A series of saturated undrained dynamic triaxial tests were carried out by using DYNTTS large-scale triaxial cyclic test system to investigate the long-term dynamic characteristics and cumulative strain evolution of round gravel foundation under cyclic subway loading. The influence mechanisms of relative density, confining pressure and dynamic stress amplitude on the long-term dynamic characteristics and cumulative deformation of round gravel were systematically investigated. A prediction model of cumulative plastic strain considering the coupling effect of 3 parameters was proposed. The results of the study show that for the subway tunnels with shallow burial depth and low relative density of the round gravel foundation, the train operation has potential safety hazards, while for the subway tunnels with deeper burial depths, the operation of the trains is relatively safe. Increasing relative density and confining pressure can effectively improve the resilient modulus of round gravel, reduce its cumulative plastic strain, elastic strain and dynamic pore-pressure ratio, and significantly enhance the foundation's liquefaction resistance. Therefore, for unfavorable working conditions such as Dr=0.3 and σ3 = 100kPa, improvement measures such as increasing tunnel depth and implementing high-pressure grouting can be adopted to reduce the settlement of the subway foundation. Based on the time-hardening theory, a cumulative plastic strain prediction model was established with comprehensive consideration of relative density, confining pressure and dynamic stress amplitude. The research results can provide theoretical support and a design basis for the dynamic stability assessment of subway round gravel foundation and its long-term settlement prediction after construction.

ABSTRACT Expansive soils exhibit high compressibility, swelling & shrinkage behaviour, moisture sensitivity, and low bearing capacity, often causing differential settlement and structural damage in pavements and foundations. Although lime and cement stabilization, their production generates significant carbon emissions. This study examines Soil-FeT geopolymer mixtures as a sustainable alternative for subgrade stabilization. Soil specimens were treated with binder contents of 10, 20 and 30%, activated using 6, 8 and 10 M alkaline solutions. Mechanical performance was evaluated through unconfined compressive strength (UCS), split tensile strength (STS), California bearing ratio (CBR), resilient modulus (MR), and hydraulic conductivity tests, while durability was evaluated through 12 wetting‒drying cycles. Geo-environmental performance was examined using leachibility, carbon emission, and energy consumption analyses. Results showed that specimens containing 10% binder activated at 10 M achieved optimum performance, with UCS increasing from 0.23 to 4.89 MPa for low-plastic soil and from 0.11 to 3.81 MPa for high-plastic soil. The soaked CBR values increased approximately 12- and 19-fold compared to untreated soils. Treated specimens exhibited durability, with mass loss below 10%, and satisified IRC subgrade requirements. Heavy metal concentrations remained below USEPA limits. Sustainability assessment indicated 82% lower CO2 emissions and 61% lower energy consumption than cement stabilization, confirming Soil-FeTgeopolymers as an eco-friendly stabilizer for applications.

Abstract The reuse of recycled materials in transportation infrastructure has gained significant popularity in recent years. Particularly, recent studies have explored the use of coal wash and rubber crumbs mixtures (CWRC) to create a synthetic, energy-absorbing railway capping layer, offering a sustainable and cost-effective alternative to traditional materials. Although previous studies have examined the mixture’s deformation characteristics, particle breakage, damping properties, and resilient behaviour under cyclic loading, its fatigue behaviour remains largely unexplored. The current research aims to investigate the effect of the applied cyclic deviator stress on the fatigue behaviour of this waste mixture, by evaluating permanent deformation, particle breakage, resilient modulus, and shear modulus through a series of cyclic triaxial tests. Three distinct zones, namely the plastic shakedown zone, critical dynamic stress zone, and low-cycle fatigue zone, are identified based on the accumulated permanent deformation and the stress–life diagram. A detailed analysis of particle breakage, resilient modulus, and shear modulus associated with these three zones is also conducted. The findings of this study will significantly impact future track design by integrating the concept of fatigue under cyclic loading based on plasticity and yield envelopes.

Recovered carbon black as a sustainable alternative to hydrated lime in asphalt mixtures: Mechanical and durability perspectives

China’s rapid urbanization has generated substantial low-strength demolition waste, creating significant environmental and disposal challenges. Recycling this waste as subgrade filler provides a viable solution given the subgrade’s lower strength requirements and high consumption capacity. This study systematically evaluated the feasibility of using unsorted rural demolition waste (URDW) as subgrade filler without separation to improve recycling efficiency. Three gradations (FF20, MF40, and CF80) with maximum particle sizes of 20, 40, and 80 mm were designed based on the characteristics of the raw materials. The composition, compaction behavior, particle breakage mechanisms, and mechanical properties of these fillers were analyzed. The experimental road subgrade employed FF20, with model tests assessing long-term performance under wet–dry (WD) cycles and field evaluations combining quality tests with settlement monitoring. Results showed that increased particle size reduced the combined content of bricks and mortar (CCBM) while raising that of concrete and stones (CCCS) in the raw materials, demonstrating particle size effects on material strength. This explains the decreasing CCBM and increasing CCCS from FF20 to CF80. Heavy compaction produced elevated particle breakage rates (Br) and irregular compaction curves, whereas surface vibration compaction generated standard curves with significantly reduced Br. Although filler compositions varied, particle size and coarse content primarily controlled Br variation during compaction. The FF20 subgrade model exhibited minimal compressive deformation under WD cycling, with transitional deformation progressively diminishing as cycling enhanced the model’s deformation resistance. After cycling, the saturated FF20 model maintained a resilient modulus (MR) above regulatory requirements. Field tests confirmed FF20-filled subgrade complies with specifications for compaction degree, deflection, and MR in both dry and saturated conditions. Forty-month monitoring recorded only 1.64 mm centerline compression, validating satisfactory deformation resistance. This study broadens potential applications for URDW and provides practical guidelines for similar projects.

To address the loose structure and insufficient dynamic stability of collapsible loess subgrades in Gangu, Gansu Province, as well as the engineering and environmental concerns associated with conventional stabilizers (high energy consumption and carbon emissions), this study proposes a composite stabilization strategy using an eco-friendly curing agent (EFCA) and P·O 42.5 Portland cement. Unconfined compressive strength (UCS) tests and dynamic triaxial tests were performed, and scanning electron microscopy (SEM) image processing together with fractal theory was employed to systematically elucidate the macroscopic dynamic response and the microscopic pore-reconstruction and evolution mechanisms of the composite-improved loess. The results indicate that the optimal mix proportion determined by an orthogonal design is "6% cement + 0.02% curing agent", yielding a 28-day UCS of 2.51MPa, which is 483% higher than that of the untreated loess. The dynamic resilient modulus (Ed) increases markedly and reaches 826.49MPa under a confining pressure of 60kPa and a dynamic stress of 30kPa (an 8.07-fold increase). Nonlinear regression analysis confirms that the Ni model, by jointly accounting for the coupled effects of mean and deviatoric stresses, provides exceptionally high predictive accuracy for Ed of the composite-improved loess, with an average relative error of only 0.026. Quantitative microstructural analysis reveals that the synergistic effects of chemical cementation and hydration products promote the transformation of loess particles into dense aggregates, resulting in a decrease in the pore fractal dimension (D) from 1.323 to 1.249. This topological reconstruction from connected macropores to discrete micropores fundamentally reduces the structural complexity of the soil. The study clarifies a cross-scale physical-mechanical mechanism whereby "microstructural pore-fractal dimensionality reduction" drives a "macroscopic surge in dynamic stiffness", providing theoretical and data support for green, low-carbon subgrade construction and long-term dynamic stability evaluation in loess regions.

This study evaluates the effect of curing modes and times on the mechanical properties of nanocomposites. Two nanocomposite resins were investigated: supra-nanohybrid (Estelite Posterior Quick; EP) and nanohybrid (Solare X; SX). They were polymerized with a light-emitting diode light-curing units (LED LCU, Valo) as follows: standard mode for 20s (ST20), high power mode for 12s (HP12), high power mode for 20s (HP20), extra power mode for 6s (XP6), and extra power mode for 20s (XP20). For Vickers microhardness (HV), disc-shaped specimens were fabricated (n=10). For the three-point bending test, bar-shaped specimens were fabricated (n=10). Flexural strength and resilience modulus were calculated. The fractured surfaces and specimen surfaces of composites were observed using scanning electron microscopy. The data were analyzed with repeated measures ANOVA, two-way variance, and Bonferroni tests (p<0.05). On the top and bottom surfaces of the EP nanocomposite resin, ST20 and HP12 revealed statistically higher HV than with XP6. Moreover, HP20 and XP20 had statistically higher HV than HP12 and XP6. For the SX nanocomposite resin, HP20 had statistically higher HV than HP12. For EP and SX, there were no significant differences in flexural strength and resilience modulus regarding the curing modes and times. Furthermore, SX demonstrated lower mechanical properties than EP. Scanning electron microscopy indicated that both nanocomposites had similar surface appearances. However, with all curing modes and times, SX exhibited layered fractures and more crack formations than EP. Different curing modes and times could influence the microhardness of nanocomposites.

Microbially induced calcium carbonate precipitation (MICP) faces persistent obstacles in transportation subgrade stabilization because rapid ureolysis can trigger premature clogging and pronounced spatial heterogeneity. This study develops a construction-oriented biocementation strategy that uses the urease inhibitor N-(n-butyl)-thiophosphoric triamide (NBPT) to regulate reaction kinetics and improve treatment uniformity under groundwater-relevant conditions. A multivariate optimization framework was established by jointly evaluating inhibitor dosage, cementation solution concentration, biological-to-chemical ratio, ambient temperature, and pH buffering. The optimal formulation was achieved at 0.1% NBPT, which delivered an unconfined compressive strength of 2.53MPa compared with 2.72MPa for the inhibitor-free control, indicating that strength was largely preserved while reaction kinetics were moderated. Calcium carbonate deposition proceeded steadily over 72h and reached a carbonate content of 11.2kg/m³ under the optimized protocol. This mineral accumulation reduced hydraulic conductivity from 1.7 × 10⁻³ m/s to 6.4 × 10⁻⁵ m/s, corresponding to a 96.3% decrease, while avoiding localized pore occlusion. Under simulated AASHTO T307 cyclic loading, the optimized treatment achieved a resilient modulus of 152MPa and limited residual deformation to 0.32mm per 1000 load cycles at an 80 kN axle load. The proposed NBPT-controlled MICP framework provides an operational window that balances strength gain, hydraulic functionality, and field implementability for subgrade applications.

Stabilizing weak soils is a well-known pavement and geotechnical engineering technique. This technique involves introducing minimal cementitious materials to improve the soil’s geotechnical characteristics. This paper investigates the use of recycled industrial polymer waste (IPW) as a reinforcement material in the presence of cementitious binders to stabilize weak silty sand soil (SM), supporting sustainable engineering practices. The randomly distributed IPW were added as percentages of 0%, 5%, and 10% to a mixture of lime soil and cement soil, with varying amounts of 0% to 6% of lime (L) and 0% to 6% of ordinary Portland cement (OPC), respectively. The laboratory experiments were conducted on natural and stabilized samples in wet (unsoaked) and submerged (soaked) conditions. The experimental program included Proctor compaction, California bearing ratio (CBR), unconfined compressive strength (UCS), durability tests, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction analyses. The resilient modulus (Mr) was estimated using an empirical equation. The outcomes of this experimental study show that adding a combination of IPW shreds with a small amount of L and/or OPC to the SM soil provides a significant increase in the UCS, CBR, durability and Mr values compared with case of SM with only L, which allows for superior characteristics and increases strength and stiffness parameters throughout any phase of earthwork construction design, resulting in stronger and stiffer subgrades. These results were reinforced by microstructural observations from SEM, EDS, and DRX, confirming the formation of cementitious gels and chemical compounds, consistent with the macro-scale mechanical improvements. The expected practical outcomes include potential reductions in pavement thickness, which can help lower pavement stabilization costs and extend its service life. Additionally, the use of waste materials to replace raw materials contributes to decreased energy consumption and emissions, although detailed assessments are needed to quantify these effects.

Characteristics of subgrade soil resilient modulus considering wide-range stress state

Asphalt is widely used in pavements, but its dependence on fossil-based resources limits long-term sustainability. Incorporating waste polyvinyl chloride (PVC) into hot mix asphalt (HMA) offers a potential solution for waste management and resource recovery. This study evaluated the mechanical performance of PVC-modified HMA, focusing on rutting resistance, stiffness, and fracture behavior. Static and dynamic creep tests, indirect tensile stiffness modulus (ITSM) tests, and semi-circular bending (SCB) fracture tests were conducted to determine tensile stiffness, resilient modulus, creep stiffness, fracture energy, fracture toughness, and flexibility index. Results showed that although PVC modification provided some rheological benefits, it did not significantly improve overall mechanical performance. Only limited advantages were observed at intermediate temperatures, where 1% PVC showed relatively better viscoelastic behavior compared to the control mixture.

Machine learning–based estimation of subgrade resilient modulus using sensor signals from in-situ modulus detector

Semi-flexible pavement (SFP) combines the flexible properties of asphalt with the rigidity of a cementitious slurry, providing improved structural and durability performance. However, the reliance on ordinary Portland cement in SFP contributes notably to global CO₂ emissions. In particular, the sugarcane and ethanol industries generate substantial amounts of sugarcane bagasse ash (SBA), which is often disposed of despite its potential as a pozzolan. Therefore, this study explores the potential use of SBA, along with nano-silica (NS), as a partial cement replacement to develop a more sustainable and higher‑performing grout for SFP applications. Control grouts were formulated with water-to-cement (w/c) ratios ranging from 0.30 to 0.45 and superplasticizer (SP) dosages of 0% to 1.5%, while modified grouts incorporated 0-20% SBA (in 5% increments) and 1% NS. The optimal control grout was achieved at a w/c ratio of 0.35 and 1% SP, satisfying flow criteria and achieving a maximum 28‑day compressive strength of 48.50MPa. The optimal sustainable grout contained 10% SBA and 1% NS, providing a 21.7% increase in compressive strength compared to the control grout. Microstructural observations confirmed a denser matrix with enhanced C-S-H formation and reduced voids, contributing to improved early strength and superior grout‑asphalt interaction. SFP specimens prepared with the optimized grout were evaluated for Marshall stability, resilient modulus (MR), indirect tensile strength (ITS), tensile strength ratio Hamburg wheel tracking, and fuel resistance. Compared to hot mix asphalt (HMA), SFP exhibited significantly enhanced performance, including approximately 88.5% higher Marshall stability, MR values exceeding 5000MPa, 70% lower rut depth, 92% moisture‑induced strength retention, less than 5% mass loss, and approximately 93% retained strength under partial and full fuel immersion. These outcomes demonstrate the potential of SBA-NS-based SFP as a sustainable and durable pavement solution suitable for heavy‑duty and fuel‑exposed applications.

The performance of soils and granular materials must be considered in relation to the main deterioration mechanisms in pavement structure design. This study aims to evaluate the use of fine-grained soils as unbound materials in pavement applications. For this purpose, physical, chemical, and mechanical characterization tests were conducted. Repeated load triaxial tests were performed following current Brazilian standards and considering different soil saturation conditions. Five widely used mathematical models were employed to assess the resilient modulus under different moisture conditions, with the Compound and Universal models showing the best correlations. As moisture content increased, a reduction in resilient modulus was observed at higher stress levels. Regarding permanent deformation, the master curve approach was applied to obtain the parameters adopted as inputs for simulations within the mechanistic-empirical pavement design software, FlexPaveBR version 2.1.5. The model results adequately represented the plastic behavior of the analyzed conditions. The simulation results indicated differences between considering bonded and unbonded granular layers. The results also indicated that the use of fine-grained soils without stabilization can be considered for subbase layers, provided that triaxial testing is conducted and the materials demonstrate satisfactory performance under mechanistic-empirical evaluation.

This study examines the effectiveness of walnut shell powder (WNSP), a sustainable, bio-based stabiliser, in enhancing the mechanical and dynamic properties of expansive high-plasticity clay (CH) and low-plasticity clay (CL) soils. Laboratory tests on soils treated with 0–15% WNSP (by dry weight) evaluated its impact on unconfined compressive strength (UCS), California Bearing Ratio (CBR), and resilient modulus ( M R ). Optimal improvements were observed at approximately 6% WNSP for CH and 9% for CL soils, with UCS increasing by 33.7% and 41.4%, and CBR by 50.7% and 87.6%, respectively. The resilient modulus, reflecting cyclic stiffness, rose by 149.3% in CH and 31.0% in CL soils. Stress-state-dependent prediction models, particularly Uzan-type formulations, effectively captured the nonlinear resilient behaviour of WNSP-stabilised soils. An extended Modified Uzan model incorporating WNSP-dependent coefficients further improved prediction accuracy. Results highlight WNSP as an eco-friendly, cost-effective stabiliser, utilising agricultural waste for sustainable pavement subgrades.

Purpose This paper presents a comprehensive systematic review of low-carbon solid waste materials applied in railway sub-ballast layers, aiming to critically assess their mechanical performance, durability, environmental benefits and regulatory readiness. The study addresses the growing need to decarbonize rail infrastructure while reducing dependence on natural aggregates, positioning sub-ballast as a strategic layer for circular economy implementation in ballasted track systems. Design/methodology/approach A PRISMA-based systematic review methodology was adopted to identify, screen and analyses peer-reviewed studies published between 2000 and 2025. The final database comprises experimental, numerical and field investigations covering mining residues, steel slags, construction and demolition waste, rubberized composites, alkali-activated materials and other industrial by-products applied to railway sub-ballast. Mechanical behavior under cyclic loading, resilient modulus, permanent deformation, hydraulic performance, durability and environmental indicators were extracted and synthesized. In parallel, an international regulatory analysis was conducted to compare sub-ballast specifications across Europe, North America, Asia-Pacific and Brazil, enabling identification of performance-regulation gaps and barriers to implementation. Findings The review demonstrates that several low-carbon waste-derived materials exhibit mechanical performance comparable to or exceeding that of conventional granular sub-ballast, particularly in terms of stiffness retention, resistance to permanent deformation and degradation under repeated loading. Steel slags, recycled concrete aggregates, slate waste and rubber-modified blends consistently show favorable resilient behavior and enhanced damping capacity, while certain mining residues and alkali-activated granular systems present promising strength and durability characteristics. Life-cycle evidence indicates substantial reductions in embodied carbon and natural aggregate consumption when these materials are adopted. However, current railway standards remain largely prescriptive and index-based, rarely incorporating cyclic performance criteria or carbon metrics, creating a structural disconnect between scientific evidence and regulatory acceptance. This gap significantly limits large-scale implementation despite growing technical maturity. Originality/value This study provides the first integrated synthesis focused exclusively on low-carbon solid waste materials for railway sub-ballast, combining mechanical performance, environmental assessment and international regulatory comparison within a unified analytical framework. By explicitly linking laboratory evidence to policy and standardization challenges, the paper advances performance-based pathways for sustainable railway substructure design. The findings offer actionable guidance for infrastructure managers, regulators and researchers seeking to accelerate the transition toward circular, low-carbon rail systems through sub-ballast innovation.

ABSTRACT Cold mix asphalt concrete (CMA) is widely used for road repair and leveling because of its simple production process and low cost. This study evaluated bottom ash (BA), a coal combustion by-product, as a partial replacement for fine aggregate in CMA. BA-incorporated CMA (BA-CMA) was assessed in terms of Marshall properties, indirect tensile strength (ITS), indirect tensile resilient modulus (ITMR), indirect tensile fatigue life (ITFL), rut depth, and skid resistance. A laser confocal microscope with image analysis was also used to examine asphalt film thickness. The results showed that BA increased asphalt film thickness because of its porous structure, rough surface texture, and low specific gravity, which enhanced asphalt emulsion absorption. Under indirect tensile loading, performance improved with increasing BA content up to 25%, at which ITS and ITMR increased by 24.2% and 26.8%, respectively, while ITFL improved at all stress levels. Fatigue modeling showed a log-log linear relationship between ITFL and tensile strain, indicating that BA mainly influenced fatigue performance through modulus modification. Under compressive loading, rut depth decreased with increasing BA content, reaching a 37.6% reduction at 50% BA. All mixtures satisfied the minimum skid resistance requirement. Although 25% BA increased cost by 17.2%, it provided the most favorable performance-to-cost balance for low- to moderate-load pavements.

This study proposed large-size cement-stabilized gravel (LCSG) mixes as a preventive measure against arch expansion in semi-rigid base asphalt pavements. LCSG mixes with weak and strong skeleton dense structures (SDSs) were designed. Their mechanical and expansion properties under different cement contents were compared with conventional cement-stabilized gravel (CCSG) mixes. Thermal expansion of LCSG and CCSG mixes was further analyzed using the discrete element method (DEM). The results indicated that compared to CCSG mixes, LCSG mixes with a strong SDS and fewer fine aggregates reduced cement content by approximately 1.0%, exhibited a 23% lower compressive resilience modulus (CRM), and a 34.5% lower expansion coefficient. Mesoscopic simulations revealed that LCSG mixes had lower temperature gradients, vertical expansion, and reduced temperature stresses than CCSG mixes, demonstrating superior effectiveness in preventing arch expansion. These findings provide theoretical guidance for applying LCSG mixes in arid regions with significant temperature fluctuations.

Objective: To determine the most suitable core build-up materials based on their mechanical and physical properties, different resin based materials were evaluated for flexural strength (FS), flexural modulus (E), modulus of resilience (R), water sorption (WS), and solubility (SO). Materials and Methods: Three dual-cure resins (CosmeCore DC Automix, CCC; Clearfil DC Core Plus, CCP; MultiCore Flow, CMC) and two bulk fill composites (Filtek One Bulk Fill Restorative, BFO; Filtek Bulk Fill Flowable, BFF) were tested, with Filtek Supreme Ultra (FSU) as the control. All tests followed ISO 4049. Beam specimens (25 × 2 × 2 mm, n = 12) were used to determine FS and E after 24 h storage in 37 °C deionized water, using a three-point bending test. Disc specimens (15 × 1 mm, n = 5) were used for WS and SO by measuring mass changes before and after water storage. Data were analysed using one way ANOVA and Tukey post hoc tests (p < 0.05). Results: CCC exhibited the highest FS and lowest WS. BFF showed the lowest E, while BFO exhibited the highest R. FSU demonstrated the lowest FS and R, along with the highest WS. No significant differences in SO were observed among groups. Conclusions: The evaluated materials showed considerable variation in mechanical and physical properties. CCC and BFO demonstrated the most favourable performance, suggesting they are the most suitable candidates for core build up procedures among the materials tested.

Fatigue fracture resistance of a short fiber-reinforced flowable resin composite in single-layer and incremental-layer blocks.