纳米碳纤维增强混凝土抗冻性能试验

对六种不同纳米碳纤维掺量的72个纳米碳纤维/混凝土试件进行了慢冻融循环试验,通过测量纳米碳纤维/混凝土经不同冻融循环次数作用后的抗剥落能力、质量损失率、相对动弹性模量和抗压强度损失率,研究了纳米碳纤维掺量对纳米碳纤维/混凝土抗冻性能的影响。另外进行了纳米碳纤维/混凝土的FE-SEM试验和压汞试验,分析了纳米碳纤维对纳米碳纤维/混凝土抗冻性能的微观改性机制。结果表明:纳米碳纤维通过改善混凝土的微观形貌,细化其孔隙结构,提高其整体性和密实度,显著改善了混凝土的抗冻性能;纳米碳纤维掺量为3vol%时,纳米碳纤维/混凝土的抗冻性能最佳。同普通混凝土相比,300次冻融循环后,纳米碳纤维/混凝土的相对动弹性模量提高了33.2%,抗剥落能力显著增强;相同冻融次数下,随着纳米碳纤维掺量的增加,纳米碳纤维/混凝土相对动弹性模量和抗压强度损失率均先增大后减小,质量损失率先减小后增大。但纳米碳纤维掺量最大为5vol%时,纳米碳纤维/混凝土的抗冻性能仍优于普通混凝土;冻融循环次数越多,纳米碳纤维对混凝土抗冻性能的改善作用越显著。      

聚合物改性碳纤维增强混凝土的动态压缩力学性能

为探究聚合物改性碳纤维增强混凝土(PMCFRC)的动态压缩力学性能,利用直径Φ100 mm分离式霍普金森压杆(SHPB)试验装置,分别对碳纤维增强混凝土及聚合物体积分数为4vol%、8vol%、12vol%的PMCFRC进行了5组不同气压下的冲击压缩试验,获得了混凝土在不同应变率下的动态应力-应变曲线和破坏形态,分析了应变率和聚合物掺量对PMCFRC动态压缩强度、变形和韧性的影响规律。结果表明：PMCFRC的动态压缩强度、变形和韧性均具有明显的应变率强化效应,聚合物对PMCFRC的动态压缩力学性能既有强化效应,也有劣化效应。随着应变率的增大,PMCFRC的动态抗压强度、动态强度增长因子(DIF)、动态峰值应变、冲击韧性均逐渐增大。随着聚合物掺量的增大,PMCFRC的动态抗压强度、DIF、冲击韧性均先增大后减小,动态峰值应变不断增大。相同应变率水平下,4%PMCFRC的动态抗压强度、冲击韧性最大,破损程度最轻;8%PMCFRC的应变率敏感性最佳,DIF最大时达到1.94,对混凝土强度的增幅最大。聚合物一方面在混凝土基体中发挥着填充、阻裂、增韧作用,另一方面改善碳纤维-混凝土基体界面的粘结...     

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.     

Fly ash-based geopolymer concrete: study of slender reinforced columns

The objectives of this paper are to present the results of experimental study and analysis on the behaviour and the strength of reinforced geopolymer concrete slender columns. The experimental work involved testing of twelve columns under axial load and uniaxial bending in single curvature mode. The compressive strength of concrete for the first group of six columns was about 40 MPa, whereas concrete with a compressive strength of about 60 MPa was used in the other six columns. The other variables of the test program were longitudinal reinforcement ratio and load eccentricity. The test results gathered included the load carrying capacity, the load-deflection characteristics, and the failure modes of the columns. The analytical work involved the calculation of ultimate strength of test columns using the methods currently available in the literature. A simplified stability analysis is used to calculate the strength of columns. In addition, the design provisions contained in the Australian Standard AS3600 and the American Concrete Institute Building Code ACI318-02 are used to calculate the strength of geopolymer concrete columns. This paper demonstrates that the design provisions contained in the current standards and codes can be used to design reinforced fly ash-based geopolymer concrete columns.     

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.     

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.     

碳纤维增强复合材料筋混凝土梁非线性力学性能

为了研究碳纤维增强复合材料(CFRP)筋混凝土梁的非线性力学性能,基于非线性理论推导了CFRP筋梁的有限元分析模型：对4个预应力CFRP筋混凝土梁进行了非线性全过程分析,考察了预应力CFRP筋、GFRP筋和普通钢筋的应力发展规律。与试验资料对比可知,计算结果与试验数据吻合良好,说明采用弥散裂缝模式、Owen屈服准则和Hinton压碎准则能较好地描述混凝土开裂、屈服和压碎特性,同时也说明了CFRP筋及其力学效应用组合单元模拟的有效性以及本文中研制程序的正确性。CFRP筋具有高强度性能,梁试件破坏时CFRP筋均未失效;与受拉区配筋为钢筋相比,GFRP筋在全过程中处于弹性阶段。     

The fracture and fatigue performance in flexure of carbon fiber reinforced concrete

The fracture parameters and fatigue performances of carbon fiber reinforced concrete is investigated by three point bending tests. In comparison with the results of quasi-static tests where no pre-cyclic loading is applied, the influence of pre-cyclic loading history on fracture parameters was researched by using compliance calibration. The test results show that the fracture parameters of carbon fiber reinforced concrete and plain concrete will be reduced if the pre-cyclic loading stress levels are higher than a certain threshold, and this threshold value for carbon fiber reinforced concrete is higher than that of plain concrete. The critical effective crack length for carbon fiber reinforced concrete is significantly larger than that of plain concrete and independent of the pre-cyclic loading history and fatigue life. Carbon fiber reinforced concrete has a considerable beneficial effect on the behaviour of concrete subjected to flexure fatigue loading.     

Theory of piezoresistivity for strain sensing in carbon fiber reinforced cement under flexure

A theory is provided for piezoresistivity in carbon fiber reinforced cement (with and without embedded steel reinforcing bars) under flexure (three-point bending). The phenomenon, which involves the reversible increase of the tension surface electrical resistance and the reversible decrease of the compression surface electrical resistance upon flexure, allows strain sensing. The theory is based on the concept that the piezoresistivity is due to the slight pull-out of crack-bridging fibers during crack opening and the consequent increase in the contact electrical resistivity of the fiber-matrix interface. This work is an extension of prior theory, which concerns the effect of uniaxial loading on the volume resistance. The extension requires modeling the surface resistance and its change under flexure. The theoretical results on the piezoresistivity, both with and without rebar, are in good agreement with prior experimental results. Differences between theoretical and experimental results are probably due to minor damage and rebar debonding during flexure.     

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。      

Fatigue behaviour of steel fiber reinforced concrete

The results of an experimental investigation on the fatigue characteristics and residual strength of steel fiber reinforced concrete (SFRC) are reported. The testing program included flexural specimens as well as split-cylinders and cubes reinforced with two fiber types at a low volume content. One of the fibers was of the deformed slit-sheet type available at aspect ratios of 45 and 60. It is shown that SFRC has a better fatigue response than plain concrete and that the deformed slit-sheet fiber has an effect almost identical to hooked-end fiber of similar dimensions. There is no increase in residual strength measured by split-tension when specimens are subjected to fatigue stress above the endurance limit. Fatigue characteristics of SFRC from this testing program as well as previous works can be interpreted as a function of the fiber factor (i.e. a parameter accounting for volume fraction, aspect ratio and fiber type) to provide design charts. More experimental work is needed to provide an acceptable database for fatigue design of SFRC.     

新型加固用智能碳纤维板及感知性能试验

结合碳纤维增强树脂的强度特性与光纤布拉格光栅的传感特性研制开发出具有变形自感知能力的智能碳纤维复合板。在介绍内嵌光纤传感器的碳纤维复合板制作工艺的基础上,利用自制张拉反力架和钢筋混凝土梁进行智能碳纤维板的感知性能试验,获取包括灵敏度、线性度、重复性、迟滞性以及准确度等传感性能指标。研究结果表明:智能碳纤维板具有良好的线性度与重复性,测试精度高,是集感知和受力、功能材料和结构材料于一体的新型土木工程智能材料,既可以方便地作为混凝土结构的加固装配件,又可作为其表面的传感器件,具有良好的工程应用前景。     

碳纤维增强树脂复合材料加固钢筋混凝土柱抗震性能的尺寸效应试验

研究了低周循环荷载下碳纤维增强树脂复合材料(CFRP)加固钢筋混凝土(RC)柱的抗震性能的尺寸效应,并以边长为150~450 mm、剪跨比均为3的三组几何相似的钢筋混凝土柱为试验研究对象,考虑了CFRP层数、构件尺寸和轴压比等变量的影响。研究结果表明:在相同的截面尺寸和轴压比下,CFRP加固RC柱的水平承载能力、耗能能力、延性和水平位移相对于未加固柱均得到了不同程度的改善,并且存在尺寸效应;CFRP加固RC柱的无量纲水平承载力会随着构件尺寸的增加而减小,尺寸效应明显;随着CFRP加固RC柱的尺寸增加,构件的安全储备系数明显减小。      

Long-term behavior of existing low-strength reinforced concrete beams strengthened with carbon fiber composite sheets

This paper presents the results of an experimental study of the short- and long-term behavior of low-strength reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). A numerical analysis model was developed and verified for the calculation of the long-term deflection and maximum allowable long-term load of such RC beams. A parametric study was also conducted and it was found that the maximum allowable long-term load of a CFRP-strengthened beam was dominated by the deflection of RC beam when the cubic compressive strength of concrete was less than a certain value. For concrete of higher strength, the maximum allowable long-term load was dominated by the stress levels of the steel bars. It was also found that the yielding load of the strengthened beams increased significantly with areas strengthened by CFRP sheets and steel bars, while the maximum allowable long-term load only increased slightly.     

Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.     

钢纤维增强粉煤灰地质聚合物单轴受压过程的声发射特性

为探索纤维增强地质聚合物宏观力学行为与细观损伤演化特征之间的关联,对不同纤维体积掺量(纤维与拌合物的体积比)的钢纤维增强粉煤灰地质聚合物复合材料进行了单轴压缩试验.基于声发射技术,对试样压缩过程的声发射行为进行监测,研究了纤维体积掺量对地质聚合物单轴受压破坏行为及声发射特性的影响.结果表明:地质聚合物的强度、延性、声发射波形的上升斜率及平均频率均随纤维掺量的提高而增大,试件破坏形态由脆性灾变逐渐向延性破坏转变;在破坏前期,纤维体积掺量为0及0.5%的试件的声发射撞击率及能量释放率(简称能量率)都保持在较高水平,最终导致试件出现灾变破坏;而2.0%的纤维体积掺量使得声发射撞击率及能量率在应力-时间曲线的拐点处达到峰值,随后缓慢下降,最终导致试件呈现延性破坏;因此,仅依据声发射撞击率及能量率的快速上升来预测灾变破坏的发生,有时可能会出现谎报的情况.     

Strain distribution along lapped joints in reinforced concrete

Strain measurements along lapped joints in reinforced concrete tension members were obtained using a method of internally strain gauging the reinforcing rods. Strain concentration gauges, installed at the lap ends of some specimens, demonstrated the localised nature of force transfer prior to crack formation.
Results from ten test specimens of varying lap length and rod diameter have been produced. The changes observed in the longitudinal strain distributions, and the associated bond and concrete stresses as cracks developed are presented.     

Fracture mechanic modeling of fiber reinforced polymer shear-strengthened reinforced concrete beam

A numerical method is developed to model shear-strengthening of reinforced concrete beam by using fiber reinforced polymer (FRP) composites. Tensile crack is simulated by a non-linear spring element with softening behavior ahead of the crack tip to model the cohesive zone in concrete. A truss element is used, parallel to the spring element, to simulate the energy dissipation rate by the FRP. The strain energy release rate is calculated directly by using a virtual crack closure technique. It is observed that the length of the fracture process zone (FPZ) increases with the application of FRP shear-strengthening. The present model shows that the main diagonal crack is formed at the support in the control beam while it appears through the shear span in the shear-strengthened beam. Another important observation is that the load capacity increases with the number of CFRP sheets in the shear span.     

Analysis of reinforced concrete columns retrofitted with fiber reinforced polymer lamina

Fiber reinforced polymer (FRP) lamina have been used widely in the last decade to enhance strength and deformation capacity of deficient reinforced concrete (RC) columns. Seismic assessment and retrofit of existing columns in buildings and bridge piers necessitate accurate prediction of the available deformation capacity. In this study, a new analytical model is proposed to represent potential plastic hinge regions of RC columns prior to and after FRP retrofit. A recently developed variable confined concrete representation is employed within the framework of fiber-discretized frame elements to model the compression zone of the FRP-confined region. Confinement distribution within this region is included through the use of a bond model, whereas the effect of lap splices are considered using an effective steel strain concept. Comparisons of analytical estimates with experimentally measured response show that the proposed model is capable of capturing essential features of the response such as strength degradation due to lap splice slippage, and failure due to FRP rupture. Furthermore, a detailed sensitivity study is conducted to determine the parameters whose uncertainty significantly affects the behavior. It is observed that, in estimating the response of existing deficient columns, parameters such as plastic hinge length, concrete strength and splice length are important sources of uncertainty. While for FRP-retrofitted columns, parameters such as jacket stiffness, dilatation strain at splice failure and yield strength of the reinforcing bars are more important sources of uncertainty.     

Effect of silica fume particle size on mechanical properties of short carbon fiber reinforced concrete

The addition of short carbon fibers (CF) in concrete leads to changes of both the mechanical and electrical properties of the hardened product. The addition of silica fume (SF) to concrete mixtures with CF facilitates an even distribution of fibers within the mixture volume and improves the strength and multifunctional properties of the product. In this work the influence of the particle size distribution of commercial SF on the strength properties of CF reinforced concrete has been studied. Also the influence of the addition of methyl-cellulose as a fiber dispersing agent that improves the homogeneity of the mixture has been studied. The results show a different behavior of concrete depending of the average SF particle size (between 1 μm and 60 μm). The evolution of the elastic compressive modulus of carbon fiber reinforced concrete (CFRC) is linked to the one of the compressive strength, with a good coincidence with the CEB (Comité Euro-International du Béton) formula values. This parameter is only slightly decreased by the addition of methyl-cellulose.