不同表面处理方法对不同全瓷与树脂粘接强度的影响

目的观察喷砂、酸蚀及硅烷偶联剂等表面处理方法对热压铸陶瓷，玻璃渗透陶瓷和氧化锆陶瓷与不同种粘结剂的粘接强度的影响。方法样本分为热压铸陶瓷组，渗透陶瓷组和氧化锆陶瓷组。每组各20片，各组中再随机分为A，B，C，D4小组，每组5片。分别应用不同种表面处理方法和粘结剂，测其剪切强度。结果方差分析表明热压铸陶瓷组中酸蚀涂硅烷树脂粘结组粘接强度最高，喷砂玻璃离子粘结组粘接强度最低。渗透陶瓷组中喷砂玻璃离子粘结组粘接强度最高，酸蚀涂硅烷树脂粘结组粘接强度最低。氧化锆陶瓷组中喷砂玻璃离子粘结组粘接强度最高，酸蚀涂硅烷树脂粘结组粘接强度最低。结论应用常规树脂粘结剂粘固热压铸陶瓷修复体取得较好的粘同效果，采用酸蚀和硅烷偶联剂的表面处理可取得最大粘结强度。但常规树脂粘结剂粘固渗透陶瓷和氧化锆陶瓷修复体取得的粘固效果不理想。临床上应根据全瓷修复体的种类选择适当的粘结剂系统和表面处理方式。     

普高与空军青航校毕业生抗荷体质差异

目的 对比青少年航空学校学员与普通高中生招飞抗荷体质相关指标差异,探索青少年航空学校抗荷体质训练成果并为训练方案优化提供科学依据。方法 随机抽取参加2022年空军招飞医学选拔定选阶段的部分青航校高三学员与同年级普通高中生作为研究对象,青航校学员按照既定培养方案,在高中阶段开展专项抗荷体质训练,并通过问卷调查收集普通高中生体育锻炼情况,纳入身体尺寸、身体成分、肌肉力量等指标进行对照研究,采用SPSS.26进行统计学分析。结果 青航校学员身高、四肢长、平静胸围、体质量均显著高于普通高中生(P＜0.05),二者坐高无统计学差异。青航校学员SMI显著高于普通高中生(P＜0.05),但体脂率对比无显著差异(P＞0.05);青航校学员背力与腿力均值/峰值显著高于普通高中生(P＜0.05);SMI与抗荷力量呈显著正相关,体脂率与抗荷力量不存在显著相关。青航校学员与普通高中生SMI分布存在差异,青航校学员的高SMI比例更高、低SMI比例更低(P＜0.05),而青航校与普通高中生体脂率分布无统计学差异(P＞0.05);结论 青航校抗荷体质训练在抗荷相关肌肉力量提高方面成效显著,但是在体脂率控制方面仍存在改进空间,未来青航校抗荷体质训练需要兼顾肌肉训练与体脂控制。     

Influence of Test Specimen Geometry on Probability of Failure of Composites Based on Weibull Weakest Link Theory

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume.     

Improved Shear Strength Prediction Model of Steel Fiber Reinforced Concrete Beams by Adopting Gene Expression Programming

In this study, an artificial intelligence tool called gene expression programming (GEP) has been successfully applied to develop an empirical model that can predict the shear strength of steel fiber reinforced concrete beams. The proposed genetic model incorporates all the influencing parameters such as the geometric properties of the beam, the concrete compressive strength, the shear span-to-depth ratio, and the mechanical and material properties of steel fiber. Existing empirical models ignore the tensile strength of steel fibers, which exercise a strong influence on the crack propagation of concrete matrix, thereby affecting the beam shear strength. To overcome this limitation, an improved and robust empirical model is proposed herein that incorporates the fiber tensile strength along with the other influencing factors. For this purpose, an extensive experimental database subjected to four-point loading is constructed comprising results of 488 tests drawn from the literature. The data are divided based on different shapes (hooked or straight fiber) and the tensile strength of steel fiber. The empirical model is developed using this experimental database and statistically compared with previously established empirical equations. This comparison indicates that the proposed model shows significant improvement in predicting the shear strength of steel fiber reinforced concrete beams, thus substantiating the important role of fiber tensile strength.     

Experimental and Analytical Research on Flexural Behavior of Concrete-Filled High-Strength Steel Tubular Members

Using high-strength steel (yield strength f_y ≥ 460 MPa) in concrete-filled steel tubes is expected to provide a superior bearing capacity by achieving light weight and efficient construction, but the existing design limitation on diameter-to-thickness (D/t) ratios for concrete-filled high-strength steel tubular (CFHST) members inevitably obstructs its wide application. In this study, aiming at the application of circular CFHST members using Q690 steel (f_y ≥ 690 MPa), a total of 15 CFHST beams were examined using a three-point loading test to investigate the failure mode, bearing capacity and plasticity evolution. Subsequently, finite element models (FEMs) were established to analyze the full-range curves, composite effect, failure mechanism and influences of key parameters including material strengths, D/t ratios, and shear-span ratios. A simplified calculation method for bearing capacity was finally proposed and verified. The results indicate that the full-range performance of tested CFHST members with out-of-code D/t ratios have ductile behavior, though they fail through the mode of steel fracture and concrete cracks in the tension zone as well as through local buckling in the compression zone; out-of-code CFHST members (e.g., D/t = 120) can perform reasonable composite behavior because of contact pressure larger than 2.5 MPa, where a thin-walled steel tube experiences an arch failure mechanism similar to core concrete at a trussed angle of 45°; the simplified bearing capacity model achieves a mean value of 0.97, and can be accepted as a primary tool to perform structural design and performance evaluation.     

Artificial Neural Network Modelling of the Effect of Vanadium Addition on the Tensile Properties and Microstructure of High-Strength Tempcore Rebars

In high-strength rebar, the various microstructures obtained by the Tempcore process and the addition of V have a complex effect on the strength improvement of rebar. This study investigated the mechanism of strengthening of high-strength Tempcore rebars upon the addition of vanadium through artificial neural network (ANN) modelling. Various V contents (0.005, 0.072 and 0.14 wt.%) were investigated, and a large amount of bainite and V(C, N) were precipitated in the core of the Tempcore rebar in the high-V specimens. In addition, as the V content increased, the number of these fine precipitates (10–30 nm) increased. The precipitation strengthening proposed by the Ashby–Orowan model is a major contributing factor to the yield-strength increase (35 MPa) of the Tempcore rebar containing 0.140 wt.% V. The ANN model was developed to predict the yield and tensile strengths of Tempcore rebar after the addition of various amounts of V and self-tempering at various temperatures, and it showed high reproducibility compared to the experimental values (R-square was 93% and the average relative error was 2.6%). ANN modelling revealed that the yield strength of the Tempcore rebar increased more significantly with increasing V content (0.01–0.2 wt.%.) at relatively high self-tempering temperatures (≥530 °C). These results provide guidelines for selecting the optimal V content and process conditions for manufacturing high-strength Tempcore rebars.     

Experimental-Numerical Study on the Flexural Ultimate Capacity of Prestressed Concrete Box Girders Subjected to Collision

Precise evaluation for flexural ultimate capacity of bridges which are subjected to the collision of over-height trucks is essential for making decisions on corresponding maintenance, strengthening or replacement. When the span of a cross-line continuous bridge with a double-box girder was hit by an overly high vehicle, the concrete floor of one girder was severely damaged, and part of the prestressed strands and reinforcements in the girder were broken. After the double-box girder was removed and separated into two single box girders, the ultimate flexural capacity of both box girders was studied by destructive tests, and a comparison was made between the damaged and undamaged girders. Moreover, finite element analysis was conducted to simulate the failure process. The results show that the flexural bearing capacity of the damaged box girder decreased by 33%, but it was still 1.07 times greater than the design bearing capacity, which basically meets the design requirements. Also, the damaged box girder showed a desirable serviceable limit state for three-axle vehicles and five-axle vehicles, but showed an undesirable serviceable limit state for six-axle vehicles. This study shows that repairing or strengthening the damaged span may be better than demolishing and rebuilding the whole superstructure bridge.     

Effects of Wax Molecular Weight Distribution and Branching on Moisture Sensitivity of Asphalt Binders

Wax is an important factor that affects the durability of asphalt binder. In order to understand the molecular weight distribution and branching of wax on the moisture sensitivity of asphalt binder, pure wax-doped asphalt binders are prepared and the performance of model asphalt binders are evaluated by surface free-energy (SFE) and binder bond strength (BBS) tests. In addition, asphaltene is regarded as an additive in this study. The results show that the addition of eicosane, triacontane, squalane and asphaltene can reduce the moisture sensitivity of asphalt, but not necessarily improve its moisture-induced damage resistance. The physical hardening effect of high-wax asphalt and its model asphalt is stronger than that of the corresponding low-wax asphalt and its model asphalt, and its moisture sensitivity is weaker than that of the low-wax asphalt. For all the model asphalts, there is a good correlation between the cohesion work, cohesion POTS (pull-off tensile strength), POTS ratio (the BBS moisture sensitivity index) and ER (the SFE moisture sensitivity index). When using the BBS test to characterize the moisture sensitivity of high-wax asphalt, it is recommended to leave the sample for some time until it is physically hardened and stable.     

Ultra-High-Performance Concrete (UHPC): A State-of-the-Art Review

The interest of researchers in UHPC has increased over the past decade. It is crucial to understand the structural behavior of reinforced UHPC (R/UHPC) components under various loading conditions before they can be used as a replacement for conventional concrete. Although several studies on ultra-high-performance concrete (UHPC) have been conducted, the knowledge is scattered, and no one can easily judge the performance and methodology of UPHC. Therefore, the purpose of this study was to review the research studies already carried out on UHPC. The review focuses on the materials’ physical and chemical composition, mechanical and durability characteristics, fire resistance, and environmental benefits of UHPC. Design considerations for effectively utilizing UHPC in structural elements are also presented. The best UHPFRC mixture is obtainable with a steel fiber content of 2–3% and a water-to-cement ratio of 0.2–0.3. The review also discusses the essentials recommendation for future research on UHPC.     

Application of Iron Tailings-Based Composite Supplementary Cementitious Materials (SCMs) in Green Concrete

How to treat the iron tailings of mining solid waste with high value is an urgent problem on a global scale. In recent years, the application of iron tailings in the building materials industry has attracted the attention of many scholars. The conversion of iron tailings into green building materials helps achieve carbon neutrality and high-value utilization of solid waste, and promotes sustainable development. Although iron tailings have been extensively studied as supplementary cementitious materials, the performance of concrete is not ideal due to its low activity. In this study, the hybrid supplementary cementitious materials system was prepared by iron tailings, phosphorus slag, and steel slag, and the effects of supplementary cementitious materials type, iron tailings content, iron tailings grinding time, and supplementary cementitious materials content on concrete performance were studied. The compressive properties, iron tailings properties, pore structure, interfacial transition zone, and element distribution of hydration products of concrete were tested by compressive strength tests, X-ray Diffractometer (XRD), X-ray Photoelectron Spectroscopy (XPS), Mercury Intrusion Porosimetry (MIP), Backscattering Electron Tests (BSE), and Energy Dispersive Spectrometer (EDS). The results show that further grinding improves the iron tailings activity. There is a synergistic mechanism between steel slag and phosphorus slag in the composite supplementary cementitious materials, which overcomes the low activity defect of iron tailings and produces concrete with a compressive strength exceeding 40 MPa. The composite supplementary cementitious materials can optimize the interfacial transition zone of the concrete interface and reduce the calcium–silicon ratio of the hydration products. However, it will deteriorate the pore structure of the concrete matrix, cause part of the concrete matrix to be damaged and lead to a loss of compressive strength, and the loss is acceptable. This work broadens the methods of comprehensive utilization of iron tailings and also provides a reference for a more detailed understanding of the properties of iron tailings-based concrete.