Soft soil subgrades often present significant geotechnical challenges under cyclic loading conditions associated with major infrastructure developments. Moreover, there has been a growing interest in employing various recycled tire derivatives in civil engineering projects in recent years. To address these challenges sustainably, this study investigates the performance of granular piles incorporating recycled tire chips as a partial replacement for conventional aggregates. The objective is to evaluate the cyclic behavior of these tire chip-aggregate mixtures and determining the optimum mix for enhancing soft soil performance. A series of laboratory-scale, stress-controlled cyclic loading tests were conducted on granular piles encased with combi-grid under end-bearing conditions. The granular piles were constructed using five volumetric proportions of (tire chips: aggregates) (%) of 0:100, 25:75, 50:50, 75:25, and 100:0. The tests were performed with a cyclic loading amplitude (qcy) of 85 kPa and a frequency (fcy) of 1 Hz. Key performance indicators such as normalized cyclic induced settlement (Sc/Dp), normalized excess pore water pressure in soil bed (Pexc/Su), and pile-soil stress distribution in terms of stress concentration ratio (n) were analyzed to assess the effectiveness of the different mixtures. Results indicate that the ordinary granular pile (OGP) with (25 % tire chips + 75 % aggregates) offers an optimal balance between performance and sustainability. This mixture reduced cyclic-induced settlement by 86.7 % compared to the OGP with (0 % TC + 100 % AG), with only marginal losses in performance (12.3 % increase in settlement and 2.8 % reduction in stress transfer efficiency). Additionally, the use of combi-grid encasement significantly improved the overall performance of all granular pile configurations, enhancing stress concentration and reducing both settlement and excess pore water pressure. These findings demonstrate the viability of using recycled tire chips as a sustainable alternative in granular piles, offering both environmental and engineering benefits for soft soil improvement under cyclic loading.
This study investigates the microhardness and geometric degradation mechanisms of interfacial transition zones (ITZs) in recycled aggregate concrete (RAC) exposed to saline soil attack, focusing on the influence of supplementary cementitious materials (SCMs). Ten RAC mixtures incorporating fly ash (FA), granulated blast furnace slag (GBFS), silica fume (SF), and metakaolin (MK) at 10 %, 15 %, and 20 % replacement ratios were subjected to 180 dry-wet cycles in a 7.5 %MgSO4-7.5 %Na2SO4-5 %NaCl solution. Key results reveal that ITZ's microhardness and geometric degradation decreases with exposure depth but intensifies with prolonged dry-wet cycles. The FAGBFS synergistically enhances ITZ microhardness while minimizing geometric deterioration, with ITZ's width and porosity reduced to 67.6-69.0 mu m and 25.83 %, respectively. In contrast, FA-SF and FA-MK exacerbate microhardness degradation, increasing porosity and amplifying microcrack coalescence. FA-GBFS mitigates the diffusion-leaching of aggressive/original ions and suppresses the formation of corrosion products, thereby inhibiting the initiation and propagation of microcracks. In contrast, FA-SF and FA-MK promote the formation of ettringite/gypsum and crystallization bloedite/glauberite, which facilitates the formation of trunk-limb-twig cracks.
Alkali-activated concrete (AAC) is a focal point in green building material research due to its low carbon footprint and superior performance. This study seeks to enhance the impact resistance of recycled aggregate concrete (RAC) by elucidating the synergistic mechanisms of alkali activation, nano-modification, and fiber reinforcement. To this end, four mix designs, incorporating NaOH and NaOH-Na2SiO3 systems with 2 % nano-SiO2(NS), were developed and assessed through setting time, compressive strength, drop hammer impact tests, and XRD/ SEM analyses. The NaOH-Na2SiO3 system exhibited a 23.5 % increase in compressive strength over NaOH, achieving 28.41 MPa, while NS refined pore structures, elevating strength to 32.2 MPa; XRD/SEM analyses confirmed mechanisms of pore refinement and interfacial enhancement. In the optimized system, the NT12-C5 formulation, incorporating polypropylene fiber (PPF) and recycled carbon fiber (RCF), exhibited superior impact resistance, with NS enhancing interfacial bonding between carbon fiber and the matrix, resulting in a 47.8 % increase in initial crack impact energy. The Weibull model validated the reliability of impact performance. Furthermore, life cycle assessment revealed that Soil Solidification Rock Recycled aggregate concrete (SSRRAC) substantially reduced carbon emissions compared to ordinary Portland cement (OPC), while maintaining competitive economic costs. This study's innovations include: (1) synergistic optimization of low-carbon AAC performance using NaOH-Na2SiO3 and NS; (2) optimized PPF/RCF formulations promoting the reuse of waste carbon fiber; and (3) application of the Weibull model to overcome conventional statistical constraints. Collectively, these findings establish a theoretical and practical foundation for the global development of sustainable building materials.
When uranium heap leaching tailings (UHLT) are used as filling aggregates, their discontinuous and non-uniform grading characteristics can easily cause segregation, settlement of the filling slurry, and deterioration of cemented body mechanical properties, seriously affecting the safety of the filling system and filling quality. To address the bimodal distribution defects of UHLT, characterized by excessively high proportions of coarse and fine particles with a lack of intermediate particle sizes, this study simulated its particle size characteristics using inert materials such as loess, fine sand, sand, and gravel. The study systematically verified the impact of grading defects on flow stability and mechanical properties. The filling slurry exhibited a spread of 222.5 mm with obvious segregation, and the uniaxial compressive strength at 28 days was 9.09 MPa. To overcome this bottleneck, this research innovatively proposed optimization strategies of qualitative reconstruction (QLR) and quantitative reconstruction (QTR). QLR involves adding medium-sized particles in stages and replacing equal amounts of coarse and fine particles, reducing the spread to 202.7 mm under an optimized quantity of 50 g, with a uniaxial compressive strength of 6.84 MPa at 3 days. However, slurry segregation still occurred. QTR established a multi-particle-size independent calculation model based on the extended Talbot gradation theory, and through the staged quantitative reconstruction of UHLT with aggregate having a grading index of 0.4, the spread decreased to 168.4 mm without segregation, achieving a uniaxial compressive strength of 5.58 MPa at 3 days and 9.11 MPa at 28 days. The study shows that both QLR and QTR can effectively improve the grading of UHLT, with QLR being simple and QTR offering precise control. The research provides new approaches for regulating filling slurries with similar discontinuous and non-uniform graded aggregates, and its innovative methodology can be extended to multiple fields such as concrete aggregate optimization.
The rapid depletion of natural aggregate resources has led to the exploration of recycled aggregates as sustainable alternatives. The steel industry annually generates 28 million tons of magnesia-based waste refractories (WMRs), making their incorporation into construction materials a potential strategy for resource conservation. However, WMR recycling poses a challenge because of its susceptibility to volume expansion during hydration. This study evaluated the feasibility of an environmentally friendly additive, lignosulfonate (LS), for stabilizing crushed waste magnesia refractory bricks (CWMR) to explore the potential application of WMR as construction aggregates. The swelling properties, including the free swell index (FSI) and the swell pressure (Ps), and mechanical properties including unconfined/uniaxial compressive strength (qU), shear wave velocity (VS), and thermal conductivity (lambda) of LS stabilized CWMR (CWMLS) were evaluated over different curing periods at varying LS contents (LSc). Hydration transformed CWMR from sandlike to highly plastic silt-like, resulting in a significant FSI of 250 % and Ps of 5.2 MPa. LS effectively stabilized CWMR, as indicated by decreased FSI and Ps, and enhanced qU and VS. Microscopic observation and mineralogy analyzes confirmed that LS stabilizes CWMR by adsorbing onto its surface. Stabilization of thermal conductivity at higher LSc over curing periods further supports these interactions. Macroscopic behavioral analyzes give stabilized effect of 94.3 % at LSc = 5 % with minimal improvement at higher LSc. These findings highlight LS as a promising stabilizer for mitigating hydration-induced expansion and improving the mechanical properties of CWMR, supporting its application as a recycled aggregate in construction.
Soil aggregate stability and pore structure are key indicators of soil degradation. Waves generated by the water-level fluctuations could severely deteriorate soil aggregates, which eventually induce soil erosion and several other environmental issues such as sedimentation and flooding. However, due to limited availability of the hydrological alteration data, there is a limited understanding of soil aggregates, intra-aggregate pore dynamics, and their relationships under periodically flooded soils. The present study has relied on long-term hydrological alteration data (2006-2020) to explore the impacts of inundation and exposure on soil aggregates and pore structure variations. Soil samples from increasing elevations (155, 160, 163, 166, 169, and 172 m) in the water-level fluctuation zone of the Three Gorges Reservoir were exposed to wet-shaking stress and determined soil structural parameters. The overall inundation and exposure ratio (OvI/E) gradually decreased from 1.87 in the lowest to 0.27 in the highest elevation, respectively. Predominant distribution of macropores was recorded in lower elevations, while micropores were widely distributed in the upper elevations. The mean weight diameter (MWD) was significantly lower in the lower (2.4-3.7 mm) compared to upper (5.3-6.0 mm) elevations. The increase in MWD has increased the proportion of micropores (PoN < 50 mu m), with R-2 = 0.59. This could suggest that the decrease in flooding intensity can create favorable conditions for plant roots growth. The strong flooding stress in lower elevations (i.e., higher values of the OvI/E) accelerated the disintegration of soil aggregates and considerably increased the formation of macropores due to slaking and cracking. The findings of the present study emphasize the need to restore degraded soils in periodically submerged environments by implementing vegetation restoration measures. This could enhance and sustain aggregate stability, which was also proved to increase functional pores under hydrological alterations.
In this experimental study, comprehensive laboratory tests were conducted to investigate the mechanical properties of tire-derived aggregate (TDA) Type A and TDA-soil mixtures applicable in the construction of drainage layer, embankment fill, and backfill materials for retaining walls, pipes, and bridge abutments. This study was an investigation of the mechanical properties of TDA, as a lightweight material, and TDA-fine-grained soil mixtures for different mix ratios of 15%, 20%, 35%, 40%, 50%, and 60% of TDA-A relative to the dry weight of the soil. Various composite samples were tested using triaxial and direct shear apparatus. Measured properties include specific gravity, Proctor maximum dry density and optimal water content, unconfined compressive strength, peak compressive strength, shear strength, and hydraulic conductivity. Test results revealed that the addition of TDA to the soil significantly improved the compressive strength under confinement and permeability of the composite specimens. Based on the test results and supporting data from intensive literature reviews, the TDA-soil mixture showed very encouraging results for use in civil engineering applications as a lightweight backfill material.
PurposeThis study aims to investigate the effects of root exudates on the aggregate stability and permeability of loess and to further reveal the mechanisms of vegetation in preventing and controlling soil erosion beyond mechanical effects.Materials and methodsWetting tests were conducted to investigate how loess aggregate stability varies with curing time and root exudate concentration; and infiltration tests were carried out to examine the influence of root exudates on the infiltration characteristics of loess with varying degrees of compaction.Results and discussionThe results showed that the stability of loess aggregates significantly increased due to the application of root exudates. Curing could enhance the stabilizing effects of root exudates on loess aggregates; however, there existed a critical curing duration. The application of root exudates reduced the stable infiltration rate and hydraulic conductivity of loess. However, untreated specimens under lower degrees of compaction exhibited lower stable infiltration rate and hydraulic conductivity due to local structural damage. The stable infiltration rate of both treated and untreated specimens decreased with curing time.ConclusionsThe effects of root exudates can be attributed to their ability to function as stabilizing agents and promote aggregation, due to their high adsorption capacities and negatively charged groups on their surfaces. On the other hand, the presence of root exudates can significantly enhance the soil microbial activity, the microorganisms and their hyphae further strengthen the soil structure, fill pores and increase the soil hydrophobicity, thereby improving the aggregate stability while reducing the soil permeability.
The cement-stabilization technique is employed on natural and recycled granular materials to improve their mechanical properties. The strength of these materials is assessed by the unconfined compressive strength on laboratory compacted specimens, typically after 7 days of curing. Standards and technical specifications specify different values of specimen height and diameter and different loading modes of testing. This makes the comparison between different materials and with the acceptance limits of technical specifications difficult. The research investigates the effect of specimen size and loading mode on the unconfined compressive strength of both natural and recycled cement-stabilized granular materials. The results revealed significant differences in strength due to variations in specimen size and loading mode. As expected, an increase in specimen slenderness resulted in a decrease in compressive strength. A linear regression model was developed to quantify the effect of the experimental variables on the compressive strength of the two cement-stabilized materials.
Enhancing the structural stability of Pisha sandstone soil is an important measure to manage local soil erosion. However, Pisha sandstone soil is a challenging research hotspot because of its poor permeability, strong soil filtration effect, and inability to be effectively permeated by treatment solutions. In this study, by adjusting the soil water content to improve the spatial structure of the soil body and by conducting unconfined compressive strength and calcium ion conversion rate tests, we investigated the effect of spatial distribution differences in microbial-induced calcium carbonate deposition on the mechanical properties of Pisha sandstone-improved soil in terms of the amounts of clay dissolved and calcium carbonate produced. The results demonstrate that improving the soil particle structure promotes the uniform distribution of calcium carbonate crystals in the sand. After microbial-induced carbonate precipitation (MICP) treatment, the bacteria adsorbed onto the surface of the Pisha sandstone particles and formed dense calcium carbonate crystals at the contact points of the particles, which effectively enhanced the structural stability of the sand particles, thereby improving the mechanical properties of the microbial-cured soils. The failure mode of the specimen evolved from bottom shear failure to overall tensile failure. In addition, the release of structural water molecules in the clay minerals promoted the surface diffusion of calcium ions and accelerated the nucleation and crystal growth of the mineralization products. In general, the rational use of soil structural properties and the synergistic mineralization of MICP and clay minerals provide a new method for erosion control in Pisha sandstone areas.