Fatigue life prediction of fiber reinforced concrete under flexural load

This paper presents a semi-analytical method to predict fatigue behavior in flexure of fiber reinforced concrete (FRC) based on the equilibrium of force in the critical cracked section. The model relies on the cyclic bridging law, the so-called stress–crack width relationship under cyclic tensile load as the fundamental constitutive relationship in tension. The numerical results in terms of fatigue crack length and crack mouth opening displacement as a function of load cycles are obtained for given maximum and minimum flexure load levels. Good correlation between experiments and the model predictions is found. Furthermore, the minimum load effect on the fatigue life of beams under bending load, which has been studied experimentally in the past, is simulated and a mechanism-based explanation is provided in theory. This basic analysis leads to the conclusion that the fatigue performance in flexure of FRC materials is strongly influenced by the cyclic stress–crack width relationship within the fracture zone. The optimum fatigue behavior of FRC structures in bending can be achieved by optimising the bond properties of aggregate–matrix and fiber–matrix interfaces.     

玄武岩纤维增韧混凝土冲击性能

采用三点弯曲冲击试验装置, 结合超声波测试技术, 研究了玄武岩纤维质量分数为0%～0.60%时, 玄武岩纤维增韧混凝土（Basalt Fiber Reinforced Concrete, BFRC）的冲击性能及其损伤演化规律, 研究了混凝土冲击破坏过程中基于超声波波速的损伤演化过程, 并应用体视显微镜观测了冲击过程中试件表面裂纹的发展, 分析了玄武岩纤维提高混凝土冲击韧性的机制。结果表明: 玄武岩纤维对混凝土的抗压强度无明显改善, 但可以显著提高混凝土的冲击韧性, 当纤维质量比为0.36%时冲击韧性提高了2.2倍。各玄武岩纤维掺量下混凝土的冲击破坏均表现出脆性特征, 但玄武岩纤维的加入有效提高了混凝土对冲击能量的吸收, 其临近破坏时损伤变量较素混凝土提高了40%～83%; 玄武岩纤维混凝土冲击破坏过程表现出多缝开裂的特征, 在最终破坏时主裂缝附近有明显的副裂缝出现。      

Investigation of microstructure and flexural behavior of steel-fiber reinforced concrete

This paper presents microstructure and flexural behavior of steel-fiber reinforced concrete produced with different steel fibers volume fraction and aspect ratio. Prismatic concrete specimens of 100 × 100 × 350 mm were prepared with and without steel fiber. Two different steel fiber types (both is hooked-end) were used by ratio of 0% (control), 0.2, 0.4, 0.6 and 0.8% by volume. Specimens were de-molded after 24 h and cured in water until 7, 28, 56, 180 and 360 days. On the prisms, flexural strength has been defined for every age. The crack widths have also been measured after maximum bearing loads. Microstructure of SFRC was studied by scanning electron microscopy and optical microscopy for 180 aged specimens. The results showed that the polarized microcopy images may be used for observing the bond characteristic of SFRC as alternatively to SEM. A good bond was observed between steel fiber and concrete matrix interface zone by using polarizing microscopy, too. Flexural strength of SFRC increased with the concrete age and fiber volume fraction. Besides, the first crack development significantly decreased by increasing of fiber volume fraction in the all concrete ages.     

玄武岩纤维网格布增强混凝土板双向弯曲性能试验

为研究玄武岩纤维网格布增强混凝土(BFT/PC)板的双向弯曲性能，首先进行了耐碱试验，通过碱液处理前后纤维的微观形貌及网格布断裂强力保持率评价了玄武岩纤维网格布的耐碱性能。在此基础上进行了双向板弯曲试验，研究了网格布层数、混凝土保护层厚度和结构型聚丙烯(PP)纤维对BFT/PC板受弯性能的影响。结果表明:玄武岩纤维网格布在NaOH溶液处理28 d后的经向、纬向断裂强力保持率分别为63%和73%，仅次于耐碱玻璃纤维网格布，具有较好的耐碱耐久性能。较小的混凝土保护层厚度可提高BFT/PC板在正常使用极限状态挠度下的残余承载力。增加网格布层数和PP纤维掺量可改善BFT/PC板的内力和应力重分布，提高正常使用极限状态挠度下的残余承载力、极限承载力和弯曲韧性，在BFT/PC板中掺入8 kg/m3 PP纤维，残余承载力、极限承载力和能量吸收值分别提高了1.40倍、0.36倍和5.10倍。本试验将PP纤维和玄武岩纤维网格布作为增强材料应用于混凝土构件的尝试为海工及近海侵蚀环境的基础设施建设提供了新思路。      

全玻璃纤维增强树脂筋混凝土电缆排管的抗弯试验

提出了采用全玻璃纤维增强树脂基复合材料（GFRP）筋混凝土电缆排管代替传统的钢筋混凝土电缆排管,该结构形式具有减少能耗的优点。通过对小尺寸和足尺GFRP筋混凝土电缆排管试件进行抗弯性能试验,研究其抗弯能力、变形及破坏特征等。试验结果表明,GFRP筋混凝土电缆排管具有与普通钢筋混凝土梁相似的力学特征,以混凝土开裂为分界点,位移-荷载曲线表现为双线性,排管侧面拉应力分布不均匀,部分区域出现了较高拉应力。提出了GFRP筋混凝土电缆排管的抗弯设计计算方法,理论计算结果与试验测试结果较为吻合。     

玄武岩纤维增强地质聚合物混凝土的高应变率力学行为

采用Φ100 mm分离式霍普金森压杆(SHPB)系统研究了玄武岩纤维增强地质聚合物混凝土(BFRGC)的高应变率力学行为,包括抗压强度、变形及能量吸收特性的应变率效应问题。结果表明：地质聚合物混凝土材料的高应变率力学性能呈现出显著的应变率相关性;BFRGC的强度特性相对于地质聚合物混凝土(GC)无明显改善,而变形及能量吸收能力较 GC有明显提高。     

Seismic flexural behavior of concrete connections reinforced with CFRP bars and grids

The corrosion of steel reinforcement in concrete and the resulting deterioration of structures prompted research on fiber reinforced polymers (FRP) as potential reinforcement for concrete members, for use in new construction. FRPs have more favorable advantages for new concrete buildings subjected to seismic loads particularly in corrosive environment.     

The diagonal tension behavior of fiber reinforced concrete beams

This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars and steel fibers. The variable parameters included the concrete compressive strength and the percentage of fibers (0%, 0.5%, 1.0% and 1.5% by volume). The beams were tested under static loads resulting in high diagonal tension stresses. The shear reinforcement was composed of stirrups instrumented with strain gages to detect the effect of the fibers on the strains. Research results indicate that as the fiber volume increases, the shear strength and the ductility of the beams increased, providing significantly higher shear strength than specified by the ACI-318 Code.     

玄武岩纤维布增强树脂基复合材料约束高温损伤混凝土轴压力学性能

对36个玄武岩纤维布增强树脂基复合材料（BFRP）约束加固的高温损伤混凝土圆柱体和15个不同高温损伤的对比试件进行了轴压试验。试验表明,BFRP侧向约束能显著改变混凝土圆柱体的破坏形态,提高混凝土圆柱体的轴压强度和变形能力。其中二层BFRP包裹的200℃、400℃、600℃和800℃高温损伤混凝土圆柱体的轴压强度分别提高了56%、82%、234%和250%,轴向变形分别提高了328%、198%、232%和136%。采用典型的纤维增强复合材料约束常温未损伤混凝土轴压强度和变形计算模型预测纤维增强复合材料约束高温损伤混凝土轴压极限强度和极限变形时存在较大的偏差。基于本文试验数据,确定了BFRP约束高温损伤混凝土极限应力和极限应变计算模型中与温度相关的参量,建议了适用于预测纤维增强复合材料约束高温损伤混凝土的极限应力计算模型和极限应变计算模型。     

纤维增强聚合物基复合材料加固锈蚀钢筋混凝土圆柱轴心受压性能

为了给纤维增强聚合物基复合材料（FRP）加固腐蚀环境下钢筋混凝土圆柱的设计和施工提供参考,促进FRP加固钢筋混凝土圆柱的应用,本文通过加速腐蚀得到类似实际环境中已锈损钢筋混凝土圆柱,采用碳纤维增强聚合物基复合材料（CFRP）条带和玻璃纤维增强聚合物基复合材料（GFRP）条带分别对锈蚀钢筋混凝土圆柱进行加固,最后对加固后圆柱进行轴心受压试验,重点研究钢筋锈蚀率、FRP层数和种类对钢筋混凝土圆柱受压承载力的影响;基于对FRP条带间隔约束效应、钢筋锈蚀对混凝土截面及钢筋力学性能影响的研究与分析,提出FRP条带间隔约束锈蚀钢筋混凝土圆柱轴心受压承载力计算模型。试验实测值与模型计算值之比的平均值为1.020,变异系数为0.063,二者符合较好。     

炭纤维增强轻质矿粉混凝土的热电行为

炭纤维增强混凝土能用来感知温度,其因在于短炭纤维的P-型传导性引起的塞贝克(Seebeck)效应所致.通过测量添加炭纤维或矿质掺和物(飞灰、硅土粉)前后六种波特兰水泥基混凝土的热电功率,研究了炭纤维增强轻质混凝土热敏的能力及其矿质掺合物对Seebeck效应的影响.结果表明: 炭纤维增强轻质混凝土具有类似于炭纤维增强标准混凝土的Seebeck效应,只是Seebeck系数因掺合了矿粉而减低.掺有矿粉的炭纤维增强轻质混凝土可用作建筑物的热传感器.     

Influence of concrete matrix and type of fiber on the shear behavior of self-compacting fiber reinforced concrete beams

Steel fibers are known to improve shear behavior. The Design Codes (Eurocode 2 (EC2), Spanish EHE-08, Model Code 2010 and RILEM approach) have developed formulas to calculate the fiber contribution to shear, mainly focused on standard FRCs, i.e. medium strength concretes with a low content of normal strength steel fibers. However, in real applications other combinations are possible, such as high or medium strength concretes with high strength steel fibers of different lengths and geometry. An experimental program consisting of 12 self-compacting fiber reinforced concrete (SCFRC) I-type beams was carried out. All the beams had the same geometry and fiber content (50 kg/m³), and they were made with two different concrete compressive strength values and five different types of steel fibers and were tested for shear. The main conclusions reached were that the type of fiber substantially affects shear behavior, even when the Design Code formulas indicate similar contributions. The combination of high strength concrete matrixes with low strength fibers does not seem to be efficient. Also, the use of high residual flexural tensile strength values (e.g. f_R3 or f_R4) does not appear to be the most accurate reference value to calculate the beam shear strength in these cases. The present Design Codes consider standard FRCs, but their formulas should be revised for concretes with fibers of different strengths, slenderness and geometry, since these properties substantially affect shear behavior.     

钢纤维对玄武岩纤维编织网增强混凝土板双向弯曲性能的影响

为了研究钢纤维对玄武岩纤维网格布增强混凝土方板双向受弯性能的影响,借鉴欧洲EFNARC标准,利用四边简支方板试验,分别对素混凝土方板、玄武岩纤维网格布增强混凝土方板、钢纤维增强混凝土方板及钢纤维与玄武岩纤维网格布混杂增强混凝土方板的弯曲性能进行研究,同时与传统钢筋网混凝土方板的弯曲性能进行对比,分析了网格布对混凝土方板的双向增强效果,探讨了钢纤维与玄武岩纤维网格布混杂使用代替传统钢筋网的可行性。结果表明:玄武岩纤维网格布可以改善方板的内力重分布,显著提高其承载力,但是破坏时脆性特征明显;钢纤维与玄武岩纤维网格布混杂使用表现出显著的正混杂效应,方板的韧性明显提高;在正常使用极限状态下,30 kg/m3的钢纤维与玄武岩纤维网格布混杂方板的弯曲性能高于传统钢筋网混凝土方板,说明钢纤维与玄武岩纤维编织网混杂使用可以代替传统钢筋网。      

Mean steel strain in reinforced concrete flexural members

Knowledge of the mean steel strain, as different from the steel strain at a crack, is of considerable significance in the calculation of crack widths and deflections which are among the important limit-states to be considered in the modern limit-state design concept. Six different theories presently available in literature for predicting the average steel strain in reinforced concrete flexural members have been compared amongst themselves and with test results. It is found that the method proposed by Rao predicts the mean steel strains more accurately than the other methods. On the basis of the earlier theories, the author proposes an empirical equation which fairly agrees with experimental results. The effect of load repetitions on the mean steel strain has been discussed.     

Punching shear behavior of concrete flat plate slab reinforced with carbon fiber reinforced polymer rods

Flat plate slab system is widely adopted by engineers as it provides many advantages . The system can reduce the height of the building, provide more flexible spatial planning due to no beams present, and further reduce the material cost. However, the main problem in practice is the brittle failure of flat plate slab under punching shear. In this paper, the punching shear behavior has been studied and an experimental work using carbon fiber reinforced polymer (CFRP) rods as shear reinforcement has been conducted in flat plate slab system.This exploratory research is to study the behavior of the flat plate slab with CFRP-rods reinforced in punching shear zone under constant gravity load and lateral displacements in a reversed cyclic manner. Three specimens of interior column-slab connection specimens were tested including one standard specimen without any shear reinforcement, the second one reinforced with CFRP-rods and the third one reinforced with stud rails as the reference to the second one. The slabs were 3000 mm long × 2800 mm wide × 150 mm deep, and were simply supported at four corners. Punching shear failure occurred for the standard specimens at a lateral drift-ratio, lateral drift divided by the length of vertical member, of approximately 5%. The specimen reinforced by CFRP-rods had significant flexural yielding and sustained deformations up to a drift ratio of approximately 9% without significant losses of strength, and punching shear was not observed in this specimen. The displacements increased up to 1.79 times larger than that of the standard specimen. And this specimen showed 42% superior ductile performance than the standard specimen and even the same capability with the stud-rail reinforced specimen. The results of the experiment indicate that CFRP-rods using in the flat slab has a better foreground.     

Compressive behavior of large-scale square reinforced concrete columns confined with carbon fiber reinforced polymer jackets

Several experimental and analytical studies on the confinement effect and failure mechanisms of fiber reinforced polymer (FRP) wrapped columns have been conducted over recent years. Although typical axial members are large-scale square/rectangular reinforced concrete (RC) columns in practice, the majority of such studies have concentrated on the behavior of small-scale circular concrete specimens. The data available for square/rectangular columns are still limited. This paper reports the results of an experimental research program on the performance of large-scale square RC columns wrapped with carbon fiber reinforced polymer (CFRP) sheets. Attention is focused on the investigation of the total effect of longitudinal and transverse reinforcement and FRP jackets on the behavior of concentrically loaded columns. A total of 20 large-scale RC columns were fabricated and tested to failure under axial loading in the structural laboratory. Three types of columns were primarily considered: unwrapped; fully wrapped; and partially wrapped. Based on the test results of RC columns, existing experimental data and procedures in the literature are also evaluated. Furthermore, stress–strain curves of the columns are successfully predicted by the analytical approach previously proposed for FRP-confined concrete.     

Stochastic analysis on flexural behavior of reinforced concrete beams based on piecewise response surface scheme

To save the computational efforts of Monte Carlo simulation together with nonlinear finite element method, the analysis framework combining the response surface method and Monte Carlo simulation is usually adopted to investigate the stochastic nonlinear behavior of structures. It is found that the traditional response surface method could not describe stochastic behavior of cracked concrete beams. In order to overcome the discontinuity caused by cracking nonlinearity, a scheme by introducing a piecewise response surface is proposed in this paper. This scheme is evaluated by the stochastic analysis of several concrete beams. The comparison between the proposed method and some traditional methods, including Monte Carlo simulation and Monte Carlo simulation with traditional response surface, shows that the proposed method could well depict the stochastic behavior of concrete beams. Finally, the probability density evolution of concrete beams from short-term deflection to a long-term deflection is analyzed.     

纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO2的钢纤维混凝土(NSFC)、钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、劈裂和抗折强度等力学性能,对不同温度热处理后的微观结构进行了SEM分析,对钢纤维与过渡区界面的相结构进行了XRD分析.结果表明:在测试温度范围内,NSFC的抗压、劈裂和抗折强度均高于SFRC和NC的强度,且在400℃时达到最大值.在常温下,NSFC的抗压、劈裂和抗折强度较NC分别提高27.01％、63.28％和54.12％,400℃高温热处理后比NC分别高35.09％、84.62％和87.23％; SEM分析表明,在钢纤维与过渡区的界面处,致密度提高,显微硬度提高.由于固相反应,使界面区结构发生变化,在钢纤维表层形成扩散渗透层(白亮层),即化合物层,呈锯齿状,XRD分析证明,白亮层主要由FeSi2和复杂的水化硅酸钙组成,从而增强了钢纤维与基体的粘结力,提高了混凝土的高温力学性能.     

Influence of fiber chemical coating on the acoustic emission behavior of steel fiber reinforced concrete

The current study focuses on the effect of chemical coating on the acoustic emission (AE) characteristics monitored during the fracture process in steel fiber reinforced concrete (SFRC). Different shapes of chemically treated and un-treated steel fibers are used to create specimens which are subjected to four point bending up to failure. Sensitive AE indices demonstrate that the coating gives distinct characteristics to the interface bonding between the fiber and the concrete matrix, which are evident mainly during the pull-out stage, after the moment of macroscopic crack formation. Specifically, AE average frequency and RA value, which defines the rising angle of the waveforms indicate that coating results in extensive matrix cracking in addition to the friction between fiber and concrete which characterizes the uncoated fibers. AE analysis can be used for interpretation of the fracturing stage and characterization of the fracture mode. It is shown that the surface conditioning of the fibers leaves a clear fingerprint on the AE signals, shedding light into the processes that occur during failure in SFRC.