Basalt fibers can significantly enhance the crack resistance of warm-mix recycled asphalt mixtures (WRAM). However, the impact of their morphology on crack resistance and damage behavior still lacks systematic comparison. This study comparatively investigates the differences in crack resistance and damage behavior of WRAM with flocculent basalt fibers (FBF) and chopped basalt fibers (CBF). Based on the edge-notch disc bending test, the impact of fiber morphology on crack resistance was evaluated using parameters such as bending index, peak load, and fracture energy. Combined with digital image correlation (DIC) technology, the strain and strain energy density changes during microcrack initiation, development, and macrocrack propagation were tracked to assess the effect of fiber morphology on external damage. Acoustic emission (AE) technology was used to analyze the changes in internal damage ring-down counts and RA-AF distribution during the external damage phase to evaluate the impact of fiber morphology on internal damage. A correlation between DIC and AE results was established to achieve a quantitative mapping of external and internal damage. Scanning electron microscopy was used to reveal the crack resistance mechanism of the fibers. The results show that FBF enhances pre-crack strength and toughness by increasing the energy threshold and the proportion of internal shear damage during the microcrack development stage. This mechanism arises from the three-dimensional network structure formed, which effectively disperses stress and inhibits microcrack development. CBF enhances post-crack toughness by increasing the energy threshold and the proportion of internal shear damage during the macrocrack propagation stage. The mechanism primarily involves fiber pullout and fracture, redistributing stress and inducing the complexity of crack path curvature. A quantitative mapping relationship between internal and external damage (R-2 > 0.98) was established to reveal the regulatory mechanism of fiber morphology on the multi-scale damage evolution behavior in WRAM.

The application of steel slag (SS) in road engineering helps conserve natural aggregates and protect the environment; however, its relatively poor volumetric stability and resistance to moisture damage limit widespread engineering application. Accordingly, the mineral composition and chemical constituents of SS aggregate were tested, and the expansion mechanism of SS aggregate was analyzed. On the basis of SS stability testing methods, a testing procedure for the expansion rate of SS aggregate was proposed. Based on expansion rate tests of SS aggregate, steel slag mixture (SSM), and steel slag asphalt mixture (SSAM), the evolution of their expansion rate with the duration of water immersion was investigated; the resistance of SSAM to moisture damage was tested, and the moisture sensitivity of SSAM was analyzed. On this basis, a volumetric stability prediction model for SSAM considering moisture sensitivity was established. The results showed that the essence of SS aggregate expansion was the co-carbonation of surface free CaO (f-CaO) and silicate minerals (C3S and C2S). The expansion of SSM was governed primarily by differences in expansion rate among SS aggregates of different sizes and by aggregate gradation, whereas the expansion of SSAM depended mainly on asphalt binder film thickness, air voids, and the expansion characteristics of SSM. The factors influencing the moisture sensitivity of SSAM were ranked as follows: the asphalt binder film thickness of SSAM, the air voids of SSAM, and the f-CaO content in SSM. Predictions from the SSAM volumetric stability model were in good agreement with the experimental results, indicating that the model not only accurately reflected the relationship between the duration of water immersion and the expansion rate but also quantified the contribution of SSM expansion to the expansion of SSAM. In addition, the model demonstrated good predictive performance for the volumetric stability of SSAM under prolonged water immersion. These findings provide a theoretical basis for predicting the volumetric stability of SSAM.

Alkali-activated materials are amorphous cementitious binders produced through the alkali activation of aluminosilicate precursors. The contents of unreacted materials and porosity significantly influence the mechanical properties of alkali-activated materials. However, conventional testing methods face challenges in quantitatively analyzing these parameters. This study focuses on alkali-activated slag-metakaolin-based materials with varying slag contents. The effects of porosity and unreacted material content on compressive strength were quantitatively analyzed using X-ray computed tomography (X-CT) and three-dimensional reconstruction analysis methods. The feasibility of these methods was validated by comparing them with conventional methods, such as heat of hydration, X-ray diffraction, and scanning electron microscopy. The results indicate that as the slag content increases, the compressive strength of alkali-activated slag-metakaolin materials first increases and then decreases, reaching a peak value of 55.88 MPa at 50% slag content. X-CT and three-dimensional reconstruction analysis methods reveal that the porosity of alkali-activated slag-metakaolin materials decreases initially and then increases with rising slag content. At 50% slag content, both porosity and maximum pore diameter reach their minimum values of 0.31% and 1519.79 mu m, respectively. The unreacted material content increases with slag content, reaching a minimum of 0.095% at 30% slag content. Conventional testing methods further demonstrate that the incorporation of slag significantly enhances the hydration rate of alkali-activated slag-metakaolin-based alkali-activated materials. However, as slag content increases, the unreacted material content also rises, leading to a trend in which the formation of C-(A)-S-H gel initially increases and then decreases. The amount of C-(A)-S-H gel formed is influenced by both the orderliness of the hydration reaction and the content of unreacted material. Results from both conventional tests and X-CT-based three-dimensional reconstruction analyses are consistent, confirming the effectiveness of these techniques in evaluating porosity and unreacted material content in alkali-activated materials. Furthermore, the combined influence of porosity and unreacted material content play a crucial role in determining the compressive strength of alkali-activated materials, highlighting the potential of these methods for rapid and quantitative evaluation of compressive performance.

In order to comprehensively reveal the freeze-thaw (F-T) damage mechanism of warm mix steel slag rubber powder-modified asphalt mixtures (CR-WSAM). This study is based on acoustic emission (AE) characteristic parameters and digital image correlation (DIC) strain cloud maps. To analyse the CR-WSAM damage evolution process under the action of different F-T cycles. A Markov chain damage model is built to predict damage states. The results show a consistent effect of synergistic monitoring of AE and DIC. F-T cycles weaken the mixture's resistance to damage, significantly reducing the acoustic emission energy and b-value, resulting in the formation of numerous primary microcracks in the specimens. The solution enters the cracks and has a stripping effect on the asphalt aggregate interface. Tensile damage dominates in pure Type I cracking, but shear damage is still present. The damage is divided into three stages, and stage III shear damage is more pronounced. Shear damage accounted for less than 10% at -10 degrees C. Shear action had a positive effect on the damage resistance of the asphalt mixtures. According to the damage stages, a damage prediction model based on the Markov chain is proposed, which can accurately reflect the damage state of asphalt mixtures.

The digital image correlation (DIC) technique was employed to capture and analyze, in real time, the strain responses of warm-mixed and hot-mixed steel slag-crumb rubber modified asphalt mixtures (CR-WSAM and CR-HSAM) during indirect tensile fatigue testing (ITFT). The spatial distribution and temporal evolution of the horizontal strain field were utilized to investigate the fatigue damage process of CR-WSAM, identify the primary cracking region, and propose a damage variable (DA) for quantitative characterization of fatigue behavior. Furthermore, the damage characteristics of CR-WSAM and CR-HSAM before and after freeze-thaw-salt erosion coupling were comparatively analyzed. Based on DA, a fatigue life prediction model was developed, and its applicability under different stress ratios and freeze-thaw-salt erosion conditions was verified. The results revealed that cracks predominantly initiated and propagated within a localized region near the specimen's mid-span. Under the coupled freeze-thaw and salt erosion conditions, the damage evolution of CR-WSAM exhibited a distinct two-stage behavior characterized by a transition from gradual accumulation to rapid propagation. When the salt solution concentration exceeded 8%, the deterioration rate tended to stabilize, suggesting the presence of a critical saturation threshold for salt-induced damage.

Asphalt pavements in frozen regions are highly susceptible to damage induced by freeze-thaw cycles, during which microcracks initiate, propagate, and ultimately evolve into macrocracks. However, warm-mixed recycled asphalt mixtures (WRAMs) exhibit limited resistance to multi-stage fracture cracking. To address this limitation, basalt fibers (BFs) with various morphologies and dosages were incorporated into WRAMs to enhance their crack-resistance mechanisms. WRAM, chopped basalt fiber-reinforced WRAM (WR-CBF), and flocculent basalt fiber-reinforced WRAM (WR-FBF) were investigated under Mode I, Mode III, and mixed Mode I/III fracture modes. Specimens were subjected to 10 and 20 water and salt freeze-thaw cycles, followed by macroscopic edge-notched disk bend (ENDB) tests, mesoscopic acoustic emission (AE) analysis, and microscopic scanning electron microscopy (SEM) observations. The results indicate that WR-FBF with a basalt fiber content of 0.4% exhibits optimal crack resistance. Fracture resistance decreases with increasing freeze-thaw cycles, with salt freeze-thaw cycles causing more severe deterioration than water freeze-thaw cycles. Chopped basalt fiber provides more stable resistance to crack propagation across different fracture modes, whereas flocculent basalt fiber primarily enhances resistance to crack initiation, particularly under fracture mode I loading. SEM observations further indicate that FBFs enhance interfacial bonding and stress redistribution through adsorption, anchoring, three-dimensional network formation, and fiber pull-out mechanisms, thereby significantly improving the overall crack resistance of WRAMs.

The interfacial transition zone (ITZ), as the weak link between the geopolymer matrix and aggregates, plays a decisive role in determining the mechanical performance of geopolymer concrete through its bonding properties. In this study, geopolymer concrete was prepared using alkali-slag-metakaolin (ASM)-based binders combined with granite, basalt, and limestone aggregates, with the objective of elucidating the ITZ formation mechanism and establishing the relationship between ITZ bonding properties and the uniaxial compressive performance of the concrete. Based on these investigations, a cohesion model for the ITZ was developed and employed to numerically simulate the uniaxial compressive damage behavior of ASM-based geopolymer concrete, with the simulation results compared against experimental data. The findings reveal that active Si, Al, and Ca species, dissolved from the aggregates under the action of the highly alkaline activator, can participate in geopolymerization reactions at the interface, thereby enhancing ITZ bonding strength. As the alkali equivalent increases, the ITZ bonding strength shows a gradual improvement. Among the mixtures tested, the geopolymer-granite ITZ exhibited the highest bonding performance, followed by basalt, whereas limestone displayed relatively lower performance. The numerical simulation results based on the ITZ cohesion model demonstrated close agreement with experimental measurements, confirming the model's applicability for simulating the uniaxial compression behavior of geopolymer concrete. Overall, this study provides a solid theoretical basis and practical guidance for optimizing the performance and advancing the engineering application of geopolymer concrete.

Traditional Portland cement-stabilized bases are extensively applied in pavement structures; however, cement production and the extraction of natural river sand are associated with significant environmental impacts and resource depletion. This study employed desert sand as fine aggregate and alkali-activated slag-fly ash waste as the binder to systematically investigate the mechanical performance and freeze-thaw durability of geopolymer stabilized desert sand (GDS) bases at high desert sand contents. Experimental results indicate that ultrafine particles in desert sand participate in geopolymerization under alkaline activation, enhancing the interfacial bonding between the geopolymer matrix and desert sand, thereby surpassing the adhesion achieved in Portland cement-desert sand systems. With increasing desert sand content, the geopolymer binder content decreases, reducing its coating and cementation effect on sand particles, while porosity increases, resulting in declines in unconfined compressive strength, compressive resilient modulus, splitting tensile strength, and flexural strength. Even at a desert sand content of 90 %, the corresponding values of these properties remained 12.03 MPa, 1034.00 MPa, 0.93 MPa, and 4.50 MPa, respectively, satisfying base layer requirements. After 60 freeze-thaw cycles, the relative dynamic modulus of elasticity remained approximately 60 %, indicating substantial freeze-thaw durability. Compared with cement stabilized desert sand (PDS) bases, GDS bases exhibited a 30-73 % reduction in carbon emissions, demonstrating pronounced environmental benefits. This study provides a theoretical foundation for the high-value utilization of industrial solid wastes and desert sand in sustainable pavement base materials.

To investigate the influence of material performance changes before and after salt freeze-thaw cycles on the cracking behavior of asphalt mixtures, this study prepared four types of asphalt mixtures for comparative cracking behavior analysis. These mixtures utilized two common materials [basalt and styrene-butadiene-styrene (SBS)-modified asphalt] and two solid waste materials (steel slag and crumb rubber-modified asphalt). By combining the semicircular bending test with digital image correlation (DIC) technology, incremental fracture energy and horizontal strain energy were adopted as evaluation metrics to analyze performance evolution from both macroscopic and mesoscopic perspectives. Additionally, asphalt-aggregate adhesion tests were conducted to assess changes in interfacial tensile strength between aggregates and asphalt, revealing the crack resistance of asphalt mixtures at the bonding level. Furthermore, the physicochemical characteristics of steel slag and its interfacial interactions with asphalt were analyzed to elucidate crack formation mechanisms under salt freeze conditions. The research findings indicate that salt freeze-thaw cycles degrade the crack resistance of asphalt mixtures, with Fourier transform infrared (FTIR) spectroscopy analysis showing that physical degradation dominates this process. Increased salt solution concentration exacerbates interfacial bonding damage during freeze-thaw cycles, while thicker asphalt films can partially mitigate this effect. The incorporation of crumb rubber and steel slag significantly improves the deformation characteristics of mixtures, enabling them to maintain excellent low-temperature deformation resistance under freeze-thaw cycles.

The utilization of steel slag as aggregate in asphalt pavement construction offers environmental benefits by reducing land resource occupation and substituting natural aggregates, contributing to sustainable development. However, in seasonally frozen regions, asphalt pavements are prone to low-temperature cracking due to freeze-thaw (F-T) cycles, significantly shortening their service life. This study employed acoustic emission (AE) to monitor the semicircular bending failure process in real time for steel-slag and crumb rubber-modified asphalt mixtures (SAMs) conditioned in aqueous and saline (8 % sodium chloride) environments. By coupling AE parameters (ring count, energy, b-value) with mechanical responses, we quantitatively characterized the multiscale damage evolution from crack initiation to interfacial failure. A Weibull-based statistical damage model was developed to describe deterioration under combined environmental and mechanical effects. Results show that as F-T cycles increase (0, 5, 10, 20), the fracture energy of SAM decreases by 20 to 50 %, with saline conditioning causing 14 % greater degradation than water conditioning. Rapid increases in AE ring count indicate interfacial debonding, whereas declines in b-value mark the transition from microcrack accumulation to macrocrack dominance. The proposed model effectively captures nonlinear damage evolution under F-T and mechanical loading, providing a framework for designing crack-resistant, durable steel slag asphalt pavements in cold regions.

Based on the acoustic emission (AE) dynamic nondestructive testing technology, a semicircular bending test (SCB) was conducted on warm-mix steel slag rubber powder modified asphalt mixture (CR-WSAM) at -10 degrees C and -20 degrees C to monitor its crack evolution behavior throughout the process. The low-temperature cracking resistance of asphalt mixtures was evaluated using the Thermal Stress Restrained Specimen Test (TSRST). By analyzing the fracture energy and AE characteristic parameters (e.g., ring count, amplitude, energy frequency) of SCB, its anticracking performance and damage evolution law were revealed. This study found that the significant increase in the cumulative energy of the acoustic emission signal and the increase in the number of RA signals indicate that the high strength and excellent surface morphology of steel slag enhance the interlocking effect between aggregates inside the mixture and improve the adhesion between asphalt and aggregate interfaces such that the energy release during crack propagation is effectively limited. The addition of SDYK warm-mix agent further improves the viscosity flow characteristics of asphalt, enhances the adhesion between asphalt and aggregate, and reduces the mixing temperature, which is manifested in the delay of the acoustic emission frequency peak and the increase in the proportion of high-frequency signals. In addition, as the temperature decreases, the growth rate in the ringing count in the acoustic emission characteristic parameters accelerates, and the peak amplitude increases significantly, indicating that the material gradually changes from a viscoelastic state to a highly elastic state; the anticracking performance of the four asphalt mixtures is ranked as follows: CR-WSAM > CR-HSAM > CR-WSBAM > CR-HSBAM.

High reclaimed asphalt pavement (RAP) content tends to deteriorate the low-temperature cracking resistance of asphalt mixtures, and fibers as additives can mitigate this problem. However, the fiber crack resistance mechanisms under different cracking modes have not been systematically elucidated in existing studies. This study investigated the effect of fibers on the low-temperature fracture behavior of warm-mix recycled asphalt mixture (WRAM) under different cracking modes. Edge notched disc bend (ENDB) and semi-circular bend (SCB) tests were conducted in conjunction with acoustic emission (AE) monitoring to analyze the cracking process. The pre-peak work of fracture (Wf) and RA-AF parameters were employed to evaluate the crack damage evolution, and the fiber crack resistance mechanisms under mode I, mode II, mode III, mixed mode I/II and mixed mode I/III were compared. The results showed that the cracking mode exerted a greater influence on the pre-peak work of fracture Wf than the asphalt mixture type. Chopped and flocculent fibers at a dosage of 0.4% significantly improved the Wf of WRAM, particularly under mixed modeI/II and mixed modeI/III conditions. RA-AF analysis revealed that tensile and shear cracks occurred simultaneously, and flocculent fibers altered the dominant crack propagation behavior. Fibers primarily acted during the crack initiation and propagation stages by increasing the crack initiation strength of WRAM and prolonging the crack propagation duration to dissipate energy. The two fiber types exhibited distinct mechanisms: chopped fibers predominantly enhanced fracture toughness, while flocculent fibers primarily improved fracture strength. For tensile-dominated cracking, fibers mainly came into play during the crack propagation stage, whereas for shear-dominated cracking, fibers predominantly acted during the crack initiation stage. The AE ringing count variance based on the critical slowing down (CSD) theory was calculated to determine the cracking warning load of WRAM. The findings provide a reference for the practical engineering application of fibers in WRAM.

This study develops an interpretable machine learning framework to elucidate the temperature-dependent structure-property relationships of crumb rubber modified asphalt (CRMA). A comprehensive dataset was constructed by systematically characterizing chemical functional groups, microstructural morphology, and nanomechanical properties of CRMA with varying aging states and rubber contents, together with rheological parameters over a wide temperature range from -30 degrees C to 80 degrees C. Predictive models including Random Forest (RF), Extreme Gradient Boosting (XGB), and Multilayer Perceptron (MLP) were established, with Bayesian optimization and nested cross-validation ensuring robustness under limited sample conditions. Results show that XGB and RF models achieve high predictive accuracy, with test R2 exceeding 0.9 in both low- and hightemperature regimes. More importantly, SHAP analysis reveals a clear transition of governing mechanisms with temperature: interfacial adhesion and molecular chain flexibility govern low-temperature behavior, multifactor coupling dominates the intermediate regime, and contact stiffness becomes the controlling factor at high temperatures. This study achieves full-temperature prediction of CRMA rheology and reveals structure-property mechanism transitions, providing an interpretable data-driven framework for asphalt design.

To investigate the influence of temperature, frequency, and RAP content on the viscoelastic properties of warm-mix recycled asphalt mixtures, dynamic modulus tests were conducted on mixtures with different RAP contents (0 φ (improved phase-angle) model were fitted synchronously with shared WLF shift factors, ensuring thermorheological consistency. The synchronous S- φ model accurately fits the variation of both dynamic modulus and phase angle with frequency.

This study investigates the volumetric expansion behavior of steel slag under immersion conditions and its subsequent effect on the low-temperature performance of asphalt mixtures. To evaluate this, stability tests on steel slag and density measurements of compacted asphalt mixtures were performed, aiming to determine the immersion expansion rates of steel slag aggregates from various regions, as well as the expansion characteristics of asphalt mixtures with different gradation types. The free calcium oxide (f-CaO) content in the aggregates was quantified using the ethylene glycol method, and a detailed analysis was conducted to examine the correlation between the immersion expansion rates of steel slag aggregates and mixtures with respect to immersion time and f-CaO content. Additionally, considering the relationship between immersion time and expansion rate, the low-temperature performance of steel slag asphalt mixtures was investigated using a restrained specimen thermal stress test. The results demonstrated that the f-CaO content, expansion rate, and stabilization time of steel slag aggregates were substantially affected by both the aggregate gradation and the source of the steel slag. Notably, fine aggregates with higher f-CaO content led to a more pronounced increase in the expansion rate of steel slag mixtures. Type I steel slag aggregates with particle sizes of 10-15 and 5-10 mm respectively contributed 46.13% and 35.06% to the relative expansion rate of Baotou SMA-13 (SMA I), while Type II steel slag aggregates with a particle size of 0-3 mm contributed 46.38% to the relative expansion rate of Shandong SMA-13 (SMA II). Fine aggregates with higher f-CaO content led to a more significant increase in the expansion rate of steel slag asphalt mixture. A comparison of the expansion rates of Type I steel slag asphalt mixtures with different gradations, including SMA I [with an asphalt content of 5.5% (81.67 g of asphalt) and a void volume of 3%] and Baotou AC-13 (ACI) [with an asphalt content of 4.3% (65.33 g of asphalt) and a void volume of 4.44%], revealed that the change in void volume due to the asphalt content was one of the factors affecting the expansion rate of steel slag asphalt mixtures. Steel slag asphalt mixtures derived from different sources but with the same gradation exhibited comparable low-temperature performance under identical expansion rates and soaking durations. Following 6 days of immersion, the low-temperature performance of SMA I and SMA II decreased by 21%-44% and 18%-60%, respectively. Notably, the hydration-induced expansion of the 0-3 mm aggregates in SMA II resulted in a more pronounced reduction in low-temperature performance compared to the coarser aggregates (10-15 and 5-10 mm) in SMA I.

Incorporating basalt fiber (BF) is an effective approach to improving the low-temperature cracking resistance of Warm-mix recycled asphalt mixtures (WRAM). However, the mechanisms by which different BF morphologies regulate crack propagation and stage-dependent damage evolution in WRAM remain unclear, which limits their scientific application in road engineering. To address this issue, low-temperature semicircular bending (SCB) tests were conducted on WRAM with 50% reclaimed asphalt pavement (RAP), chopped basalt fiber-reinforced WRAM (CBF-WRAM), and flocculent basalt fiber-reinforced WRAM (FBF-WRAM). Acoustic emission (AE) monitoring and variational mode decomposition (VMD) were further employed to analyze the damage evolution characteristics during crack propagation. The results showed that CBF and FBF increased the toughness index of WRAM by 92% and 238%, respectively. In the post-peak stage, CBF delayed crack propagation through energy-dissipating mechanisms such as fiber pull-out and fracture. Although this was accompanied by more high-frequency AE events, it reduced the release level of high-frequency energy. In comparison, FBF exhibited a stronger inhibitory effect on high-frequency AE activity, with the energy release level of its high-frequency AE events being less than 10% of that of WRAM. The study provides a mechanistic basis for selecting fiber types in high-RAP Warm-mix recycled asphalt mixtures and offers new insight into improving their low-temperature durability.

To investigate the adhesion characteristics of different asphalt-aggregate (A-A) interfaces under salt solution erosion. Clean water and 8% saline solution were selected, and molecular dynamics (MD) simulation was performed to establish a model of rubber powder-modified asphalt (CR)/SBS modified asphalt (SBS)-water/salt solution-steel slag/basalt interface. Study on the mechanism of interaction between asphalt molecules and aggregate minerals through diffusion coefficient, RDF and interfacial adhesion energy density. Based on the surface energy theory, the adhesion work between different asphalt and aggregate was calculated to verify its rationality. The results show that the degradation effect of Cl- and Na+ in salt solution on the interfacial adhesion of A-A system is much stronger than that of water molecules. The denser molecular aggregation makes the A-A system have better adhesion characteristics. Regardless of water or salt solution, the adhesion of the A-A interface is CR-steel slag > SBS-steel slag > CR-basalt > SBS-basalt.

Incorporating steel slag as the dominant aggregate in asphalt mixtures has garnered increasing attention for its both mechanical and environmental benefits. However, the moisture-induced expansion of steel slag under longterm vapor exposure degrades the integrity of asphalt mixtures. The steel slag asphalt mixture exhibits crack propagation during its service life. To address this issue, we developed a fracture modeling framework that incorporates a moisture-cohesive zone model (M-CZM) and a moisture-induced slag expansion equation (M-SE) within a finite element (FE) model of asphalt mixtures. This framework simultaneously defines the decrease in adhesion/cohesion and steel slag expansion as a function of moisture content. We conducted a semi-circular bending test on asphalt mortar specimens with different vapor exposure time to validate the fracture modeling framework. The results showed that the simulation results had little difference with the measured ones. Based on the framework, we simulated the fracture process of steel slag asphalt mixtures under long-term vapor exposure. The coupled damages of moisture-induced adhesion/cohesion reduction and slag expansion were quantified. The results indicated that adhesive damage precedes cohesive damage under moisture-induced slag expansion. After 100 days of vapor exposure, the asphalt mixture's peak force decreases by 35 %. The coupled effects of moisture-induced adhesion/cohesion reduction and steel slag expansion shift the crack initiation point from the middle toward the edges of the specimen. This work provides insight into the fracture evolution of steel slag asphalt mixtures under long-term vapor exposure, offering evidence for assessing the durability of such mixture design.