Investigation of insulated FRP-wrapped reinforced concrete columns in fire

Research has demonstrated that fibre-reinforced polymers (FRPs) can be used efficiently and safely in strengthening and rehabilitation of reinforced concrete structures. However, the use of FRPs in buildings has been limited because relatively little is known about the behaviour in fire of reinforced concrete structural members that have been strengthened with FRP systems. This paper presents the recent results of an ongoing experimental study of the fire performance of FRP-wrapped reinforced concrete circular columns. The results of fire tests on two columns are presented, one of which was tested without supplemental fire protection, and one of which was protected by a supplemental fire protection system applied to the exterior of the FRP-strengthening system. The primary objective of these tests was to compare the fire behaviour of the two FRP-wrapped columns and to investigate the effectiveness of the supplemental insulation system. The thermal and structural behaviour of the two columns are discussed. The results show that, although FRP systems are sensitive to high temperatures, satisfactory fire endurance ratings can be achieved for reinforced concrete columns that are strengthened with FRP systems by providing adequate supplemental fire protection. In particular, the insulated FRP-strengthened column in this study was able to resist elevated temperatures during the fire tests for at least 90 min longer than the equivalent uninsulated FRP-strengthened column.     

FRP加强R/C梁防火性能调查研究

对纤维加强聚合物(FRP)材料加强钢筋混凝土梁在火灾中的防火性能进行了研究。试验结果显示,防火材料能够帮助构件达到标准防火性能,同时对不断增长的FRP应用也很重要。介绍了不同文献对FRP在高温下的性能所做的研究以及目前标准对建筑构件的耐火极限要求。对两个大型试验进行了介绍。试验材料为混凝土梁构件,外部用FRP板加强,并且三分之二用专利耐火隔热材料保护。给出了试验结果,并重点强调了曝火状态下所测得的构件温度。试验数据验证了热传导数值模型,该模型可用于预测加强并绝缘钢筋混凝土梁构件内的温度。试验结果和模型数据显示,正确设计和进行绝缘保护的FRP加强梁构件耐火极限能够达到4h或者以上。     

TESTS ON STRUCTURALLY DEFICIENT RC SLABS STRENGTHENED WITH FIBRE REINFORCED POLYMER (FRP) COMPOSITES

This paper reports the results of a series of tests on fibre reinforced polymer (FRP) strengthened reinforced concrete (RC) slabs, which were recently undertaken at the University of Technology,Sydney. The slabs were reinforced with high-strength low-ductile steel reinforcement and strengthened with either carbon FRP (CFRP) or glass FRP (GFRP) composites. The unstrengthened control slabs failed by fracture of the steel tension reinforcement while the FRP strengthened slabs failed by debonding of the FRP followed by rupture of the tension steel. The FRP-strengthened slabs were stronger than their unstrengthened counterparts and displayed considerable ductility.     

Experimental investigation of load carrying capacity of the slender reinforced concrete columns wrapped with FRP

Axial compression test was conducted on 6 elliptical modified rectangular slender reinforced concrete columns wrapped with FRP, with a slenderness ratio L/b ranging between 4.5 and 17.5. The test result showed that the effect of the slenderness ratio on the load carrying capacity of FRP-wrapped concrete columns is more significant than that of ordinary reinforced concrete columns. The strengthening effect decreases with increase of the slenderness ratio. When the slenderness ratio is less than 17.5, the load carrying capacity of FRP-wrapped columns is still 20% higher than that of ordinary reinforced concrete column. The columns were then successfully analyzed with the commercial finite element software ANSYS, taking into account of the constitutive relationship of concrete reinforced with FRP, the failure criterion and the initial geometrical imperfections. The effect of the cross-sections, steel ratio and the confinement ratio is investigated and a simplified formula for the stability coefficient to facilitate design of FRP-wrapped columns is derived.     

受火后加固钢筋混凝土偏压格构柱受力性能试验研究

为研究受火后加固钢筋混凝土格构柱的受力性能,通过5根钢筋混凝土格构柱的偏心受压试验,分析了受火时间和填实加固对钢筋混凝土格构柱的破坏形态和承载力的影响.基于条带法提出了受火后未加固及受火后加固修复格构柱的受压承载力计算方法.结果表明:未受火对比格构柱和受火后未加固格构柱均发生柱肢混凝土压溃破坏,受火后填实加固格构柱发生...     

FRP约束钢筋混凝土圆柱力学性能的试验研究

以长细比和偏心率为变化参数,共进行了16个圆形截面钢筋混凝土柱试验,其中8个试件在环向包裹了FRP(纤维增强复合材料),另8个试件未包裹FRP,以作力学性能对比。试验结果表明,由于FRP的约束作用,包裹FRP试件较相应未包裹FRP试件的承载力提高14%～68%,同时延性也有不同程度的提高,证明FRP约束对提高钢筋混凝土柱(包括长柱及偏压柱)力学性能的有效性。最后,提供了环向包裹FRP后圆形截面钢筋混凝土柱承载力计算方法,计算结果和试验结果基本吻合。     

FRP约束方形截面钢筋混凝土偏压长柱的试验研究

以偏心率为变化参数,共进行了8个长细比Lb为20.4的方形截面钢筋混凝土偏心受压长柱的试验,其中有4个试件沿环向包裹了单向FRP(纤维增强复合材料),另4个未包裹FRP的试件用作力学性能对比。试验结果表明,在长细比较大的情况下,FRP约束虽没有明显提高方形偏压柱的承载力,但在一定程度上改善了构件的延性,尤其是对偏心距较小的构件。当需要同时提高方形截面长柱承载力及延性的时候,建议采用双向FRP对构件进行包裹,或采取其他有效措施。     

Finite element simulation of debonding in FRP-to-concrete bonded joints

The behaviour of reinforced concrete (RC) structures strengthened with externally bonded fibre-reinforced polymer (FRP) reinforcement in the forms of thin plates or sheets is often dominated by debonding of the FRP reinforcement from concrete. As a result, a large number of studies have addressed debonding failures in FRP-strengthened RC structures, with many of them being focussed on understanding the behaviour of simple FRP-to-concrete bonded joints in which an FRP plate/sheet is bonded to a concrete prism and is subject to a tensile force. Despite the many existing studies, there are still substantial uncertainties and difficulties with the finite element modelling of debonding failures due to the complex behaviour of cracked concrete. This paper explores the use of different crack models in modelling and compares their predictions of the debonding behaviour of FRP-to-concrete bonded joints. The results presented in this study show that a non-coaxial rotating angle crack model (RACM) is required to accurately predict this debonding behaviour. Furthermore, the debonding mechanism is also examined using results obtained with a non-co-axial RACM.     

纤维增强复合材料加固混凝土柱的抗震性能研究现状

阐述纤维增强复合材料(FRP)加固混凝土柱的研究进展及成果,包括FRP加固混凝土柱的原理、破坏形式、试验研究、有限元模拟、抗震性能评估以及未来发展趋势等。在抗震性能评估方面,介绍了延性的量化设计指标:位移延性系数和曲率延性系数。介绍计算位移延性系数的3种实用方法,并用曲率增大系数来衡量曲率延性系数,以期为FRP抗震加固的进一步研究提供参考。     

CFRP加固钢筋混凝土梁耐火性能试验研究

采用厚型防火涂料和硅酸钙防火板对3根CFRP加固钢筋混凝土梁进行了不同方法的防火保护,在ISO834标准升温条件下进行耐火性能对比试验。分析了不同防火方法及端部锚固性能对高温下CFRP加固梁的温度场变化规律、跨中挠度、破坏形态及耐火极限的影响。试验研究表明:采用50mm厚防火涂料和40mm硅酸钙防火板对加固梁进行三面U型防火保护,耐火极限均可达到2.0h;提高CFRP的端部锚固性能可有效地改善加固梁的耐火性能;防火涂层中增设钢丝网片约束防火涂料、防止开裂和脱落效果明显。试验结果分析表明:CFRP加固混凝土梁防火保护的重点并非是CFRP材料本身,而是钢筋混凝土梁;可以通过延缓钢筋与混凝土材料的温度增长来推迟加固梁达到极限状态的时间,从而获得较好的耐火性能。     

Evaluation of debonding failure of reinforced concrete girders strengthened in flexure with FRP laminates using finite element modeling

The use of Fiber-Reinforced Polymer (FRP) materials dates back to the early 1940s when they were used in aerospace and naval applications. During the 1970s and early 1980s, FRP started being used in civil engineering applications for new construction, but more importantly for repair and strengthening of existing structures. However, experimental research showed that the typical failure mode of reinforced concrete (RC) structures strengthened with FRP composite materials is due to the debonding that occurs at the interface between concrete and FRP. The bond between FRP and concrete is therefore the key factor controlling the behavior of these structures since it limits the full use of the FRP strength. The paper evaluates the effect of the debonding failure on the response of FRP-strengthened RC beams. A nonlinear RC beam element with bond-slip between the concrete and the FRP laminates is developed and used to analyze several test specimens and to investigate their corresponding failure mode. The model was also used to study the reduction factor of FRP tensile strength of simply supported strengthened RC girders due to debonding failure. This reduction factor proved to be affected by several parameters: (a) the bond strength between FRP and concrete interface; (b) the concrete strength; (c) the thickness of FRP; (d) the modulus of FRP; (e) the width of FRP laminate; and (f) the development length of the FRP sheet. A large number of beam specimens were analyzed in order to conduct a thorough evaluation of debonding failure of RC beams strengthened with FRP laminates. Based on these studies, new equations that account for the aforementioned parameters were proposed to address the reduction in FRP strength due to debonding failure.     

The performance of near-surface mounted CFRP strengthened RC beam in fire

Carbon fiber reinforced polymers (CFRP) have been widely used in the retrofitting and strengthening of concrete structures. However, the layouts of CFRP in externally bonded (EB) strengthening system and near surface mounted (NSM) system result in the vulnerability of fire resistance. To explore the fire resistance potential of NSM-CFRP strengthening system, 15 RC beams strengthened with CFRP and 2 reference beams were loaded to failure either under the ISO834 standard fire or at ambient temperature. In test, the following factors were considered: the strengthening method (near surface mounted technique/externally bonded technique), adhesive (epoxy and magnesium-oxychloride cement), fire protection materials (thin intumescent fire retardant coating and thick fire retardant coating) and location of fire protection (local patch protection/single-sided protection/U-shaped protection). With appropriate fire protection, the RC beams strengthened with NSM-CFRP resisted the standard fire for more than 3 h at high load level. Besides the effects of the aforementioned factors, the friction stress at the CFRP/matrix interface when CFRP is in globally slipping is found playing a crucial role for the fire resistance of NSM-CFRP strengthening system. The experimental observation sheds some light on how the NSM-CFRP strips can retain the contribution at elevated temperatures and why NSM-CFRP strengthening outperforms EB-CFRP strengthening when exposed to fire.     

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.     

Post-earthquake recoverability of existing RC bridge piers retrofitted with FRP composites

The novel concept of this paper is to investigate the required recoverability of existing important reinforced concrete (RC) bridges retrofitted with fiber-reinforced polymers (FRP) to restore their original functions after a moderate or strong earthquake. Hence, this paper presents an up-to-date literature search on the inelastic performance of 109 FRP-retrofitted columns with lap-splice deficiency, flexural deficiency, or shear deficiency. The study is conducted in the following steps: using post-yield stiffness as a seismic index, the effectiveness of FRP jackets in enhancing the inelastic stage performance of non-ductile reinforced concrete columns is scrutinized for the available database; the performance of columns which successfully achieved post-yield stiffness is categorized in accordance with the required recoverability after an earthquake; and according to the definition of a controllable recoverable structure, the appropriate composite jacket thickness is calculated. In the view of a proposed mechanical model of an FRP–RC damage-controllable structure, 61 columns of the available database exhibited idealized lateral performance with stable post-yield stiffness, or secondary stiffness. Lateral drift at the end of the recoverable state is defined from the hysteretic responses of 39 columns and is visualized as a ratio of column lateral drift by the end of the post-yield stiffness with explicit consideration for the effect of both column cross-section shape and deficiency. Finally, suitable FRP design assumptions and concepts certifying the reality of post-yield stiffness are given. Furthermore, in the light of Seismic Design Specifications of Highway Bridges in Japan, a FRP strengthening design guideline that considers and evaluates structural recoverability is proposed.     

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.     

Fatigue-loading effect on RC beams strengthened with externally bonded FRP

External bonding of fiber-reinforced polymers (FRP) on concrete beams is particularly attractive for the strengthening of civil engineering structures in order to increase their strength and stiffness. Principles for design of such strengthening methods are now established and many guidelines exist. However, fatigue design procedure is still an ongoing research topic.This paper focuses on the damage behavior of FRP-strengthened reinforced concrete (RC) structures subjected to fatigue loading.In order to design bonded reinforcements, an iterative computational method based on section equilibrium and material properties (concrete, steel, adhesive and composite) has been previously developed by authors [1], [2], [3]. In the present study, this method is extended to describe the fatigue behavior of RC beams.A specific modeling coupled with an experimental investigation on large-scale beams made it possible to compare the theoretical and experimental fatigue behaviors of RC beams with and without composite reinforcements. The model is developed and calibrated using data of the literature or recorded during experiments specifically carried out for this study. Results showed that the beam deflection and the strain in each material could be calculated with a sufficient accuracy, so that the fatigue behavior of the FRP-strengthened beams was correctly estimated by the model.     

Creep behaviour of CFRP-strengthened reinforced concrete beams

The strengthening of reinforced concrete structures with externally bonded fibre reinforced polymer (FRP) laminates has shown excellent performance and, as a result, this technology is rapidly replacing steel plate bonding techniques. The numerous studies that have been carried out to date on FRP-strengthened concrete elements have mainly focussed on the static and short-term responses; very little work has been done regarding the long-term performance. This paper addresses this issue, and presents results from a series of experiments on the time-dependent behaviour of carbon FRP-strengthened concrete beams. Twenty-six reinforced concrete beams with dimensions 100 × 150 × 1800 mm, with and without bonded CFRP laminates, were investigated for their creep behaviour. Different reinforcement ratios were used to evaluate the contribution of the external reinforcement on the creep resistance of the beams. High levels of sustained load were used in order to determine the maximum sustained load that can be applied without any risk of creep failure. The applied sustained loads varied from 59% to 78% of the ultimate static capacities of the un-strengthened beams. For most of the long-term tests, the applied sustained loads were higher than the service loads. This was done to account for the fact that strengthening is typically required when a structure is expected to carry increased service loads. The main parameters of this study were (i) the level of sustained load and (ii) the strengthening scheme. The results confirm that FRP strengthening is effective for increasing the ultimate capacities of the beams; however, there is virtually no improvement in performance with regard to the long-term deflections.     

碳纤维布加固钢筋混凝土板的耐火性能试验研究

在ISO834标准升温条件下,进行3块设置不同防火材料的碳纤维布加固钢筋混凝土板,以及1块未加固板的耐火性能对比试验。分析不同防火措施对高温下碳纤维布加固钢筋混凝土板的破坏形态、跨中挠度及耐火极限的影响。试验结果表明:(1)只要采取适当的防火措施,在使用荷载明显增大的情况下,碳纤维布加固钢筋混凝土板仍具有与未加固板大体相当甚至更长的耐火极限;(2)与薄型防火涂料相比,水泥砂浆的防火保护效果相对较弱;(3)厚度5 mm的薄型防火涂料的防火效果相对较好。为确保实际工程中碳纤维布加固钢筋混凝土板具有足够的耐火能力,科学合理地选择防火材料及其厚度是十分重要的。     

Shear capacity of FRP-strengthened RC beams: FRP debonding

Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre-reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.