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In order to obtain the asphalt pavement texture information in real time and accurately monitor the anti-skid performance of the road pavement, an automatic close-range photogrammetry system (ACRP System) was proposed and established based on the three cameras close-range photogrammetry (CRP) technology. Automatic image acquisition and 3D reconstruction were achieved by the ACRP system. Firstly, the collected pavement texture images were digitized. Secondly, a 3D model of asphalt pavement with surface texture was established in the 3D reconstruction software module based on MATLAB and Python joint programming, then the surface elevation data of asphalt pavement were extracted. Finally, the calculation of the road surface texture index parameters were performed in 3D reconstruction software module. Sand patch testing method and laser scanning method (ZGScan) were used to collect the on-site data as comparison test of the asphalt pavement texture. The mean texture depth (MTD) and the root mean square roughness (RMSR) were chosen as the statistical indicators of pavement surface texture. The results show that the texture data obtained by the ACRP system has relatively higher accuracy and efficiency, and the recognition accuracy is close to 0.02 mm. The ACRP system improves the efficiency and accuracy of traditional close-range photogrammetry and provides real-time and effective road surface anti-skid information for subsequent safety braking of autonomous vehicle.

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.

Black cotton soils (BCSs) were stabilized with various percentage of lime, natural volcanic ash (VA) and their combinations. The laboratory tests were performed to evaluate the influence of the stabilizers on the physical-mechanical properties of BCS. These laboratory tests included Atterberg limits, California bearing ratio (CBR), swell percent and unconfined compressive strength (UCS). The changes of minerals were also derived by performing X-ray diffraction, Infrared spectroscopy and scanning electron microscope, which were employed to explain the stability mechanism of BCS together with pH test. Results revealed that the added stabilizers improved greatly the physical-mechanical properties of BCS. The use of combinations of lime and VA showed superior results when compared with the single stabilizer. BCS can meet the performance requirements of roadbed materials referring to JTG D30-2015 just by mixing with 3% lime and 15% VA. The increased pH of the stabilized BCS indicated that solubility of the silicate and the aluminate increased, which accelerated the pozzolanic reaction between clay soils and stabilizers. The intrinsic lamellar structures of clay mineral were destructed in the reaction process. Moreover, several new minerals were produced to stabilize the soil fabric. Overall, the use of VA can reduce the consumption of lime in BCS stabilization and actualize the utilization of vast resources BCS as a low-cost roadbed material.

In this paper, the mechanical properties of excess-sulfate phosphogypsum slag cement (PPSC) were studied by experimental data, microscopic tests and molecular dynamics (MD) simulations. Specifically, the properties of PPSC in terms of setting time, compressive and flexural strengths were investigated by experiment. The hydration products of PPSC were tested and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furtherly, at the atomic scale, the relationship between nanoscale and macroscopic mechanical properties of PPSC was established. The mechanism of the effect of chemical composition on the mechanical properties of PPSC was studied at multiple scales, and the compound synergistic effect between chemical compositions was further studied. The results show that the mechanical properties of PPSC increase with the increase of CaO/SO3, and decrease with the increase of SiO2/Al2O3. CaO and Al2O3 can improve the mechanical properties of PPSC. The high mobility and structural instability lead to CaO/SO3 has greater effect on the mechanical properties of PPSC than that of SiO2/Al2O3. When CaO/SO3 is 1.8 ∼ 2.0 and SiO2/Al2O3 is 3.5 ∼ 3.7, the mechanical properties of PPSC are better. Moreover, when CaO/SO3 is 2.0 and SiO2/Al2O3 is 3.5, the mechanical properties of PPSC are the best. At this time, the compound synergistic effect of alkaline activator and sulphate activation is the best. This paper provides an important reference for the composition design and application of PPSC.

Expansion joint failure is one of important causes to cause concrete pavement damages. To develop a new sealant to effectively seal expansion joint on concrete pavement, titanium dioxide/shape memory polyurethane (TiO2/SMPU) composites with a tailored transition temperature (Tt) was prepared, and effects of TiO2 content on various properties of TiO2/SMPU composites were discussed. The results indicate that TiO2 nanoparticles are uniformly physically filled in SMPU pores and wrapped by SMPU to form a compact skeleton structure when TiO2 content is suitable. The tailored shape memory Tt of TiO2/SMPU composites can be used as the shape memory switching temperature to match the working temperature of expansion joint in China. Also, TiO2/SMPU composites include amorphous phases or microcrystal structures which are dominated by the presence of crystalline rutile TiO2. The incorporation of TiO2 leads to the decrease in crystallinity of TiO2/SMPU composites. Additionally, a suitable content of TiO2 can obviously increase the absorptivity of SMPU to UV light in the wavelength range from 300 nm to 400 nm, and also improve the reflectivity to visible light, lowering photo-aging properties of SMPU. Further, TiO2 content shows a slight effect on shape fixity ratio of SMPU, but has an obvious influence on shape recovery ratio. Prepared TiO2/SMPU composites show satisfactory shape memory property after a two-stage biaxial programming process. Finally, mechanical properties of SMPU are improved by a suitable TiO2 content. The improvement in mechanical properties of TiO2/SMPU composites can better accommodate the working conditions of sealant. TiO2 content of 3% is proposed to prepare TiO2/SMPU composites with a specially tailored shape memory Tt, which can meet engineering requirements of expansion joints when used as a sealant of concrete pavement.

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.

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.

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.

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.

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.

This paper proposed a new type of fly ash-based geopolymer foams (FAGF) designed for wall insulation in buildings. The first part investigated the pore structures and properties of FAGF such as porosity, average Feret diameter (AFD), and roundness via a variation of the mix ratios and contents of precast foam. Then the results were further supported by the microstructures of pores, leading to the optimization of mix formulations of precast foam in terms of FAGF. The second part of this study explored the relationship between precast foam properties and FAGF performance. Then the influence of foam content on the pore structure parameters, fluidity, dry density, compressive strength and thermal conductivity of FAGF were measured and analyzed, revealing the mechanism of pore forming within FAGF. The results indicated that the foam in the paste was mainly controlled by five kinds of forces: surface tension Fst, internal gas pressure Pi, drainage force Fd extrusion binding force Fc and buoyancy Fb. When these forces were balanced, the foam was in a relatively stable state in the paste, resulting in the uniform pore size distribution and the excellent macro properties of FAGF.

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.

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.

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.

Bituminous material is widely utilized in building waterproof, pavement engineering, etc. However, when the fire happens, hazardous fumes are released due to bitumen decomposition, affecting adversely the environment and human health. The thermal decomposition behavior of bitumen is the first step of its conversion process, such as combustion, and carbonization. To better understand the bitumen conversion when exposed to fire, the decomposition characteristics of bitumen was investigated and the dynamic evolution of gaseous products was characterized in this study. The results from thermogravimetry (TG) indicate that the decomposition of bitumen shows a single-step decomposition mechanism. The spectrum profiles from Fourier transform infrared spectroscopy (FTIR) further characterizes the decomposition mechanism and the overall evolution of volatiles from bitumen at flash and fire points. The evolved volatiles include carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ), water (H 2 O), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), hydrocarbon, methanol, formic acid, phenols, aromatic compounds, etc. The vapor pressure is a main factor to determine the concentration and hazards of gaseous products during bitumen decomposition. It was found that the evolution of each gaseous product was concentrated from 60 min to 110 min and each volatile was emitted from different chemical compositions of bitumen at various temperature ranges.

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.

The shape, size, angularity and surface texture properties of coarse aggregates were studied in this paper. Aggregates with different shapes were separated manually and the percentage by number of each shape was computed. The three sizes (length, width and thickness) of aggregates were extracted using an image analysis approach and their statistical distributions were studied. An indicator called angularity and surface texture (AT) index was developed to characterize the combined effect of the coarse aggregate angularity and surface texture based on two-dimensional aggregate images. The statistical distributions of the AT indices of different sized limestone and basalt aggregates and their composite AT indices were studied. The void contents of different sized limestone and basalt aggregates in loose condition were tested to validate the AT index.

The field measurement and laboratory tests were conducted to investigate potential causes of in-place rutting. The results indicate that the major rutting is attributed to the decrease in thickness of middle and lower layer, and the driving lane shows a severer rutting. Inadequate compaction is a major cause for the final rutting depth. Also the aggregate gradation has a major contribution to rutting. The gradation between 1.18 and 4.75 mm in sieve size becomes finer for the three layers. The upper and middle layer show an increase in asphalt content, but the lower layer presents a decrease in asphalt content.

A micromechanical model was built in this paper to investigate the stiffness anisotropy of asphalt concrete (AC) using the discrete element method. Four three-dimensional cubic AC digital samples with different aggregate particle orientations were built using discrete element software PFC3D. The aggregate gradation and shape, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the four cubic samples loaded in three different directions. Dynamic stiffness in different directions was calculated from the compression stress–strain responses. Results show that the AC stiffness is significantly dependent on preferential orientation of aggregate particles. The AC stiffness in the long-axis direction of aggregate particles is shown to be up to 43% higher than the stiffness in the particle short-axis direction. The stiffness anisotropy of AC decreases as the mixture temperature drops.