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• A simplified skeleton method for coarse aggregates analysis of asphalt mixtures was proposed. • The stress transfer paths and load transfer efficiencies of three typical gradations were studied. • Eff skeleton reflected the load transfer efficiency of the coarse aggregate skeleton under no-load conditions. • The DisP value reflected the load transfer efficiency of the coarse aggregate skeleton under a specific load. Studies on asphalt mixture skeletons have primarily focused on the statistical skeleton morphology while neglecting internal stress transfer. This study therefore proposed a simplified reconstruction-based coarse aggregate skeleton method for analyzing asphalt mixes using the force chain concept from granular matter mechanics. The stress transfer paths and load transfer efficiencies were investigated for three typical aggregate gradations (AC-13, SMA-13, and OGFC-13) and an analysis of their correlation with uniaxial penetration test results was performed. The AC-13 mixture exhibited the largest uniaxial penetration strength (3.30 kN on average) at the lowest failure displacement (4.1 mm on average). As the characteristics of the simplified skeleton were shown to be related to the number of contact points provided between large aggregate particles, the force chain morphology was closely linked to the mixture gradation. The ratio of strong to weak force chains in the AC-13 mixture was approximately 0.77, whereas those of the SMA-13 and OGFC-13 mixtures were significantly different at 0.90 and 1.13, respectively. The transfer efficiencies of the AC-13, SMA-13, and OGFC-13 specimen skeletons considering morphology alone (i.e., under the no-load condition) were 20.94 %, 62.34 %, and 73.57 %, respectively. However, the correlation between the load transfer efficiency of the coarse aggregate skeleton under a specific load and the observed penetration strength reached 0.9. Thus, the simplified skeleton analysis method proposed herein was shown to satisfactorily retain the skeletal characteristics of the mixes. [ABSTRACT FROM AUTHOR]

• The variation laws of mortar indicators were investigated by performance tests. • The correlation between mortar performance and mixture performance was analyzed. • Mixture with excellent anti-rutting and deformation performance was designed. • The design method can realize the coordination of rutting and crack resistance. The anti-rutting and anti-cracking performance of asphalt mixtures are a pair of contradictions that are difficult to coordinate. In order to ensure the rutting resistance of asphalt mixture to meet the requirements of the specification, while improving its deformation capacity and crack resistance. Based on the mortar performance, the Anti-rutting and High Ductility Asphalt Mixture with excellent anti-rutting performance and deformation performance was designed in this paper. With the filler-asphalt ratio as the control index, SBS modified asphalt and SHV modified asphalt was selected to design five asphalt mortars with different filler-asphalt ratios of 0, 0.5, 1.0, 1.5, 2.0. Variation laws of mortar technical indicators were investigated by rotational viscosity test, cone penetration test, and BBR test. Asphalt mixtures were prepared and a rutting test and small beam bending test were carried out to ensure the consistency of the mixture skeleton state by grouting molding. The influence of the mortar performance index on the road performance of the mixture was evaluated by conducting correlation analysis, and then an anti-rutting and high-ductility asphalt mixture was proposed based on mortar performance. The obtained results showed that the technical indicators of asphalt mortar had a good correlation with the filler-asphalt ratio, and the developed regression model could be used to determine the correlation and design of the mortars. The technical performance index of mortar was found to be directly correlated with the road performance index of the mixture. The developed anti-rutting and high-ductility asphalt mixture not only had good anti-rutting performance but also had good deformation capacity and cracking resistance. Its anti-rutting performance could meet the specification requirements, and the maximum flexural strain could reach 7161 με. The performance of asphalt mortar determines the road performance of the mixture. The rutting resistance and cracking resistance of the mixture could be coordinated by designing asphalt mortar to improve its comprehensive road performance. To guarantee that the dynamic stability of the mixture met the required specifications and the deformation ability was good, the critical value of each technical index of asphalt mortar was recommended. [ABSTRACT FROM AUTHOR]

• The tangential elastic stiffness of new-new, old-old and new-old interface contact types were obtained. • The ks regression model established has high accuracy. • The shear modulus model takes into account the classification of contact points in the skeleton and the mechanical characteristics of aggregate interface. • There is a good correlation between the calculated results of the shear modulus model and the laboratory test results. The mechanical characteristics, quantity, and distribution of contact interfaces in the composite skeleton of recycled asphalt mixture affect the consistency of deformation of new and old aggregate composite skeleton, further affecting the macroscopic mechanical properties of material. To quantitatively analyze the effect of reclaimed asphalt pavement (RAP) content on the mechanical characteristics of contact interface and shear performance of recycled asphalt mixture, the tangential elastic stiffness of interface of new-new, new-old, and old-old contact types is defined as k s n e w - n e w , k s n e w - o l d , and k s o l d - o l d , respectively; interfacial shear tests were carried out for new-new, new-old, and old-old contact types. Digital image technology and statistical analysis were used to obtain the proportion distribution of α n e w - n e w , α n e w - o l d , α o l d - o l d of three aggregate contact types under different RAP contents. Combined with the calculation results of interface tangential elastic stiffness and shear modulus with different RAP contents, an interface tangential elastic stiffness estimation model and a shear modulus calculation model were established, and the rationality of the model was verified. The results show that different interface contact types significantly affected the shear test results. The tangential elastic stiffness of old-old contact type is the highest, 0.3416 × 106 N/m. The tangential elastic stiffness of new-old and new-new contact types are 0.2254 × 106 N/m and 0.0992 × 106 N/m, respectively. The results of contact interface shear test of recycled asphalt mixture with different RAP contents show that the overall trend of shear curve is similar, and the peak value and mean value of shear force do not change consistently with the increase of RAP content. Using the least square mathematical analysis method for overdetermined equation, the goodness-of-fit of k s regression model with different RAP contents is 0.85. There is a good correlation between the shear modulus calculation results and uniaxial penetration test results, and the correlation coefficient is 0.80. [ABSTRACT FROM AUTHOR]

• Analyzed interfacial bonding effect on SFP via silane solution soak of PAM. • Examined grouting's effect on SFP compressive strength through compression test. • Estimated grouting's contribution to SFP's low-temp splitting and shear strength. • Suggested cost-effective ways to improve SFP's mechanical properties. Semi-flexible pavement (SFP) materials, composed of porous asphalt mixture (PAM) and cement-based grouting material, are multiphase composites commonly used in roadway construction. However, damage frequently occurs at the composite interface of two-phase materials, and most SFP research has focused on the design of grouting material, with insufficient attention given to the composite interface. This paper aims to investigate the effect of bonding properties between organic PAM and inorganic grout on the mechanical properties of SFP. To investigate the different bonding properties of the SFP asphalt-grout interface, a simple method was employed, in which PAM specimens were immersed in a silane coupling agent solution to form a layer of coupling agent on the PAM surface. A series of SFP specimens were created using PAM with a porosity of 28–30%, and different strength grouting materials (40 MPa, 60 MPa, 70 MPa), and three laboratory tests were conducted to evaluate their mechanical properties. The results demonstrated that the interfacial bonding strength between asphalt and grouting material significantly influences the low-temperature splitting tensile strength, uniaxial penetration strength and uniaxial compression strength of SFP. Compared to specimens formed from high-strength grouting materials, SFP specimens immersed in a silane coupling agent solution exhibited superior low-temperature splitting tensile strength (2.06 MPa > 1.97 MPa, 2.63 MPa > 2.34 MPa) and shear strength (7.28 MPa > 6.46 MPa, 7.97 MPa > 7.26 MPa). As a result, this study highlights the importance of considering the bonding properties of the asphalt-grout interface in the design and preparation of SFP. The results of this study can be useful for enhancing the performance and durability of SFP materials in a more economically efficient way. [ABSTRACT FROM AUTHOR]

• A two-dimensional (2D) finite element model was developed to simulate uniaxial compression tests. • DIP technology was used to distinguish the virgin aggregate and RAP aggregate in recycled asphalt mixture. • The concept of force chain was introduced to evaluate the homogeneity of mixture. • A comparison of the two homogeneity assessment methods reveals different emphases. Numerous studies have demonstrated a direct link between the homogeneity of asphalt mixture and the rutting performance, water stability, tensile strength, and service life of the asphalt pavement. Due to the existence of RAP clusters, poor homogeneity of the recycled asphalt mixtures is more severe than natural ones. However, previous studies have mainly focused on analyzing the distribution of aggregates, ignoring the transfer of the internal stress in the mixture. This paper proposed a novel approach for the analysis of the homogeneity of recycled asphalt mixture using the efficiency of force chain transferring stress. Therefore, to prepare gyratory compacted samples, diabase in green color was used as the virgin aggregate and marble in white color as the RAP aggregate. Moreover, to obtain the vertical section images, the samples were cut and processed by digital image processing (DIP). This included distinguishing the virgin and RAP aggregates by global gray threshold segmentation, adding asphalt film around the aggregates, and constructing a force chain network relied on the aggregate contact information. Also, the intensity of the force chain was determined using the finite element method (FEM). In addition, the efficiency of force chain transferring vertical stress (ETVS) was introduced and the homogeneity index D of the recycled asphalt mixture was proposed based on the variation coefficient of ETVS. Furthermore, uniaxial compression tests were carried out to mathematically analyze the homogeneity index D proposed in this paper and the homogeneity index D ′ proposed based on the aggregate area ratio in previous studies. The obtained result showed that the homogeneity index D increased first and then decreased with the increase of RAP content. Moreover, the correlation analysis demonstrated the compressive performance of the mixture to be mainly reflected by the homogeneity index D. The correlation coefficients of the compressive strength and the compressive resilient modulus were found to be 0.9010 and 0.8464, respectively. Finally, the homogeneity index D ′ was observed to mainly reflect the stability of the compressive performance of the mixture, with the correlation coefficients of the variation coefficient of the compressive strength and the compressive resilient modulus of 0.8745 and 0.8869, respectively. [ABSTRACT FROM AUTHOR]

• Exploring the thermal performance of phase change asphalt mixture. • Equal volume replacement of mineral filler and fine aggregate with CPCMs. • Exploring the mechanical strength of phase change asphalt mixture. Phase change materials (PCMs) have the capabilities of thermal energy storage and temperature regulation. However, the application of PCMs in asphalt pavements is still limited due to the severe leakage and low thermal performance problem. In this research work, the phase change asphalt mixture (PCAM) with Polyethylene glycol/hydrophobic fumed silica composite phase change material (PEG/HFS) and Polyethylene glycol/SiO 2 composite phase change material (PEG/SiO 2) replacing mineral filler and fine aggregate was prepared and tested. The leakage experiment illustrated that the PEG/HFS with PEG content of 45% (wt) and PEG/SiO 2 with PEG content of 70% (wt) had good thermal exudation stability. The results of XRD and FTIR showed that the preparation process of composite phase change materials (CPCMs) was a pure physical mixing process without chemical reactions. In addition, based on the results of the indoor radiation test, Marshall test, and rutting test, we found that when mineral filler and fine aggregate were replaced by 25% of the same volume of CPCMs, the PCAM exhibited a better cooling effect and relatively small decrease in mechanical strength. Therefore, the PCAM developed in this research has great potential for improving the thermal performance of asphalt steel bridge deck. [ABSTRACT FROM AUTHOR]

• An asphalt-filler volume equivalent substitution method is proposed. • When stone volume is constant, filler is replaced with equal volume asphalt. • Influence of filler-asphalt ratio reduction is studied on skid resistance. • Influence of the proposed method on the road performance of mixtures is evaluated. • Mixture with excellent anti-rutting and Low-temperature deformation is designed. The asphalt mixture designed with the Marshall mix design method has stable quality and convenient technical management, but the design range of the asphalt-aggregate ratio is small and difficult to adjust, and is therefore difficult to adapt to the design of functional pavement materials with different requirements. In order to solve these problems, the asphalt-filler volume equivalent substitution method (the volume equivalent substitution method) has been proposed. Based on the optimum asphalt-aggregate ratio of the Marshall method, the asphalt-aggregate ratio in the mixture is increased by the equal volume substitution of asphalt and filler. The increase in the asphalt volume is equal to the decrease in the filler volume, and vice versa. With the condition of constant aggregate gradation, the modified asphalt of asphalt mixtures with filler contents of 6.0%, 4.5%, 3.0%, 1.5%, and 0% is designed based on the AC-13 gradation. Based on this, a high-temperature stability test, a low-temperature crack resistance test, a four-point bending fatigue test, and other tests are carried out for the asphalt mixtures. The results show that the dynamic stability of each mix proportion designed with the volume equivalent substitution method is 4924 times/mm even under the most unfavorable conditions (the filler dose is 0%). The maximum bending tensile strain of the specimen with a 0% filler dosage can reach 9667 μ, which is more than twice that of the specimen with a 6% filler dosage. The fatigue test shows that increasing the asphalt content in the mixture is beneficial for improving fatigue durability. The research results show that the material designed by the volume equivalent substitution method has excellent road performance in each proportion. This paper provides a method for the flexible adjustment of the asphalt-aggregate ratio and the asphalt mixture comprehensive road performance. Using this method, the optimum asphalt-aggregate ratio needs to be determined comprehensively according to the stress characteristics and functional requirements of the pavement and the pavement performance index of the mixture, and further research is still needed. [ABSTRACT FROM AUTHOR]

• Aromatic and saturate are agglomerated near SWCNT, followed by resin and asphaltene. • Van der Waals energy dominates interfacial interactions between CNTs and asphalt. • MWCNTs are more appropriate to be embedded into asphalt. • The addition of MWCNT to asphalt significantly increases aggregate-asphalt adhesion ability. In this work, a comprehensive study has been conducted on the interfacial contact behavior of carbon nanotubes (CNTs) and asphalt binders as well as the influence of multi-walled carbon nanotubes (MWCNTs) embedded into asphalt on the adhesion ability of asphalt on aggregate surface at atomic scale. CNT/asphalt composite and asphalt-aggregate interface models were investigated by molecular dynamics (MD) simulations to evaluate the molecular interactions of materials. The interfacial adhesion characteristics of functionalized single-walled carbon nanotubes (SWCNTs) and MWCNTs with asphalt binders were investigated by performing pull-out simulations and the interactions of SWCNTs and asphalt components were analyzed. The obtained results showed that asphalt components with low molecular weight were more easily adsorbed by SWCNTs than higher weight components. For CNTs/asphalt composite model, Van der Waals (VdW) energy is much higher than electrostatic energy accounting for a great portion of interaction energy. Although functional groups on the surface of CNTs can increase interfacial interaction, this increase is far less than the enhancement of interaction energy and interfacial shear stress between MWCNTs and asphalt binders. Therefore, MWCNTs are regarded as a better additive to be inserted into asphalt binders to enhance the interfacial adhesion of CNTs/asphalt composites. In addition, the results of adhesion energy change of asphalt-aggregate interface models indicated that MWCNT addition into asphalt significantly increased the adhesion ability of asphalt on aggregate surface (especially for alkaline aggregates). [ABSTRACT FROM AUTHOR]

[Display omitted] • Influence of aggregate Strength and shape on the performance of asphalt mixture was evaluated. • The CAVF design method was used for gradation design, and iPAS was used to explore contact characteristics. • Internal skeleton index has an excellent linear relationship with mechanical properties. • The strength of aggregate has a minimal effect on the performance of the asphalt mixture. The strength and shape of aggregates significantly affect the internal skeleton structure (ISS) of the asphalt mixtures, which affects the service life of asphalt pavement. ISS is a critical factor that decides the load transfer path in the asphalt mixture. To improve the performance of the asphalt mixture, three aggregates Granite, Diabase, and Limestone with different strengths and shapes were used. Two-dimensional image processing was used to acquire the ISS including contact properties, distribution and orientation of aggregates. Marshall, wheel tracking and three-point bending beam tests were carried out to evaluate the relationship between skeleton indexes with mechanical properties. Results indicated that the coarse aggregate voids filling method could obtain mineral gradations with the same volume combinations for the three aggregates. The crushing value, loss-angles value, roundness, and flakiness index have an excellent relationship with contact characteristics, showing that strength and shape significantly affect the ISS of the asphalt mixture. The results of Marshall, rutting, and bending stiffness modulus are consistent with the internal structural index, proving that low-temperature and high-temperature performance of asphalt mixture are directly affected by aggregate contact properties. There is a weak linear relationship of strength parameters with mechanical properties because Diabase is three times stronger than Granite but has a lower performance demonstrating that the strength of aggregate has a minimal effect on the performance and stability of the asphalt mixture. The results obtained in this study could be used to improve the utilization efficiency of aggregate. [ABSTRACT FROM AUTHOR]