Search Results (1,177)

Fiber-reinforced concrete (FRC) has become increasingly important in recent decades due to its superior mechanical properties, especially flexural strength and toughness, compared to normal concrete. FRC has also received significant attention because of its superior fire resistance performance compared to non-fiber concrete. In recent years, studies on the mechanical performance, fire design, and post-fire repair of thermally damaged fibrous and non-fibrous concrete have gained importance. In particular, there are very few studies in the literature on the mechanical performance and flexural behavior of steel and basalt hybrid fiber concretes after high temperature and water re-curing. This study aims to determine the mechanical properties and toughness of concrete containing steel fiber (SF) and basalt fiber (BF) after ambient and high temperature (400 °C, 600 °C, and 800 °C). Additionally, this study aimed to examine the changes in fire-damaged FRCs as a result of water re-curing. In this context, high temperature and water re-curing were carried out on non-fibrous concrete (control) and four different fiber compositions: in the first mixture, only steel fibers were used, and in the other two mixtures, basalt fibers were substituted at 25% and 50% rates instead of steel fibers. Furthermore, in the fifth mixture, basalt fibers were replaced by polypropylene fibers (PPFs) to make a comparison with the steel and basalt hybrid fiber-reinforced mixture. This study examined the effects of different fiber compositions on the ultrasonic pulse velocity (UPV) and compressive and flexural strength of the specimens at ambient temperature and after exposure to elevated temperatures and water re-curing. Additionally, the load–deflection curves and toughness of the mixtures were determined. The study results showed that different fiber compositions varied in their healing effect at different stages. The hybrid use of SF and BF can improve the flexural strength before elevated temperature and particularly after 600 °C. However, it caused a decrease in the recovery rates, especially after re-curing with water in terms of toughness. Water re-curing provided remarkable improvement in terms of mechanical and toughness properties. This improvement was more evident in steel–polypropylene fiber-reinforced concretes. Full article

The impact of introducing a nonwoven polyamide PA 12-E material on the mechanical properties of polymer composite materials based on epoxy autoclave prepreg T107 has been investigated. This study demonstrates that the incorporation of nonwoven fabric does not lead to a decrease in the mechanical properties of the composites. A significant advantage of composites reinforced with nonwoven fabric is their enhanced impact resistance. During a free impact with an energy of 6.67 J per 1 mm of the sample, complete breakdown with fiber destruction occurs in samples without nonwoven material. In contrast, samples containing nonwoven material exhibit damage characterized by stratification without compromising the fibers. The compressive strength after impact increased from 260 to 320 MPa with the addition of nonwoven material. Consequently, the proposed modification of the commercial prepreg will expand the material’s range of applications and enhance safety, particularly in aircraft structures. Full article

Exposure to elevated temperatures leads to the deterioration of the mechanical properties of cementitious materials. However, the inclusion of fibers can mitigate, to some extent, the negative effects of high temperatures on these materials. Specifically, polypropylene (PP) fibers, a synthetic fiber type, have been demonstrated to improve the performance of cement-based composites. Therefore, it is essential to investigate the impact of temperature on the behavior of fiber-reinforced mortar for its broader application in construction. This study explores the effects of varying PP fiber contents (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1%) and different temperature exposures (25 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C) on the performance of cement mortar. The experimental results show that elevated temperatures significantly degrade both the mechanical and thermal properties of fiber-reinforced mortar. As the temperature and fiber content increase, both the quality and thermal conductivity of the mortar decrease. Between 25 °C and 200 °C, the incorporation of PP fibers (ranging from 0% to 0.2%) significantly enhances the compressive and flexural strengths of the mortar. However, this improvement becomes less pronounced as the fiber content exceeds 0.2%. At temperatures above 200 °C, further increases in temperature, coupled with higher fiber contents, consistently lead to a reduction in the compressive and flexural strengths. Based on the principles of continuous damage mechanics (which describes the degradation and fracture of materials under loading) and the dual-parameter Weibull distribution theory, a constitutive model is proposed to describe the damage behavior of high-temperature PP-fiber-reinforced mortar under uniaxial compressive stress. Full article

The flexural behavior of geogrid-reinforced foamed lightweight soil (GRFL soil) is investigated in this study using unconfined compressive and four-point bending tests. The effects of wet density and reinforcement layers on flexural performance are analyzed using load–displacement curves, damage patterns, load characteristics, unconfined compressive strength, and flexural strength. A variance study demonstrates that increasing the wet density significantly increases unconfined compressive strength. Bond stress mechanisms enable geogrid integration, efficiently reroute stresses internally, and greatly increase flexural strength. With a maximum unconfined compressive strength of 3.16 MPa and a peak flexural strength increase of 166%, this reinforcement increases both strength and ductility by changing the damage pattern from brittle to ductile. The principal load is initially supported by the foamed lightweight soil, and in later phases, geogrids take over load-bearing responsibilities. Additionally, the work correlates the ratio of unconfined compressive to flexural strength with wet density and informs the development of predictive models for unconfined compressive strength as a function of reinforcing layers and wet density. Full article

Pile foundation failures during earthquakes can cause severe structural damage, emphasizing the importance of accurate strength evaluation. This study focuses on concrete-filled steel pipe pile heads with inner ribs, which play a crucial role in resisting compressive loads. Compression tests were conducted on specimens simulating pile heads to investigate stress transfer between the steel pipe and infill concrete. A numerical analysis model was developed using ABAQUS 6.14 and validated against experimental results, successfully reproducing load-deformation relationships and stress transfer mechanisms. Simulations extended the study by analyzing the bearing strength of the infill concrete under rib-induced pressure, with varying diameter-to-thickness ratios D/t. The results show that the compressive strength is primarily governed by the combined effects of steel pipe buckling resistance and concrete bearing resistance of a single layer of inner ribs. The proposed evaluation formula provides a lower-bound estimate of compressive strength and effectively captures key parameters influencing performance. Full article

China has an existing building area of 80 billion square meters, where reinforced concrete structures have a large quantity and a wide surface area. The risk of structures being subjected to blast loading is relatively high. Reactive powder concrete has the specialties of ultra-high toughness, super strength, and a high strength to ponderance ratio. Reinforced concrete (RC) structures strengthened by RPC are called RPC-RC structures, which can easily elevate the explosive load resistance of building structures while also strengthening the building. It is a significant method used in avoiding the collapse of structures under explosive loads. The dynamic reaction and damage evaluation approaches of RPC-RC columns under explosive load have not been deeply studied. For addressing this issue, numerical simulation of RPC strengthened RC columns under explosive load was carried out by LS-DYNA (R10), and the correctness of the numerical simulation was verified by comparing it with relevant experimental results. In this paper, a finite element model of an RPC-RC column was established, and the main factors affecting the anti-explosion performance of an RPC-RC column were studied. The influence of the RPC reinforcement layer parameters (RPC thickness, RPC strength, longitudinal reinforcement ratio, and stirrup ratio) on the dynamic reaction and damage degree of RPC-RC columns was examined. The consequences indicated that the failure mode of the columns after RPC reinforcement can alter from bending shear damage to bending damage. As the thickness and strength of the RPC increases, the longitudinal reinforcement ratio increases, the stirrup ratio increases, and the maximum horizontal deformation of the center point of the RPC reinforced RC columns decreases. For RPC-RC columns with a height of 3–4 m and a width of 300–400 mm under blast loading, columns with an axial compression ratio greater than 0.3 will collapse, while columns with an axial compression ratio less than 0.3 are less likely to collapse. In the light of the calculation outcomes, a formula for reckoning the damage index of RPC-RC columns was proposed, taking into account factors such as proportional distance, axial compression ratio, RPC thickness, longitudinal reinforcement ratio, and stirrup ratio. Full article

Red mud is a kind of solid waste in the production process of the aluminum industry. The long-term stockpiling of red mud not only occupies a large amount of land but also causes environmental pollution. In order to improve the strength, reduce the alkalinity and toxicity of red mud, and study its durability under freeze–thaw cycles, CGFPA binders, whose components were calcium carbide residue, ground granulated blast furnace slag, fly ash, phosphogypsum, and graphene, were adopted to solidify/stabilize red mud in this paper. The effects and the mechanism of freeze–thaw cycling on the unconfined compressive strength, pH value, and toxic leaching of the solidified/stabilized red mud was investigated. The micro-mechanism was analyzed by XRD, SEM-EDS, and FT-IR. The results of the study showed that the mass, unconfined compressive strength, and pH of the solidified/stabilized red mud decreased gradually with an increase in the number of freeze–thaw cycles, while the leaching concentration of pollutants increased gradually. The rate of loss of unconfined compressive strength satisfies an exponential function with the number of cycles, and the logarithm of pollutant concentration satisfies a linear relationship with the number of cycles. The cumulative loss of mass was 6.7%, 5.4%, 3.6%, and 3.3%, and the cumulative loss of unconfined compressive strength was 50.6%, 47.5%, 32.2%, and 25.3%, and the pH value was reduced to 9.42, 9.54, 9.80, and 9.92, respectively, after 10 freeze–thaw cycles at binder mixing ratios of 15%, 20%, 25%, and 30%, while the leaching concentrations of Cu, Zn, Cr, Ni, As, Pb, and Cd increased from 7.4 μg/L, 87.2 μg/L, 5.2 μg/L, 7.0 μg/L, 6.9 μg/L, 3.7 μg/L, and 0.7 μg/L to 17.5 μg/L, 123.5 μg/L, 10.2 μg/L, 15.7 μg/L, 11.4 μg/L, 5.6 μg/L, and 4.9 μg/L, respectively, under the condition of a 30% incorporation ratio. The gelling products generated by the hydration reaction of the binders were mainly C-S-H, C-A-S-H, C-A-H, AFm, etc. Under the action of freeze–thaw cycles, the lattice-like structure of the solidified/stabilized red mud was damaged, resulting in a decrease in its unconfined compressive strength and an increase in pollutant leaching concentration. The research results can provide a theoretical basis for the use of red mud in permafrost regions. Full article

The excavation of the rock mass at the tunnel entrance in regions characterized by high altitudes and elevated stress levels results in the direct exposure of the surrounding rock to atmospheric conditions. This surrounding rock is subjected to the compounded effects of excavation-induced unloading damage and freeze–thaw erosion, which contribute to the degradation of its mechanical properties. Such deterioration has a negative impact on production and construction operations. Following tunnel excavation, the lateral stress exerted by the surrounding rock at the tunnel face is reduced, leading to a predominance of uniaxial compressive stress. As a result, the failure mode and mechanical behavior of the rock exhibit characteristics similar to those observed in uniaxial loading tests conducted in controlled laboratory environments. This study conducts laboratory-based uniaxial loading and unloading tests, as well as freeze–thaw tests, to examine the strength, deformation characteristics, and fracture attributes of unloading sandstone subjected to freeze–thaw erosion. A damage deterioration model for unloading sandstone under uniaxial conditions is developed, and the patterns of damage response are further analyzed through the identification of compaction points and the definition of damage response points. The results indicate that (1) as the degree of freeze–thaw erosion increases, the failure threshold of the sandstone significantly decreases, with the residual rock fragments on the fracture surface transitioning from hard and sharp to soft and sandy; (2) freeze–thaw erosion has a pronounced negative impact on the cohesion of the sandstone, while the reduction in the internal friction angle is relatively moderate; and (3) the strain induced by damage following three, six, and nine freeze–thaw cycles exhibits a gradual decline and appears to reach a state of stabilization when compared to conditions without freeze–thaw exposure. Investigating the mechanical properties and deterioration mechanisms of the rock in this specific context is crucial for establishing a theoretical foundation to assess the stability of the tunnel’s surrounding rock and determine the necessary support measures. Full article

Understanding the mechanical and physical behavior of aged CPB under cyclic loading is a significant area of research. Many parameters such as cementation (hydration) and the microstructure, which dictate the arrangement of particles and permeability, affect the mechanical features of cemented paste backfill (CPB). The impact of a wide range of external energy sources within the mining environment, such as cyclic loading resulting from long-term blasting, can significantly alter the applied stresses on the backfill mass. This paper aims to delve into this crucial area of research. A series of uniaxial cyclic tests were conducted on CPB, utilizing samples made from tailing materials sourced from a copper mine in South Australia. Different loading levels were applied at various curing times. All samples exhibited cyclic loading hardening behavior for cyclic loading levels between 80% and 93% of monotonic unconfined compressive strength (UCS), and a cyclic loading damage behavior was observed for 96% of UCS loading level for both 14- and 28-day curing periods. To further investigate these findings, scanning electron microscope analysis as well as sonic velocity tests were conducted for capturing microstructural changes in the samples before and after tests. These findings can be used to indicate a safe firing distance to a filled mass. Full article

To enhance the improvement effect of Enzyme-Induced Carbonate Precipitation (EICP) technology more effectively, an abundant renewable resource—lignin—was introduced as an additive during the EICP modification process of silty clay. The mechanical properties of the improved soil specimens were analyzed from a macroscopic point of view by using unconsolidated undrained (UU) triaxial tests and unconfined compressive strength (UCS) tests to determine the optimal lignin content and curing time. The micro-mechanism of the improved soil specimens was elucidated from the microscopic point of view by combining scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The experimental results showed that lignin synergized with EICP could effectively improve the mechanical properties of the soil, and the mechanical properties of the co-consolidated soil specimens were better than those of the single consolidated and untreated soil specimens as a whole. The single EICP-consolidated soil specimen had undergone brittle damage; lignin could enhance the toughness of the soil and weaken its brittle characteristics. With the increase of lignin content, the mechanical indicators of co-consolidated soil specimens showed the trend of increasing and then decreasing, and reached the optimum at 0.75%. Moreover, the addition of lignin significantly increased the cohesive force, while the friction angle was less affected. With extended curing time, the mechanical indicators of the co-consolidated soil specimens increased overall, and tended to stabilize after 7 days of curing, hence selecting 7 days as the optimal curing time. From the microscopic point of view, lignin provides nucleation sites for the calcium carbonate precipitates generated by EICP, and the joint action of the two can fill the soil pores and cement the soil particles, thereby improving the overall strength of the soil. The results of the study can provide a theoretical basis and practical reference for the construction of foundation projects in silty clay areas. Full article

Heavy metal powders driven by explosions can enhance the near-field lethality of explosive warheads by forming a quasi-pressure field while reducing collateral damage at medium and long ranges. Incorporating polymers into high-content metal powders prevents powder sintering under explosive high pressure, enhancing dispersion uniformity and making them promising for controllable warhead applications. To describe the mechanical behavior of materials under impact loading, this paper investigates the dynamic and static mechanical properties and constitutive modeling of tungsten powder/polytetrafluoroethylene (PTFE) composites. Quasi-static compression tests and split Hopkinson pressure bar (SHPB) dynamic tests were conducted on composites with varying tungsten contents (0 wt%, 70 wt%, 80 wt%, and 90 wt%) and particle sizes (200 μm, 400 μm, and 600 μm), obtaining compressive stress–strain curves over a strain rate range of 0.001 to 3610 s⁻¹. The compressive strength of the composites slightly decreased with increasing tungsten particle size but increased with higher tungsten content. Under quasi-static compression, the compressive strength of the composites with 70 wt% and 80 wt% tungsten was lower than that of pure PTFE. This was due to the bonding strength between the tungsten particles and the resin being weaker than the cohesion within the resin. Additionally, the random distribution of the tungsten particles in the matrix led to shear cracks propagating along the phase interfaces, reducing the compressive strength. The compressive strength of the composites with 90 wt% tungsten exceeded that of pure PTFE, as the packed arrangement of the tungsten particles increased the material strength through particle extrusion and friction during compression. Under dynamic impact, the compressive strength of the composites was higher than that of pure PTFE, primarily due to particle extrusion and friction effects. The composites exhibited significant strain rate sensitivity, with both the compressive strength and critical strain increasing quasi-linearly with the strain rate. Based on the experimental data, a damage-modified Zhu–Wang–Tang (ZWT) viscoelastic model was employed to fit the data, effectively characterizing the uniaxial compressive constitutive behavior of tungsten powder/PTFE composites. Full article

Due to its high axial bearing capacity and good ductility, the embedded steel plate composite shear wall structure has become one of the most widely used lateral force-resisting structural members in building construction. However, bending failure is prone to occur during strong earthquakes, and the single energy dissipation mechanism of the plastic hinge zone at the bottom leads to the concentration of local wall damage. To improve the embedded steel plate composite shear wall structure, the plastic hinge zone of the composite shear wall is replaced by fiber-reinforced concrete (FRC) and analyzed by ABAQUS finite element simulation analysis. Firstly, the structural model of the embedded steel plate composite shear wall structure with FRC in the plastic hinge zone is established and the accuracy of the model is verified. Secondly, the effects of steel ratio, longitudinal reinforcement ratio, and FRC strength on the bearing capacity of composite shear walls are analyzed by numerical simulation. Finally, a method for calculating the embedded steel plate composite shear wall structure with FRC in the plastic hinge zone is proposed. It is shown that the displacement and load curves and failure modes of the model are basically consistent with the experimental results, and the model has high accuracy. The axial compression ratio and FRC strength have a great influence on the bearing capacity of composite shear walls. The calculation formula of the normal section bending capacity of the embedded steel plate composite shear wall structure with FRC in the plastic hinge zone is proposed. The calculated values of the bending capacity are in good agreement with the simulated values, which can provide a reference for its engineering application. Full article

Corrosion of traditional cement mortar is a critical issue in karst areas. Composite salt, i.e., sulfate–chloride salt, represents a typical corrosion agent due to the abundance of Cl⁻ and SO₄²⁻ ions in such geological environments. In this study, we used nano-metakaolin to enhance the physical and mechanical properties of cement mortar in the early aging stages, simulating groundwater corrosion by a compound salt solution in the karst region. The appearance and the change in the flexural/compressive strength of cement mortar upon the nano-metakaolin addition in the early aging stages under dry and wet cycling conditions were analyzed and combined with the results of scanning electron microscopy, thermogravimetric analysis, and other methods, revealing the underpinning mechanism behind the function changes of nano-metakaolin-modified cement mortar. The results show that nano-metakaolin effectively promotes cement hydration in the early aging stages. The flexural/compressive strength after 7 days of aging with 1% of added nano-metakaolin increased by 10.38% and 4.41%, respectively, compared to ordinary cement mortar. Furthermore, adding 1–5% of nano-metakaolin under dry and wet cycling and the coupling effect of chloride and sulfate erosion effectively reduce the damage of harmful ions on the cement mortar, leading to evident corrosion inhibition. The generation of hydration products increased after adding the Ghanaian metakaolin, filling the microcracks and micropores, and increasing the overall microstructural compactness. Full article

Despite decades of extensive studies, the mechanism of concrete creep remains a subject of debate, mainly due to the complex nature of cement microstructure. This complexity is further amplified by the interplay between water and the cement microstructure. The present study aimed to better understand the creep mechanism through creep tests on microprisms of cement paste at hygral equilibrium. First, microprisms with dimensions of 150 mm × 150 mm × 300 mm were prepared by precision cutting from a cement paste specimen with a water-to-cement ratio of 0.4. Subsequently, uniaxial compression and creep tests were carried out on these microprisms in a chamber with controlled relative humidity (RH). To mitigate the impact of plasticity and damage, the applied peak load was set to generate a stress level that was approximately 40% of the compressive strength. Moreover, an analytical coefficient φ was formulated to account for the foundation effect on microprism creep, agreeing with the numerical analysis employing the finite element method. Our findings showed that the microscale creep compliance varied when the RH level was changed from 90% to 11%. Furthermore, logarithmic and power-law models were both applied to simulate creep curves. Lastly, the modeled creep behaviors were compared with those obtained by microindentation experiments in previous studies. Full article

Ensuring the mechanical performance of backfill materials while reducing cementation costs is a key challenge in mine backfill research. To address this, fiber materials such as polypropylene (PP) fiber and rice straw (RS) fiber have been incorporated into cement-based mixtures for mine backfilling. This study investigates the effects of PP and RS fibers on the mechanical properties, flow characteristics, and microstructure of Tailings and Wasted Stone Mixed Backfill (TWSMB). A series of orthogonal experiments were designed to evaluate the influence of variables, including the cement–sand ratio, solid mass concentration, wasted stone mass concentration, fiber content, and fiber length on the TWSMB properties. The results indicate that the influence of cement–sand ratio and solid mass concentration have a more significant impact on strength than fibers, though the fibers show a stronger effect than the wasted stone mass concentration. Both fiber types enhanced the strength of the specimens, with PP fiber exhibiting a stronger reinforcing effect than RS fiber. Furthermore, the effect of PP fiber content was more pronounced than that of fiber length, whereas the opposite trend was observed for RS fiber. The optimum fiber parameter levels were determined for each type: PP fiber performed best at a mass concentration of 1.5% and a length of 6 mm, while RS fiber showed optimal performance at a mass concentration of 1.0% and a length of 5–10 mm. Macroscopic damage analysis indicated that the structural integrity and residual compressive strength of the TWSMB specimens were preserved even after surpassing the ultimate compressive strength, due to the crack-bridging effect of the fibers. Microstructural analysis showed that PP fiber-reinforced specimens exhibited a dense structure formed through reactions with other hydration products. In contrast, the surface of RS fibers was nearly fully encapsulated by hydration products, resulting in the formation of a physical skeleton structure. This study provides new insights into minimizing cement consumption and reducing backfilling costs in mining operations. Full article