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The stabilized crumb rubber modified asphalt is gaining attention from the researchers because of its excellent road performance and the environmental-friendly characteristic. Traditionally, the high-temperature performance of the hot-recycling asphalt binder was improved by adding low-viscosity recycling agent. In this paper, the stabilized crumb rubber modified asphalt was studied to investigate its improvement on physiochemical and rheological properties of the hot-recycling asphalt binder, especially high-temperature performance compared with traditional low-viscosity recycling agent. Three types of asphalt with different ingredients were investigated in this paper: (1) asphalt binder contains aged asphalt and crumb rubber modified asphalt; (2) asphalt binder contains aged asphalt and recycling agents; (3) asphalt binder contains aged asphalt, crumb rubber modified asphalt and recycling agents. The matrix asphalt, aged asphalt and stabilized crumb rubber modified asphalt was set as the control group. The physiochemical properties were evaluated by Fourier Transform Infrared Spectroscopy (FTIR), Gel Permeation Chromatography (GPC) and Atomic Force Microscopy (AFM). Moreover, the rheology experiments such as Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) were carried out to assess the practical performance of the three types of asphalt binder. In conclusion, it could be found that the mixing condition of different ingredient in the asphalt binder was mainly caused by physical mixing. Asphalt binder contains crumb rubber modified asphalt and aged asphalt was recommended considering the high and low-temperature performance. The optimal proportion was: 50% crumb rubber modified asphalt and 50% aged asphalt considering the economic benefits and rheological behaviors. In the extremely low-temperature environment, the high and low performance of crumb rubber modified asphalt and recycling agents in hot-recycling field still needs more investigation.

The objective of this study was to determine the effects of flow rates and layer thicknesses for the aggregates on the estimation of the aggregate gradation of an asphalt mixture based on machine vision. The watershed algorithm based on distance transform and concave point detection algorithm were synthetically used to split the touching particles. Increasing the flow rate overall decreased the prediction accuracy of aggregate gradation by image segmentation; the prediction accuracy decreased with the increase of the layer thickness. The prediction accuracy of the segmentation was better than that of the online detection of the asphalt mixture plant.

The gradation and mechanical properties of aggregates impressively affect the load carrying capacity and rutting resistance of asphalt mixtures. They are able to provide some fundamental parameters that are linked to mixture performance. In order to accurately detect critical controlled sizes and obtain more fundamental parameters that can characterize load carrying capacity and rutting resistance of asphalt mixtures, this paper aims to develop a mechanics-based approach to evaluate aggregate gradation, which contains the theoretical packing analysis and discrete element simulation. An extended theoretical morphology framework is developed by checking the local and global interlock of graded aggregates in the theoretical packing analysis. The sieve size range of primary structure which has an interactive interlock can be determined in the local interlock check model. The relationship between the transferred force and the induced force is proposed in the densest and loosest packing arrangements, respectively. The morphological parameters of disruption factor and weighted average size for the global scale are also proposed in theoretical packing analysis. Then the discrete element method is used to validate the theoretical framework. The advanced gradation input algorithm procedure is established in the discrete element simulation to simulate the compression test of graded aggregates. The mechanical parameters of contact force, contact points and stress–strain curves are extracted from the simulation results. The results show that the discrete element method can validate the theoretical framework and output the performance related parameters. The contact force analysis shows that the aggregates retaining on sieve sizes of 2.36 and 4.75 mm provide more than 50% contribution to resist load, and the aggregates retaining on sieve sizes of 1.18, 0.6 and 0.3 mm provide more than 50% contribution to stabilize the structure. The coordination number analysis suggests that the gradation with more fine aggregates might lead to a greater number of voids but smaller air-void size when the asphalt content and porosity remain constant. The stress–strain curve analysis shows that the modulus and secondary strain are highly related to the rutting performance of an asphalt mixture.

The rejuvenator is typically utilized to partially recover the mechanical properties of reclaimed-asphalt pavement (RAP). Although the rejuvenator works effectively in softening the highly ...

The air voids distribution has a significant effect on the pavement performance related distresses such as rutting, cracking, moisture damage and permeability. However, most studies only quantified the air voids by average content, ignoring the heterogeneity inside the materials. This study focuses on the heterogeneity of air voids distribution inside the open-grade asphalt mixture based on 2D image analysis. Equivalent aperture is proposed to measure the area of each air void. Results showed that along both vertical and horizontal sections inside the open-grade asphalt mixture, the number of voids with 0~2 mm equivalent aperture would have a great impact on the total number of voids, while large amounts of small voids would not significantly affect the total voids number. Additionally, voids with 0~4 mm equivalent aperture account for the largest proportion, and when the equivalent aperture is beyond 4 mm, the number of voids would decrease as the equivalent aperture increases, regardless of the void areas. Furthermore, in both vertical and horizontal sections, as the equivalent aperture increases, the speed to accumulate voids area would firstly increase and then decrease. As the equivalent aperture increases, the contribution to the total voids area would increase accordingly, and it would approach the peak when the equivalent aperture reaches about 8 mm.

Based on the discrete element method, the paper conducted a virtual simulation of static compaction test to investigate the motion of coarse aggregates within asphalt mixture. Considering the irreg...

To investigate the impacts of inner air-void structures on the high temperature performance of dense-graded asphalt concrete, the digital models of rutting plate composed of asphalt mastics, coarse aggregates and porosity were constructed by the discretized emulating software PFC 3D. Then, curves of rutting depth during virtual rutting tests were recorded, which matched with the laboratory-measured results. The similarity between these test curves verified that the virtual rutting test can be a feasible way to predict the rutting behavior of asphalt concrete. After that, the inner structure characteristics of air voids including porosity, size, horizontal and vertical distribution were put into the emulation software to investigate their effects on rutting behavior. Results show that larger porosity in asphalt concrete leads to worse rutting resistance, and this is especially true when air voids are concentrated in the middle section of the testing sample. In fact, the middle section contributes most to the rutting formation compared with other sections. In terms of vertical distribution, air voids concentrated in the mid-upper region of rutting plate have adverse effects on the rutting resistance. Moreover, sizes of air voids also influence the rutting behavior and the impact depends on aggregate gradations.

The traditional method for determining mixing and compaction temperatures of asphalt binders, such as Brookfield viscosity test, often yields excessively high temperatures when used in many modified asphalts, and underestimates the temperature decreasing effect of the additives when used in warm-mixing asphalts. This study proposed a new viscosity measurement method, namely rotational plate viscosity (RPV) testing, to overcome these issues. The new method uses a Dynamic Shear Rheometer (DSR) with a 25-mm-diameter parallel plate geometry and a fixed shear rate 40 s−1 to measure the viscosity-temperature curve of modified asphalts, which minimizes the effect of the shear-thinning behavior. In addition, there are two main advantages of this new method. First, it can be carried out using an existing standard DSR machine typically used in most asphalt binder laboratories. Second, it can greatly improve the accuracy and efficiency of testing with sophisticated sensors and programmed settings.

In this study, the underlying causes of the performance degradation of three typical waste-oil-rejuvenated asphalts (i.e., asphalts rejuvenated by waste cooking oil (WCO), waste bio-oil (WBO), and waste engine oil (WEO)) after secondary aging are explored via SARA fractions analysis. The rejuvenation provided by WCO is found to be mainly based on supplementing aged asphalt with saturates, which causes the newly formed colloids to easily lose light components and exhibit low stability. Further, WEO contains some metal residues that catalyze the oxidation and polycondensation process of asphalt. Consequently, the performance of the asphalts rejuvenated separately by WCO and WEO is severely degraded during aging. By contrast, WBO has a stable colloidal structure and lacks ashes; thus, WBO-rejuvenated asphalt exhibits the most stable and reasonable performance degradation in the aging process. Our research suggests that the effect of rejuvenation on aged asphalt cannot be judged only by the difference between the performances of the rejuvenated asphalt and the original asphalt. The performance maintenance of the rejuvenated asphalt during the secondary aging process requires further investigation.

This study aims to explore the possibility of utilizing limestone in porous asphalt concrete (PAC), targeting to the lower layer of the double-layer porous asphalt pavement. Limestone aggregate, which features relatively lower strength and desirable adhesion with asphalt, is used to replace basalt aggregate in PAC due to the shortage and high price of basalt. Both PAC with limestone and PAC with basalt are formulated with Marshall design method. Engineering performance evaluation verified that in comparison with PAC using basalt, PAC using limestone has less optimum asphalt content, lower Marshall compaction numbers, inferior drainage capacity, superior low-temperature cracking resistance, comparable rutting resistance and moisture stability, while satisfy the specification requirements. Furthermore, Superpave compaction model is employed to simulate the site compaction process. The results demonstrated that Superpave compaction process is divided into initial and over densification stages, in which the voids reduction in PAC with limestone is accelerated compared to that in PAC with basalt. The dynamic modulus of PAC increases with loading frequency, decreases with test temperature, and remarkably increases with gyration numbers. The master curves of dynamic modulus for PAC are constructed, which are approximately consistent for both PACs at equivalent compaction numbers. The tensile strength of PAC is found to increase with gyration numbers and both PACs present comparable tensile strength at the given compaction numbers. It is recommended to compact PAC using limestone with 40 gyrations. Therefore, limestone PAC with appropriate design and compaction parameters is prospective as the lower layer of the double-layer porous asphalt pavement.

Mix segregation is generally described as localized non-uniform zones of mix that do not conform to the original Job Mix Formula in gradation or asphalt content. Segregation may cause field performance issue such as rutting but no good trend between segregation and field/laboratory rut depth is observed. In this paper, a field road with segregation was monitored and those reported parameters used to detect segregation were collected. The segregated areas and the control non-segregated areas were identified by visual assessment. Rut depth were measured within one year after the road was opened to traffic. Rut depth from segregated areas and non-segregated areas are compared to evaluate the effect of segregation on rutting performance. The possibility of using different parameters such as pavement texture and construction temperature to detect segregation is also analyzed. Acknowledging that the rut depth may be affected by multiple parameters, a statistical Partial Least Squares (PLS) regression method is applied to select predictor variables that have the most significant effect on rut depth, and to develop rut depth prediction model. Results indicate that rut depth from segregated areas (visual) are statistically higher than rut depth from the non-segregated areas (visual). It is also found that pavement texture and construction temperature are the potential factors to detect segregation since they are correlated well with both visual assessment and rut depth. The developed rut depth prediction model shows good predictability and is well validated by considering segregation (mean texture depth), climatic effect (aging days), material property (HMA layer moduli), and pavement structure (entire HMA thickness).

Performance of asphalt pavement degrades after opening to traffic. The degradation of pavement structure is difficult to be evaluated because of the difficulties in in-situ testing and large scattering of testing results. Mechanical properties of asphalt mixtures were tested in this study to indirectly evaluate the degradation of pavement structure. Firstly, different testing methods were discussed from specimen preparation to testing results simulation of pavement structures. Indirect tensile test was finally chosen because of its easy testing process, clear stress state, stable testing results and convenience in preparation of field cored specimen. Secondly, asphalt mixtures with the same material and gradation as that of Nanjing Airport Expressway asphalt pavement were aged from 2 h to 24 h to simulate pavement at different aging period and tested. All testing results are sensitive to aging when the aging time is short and keep relatively stable when the aging time is longer. Elastic modulus of asphalt mixtures is more sensitive to aging time than that of strength and modulus at failure. Degradation Index (Id) was defined as the ratio of elastic modulus of original asphalt mixture to that of aged mixture which shows the aging degree of asphalt mixtures. Thirdly, fatigue tests were conducted on mixtures of different aging time to determine their fatigue life. Indirect tensile tests were performed on specimens with 20%–80% of fatigue life and different aging time to check the combined effect of climate and traffic loading. Id was modified based on the testing results.

This study investigated the simulation of wheel tracking test and the high-temperature rutting behaviour of an asphalt mixture by using the discrete element method (DEM). Based on the DEM software named as Particle Flow Code in three dimensions (PFC3D), a micromechanical model of an asphalt mixture composed of coarse aggregates, asphalt mastic, and air voids was built and a two-dimensional virtual wheel tracking test was simulated. Based on the virtual wheel tracking test, the distribution of displacement and contact forces within test specimen were analysed. It is proved that the built virtual wheel tracking test can capture the rutting deformation caused by the combination of densification and lateral flow deformation. It is also indicated that, within an asphalt mixture, the aggregate skeleton is the main bearing body for the compressive forces, while the compressive and tensile forces between aggregate and mastic are severer than that within asphalt mastic. It is important to guarantee the stability o...

Property characterization of high modulus recycled asphalt concretes modified by high modulus agents based on experimental evaluation and comparison with normal recycled asphalt concretes was performed in this paper. Both normal recycled asphalt concretes and high modulus recycled asphalt concretes with percentages of reclaimed asphalt pavement (RAP) varying from 20% to 60% were prepared for property evaluation including dynamic modulus by simple performance tester (SPT), rutting stability by wheel loading test, moisture susceptibility by Marshall test and indirect tensile test, thermal cracking resistance by three-point beam bending test, and fatigue cracking resistance by four-point beam bending test. It is found that, both RAP and high modulus agents can improve the dynamic modulus and rutting stability of asphalt concrete but jeopardize the thermal cracking resistance and fatigue cracking resistance of recycled asphalt concrete. However, the high modulus agents can enhance the moisture susceptibility of recycled asphalt concrete while the RAP could harm the moisture susceptibility of recycled asphalt concrete. It is also noticed that the high modulus agents can reduce the harmful effects by RAP on the properties of recycled asphalt concrete. Meanwhile, increasing the asphalt content of high modulus recycled asphalt concrete can further improve its moisture susceptibility, thermal cracking resistance and fatigue cracking resistance while keep its dynamic modulus and rutting stability still much better than the normal recycled asphalt concrete. The evaluation results show that the high modulus asphalt concrete have more tolerance than the normal asphalt concrete for negative effects of RAP on the engineering properties. It is more promising to use large percentage of RAP in the high modulus recycled asphalt concrete than in the normal recycled asphalt concrete.

The high-temperature creep behavior of asphalt mixture was investigated based on micromechanical modeling and virtual test by using three-dimensional discrete element method (DEM). A user-defined micromechanical model of asphalt mixture was established after analyzing the irregular shape and gradation of coarse aggregates, the viscoelastic property of asphalt mastic, and the random distribution of air voids within the asphalt mixture. Virtual uniaxial static creep test at 60 °C was conducted by using Particle Flow Code in three dimensions (PFC3D) and was validated by laboratory test. Based on virtual creep test, the micromechanical characteristics between aggregates, within asphalt mastic, and between aggregate and asphalt mastic were analyzed for the asphalt mixture. It is proved that the virtual test based on the micromechanical model can efficiently predict the creep deformation of asphalt mixture. And the high-temperature behavior of asphalt mixture was characterized from micromechanical perspective.

This study conducted performance evaluation of deicing asphalt mixtures with anti-icing additives and proposed a solution to reduce the negative influences of anti-icing additives on the engineering properties of asphalt mixtures. Engineering property tests, including high-temperature wheel tracking test, low-temperature three-point bending beam test, Marshall test and splitting test were conducted to evaluate the engineering properties of deicing asphalt mixtures. It is found that the anti-icing additives have negative effects on the rutting stability, thermal cracking resistance and moisture susceptibility of asphalt mixture. The addition of polyester fibers can compromise the negative effects by anti-icing additives to get fiber-reinforced deicing asphalt mixtures with satisfied engineering property. The anti-icing capacity of deicing asphalt mixture was also evaluated based on testing road observation and laboratory tests such as ice melting test, ice-mixture bond observation test, ice layer rupture test and ice layer pull-out test. It is proved that the anti-icing additives can be released from asphalt mixtures to help the melting of snow/ice and weaken the bond between ice and mixture effectively, which is helpful to keep the skid resistance of asphalt pavement. The findings indicate the fiber-reinforced deicing asphalt mixtures provide a promising way to build self-ice-melting asphalt pavement with well performance.

Dynamic shear modulus of an asphalt mastic has a remarkable effect on the mechanical performance of an asphalt pavement, and particulate composite micromechanical models are proven to be su...

Porous asphalt mixture has been extensively exploited as a surfacing layer on highways because it contributes to controlling pavement runoff, enhancing driving safety, mitigating urban-heat island, and reducing tire-pavement noise. This study proposes a methodology to design two types of high-viscosity modified porous asphalt mixtures with target air voids of 20% and 22% (PAC-10 and PAC-16) for the double-layer porous asphalt pavement, involving in fabricating high-viscosity asphalt, formulating aggregate gradations, and optimizing asphalt-aggregate ratio. Firstly, high-viscosity asphalt was manufactured by incorporating high-viscosity modifier into the base asphalt by 12% and 15% (Base-12, Base-15) and crumb rubber-modified asphalt by 8% (CR-8). The physical properties, 60 °C dynamic viscosity, rotational viscosity and viscoelastic properties of high-viscosity asphalt were characterized. Next, based on orthogonal design method, two aggregate gradations were optimized for PAC-10 and PAC-16 mixtures. Furthermore, the Draindown and Cantabro tests were carried to determine the optimum asphalt-aggregate ratios. Finally, the performance metrics of air void content, permeability coefficient, high-temperature and low-temperature performance, and moisture susceptibility was evaluated on PAC-10 and PAC-16 mixtures. The results show that with the addition of high-viscosity modifier into asphalt binder, the softening point, ductility, 60 °C dynamic viscosity, rotational viscosity, complex shear modulus, phase angle, and rutting factor of the binder are increased while the penetration is decreased. The formula of voids volume of the mixture as a function of aggregate gradation is developed, which facilitates the determination of aggregate gradation. Furthermore, the optimum asphalt-aggregate ratio is found to be 4.7% for PAC-10 mixture and 4.1–4.4% for PAC-16 mixture. Both PAC-16 and PAC-10 mixtures with CR-8 are recommended to construct the double-layer porous asphalt pavement.

A Simplified Triaxial Repeated Load Test (STRT) is reported to evaluate the rutting resistance of asphalt mixtures under field conditions. The laboratory tests and field measurements are conducted to verify the feasibility of the newly developed test method to characterize the permanent deformation of asphalt mixtures. The sensitivity of D1, λ (regression parameters in the Stephen Price model) and flow number FN measured by conventional Triaxial Repeated Load Test (TRT) and STRT is compared. Then the correlations between D1, FN and the rutting depth of a selected asphalt pavement are analyzed. The results indicate that the varying confinement in the STRT has significant effects on the permanent deformation of asphalt mixture. Repetitive tests prove that parameters measured by STRT share the similar stability with TRT. D1 can be used as an alternative indicator to assess the rutting resistance of mixtures, particularly for the polymer modified asphalt mixtures. D1 and FN measured by both STRT and TRT have good relevance with the rutting depth of the asphalt pavement, while the STRT makes a better distinction of permanent deformation between field cores with different rutting depths. As a result, the newly developed STRT turns out to be feasible and can better understand the dynamic creep properties of asphalt mixtures.

The objective of this work is to model the microstructure of asphalt mixture and build virtual test for asphalt mixture by using Particle Flow Code in three dimensions (PFC3D) based on three-dimensional discrete element method. A randomly generating algorithm was proposed to capture the three-dimensional irregular shape of coarse aggregate. And then, modeling algorithm and method for graded aggregates were built. Based on the combination of modeling of coarse aggregates, asphalt mastic and air voids, three-dimensional virtual sample of asphalt mixture was modeled by using PFC3D. Virtual tests for penetration test of aggregate and uniaxial creep test of asphalt mixture were built and conducted by using PFC3D. By comparison of the testing results between virtual tests and actual laboratory tests, the validity of the microstructure modeling and virtual test built in this study was verified. Additionally, compared with laboratory test, the virtual test is easier to conduct and has less variability. It is proved that microstructure modeling and virtual test based on three-dimensional discrete element method is a promising way to conduct research of asphalt mixture.

This study evaluated the effects of warm mix asphalt (WMA) additives on the compaction temperature and properties of crumb rubber modified (CRM) asphalt binder and mixture. Two different WMA additives (named as Sas and Evm) were used to prepare warm-mix CRM asphalt binder and mixture. The viscosity of different warm-mix CRM asphalt binders and mastics was measured at different temperatures. The rheological and mechanical properties of different warm-mix CRM asphalt binders were tested. At the mixture level, the volumetric properties of different warm-mix CRM asphalt mixtures were experimented by Gyratory compactor at different temperatures and the performance of different warm-mix CRM asphalt mixtures were evaluated. It was found that both of the two WMA additives could lower the compaction temperatures of CRM asphalt mixtures by 10–20 °C. However, they have different influences on rheological properties of CRM binder and performance of CRM mixture. The Sas warm-mix additive can improve the anti-rutting performance of CRM mixture but may degrade its low-temperature performance and moisture stability. The Evm warm-mix additive has no adverse effects on the high-temperature and low-temperature performance of CRM asphalt mixtures and can improve its moisture stability.

This study focused on the design of heat-resistant asphalt mixture for permafrost regions. Vermiculite powder with low thermal conductivity was used to replace some of the fine aggregates in the asphalt mixture to lower the thermal conductivity of asphalt mixture. Asphalt mixtures with different mass ratios (0, 3, 6, 9 and 12 %) of vermiculite powder were prepared for performance evaluation and thermal property evaluation. Wheel tracking test, low-temperature bending beam test, freeze–thaw splitting test and fatigue test were conducted to evaluate the influences of vermiculite powder on the high-temperature rutting resistance, low-temperature cracking resistance, moisture stability and anti-fatigue performance of asphalt mixture. Mathis TCI analyzer was used to analyze the influences of vermiculite powder on the thermal conductivity of asphalt mixture. Temperature monitor system was used to figure out the influences of vermiculite powder on the inside temperature of asphalt mixture. It is proved that vermiculite powder has no significant influences on the performances of asphalt mixture, while it obviously affects the thermal property of asphalt mixture. By the addition of 9–12 % vermiculite powder in asphalt mixture, the performances of asphalt mixture can still well meet the performance requirements for permafrost regions; the thermal conductivity can be reduced by 40–55 %; and the inside temperature of asphalt mixture can be lowered by 1–2 °C. It proves the feasibility of using vermiculite powder to produce heat-resistant asphalt mixture for permafrost regions.

This study focused on the effects of different parameters related to air voids on the creep behavior of asphalt mixture based on micromechanical modeling and virtual test through three-dimensional discrete element method (DEM). By using DEM software named as Particle Flow Code in three dimensions (PFC3D), user-defined micromechanical modeling was conducted for asphalt mixture. And virtual uniaxial static creep test of asphalt mixture was conducted by PFC3D and verified by laboratory creep test. The effects of air void content, microstructure parameters including size, elongated index and orientation of air voids, and air void distribution on the creep behavior of asphalt mixture were investigated based on virtual creep test. It is proved that bigger content, size and elongated index of air voids lead to greater creep strain of asphalt mixture. Compared to vertical orientation, horizontal orientation of air voids is more harmful to the creep behavior of asphalt mixture. Nonuniform distribution of air voids within asphalt mixture also has negative effects on the creep deformation of asphalt mixture, especially for vertical nonuniform distribution of air voids. Therefore, guaranteeing and improving the content, distribution and microstructure of air voids within asphalt mixture are important to keep the high-temperature stability of asphalt mixture. Keywords: Asphalt mixture, Creep behavior, Air void, Discrete element method

This study investigated the feasibility of using high-content reclaimed asphalt pavement (RAP) in high modulus asphalt concrete (HMAC). The design method of HMAC was adopted from the French Standard in which dynamic modulus were used to determine the optimum binder content and aggregate gradation. Laboratory performance tests, including high-temperature wheel tracking test, low temperature bending beam test, moisture stability test, and fatigue test, were conducted to evaluate mechanical performance of HMAC with different RAP contents. The effects of RAP content on performance of recycled high modulus asphalt concrete (RHMAC) depend on the specific RAP content and the performance indicator. In general, the RAP content shows significant influence on dynamic modulus and failure strain when the RAP content increases to 40%, while the RAP content shows significant impact on dynamic stability and tensile strength ratio when the RAP content increases to 50%. The RHMAC can be used in asphalt binder layer and base layer to provide good rutting resistance in long-lasting pavement (perpetual pavement) due to its high modulus and stability at high temperatures. However, considering the influences of RAP on low temperature performance and moisture stability, RHMAC is not suggested to be used in the surface layer with high RAP contents.

This study focused on the diffusion and distribution of the rejuvenator in the aged asphalt of reclaimed asphalt pavement (RAP) materials during hot recycling. Diffusion tests were designed based on the penetration test and the dynamic shearing rheometer (DSR) test to evaluate the diffusivity of the rejuvenator in aged asphalt. The phased extraction and recovery test and the asphalt mortar transfer test were designed to determine the distribution of the rejuvenator during the recycling of asphalt and the distribution of the recycled asphalt within the asphalt mixture during recycling, respectively. Performance tests, including the wheel tracking test, low-temperature bending beam test and freeze-thaw splitting test, were conducted to evaluate the influences of the rejuvenator distribution on the performances of the asphalt mixture during recycling. The viscosity, components and rejuvenator thermal stability are found to have important influences on the diffusivity and distribution of the rejuvenator in aged asphalt. During the short blending process of recycling the asphalt mixture, it is difficult for the rejuvenator to fully diffuse into the aged asphalt. Therefore, the property and distribution of the recycled asphalt tends to be nonuniform in the asphalt mixture during recycling. This nonuniformity can lead to poor high-temperature anti-rutting performance, poor low-temperature cracking resistance and poor moisture stability of the recycled asphalt mixture.

In order to investigate rutting behavior in long and steep section of mountainous highway, this paper selected generalized Kelvin model as the constitutive relationship for asphalt mixture, proposed a new method for material parameter identification, provided the tangential stiffness matrix for the generalized Kelvin model, and applied the constant power vehicular loads on the finite model. The results show that the overload, operational speed, and the tangential and vertical forces of the vehicle have a significant impact on the rutting of asphalt pavement. Based on rutting sensibility analysis, this paper also develops a confinement method of the slope grade and length in long and steep section of mountainous highway.

Cold recycling technology is getting more and more attention due to economic and environmental benefits by reduced energy consumption and resource conservation. However, its application has been limited to the base and subbase layer because of complicated components and poor crack resistance for the last decades. The design method has shortcomings especially the traditional mixing order, which may be one of the reasons for poor crack resistance. To support the viewpoint above, synthetic composite interfaces were designed to simulate the worst situation during the traditional mixing process and observed in micro scale by scanning electron microscopy. In the microstructure of the traditional mixing order, it was apparent that cement paste had a number of microdefects, a signal of lower interfacial adhesive strength. Moreover, based on the microscopic observation, the adding sequence of cement ought to be changed and two optimized mixing orders were designed of which the difference was verified by the SEM observation of synthetic composite interfaces and the mechanical experiments for different curing time. It can be summarized that mechanical performance was consistent with the microscopic observation. The traditional mixing order was the worst one both in the strength and moisture sensitivity. Finally, the optimal mixing order is put forward to decrease the possibility of the adverse interface, that is, the graded aggregates are mixed with additional water first to reach the workability, while cement, asphalt emulsion and mineral powder are mixed to form cement-asphalt emulsion mortar, finally mixing them all up.

Cold recycling technique is an effective strategy of cost reduction, resource conservation and energy consumption in pavement maintenance and rehabilitation. However, the mechanical mechanism of cold recycled mixture is not clear in the process of strength formation. The research was to put forward practical suggestions for mechanical property improvement of cold recycled mixture and generalize the application more efficiently to reach an ultimate purpose of energy and resource conservation. Various mixing orders were designed to construct different interfacial features and reflect on mechanical properties, and two-phase “aggregate-binder” interfaces were considered as the main interface forms to respectively reveal the mechanical and microstructural characteristics of cold recycled mixture. The mechanical properties of different mixing orders were compared and analyzed by the indirect tensile tests and dynamic modulus tests. Besides, the microstructure was observed by scanning electron microscope and the backscattered images were captured to extract microvoid parameters. Results indicated that different mixing orders and interface forms had a great influence on the mechanical properties of cold recycled mixture. Within the range of binder content applied in cold recycled mixture, the strength of the interface involving the recycled asphalt particles was larger than that of the interface involving the new aggregates. The indirect tensile strength of the interface involving asphalt emulsion was larger than that of the interface involving cement, while the microvoidage was just on the contrary. In conclusion, mixing cement before adding asphalt emulsion is not conducive to the improvement of mechanical properties. It is suggested that cement and asphalt emulsion should be fully mixed before the contact between aggregate and mortar in order to realize the optimal performance of cold recycled mixture; otherwise, mineral aggregates mixed with asphalt emulsion first can also be a better alternative.

In order to study the adhesion between tire and asphalt pavement, we established a finite element model of a hydroplaning, inflatable, patterned tire based on the coupled Eulerian–Lagrangian method and then validated the model’s applicability. We numerically calculated tire-pavement adhesion curves for three types of pavement: asphalt concrete (AC), stone mastic asphalt (SMA), and open-graded friction course (OGFC). In accordance with adhesion characteristic theory with regard to tires and asphalt pavements, we analyzed the influential factors that affect the adhesion characteristics of the tire-asphalt pavement interface in an antilock braking system and under damp conditions. The results show that the adhesion between tire and pavement is related to the movement of the tire. In this study, the longitudinal adhesion coefficient for the tire-pavement interface initially increased with an increase in the slip rate and then decreased. Once the slip rate was about 20 percent, the longitudinal adhesion reached its maximum value. In addition, we found that a deep surface macrotexture improved the hydroplaning speed of the tire when the water film was not too thick and the inflation pressure was high. Also, dry pavement led to better adhesion than a wet state in terms of specific mean profile depth. With the same water film thickness, the adhesion coefficient decreased with an increase in driving velocity. The OGFC pavement offered better skid resistance than both AC pavement and SMA pavement.

In order to improve the thermal stability of wide embankment for the Qinghai-Tibet Expressway (QTE) to be constructed in permafrost zones on Qinghai-Tibet plateau, a new kind of reflective-resistant-ventilated coupling structure (RRVCS) was proposed and designed. A numerical model was then developed through ABAQUS to investigate the influences of RRVCS on the thermal regime of wide embankment. The temperature field and thawing depth of normal wide embankment without protective measures, wide embankment with crushed stone layer, and wide embankment with RRVCS were compared to verify the cooling effectiveness of RRVCS based on numerical analysis. Vermiculite powder has great influences on the thermal property of asphalt mixture, and it can be reduced by 22.8%, 29.5%, 37.3%, and 50.6% after the addition of 4%, 6%, 8%, and 10% vermiculite powder, respectively. Under the same condition, the temperature of RRVCS embankment is the lowest and its thermal stability is the best. Setting crushed stone layer can improve the thermal stability of embankment. However, the improvement is limited for wide embankment with width exceeding 26 m. RRVCS has the best protective effects on the thermal stability of wide embankment and is recommended as a protective measure for wide embankment of expressway in permafrost regions of Qinghai-Tibet Plateau.

Aggregate skeleton in asphalt concrete mixtures plays a significant role in the pavement performance. However, the mechanical roles for different-sized particles in the aggregate skeleton have not been fully revealed due to the limitations of physical techniques. This study utilizes a numerical simulation approach to evaluate aggregate structure characteristics and their resistance to deformation. Using the discrete element method (DEM), a three-dimensional (3D) aggregate blend was generated with due consideration of the gradation. The blend was employed to simulate the penetration test of aggregate blends with different friction coefficients of 0.1, 0.2, 0.3, 0.4 and 0.5, respectively. The determination of the friction coefficient and the validation of DEM model were further conducted by laboratory experiments. Using the validated DEM model, penetration tests of five uniform blends and nine graded blends were simulated. External applied force (i.e., penetration force), penetration displacement, contact force, contact number and force taken by aggregate particles of different sizes were calculated using the DEM simulation to explore the mechanical characteristics of aggregate particle. This study demonstrated that the aggregate size plays an important role on the mechanical performance of aggregate skeleton. The results of this research can be applied to optimize the mix design of asphalt concrete and to further improve the aggregate gradation design.

This paper investigated the modification mechanism of the desulfurized rubber asphalt, and compared its performances with those of crumb rubber asphalt. The microstructures of desulfurized rubber asphalt and crumb rubber asphalt were identified by using the scanning electron microscope technology, component analysis and infrared spectrum analysis. Conventional and Strategic Highway Research Program (SHRP) performance tests were conducted to evaluate the performances of desulfurized rubber asphalt and crumb rubber asphalt. Mixture performance tests were also performed to characterize the high temperature rutting resistance, low temperature cracking resistance, moisture stability and fatigue life of the desulfurized rubber asphalt mixture and crumb rubber asphalt mixture. It was found that the modification mechanisms of desulfurized rubber asphalt and crumb rubber asphalt are different. The desulfurized rubber can integrate into the asphalt while the crumb rubber mainly exists incompatibly in asphalt. This difference results in the various performances between the desulfurized rubber asphalt and crumb rubber asphalt. Both crumb rubber and desulfurized rubber can improve the performance of asphalt binder and mixture, but the desulfurized rubber asphalt has much lower viscosity and better storage stability than the crumb rubber asphalt. The preparing temperature of the desulfurized rubber asphalt mixture can be reduced by 10∼20°C compared to the crumb rubber asphalt mixture. Moreover, without the interference of solid rubber particles, the desulfurized rubber asphalt can be used in dense-graded asphalt mixture while crumb rubber asphalt mixture tends to use gap gradation with high asphalt content.

The micro-mechanical response of asphalt mixtures was studied using the discrete element method. The discrete element sample of stone mastic asphalt was generated first and the vehicle load was applied to the sample. A user-written program was coded with the FISH language in PFC3D to extract the contact forces within the sample and the displacements of the particles. Then, the contact forces within the whole sample, in asphalt mastic, in coarse aggregates and between asphalt mastic and coarse aggregates were investigated. Finally, the movement of the particles in the sample was analyzed. The sample was divided into 15 areas and a figure was drawn to show how the balls move in each area according to the displacements of the balls in each area. The displacements of asphalt mastic balls and coarse aggregates were also analyzed. The experimental results explain how the asphalt mixture bears vehicle load and the potential reasons why the rutting forms from a micro-mechanical view.

To investigate the combustion mechanism of asphalt in this study, asphalt binder was separated into four components, namely, saturate, aromatic, resin, and asphaltene. Thermo-gravimetric analysis coupled with mass spectrometry (TG–MS) was implemented to analyze the combustion behavior of the four components of asphalt in a simulated air environment. The Coats–Redfern Integral Model (CRIM) and Distributed Activation Energy Model (DAEM) were used to calculate the activation energies of the four components during the combustion process, respectively. During the combustion of each component, TG–MS was employed to investigate the weight loss and to identify the products. According to the mass loss rates at different temperature range, the combustion process was divided into several stages at a heating rate of 15 K/min. DAEM was utilized to calculate nine activation energies during the entire combustion process at three heating rates of 10, 15, and 20 K/min, respectively. Comparison of the activation energies obtained with the two methods indicates that the four components have entirely different combustion mechanisms, which demonstrates the importance of the component separation in the investigation of the combustion mechanism of asphalt.

This study focused on the key factors affecting the tensile strength of cold recycled asphalt mixture with cement and emulsified asphalt. The specific surface areas and strength of RAP were analyzed. The interaction between the emulsified asphalt and cement was observed. Comprehensive laboratory testing was conducted to evaluate the influences of RAP, emulsified asphalt, and cement on the tensile strength of cold recycled asphalt mixture. It is found that although RAP is used as aggregates, its inner structure and strength are much different from real aggregates. The strength of RAP has decisive effect on the strength of cold recycled asphalt mixture. New aggregates and fine gradation design can help improve the bonding between RAP and binder. For emulsified asphalt, slow setting of asphalt can give sufficient time for cement to hydrate which is helpful for strength formation in the cold recycled asphalt mixture. The high viscosity of asphalt can improve the early strength of cold recycled asphalt mixture that is important for traffic opening in the field. Cement is an efficient additive to improve the strength of cold recycled asphalt mixtures by promoting demulsification of emulsified asphalt and producing cement hydrates. However, the cement content is limited by RAP.

This study focused on the property characterization of asphalt and asphalt mixtures modified by different types of crumb rubber (normal and desulfurized). This characterization was based on...

This study aims to characterize the mechanism and performance of different crumb rubber modified asphalt including the desulfurized crumb rubber asphalt and the common crumb rubber asphalt. Component analysis and Gel Permeation Chromatography test were used to reveal the performance mechanism of different crumb rubber asphalt. Superpave tests were conducted for performance evaluation of different crumb rubber asphalt at the low and high temperatures. Separation test was conducted to compare the storage stability of different crumb rubber asphalt. Mix designs and performance tests were conducted to evaluate the high temperature rutting resistance, low temperature cracking resistance, moisture stability, and fatigue life of different rubber asphalt mixtures. The results show that the crumb rubber mainly exists in asphalt independently while most of the desulfurized rubber can be digested into asphalt and partially changes the components and microstructure of asphalt. The most obvious difference between the crumb rubber asphalt and the desulfurized rubber asphalt was observed for viscosities. The crumb rubber asphalt show better performance than the desulfurized rubber asphalt based on Superpave performance testing results, while the desulfurized rubber asphalt shows better storage stability. Compared to the crumb rubber asphalt mixture, the compaction temperature of desulfurized rubber asphalt mixture can be lowered by 10 to 20℃ to achieve the same performance requirements.

This paper focused on the design and evaluation of an asphalt mixture with low thermal conductivity. Based on a designed SMA13 asphalt mixture, vermiculite powder was used to replace part of the fine aggregates to get an asphalt mixture with lower thermal conductivity. The Marshall test, wheel tracking test, low temperature bending beam test, and freeze-thaw splitting test were conducted to obtain a full evaluation of asphalt mixtures with different contents of vermiculite powder. A Mathis TCI thermal analyzer was used to test the thermal conductivity of asphalt mixtures, and the T-monitor temperature measuring system was used to compare the inside temperature of asphalt mixtures with and without vermiculite powder. It was found that with 10% vermiculite powder in the asphalt mixture, all of the road performances were satisfied, while thermal conductivity was lowered by 40%, and the inside temperature of the asphalt mixture was lowered by up to 2℃. The findings in this paper proved the feasibility of using vermiculite powder to reduce the thermal conductivity of asphalt mixtures.

The fatigue properties of asphalt mixtures are important inputs for mechanistic-empirical pavement design. To understand the fatigue properties of asphalt mixtures better and to predict the fatigue life of asphalt mixtures more precisely, the energy-controlled test mode was introduced. Based on the implementation theory, the laboratory practice for the energy-controlled mode was realized using a four-point-bending fatigue test with multiple-step loading. In this mode, the fatigue performance of typical AC-20 asphalt specimens with various reclaimed asphalt pavement (RAP) contents was tested and evaluated. Results show that the variation regulation of the dissipated energy and accumulative energy is compatible with the loading control principle, which proves the feasibility of the method. In addition, the fatigue life of the asphalt mixture in the energy-controlled mode was between that for the stress-controlled and strain-controlled modes. The specimen with a higher RAP content has a longer fatigue life and better fatigue performance.

This research examines reinforcing effect of different fibers on asphalt binder under low temperature. Laboratory test of multiple-fiber pullout (MFPT) from asphalt matrix was conducted to investigate the influence of fibers (fiber type and embedment length) and matrix (asphalt binder type) on the fiber pullout strength. Furthermore, the influence of fiber (with or without fiber, fiber length and dosage) and matrix (asphalt binder type and temperature) on the tensile properties of fiber reinforced asphalt (FRA) binder was investigated via direct tensile test (DTT). From the MFPT test it was found that fiber pullout strength can actually exceed the fiber strength provided by the manufacturer, presumably due to coating effect of the asphalt on the surface of fibers. Aramid fiber requires much longer embedment length to fully activate its bond with asphalt than the polyester fiber does. From the DTT test it was observed that addition of adequate polyester fiber can greatly enhance the tensile properties of the FRA, particularly in terms of failure tensile strain. While asphalt matrix becomes more brittle when temperature drops, FRA mains its tensile ductility with decreasing temperature, which deserves further investigation.

When a fire takes place in a tunnel, the surface of the asphalt pavement will burn and release a large amount of smoke, which is toxic to human health. Thus, in order to prevent the combustion of the asphalt pavement under fire, it is necessary to propose some methods to retard its physical and chemical reaction under the high temperature. In this study, ten different combinations of fire retardants and a control case where no fire retardant was applied were prepared for evaluation. The thermogravimetric (TG)&ndash, mass spectrometry (MS) tests were used to evaluate their effect on the fire retardance from mass and energy perspectives and the Fire Dynamics Simulator (FDS) software was used to evaluate the fire retardance from temperature and smoke distribution perspectives. In experimental analysis, the TG (thermogravimetric) and DTG (differential thermogravimetric) curves were used to analyze the mass loss rate and residual mass of the asphalt and the activation energy was calculated and analyzed as well. In addition, decay rate of mass loss rate and increasing rate of activation energy were proposed to evaluate the ease of combustion of the asphalt with and without fire retardants. The results show that in laboratory experiments, the fire retardant combination which includes 48% aluminum hydroxide, 32% magnesium hydroxide, 5% expanded graphite, and 15% encapsulated red phosphorous would lead to an improved effect of fire retardance. In numerical modeling, the temperature and smoke height distribution over time were adopted to evaluate the fire retardance effect. The temperature distribution was found to be symmetrical on both sides of the combustion point and the same combination as proposed in experimental analysis was found to have the best effect on fire retardance due to the largest decrease in temperature. Additionally, because of the highest smoke height distribution, an improved effect on smoke suppression was also found when this combination was applied.

This paper proposes a new modeling method to reconstruct hollow shapes of aggregate particles using the discrete-element method (DEM) to accurately characterize the microstructures of aggre...

To investigate suitability of the perpetual pavement under ultra-heavy loads, a test road was constructed on expressway in Shandong province of China. There were five pavement structures include semi-rigid asphalt pavement, each was instrumented with gages for measuring the strains of asphalt base layer, the vertical stress of subgrade, temperature of asphalt layers. The analysis of the strain data indicated that the strain values are affected by the temperature, the vehicle load, axle type, and the pavement structure combination. To research the response of different structure, tested different axle and load at different temperature, then different pavement response models were developed that accounts for layer thickness, axles load, pavement temperature and equivalent modulus of pavement foundation. The models provides good references under heavy vehicle loading and China local climate, it will be useful for perpetual pavement design.

Based on the data got from actual measurement of mineral filler and aggregate surface area, this paper is going to present a new method, which is different from conventional approaches, to help to determine aggregate surface area. A formula that can be used to calculate aggregate surface area will be obtained. Currently, many methods are available to calculate asphalt film thickness. These methods are not perfect and deserve a new survey. Therefore, by referring to and analyzing those existing methods, this paper will put forward a revised, updated and instructive calculating formula to guarantee the accuracy of calculation.

The main objective of this work is to propose new mixture response parameters and to compare correlations with rut depths and sensitivity of permanent deformation response parameters based on field extracted cores and lab-mixed duplicates. A new “mix-confined” test is developed and four new parameters for this test are proposed. Correlation coefficients with rut depths and coefficients of variation (sensitivity) are compared between the four new and two existing parameters. Some parameters are recommended to be used for the newly developed test. The results show that, newly developed test can capture the changes of permanent deformation of asphalt mixtures. Only one new parameter (D 1 of Stephen Price model) and one existing parameter (flow number, F n) have strong correlations with rut depths of asphalt pavements (R 2 greater than 0.7) and have relative small sensitivity (coefficient of variation, COV, less than 30%). For polymer modified asphalt mixtures, the parameter D 1 rather than F n should be used. These findings can be used to check the permanent deformation of asphalt mixture during the mix design.

An algorithm for generating three-dimensional aggregates was developed first in this study. This algorithm can model the various shapes, sizes, and angularity of coarse aggregates. The shapes, sizes, and angularity of the generated aggregates can be correlated to those of actual aggregates. Then, algorithms were developed to model coarse aggregate gradation accurately and place the coarse aggregates randomly within the sample. Finally, algorithms were developed to generate the asphalt mastic. The triaxial compression test of the discrete-element sample of the asphalt mixture with the nominal maximum aggregate size of 16 mm was simulated to predict its dynamic modulus. The results indicate that the discrete-element model generated using the algorithms developed in this paper can predict the mixture modulus across a range of loading frequencies.