Archives of Civil and Mechanical Engineering最新文献

The paper presents experimental and numerical investigations on structural-sized glued laminated timber beams made of coniferous wood with different reinforcing bars configurations (steel, BFRP and GFRP). Thirty-nine specimens were analysed in a four-point bending testing scheme. The highest improvements in stiffness (23.9% and 38.9%) and ultimate load-carrying capacity (13.6% and 29.4%) were observed for beams reinforced with steel bars. However, plasticisation in the steel occurred before wood failure. Beams reinforced with BFRP bars showed intermediate improvements in stiffness (8.2%, 14.2%) and ultimate load (9.6%, 17.5%), followed by those with GFRP bars (5.1% and 9.0%, and 6.2% and 10.7%). Most times, final failure occurred in the tension zone because of cracking at wood defects such as knots and was preceded by ductile failure in the compression zone. The entire experimental programme enabled to validate the FE model, based on the orthotropic Hill yield criterion for wood and cohesive surface for the adhesive behaviour. The proposed FE model shows the ability to predict accurately the failure load and the mid-span deflection. In addition, it can reliably forecast failure modes in both compression (upper lamella) and tension (lower lamella), as well as shear failure because of excessive shearing stresses in the adhesive. The model also supports the analysis of different reinforcement configurations and materials within the tension zone, accounting for varying wood properties.

Incorporating zinc oxide nanoparticles (N-ZnO) is possible in lightweight foamed concrete (LWFC) construction. This paves the way for studies of the thermal, mechanical, pore structure, and other aspects of the LWFC that make use of the N-ZnO. For this purpose, six LWFC mixtures were formulated as cement additives ranging from 0 to 1% N-ZnO. Fresh state attributes included setting time, workability, plastic, and dry density. The transport parameters, specifically sorptivity, water absorption, and intrinsic permeability, were all analyzed. Compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, and dry shrinkage were among the mechanical parameters investigated. Thermal properties, pore organisation, SEM analysis, and dispersion were also studied. N-ZnO has been shown to minimise slump flow diameter, initial, and final setting times while increasing LWFC dry density in laboratory trials. By increasing N-ZnO to 0.6%, transport, mechanical, and pore characteristics were enhanced. After 28 days, the LWFC with 0.6% N-ZnO had 70% higher compressive strength, 82% higher flexural strength, and 84% higher splitting tensile strength than the control mix. In terms of SEM and pore dispersion, N-ZnO-based concrete was the superior pore filler, accounting for up to 0.6% more volume. All density, thermal conductivity, and diffusion conductivity improved for all N-ZnO-based specimens, especially at 0.6% N-ZnO.

Shotcrete is a widely used material for tunneling engineering due to its multiple advantages including formwork-free pouring, fast setting and hardening, and convenient construction technology. The extensive application of shotcrete in engineering has evolved many challenges such as pipe plugging, inadequate build-up thickness, high rebound loss, and poor early strength, which significantly affect its engineering application and economics. The key to solving these challenges lies in improving shotcrete shootability upfront. To advance the understanding and development of this research field, this paper first reviews the definition of shotcrete shootability based on concrete performance changes during the spraying process. Then, the characterization methods of shotcrete shootability were analyzed with a focus on rebound loss. Finally, the factors influencing shotcrete shootability were discussed and measures for improvement were highlighted. This review aims to offer new insights and attract the attention of researchers to facilitate the development of shotcrete process technology.

Solid-state friction welding of ultra-fine grained materials (UFG) is a complex process to implement. Exceeding the recrystallization temperature during the process causes degradation of the material’s properties. The process temperature can be reduced by limiting the heat power. Using standard friction welding machines and parameters, it is difficult to achieve an appropriately low heat power. The welding tests of Cu-ETP copper with an UFG structure were carried out on a prototype rotary friction welding machine. It enables welding materials with extremely short friction times and high pressure forces. A given set of non-standard/sharp parameters influences the generated heat power. Its characteristic course is called the friction heat impulse (FHI). The use of FHI allows the energy to be reduced to the minimum necessary to obtain a joint. In order to obtain better joint conditions in a short welding time, a conical surface was used on one of the welded samples. The friction welding tests carried out using the FHI method and a prototype machine showed that the average hardness value of the joints increased slightly from 129 HV0.2 to 130 HV0.2. Additionally, the tensile strength remained at the same level as the UFG base material. The results proved that it is possible to weld UFG copper rods with conical face under recrystallization temperature without any signs of degradation of the UFG structure.

Mini-tubes with a 180 µm wall thickness were prepared by electrical discharge machining (EDM) from hydrostatically extruded pure zinc. Given zinc’s low thermal stability, microstructural analysis of the EDMed tubes was performed using electron backscattered diffraction (EBSD). The impact of variable pulse currents on microstructure and surface quality was assessed, considering different material states before EDM, including solid and predrilled rods. Surface quality was determined based on scanning electron microscopy observations and roughness measurements. Finally, mechanical properties were evaluated using static tensile tests. To observe the effect of different EDM parameters, the microstructure and mechanical properties of the mini-tubes were compared to pure zinc in rod form. The study revealed that pulse currents in the range of 3.2–2.6 A resulted in the formation of new grains, which were found across the cross-section of the mini-tubes, suggesting that the entire wall thickness was a heat-affected zone where recrystallization processes occurred. In the case of the lowest applied pulse current, grain size remained unchanged; however, some twins and an increased share of low-angle grain boundaries were observed. Surface quality deteriorated with increasing pulse current, with the thickest recast layer and highest roughness observed in hydrostatically extruded pure zinc, machined with the highest pulse current. It was also demonstrated that the initial form of the material had a slight effect on these features. The mechanical properties of the mini-tubes remained comparable to those of the rods. Overall, the EDM process shows promise for fabricating semi- or final products of absorbable zinc-based stents.

Within the realm of micro/nanomechanics, thermoelastic damping (TED) is acknowledged as a contributing factor to energy dissipation in mechanical structures. Consequently, the development of an accurate model for this phenomenon is crucial to get the best performance out of extremely small resonators. Considering the superiority of multi-dimensional heat transfer modeling compared to one-dimensional (1D) modeling as well as the definiteness of the size effect on both mechanical and thermal fields, this paper establishes a mathematical framework to appraise TED in circular micro/nanoplates with two-dimensional (2D) heat conduction by leveraging the capabilities of the modified couple stress theory (MCST) and Moore–Gibson–Thompson (MGT) heat equation. To initiate the investigation, the constitutive and heat equations for circular plates are formulated based on the MCST and MGT model. Through the solution of 2D heat equation, the spatial distribution of temperature within the plate is determined. Employing the previously obtained couple stress-based constitutive equations and temperature distribution function, the mathematical expressions of wasted and elastic energies within a single vibration cycle are derived. Ultimately, the substitution of the derived expressions into the formula of energy dissipation (ED) approach yields an infinite series solution for the computation of TED in small-sized circular plates. Following a comparative analysis of the proposed model with prior studies, convergence studies are conducted to identify the optimal number of terms needed for trustworthy outcomes. A range of numerical results with the aid of simulated model are also prepared, with a focus on analyzing the distinctions between 1D and 2D models as well as the implications of using the MCST and MGT model. The findings betoken apparent deviations between the predictions of the new 2D size-dependent model and the 1D traditional formulation, especially for tiny and relatively thick circular plates.

This study explored the combined effects of using attapulgite (ATP) as a partial cement replacement and basalt fibers (BF) as reinforcement in the development of high-performance foam concrete (FC) with 100% pumice aggregate. The experimental program included preparing FC mixtures with ATP replacements at 10%, 20%, and 30% by cement weight, and adding BF at volume fractions of 0.5%, 1.0%, and 2.0%. Key properties assessed were fresh flowability, compressive and flexural strengths, stress–strain behavior, thermal conductivity, and durability under sulfate exposure and high temperatures. Findings revealed a synergistic effect between ATP and BF, leading to significant performance enhancements across various parameters. The mixture with 30% ATP and 0.5% BF exhibited the highest compressive strength, reaching 19.45 MPa at 28 days and 22.11 MPa at 90 days, indicating improvements of 129.3% and 85.3% over the reference mix, respectively. This combination also achieved the lowest sorptivity, improved thermal stability, and better sulfate resistance, making it highly suitable for structural applications in harsh environments. In addition, the mixture with 10% ATP and 0.5% BF demonstrated the lowest thermal conductivity, reducing heat transfer by 4.2% compared to the control, which is beneficial for thermal insulation in building materials. Microstructural analysis using Scanning Electron Microscopy (SEM) showed that ATP’s pozzolanic reactivity led to a denser microstructure with stronger bonding, while BF effectively bridged micro-cracks, enhancing the FC matrix's durability. Overall, these results highlighted the potential of ATP and BF to significantly enhance FC’s mechanical, thermal, and durability properties, providing an eco-friendly solution with lower cement use and greater resilience to environmental stressors. This study contributes to sustainable construction technology by showcasing how ATP and BF can optimize FC performance, supporting its wider use in the construction industry.

Over the past few decades, the demand for retrofitting reinforced concrete members has risen dramatically. Retrofitting reinforced concrete slabs with carbon-fiber-reinforced polymer (CFRP) rods and ultra-high-performance-fiber-reinforced-concrete (UHPFRC) jackets is the one of significantly effective techniques. However, the main challenge of employing CFRP rods and UHPFRC jackets technique to strengthen existing reinforced concrete members is the efficiency of using CFRP rods and the debonding issue between old and fresh new concrete jacketing. Debonding mostly occurs between CFRP rods and old concrete, as well as the surfaces of old and new concrete, and is especially prevalent when the slab is subjected to daily repetitive loads such as cyclic loads. In this study, a new retrofitting system utilizing a mechanical anchor system was proposed to improve the bond between the UHPFRC layer and existing slab. This mechanical system incorporates an expandable anchorage bolt and steel plates. Therefore, a benchmark RC slab and two retrofitted RC slabs were experimentally tested under a five-point incremental repeated load (cyclic load) employing the dynamic actuator. The influence of embedded CFRP rods into the jacket of UHPFRC on the performance of system were studied. The experimental results showed that the newly proposed approach was significantly effective in preventing early debonding. In addition, the mechanical system played an essential role by improving the attachment between the jacket and the slab, ensuring better load transfer. A new proposed retrofitting technique improved the capacity of slabs from 164 to 298kN, illustrating an improvement of over 82%. On the other hand, the FE models have been developed to provide both practical validation and deeper analytical insights, ensuring a comprehensive evaluation of the proposed retrofitting system. Experimental data were used to validate the results of the finite-element models, which showed good agreement and high accuracy. Finally, a parametric study was executed to evaluate the impact of various parameters on the performance and efficacy of the new suggested strengthening technique, and to optimize the proposed system parameters, including the diameter of bolts, a normal strength concrete (NSC) jacket with various grades rather than the UHPFRC, applying the proposed retrofitting technique on a compressive side instead of a tension zone, and rebar steel of varying diameters in the jacket instead of CFRP rods. Findings indicated (parametric study) that using anchor bolts with a diameter greater than 12 mm improved the slabs’ ultimate load capacity.

The adoption of geopolymer, an inorganic amorphous material, reduces carbon dioxide emissions associated with ordinary portland cement (OPC) concrete and increases the usage of fly-ash (FA). Geopolymer concretes (GPCs) hold a lot of potential as cement substitutes as they can provide high early strength and resistance under aggressive environments. This paper reviews the compositions, curing regimes, mix designs, predictive models, and durability issues of FA-based GPC, drawing on recent credible publications. The role of microstructure on the strength and durability parameters is highlighted. Recent attempts, such as the utilization of multi-layered GPC–OPC, curing of geopolymer paste before casting, and the effect of mechanical milling of FA, are discussed. It was inferred that the porosity decreases and the microstructure gets denser as the volume fraction of nanomaterials increases. Research on durability issues indicates that the alumina–silicate determines the structural integrity of GP binders, exhibiting higher early age strength, reduced creep and shrinkage, and enhanced durability against hostile acids and sulphates. The primary carbonation and efflorescence reaction products for FA-based GPC are highly soluble Na₂CO₃ and K₂CO₃, which can increase the porosity of the concrete. Furthermore, it was found that deep residual networks and artificial neural network models were effective tools for predicting compressive strength, and the hybrid ensemble machine learning models outperformed conventional machine learning models. Reviewing large data might provide crucial information for the general use of FA-based GPC with suitable mechanical and durability features.

The research concerns the oxidation process of SPS-sintered zirconium-based materials. The sintered materials are characterized by residual porosity, which can affect the intensification of the oxidation process, compared to melted materials. On the basis of TGA tests carried out at temperatures of 500, 600, 700, and 800 °C, activation energies in air of sintered Zr, Zr–2.5Cu, Zr–2.5Nb, and Zr–2.5Mn, respectively, were calculated. Based on the studies of the microstructure and phase composition of oxidized materials, degradation mechanisms were analyzed for materials after annealing at 700 °C. In sintered Zr, Zr–2.5Cu, and Zr–2.5Nb oxidized at 700 °C, Si₃N₄ was detected. Si₃N₄ is formed at the zirconium oxide–zirconium boundary, in oxygen-deficient conditions. Zr–2.5Cu demonstrated the lowest tendency to oxidize despite the low values of activation energy for the oxidation process in air of 61 kJ/mol. This material showed the highest resistance to oxidation in air.