt mixtures has recommended the use of 4%, and 6% of zeolite with and without 8% of CR as the most ideal percentages of employing these additives, taking into account their correct performance against cracking, moisture damage, and rutting damage.

Research on the Moisture Stability of Asphalt Mixtures with Three Solid Waste Fillers

Widespread interest has been drawn to the use of solid waste fillers as a partial replacement for natural fillers in high-performance asphalt mixtures in recent years. However, variations in the material properties of solid waste fillers remain a problem for the recycling method. To address this issue, the limestone powder in asphalt mixtures was replaced with three solid waste fillers, including steel slag powder, tailings powder and calcium carbide slag powder in this study. The chemical composition of the fillers was first characterized to assess the homogeneity of the material. Then, a dense-graded asphalt mixture (AC) and a stone matrix asphalt (SMA) mixture were designed, produced and characterized for wet stability. The results show that the asphalt mixtures with solid waste fillers were superior to limestone powder (LP) asphalt mixtures in terms of resistance to water damage, and the steel slag powder showed the best improvement in moisture stability of the asphalt mixtures. The optimum substitution of solid waste filler for limestone filler was 25%. With the addition of anti-stripping agents, the moisture stability of the asphalt mixture with limestone filler was also greatly enhanced. On the contrary, a marginal enhancement was observed in the moisture stability of asphalt mixtures using solid waste fillers. Solid waste fillers can be used in asphalt mixtures and have a similar function as that of anti-stripping agents. In summary, the use of solid waste fillers to replace mineral fillers in asphalt mixtures is a reliable, value-added recycling option.

Research on properties of basalt fiber-reinforced asphalt mastic

A basalt fiber-reinforced asphalt mixture can improve the engineering properties of asphalt pavement and prolong the service life of the road. However, few studies have systematically examined the composition of asphalt mixtures or the optimal ratio of fiber asphalt mastic suitable for different structural types. The effects of fiber content, filler–asphalt ratio, and asphalt viscosity on the properties of fiber asphalt mastic were investigated by orthogonal experiments to explore the reinforcement effect of basalt fiber on asphalt mastic. The optimal ratio of fiber asphalt mastic suitable for gap-graded and dense-graded asphalt mixtures was obtained by the fuzzy comprehensive evaluation (FCE) method. Meanwhile, the reinforcement effects of bundled basalt fiber (BBF), flocculated basalt fiber (FBF), polyester fiber (PF), and lignin fiber (LF) on asphalt mastic were compared and analyzed based on the optimal ratio of FBF asphalt mastic. The results showed that the optimal fiber asphalt mastic ratio suitable for gap-graded and dense-graded asphalt mixtures were that fiber content, filler–asphalt ratio, and asphalt viscosity were 3%, 1.8, and 1.1 Pa·s and 2%, 1.0, and 0.7 Pa·s, respectively. Analyzing the properties of different types of fiber asphalt mastic revealed that FBF could effectively enhance the high-temperature rheological properties and low-temperature tensile properties of asphalt mastic compared with other fibers. FBF asphalt mastic improved the asphalt rutting factor by more than four times. The tensile fracture energy of fiber asphalt mastic was more than three times that of the corresponding asphalt. The reinforcement effect of BBF was poor; it was recommended to be broken up before use.

System Identification Framework for Modeling of Static Creep and Dynamic Creep Behavior of Dense-Graded Asphalt Mixtures

The response of two dense-graded asphalt concrete (AC) mixtures under static and dynamic creep tests conducted in compression is modeled using the principles of the system identification framework in this study. The AC materials under static and dynamic creep loading conditions exhibit complex behavior patterns such as history dependency, plasticity, and temperature dependency. It is desired to formulate a constitutive model describing these complex phenomena. The empirical approach is outside the scope of this study. A viscoelastic continuum damage model requires individual experimentation to characterize linear and damage behavior. A viscoplastic model requires incremental deviator stress for plastic deformation. Both static and dynamic creep tests do not satisfy this condition. Therefore, the “system identification” approach is used in this study. Two methods of system identification approach—that is, black box and grey box modeling—are adopted to characterize static and dynamic creep behavior of AC. Of these two models under consideration, the black box models were versatile in modeling complex loading situations with a high degree of accuracy but lacked physical interpretation. While the grey box models have a physical interpretation, they are not as versatile as black box models.

Influence of Deicer on Water Stability of Asphalt Mixture under Freeze–Thaw Cycle

In seasonal frozen soil areas, the repeated freeze–thaw cycle of internal moisture in asphalt mixture in winter and spring will accelerate the peeling of asphalt film and aggravate the water damage of asphalt pavement. It is of great significance to carry out the attenuation law of mechanical properties of asphalt mixture under freeze–thaw cycles to prevent and reduce the economic losses caused by water damage to asphalt pavement. This study will investigate the impact of deicer application on the water stability of asphalt mixtures within the climatic conditions prevalent in Northwest China. Specifically, freeze–thaw cycle tests were administered to two types of dense-graded asphalt mixtures under three distinct deicer solutions and three disparate low-temperature environments. The Marshall water immersion test and freeze–thaw splitting test were employed to evaluate the water stability of asphalt mixtures subject to multiple factors, and the relative importance of each factor was statistically analyzed using the acquired data. Results demonstrated that AC-13 and AC-16 asphalt mixtures (AC is asphalt-concrete, which is asphalt concrete, and 13 or 16 represents the maximum particle size of aggregate (13 mm or 16 mm)), saturated in 15% CH4N2O, 20% NaCl, and 20% CH2CH3OH solutions, underwent a varying number of freezing–thawing cycles (0, 5, 10, 15, 20, 25, and 30) at temperatures of −5 °C, −15 °C, and −25 °C, respectively, displayed a discernible decline in their residual stability MS0 and freeze–thaw splitting tensile strength ratio TSR. This decline was particularly marked when temperatures dropped below the solution’s freezing point. Disregarding the fixed factors of weather variation (different low-temperature environments) and road service duration (number of freezing–thawing cycles), the aggregate grading imposed a more pronounced influence on asphalt mixture water stability than the presence of deicers.

Characterization of internal pore size distribution and interconnectivity for asphalt concrete with various porosity using 3D CT scanning images

The open pore characteristics of asphalt mixtures play a crucial role in their resistance to water-induced damage. Accurate extraction and analysis of the pore structures at a micro level can reveal the seepage characteristics of asphalt pavements. However, existing studies mainly focus on the through-pore characteristics of porous asphalt mixtures and there is a lack of research on the open pore characteristics and interconnected paths of dense graded asphalt mixtures. This study aims to fill this gap by using 3D X-ray computed tomography technology to analyze the digital images of three typical asphalt mixtures with different porosities. The study quantitatively and statistically analyzes several new pore structure parameters at a micro level, such as porosity, equivalent diameter, coordination number, pore throat dimension, and tortuosity. The interconnected paths of the pores are also compared and analyzed. The results show that as the total pores content increases, the difference between the pore fraction and open pores fraction decreases. The porosity of porous asphalt mixtures is high, but mainly composed of interconnected pores with only a small proportion of closed pores. The coordination number determines the connectivity and thus the porosity of the asphalt mixture, and larger open pore channels lead to larger pore diameters and greater permeability. As the porosity of the asphalt mixture increases, the average coordination number of its internal pores also increases, leading to better pore connectivity and a more complex pore network.

Effect of Pore Characteristics on Sound Absorption Ability of Permeable Pavement Materials

Permeable pavement materials are attracting great attention due to the superior benefits in fast drainage, driving safety, and noise reduction effectiveness. This study investigated the sound absorption behaviors of permeable pavement materials, including permeable cement concrete (PCC) and open-graded friction course asphalt mixture (OGFC). The sound absorption coefficients (SACs) of dense-graded cement concrete and dense-graded asphalt mixture were also measured for comparison. The sound absorption test was conducted by the impedance tube method according to ASTM E1050-19. Computed tomography (CT) scanning tests were conducted to obtain the pore characteristics of PCC and OGFC, including porosity distribution, pore size, and coordination number (CN). The porosity distribution, pore size distribution, and CN distribution presented a symmetrical trend of decreasing, then increasing, and then decreasing with the increase of specimen height. When OGFC and PCC showed similar porosities, the pore number of PCC was significantly larger than that of OGFC. Comparisons of the peak SAC and the average SAC were made between PCC and OGFC. Correlations were made between the SAC and the pore characteristics. Results indicated that with increasing porosity, the peak SAC and the average SAC both increased significantly for PCC and OGFC. The characteristic pore size, pore number, and CN distribution together affected the SAC. At the same porosity, OGFC mixtures presented larger average SAC compared to PCC, which was ascribed to the fact that more pores with larger size and a better pore connectivity existed in OGFC mixtures.

Neural Network Aided Homogenization Approach for Predicting Effective Thermal Conductivity of Composite Construction Materials.

Thermal conductivity is a fundamental material parameter involved in various infrastructure design guides around the world. This paper developed an innovative neural network (NN) aided homogenization approach for predicting the effective thermal conductivity of various composite construction materials. The 2-D meso-structures of dense graded asphalt mixture, porous asphalt mixture, and cement concrete were generated and divided into 2n × 2n square elements with specific thermal conductivity values. A two-layer feed-forward neural network with sigmoid hidden neurons and linear output neurons was built to predict the effective thermal conductivity of the 2 × 2 block. The Levenberg-Marquardt backpropagation algorithm was used to train the network. By repeatedly using the neural network, the effective thermal conductivities of 2-D meso-structures were calculated. The accuracy of the above NN aided homogenization approach was validated with experiment, and various factors affecting the effective thermal conductivity were analyzed. The analysis results show that the accuracy of the NN aided approach is acceptable with relative errors of 1.92~4.34% for the dense graded asphalt mixture, 1.10~6.85% for the porous asphalt mixture, and 1.13~3.14% for the cement concrete. The relative errors for all the materials are lower than 5% when the heterogeneous structures are divided into 512 × 512 elements. Ignoring the actual material meso-structures may lead to significant errors (134.01%) in predicting the effective thermal conductivity of materials with high heterogeneity such as porous asphalt mixture. While proper simplification is acceptable for dense construction composite materials. The effective thermal conductivity of composite cement-asphalt mixtures increases with higher saturation of grouted material. However, the improvement effect of the high-conductive cement paste on the composite cement-asphalt mixtures could be significantly reduced when the cement paste concentrates at the bottom of the mixture. Cracked aggregates and segregation of material components tend to decrease the effective thermal conductivity of construction materials. The NN aided homogenization approach presented in this paper is useful for selecting the effective thermal conductivity of construction materials.

Evaluation of the Factors Affecting the Performance of Fiber-Reinforced Asphalt Mixtures

This paper summarizes the results of the first comprehensive laboratory study that was conducted to evaluate the effect of using different aramid fiber properties (types, lengths, and dosages) with different combinations on the performance of dense-graded asphalt mixtures reinforced with aramid fibers. Asphalt mixtures with two different aramid fibers (Fiber A and Fiber B) were prepared. For each type of fiber, two lengths and four doses were considered. The semicircular bend (SCB) test, indirect tensile asphalt cracking (IDEAL-CT) test, and the overlay tester (OT) were conducted to evaluate fracture resistance and reflection cracking resistance of prepared mixtures, along with the asphalt concrete cracking device (ACCD) test for evaluating the low-temperature cracking resistance, the Hamburg wheel-tracking (HWT) test for evaluation of rutting susceptibility. Cost analysis was also done to evaluate the feasibility of adding aramid fibers to the asphalt mixtures. Results of the conducted tests indicated that the effects of adding aramid fibers to asphalt mixtures vary depending on the type of aramid fiber and its properties. In general, the addition of the Type A and Type B aramid fibers to the unmodified performance grade (PG) 64-22 mixtures resulted increasing their fatigue lives by 104% and 65%, respectively. Adding Type B fiber [short aramid/polyolefin fiber blend with 12.5 mm (0.75-in.) length] or long Type A fiber (wax-treated aramid fiber with a length of 1.5 in.) to the PG 64-22 mixtures resulted in improving the cracking resistance of reinforced mixtures. In addition, doubling the suppliers’ recommended dosage of each fiber type resulted in significantly lower cracking resistance values. The fibers did not improve the resistance to low-temperature cracking for asphalt mixtures. However, the rutting and moisture susceptibility results for fiber-reinforced mixtures were satisfactory compared with the unreinforced mixtures. Results of cost analysis indicated that the addition of aramid fibers to the mixtures resulted in a significant reduction of construction cost for these mixtures to the values between 27% and 37%. The optimum formulation of both aramid fibers for the purpose of improving cracking resistance of asphalt mixtures was found as 1.5-in. length at the supplier recommended dosage for Type A aramid fiber, and 0.75-in. length and the supplier recommended dosage for Type B aramid fiber.

Effect of low dosage crumbed rubber on the mechanical properties of a dense graded asphalt mixture

The benefits of crumb rubber modification of bituminous binder for asphalt mixture production is well established. Although most research has used high (20–30% by mass of the bitumen) crumb rubber dosages, there is now interest in low (5–15%) dosages for use in local road surfacing, where mechanical properties are less important. This paper quantifies the effects of low-dosage crumb rubber on the mechanical properties of a typical dense graded local road surface mixture. It was concluded that 5–10% crumb rubber dosage significantly increased the deformation (rutting) resistance and crack (fatigue) resistance of the asphalt. The effect of crumb rubber on mixture stiffness (modulus), moisture damage (stripping) resistance and workability (compactability) was mixed but was unlikely to present any practically important benefit or detriment. Consequently, 5–10% was concluded to be the preferred range of crumb rubber dosage for dense graded asphalt for local road surfacing.

Microscopy-based approach for measuring asphalt film thickness and its impact on hot-mix asphalt performance

The thickness of the asphalt binder’s film surrounding the aggregate particles in Hot-Mix Asphalt (HMA) plays a significant role in the durability of the pavement. This study introduces a new approach for determining the film thickness of the asphalt binder using a reflective digital stereo microscope. Image analysis was performed using the thresholding process to distinguish the aggregates, the voids, and the asphalt binder from each other using ImageJ software. Six different mixtures were investigated using this approach. The same materials were used to fabricate those mixtures but with different aggregate gradations. The thickness of the asphalt film was correlated to the dynamic modulus (E*) and rutting measured by flow number. The analysis showed that asphalt film thickness plays a major role in the dynamic modulus measurements at intermediate and high-temperature ranges. The correlation of asphalt film thickness with rutting showed a significant decrease in the flow number as the asphalt film thickness increased. This was more significant for gap-graded and Bailey’s gradation asphalt mixtures compared to the dense-graded asphalt mixtures. Finer dense gradations were slightly less affected by the asphalt film thickness increase compared to the coarser gradations.

Mechanical properties of coarse asphalt Mixture-Paved trackbed under High-Speed moving train loads

The long-term stability of the substructure is a critical guarantee for the safety and ride comfort of high-speed railway (HSR) operations. The asphalt mixture-paved trackbed (APT) can not only play a favorable role in preventing and draining water on the top of the subgrade but also be beneficial to the gradual change of vertical stiffness between layers. This paper determines a coarse but dense-graded asphalt mixture suitable for the APT and numerically analyzes the mechanical properties of APT under high-speed moving train loads. The results indicate that the SBS-modified asphalt mixture (SBS-25) with the optimum asphalt content (OAC) of 4.2 % is more appropriate than the crumb rubber-modified asphalt mixture (CRMA-25) for the APT. The dynamic stress in the subgrade of APT is significantly determined by the conditions of the AC layer. Meanwhile, the vibration attenuation of APT is directly related to temperature change. Compared with the traditional ballastless track, the vibration acceleration of APT with trackbed thickness of 300 mm reduces by 20.1 % at 20 ℃ while only reducing by 1.7 % at 40℃. Moreover, the greater the structural stiffness of the APT, the stronger the control on dynamic deformation of each layer in the subgrade, and the influence on the formation is weaker than that on the bottom layer of the subgrade. The findings hopefully contribute to the APT substructure design; however, further verification of real engineering still needs to be conducted by field or full-scale tests.

Fabricating and determining representative volume elements of two-dimensional random aggregate numerical model for asphalt concrete without damage

In order to optimize the fabricating method of representative volume element (RVE) for two-dimensional (2D) random aggregate numerical model of asphalt concrete, and determine the minimum size of RVE corresponding to the 2D random aggregate numerical model of typical graded asphalt concrete in the nondestructive state. The authors first proposed an RVE fabricating method that includes “aggregate region identification” algorithm and can place “partial aggregate”. Then, by using this method, 9 different sizes of RVEs corresponding to AC-13C, AC-20C and AC-25C asphalt concrete (AC-13C, AC-20C and AC-25C asphalt concrete are dense-graded asphalt mixtures, and their nominal maximum aggregate sizes are 13.2 mm, 19 mm and 26.5 mm respectively) were fabricated. Subsequently, the geometric characteristics (aggregate area proportion, aggregate angle, “density” homogeneity) of these RVEs were analyzed. After that, virtual dynamic modulus tests were performed on these RVEs. Finally, the results of geometric analysis and virtual dynamic modulus test were comprehensively analyzed. The results show that the “aggregate region identification” algorithm can effectively improve the aggregate placement efficiency. By placing “partial aggregate”, the RVEs with small-sizes that meet the requirements of aggregate area proportion in each particle size range can be fabricated. In the nondestructive state, the minimum sizes of 2D random aggregate numerical model’s RVE corresponding to AC-13C, AC-20C and AC-25C asphalt concrete are 20 mm, 30 mm and 40 mm respectively.

Performance assessment of asphalt mixtures produced with a bio-binder containing 30% of lignin

Lignin is one of major by-product from wood and paper industry. As natural biopolymer, it could be a valid alternative as bitumen replacement and/or modifier to enhance the relevant performance of asphalt mixtures, promoting concurrently environmental benefits. In this context, the study focused on the effects of two different binders containing 30% of lignin (by weight) on the mechanical properties of bio-asphalt mixtures. Specifically, two different lignins were blended with two plain bitumens having different consistencies so obtaining two bio-binders with a similar consistency to that of a reference plain bitumen. The two bio-binders and the reference bitumen were used to produce in the laboratory three dense graded asphalt mixtures for binder courses according to the Italian Technical Specifications. The binder-aggregate adhesion, Indirect Tensile Strength, Stiffness Modulus, thermal susceptibility, fatigue resistance, and low-temperature cracking of mixtures were analysed in both unaged and long-term aged conditions. Overall, this investigation demonstrates that bio-mixtures containing lignin offer comparable results to the reference asphalt mixture. Moreover, the bio-asphalt mixtures exhibit a lower thermal susceptibility, as well as lower aging susceptibility. The findings of this research highly encourage the use of lignin as partial replacement of bitumen in asphalt pavements.