The substitution of slag-based geopolymer materials for cement-stabilized macadam in road bases is in line with the demand for green and sustainable development in the transportation industry. Thus, slag/fly-ash-based geopolymer materials were prepared to stabilize macadam of road bases in this study. The mechanical properties, freeze-thaw resistance, and dry shrinkage durability of slag/fly-ash-based stabilized macadam materials were studied to analyze the influence of geopolymer dosage on these properties of stabilized macadam. Microscopic tests such as XRD, FITR, and SEM were carried out to explore the formation mechanism of strength and the characteristic of interface transition zone (ITZ). Results show that the 28 d compressive strength, compressive elastic modulus, and tensile strength of slag/fly-ash-based stabilized macadam increase linearly with the increase of geopolymer content. When the dosage of slag-based geopolymer is 4%, the 7 d unconfined compressive strength of slag/fly-ash-based stabilized macadam reaches 8.76 MPa, and the strength still reaches 14.84 MPa after five freeze-thaw cycles (28 d), which satisfy the application requirements of expressway and first-class highway base (JTG/T F20-2015). The dry shrinkage property of slag/fly-ash-based stabilized macadam is better than that of cement-fly-ash-stabilized macadam. When the amount of geopolymer is 3%, the dry shrinkage strain of slag/fly-ash-based stabilized macadam is 231.2 με, which is smaller than that of cement-fly-ash-stabilized macadam (261.3 με). No obvious porosity around the ITZ was detected, indicating good binding between the geopolymer and the aggregate.

To overcome the shortcomings of traditional wet styrene-butadiene-styrene (SBS) modification technology, such as its high energy consumption and thermal decomposition, a warm mix and fast-melting SBS modifier was developed. Based on the theory of rheology, a dynamic shear rheometer (DSR) was applied to investigate the viscoelastic properties of the warm mix and fast-melting SBS-modified asphalt using a frequency scanning test. Atomic force microscopy (AFM) was used to reveal the modification mechanism of the SBS-modified asphalt. An investigation of the thermal stability of the asphalt binder was conducted using a thermogravimetric test (TG). The results exhibited that the SBS-modified asphalt had good viscoelastic properties, as well as thermal stability. The “bee structure” of the SBS-modified asphalt was finer and more stable. In addition, the adhesion and the Derjaguin–Muller–Toporov (DMT) modulus of the SBS-modified asphalt at a warm mixing speed was higher than that of regular SBS-modified asphalt.

Warm Mix Asphalt (WMA) technology can effectively reduce carbon emissions and energy consumption during road project construction. However, it may have a negative impact on the binding properties of asphalt mixtures. In order to effectively evaluate the adhesion performance of asphalt binders and aggregates under the combined influence of WMA and traditional polymer-modified asphalt, this paper provides a comprehensive evaluation at the micro and macro levels. The adhesion between three different modified asphalts (warm mix crumb rubber/ Styrene-Butadiene-Styrene (SBS) composite modified asphalt, warm mix crumb rubber asphalt, and warm mix SBS modified asphalt) and two different aggregates (limestone and granite) under both virgin and short-term aging conditions were analyzed. Regardless of the type of modified asphalt, the results showed that limestone aggregates have better adhesion properties with asphalt binders. In addition, the short-term thermal oxidation aging behavior is conducive to enhancing the asphalt-aggregate adhesion characteristics. Furthermore, WMA additives, crumb rubber, and SBS compound modification can improve the adhesion performance between asphalt and aggregate.

To comprehensively understand the impact of various environmental factors on the self-healing process of graphene-modified asphalt, this study employs molecular dynamics simulation methods to investigate the effects of aging degree (unaged, short-term aged, long-term aged), asphalt type (base asphalt, graphene-modified asphalt), healing temperature (20 °C, 25 °C, 30 °C), and damage degree (5 Å, 10 Å, 15 Å) on the self-healing performance of asphalt. The validity of the established asphalt molecular models was verified based on four physical quantities: density, radial distribution function analysis, glass transition temperature, and cohesive energy density. The simulated healing time for the asphalt crack model was set to 200 ps. The following conclusions were drawn based on the changes in density, mean square displacement, and diffusion coefficient during the simulated healing process under different influencing factors: Dehydrogenation and oxidation of asphalt molecules during the aging process hinder molecular migration within the asphalt crack model, resulting in poorer self-healing performance. As the service life increases, the decline in the healing performance of graphene-modified asphalt is slower than that of base asphalt, indicating that graphene-modified asphalt has stronger anti-aging properties. When the vacuum layer in the asphalt crack model is small, the changes in the diffusion coefficient are less pronounced. As the crack width increases, the influence of various factors on the diffusion coefficient of the asphalt crack model becomes more significant. When the crack width is large, the self-healing effect of asphalt is more dependent on these influencing factors. Damage degree and oxidative aging have a more significant impact on the healing ability of graphene-modified asphalt than healing temperature.

The indirect tensile test plays a crucial role in experimental investigations of brittle material properties. In this study, a mechanical analysis model of the rectangular test block is established based on the theory of elastic mechanics for the characteristics of the indirect tensile test. The theoretical solution of the triangular series is derived for the rectangular test block under the locally distributed load. The finite element simulation results and splitting test results were compared with the theoretical results. The results of the study verify the accuracy of the theoretical solutions. Based on the proposed analytical solution, the effects of loading width and length-to-height ratio (h/l) of local loading on the measured tensile strength of test block are discussed. The results demonstrate that the tensile strength of the test block increases as the loading width expands, and the rate of growth in the recorded tensile strength gradually stabilizes. The variation in loading width affects the location of crack initiation points during the concrete test block splitting tests. When the loading width exceeds 6% of the side length of test block, the cracking point is positioned at the center of test block, ensuring the effectiveness of the splitting test. As the length-to-height ratio of the test block increases, there is a general upward trend in the measured tensile strength. When h/l < 0.6, the measured tensile strength initially increases before decreasing. However, when h/l > 0.6, the measured tensile strength consistently increases, with the rate of increase gradually diminishing until it stabilizes. The length-to-height ratio also significantly influences the location of the cracking point in the test block. As the length-to-height ratio increases, the cracking point initially shifts from around the center to the central point and then further from the center toward the edge. To ensure that the location of the crack initiation point is in the center of the specimen and that the tensile strength is close to the measured result, the length to height ratio can be chosen at around 0.85.