Search Results (691)

Crack control and deformation performance are critical to the serviceability of concrete box girders. This study investigates the structural behavior of concrete box girders reinforced with polypropylene fibers through four-point bending tests and clarifies the influence of fiber volume fraction on cracking, deformation, and strain responses. The results show that polypropylene fibers effectively reduce the number of cracks, maximum crack width, and equivalent crack area of the box girders, while improving crack closure. At the service-level load of 0.7 P, increasing the fiber volume fraction from 0 to 0.3% reduced the number of cracks from 11 to 4, the maximum crack width from 0.16 mm to 0.12 mm, and the equivalent crack area from 653.97 mm² to 177.50 mm². The initial stiffness of the box girders did not exhibit a monotonic relationship with fiber volume fraction; however, at higher load levels, all fiber-reinforced specimens exhibited greater secant stiffness than the plain-concrete specimen. At a load of 315 kN, compared with the specimen without fibers, the specimen containing 0.2% polypropylene fibers showed an increase in secant stiffness from 15.021 kN/mm to 20.875 kN/mm, accompanied by a reduction in mid-span displacement from 20.971 mm to 15.090 mm. Fiber content also affected the transverse and longitudinal surface strains of the bottom slab at the mid-span section, as well as the longitudinal strains of the webs, with these effects becoming more evident at higher load levels. Overall, a fiber volume fraction of 0.2% provided a favorable balance between crack control and high-load deformation response, whereas 0.3% exhibited the strongest crack-control performance. Full article

This study investigates the influence of controlled fiber orientation on the mechanical and microstructural properties of cellulose excelsior fiber cementitious composites with an excelsior content of 75% by volume. Two different sets of composites were fabricated. In one set, no effort was made to orient the fibers, while in the other set fibers were preferentially aligned through manual placement and compressive consolidation. The mechanical performance, including elastic modulus and flexural strength, was evaluated. The internal structure, specifically porosity and 3D fiber orientation, was quantified using X-ray Computed Tomography (XCT) and subsequent 3D image analysis. The results demonstrate that the composites with aligned fibers exhibited a 20% higher bulk density and a significantly lower porosity (5.1%) compared to the non-aligned composites (9.0%), representing a 43% reduction in void volume. Further image analysis showed distinct differences in fiber orientation relative to the axis of the specimen. These different distributions led to a 120% increase in elastic modulus and a 58% increase in flexural strength. These results demonstrate how, within limits, the mechanical properties of the composite system can be controlled to meet application demands. Full article

This study investigates the effects of low-volume steel microfibers (0–0.4 vol.%) and superplasticizer (SP) dosage (0.8 wt.% and 1.5 wt.%) on the mechanical and durability properties of self-compacting concrete (SCC) for railing structures, with a constant water-binder ratio of 0.28. Fresh (slump flow), mechanical (compressive strength up to 90 days, 28-day flexural strength), durability (drying shrinkage, freeze–thaw resistance after 200 cycles), and microstructural (mercury intrusion porosimetry) properties were evaluated. SP enhances flowability while steel fibers reduce it. All mixtures except that with 0.8% SP and 0.4% fibers meet the workability requirements of Chinese standard JGJ/T 283-2012 for SCC. Compressive and flexural strengths generally increase with fiber content but decrease when the SP dosage rises from 0.8% to 1.5%. Steel fibers effectively reduce drying shrinkage and improve freeze–thaw resistance, as indicated by higher relative dynamic elastic moduli and lower mass loss after 200 cycles. Microstructural analysis reveals that the higher SP dosage (1.5 wt.%) significantly increases porosity, which explains the observed higher shrinkage and lower strength. Considering mechanical properties, durability, and castability, the SCC mixture with 0.3 vol.% steel fibers and 0.8 wt.% SP is recommended for railing structure applications. Full article

To solve the problems of explosive spalling and sharp deterioration of mechanical properties of ultra-high performance sprayed concrete (UHPS) under high-temperature conditions during tunnel fires, four groups of specimens with different volume contents (0%, 0.2%, 0.3%, 0.4%) of polypropylene fiber (PPF) were designed based on the optimal mix proportion at room temperature. Multi-gradient high-temperature tests at 20 °C, 200 °C, 400 °C, 600 °C and 800 °C were conducted to explore the effects of PPF on the high-temperature damage evolution and spalling resistance of UHPS. The test results show that no obvious spalling occurs in specimens exposed to temperatures of 400 °C and below. Surface peeling appears in the group without PPF addition at 400 °C, and severe explosive spalling happens in the 0% PPF group at 600 °C to 800 °C, while all PPF-incorporated groups maintain structural integrity. The mass loss rate increases with the rise in temperature and PPF content, reaching 15% in the 0.4% PPF group at 800 °C. In terms of mechanical properties, compressive strength, splitting tensile strength, and flexural strength all rise first and then decline with increasing temperature, and the 0.3% PPF group reaches peak values at 400 °C (compressive strength: 135.95 MPa, splitting tensile strength: 23.24 MPa, flexural strength: 27.42 MPa). Flexural toughness decreases continuously as temperature rises, and the 0.2% PPF group exhibits the best toughness retention. This study clarifies the high-temperature modification effect of PPF on UHPS and its optimal content range, providing important theoretical support and an experimental basis for the fire safety protection design of tunnel lining concrete. Full article

This study investigates the direct tensile mechanical behavior of cement mortar plates reinforced with a galvanized steel mesh and randomly incorporated alkali-resistant glass fibers. An experimental program was executed using direct tensile tests on thin mortar specimens containing fiber volumetric fractions of 0%, 4%, 6%, 8%, and 10% relative to the cement volume. To rigorously characterize the mechanical response, the study quantified the apparent initial stiffness, 0.2% offset stress, ultimate tensile strength, and post-offset energy absorption capacity. Results indicate that increasing alkali-resistant glass-fiber content systematically modified the global tensile response of the composite system. At 10% glass-fiber content, the mean crosshead-derived apparent initial tensile stiffness was 10.43 times that of the reference group without glass fibers. The characteristic stress determined using the adopted 0.2% offset criterion and the ultimate tensile strength increased by 154.5% and 74.1%, respectively, while the apparent post-offset energy absorption increased by 68.4%. Because strain was derived from crosshead displacement, the apparent stiffness and energy-absorption parameters represent the global specimen–grip–machine response rather than intrinsic material properties. The experimental results exhibited acceptable repeatability, although the apparent tensile stiffness showed greater variability than the strength-related parameters. These findings support the continued development of the investigated composite configuration for thin cementitious elements requiring improved tensile response and damage tolerance. Full article

This study investigates the strength development, crack evolution and toughening mechanism of basalt fiber-reinforced desert sand high-strength concrete. An L9(3³) orthogonal design was first used to optimize the reference mixture, after which basalt fibers with volume fractions of 0, 0.3%, 0.4% and 0.5% were incorporated. Mechanical testing, digital image correlation, SEM, XRD, TG and FTIR were combined to clarify the relationship among fiber dosage, crack propagation and microstructural reinforcement mechanisms. The optimized matrix mixture was obtained with a water-to-binder ratio of 0.32, a desert sand replacement ratio of 40% and a fly ash content of 20%. The incorporation of basalt fiber had little influence on the 28 d compressive strength, whereas the splitting tensile strength was markedly improved. The highest splitting tensile strength was observed in the 0.4% fiber group, reaching 5.46 MPa, which was 12.81% higher than that of the reference mixture. DIC results showed that basalt fiber reduced strain localization and limited crack opening. The 0.5% group had the lowest COD, while the 0.4% group showed a better balance among tensile strength, strain redistribution and crack-opening control. SEM observations showed fiber bridging and fiber–matrix interaction near the fracture region. Meanwhile, XRD, TG-DTG and FTIR showed no obvious changes in the main phases or functional groups, indicating that the improvement was mainly related to the physical crack-control effect of basalt fibers rather than chemical modification of the matrix. Overall, 0.4% basalt fiber was identified as the preferred dosage for the present system. Full article

In the manufacturing of carbon fiber/epoxy composites, different stacking sequences have different effects on the void inside materials. In this paper, the double-vacuum-bag (DVB) process was utilized to fabricate laminates with different stacking sequences, including different angles, different thicknesses and plain weave prepregs, and the single-vacuum-bag (SVB) process was set as the control group. The void content of different laminates in the cross-section was counted by image analysis software, and material thickness, density, and fiber volume fraction were measured by experiments. Three-point bending and short-beam-shear tests were conducted to evaluate the material mechanical properties. The results show that the void contents of laminates prepared by the DVB process are all less than 1% with different stacking sequences, while the laminates manufactured by the SVB process contain a large number of voids inside. The density and fiber volume fraction of the DVB process are higher than those of the SVB process. In terms of mechanical properties, the flexural strength and interlayer-shear-strength (ILSS) of the DVB process are higher than those of the SVB process. The results of this paper expand the application of the DVB process and provide a reference for low-cost manufacturing of composite materials. Full article

Thin spray-on liner (TSL) technology provides rapid and highly automated surface support for underground coal mine roadways. However, in deep roadways affected by high in situ stress, mining-induced disturbances, and fractured surrounding rock, conventional TSL materials require improved tensile–shear resistance, deformation compatibility, and support adaptability. Although fiber reinforcement is an effective method for enhancing polymer composites, systematic studies on the effects of fiber type and dosage in reactive polymer-based TSL materials remain limited. In this study, a commercially available two-component polyurea-silicate-based TSL matrix was reinforced with polyvinyl alcohol (PVA) fibers, polypropylene mesh fibers, and toughened polypropylene fibers at volume fractions of 0.25–1.50%. A stepwise experimental program, including uniaxial compression, variable-angle shear, tensile, circular-indenter buffered shear, and true triaxial tests, was conducted to evaluate the mechanical behavior and support-related performance of the fiber-reinforced TSL materials. The basic mechanical tests showed that the 0.75% toughened polypropylene fiber group maintained favorable compressive and shear resistance, achieving a cohesion of 8.65 MPa and an internal friction angle of 24.12°. PVA fibers exhibited higher tensile reinforcement efficiency at relatively low contents, with the 0.25% PVA fiber group reaching a peak tensile stress of 13.61 ± 1.00 MPa. The 1.0% PVA fiber group showed good deformation coordination, with a compressive strength of approximately 49.87 MPa. In the circular-indenter buffered shear test, the 1.0% PVA fiber group reached a peak load of 0.636 ± 0.055 kN and an absorbed energy of 3.118 ± 0.832 J at 10 mm displacement. Under true triaxial loading, the 1.0% PVA fiber group absorbed 311.4 J of energy at a displacement of 10 mm, approximately 5.5% higher than that of the 0.75% toughened polypropylene fiber group. Therefore, 1.0% PVA fiber reinforcement is recommended as the optimal reinforcement scheme for polymer-based TSL materials used in deep, fractured, and large-deformation coal mine roadways. Full article

Fiber-reinforced high-water-resistance concrete backfill (FHWCB) is a rapid-setting cementitious backfill system used for underground support and backfilling, but its stability is strongly affected by mixture water content and fiber dispersion. This study investigated the fresh-state behavior, mechanical performance, acoustic emission (AE) fractal characteristics, b-value response, and energy evolution of FHWCB. Mixtures with water-to-cement ratios (w/c) of 1.0–1.8 and fiber volume fractions (Vf) of 0–0.5% were prepared and tested using fresh property measurements, unconfined compression, thermogravimetry, AE monitoring, correlation dimension analysis, b-value analysis, and strain energy partitioning. Increasing w/c improved flowability and delayed setting, but weakened the hydration skeleton and reduced early-age compressive strength by approximately 56–61%. Fiber reinforcement showed a non-monotonic effect: Vf = 0.3% increased compressive strength by approximately 16–26%, whereas excessive fiber addition reduced strength because of fiber clustering and weak local zones. AE amplitude sequences exhibited measurable fractal characteristics. A higher correlation dimension indicated distributed microdamage, while decreasing correlation dimension and b-value reflected the transition toward localized macrocrack growth. Energy analysis showed that the peak elastic strain energy density decreased from approximately 0.60 to 0.39 MJ/m³ as w/c increased. The proposed AE fractal–b-value–energy framework provides a quantitative basis for tracking damage progression and optimizing FHWCB for underground engineering. Full article

This study develops a finite element analysis (FEA) framework to quantify hydrostatic consolidation of ultra-high-molecular-weight polyethylene (UHMWPE) fibril arrays containing interstitial voids. Hexagonal and stochastic representative volume elements (RVEs) were constructed from experimentally measured fibril diameters and embedded in a nearly incompressible Mooney–Rivlin medium to transfer hydrostatic pressure to irregular fibril surfaces. Molecular dynamics (MD)-derived fibril properties at 300 K and 400 K were used to evaluate the effects of temperature, elastic–plastic deformation, and stochastic microstructure on void collapse and effective fiber properties. The 300 K elastic model required pressures approaching 1.3 GPa to reach approximately 1–2% void content, whereas the combined effects of elevated temperature, elastic–plastic fibril behavior, and stochastic fibril packing reduced the corresponding pressure to approximately 160 MPa. The stochastic RVE exhibited a higher initial void content, earlier plastic dissipation, lower initial effective stiffness, and nonuniform fibril–fibril contact evolution compared to the idealized hexagonal RVE. As void content decreased, both models converged toward the dense fibril response. The framework establishes a processing–microstructure–property relationship linking consolidation pressure to the evolving void morphology, fibril shape and contact development, and the resulting effective plane-strain bulk modulus and the transverse compressive stress–strain response, including the Young’s modulus and Poisson’s ratio, of UHMWPE fibers. Full article

Mechanically aerated yeast-free dough is particularly susceptible to freeze–thaw damage because its porous structure is formed before freezing and cannot be restored during thawing due to the absence of fermentation. This study investigated the combined effects of apple powder incorporation and technological processing conditions on the rheological, structural, physicochemical, nutritional, and sensory properties of whipped yeast-free frozen dough and the resulting bread. Apple powder was incorporated at three formulation levels (50, 100, and 150 g per batch), while whipping speed (450–900 rpm), whipping time (3–7 min), freezing temperature (−14 to −38 °C), and microwave thawing time (4–8 min) were optimized using response surface methodology based on a Draper–Lin composite design. Dough properties were evaluated using Mixolab analysis and structural–mechanical measurements, whereas bread quality was assessed by specific volume, porosity, physicochemical characteristics, biochemical composition, amino acid profile, microbiological safety, and sensory evaluation. The developed regression models adequately described the effects of technological variables on dough quality (R² > 0.95). Deep freezing at −38 °C followed by 4 min of microwave thawing minimized structural deterioration and improved dough stability after freeze–thaw treatment. Apple powder increased the nutritional value of the bread by enhancing the dietary fiber (4.8–7.3%), potassium (125.6–156.7 mg/100 g), iron (2.45–3.20 mg/100 g), and vitamin C (0–2.2 mg/100 g) contents. Although the highest level of apple powder provided the greatest nutritional enrichment, it also reduced the dough rheological stability and produced a less homogeneous crumb structure. Overall, the formulation containing 100 g of apple powder per batch combined with a whipping speed of 900 rpm, whipping time of 7 min, freezing at −38 °C, and microwave thawing for 4 min provided the best balance between rheological stability, freeze–thaw resistance, bread quality, nutritional enhancement, microbiological stability, and sensory acceptability. These findings demonstrate that simultaneous optimization of formulation and processing conditions is an effective strategy for improving mechanically aerated yeast-free frozen bakery products and provides a scientific basis for the development of functional frozen bread technologies. Full article

Cover cracking induced by steel corrosion is a critical issue governing the durability degradation of reinforced concrete structures in marine environments. The crack initiation and propagation processes dominated by non-uniform rust expansion stress fields urgently require high-resolution continuous monitoring techniques. In this study, based on the principle of Rayleigh backscattering, distributed optical fibers were embedded along the upper surface of specimens to conduct in situ monitoring of surface strain in concrete. The effects of specimen length, biochar content, cover thickness, and rebar diameter were systematically investigated. The results indicate that the surface strain evolution follows a two-stage pattern—a slow growth stage followed by a rapid rise stage—corresponding respectively to the early-stage filling of interfacial pores and stress accumulation, and the later-stage propagation of macroscopic cracks. Increasing specimen length significantly amplifies the spatiotemporal non-uniformity of strain, characterized by “locally high peak strains but low overall mean values,” with the onset time of strain surges differing by more than 50 h across different cross-sections. The incorporation of 0.5% biochar reduces the average strain by approximately 17% and delays crack initiation to 260 h. Increasing cover thickness from 25 mm to 40 mm exhibits the most pronounced inhibitory effect, achieving a 39% reduction in strain and delaying crack initiation to 320 h, primarily attributed to the extended chloride transport path and enhanced hoop confinement stiffness. Reducing rebar diameter from 20 mm to 12 mm decreases the peak strain to 79% of that of the standard specimen, owing to reduced rust product volume and increased relative cover thickness. The macro-cell effect driven by chloride concentration gradient transition zones is identified as a key factor governing crack initiation locations. Theoretical crack widths derived from strain integration of optical fiber data are slightly lower than measured values, yet the overall trends remain consistent. This study provides a quantitative basis for continuous monitoring and durability assessment of corrosion-induced cracking in marine environments. Full article

To overcome the limitations of conventional numerical simulations of cemented gangue backfill (CGB), this study developed a refined PFC2D model that incorporates the actual particle size distributions of coal gangue and river sand. Randomly distributed steel fibers were generated using FISH programming. Based on uniaxial compression tests and scanning electron microscopy (SEM) observations, the influence of steel fibers on the mesoscopic damage mechanism of CGB is systematically investigated. The results indicate that: (1) the refined model significantly improves the reliability of numerical simulations, accurately reproducing stress concentration within coarse aggregates and the steel fiber “bridging effect”; (2) a steel fiber volume fraction of 0.8% optimizes force chain distribution and suppresses crack propagation, promoting a transition in failure mode from brittle shear failure to ductile compressive–extrusion failure mode, with the peak strength and residual strength increased by 23.7% and 40.2%, respectively, compared with the fiber-free specimen; (3) PFC simulations reveal that steel fibers markedly retard damage accumulation by modifying the force chain network and crack propagation paths; and (4) SEM analysis demonstrates that steel fibers enhance the toughening effect through the interfacial transition zone, whereas excessive fiber content (1.2%) leads to fiber agglomeration and a 62.5% increase in porosity, resulting in performance deterioration. This study provides a robust theoretical framework for gradation reconstruction and refined fiber modeling in the design of roadside backfill materials. Full article

Bamboo is a natural fiber-reinforced composite whose macroscopic mechanical properties depend on the microscale synergistic deformation of fibers and parenchyma. However, existing in situ SEM testing techniques are mainly designed for metallic tensile testing and are unsuitable for analyzing bamboo’s meso-scale behavior. To address this, we developed an in situ SEM three-point bending instrument specifically for natural fiber materials. The instrument keeps the region of interest (ROI) stably centered in the SEM field of view through a stationary central indenter and symmetrically moving supports. It offers a 0–450 N load range, 0.5N force resolution, 1 μm displacement resolution, and is compatible with a Tescan Vega 4 SEM chamber. Using this instrument, in situ bending tests were performed on Moso bamboo (Phyllostachys edulis) with fiber volume fractions of 23%–42%, combined with digital image correlation for full-field strain measurement. Results show that flexural modulus, strength, and fracture work all increase significantly with fiber content. A microstructural failure classification framework was established based on in situ SEM observations, categorizing the observed failure modes according to the local arrangement of fibers and parenchyma. The proportions of these failure modes were found to be closely associated with the gradient distribution of strength and toughness across the culm wall. Three extrinsic toughening mechanisms were identified: fiber-induced crack deflection, parenchyma cell collapse densification, and fiber–parenchyma interfacial debonding. The developed instrument and analysis method offer a promising experimental platform for multi-scale mechanical characterization of natural composites. Full article

Rubber aggregate concrete (RAC) offers excellent frost resistance and impact toughness, but the incorporation of rubber particles substantially reduces its compressive strength, limiting structural applications. Existing improvement strategies have mainly relied on a single modification route, whereas the dynamic compressive response of RAC containing both ceramic balls and steel fibers remains insufficiently clarified. Here, a ceramic ball–steel fiber rubber aggregate concrete (CBSFRC) was investigated using quasi-static compression, a Φ120 mm split Hopkinson pressure bar system, and high-speed photography. Three steel-fiber volume fractions (1.0%, 1.5%, and 2.0%) were tested. The CBSFRC waveform displayed a characteristic ‘low-first, high-second’ double-peak pattern, in contrast to the ‘high-first, low-second’ pattern of the reference rubber aggregate concrete (CRC). At the common interpolated strain rates of 40, 45, and 50 s⁻¹, the dynamic compressive strengths of CBSFRC were 35.9–53.6% higher than those of CRC; no extrapolation was used. The largest quasi-static strength increase among the tested mixtures was 51.7%. The observed CBSFRC ultimate strains and strain energy densities ranged from 11.1–17.5 × 10⁻³ to 8.1–14.1 × 10⁵ J/m³, respectively. Matched-rate analysis showed that the largest DIF increment was 18.8% for SF-1CBRC at 40 s⁻¹, whereas the largest strain-energy-density increment was 70.2% for SF-1.5CBRC at 50 s⁻¹. These results describe the tested range and do not establish a universal optimum steel-fiber content. Full article