FRP筋异强混凝土叠浇梁挠度与延性研究

为提高纤维增强聚合物(FRP)筋异强混凝土叠浇梁的抗弯性能与延性,研究了钢纤维掺量、钢纤维混凝土叠浇层厚度对FRP筋异强混凝土叠浇梁的影响。以钢纤维体积掺量(0%、0.5%、1.0%、1.5%)与钢纤维混凝土叠浇层厚度(0 mm、180 mm、210 mm、300 mm)为变量,对6根FRP筋异强混凝土叠浇梁进行三分点弯曲试验,并对试验梁的破坏过程、破坏形态、裂缝宽度以及跨中挠度进行分析。研究结果表明：钢纤维的掺入改善了FRP筋异强混凝土叠浇梁的受力性能,使其由脆性破坏向延性破坏发展;随着钢纤维掺量、钢纤维混凝土叠浇层厚度的增加,FRP筋异强混凝土叠浇梁的极限承载力提高了9%～33%,抗弯性能提升了4%～21%,延性提升了22%～89%。基于试验与理论分析,建立了钢纤维作用下的FRP筋异强混凝土叠浇梁挠度计算公式与延性评价方法。     

Flexural performance of ring-type steel fiber-reinforced concrete

Steel fiber-reinforced concrete (SFRC) with randomly dispersed, short straight steel fibers hardly fails by fiber yielding, and the postpeak behavior is governed by mechanisms related to fiber pullout. It would be more desirable if more fracture energy could be consumed by fiber yielding at failure. It has been experimentally demonstrated in this research that SFRC with the ring-type steel fibers failed by more energy consuming mechanisms other than fiber pullout. Consequently, significant improvements in flexural toughness were obtained as compared to that of SFRC with conventional straight steel fibers.     

Fracture behavior and analysis of fiber reinforced concrete beams

Fiber reinforced concrete beams with varying notch depths and different volume fractions of steel fibers were tested. The results were analyzed to examine the applicability of various fracture mechanics approaches including: critical stress intensity factor, J-integral, critical crack opening displacement, compliance technique for determining the slow crack growth and R-curve analysis. Attempts were made to identify a fracture parameter which is independent of test-specimen geometry and which can correctly predict the effects of fiber addition. R-curve method appears to be promising.     

Cohesive fracture model for functionally graded fiber reinforced concrete

A simple, effective, and practical constitutive model for cohesive fracture of fiber reinforced concrete is proposed by differentiating the aggregate bridging zone and the fiber bridging zone. The aggregate bridging zone is related to the total fracture energy of plain concrete, while the fiber bridging zone is associated with the difference between the total fracture energy of fiber reinforced concrete and the total fracture energy of plain concrete. The cohesive fracture model is defined by experimental fracture parameters, which are obtained through three-point bending and split tensile tests. As expected, the model describes fracture behavior of plain concrete beams. In addition, it predicts the fracture behavior of either fiber reinforced concrete beams or a combination of plain and fiber reinforced concrete functionally layered in a single beam specimen. The validated model is also applied to investigate continuously, functionally graded fiber reinforced concrete composites.     

基于分形理论和扩展有限元法的钢纤维混凝土损伤破坏机理

为研究钢纤维混凝土损伤破坏过程和裂纹发展演化机理,基于分形理论和扩展有限元法,建立钢纤维混凝土立方体抗拉试验细观有限元模型和切口梁三点弯曲试验有限元模型,以相关试验测试结果为基础,比较验证了所建有限元分析模型的可靠性。以裂纹分形维数表征钢纤维混凝土损伤演化过程,考察不同钢纤维体积掺量和长度、粗骨料形状等重要因素对钢纤维混凝土损伤演化过程的影响。结果表明,基于裂纹分形维数的损伤值可以较好地反映钢纤维混凝土的损伤演化过程及特征,钢纤维体积掺量、长度的增加和骨料形状的不规则化会延缓钢纤维混凝土立方体试件的损伤演化过程,钢纤维体积掺量、初始裂纹距跨中距离的增加和初始裂纹缝高比的减小可在较小程度上延缓钢纤维混凝土切口梁的损伤演化过程。     

玄武岩纤维钢筋混凝土梁弯曲性能试验研究

通过玄武岩纤维钢筋混凝土梁四点弯曲试验,分析其破坏形态和破坏机理.通过四点对称加载方式研究不同玄武岩纤维掺量下梁的承载力、挠度、韧性、混凝土应变、钢筋应变等变化规律.试验结果表明:玄武岩纤维掺入对钢筋混凝土梁的开裂荷载和极限荷载都有一定的提高,开裂荷载最大提高幅度为32％,极限荷载最大提高幅度为6.5％.与普通钢筋混凝土梁相比玄武岩纤维钢筋混凝土梁的挠度、韧性均有所提高.玄武岩纤维对梁的受压区混凝土具有阻止裂缝扩展的能力,当梁上部受压区混凝土被压碎时,混凝土碎块会在纤维的桥接作用下不剥落,梁仍保持较好的整体性.     

Relation between fibre distribution and post-cracking behaviour in steel fibre reinforced self-compacting concrete panels

In this research, the influence of the fibre distribution and orientation on the post-cracking behaviour of steel fibre reinforced self-compacting concrete (SFRSCC) panels was studied. To perform this evaluation, SFRSCC panels were cast from their centre point. For each SFRSCC panel, cylindrical specimens were extracted and notched either parallel or perpendicular to the concrete flow direction, in order to evaluate the influence of fibre dispersion and orientation on the tensile performance. The post-cracking behaviour was assessed by both splitting tensile tests and uniaxial tensile tests. To assess the fibre density and orientation through the panels, an image analysis technique was employed across cut planes on each tested specimen. It is found that the splitting tensile test overestimates the post-cracking parameters. Specimens with notched plane parallel to the concrete flow direction show considerable higher post-cracking strength than specimens with notched plane perpendicular to the flow direction.     

芳纶纤维加固钢筋混凝土梁抗弯疲劳性能试验研究

本文采用芳纶纤维加固钢筋混凝土梁进行室内疲劳试验,分析了混凝土和钢筋的应变滞回变化规律及加固构件刚度随循环次数的衰减变化规律.试验表明采用芳纶纤维进行加固后,梁的疲劳抗裂性能得到极大改善,有效地延长了损伤混凝土结构的使用寿命,验证了芳纶纤维用于加固承受疲劳荷载结构的可靠性.     

Non-oxide CMC fabricated by Fused Filament Fabrication (FFF)

The present study introduces an approach to additively manufacture non-oxidic Ceramic Matrix Composites (CMCs) by the Material Extrusion (MEX)-based Fused Filament Fabrication. Therefore, highly SiC particle filled thermoplastic filaments were developed containing short fibers and/or long fibers to maintain shape during the thermal treatment and exhibiting flaw tolerant behavior. Polymer Infiltration and Pyrolysis (PIP) was applied to densify the microstructure. It is shown that a low content of long fibers of about 5 % can already exhibit damage tolerance comparable to steel reinforced concrete. Thus, the mechanical behavior is different to conventional fiber composites with significant higher fiber volume content. Nevertheless, these results enable new opportunities for producing advanced complex CMC components by means of Additive Manufacturing (AM).     

The bond of near-surface mounted reinforcement to low-strength concrete

This paper focuses on the bond between near-surface mounted (NSM) reinforcement and low-strength concrete. In order to investigate this, eight beams made of low-strength concrete were made. The compressive strength of this concrete varied from 14.22?MPa to 16.83?MPa. These beams were then tensioned under monotonic loading until failure. The test setups differed in terms of their groove size and the type of reinforcement (a rod and plate of carbon fiber reinforced polymer, prestressing steel). Based on the achieved results and analysis, it was found that the NSM method can be applied to low-strength concrete. Furthermore, the application of a NSM reinforcement rod and plate, made of the carbon fiber reinforced polymer, and prestressing steel showed a satisfactory bond strength when compared to low-strength concrete. However, the carbon plates performed better in terms of failure load and rate use than the rods made of carbon and the prestressing steel. Moreover, the results showed that the increase of groove size for the near-surface mounted reinforcement made of prestressing steel did not have an effect on the failure mode. In addition, a significant increase of the failure load was observed for the prestressing steel. Finally, the effect of concrete strength was analyzed and compared with the results found in literature.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

Predicting the pullout response of inclined hooked steel fibers

Steel fiber reinforced concrete (SFRC) is symptomatically an anisotropic material due to the random orientation of fibers within the cement matrix. Fibers under different inclination angles provide different strength contributions at a given crack width. Therefore the pullout response of inclined fibers is a paramount subject to understand and quantify SFRC behavior, particularly in the case of fibers with hooked ends, which are currently the most widely used. Several experimental results were considered to validate the approach and to assure its suitability on distinct material properties and boundary conditions. The good agreement on predicting the pullout behavior of these fibers encourages its use towards a new concept of design and optimization of SFRC.     

Synergistic effect of combined fibers for spalling protection of concrete in fire

This study demonstrates the synergistic effect of some particular combination of fibers that can provide significantly better spalling protection of concrete in a fire than single fiber by themselves at the same fiber content level. Various combinations of polypropylene, polyvinyl alcohol, cellulose and nylon fibers were investigated. Fire tests were conducted in accordance with ISO-834. The combination of nylon (9 mm length) and polypropylene (19 mm length) fibers found to provide the most optimum results. By combining these two fibers, the same level of spalling protection was achieved by three times less fiber content than the single type of 0.10% polypropylene fiber commonly prescribed. A “fiber effectiveness parameter” is proposed which is a function of total number of fibers per unit volume and length of fiber. This parameter is useful in providing quantitative explanations of various fiber additions and their spalling results in fire.     

Simple tools for fiber orientation prediction in industrial practice

In this paper, the two origins of the preferred orientation of fibers are first reviewed. We then propose a definition of what to call an oriented fiber from a practical point of view in the cementitious material field. Considering typical industrial flows and materials, we identify the dominant phenomena and orientation characteristic time involved in the fiber orientation process in the construction industry. We show that shear induced fiber orientation is almost instantaneous at the time scale of a typical casting process. We moreover emphasize the fact that shear induced orientation is far stronger in the case of fluid materials such as self compacting concretes. The proposed approach is validated on experimental measurements in a simple channel flow. Finally, a semi-empirical relation allowing for the prediction of the average orientation factor in a section as a function of the rheological behavior, the length of the fibers and the geometry of the element to be cast is proposed.     

Influence of steel fibers on the development of alkali-aggregate reaction

This work presents the results of an experimental research concerning the use of fibers in mortar specimens subjected to alkali-aggregate reaction (AAR). Two types of steel fibers (0.16 mm diameter and 6.0 mm length, and 0.20 mm diameter and 13.0 mm length) were used with fiber volume contents of 1% and 2%. Besides the expansion accelerated tests, compressive tests and flexural tests have also been carried out to display the main mechanical characteristics of the fiber-reinforced mortars after being subjected to AAR. Moreover, the microstructure of the specimens was analyzed by scanning electron microscopy and energy dispersive X-ray. The results shown that the addition of steel fibers reduced the expansion due to AAR for the experimental conditions studied in this paper. The most expressive benefit corresponded to the addition of 13.0 mm fibers in the mixture containing 2% fiber content. This fiber volume content also corresponded to the maximum increment in the mechanical properties compared to the reference mortar, mainly for the post-cracking strength and for the toughness in bending. It was observed that the fibers have a beneficial effect on the material, without compromising its main mechanical properties.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

BFRP筋再生混凝土梁抗弯性能试验研究

通过7根玄武岩纤维(BFRP)筋再生混凝土梁和1根钢筋再生混凝土梁的抗弯性能试验,研究了不同纵筋配筋率与再生骨料取代率对BFRP筋再生混凝土梁受弯性能的影响,分析其承载力变化过程、破坏形态、挠度变形与裂缝发展情况,并与钢筋再生混凝土梁进行对比。结果表明,BFRP筋再生混凝土梁的破坏形式有少筋和超筋破坏两种,分别由BFRP筋拉断和受压区再生混凝土压碎控制。合理配筋的BFRP筋再生混凝土梁破坏前产生的挠度较大,且受拉BFRP筋的应变也较大,说明合理配筋的BRPP筋再生混凝土梁具有一定的延性,能较好地发挥两种材料的性能。BFRP筋再生混凝土梁的裂缝宽度和裂缝条数受再生骨料取代率影响较小,而受配筋率影响较大。此外,BFRP筋再生混凝土梁的初裂荷载相比钢筋再生混凝土梁略低,但极限荷载却有明显提高。     

端钩型钢纤维UHPC抗压强度的试验研究

本文对5组超高性能混凝土(UHPC)立方体试件进行了轴心受压试验,观察不同纤维掺量及不同尺寸UHPC试件的破坏过程及破坏形态,研究了端钩型钢纤维及不同尺寸对UHPC受压性能的影响,比较各纤维体积掺量立方体试块的荷载-位移曲线,给出了纤维约束系数,分析了其对UHPC立方体抗压强度及压缩耗能的影响,建立了UHPC立方体抗压强度的预测模型。结果表明:与未掺纤维UHPC试件相比,掺入端钩型钢纤维的试件,在载荷达到极限荷载的40%左右时,试件开始发生损伤,在载荷接近极限荷载时,试件内部发生持续的快速断裂声响;掺入端钩型钢纤维的UHPC试件最终破坏呈多条斜向裂缝,且最终破坏时试件仍能保持完整形态,呈现“裂而不碎”的状态;随着端钩型钢纤维体积掺量的增加,试件的受压峰值荷载增加,且伴随着试件的变形增大;与未掺纤维UHPC试件相比,随着纤维掺量的增大,尺寸效应对UHPC的受压性能的影响逐渐减小。基于纤维约束指数,建立了UHPC立方体抗压强度的预测模型,预测结果与试验结果吻合度较高。     

The relation between fiber orientation and tensile behavior in an Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC)

In this study, the effect of the fiber orientation distribution on the tensile behavior of Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC) was investigated. The tensile behavior was explored separately in two stages; pre-cracking and post-cracking tensile behaviors. Pre-cracking tensile behavior is expressed using the mechanism of elastic shear transfer between the matrix and the fiber in the composites. Post-cracking tensile behavior was expressed as the combined behavior of the resistance by the fibers and the matrix, considering a probability density distribution for the fiber orientation distribution across crack surface and a pullout model of steel fiber. The effect of the fiber orientation distribution was found to be very small on pre-cracking behavior, but to be significant on post-cracking behavior of UHPFRCC. The predicted results were compared with the experimental results, and the comparison presented satisfactory agreement.     

Effect of shrinkage reducing admixture on tensile and flexural behaviors of UHPFRC considering fiber distribution characteristics

This study investigated the effect of shrinkage reducing admixture (SRA) on the properties of ultra high performance fiber reinforced concrete (UHPFRC) including fluidity, compressive, single fiber pullout, tensile and flexural behaviors. In addition, the influence of fiber distribution characteristics such as fiber orientation, fiber dispersion, number of fibers, and packing density on the flexural behavior of UHPFRC was evaluated according to the amount of SRA, using an image processing technique that was developed. Three different SRA to cement weight ratios (0%, 1%, and 2%) were investigated on UHPFRC with 2 vol.% of steel fibers. The specimen without SRA exhibited the best performance in compressive, single fiber pullout, and tensile behaviors including load carrying capacity, strain capacity, and energy absorption capacity and had a highly densified interfacial transition zone between fiber and matrix. In particular, the flexural strength of UHPFRC varied with the fiber distribution characteristics, rather than the amount of SRA.