Search Results (366)

Silicon carbide (SiC) ceramics are regarded as high-performance structural materials due to their excellent high-temperature strength, wear resistance, and thermal stability. However, their inherent high brittleness, low fracture toughness, and difficulty in densification have limited their wider application. To overcome these challenges, introducing a second phase and/or sintering aids is necessary. In this paper, SiC–AlN–VC multiphase ceramics were fabricated via spark plasma sintering at 1800 °C to 2100 °C. The interface, mechanical, and thermal properties were examined. It was found that the VC particles effectively pin the grain boundaries and suppress the abnormal growth of SiC grains. At temperatures exceeding 1800 °C, the N atoms released from the decomposition of AlN diffuse into the VC lattice, forming a V(C,N) solid solution that enhances both the toughness and strength of the ceramics. With increasing sintering temperature, the mechanical properties of the SiC multiphase ceramics first improve and then deteriorate. Ultimately, a nearly fully dense SiC multiphase ceramic is obtained. The maximum hardness, flexural strength, and fracture toughness of SAV20 are 28.7 GPa, 508 MPa, and 5.25 MPa·m^1/2, respectively. Furthermore, the room-temperature friction coefficient and wear rate are 0.41 and 3.41 × 10⁻⁵ mm³/(N·m), respectively, and the thermal conductivity is 58 W/(m·K). Full article

This study investigates the flexural performance of geopolymer (zero-cement) concrete (ZCC) beams compared to normal concrete (NC) under monotonic and cyclic loading. Sixteen reinforced beams with compressive strengths of 20 and 30 Mpa and reinforcement configurations of 2Ø10 and 3Ø12 were tested to evaluate load–deflection behavior, ductility, energy absorption, and cracking characteristics. Under monotonic loading, ZCC beams achieved 9–17% higher ultimate strength and 5–30% greater mid-span deflection than NC beams, indicating superior ductility and energy dissipation. Under cyclic loading, ZCC beams demonstrated more stable hysteresis loops, slower stiffness degradation, and 8–32% higher cumulative energy absorption. ZCC specimens also sustained 8–12 cycles, corresponding to 70–90% of the monotonic displacement, whereas NC beams generally failed earlier at lower displacement levels. Increasing reinforcement ratio enhanced stiffness and load capacity but reduced deflection for both materials. Crack mapping showed finer and more uniformly distributed cracking in ZCC beams, confirming improved bond behavior between steel reinforcement and the geopolymer matrix. In addition, geopolymer concrete beams exhibited a significant enhancement in ductility, with the ductility coefficient increasing by nearly 50% compared to normal concrete under cyclic loading. Overall, the findings indicate that ZCC provides comparable or superior structural performance relative to NC, supporting its application as a sustainable, low-carbon material for flexure- and shear-critical members subjected to static and cyclic actions. Full article

Ultra-high-performance concrete (UHPC) is an advanced cementitious composite material with high durability and the strength properties exceeding those of conventional concrete. This paper presents the results of experimental testing assessing the freeze–thaw durability of UHPC specimens with varying fiber types (13 mm straight microfibers and 30 mm hooked-end fibers) and fiber percentages, as well as pre-existing cracks. The performance of all specimens was evaluated by measuring resonant frequency at intervals during testing and residual flexural strength after the completion of 350 freeze–thaw cycles. All specimens showed no degradation of resonant frequency over time. However, the pre-cracked specimens showed an increase in resonant frequency over the course of testing. The uncracked straight fibers specimens exposed to freeze–thaw cycles had the highest flexural strength, but the flexural resistance of the pre-cracked straight fibers specimens increased compared to the control specimens after 350 freeze–thaw cycles. The pre-cracked hooked fiber specimens showed higher first cracking strength and similar ultimate strength to the uncracked specimens after freeze–thaw exposure. Full article

Basic magnesium sulfate cement (BMSC) exhibits rapid setting, early strength development, high ultimate strength, and good durability, making it a promising construction material for the extreme environments of Mars. Following the principle of in situ resource utilization (ISRU), this study employs the Martian regolith simulant NUAA-1M, developed by Nanjing University of Aeronautics and Astronautics, as both a mineral admixture and aggregate to prepare Martian basic magnesium sulfate cement (M-BMSC) and Martian basic magnesium sulfate cement concrete (M-BMSCC). The effects of NUAA-1M fines on the setting time, compressive strength, hydration heat evolution, hydration products, microstructure, and pore structure of M-BMSC were systematically investigated. Moreover, the fundamental physical and mechanical properties of M-BMSCC incorporating NUAA-1M as an aggregate were evaluated, and an empirical correlation model was established between its compressive strength (f_cu), flexural strength (f_t), and splitting tensile strength (f_sp). Results indicate that with increasing NUAA-1M fines content, the setting time of M-BMSC was prolonged, while its compressive strength initially increased and then decreased. The incorporation of NUAA-1M fines modified the hydration process and phase assemblage of M-BMSC, promoting the formation of magnesium (alumino)silicate hydrate (M-(A)-S-H) gels and refining the pore structure. Hydration monitoring within 24 h confirmed the rapid hydration characteristics of M-BMSC, demonstrating its suitability for Martian conditions. M-BMSCC exhibited excellent early- and high-strength performance, achieving a 28-day compressive strength of 59.2 MPa at a binder-to-aggregate ratio of 2:1, corresponding to a total NUAA-1M content of 84.75% in the mixture. This work provides a novel ISRU-based material strategy for the construction of Martian bases and infrastructure. Full article

Reinforced concrete (RC) structural walls are essential for ensuring adequate lateral stiffness and strength in buildings located in seismic regions. However, many older structures incorporate nonconforming walls constructed with low-strength concrete, plain longitudinal reinforcement, and insufficient boundary confinement, and experimental data on such systems remain limited. This study investigates the seismic performance of two full-scale, relatively slender nonconforming RC wall specimens representative of older construction: one with no boundary confinement (SW-NC-FF) and one with insufficient confinement (SW-IC-FF). Both specimens exhibited flexure-controlled behavior, with initial yielding of boundary longitudinal bars occurring at an approximately 0.30% drift ratio and maximum reinforcement tensile strains of 0.006 (SW-IC-FF) and 0.015 (SW-NC-FF). Rocking governed the lateral response due to progressive debonding of the plain bars along the wall height, producing pronounced pinching and self-centering behavior. Failure occurred through longitudinal bar buckling and concrete crushing, with ultimate drift ratios of 2.0% and 1.5% and displacement ductility values of 4.0 and 4.3 for SW-IC-FF and SW-NC-FF, respectively. Experimental results were compared with backbone predictions from ASCE 41:2023, NZ C5:2025, and EN 1998-3:2025. While all three guidelines captured initial stiffness and yield rotations, their rotation-capacity predictions diverged, underscoring the need for improved assessment approaches for rocking-dominated, plain-reinforced walls. Full article

Currently, unreinforced steel fiber-reinforced concrete (USFRC) has not been widely adopted in underground engineering within China. However, extensive research has demonstrated that incorporating steel fibers can effectively enhance the mechanical properties of concrete, such as tensile strength, shear strength, residual flexural tensile strength, and also improve its durability. This study, based on the Qiandong experimental section of Dalian Metro Line 4, aims to investigate the failure modes, bearing capacity, and calculation methods for reinforced concrete (RC) and USFRC lining segment joints under compression-bending loading. The objective is to provide a reference for the application of USFRC lining segments in domestic underground engineering. The main conclusions are as follows: (1) The primary failure mode of RC segment joints is large-area crushing of concrete on the outer curved surface, with tensile crack widths on the inner curved surface less than 0.20 mm. The failure mode of USFRC segment joints is characterized by a 2.50 mm wide tensile crack below the loading point. (2) The bolt strain at failure for RC segment joints is approximately twice that of USFRC joints, with both reaching the yield strength and entering the plastic deformation stage. The bolt stress versus bending moment curve exhibits two distinct growth stages. USFRC can effectively control bolt deformation and stress, thereby enhancing bearing capacity. (3) The joint rotation angle versus bending moment curve follows a bilinear model. Under identical bending moments, the rotation angle of RC segment joints is significantly larger than that of USFRC joints. In the two stages, the rotational stiffness of USFRC joints is 367.13% and 763.82% of that of RC joints, respectively. (4) Bolts do not influence the bearing capacity of the segment joints. Existing calculation models in current design codes can accurately predict the ultimate bearing capacity of both RC and USFRC segment joints, demonstrating high prediction accuracy. Full article

Concrete remains the most widely used construction material globally; however, its intrinsic limitations—low tensile strength, brittle behavior, and susceptibility to microcracking—necessitate performance enhancement for demanding structural applications. Hybrid fiber-reinforced concrete (HFRC) offers a promising solution, yet the optimal balance of steel fibers (SF) and polypropylene fibers (PF) for structural elements such as slabs remains insufficiently understood. This study experimentally investigates the flexural behavior of 42 reinforced concrete slabs (21 one-way and 21 two-way) incorporating systematically varied SF–PF volumetric ratios, advancing current knowledge by identifying performance-optimal hybrid configurations for each slab type. One-way slabs were tested under four-point bending and two-way slabs under three-point bending, with structural responses evaluated in terms of load capacity, cracking behavior, deflection characteristics, and failure modes. The results demonstrate that fiber dosage does not proportionally enhance strength, as excessive content leads to fiber balling and reduced workability—highlighting the need for optimized hybrid proportions rather than indiscriminate addition. Quantitative findings confirm significant performance gains with properly tuned hybrid mixes. For one-way slabs, the optimal combination of 0.7% SF + 0.9% PF achieved 115% of the ultimate load of the control specimen, demonstrating a substantial improvement in flexural resistance. Two-way slabs exhibited even greater enhancements: first-crack load increased by up to 213%, and ultimate load improved by 40.36%, while deflection capacity rose by 44.81% at first crack and 39.80% at ultimate load with the optimal 0.9% SF + 0.1% PF mix. Comparatively, two-way slabs outperformed one-way slabs across all metrics, benefiting from multidirectional stress distribution that enabled more effective fiber engagement. Overall, this study provides new insight into hybrid fiber synergy in RC slabs and establishes quantified optimal SF–PF combinations that significantly enhance load capacity, ductility, and crack resistance for both one-way and two-way systems. Full article

Traditional concrete materials have limitations in terms of load-bearing capacity and ductile failure. In contrast, Engineered Cementitious Composites (ECCs), with their superior strain-hardening behavior and multiple cracking characteristics, have attracted widespread attention in the field of high-performance materials. In this study, ECC specimens incorporating different types of fibers (polyethylene (PE) fibers, polyvinyl alcohol (PVA) fibers) at varying contents were tested to systematically analyze their influence on mechanical properties. Compressive, flexural, and uniaxial tensile strength tests were conducted to evaluate the mechanical performance of ECCs. In addition, scanning electron microscopy (SEM) was employed to examine the fracture surfaces of the fibers, providing deeper insights into the interfacial behavior and fracture morphology of the different fiber-reinforced systems. Fracture surface analysis reveals that the interfacial bonding characteristics between different fibers and the matrix significantly influence fracture behavior. Moreover, as the tensile performance of ECCs is influenced by the interaction of multiple factors, traditional constitutive models exhibit limitations in accurately predicting its complex nonlinear behavior. To address this limitation, an Artificial Neural Network (ANN) approach was adopted to develop a predictive model based on bilinear stress–strain relationships. The model was constructed using ten key input parameters, including matrix composition and fiber properties, and was able to accurately predict the first cracking strain, first cracking stress, ultimate strain, and ultimate stress of ECCs. Sensitivity analysis revealed that fiber tensile strength and fiber content were the most significant factors influencing the tensile behavior. The predicted tensile curves showed strong consistency with the experimental results, thereby confirming the reliability and applicability of the proposed ANN-based model. Full article

Injection molding is a high-volume manufacturing process widely used for producing polymer components; however, its process parameters strongly influence residual stress, warpage, and the resulting mechanical performance. This work presents a comprehensive factorial design and ANOVA to evaluate the simultaneous effects of the injection temperature, packing pressure, packing time, and specimen orientation on the warpage, hardness, tensile, and flexural properties of polypropylene plates. The results demonstrate that the injection temperature and packing pressure are the dominant factors affecting the hardness and ultimate tensile strength, whereas warpage is mainly governed by the injection temperature and orientation. Under the tested conditions, certain combinations of injection temperature and packing pressure led to an improved mechanical performance; however, these adjustments also produced reductions in other properties, indicating that the balance among parameters depends on the targeted application rather than a single optimal set. Conversely, the parameter combination that produced the lowest warpage still yielded a significant increase in $E_{sec}$, indicating that reducing the warpage does not necessarily compromise the tensile stiffness. Interestingly, variations in the stress distribution between the tensile and bending tests suggest that the solidification-induced structure of the material influences its mechanical response, with specimens that showed a lower tensile strength exhibiting a comparatively higher resistance under bending. These findings provide new insights into the trade-offs between dimensional accuracy and mechanical performance and offer practical guidelines for optimizing polypropylene injection molding processes. Full article

The traditional methods for calculating the cracking loads of concrete members require the introduction of a semi-empirical inelastic influence coefficient of the section resistance moment to reflect the influence of sectional inelastic deformation development, and its value needs to be corrected according to the sectional characteristics and material properties. In contrast, emerging machine learning models for predicting the cracking loads of concrete members lack clear mechanical mechanisms, making their predictions difficult to interpret. Based on this, member cracking is reinterpreted as a change in the macroscopic performance of the member, and the strain energy variation before and after the fracture of plain concrete axially tensioned members is analyzed. A viewpoint is proposed that employs the incremental change in strain energy to characterize member cracking. With the internal tensile deformation representing the total strain energy of the member, a calculation method for the member cracking loads is established based on the critical condition that the second-order differential of this strain energy with respect to the member deformation equals zero. This method is applicable to both axially tensioned and flexural members of steel- and FRP-reinforced concrete that primarily undergo axial tensile deformation. The method is applied to analyze the cracking of axially tensioned and flexural members under different concrete strength grades, reinforcement types, and reinforcement ratios. It successfully explains the mechanism whereby the strain in the tension zone at member cracking exceeds the material ultimate tensile strain. Calculations verified against experimental data from four sets of tests on steel- or FRP-reinforced concrete beams demonstrate that the proposed method can be accurately applied to both normal- and high-strength concrete, as well as to both steel and FRP reinforcement, with a relative error of only 1% and a coefficient of variation of 0.12. Full article

The long-term flexural performance of steel-concrete composite beams after creep is influenced by multiple factors such as the degree of shear connection, cross-sectional form, and boundary conditions. The engineering community has an ambiguous understanding of the coupling effects of these factors. To address this issue, this paper conducts systematic experimental research: six simply supported beams (three box-shaped, three I-shaped) and four continuous beams (two box-shaped, two I-shaped) were designed with three degrees of shear connection (0.57, 1.08, 1.53). These beams first underwent simulated creep tests (24 °C, 80% relative humidity, 10 kN load, 180 days), followed by monotonic bending tests. The results indicate: (1) A high degree of shear connection (1.53) reduces creep deflection by 15–20% compared to partial connection (0.57) and delays the initiation of interface slip to 30% of the ultimate load; (2) Box sections exhibit 10–15% lower creep deflection than I-sections, though both experience 40–60% stiffness reduction after creep; (3) Continuous beams show a 25% improvement in crack resistance in the negative moment region and a 50% increase in flexural capacity at mid-span compared to simply supported beams; (4) After creep, the elastic modulus of concrete decreases by 40–60% (inversely related to the degree of shear connection), with fully connected specimens retaining 55–61% of their strength, while partially connected specimens retain only 43–49%. This study quantifies the degradation patterns of concrete performance, clarifies the influence mechanisms of key structural factors, and provides theoretical and experimental support for the long-term performance design of composite beams. Shear connection design is crucial for mitigating creep effects. Full article

Significant fluctuations in reservoir water levels occur seasonally during the flood period, adversely affecting the stability of bank slopes. In this paper, a modified mechanical model for the flexural toppling of anti-dip rock slopes under water level fluctuations is established, and an actual deflection equation for rock slabs is derived. The critical length for the flexural toppling failure of rock slabs is calculated, which can be used to evaluate slope stability. Multiple linear regression analysis reveals the relative degree of the influence of each parameter (such as rock slab thickness, rock layer dip angle, water level height, etc.) on the critical length. The results indicate that rock slab thickness plays a controlling role in slope stability. The failure mechanisms of the slope under the influence of water level fluctuations are revealed through fluid–solid coupling numerical simulations. The results indicate that the rise in water level reduces the strength of the rock mass in the submerged zone and generates significant water pressure on the rock mass at the slope toe, leading to its cracking. A rapid drop in water level generates seepage forces detrimental to slope stability and carries away fractured rock particles at the slope toe, ultimately causing slope failure. Finally, the reliability and applicability of the proposed method are validated through numerical simulations, case studies, and comparisons with existing analytical solutions. Full article

Recently, significant attention has been paid to the use of multirole materials in additive manufacturing (AM). Polyvinylidene fluoride (PVDF) is an ideal candidate material that has been selected for examination because of its unique characteristics. This study establishes a correlation between the macroscopic mechanical behavior and microscopic structural mechanisms, enabling the utilization of the deformation rate in tailoring the mechanical response of printed PVDF components. This research focuses on testing AM PVDF samples under different strain rates (10–300 mm/min), aiming to report their behavior under loading conditions compatible with the stochastic nature of real-life applications. The thermal (thermogravimetric analysis and differential scanning calorimetry) and rheological (viscosity and melt flow rate) properties were investigated along with their morphological characteristics (scanning electron microscopy). The response under combined dynamic and thermal loading was investigated through dynamic mechanical analysis, and the structural characteristics were investigated using spectroscopic techniques (Raman and energy-dispersive spectroscopy). The properties examined were the ultimate and yield strengths, modulus of elasticity, and toughness. Sensitivity index data are also provided. For completeness, the flexural strength, Charpy impact strength, and Vickers hardness were also evaluated, suggesting that the AM PVDF samples exhibit a resilient nature even when subjected to extremes regarding their strain rate versus their overall mechanical characteristics. PVDF exhibited a strain-hardening response with an increase in its strength of up to ~25% (300 mm/min) and a stiffness of ~15% (100 mm/min) as the loading speed of testing increased. Full article

To overcome the corrosion issues of conventional steel reinforcement and the brittleness of fiber-reinforced polymer (FRP) materials, steel-FRP composite bars (SFCBs) offer an innovative solution by combining the ductility of steel with the high strength and corrosion resistance of FRP. However, existing research primarily focuses on experimental investigations, with insufficient numerical simulations of SFCB-reinforced concrete beams, particularly regarding bond-slip effects at the SFCB-concrete interface—a critical mechanism governing composite action and structural performance. This study develops a finite element (FE) model incorporating SFCB-concrete bond-slip effects to analyze the influence of outer FRP layer thickness (0, 3, 5, and 7 mm) on the flexural performance of concrete beams. The FE model demonstrates good predictive accuracy, with errors in ultimate capacity and mid-span displacement within 7% and 8%, respectively. Both cracking and yield loads increase with FRP thickness, while the ultimate load peaks at 5 mm. At 7 mm, concrete crushing occurs before the SFCB reaches its ultimate strength. The ductility index decreases with greater FRP thickness due to increased elastic energy without enhanced plastic energy (fixed steel core area), thereby reducing overall ductility. These findings provide a theoretical basis for optimizing SFCB-reinforced concrete structural design. Full article

Reinforced concrete (RC) joist slabs are common in Middle Eastern buildings, where architectural needs often necessitate planting columns on shallow beams. Although such beams typically satisfy flexural and shear design requirements, their serviceability is frequently compromised by excessive deflections. This study experimentally investigated the effectiveness of polymer-bonded/bolted steel plates versus an Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) overlay, applied to the compression face, in controlling the deflection of shallow beams with planted columns. Four half-scale beams were tested under single-point loading, including two unstrengthened specimens to be used as reference beams. The first control beam reflected typical design practice—adequate in strength but exceeding code deflection limits—while the second specimen was designed to achieve similar flexural capacity with serviceable deflection. The remaining two beams were externally strengthened using either steel plates or UHPFRC overlay. Experimental results were analyzed in terms of failure mode, peak load, and deflection response. Both strengthening methods improved bending performance, stiffness, and load capacity, with UHPFRC showing superior effectiveness. Simplified analytical equations provided reasonable predictions of deflection and ultimate load. The findings highlight the potential of compression-side strengthening, particularly using UHPFRC, to enhance the serviceability of shallow RC beams supporting planted columns. Full article