Search Results (2,421)

Natural aggregate concrete is prone to crack propagation and overall crushing under impact load, making it difficult to satisfy the service requirements for collapse resistance in building structures. In order to improve the impact resistance of concrete, this study investigated the impact of the mechanical behavior of steel–polypropylene hybrid fiber-reinforced concrete (SPFRC) using a split Hopkinson pressure bar apparatus. The effects of steel fiber content, polypropylene fiber content, and strain rate on the fractal dimension were examined, and the relationship between total energy dissipation and fractal dimension was established. The results show that the mean fragment size of the crushed specimens decreases linearly with increasing driving voltage, whereas it increases with fiber content. The fractal dimension monotonically increases with an increasing strain rate and decreases as the fiber content increases. Under a driving voltage of 1200 V and steel fiber content of 0.5%, increasing the polypropylene fiber content from 0% to 0.1% yields the largest reduction in the fractal dimension (15.00%) for specimen S0.5P0.1, exceeding the reductions from increments of 0.1%–0.25% and 0.25%–0.5%. Comparative results demonstrate that SPFRC exhibits superior impact failure resistance compared with concrete reinforced by a mono type of fiber. Exploring the correlation between fractal features and total energy dissipation can realize a more systematic and comprehensive performance assessment of concrete materials. The research results can provide quantitative theoretical support for the impact resistance evaluation and ratio optimization of SPFRC. 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

Tunnel lining concrete is prone to early-age cracking and leakage during service, which provides pathways for water and harmful ions and consequently threatens the safety and durability of tunnel structures. To improve the crack resistance and water penetration resistance of lining concrete, this study investigates the effects of steel fibers, a CaO-MgO composite expansive agent, and fly ash–slag replacement on workability, mechanical properties, ultrasonic pulse velocity, early-age cracking, and water penetration resistance. The results show that steel fibers had the most pronounced effect on crack-width control; at 1.0% steel fiber content, the total cracking area and water penetration height decreased by 71.9% and 68.2%, respectively. The composite expansive agent showed an optimum dosage of 10%, at which cracking resistance and water penetration resistance were both improved. The largest reduction in water penetration height was observed when slag was fully replaced by fly ash, with an 87.5% decrease. These findings provide a material-design reference for improving the crack resistance and durability of tunnel lining concrete under restrained early-age conditions. Full article

Glass fiber-reinforced polymer (GFRP) is increasingly used in civil engineering because of its high strength-to-weight ratio, corrosion resistance, durability, and low maintenance requirements. However, its inherently brittle behavior and limited ductility restrict its application in seismic regions due to poor energy dissipation capacity. To address this limitation, this study proposes a novel hybrid system comprising pultruded GFRP profiles and a replaceable steel link with ductile pipe sections. The pipe element confines inelastic deformation to the steel components while keeping the GFRP members elastic. Numerical results demonstrate stable hysteretic behavior with no significant degradation in strength or stiffness, confirming the effectiveness of the proposed system. Also, increasing the ratio of the pipe thickness to the flange thickness of the steel link (β) ensures suitable performance provided that plastic hinge formation remains confined to the ductile pipe element and replaceable steel link. Adding the pipe element to the I-shaped steel link increases web stress when β ≤ 1.0 (leading to web yielding), while stresses in the flange, GFRP beam, and GFRP columns are reduced by 46–51%, 17–60%, and 15–40%, respectively. However, for β > 1.0, stresses in GFRP components are not reduced but slightly increase by 1–9% (negligible), making β > 1.0 not recommended. Also, by changing the $\beta =0.50$ to $0.75$, $1.00$, $1.25$, and $1.50$, the flexural capacity, stiffness, and energy dissipation are enhanced by 1.51 times to 2.46 times, 1.18 times to 1.38 times, and 1.35 times to 1.68 times, respectively. Finally, the necessary design equations for the proposed system are presented. Full article

The weak mechanical behavior of recycled aggregate concrete (RAC) stems from microstructural defects in its composition. This study investigates the reinforcement of RAC through steel fiber incorporation. Using RAC replacement ratios and steel fiber contents as variables, this study examined their effects on compressive strength, splitting tensile strength, elastic modulus, and Poisson’s ratio. The axial compressive stress–strain curves of steel-fiber-reinforced RAC specimens were systematically measured. Scanning electron microscopy was employed to elucidate the modification mechanisms of steel fibers in RAC. The findings indicate that although greater replacement ratios weaken the mechanical performance of steel-fiber-reinforced RAC, an increase in fiber dosage enhances its strength. The most significant enhancement occurs when fiber content increases from 0.5% to 1.0%; at a replacement ratio of 0, the splitting tensile strength achieves the highest improvement of 11.14%. By considering the influencing factors, including the replacement ratio and steel fiber content, mechanical performance indices such as cube compressive strength were determined. Furthermore, the quantitative correlations linking the transformed values of various indices and the governing variables, together with the complete stress–strain curve formulations, were developed. The enhancement in RAC performance can be ascribed to the crack-bridging and crack-resisting effects provided by the embedded steel fibers. This research provides crucial experimental evidence supporting the engineering applications of steel-fiber-reinforced RAC. This study offers essential empirical data that underpin the practical implementation of steel-fiber-enhanced RAC. Full article

The use of Glass Fiber-Reinforced Polymer (GFRP) bars to reinforce concrete beams can provide high resistance to corrosion, high performance, high sustainability, and reasonable strength but with low ductility. This study focused on the structural behavior and indirect evaluation of the ductility index of hollow and solid beam specimens reinforced with GFRP bars, steel bars, or both (hybrid). Eight simply supported beam specimens with dimensions of 1200 mm (length), 150 mm (height), and 100 mm (width) were made using self-compacted concrete (SCC) and tested using two-point repeated loading. The tests results showed that the ultimate load capacity of the tested solid and hollow beams reinforced with GFRP bars were 78% and 67% higher than that of the corresponding solid and hollow beam specimens with steel-bar reinforcement. The measured energy absorption is “instantaneous” energy absorption because the residual stress disappears after the load is removed at the end of the test and any cracks will close due to the semi-linear response of the beam specimens reinforced with GFRP bars. Regarding the solid beam specimens, those containing GFRP bars showed an increase in energy absorption of 64–127% compared with the corresponding reference beams. The hollow beam specimens containing GFRP bars showed an increase in energy absorption of 21–68% compared with the corresponding reference beam containing three steel bars. 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

An experimental study was conducted to investigate the flexural behavior of continuous beams strengthened with precast concrete plates. Ten rectangular concrete beams with a cross-section of 210 × 150 mm and a total length of 2400 mm were tested under four-point loads. One specimen, without any strengthening, acted as the control, while the remaining nine were strengthened at both the positive and negative moment zones. The variables in this study were: strengthening plate thickness, length, type of bonding (epoxy or mechanical connector), bonding method (surface bonding or 10 mm grooving), type of concrete used (UHPC, SFRC, or SIFCON), and finally, the steel fiber ratio. The failure mode, cracking modes, ultimate load, load–deflection curve, stiffness and ductility were analyzed. The results showed the effectiveness of the strengthening methods, as they improved the flexural strength of the beams by 15.7% to 53%, as well as their stiffness by 15% to 173.8%, and reduced crack propagation. Also, decreasing the thickness and length of plates reduced the flexural strength by 7.28% and 23.5%, respectively. When the bonding methods were compared, the beam with mechanical bonding showed 5.7% more flexural strength than the one using epoxy. However, it was noted that all cracks in the strengthening plates were located at the bolt positions. Additionally, the use of SIFCON plates enhanced flexural strength more than UHPC and SFRC plates. However, for the SFRC plate, increasing the steel fiber content from 1.5% to 2% improved the strength by 1.2%, but this high percentage also caused cracking in the SFRC plate due to the inhomogeneity of the concrete mixture. As for the initial stiffness, the sample in which epoxy was used showed the highest value, with an increase of 173.8%, due to the uniform bonding at the connection surface. Finally, it was observed that the reference beam had the highest ductility due to the high ultimate displacement resulting from the numerous cracks that occurred in the beam, which were reduced in the strengthened beams. Full article

Fiber-reinforced self-compacting concrete (FR-SCC) incorporating granite powder (GP), an expansive agent (EA), steel fibers (SFs), and polypropylene fibers (PPFs) was investigated for potential pre-cast tunnel-segment applications. Sixteen mixtures, covering GP replacement ratios of 0–18%, EA dosages of 0–8% by binder mass, and SF and PPF volume fractions of 0–0.75% and 0–0.15%, were tested in uniaxial compression on 100 mm × 100 mm × 300 mm prisms with acoustic emission (AE) monitoring. Within the tested range, 12% GP and 8% EA gave the most favorable binder composition. XRD and SEM analyses indicated that GP acted predominantly as an inert filler with no detectable portlandite consumption, while the expansive agent was associated with additional ettringite formation. At this composition, hybrid SF/PPFs increased the post-peak energy by a factor of 7.66 relative to the fiber-free mixture, mainly improving the post-peak rather than the pre-peak behavior. Among the Carreira–Chu, GB 50010, and modified Weibull formulations, the GB 50010 piecewise model best reproduced the full stress–strain curves and was used as the primary constitutive model. Two-variable regressions were established to separate the apparent effects of the SF and PPF volume fractions on the ascending- and descending-branch shape parameters, and a ductility-calibrated expression was developed for the descending-branch parameter. The Pearson coefficient between the descending-branch parameter and the AE characteristic strain was −0.904, while that between the AE characteristic strain and the macroscopic residual strain was +0.983. These results link constitutive modeling, AE damage evolution, and macroscopic post-peak ductility for FR-SCC within the tested range of mix proportions. Full article

To enhance the crack resistance and toughness of cement mortar, steel fibers (SF), carbon fibers (CF), and their hybrid combinations were incorporated at various volume fractions. Cubic specimens with single or double prefabricated holes were tested using compressive and splitting tensile tests combined with digital image correlation technology (DIC). to evaluate mechanical properties, crack propagation, and failure patterns. Results indicate that steel fibers primarily suppressed macrocracks via bridging and improved strength, while carbon fibers inhibited microcrack initiation and promoted uniform strain distribution. Hybrid fibers achieved combined reinforcement, with the S2C02 mixture exhibiting the best overall performance: compressive strength increased by 39.7–55.1%, tensile strength by 70.45–75.9%, and the toughness and tensile/compressive ratio were enhanced. DIC analysis showed reduced strain concentration, more uniform strain fields, delayed crack propagation, and a transition from brittle to quasi-ductile failure. These findings demonstrate an optimal hybrid fiber dosage and reveal the combined mechanism of steel–carbon fibers in defect-containing cement mortar, providing guidance for material design and performance optimization. Full article

The seismic assessment of existing strategic and highly occupied reinforced concrete buildings, such as schools located in moderate-to-high seismic regions, is a critical task, especially because they have been constructed in periods lacking performance-based seismic design requirements. Following a review of possible local and global structural strengthening strategies, this study presents the seismic evaluation and retrofit design of an existing reinforced concrete school building designed before the introduction of modern seismic codes. The assessment is supported by an in situ investigation campaign including destructive and non-destructive materials testing, geotechnical investigations, and a detailed survey of the original construction details. A three-dimensional numerical model, calibrated on the in situ survey and material test results, enables the evaluation of the building’s seismic performance in both the pre- and post-intervention configurations. The model also supports the optimization of retrofit costs: a constraint of relevance because the intervention was funded through a dedicated public budget allocated by the national authority and subject to public validation. This represents a procurement framework that explicitly links the achievable retrofit performance level to a fixed cost ceiling, unlike standard practice in most seismic-prone countries. A global strengthening strategy was implemented including new reinforced concrete shear walls structurally connected to the foundations of the existing walls, whose capacity was enhanced through the installation of micropiles. Fiber-reinforced polymer (FRP) wrapping was applied to improve both flexural and shear beam capacity, while steel jacketing was adopted for some vertical elements. The combined interventions significantly improved the seismic performance of the building, ensuring a safer and more reliable response under future seismic events, respecting the initial publicly available budget. Full article

Precast concrete segmental bridges rely on keyed dry joints to transfer shear forces, yet existing formulas have insufficient accuracy for normal concrete (NC) and ultra-high-performance concrete (UHPC) joints. In this study, a unified database containing 158 specimens of keyed dry joints was established and systematically screened, and 134 specimens were retained for machine-learning modeling and formula development. Among six machine-learning models, Categorical Boosting (CatBoost) showed the best prediction performance. SHapley Additive exPlanations (SHAP) analysis indicated that confining stress, concrete compressive strength, the area of the base of all keys in the failure plane, and the area of contact between smooth surfaces on the failure plane govern shear capacity. Based on these key variables, multiplicative and additive explicit formulas were developed, and the additive formula showed better overall performance, with average experimental-to-predicted ratio (Avg), coefficient of variation (CoV), and R² values of 1.08, 0.22, and 0.94, respectively. Although the additive formula stably predicted NC specimens, it showed insufficient accuracy for UHPC specimens. After introducing steel fiber volume fraction into the root shear term, the UHPC subset’s Avg and CoV improved from 1.30 and 0.24 to 1.09 and 0.21, respectively. The proposed framework integrates data-driven prediction and physical interpretability, providing an engineering-oriented method for shear-capacity prediction of keyed dry joints. Full article

This study develops a rapid-hardening and high-fluidity ultra-high-performance cement (UHPC)-based grout by incorporating calcium sulfoaluminate (CSA) cement as an early strength component, along with steel and wollastonite fibers as hybrid reinforcements. The UHPC grout proportions containing different contents of CSA cement and wollastonite fibers were designed to investigate fluidity, mechanical properties, hydration kinetics, microstructure and chloride resistance. The results showed that both CSA cement and wollastonite fibers significantly enhanced the compressive and flexural strength of UHPC grout. The incorporation of CSA cement led to rapid compressive strengths of 23 MPa at 6 h and 75.9 MPa at 1 day, marking a significant enhancement compared to the reference group and indicating excellent early-age performance. CSA cement accelerated the hydration process of the UHPC grout and promoted formation of more ettringite. Wollastonite fibers and U-type expansive agent (UEA) further improved the mechanical performance through bridging and physical filling effects. Moreover, CSA cement and wollastonite fibers effectively optimized expansion behavior and refined pore structure of the UHPC grout, and improved its chloride penetration resistance. Although both components influenced the fluidity of the grout, the UHPC grout still maintained high fluidity, offering a promising outlook for its potential use in demanding engineering applications. Full article

This study investigates the flexural behavior, load-carrying capacity, and crack propagation of concrete beams reinforced with hybrid glass-fiber-reinforced polymer (GFRP) and steel bars. To evaluate the structural performance, concrete beam specimens with cross-sectional dimensions of 150 mm × 300 mm and a total length of 2050 mm were fabricated using a design concrete compressive strength of 35 MPa and tested under flexural loading. Each tested specimen featured a distinct hybrid reinforcement configuration to investigate the influence of bar arrangement on the mechanical behavior. Flexural cracks were systematically monitored using a crack-width comparator gauge at specific loading stages, accounting for key milestones such as ultimate load capacity and sudden load drops. The experimental findings were complemented by an analytical model to validate the performance parameters and predict the ultimate capacity. The results demonstrate that the specific configuration and arrangement of hybrid reinforcement significantly influence the post-cracking stiffness and crack growth. Specifically, the hybrid configuration effectively balances the ductile response of steel with the brittle behavior of GFRP, achieving significant control over serviceability crack widths and an enhanced ultimate load-carrying capacity. Experimental results indicated that for elements exhibiting identical axial stiffness, the reinforcement layering configuration provided a 66% improvement in the deformability factor alongside a 10% enhancement in the load-carrying capacity. It is recommended that the steel tension reinforcement be positioned in the inner layer at a spacing of about two times the GFRP bar diameter to mitigate corrosion risks. Additionally, it was established that the theoretical load capacity accounted for 70% to 86% of the experimental load capacity. Full article

Carbon fiber-reinforced polymer (CFRP) repair is widely used to rehabilitate corroded steel pipelines; however, the relative influence of CFRP repair geometry on stress reduction remains insufficiently quantified. This study investigated the effects of CFRP thickness and repair length on the hoop stress response of steel pipelines containing circumferentially uniform longitudinal corrosion defects under internal pressure. An axisymmetric finite element model was developed in ABAQUS and verified against an analytical multilayer cylinder solution based on the Lamé thick-cylinder theory. The model was based on an idealized circumferentially uniform corrosion defect, linear elastic material behavior, and perfect bonding between the steel pipe, epoxy filler, and CFRP repair layer. A parametric study was performed by varying the defect depth, defect length, CFRP thickness, and repair length. The results showed that CFRP thickness was the dominant parameter controlling the repair effectiveness. For the deepest defect case, increasing the CFRP thickness ratio from 0.25 to 0.75 increased the hoop stress reduction from approximately 40% to more than 58% for the shorter defect and from approximately 40% to more than 62% for the longer defect case. In contrast, increasing the repair length beyond full defect coverage produced only marginal additional stress reduction. Based on a 10% stress-tolerance criterion relative to the intact pipe response, the required CFRP thickness-to-defect-depth ratio increased with defect severity. These findings support the preliminary CFRP repair sizing by prioritizing repair thickness over excessive repair length. Full article