BFRP筋钢纤维高强混凝土梁受弯承载力试验与理论

通过11根玄武岩纤维增强聚合物复合材料（BFRP）筋钢纤维高强混凝土梁的受弯性能试验,研究了钢纤维混凝土层厚度、钢纤维体积分数和BFRP筋配筋率对BFRP筋钢纤维高强混凝土梁受弯破坏形态及其承载力的影响。结果表明,BFRP筋钢纤维高强混凝土梁的破坏模式可分为受压破坏、受拉破坏和平衡破坏3种;钢纤维混凝土层厚度和钢纤维体积分数的变化对于BFRP筋钢纤维高强混凝土梁受弯承载力具有一定程度的影响,当BFRP筋配筋率为0.77%时,掺加体积分数为1.0%钢纤维的梁受弯承载力较无钢纤维梁提高了22.7%,在受拉区0.57倍截面高度内掺加1.0vol%钢纤维的梁受弯承载力达到全截面钢纤维混凝土梁受弯承载力的86.7%;增大BFRP筋配筋量可显著提高BFRP筋钢纤维高强混凝土梁的受弯承载力,BFRP筋配筋率为1.65%的试验梁受弯承载力较配筋率为0.56%的试验梁提高了39.4%。针对不同的破坏模式,提出了BFRP筋钢纤维高强混凝土梁受弯承载力和平衡配筋率的计算方法,并结合安全配筋率的概念对试验梁的破坏模式进行了预测,试验结果与分析结果吻合良好。     

Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

Effects of stirrup,steel fiber,and beam size on shear behavior of high-strength concrete beams

This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.     

预制装配型钢混凝土梁受剪承载力试验与计算方法研究

为了深入研究预制装配型钢混凝土梁的受剪机理并提出可准确预测其受剪承载力的计算公式,该文完成了2个足尺预制装配型钢混凝土梁试件的静力剪切性能试验。通过分析试件的破坏过程、荷载-位移曲线和应变发展规律,对不同剪跨比下试件的破坏形态和承载能力进行了研究。基于变形协调桁架-拱模型和Nakamura模型建立了该种预制装配型钢混凝土梁及普通现浇型钢混凝土梁共同适用的受剪承载力计算模型。通过与75个发生剪切破坏的型钢混凝土梁试验结果对比可得:该文提出的计算方法可较好反映型钢混凝土梁的剪切破坏机理,试验值与计算值吻合良好;规范AISC 360-2010和JGJ 138-2016建议的受剪承载力计算公式较为保守。     

基于MCFT理论的钢纤维混凝土梁的截面分析

根据钢纤维混凝土的特性,对MCFT理论的裂后混凝土平均主应力-平均主应变关系进行了修正。在Vecchio和Collins对钢筋混凝土板在纯剪作用下截面分析的基础上,叠加了弯矩的作用,建立了钢纤维混凝土梁在弯剪复合作用下的截面分析模型。利用作者以及其他研究者的试验对该模型进行了验证,结果表明计算得到的钢纤维混凝土梁的剪力-箍筋应变曲线和极限荷载与实测结果吻合良好。该文还利用该模型对钢纤维和箍筋对梁抗剪性能的影响效率进行了比较。     

Finite element analysis of partially prestressed steel fiber concrete beams in shear

This paper presents a finite element formulation for the modeling of the behavior of partially prestressed steel fiber concrete beams in shear. Based on a secant modulus approach, the formulation treats steel fiber concrete as an orthotropic material, characterized by appropriate constitutive relations in the principal compressive and tensile directions. An experimental program with the partial prestressing ratio, the shear span:effective depth ratio, and the volume fraction of steel fibers as test variables was carried out and the deflections of the beam, concrete, and steel strains were monitored and compared with the results of the finite element analysis. The finite element formulation was found to predict the deformational characteristics and the ultimate load of the test beams well. Steel fibres were observed to improve the beam stiffness after the occurrence of first shear crack and to enhance the shear strength of partially prestressed concrete beams significantly.     

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.     

剪跨比对CFRP加固无腹筋混凝土梁剪切破坏及尺寸效应的影响研究

剪跨比对FRP抗剪加固梁的裂缝开展和破坏模式有重要影响,但对FRP加固梁抗剪强度及尺寸效应的影响研究较少.采用三维细观数值模拟方法,考虑混凝土细观组成的非均质性及碳纤维布(CFRP)与混凝土之间的相互作用,建立了CFRP加固无腹筋钢筋混凝土梁剪切破坏力学分析模型.在验证细观模拟方法合理性的基础上,拓展模拟与分析了剪跨比...     

应变速率型RC梁柱显式分析单元及其在ABAQUS软件中的实现

为准确、高效地分析钢筋混凝土(RC)梁柱结构在强动载作用下的损伤破坏甚至倒塌,建立了能够描述大变形、大应变的空间纤维Timoshenko梁单元,并将其与考虑率相关效应的钢筋、混凝土材料模型相结合,通过用户显式单元子程序在ABAQUS中实现。基于所建立的应变速率型3D纤维梁显式单元,借助ABAQUS的前后处理及求解功能,对爆炸荷载作用下的RC梁柱构件的动态响应和破坏模式及RC框架结构的连续性倒塌进行分析。结果表明:建立的应变速率型3D纤维梁柱单元能够合理描述RC构件的变形特性及钢筋、混凝土的应变速率效应;可以模拟爆炸作用下RC构件的弯曲、弯剪及直剪破坏模式,以及RC框架的连续倒塌过程;将纤维梁柱单元与率相关模型相结合于ABAQUS软件提供了一种强动载作用下高效、精确的RC结构非线性分析方法。     

型钢混凝土深梁受剪性能及承载能力研究EI北大核心CSCD

为了研究型钢混凝土深梁的受剪机理,以剪跨比、型钢截面高度比、翼缘宽度比为影响因素,设计7个试件进行了跨中集中荷载作用下的抗剪性能试验,对受力过程、荷载-位移曲线、破坏形态、受剪承载力等进行了比较分析,基于修正压力场理论提出了型钢混凝土深梁受剪承载力计算模型。结果表明:剪跨比是型钢混凝土深梁破坏形态主要影响因素,较大翼缘宽度比的试件具有更高的受剪承载力;基于修正压力场理论的计算模型能够综合考虑剪跨比和翼缘宽度比的影响,模型计算值与试验结果吻合较好。     

Cracking characteristics of reinforced steel fiber concrete beams under short- and long-term loadings

In this paper, an analytical method for the prediction of maximum crack width in reinforced steel fiber concrete (SFC) beams under short-term loading is first presented. The method accounts for the enhanced cracking strength, restraint against crack growth, and reduced tensile steel strains due to the presence of steel fibers. Based on a correlation analysis, a semiempirical formula for the long-term crack widths in reinforced SFC beams under sustained loads is also proposed. Tests were carried out on 10 beams to investigate the effect of steel fiber content on the cracking characteristics in both the short- and long-term. The results indicated that the use of steel fibers greatly reduced the maximum crack widths in reinforced concrete beams. Good agreement was generally obtained between the analytical predictions and test results.     

玄武岩格栅增强水泥基复合材料单轴拉伸力学性能试验及本构关系模型

考虑玄武岩纤维增强树脂合物基复合材料(BFRP)格栅层数和水泥基复合材料(ECC)配比等因素,对BFRP增强大掺量粉煤灰/矿粉ECC棒骨试件进行了静力单轴拉伸试验,研究掺加增强粉煤灰/矿粉ECC的抗拉力学性能。结合试验数据,基于Richard和Abbot的弹塑性应力-应变公式提出掺加增强ECC的应力-应变本构关系模型。试验结果表明:随着掺加层数的增加,格栅增强ECC的极限抗拉强度显著增大。同配合比掺矿粉制成的ECC抗压强度、开裂应变及应力高于掺粉煤灰制成的ECC。掺加增强掺矿粉ECC试件相对掺粉煤灰ECC试件具有较好的抗拉力学性能。计算结果表明,建立的单轴受拉本构关系模型可以有效地预测掺加增强ECC的应力-应变关系和极限抗拉强度。     

Effective strain of RC beams strengthened in shear with FRP

The failure modes of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) sheets or strips are not well understood as much as those of RC beams reinforced with steel stirrups. When the beams are strengthened in shear with FRP composites, beams may fail due to crushing of the concrete before the FRP reaches its rupture strain. Therefore, the effective strain of the FRP plays an important role in predicting the shear strength of such beams. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams strengthened in shear with FRP composites and internally reinforced with conventional steel stirrups. Ten RC beams strengthened with varying FRP reinforcement ratio, the type of fiber material (carbon or glass) and configuration (continuous sheets or strips) were tested. Comparisons between the observed and calculated effective strains of the FRP in the tested beams failing in shear showed reasonable agreement.     

Analytical and experimental investigations on the fracture behavior of hybrid fiber reinforced concrete

In the present study, Mode-I fracture tests of hybrid fiber reinforced concrete (HFRC) composite beams were conducted and the fracture properties and other post peak strength characteristics of the HFRC composites were evaluated and analyzed. The HFRC composite was produced using three types of fibers namely steel, Kevlar and polypropylene. A total of 27 HFRC composite beam specimens were cast and tested using the RILEM recommended three point bending test. The main variables were the fiber volume content and combinations of different fibers. The load versus crack mouth opening displacement (CMOD) curves of HFRC composite beams were obtained. Inverse analysis was carried out to determine the tensile strength and crack opening relationship. Analytical models based on comprehensive reinforcing index were developed for determining the influence of the fibers on fracture energy, flexural tensile strength, equivalent tensile strengths and residual tensile strengths of HFRC composites. Based on the experimental results and inverse analysis, a model for predicting the tensile softening diagram of HFRC composite mixes was also developed. The analytical models show conformity with the experimental results.     

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.     

Revisiting the shear design equations for concrete beams reinforced with FRP rebar and stirrup

Current design guidelines and codes use modified shear equations for calculating the shear strength contribution of fiber reinforced polymer (FRP) transverse reinforcement (stirrup) in concrete beams reinforced with longitudinal steel or FRP rebar. These equations, originally developed for steel as longitudinal and transverse reinforcement, are semi-empirical in nature and were developed with a core analytical equation where the coefficients were determined from regression analysis. Here, a comparative study among various code equations was conducted to predict the shear strength of FRP reinforced concrete beams. To facilitate this comparison a database was established from the published literature, which was composed of slender concrete beams (shear span to depth ratio, a/d >2.5) with FRP longitudinal and transverse reinforcement. The database contained 114 beams without transverse reinforcement and 46 beams with transverse reinforcement. The guidelines, codes and models that were implemented and compared in this study consisted of ACI 440.1R-06, JSCE 1997, CNR-DT 203, CSA S806-02, CSA S6-06, unpublished CSA S6-06 Addendum, ISIS-M03-01, simplified Modified Compression Field Theory, cracked section analysis model and modified Zsutty equations. It was observed from the statistical analysis that the CSA S806-02 produced greater coefficients of variation than the CSA S6-09. The ACI 440.1R-06 and JSCE 1997 produced more conservative results in calculating the transverse shear strength. The CSA S6-09 Addendum exhibited the best all-around performance in predicting the shear contribution of FRP reinforced beams compared to that of other design codes and guidelines.     

Behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening

This paper summarizes the results of experimental and analytical studies on the flexural strengthening of reinforced concrete beams by the external bonding of high-strength, light-weight carbon fiber reinforced polymer composite (CFRPC) laminates to the tension face of the beam. Four sets of beams, three with different amounts of CFRPC reinforcement by changing the width of CFRPC laminate, and one without CFRPC were tested in four-point bending over a span of 900 mm. The tests were carried out under displacement control. At least one beam in a set was extensively instrumented to monitor strains and deflections over the entire range of loading till the failure of the beam. The increase in strength and stiffness provided by the bonded laminate was assessed by varying the width of laminate. The results indicate that the flexural strength of beams was significantly increased as the width of laminate increased. Theoretical analysis using a computer program based on strain compatibility is presented to predict the ultimate strength and moment–deflection behavior of the beams. The comparison of the experimental results with theoretical values is also presented, along with an investigation of the beam failure modes.     

Concrete beams with externally bonded flexural FRP-reinforcement: analytical investigation of debonding failure

This paper studies the problem of early concrete cover delamination and plate-end failure of reinforced concrete beams strengthened with externally bonded FRP-reinforcement. The accuracy of analytical models and finite element (FE) methods for predicting this type of failure is assessed against published experimental data. Two design approaches based on the maximum concrete tensile strength and the shear capacity of concrete beams were examined first and it was found that linear elastic analysis cannot accurately predict the brittle plate-end concrete failure. It was also found that the extent of strengthening that can be achieved is limited by the shear capacity of concrete beams. The FE analysis is used to examine the effects of internal tensile reinforcement on the magnitude of principal tensile stresses in the critical region. The non-linear behaviour of FRP-strengthened beams is also examined in the FE analysis using the smeared crack model for concrete which is shown to adequately display the inelastic deformation of the beam. Finally, the mixed mode of failure due to the combined shear and concrete cover delamination is addressed through modelling plate-end and shear crack discontinuities using the discrete crack approach.     

Prestressed fiber reinforced concrete beams with reduced ratios of shear reinforcement

This paper presents an analysis of the influence of prestress and fibers on the shear behaviour of thin-walled I-section beams with reduced shear reinforcement ratio. Reduction of shear reinforcement in prestressed precast beams can make the reinforcement simpler and may increase the productivity in long line precasting beds. The use of short fibers can improve the shear strength and ductility. Nine concrete beams were built (six with prestressing forces) with three different mixtures: without fibers, with steel fibers, and with polypropylene fibers. Shear reinforcement ratios varied from 0 to 0.225% (geometric ratio). It was noted that prestressing increases cracking strength (both in bending and shear), extends the non-cracked area, and makes the compression struts less inclined. In the case of fiber reinforced concrete beams, control of cracking is more effective and consequently deflections are smaller. Ductility is also increased. Both fibers and prestressing reduce stresses in the stirrups and increase shear strength.     

Shear enhancement of reinforced concrete beams strengthened with FRP composite laminates

This paper presents the results of an experimental investigation on shear strength enhancement of reinforced concrete beams externally reinforced with fiber-reinforced polymer (FRP) composites. A total of nine full-scale beam specimens of three different classes, as-built (unstrengthened), repaired and retrofitted were tested in the experimental evaluation program. Three composite systems namely carbon/epoxy wet layup, E-glass/epoxy wet layup and carbon/epoxy precured strips were used for retrofit and repair evaluation. Experimental results indicated that the composite systems provided substantial increase in ultimate strength of repaired and strengthened beams as compared to the pre-cracked and as-built beam specimens. A comparative study of the experimental results with published analytical models, including the ACI 440 model, was also conducted in order to evaluate the different analytical models and identify the influencing factors on the shear behavior of FRP strengthened reinforced concrete beams. Comparison indicated that the shear span-to-depth ratio (a/d) is an important factor that actively controls the shear failure mode of beam and consequently influences on the shear strength enhancement.