Search Results (113)

Ultra-high-performance concrete (UHPC) reinforced with recycled tire steel fibers (RTSFs) was studied to evaluate its mechanical properties and cracking behavior. Using acoustic emission (AE) monitoring, researchers tested various RTSF replacement rates in compression and flexural tests. Results revealed a clear trend: mechanical properties initially improved then declined with increasing RTSF content, peaking at 25% replacement. AE analysis showed distinct patterns in energy release and crack propagation. Signal timing for energy and ringing count followed a delayed-to-advanced sequence, while b-value and information entropy changes indicated optimal flexural performance at specific replacement rates. RA-AF classification demonstrated that shear failure reached its minimum (25% replacement), with shear cracks increasing at higher ratios. These findings demonstrate RTSFs’ dual benefits: enhancing UHPC performance while promoting sustainability. The 25% replacement ratio emerged as the optimal balance, improving strength while delaying crack formation. This study provides insights into the mechanism by which waste tire steel fibers enhance the performance of UHPC. This research provides valuable insights for developing eco-friendly UHPC formulations using recycled materials, offering both environmental and economic advantages for construction applications. Full article

Predicting the flexural behavior of fiber-reinforced ultra-high-performance concrete (UHPC) remains a significant challenge due to the complex interactions among numerous mix design parameters. This study presents a machine learning-based framework aimed at accurately estimating the modulus of rupture (MOR) of UHPC. A comprehensive dataset comprising 566 distinct mixtures, characterized by 41 compositional and fiber-related variables, was compiled. Seven regression models were trained and evaluated, with Random Forest, Extremely Randomized Trees, and XGBoost yielding coefficients of determination (R²) exceeding 0.84 on the test set. Feature importance was quantified using Shapley values, while partial dependence plots (PDPs) were employed to visualize both individual parameter effects and key interactions, notably between fiber factor, water-to-binder ratio, maximum aggregate size, and matrix compressive strength. To validate the predictive performance of the machine learning models, an independent experimental campaign was carried out comprising 26 UHPC mixtures designed with varying binder compositions—including supplementary cementitious materials such as fly ash, ground recycled glass, and calcium carbonate—and reinforced with mono-fiber (straight steel, hooked steel, and PVA) and hybrid-fiber systems. The best-performing models were integrated into a hybrid neural network, which achieved a validation accuracy of R² = 0.951 against this diverse experimental dataset, demonstrating robust generalizability across both material and reinforcement variations. The proposed framework offers a robust predictive tool to support the design of more sustainable UHPC formulations incorporating supplementary cementitious materials without compromising flexural performance. Full article

To address the issues of high carbon emissions from concrete and high energy consumption in the manufacturing of traditional steel fibers, this study investigates the feasibility of replacing industrial steel fibers (ISF) with recycled tire steel fibers (RSF) in high-performance cement-based materials. The study examines the effects of fiber type and dosage on the mechanical properties within the systems of ultra-high-performance concrete (UHPC) and slurry-infiltrated fiber concrete (SIFCON) and analyzes the carbon emission levels using the Life Cycle Assessment (LCA) method. Research results indicate that the compressive and tensile strengths of SIFCON are significantly higher than those of UHPC. Under the same conditions, RSF has little difference in tensile performance when compared with ISF, suggesting a great substitution potential. Carbon emission analysis shows that although the total carbon emissions of the SIFCON system are relatively high, its performance improvement is remarkable. Both the carbon emission per tensile strength and carbon emission per compressive strength are lower than those of UHPC, demonstrating a high degree of environmental friendliness. Overall, this study shows that RSF can not only effectively enhance the performance of high-performance cement-based materials but also reduce carbon emissions, making it a reinforcing material with both excellent performance and sustainability. Full article

To develop an ultra-high performance concrete (UHPC) wall structure suitable for nuclear power plant applications, this study establishes a finite element model to evaluate the ultimate bearing capacity of UHPC walls under eccentric compression with single-sided thermal loading during accident conditions. The accuracy and reliability of the finite element analysis (FEA) method were rigorously validated by simulating and replicating experimental results using the same modeling approach adopted in this study. Based on the validated model, the influence of single-sided thermal loading on the ultimate bearing capacity of UHPC walls under nuclear power plant accident conditions was thoroughly investigated. Key parameters—including the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade—were systematically analyzed to determine their effects on the ultimate bearing capacity of UHPC wall specimens. The results demonstrate that the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade significantly affect the degradation rate of the ultimate load of UHPC walls as the temperature increases. Additionally, this paper proposes a calculation method for the normal section bearing capacity of rectangular cross-sections in UHPC large eccentric compression members under single-sided thermal loads. These findings provide theoretical support and scientific evidence for the design of new UHPC structural specimens in nuclear power plants. Full article

Ultra-high-performance concrete (UHPC) is a material with high mechanical properties that requires the use of fibers to overcome its brittleness, but the use of only one type of fiber may not improve UHPC performance enough. This study investigates the hybrid use of steel and glass fibers to achieve ultra-high strength along with improved ductility and impact resistance. A total of 22 concrete samples, including both plain (unreinforced) and fiber-reinforced types, were produced using micro straight-steel fibers, hooked steel fibers, and micro glass fibers, either individually or in combination. The mechanical properties, ductility, and impact behavior of the concrete samples were evaluated through experimental testing. The inclusion of microfibers had little impact on the compressive strength of concrete, which remained in the range of 130–150 MPa. However, it significantly enhanced the tensile strength, as evidenced by a flexural strength increase of up to 163% compared to the control concrete without microfibers. Numerical simulations were carried out to complement and validate the experimental investigation of projectile penetration. The depth of projectile penetration (DOP) test results were compared with existing empirical models from the literature. The incorporation of hooked steel fibers in hybrid blends significantly improved ductility and enhanced penetration resistance. In addition, previously proposed models from the literature were found to be highly conservative in predicting DOP at high projectile velocities. Full article

The application of rebar reinforced ultra-high-performance concrete (R-UHPC) has been increasingly adopted in engineering structures due to its exceptional mechanical performance and durability characteristics. Nevertheless, when subjected to combined saline and stray current conditions, R-UHPC remains vulnerable to severe corrosion degradation. This investigation examined the corrosion performance and tensile behavior evolution of R-UHPC containing 2.0 vol% copper-coated steel fiber content and HRB400 steel rebar with a reinforcement ratio of 3.1%. The accelerated corrosion process was induced through an impressed current method, followed by direct tensile tests at varying exposure periods. The findings revealed that the embedding of rebar in UHPC led to the formation of fiber-to-rebar (F-R) conductive pathways, generating radial cracks besides laminar cracks. The bonding between rebar and UHPC degraded as corrosion progressed, leading to the loss of characteristic multiple-cracking behavior of R-UHPC in tension. Meanwhile, R-UHPC load-bearing capacity, transitioning from gradual to accelerated deterioration phases with prolonged corrosion, aligns with steel fibers temporally. During the initial 4 days of corrosion, the specimens displayed surface-level corrosion features with negligible steel fiber loss, showing less than 4.0% reduction in ultimate bearing capacity. At 8 days of corrosion, the steel fiber decreased by 22.6%, accompanied by an 18.3% reduction in bearing capacity. By 16 days of corrosion, the steel fiber loss reached 41.5%, with a corresponding bearing capacity reduction of 29.1%. During the corrosion process, corrosion cracks and load-bearing degradation in R-UHPC could be indicated by the ultrasonic damage factor. Full article

This study assesses the punching shear characteristics of ultra-high-performance concrete (UHPC) slabs in two phases. The initial phase involved experimental tests to determine the critical thickness differentiating punching shear failure and flexural failure modes. Subsequently, the second phase further explored the punching shear behavior of UHPC slabs by analyzing various key parameters. The experimental findings indicated that as the thickness of the slabs increased, the punching shear capacity exhibited nearly linear enhancement, surpassing the improvement seen in bending capacity. Thus, a critical thickness of at least 100 mm was identified as the threshold distinguishing punching shear failure from flexural failure. Additionally, an increase in slab thickness significantly elevated the cracking load of the UHPC slabs. While a higher reinforcement ratio of 3.5% slightly increased the first cracking load, it greatly enhanced the ultimate capacity. The addition of steel fibers also contributed to improvements in both cracking and ultimate loads, albeit to a limited extent. The use of a granite powder substitute, comprising 10% of the mass of silica fume, had minimal impact on the punching shear capacity of the UHPC slabs. Finally, a comparison is drawn between the experimental results for punching shear capacity and those obtained from various theoretical models. This comparison highlights significant discrepancies in the results, stemming from the differing parameters employed in the proposed theoretical models. Among the prediction models, the JSCE model provided the most balanced and conservatively accurate estimation of punching shear capacity, effectively incorporating the effects of slab thickness, reinforcement ratio, and fiber content, thus highlighting its potential as a reliable reference for future design recommendations. This information will serve as a valuable reference for future research and practical applications related to UHPC bridge decks and slabs. Full article

The water absorption behavior of concrete is a critical indicator of its durability, and a comprehensive understanding of water transport characteristics can significantly enhance concrete performance. This study investigates the capillary water absorption properties of steel fiber-reinforced concrete across various strength grades by combining mercury intrusion porosimetry (MIP) and ¹H low-field nuclear magnetic resonance (¹H low-field NMR) techniques. Key findings reveal that the capillary water absorption of steel fiber-reinforced concretes occurs in the following two distinct stages: an initial rapid absorption phase (0 min to 6 h) and a subsequent slow absorption phase (1 day to 12 days). Modifications to the concrete matrix composition substantially reduce capillary water absorption rates, with ultra-high-performance concrete (UHPC) exhibiting exceptionally low absorption levels (the cumulative capillary water absorption of UHPC accounts for only 4.5–5.7% of that of C30 concrete). Additionally, for higher-strength concrete and extended absorption durations, the capillary water absorption rate deviates from the linear relationship with the square root of time. This deviation is attributed to the interaction of gel pore water with unhydrated cement particles, generating more hydration products, which refine the pore structure, reduce capillary pore connectivity, and increase pore tortuosity. Furthermore, steel fibers influence water transport through the following two primary mechanisms: interfacial interactions between the fibers and the matrix and a physical blocking effect that impedes water movement. Full article

The utilization of aeolian sand (AS) as a substitute for river sand (RS) in ultra-high-performance concrete (UHPC) offers a sustainable solution to address natural sand resource shortages while enhancing AS utilization. This study systematically evaluates the influence of AS content (0–100% RS replacement by mass) on the workability, mechanical properties, and microstructure of UHPC under different curing regimes. All mixtures incorporate 0.65% by volume of straight steel fibers to ensure adequate fiber reinforcement. The results reveal that the spherical morphology, smooth surface nature, and fine particle size of AS enhance the matrix fluidity and reduce the early autogenous shrinkage of UHPC. By employing steam curing at 90 °C for 2 d followed by standard curing for 7 d (M3), UHPC samples with a 60% and 80% AS substitution achieve a compressive strength of 132.4 MPa and 130.8 MPa, respectively; a flexural strength exceeding 18 MPa; a porosity below 10%; and a gel pore content exceeding 60%. The steel fiber reinforcement contributes significantly to the flexural performance, with the fiber–matrix interface quality maintained even at high AS replacement levels. These findings highlight the feasibility of AS as an alternative fine aggregate in UHPC. Full article

This study investigates the use of Ultra-High Molecular Weight Polyethylene (UHMWPE) fibers and fabric to enhance the flexural performance of Ultra-High-Performance Concrete (UHPC). A total of 45 specimens were tested to examine the effects of fiber type, fabric material, adhesive, and various combined strengthening techniques. The main findings are that incorporating UHMWPE fiber into the ultra-high-strength mortar (HSM) matrix provides superior performance compared to steel fiber, particularly in enhancing crack resistance and energy absorption. UHMWPE fiber-reinforced UHPC achieved a flexural toughness of 307 KJ/m³, over three times higher than that of steel fiber-reinforced UHPC (98 KJ/m³). The use of UHMWPE fabrics was more effective in improving the ductility and toughness of the composites than the use of glass fabrics. The bonding effect of using epoxy resin with UHMWPE fabric is better than using magnesium phosphate cement (MPC). Increasing the number of fabric layers improved the flexural properties of externally bonded fabric but had no impact on internal reinforcement techniques. The best strengthening method in this study was a combination of incorporating UHMWPE fiber internally and externally bonded fabric on a concrete surface, yielding the highest toughness of 580 KJ/m³. Full article

This manuscript delivers a comprehensive evaluation of five different ultra-high-performance concrete (UHPC) shear resistance models: FHWA-HRT-23-077 (2023), ePCI report (2021), French Standard NF-P-18-710 (2016), Canadian Standards A23.3-04 (2004), and Modified Eurocode2/German DAfStb (2023). The models differ in accounting for the steel fiber and shear reinforcement contribution and determining the angle of inclination of the diagonal compression strut. The evaluation was carried out using an experimental database of 198 UHPC specimens and focused on accuracy, conservatism, and ease of use for each considered model. The database included beams with prestressed and steel reinforcement, different shear reinforcement ratios, and a wide range of geometrical and material properties. In order to apply the FHWA method, a utilization tensile stress (f_t,loc) prediction equation was developed. Generally, the FHWA method showed superior performance to the other models in terms of statistical measures and consistent prediction conservatism across variable ranges. Although the ePCI methods yielded the highest conservatism, it can be said that the ePCI, AFGC, and CSA methods showed similar behavior with different degrees of conservatism. The DAfStb method showed the lowest prediction accuracy and the greatest scatter of data. Except for the FHWA method, all methods showed a reduction in conservatism at a high transverse reinforcement ratio. Full article

This study investigates multi-scale reinforcement of Ultra-High-Performance Concrete through targeted modifications of its mechanical and fracture-resistant properties via carbon microfibers and carbon nanotubes. The research employed comprehensive characterization techniques including workability tests, mercury porosimetry for microscale porosity analysis, and X-ray tomography for macro-scale pore evaluation. Mechanical performance was assessed through compression strength, tensile strength, and fracture energy measurements. Results demonstrated significant performance enhancements testing UHPC samples with 6 mm carbon microfibers (9 kg/m³) and varying carbon nanotubes dosages (0.11–0.54 wt%). The addition of carbon microfibres improved compressive strength by 12%, while incorporating 0.54 wt% carbon nanotubes further increased strength by 24%. Remarkably, the combined reinforcement strategy yielded a 313% increase in tensile strength compared to the reference mixture. The synergistic effect of carbon fibers and carbon nanotubes proved particularly effective in enhancing concrete performance. This multi-scale reinforcement approach presents a promising alternative to traditional steel fiber reinforcement, offering superior mechanical properties and potential advantages in corrosive environments. Full article

To investigate the strengthening effectiveness of Fabric-Reinforced Cementitious Matrix (FRCM) composites on shield tunnel segments, this study conducted four-point bending tests on FRCM composite panels. The influence of different cementitious matrices (engineered cementitious composite, ECC; ultra-high-performance concrete, UHPC) on the flexural behavior of FRCM panels was systematically analyzed. Numerical simulations were additionally conducted to analyze deformation behavior, damage progression, and stress variations in steel reinforcements within standard structural segments strengthened with FRCM composite panels. A parametric analysis was performed to assess the effects of cementitious matrix type, panel thickness, and carbon fiber-reinforced polymer (CFRP) grid layers on the reinforcement efficiency. The experimental results demonstrated that FRCM composite panels exhibit superior flexural performance. Specimens with UHPC matrices exhibited higher cracking stresses and enhanced flexural stiffness during the elastic phase, while those with ECC matrices demonstrated advantages in post-peak hardening behavior and energy dissipation capacity. Both matrix types achieved similar cracking strains and comparable ultimate flexural strengths. Numerical simulations revealed that FRCM strengthening significantly improves the ultimate flexural bearing capacity of segments while effectively controlling deformation. For UHPC-based FRCM reinforced segments, the ultimate bearing capacity increased with both UHPC thickness and CFRP layer quantity. In contrast, ECC-based FRCM reinforced segments exhibited capacity enhancement primarily correlated with CFRP layer addition, with negligible sensitivity to ECC thickness variations. Full article

The interfacial shear performance between ultra-high-performance concrete (UHPC) and normal concrete (NC) is a critical factor in determining the overall performance of composite structures. This paper systematically reviews the research progress on the interfacial shear performance of UHPC-NC, revealing the core mechanisms of bond strength (dominated by mechanical interlocking with chemical bonding as a supplementary factor). It compares the advantages and disadvantages of single-shear, Z-shaped shear, double-shear, and inclined shear tests, clarifying the influence patterns of key parameters such as interface roughness, matrix wetness, curing conditions, and fiber content. This study found that interface treatment is the most significant factor in improving shear strength. Roughening or grooving treatments can increase the strength by more than 40%~80%, while the combination of rebar planting and grooving can further enhance ductility. The matrix wetness (saturated and moist) and UHPC age (within 7 days) need to be strictly controlled to avoid differences in shrinkage stress. Prediction models based on mechanics, finite element analysis, and experimental data each have their advantages and disadvantages and should be selected based on actual working conditions. To address common issues in practical engineering, such as insufficient interface roughness, shrinkage cracking, and fatigue degradation under cyclic loading, it is recommended to adopt composite interface treatment techniques (such as roughening + rebar planting), prestressing design, and optimized fiber distribution (with a steel fiber content of 1.5%~2.5%). This paper provides the theoretical basis and practical guidance for the design optimization and construction control of UHPC reinforcement projects and composite structures. Full article

The addition of high volume fractions of fibers in ultra-high-performance concrete (UHPC) presents specific durability-based challenges due to the high content of interfacial transition zones (ITZ) between the fibers and surrounding mortar, along with the production cost. Hence, this study explored a novel coating approach on the surface of micro steel fibers, considering various nanomaterials, including graphene oxide (GO), nano-aluminum oxide, and nano-calcium carbonate. Poly(vinyl alcohol) (PVA) was employed as a coupling agent. UHPC mixtures containing coated fibers were compared with reference uncoated fiber-reinforced UHPC and UHPC containing GO. The proficiency of the proposed technique was measured through compressive strength, direct tensile, and flexural tests. A microstructure analysis was conducted using scanning electron microscope (SEM) images to determine the ITZ depth surrounding the coated fibers. Findings indicated improvements ranging from 10.7% to 21% for compressive strength, 11.2% to 38% for tensile strength, and 26.6% to 60% for flexural capacity. Full article