Residual flexural strength after low-velocity impact in glass/polyester composite beams

This work focuses on an experimental study of flexural after impact behaviour of glass/polyester composite beams. The influence of impact energy, beam width, and impactor-nose geometry on the residual flexural strength was evaluated. Two widths of plain woven laminate specimens were selected. For each specimen width, the geometries of two impactor-noses (Charpy and hemispherical) were chosen to carry out impact tests using a three-point bending device, so that four different test configurations were executed. The residual flexural strength of damaged specimens, evaluated by quasi-static three-point bending tests, was found to depend on the extent of the damage, so that the residual flexural strength was lower in the specimens in which the damage reached the edges of the beam. For this reason, the residual strength was lower in specimens impacted with a Charpy-nose impactor than in the specimens impacted with a hemispherical-nose impactor. Analogously, the narrower specimens presented a lower residual flexural strength than did the wider ones.     

CFRP筋增强ECC梁弯曲性能试验研究

为研究碳纤维增强树脂复合材料(Carbon fiber reinforced polymer,CFRP)筋/超高韧性纤维增强水泥基复合材料(Engineered cementitious composite,ECC)梁的抗弯性能,对3根CFRP筋/ECC梁、1根玻璃纤维增强树脂复合材料(Glass fiber reinforced polymer,GFRP)筋/梁和1根CFRP筋混凝土梁进行了四点弯曲试验,分析了配筋率、纤维增强树脂复合材料(Fiber reinforced polymer,FRP)筋类型和基体类型对梁抗弯性能的影响。试验结果表明:CFRP筋/ECC梁与GFRP筋/ECC梁和CFRP筋混凝土梁类似,均经历了弹性阶段、带裂缝工作阶段和破坏阶段;配筋率对CFRP筋/ECC梁的受弯性能影响较大。随着配筋率的增加,CFRP筋/ECC梁的承载能力不断提高,延性性能逐渐减弱;ECC材料优异的应变硬化能力和受压延性,使得CFRP筋/ECC梁的极限承载能力和变形能力均优于CFRP筋混凝土梁;由于ECC材料多裂缝开裂能力,CFRP筋/ECC梁开裂后,纵筋表面应变分布比CFRP筋混凝土梁更均匀; 由于聚乙烯醇(Polyvinyl alcohol,PVA)纤维的桥联作用,CFRP筋/ECC梁破坏时,其表面出现了大量的细密裂缝,且能保持较好的完整性和自复位能力;正常使用阶段,CFRP筋/ECC梁的最大弯曲裂缝宽度均小于CFRP筋混凝土梁。最后,根据试验结果,建立了基于等效应力图的CFRP筋/ECC梁弯曲承载力简化计算模型,确定模型中的相关系数。由简化模型计算的极限承载力与试验结果具有较好的相关性。      

Development of kenaf fibre reinforced polymer laminate for shear strengthening of reinforced concrete beam

Fabrication of green fibre composite laminate for strengthening of reinforced concrete structure is one of the current interests in the field of construction industry. The aim of this research was to develop kenaf fibre reinforced polymer (KFRP) laminate for shear strengthening of reinforced concrete beam. Comprehensive design and theoretical models were also proposed for KFRP laminate shear strengthened beam. In the experimental programme, KFRP laminate had been fabricated with various fibre content to obtain optimal mix ratio. Physical and mechanical properties of KFRP laminates were experimentally investigated. Three reinforced concrete beam specimens were prepared for structural investigations. Results showed that KFRP laminate with maximum fibre content had the highest tensile strength and the laminate was found to be elastic isotropic in nature. The KFRP laminate strengthened beam had 100 % higher shear crack load and 33 % ultimate failure load as compared to un-strengthened control beam. It reduced the numbers and width of cracks and had shown strain compatibility behavior with shear reinforcement. The failure load, ductility, crack patterns and strain characteristics of KFRP laminate strengthened beam were found to be closely comparable with CFRP laminate strengthened beam. The experimental results satisfactorily verified the proposed design and theoretical models.     

Behaviour of hybrid FRP–UHPC beams subjected to static flexural loading

This paper provides the experimental results of a new hybrid beam intended for use in bridge applications. The hybrid beams were made up of pultruded Glass Fibre Reinforced Polymer (GFRP) hollow box section beams strengthened with a layer of Ultra-High-Performance-Concrete (UHPC) on top and either a sheet of Carbon FRP (CFRP) or Steel FRP (SFRP) on the bottom of the beam. Four hybrid FRP–UHPC beams were tested along with one control GFRP hollow box beam under four-point static flexural loading. Two types of beams were tested (Phase I and Phase II), which incorporated different connection mechanisms at the GFRP–UHPC interface. It was concluded that the hybrid beams had higher flexural strength and stiffness than the control beam, where the beams reinforced with SFRP showed greater percent cost effectiveness than beams reinforced with CFRP. In addition, the improved connection mechanism used in Phase II beams was found to provide adequate interface bond strength to maintain full composite action until ultimate failure.     

Mechanical performance of composite-strengthened concrete structures

The interest of using fibre reinforced plastic (FRP) materials in rehabilitating damaged concrete structures respectively has been increased rapidly in recent years. In this paper, the structural behaviours of the glass–fibre composite strengthened concrete structures subjected to uni-axial compression and three point bending tests are discussed through experimental studies. Two types of concrete structure are used in present study, they are concrete cylinder and rectangular concrete beam. Discussion on the environmental effects of composite strengthened reinforced concrete (RC) structures is also addressed. Experimental results show that the use of glass–fibre composite wrap can increase the load carrying capacity of the plain concrete cylinders with and without notch formation. The flexural load capacity of the concrete beam increases to more than 50% by bonding 3 layers of glass–fibre composite laminate on the beam tension surface. Direct hand lay up method gives better strengthening characteristic in term of the ultimate flexural load compared with pre-cured plate bonding technique. The flexural strengths of composite strengthened RC beams submerged into different chemicals solution for six months are increased compared with the RC beams without strengthening. The strength of the concrete structure is seriously attacked by strong acids.     

Shear Strength Evaluation of Concrete Beams Reinforced with BFRP Bars and Steel fibers without Stirrups

This paper presents experimental and analytical investigations on concrete beams reinforced with basalt fiber reinforced polymer (BFRP) and steel fibers without stirrups. Independent behaviour of BFRP reinforced beams and steel fiber reinforced beams were evaluated and the effect of combining BFRP bars and steel fiber was investigated in detail. It is found that combining steel fibers with BFRP could change the shear failure of BFRP reinforced beam to flexural failure. Further, the existing analytical models were reviewed and compared to predict the shear strength of both FRP reinforced and steel fiber reinforced beams. Based on the review, the appropriate model was chosen and modified to predict the shear strength of BFRP reinforced beam along with steel fibers.     

钢筋混凝土梁受弯破坏及尺寸效应的细观模拟分析

钢筋混凝土构件破坏的尺寸效应取决于混凝土材料的非均质性以及钢筋/混凝土相互作用。该文借助混凝土细观结构特征,基于非线性弹簧单元来描述钢筋与混凝土之间的相互作用,建立了钢筋混凝土梁破坏行为模拟的三维细观数值模型。在模拟结果与试验结果吻合良好的基础上,拓展模拟了更大尺寸梁的弯曲大变形破坏行为,并分析了单调及循环加载模式对不同尺寸悬臂梁受弯破坏及名义抗弯强度影响规律。模拟结果分析表明：1)该文工况下钢筋混凝土悬臂梁的弯曲破坏存在尺寸效应,弯曲强度随梁深增大而减小; 2)循环加载下,混凝土、钢筋以及两者间的粘结性能由于低周疲劳而使得梁的弯曲破坏呈现出脆性特征; 3)相比于单调加载,循环加载条件下,悬臂梁的破坏具有更强的脆性,名义抗弯强度尺寸效应更明显。     

Use of permanent ferrocement forms for concrete beam construction

The results of an experimental investigation to examine the feasibility and effectiveness of using precast U-shaped ferrocement laminates as permanent forms for construction of reinforced concrete beams are presented in this paper. The precast permanent ferrocement forms are proposed as a viable alternative to the commonly used wooden and/or steel temporary forms. The experimental program comprised casting and testing of three control reinforced concrete beams of dimensions 300?×?150?×?2000?mm and eighteen beams of total dimensions of 300?×?150?×?2000?mm consisting of a reinforced concrete core cast in a precast U-shaped permanent ferrocement form of thickness 25?mm. Each control beam was reinforced with two steel bars of 12?mm diameter at the top and bottom of the beam and stirrups of 10?mm diameter placed at 200?mm intervals. The concrete core of the beams incorporating permanent ferrocement forms was reinforced with two steel bars of 12?mm diameter placed at the tension side of the beam without any stirrups. Three types of steel mesh were used to reinforce the ferrocement laminate. These types are: woven wire mesh, ×8 expanded wire mesh, and EX156 expanded wire mesh. Single layer and double layers of each type of the steel mesh were employed. All specimens were tested under three-point flexural loadings. The performance of the test beams in terms of strength, stiffness, cracking behavior and energy absorption was investigated. The results showed that high serviceability and ultimate loads, crack resistance control, and good energy absorption properties could be achieved by using the proposed ferrocement forms.     

Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates

Existing experimental studies showed that the reinforced concrete (RC) beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates had three possible flexural failure modes (including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio. Theoretical formulas based on the compatibility of strains and equilibrium of forces were presented to predict the nominal flexural strength of strengthened beams under the three failure modes, respectively, and a limitation on the tensile strain level developed in the prestressed CFRP plate was proposed as the debonding failure occurred. In addition, the calculation methods for cracking moment, crack width and deflection of strengthened beams were provided with taking into account the contribution of prestressed CFRP plates. Experimental studies on five RC beams strengthened with prestressed CFRP plates and a nonlinear finite element parametric analysis were carried out to verify the proposed theoretical formulas. The available test results conducted by other researchers were also compared with the predicted values.     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.     

Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.     

模拟不中断交通状况下粘贴碳纤维布加固钢筋混凝土梁试验研究

为了调查动荷载作用下碳纤维布与钢筋混凝土梁的粘贴性能及加固效果,进行了5根模拟交通荷载(等幅正弦波形动荷载)作用下粘贴碳纤维布加固钢筋混凝土梁和1根保持荷载下粘贴碳纤维布加固钢筋混凝土梁以及2根对比梁的试验研究.试验中考虑了混凝土等级、配筋率、有无锚固条、粘贴长度、荷载幅值5个变化参数.试验结果表明,在模拟交通荷载的作用下,碳纤维布加固的钢筋混凝土梁粘贴性能满足要求,粘贴效果良好.在模拟交通荷载后的静载作用下,试验梁的抗弯承载力提高较多,加固效果明显,进一步验证了粘贴效果.     

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。     

粘贴钢板加固钢筋混凝土梁的分离式有限元模型

提出了外粘钢板加固受弯钢筋混凝土梁的非线性有限元模型。该模型中采用了一种特殊的、具有剥离破坏功能的界面单元来模拟混凝土梁和外粘钢板之间的粘结层,这种剥离破坏主要发生在粘贴钢板端部区域和弯曲、剪切裂缝附近。影响这种剥离破坏的主要因素有两个:一是粘贴钢板的端部与加固梁支座距离;二是粘贴钢板的厚度。传统的梁理论不能描述这种加固梁破坏模式,采用有限元方法能全方位地描述这种加固梁的各种性状和破坏模式。数值计算结果与粘贴不同厚度钢板加固梁的试验结果相吻合。     

碳纤维布加固已承受荷载的钢筋混凝土梁抗弯性能试验研究及抗弯承载力计算

进行了6根碳纤维布加固已承受荷载的钢筋混凝土梁和2根对比混凝土梁的抗弯性能试验研究,分析了碳纤维布加固已承受荷载的钢筋混凝土梁的破坏机理,研究了荷载历史对加固梁极限荷载的影响.试验结果表明,粘贴碳纤维布可以有效地提高加固梁的抗弯承载能力.无论荷载历史如何,只要梁承受的初始荷载相同,梁破坏时的极限荷载基本相同.梁端锚固对加固梁的极限荷载影响不明显.根据不同的破坏模式,提出了碳纤维布加固已承受荷载的钢筋混凝土梁的承载力计算方法,给出了工程实用计算公式.     

FRP筋混凝土T形和矩形截面梁抗弯承载力计算方法

为建立纤维增强复合材料(fiber-reinforced polymer,FRP)筋混凝土T形和矩形截面梁抗弯承载力简化计算方法,根据平衡破坏状态下的截面分析,定义了等效FRP配筋率ρ_ef和相应的平衡配筋率ρ_{ef, b}。在此基础上,基于257根FRP筋混凝土梁试验结果的统计分析,改进了受拉破坏和受压破坏皆可能发生的过渡区范围(ρ_{ef, b} <ρ_ef≤1.5ρ_{ef, b})。编制了受拉破坏控制截面的非线性分析程序,考虑多个设计参数的影响,开展了25 344个截面的参数分析。通过对参数分析结果的多元回归分析,推导了受拉破坏控制截面的抗弯承载力简化计算公式。此外,基于截面内力平衡和协调条件,推导了受压破坏控制截面的抗弯承载力计算公式。以国内外257根梁抗弯承载力试验结果,验证了所提方法的适用性。     

玄武岩纤维复合材料性能提升及其新型结构

为建立纤维增强复合材料(fiber-reinforced polymer,FRP)筋混凝土T形和矩形截面梁抗弯承载力简化计算方法,根据平衡破坏状态下的截面分析,定义了等效FRP配筋率ρ_ef和相应的平衡配筋率ρ_{ef, b}。在此基础上,基于257根FRP筋混凝土梁试验结果的统计分析,改进了受拉破坏和受压破坏皆可能发生的过渡区范围(ρ_{ef, b} <ρ_ef≤1.5ρ_{ef, b})。编制了受拉破坏控制截面的非线性分析程序,考虑多个设计参数的影响,开展了25 344个截面的参数分析。通过对参数分析结果的多元回归分析,推导了受拉破坏控制截面的抗弯承载力简化计算公式。此外,基于截面内力平衡和协调条件,推导了受压破坏控制截面的抗弯承载力计算公式。以国内外257根梁抗弯承载力试验结果,验证了所提方法的适用性。     

GFRP筋橡胶集料混凝土梁受弯性能

为提高纤维增强聚合物复合材料(FRP)筋混凝土梁抗裂性能,改善其脆性破坏特征,将玻璃纤维增强聚合物复合材料(GFRP)筋与橡胶集料混凝土共同应用于梁构件中。采用ABAQUS对GFRP筋橡胶集料混凝土梁的受弯性能进行有限元模拟及参数分析,探究了橡胶掺量、GFRP筋配筋率、混凝土强度等级及截面高度对梁受弯性能的影响。结果表明:增加混凝土中橡胶颗粒的掺量可提高梁的开裂荷载,当橡胶掺量为15%时,开裂荷载提高了29%;增加配筋率可提高梁的开裂荷载和承载力,当受拉筋直径由10 mm增加至18 mm时,橡胶掺量为10%的GFRP筋橡胶混凝土梁开裂荷载提高了约15%,承载力提高了约85%,但配筋率增加至一定数值后,其影响不再明显;提高橡胶混凝土强度等级,可提高梁的开裂荷载及承载力,当橡胶混凝土强度等级由C25提高至C40时,开裂荷载提了高约53.7%,承载力提高了约23%;为更好地满足正常使用极限状态,GFRP筋橡胶混凝土梁的截面高度宜适当增加。      

Flexural performance of layered ECC-concrete composite beam

The pseudo strain-hardening behavior of fiber reinforced engineered cementitious composites (ECC) is a desirable characteristic for it to act as a substitute for concrete to suppress brittle failure. The use of ECC in the industry is, however, limited by its high cost. To achieve higher cost/performance ratio, ECC can be strategically applied in parts of a structure that is under relatively high stress. In this paper, layered ECC-concrete beams subjected to flexural load are investigated from both theoretical and experimental aspects. Four-point bending tests are performed on beam members with ECC layer at its tensile side. The application of ECC layer leads to increase in both the flexural strength and ductility, and the degree of improvement is found to increase with the ECC thickness. A semi-analytical approach for modeling the flexure behavior of layered ECC-concrete beams is also developed. In the model, the stress–crack width relation of both concrete and ECC are employed as fundamental constitutive relationships. The model and experimental results are found to be in good agreement with one another. Simulation with the model shows that when the ECC thickness goes beyond a certain critical value, both the flexural strength and ductility (reflected by crack mouth opening and crack length at ultimate load) will significantly increase. The critical ECC thickness is hence an important design parameter, and it can be determined with the theoretical approach developed in the present work.