IC debonding resistance of groups of FRP NSM strips in reinforced concrete beams

The use of fibre reinforced polymer (FRP) externally bonded (EB) plates in the form of pultruded and wet lay-up plates is now generally accepted as an efficient and unobtrusive technique for retrofitting reinforced concrete structures and is applied worldwide. However, EB plates, and in particular EB pultruded plates, tend to debond at strains much lower than their fracture strains. An alternative technique of adhesively bonding pultruded plates or strips in narrow grooves sawn into the concrete cover, that is near surface mounted (NSM) plates or strips, is now gradually gaining acceptance as tests have shown that the debonding strains can be much higher than that for EB plates. However, tests have also shown that NSM plates can interact with adjacent parallel NSM plates to cause intermediate crack (IC) debonding of groups of NSM strips at reduced strengths. This paper develops a mathematical model for the IC debonding resistance of groups of NSM plates for use in the flexural and shear strengthening of reinforced concrete beams.     

钢板加强的钢筋混凝土梁的强度破坏模型

将钢板加固在钢筋混凝土梁的受拉表面已经成为一种弯曲加强的主要方法。十多年来工程师和研究者在钢板加强梁的性能方面做了大量的研究。这些研究表明钢板常由于粘合剂的脱落而过早的失效。本文首次对现有的板端脱落模型进行了分类总结,为广大研究者提供了一个有力的参考依据。     

FRP-strengthened RC slabs anchored with FRP anchors

An abundance of tests over the last two decades has shown the bending capacity of flexural members such as reinforced concrete (RC) beams and slabs to be enhanced by the bonding of fibre-reinforced polymer (FRP) composites to their tension face. The propensity of the FRP to debond, however, limits its effectiveness. Different types of anchorages have therefore been investigated in order to delay or even prevent debonding. The so-called FRP anchor, which is made from rolled fibre sheets or bundles of lose fibres, is particularly suitable for anchoring FRP composites to a variety of structural element shapes. Studies that assess the effectiveness of FRP anchors in anchoring FRP strengthening in flexural members is, however, limited. This paper in turn reports a series of tests on one-way spanning simply supported RC slabs which have been strengthened in flexure with tension face bonded FRP composites and anchored with different arrangements of FRP anchors. The load-deflection responses of all slab tests are plotted, in addition to selected strain results. The behaviours of the specimens including the failure modes are also discussed. The greatest enhancement in load and deflection experienced by the six slabs strengthened with FRP plates and anchored with FRP anchors was 30% and 110%, respectively, over the unanchored FRP-strengthened control slab. The paper also discusses the strategic placement of FRP anchors for optimal strength and deflection enhancement in FRP-strengthened RC slabs.     

CFRP板与钢梁粘结剥离破坏的试验研究

粘结层是碳纤维增强复合材料(CFRP)板加固钢梁的最薄弱环节。通过对4根加固梁的静载试验及6根加固梁的疲劳试验研究了CFRP板加固钢梁的剥离破坏。在CFRP板底端部贴的应变片记录了试验过程中的应变变化,并结合理论分析,探讨了各项因素对板端应变的影响及CFRP板的剥离破坏过程。静载试验中记录的随外荷载变化的应变曲线以及疲劳试验中记录的随疲劳加载次数变化的应变曲线都能分为4个阶段。阶段Ⅰ为板端溢胶对板端作用的拉力的衰退阶段,板端溢胶逐渐失去作用;阶段Ⅱ为正应力衰退阶段,正应变开始减小,而剪应力继续增加,使得压应变的增加更快;阶段Ⅲ为剪应力衰退阶段,在达到压应变峰值后,剪应力和应变减小,直到应变为0;阶段Ⅳ为剥离破坏阶段。根据这一规律,利用板端的应变片可以很好地监控加固梁粘结层的性能变化及裂缝的萌生。     

Interfacial stress analysis of plated beams under symmetric mechanical and thermal loading

Extensive research has shown that bonding a thin plate to the tension face of a beam can significantly improve its flexural performance. Such a plated beam often fails by the premature debonding of the thin plate from the original beam in a brittle manner. A sound understanding of the mechanism of such debonding failures is important for the effective use of this strengthening technique. This paper presents a new analytical solution for the interfacial stresses in plated beams subjected to arbitrary mechanical and thermal loads which are symmetrically positioned about the mid-span. While the bonding of fiber reinforced polymer (FRP) plates to reinforced concrete (RC) beams represents the most common application of the plate bonding technique, the present solution is generic in terms of the materials of the beam and the plate. The solution is represented by Fourier series and is based on the minimisation of complementary energy. The new solution takes into consideration the non-uniform stress distribution in the adhesive layer and the stress-free boundary condition at the ends of the plate. In addition, it correctly predicts the drastic difference in the interfacial normal stress between the plate-to-adhesive interface and the adhesive-to-beam interface. The solution is general in nature and may be applied to the analysis of other types of composite structures.     

外部粘贴碳纤维或钢板加固梁中粘结界面应力分析

钢筋混凝土梁可采用外贴碳纤维或钢板条于梁底的方法进行补强加固。但是 ,在板条与梁底间的粘结界面端部将产生较大的沿粘结面的剪应力和垂直粘结面的正应力 ,这些应力将有可能造成外贴板条端部锚固失效 ,从而 ,使梁因板条的材料强度得不到充分发挥而提前破坏。目前 ,已有的计算界面应力公式都是在弹性理论基础上假定应力沿胶层厚度为常量 ,并且 ,忽略了板条端部的应力奇异现象而得到的。为此 ,对该类加固梁的胶层应力分布 ,特别是板条端部的应力 ,采用LUSAS有限元分析程序进行了详细的计算研究 ,对影响胶层应力分布的主要因素也进行了分析研究     

Numerical simulation of steel-plate strengthened concrete beam by a non-linear finite element method model

Strengthening with epoxy bonded steel plate is one of the most widely used techniques for flexural upgrading of reinforced concrete (RC) beams. However, debonding failure at the plate cut-off zone and or in the vicinity of flexure and shear cracks leads to catastrophic failure of the upgraded beams. This particular failure depends on several factors such as the distance of plate curtailment from the support, plate thickness and the provision of end anchors. Since the conventional beam theory cannot predict the debonding failure of such beams, a finite element model capable of predicting the overall behavior of strengthened beams including different failure modes accurately is developed. This paper presents the formulation of finite elements and material models and simulation results of some RC beams tested for flexural strengthening with epoxy bonded steel plates.     

预应力碳纤维复合板加固钢筋混凝土梁的抗弯响应预测

现有的试验研究显示,碳纤维复合材料用量不同时,采用预应力碳纤维复合板加固钢筋混凝土梁可能出现3种弯曲破坏模式(包括受压破坏、受拉破坏、剥离破坏)。基于应变和力平衡的协调性,提出理论公式对3种破坏模式下的名义抗弯强度进行预测。当出现剥离破坏时,提出预应力碳纤维复合板中的受拉应变的极限。此外,考虑预应力碳纤维复合板的作用,提出开裂弯矩、裂缝宽度和加强梁的挠度计算方法。对5根采用预应力碳纤维复合板加固的钢筋混凝土梁进行试验,并通过非线性有限元分析验证所提出的理论公式。同时,预测值与其他研究者的试验结果也进行了比较。     

Structural behaviour of babadua reinforced concrete beams

The strength and deformation characteristics of concrete beams reinforced with babadua bars ranging from 2.87 to 12.13% were examined from tests performed on the beams. The beams were tested to failure mostly under third-point loading. Collapse of the beams occurred mostly through either flexural failure of concrete in compression or diagonal tension failure. The experimental failure loads averaged, respectively, 6.40 and 2.62 times the theoretical flexural strength and theoretical shear strength of the unreinforced concrete section. Also, the experimental failure loads were only approximately 1.18 times the theoretical flexural strength of the reinforced concrete and 1.05 times the theoretical shear strength of the concrete sections taking into consideration the resistance of the tension reinforcement.     

Moment redistribution in continuous plated RC flexural members. Part 1: neutral axis depth approach and tests

It is now common practice to retrofit or rehabilitate existing reinforced concrete beams and slabs by adhesively bonding fibre reinforced polymer (FRP) or metal plates to their surfaces. Advanced design rules are available for quantifying the various plate debonding mechanisms and consequently the shear and flexural capacities of the plated sections. These design rules show that even though the required increase in strength can be obtained by plating, plate debonding can severely reduce the ductility of a flexural member to such an extent that plating guidelines often exclude moment redistribution. This exclusion may reduce the application of plating, in particular to retrofitting buildings where ductility is often a requirement, or it may require the development of a radically different approach to design that does not rely implicitly on ductility. In this paper, it is shown that the commonly used neutral axis depth approach for moment redistribution in RC flexural members cannot be used for most plated structures because plate debonding often occurs before the concrete crushes. Tests on plated flexural members are also reported which show that moment redistribution can occur. In Part 2 of this paper, a moment redistribution analysis procedure is developed that can cope with plate debonding of externally bonded plates.     

钢板-混凝土组合板受弯性能试验研究

以钢板代替受力钢筋,通过栓钉将外侧钢板与内部混凝土板相连,二者共同作用形成钢板-混凝土组合板。根据钢板布置形式的不同,可将钢板-混凝土组合板分为单面钢板-混凝土组合板（SSC）和双面钢板-混凝土组合板（DSC）。通过对4个SSC试件和3个DSC试件的受弯试验研究,分析了不同钢板厚度、抗剪连接程度以及构造钢筋配置对组合板受弯性能和破坏形态的影响。试验结果表明,按完全抗剪连接设计的试件破坏形态与适筋梁相似,具有良好的受弯承载能力和延性;当受拉区钢板采用部分抗剪连接设计时,剪跨区栓钉易剪断导致承载力明显降低;当受压区钢板采用部分抗剪连接设计时,顶层钢板易发生局部屈曲,导致试件承载力和延性有所降低。基于试验结果,给出了钢板-混凝土组合板的受弯承载力计算式,计算值与试验值吻合较好。     

Flexural behavior of strengthened steel–concrete composite beams by various plating methods

The effect of pre-intermediate separation on the flexural behavior of strengthened steel–concrete composite beams by either adhesively bonded carbon fiber reinforced polymers (CFRP) sheet or welded/bonded steel plate was studied. In the case of strengthened by CFRP sheet, two different attachment patterns, namely, CFRP sheet wrapped around the flange of the I-beam and CFRP sheet wrapped around the flange along with a part of the web, were examined by testing four different strengthened steel–concrete composite beams under four point bending (4PB). Two of these beams were strengthened by fully bonded CFRP sheet with the two different patterns, while, the others are similar but have pre-intermediate debonding area of 50 mm length × flange width at the bottom surface of the lower flange. In the case of strengthened by steel plate, three different attachment patterns of steel plate to the soffit of the beams, namely, discontinuously welded, end welded, and bonded/welded steel plates, were also tested under 4PB.The experimental results showed that, there is no growth of the intermediate debonding before the yield of the lower flange occurred for all strengthened beams by CFRP sheet. After yielding, the beams with pre-debonding area showed lower flexural capacity than those with fully bonding due to the rapid growth of the intermediate debonding. On the other hand, there is a difference in the yield load between the three different patterns of the welded steel plates with a marginal difference in the elastic stiffness.     

芳纶纤维筋有黏结部分预应力混凝土梁受弯性能研究

对以芳纶纤维筋为有黏结预应力筋、环氧涂层钢筋为非预应力筋的部分预应力混凝土梁的受弯性能进行研究,共进行了10根梁从混凝土开裂、裂缝开展直至梁受弯破坏的全过程试验。测定了梁的开裂弯矩,梁截面应变分布,变形的发展,裂缝出现、发展及分布情况,提出区分芳纶纤维筋破断及混凝土压坏的界限等效配筋率或界限中和轴高度,推导出芳纶纤维筋拉断和混凝土压坏同时发生,混凝土压坏、芳纶纤维筋未拉断及芳纶纤维筋拉断、混凝土未压坏等情况的受弯承载力计算方法,并提出芳纶纤维筋有黏结部分预应力混凝土梁短期刚度及裂缝宽度的计算方法。     

Strengthening of reinforced concrete beams in shear by the use of externally bonded steel plates: Part 1 – Experimental programme

This paper describes a programme of research to investigate the behaviour of reinforced concrete beams strengthened in shear by means of externally bonded steel plates and hence to provide engineers with appropriate design guidance. The shear strengthening and testing to failure of 15 rectangular section reinforced concrete beams was conducted. To account for the different potential types of shear failure (diagonal tension, shear compression and deep beam failure) three shear span to depth ratios were used. A further 15 experiments were then conducted using ‘T’section beams. In most cases the addition of externally bonded shear plate reinforcement provided enhancement in shear capacity comparable with that provided by conventional internal shear links designed to BS8110 [British Standards Institute (BSI). Structural use of concrete, Part 1: Code of practice for design and construction. BS 8110: Part 1: BSI. London; 1997]. Measurement of the principal strains in the plated beams showed that as well as tensile and compressive strains due to bending, strains arising from an underlying shear crack are evident. Hence the plates serve to control crack opening and catastrophic shear failure up to a certain load level, when bond failure occurs in the concrete. The use of partial depth plates with T section beams does not appear to be as successful as the use of full depth plates on rectangular section beams.     

Nonlinear finite element analysis of concrete filled steel plate composite coupling beams: force and deformation mechanics

在外包钢板-混凝土组合连梁试验研究的基础上,对组合连梁试件和不同参数取值的补充模型进行了非线性有限元分析。有限元分析得到的滞回曲线与试验曲线吻合良好,能较好地模拟组合连梁的受力行为。通过应力分析,发现内填混凝土的工作机制类似于斜压杆,其压力沿着连梁对角线方向从一端的受压侧传递到另一端的受压侧。外包钢板承担的剪力比例和弯矩比例的范围分别为0.36~0.76和0.57~0.82;混凝土的剪力与轴压力之比和连梁的高跨比近似相等。连梁的侧向位移由节点区变形引起的梁端转角、连梁的弯曲变形与剪切变形产生;在常用的跨高比范围内,剪切变形产生的侧向位移在总位移中占有较大比例,且受跨高比的影响很大。     

A simple and rational beam segment model for analyzing intermediate crack-induced debonding in FRP-strengthened beams

Intermediate crack (IC) induced debonding failure of fibre reinforced polymer (FRP) plate strengthened reinforced concrete (RC) beams is one of the common debonding modes, in which debonding initiates at a major flexural crack and then propagates toward a plate end. Finite element (FE) method has been used to analyze the IC debonding failure and a whole strengthened beam has been taken as a mechanical model in existed researches. In view of that it is difficult to appropriately simulate all the cracks in an entire strengthened beam and consequently to carry out a parameter study, a beam segment between two major adjacent flexural cracks in a flexural-shear zone of a strengthened beam was taken as a new simple mechanical model. The new simple “beam segment model” was validated to be rational by using Saint–Venant Principle and a numerical study. In the light of this, a parameter study was performed through a three dimensional nonlinear FE analysis of the proposed beam segment model. The influences of the load level, the bond-slip (δ–τ) relationship of FRP-to-concrete interface, and the geometry of the beam segment on the IC induced debonding were investigated. The numerical analysis shows that the influence of δ–τ relationship of FRP-to-concrete interface on debonding is fairly more significant than that of crack spacing of strengthened beams. Comparisons between analytical and experimental results show that the proposed beam segment model is appropriate for estimating the IC induced debonding.     

Flexural and shear capacities of concrete beams reinforced with GFRP bars

This paper reports test results of 12 concrete beams reinforced with glass fibre-reinforced polymer (GFRP) bars subjected to a four point loading system. All test specimens had no transverse shear nor compression reinforcement and were classified into two groups according to the concrete compressive strength. The main parameters investigated in each group were the beam depth and amount of GFRP reinforcement. Two modes of failure were observed, namely flexural and shear. The flexural failure is mainly occurred due to tensile rupture of GFRP bars either within the mid-span region or under the applied point load. The shear failure is initiated by a major diagonal crack within the beam shear span. This diagonal crack extended horizontally at the level of the GFRP bars indicating bond failure.Simplified methods for estimating the flexural and shear capacities of beams tested are presented. The flexural capacity is estimated based on the compatibility of strains and equilibrium of forces. Comparisons between the flexural capacity obtained from the theoretical analysis and that experimentally measured in the current investigation and elsewhere show good agreement. To predict the shear capacity of the beams tested, four methods recently proposed in the literature for GFRP-reinforced concrete beams are used. These methods have been developed by modifying the ACI 318-99 shear capacity formula for steel-reinforced concrete beams to account for the difference in the axial stiffness of GFRP and steel bars. It has been shown that the theoretical predictions of the shear capacity obtained from these methods are inconsistent and further research needs to be carried out in order to establish a rational method for the shear capacity calculation of GFRP-reinforced concrete beams.     

端部锚固CFRP板加固RC梁IC剥离过程非线性分析1

端部锚固预应力碳纤维增复合板(Carbon Fiber Reinforcement Polymer Laminate,简称CFRP板)加固钢筋混凝土结构目前最为常见的抗弯加固方式,为探讨端部锚固CFRP加固RC梁IC剥离过程的起始和演化,文章基于部分黏结作用复合梁理论,利用内聚力开裂模型模拟界面层的剥离过程,通过引入含残余强度的四线性黏结滑移本构模型模拟CFRP板与钢筋混凝土梁之间相对滑移带来的界面的变形协调和应力变形行为的非线性特征和软化特性,得到单一裂缝模式下中间裂缝剥离过程中相应的界面黏结剪应力,相对滑移量、CFRP板轴力分布以及加固构件整体荷载 跨中位移,荷载 跨中CFRP板应变的响应曲线,并与文献提供的试验数据进行对比。结果表明,理论计算结果与试验结果基本一致,可以反映端部锚固CFRP板加固混凝土梁的实际受力状态,为端部锚固CFRP板加固混凝土梁的受力分析和设计提供了理论参考。     

带肋钢板加固腹板开大洞口RC梁的受剪承载力试验研究

设计并制作24根腹板开矩形大洞口钢筋混凝土梁,分别进行外贴钢板和带肋钢板加固,对比分析不同开洞尺寸、开洞位置、多洞口分布和锚栓构造等因素对钢板加固有效性的影响.结果表明:带肋钢板加固能有效改善洞口角部的受力性状,约束钢板不发生平面外屈曲失稳,使得钢板加固梁的洞口上肢整体压溃的脆性破坏模式转变为混凝土局部压碎的延性破坏模...     

Post-tensioned self-centering moment connections with beam bottom flange energy dissipators

This work presents results of experimental and analytical studies of self-centering moment connections. The connection subassembly consists of post-tensioned steel beams, a reinforced concrete column, and energy dissipators placed only below the beam bottom flange for simplicity of construction, ease of replacement, and no interference with the composite slab. Two types of steel energy dissipators are proposed: one includes a reduced section plate restrained by two flat plates, and the other uses cross-shaped steel plates. Cyclic tests are conducted on three full-scale post-tensioned connection subassemblies and six energy dissipators. Finite element analysis is performed to investigate the cyclic performance and likelihood of fracture at critical regions in the energy dissipators. Cyclic test results show that (1) energy dissipation, moment, and flexural stiffness of the beam in positive bending are larger than those of the beam in negative bending, (2) the location of the compression toe at the end of the beam stabilizes at the junction between the beam flange and web after an interstory drift of 1.5%, in which the gap opening angles of the beams are similar in both bending directions, and (3) the shoulder radius equal to 2.5 times plate thickness results in a premature fracture along the shape transition of the reduced section plate. This study also develops an iterative analytical procedure for predicting un-symmetrical cyclic responses of post-tensioned connection subassemblies.