EB-FRP及HB-FRP加固预裂RC梁对比试验研究

为验证FRP螺栓加固新技术的合理性,结合现役钢筋混凝土桥梁带裂缝工作的特点,采用表面粘贴FRP（EB-FRP）加固技术和FRP螺栓混合加固技术（HB-FRP）,分别对不同预裂度的钢筋混凝土简支梁进行抗弯加固,通过两点对称加载抗弯试验研究加固预裂梁的受力特性和破坏模式,分析裂缝分布对加固预裂梁的剥离荷载及FRP材料利用率的影响。试验结果表明,与表面粘贴FRP加固技术相比较,采用FRP螺栓混合加固的预裂梁,抗弯极限承载能力提高30%～44%,加固后结构的破坏表现为明显的延性,改善了FRP脆性剥离破坏模式;另外,钢筋混凝土梁裂缝对FRP的剥离荷载、FRP材料的利用率及梁的破坏模式影响显著。     

钢筋混凝土T梁HB-FRP抗剪加固试验和数值模拟

基于室内模型试验,开展了混合黏结纤维增强复合材料(HB-FRP)抗剪加固钢筋混凝土T梁的受力性能研究.对未加固、外贴纤维增强复合材料(EB-FRP)加固及HB-FRP加固T梁进行了破坏性对比试验,并采用Abaqus软件建立了精细化有限元模型,对比分析了试验和数值计算结果,验证了有限元模型的准确性;在此基础上进行参数分析,研究了混凝土强度、箍筋间距、FRP条带数量及FRP厚度对加固梁抗剪承载能力的影响.结果 显示:HB-FRP抗剪加固梁的剪切裂缝间距要小于EB-FRP加固梁的裂缝间距;EB-FRP加固梁发生黏贴区域大面积剥离,而HB-FRP加固梁仅在相邻钢扣件间有裂缝的区域出现了剥离,钢扣件有效抑制了裂缝剥离扩展;HB-FRP抗剪加固梁的FRP应变水平为EB-FRP抗剪加固梁的2倍,表现出了较好的延性.综合考虑未加固梁、EB-FRP加固梁及HB-FRP加固梁的参数分析结果,对钢筋混凝土T梁抗剪承载能力的影响因素按照重要性降序为:混凝土强度、箍筋间距、FRP间距、FRP厚度.     

FRP筋与混凝土黏结破坏性能研究

对27个黏结试件进行拔出试验,试验参数为筋直径、黏结长度和混凝土强度。研究了FRP筋与混凝土的黏结破坏形式,试验结果表明:黏结长度较小、中等和较大时黏结破坏形式分别为筋拔出、混凝土劈裂和筋拉断破坏。混凝土强度和FRP筋直径等因素对黏结破坏形式没有直接影响。研究结果为确定FRP筋的黏结锚固长度和制定相关规范提供试验依据。     

盐腐蚀环境下内嵌FRP筋加固混凝土界面黏结性能试验研究

为了研究盐腐蚀环境下内嵌FRP筋加固混凝土界面的黏结性能,对27个内嵌FRP筋加固混凝土试件进行盐腐蚀后的单端拉拔试验,分析试件的受力过程和破坏模式,研究内嵌FRP筋黏结长度、腐蚀时间和FRP筋类型对界面黏结性能的影响。结果表明：盐腐蚀的试件破坏模式分为结构胶劈裂、FRP筋拉断和结构胶劈裂且FRP筋弯折等3种,且以结构胶劈裂破坏为主。盐腐蚀环境下内嵌FRP筋混凝土试件的黏结应力与黏结长度、破坏模式与腐蚀时间有关。盐腐蚀环境会影响混凝土、黏结材料及FRP筋的力学性能,加剧黏结界面失效破坏。腐蚀时间为30 d和90 d的内嵌BFRP筋加固混凝土试件的耐盐腐蚀能力高于内嵌GFRP筋加固混凝土试件的,腐蚀时间为60 d的内嵌GFRP筋加固混凝土试件的耐盐腐蚀能力优于内嵌BFRP筋试件的。根据试验数据拟合了盐腐蚀环境下内嵌FRP筋加固混凝土界面黏结-滑移本构关系,其拟合优度达到0.988 0。     

FRP-螺栓联合加固RC梁粘结性能试验研究

通过对17根跨中留有预切缝的试验梁进行破坏性加载试验,研究了FRP-螺栓联合加固技术的粘结性能、锚固间距和合理锚固长度。结果表明,HB-FRP(FRP-锚钉联合加固技术)体系较EB-FRP(外贴FRP加固法)体系的延性、承载力、FRP的剥离应力等都有较大的提高。在试验数据及理论分析的基础上,提出了HB-FRP和EB-FRP中部裂缝引起的剥离强度计算公式以及较为合理的锚固间距估算公式,计算结果与试验结果吻合较好。所得结果可为联合加固的进一步研究及工程上的应用提供一定的依据。     

SWR-PM加固层与混凝土界面黏结性能试验研究

进行了钢丝绳-聚合物砂浆（SWR-PM）加固层与混凝土的单面剪切试验,研究了混凝土强度、加固层黏结长度和加载端受压高度对试件破坏形态及剥离承载力的影响规律.结果表明：试件的破坏形态包括界面剥离破坏、混凝土拉剪破坏及混合破坏;界面剥离承载力随着黏结长度的增加而增长,直至黏结长度超过有效黏结长度后,剥离承载力才趋于稳定;试件的破坏荷载随着混凝土强度的增大而提高.建立了加固层黏结长度与界面剥离承载力的关系计算式,计算结果与试验结果吻合良好.     

不同FRP增强混凝土梁断裂性能试验研究

为了探讨不同种类纤维增强复合材料（FRP）增强带裂缝混凝土的断裂性能,开展了芳纶纤维增强复合材料（AFRP）、碳纤维增强复合材料（CFRP）和玻璃纤维增强复合材料（GFRP）增强带裂缝混凝土梁的三点弯曲试验,分析了其断裂性能参数.结果表明：相对于普通混凝土梁试件,FRP对带裂缝混凝土梁的阻裂加固效果更明显;CFRP增强混凝土梁的起裂荷载和失稳荷载均大于AFRP与GFRP增强混凝土梁,CFRP的阻裂增强效果最佳;AFRP增强混凝土梁和CFRP增强混凝土梁的破坏形式均为试件底部混凝土 FRP界面的剥离破坏,GFRP增强混凝土梁的破坏形式为试件底部GFRP的拉断破坏;通过对不同FRP增强混凝土梁阻裂加固机理的分析,计算得出CFRP增强混凝土梁的起裂韧度和失稳韧度最大,且CFRP价格适中,因此使用CFRP对带裂缝混凝土梁进行增强加固的性价比最优.     

HB FRP加固混凝土结构组合界面黏结特性

为揭示HB-FRP(hybrid bonded fiber reinforced polymer)加固混凝土结构多作用组合工作机制,设计了5组黏结作用组合试验.基于实测荷载-滑移关系、应变分布、黏结-滑移关系开展了界面黏结特性研究,提出组合界面黏结-滑移统一模型和黏结荷载表达式.结果表明:HB-FRP加固混凝土组合作用可拆分为FRP黏结混凝土、侧压力和FRP黏结钢板;组合界面的黏结应力发展不同步,FRP板下表面与混凝土的剥离早于FRP板上表面与钢板的剥离,叠合工作时序不同;由侧压力引起的界面摩擦应力随界面应力的发展而增加并趋于稳定;理论模型结果与试验结果具有较好的一致性,可用于计算组合界面的黏结荷载.     

FRP补强加固RC梁粘结破坏机理研究

根据FRP(Fiberreinforcedplastic)补强加固RC梁的试验结果,总结了FRP补强加固钢筋混凝土受弯构件早期破坏形态的过程和特点。FRP加固钢筋混凝土抗弯构件粘结界面的剪应力的分布在纤维截断点处存在较高的应力集中,随着离截断点距离的增大剪应力分布逐渐趋于均匀,粘结锚固长度不足和过高的应力集中是造成FRP加固钢筋混凝土构件早期破坏的主要原因。采用“齿”状块体力学计算模型和混凝土裂缝理论分析了FRP加固钢筋混凝土受弯构件时需要的有效锚固长度,并通过简化修正得出了FRP加固钢筋混凝土受弯构件最小锚固长度的计算公式,提出了FRP的容许应变值和避免FRP早期破坏应采取的措施,可供FRP加固工程设计和施工参考。     

织物增强混凝土与再生保温混凝土界面黏结性能试验研究

为研究织物增强混凝土与再生保温混凝土界面的黏结性能,设计了3组6种不同界面黏结推出试件,分析不同处理方式下推出件的荷载-位移曲线及试件的破坏形态。试验结果表明:预留沟槽推出试件对阻止界面滑移和剥离影响效果明显,在极限荷载时,预制纤维编织网增强混凝土(TRC)板肋被剪断,试件整体破坏形式呈脆性破坏;带纤维增强复材(FRP)剪力连接件剪切构件承载能力相对较好,荷载-位移曲线下降段较为平缓,试验过程中产生多条裂缝,沿着剪切面呈延性破坏,整个试验过程并未发生FRP连接件脆断、界面剥离现象;本试验合理的推出件设计和加载方案避免了对刚性模量较低夹芯层进行直接加载导致位移-荷载曲线不符合实际的试验要求。     

疲劳荷载下FRP粘结系统的性能

研究和比较疲劳荷载下,纤维增强复合材料和混凝土各种粘结系统粘结性能。FRP粘结系统包括外部粘结FRP,表面嵌入粘结FRP,纤维锚固FRP以及最近发展起来的混杂粘结FRP系统。在两种力学参数不变的情况下,对5组混杂粘结FRP系统进行了研究。为达到研究的目的,改进了双剪粘结试验(直接拉出试验)。通过疲劳和单调试验,对破坏机制、荷载-位移曲线以及应变进行分析,探讨了荷载大小和循环周期对粘结性能的影响。基于试验结果,根据疲劳粘结性能对不同粘结系统的优缺点进行论述。     

FRP与混凝土的粘结性能研究进展

FRP与混凝土的粘结性能是外贴纤维加固混凝土结构技术的关键问题.本文介绍了近10余年来,国外学者对FRP与混凝土的粘结性能的试验和理论研究进展,着重论述了试验方法、传力机理、粘结界面破坏形式、粘结强度及影响因素、FRP应变及粘结剪应力的分布、粘结应力-滑移关系以及粘结强度计算模型等,总结以往研究成果并提出了有关建议,供实际加固工程应用参考.     

FRP筋复合式锚具锚固性能的试验研究

当复合材料FRP(Fiber Reinforced Polymer/Plastics)筋或拉索应用到缆索承重桥梁的拉索体系中时,粘结式锚具和夹片式锚具均有其自身的局限性。对此,根据普通拉索锚固体系的特点,并结合FRP筋夹片式锚具和粘结式锚具的研究成果,提出了锚固FRP筋的复合式锚具。复合式锚具由后部的楔紧锚固和前部的粘结锚固组成,其中楔紧锚固部分包括锚杯、带有凹齿的夹片、铝套管以及塑料薄膜,粘结锚固部分包括钢套筒和粘结介质-活性粉末混凝土RPC(Reactive Power Concrete)。静载试验研究了锚杯长度、钢套筒长度、夹片预紧力、筋材预张拉力等参数对复合式锚具锚固性能的影响。结果表明:复合式锚具试件中的极限荷载最大为208kN,相应的锚固效率系数为104%,大于95%,满足规范要求。复合式锚具两种锚固形式的锚固长度较合理组合为:对不预张拉锚具,可取锚杯长度40mm以及粘结锚固长度100mm;对预张拉锚具,可取锚杯长度60mm以及粘结锚固长度60mm。提出的复合式锚具极限荷载计算式具有较好的适用性。     

RECENT DEVELOPMENTS IN FRP STRENGTHENING TECHNIQUES

This paper mainly gives a State-of-the-Art report of recent development in FRP strengthening techniques for structural rehabilitation by starting with a brief review on some achievements in clarifying bonding/debonding mechanisms and developing evaluation/design methodology for predicting debonding failure caused by intermedioate flexural cracks of con crete.Due to some drawbacks of current FRP bonding technique,two effective FRP strengthening methods,i.e.FRP prestressing and hybrid strengthening techniques,are investigated in detail to make full advantage of FRP composite materials.Combining with the newly develiped PBO fiber sheets,a more effective prestressing method is established as compared with carbon fiber sheets,a more effective prestressing method is established as compared with carbon fiber sheets,where several anchorage treatments are also established to prevent anchorage bond failure due to high shear stress concentration after release of prestressed FRP ends.In addition,the fatigue performance of RC beams externally strenghened with prestressed PBO fiber sheets is experimentally studied.For the developments of hybrid composites and their strengthening methods,both experimental and analytical studies are performed to clarify the hybrid strengthening behavior in structures.     

Bond behaviour of FRP-to-clay brick masonry joints

The out-of-plane bending and in-plane shear response of unreinforced modern clay brick masonry walls retrofitted with fibre-reinforced polymer (FRP) strips is often governed by debonding failure mechanisms. Hence, it is necessary to quantify the fundamental interface bond–slip model, which describes the debonding behaviour of the FRP-to-masonry interface. This paper presents the results of a series of 29 pull tests investigating the use of externally bonded (EB) and near surface mounted (NSM) retrofitting techniques. Test variables included: surface preparation; geometric properties; location of FRP (relative to perpend joints and cores); bonding agent of bed joints (mortar and quick drying paste); bonding method for glass fibre sheets (plate bonding and dry lay-up); and FRP material. A discussion of the test results and preliminary practical recommendations are also provided. A model used to predict the intermediate crack debonding resistance was verified against the test data. The model is generic in that it is applicable to both the EB and NSM retrofitting techniques. This generic model was shown to give very good ultimate strength predictions for the series of 29 pull tests conducted as part of this research.     

Numerical Analyses of Hybrid-Bonded FRP Strengthened Concrete Beams

Abstract:   To overcome the problem of debonding, a new hybrid-bonded fiber-reinforced polymers (HB-FRP) strengthening system has recently been developed. This HB-FRP technique can increase the interfacial bond strength several folds. In this work, a finite element (FE) model for HB-FRP strengthened beams is developed to simulate the responses of the new structural system. The interfacial behaviors between the FRP and the concrete are simulated with a shear friction model. Comparisons between the predictions from this model and the test results demonstrate the accuracy of the FE model. The mechanism of the HB-FRP system and failure modes are then studied using the proposed model.     

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.     

FRP筋与混凝土粘结性能的试验研究

进行了28个纤维聚合物(FRP)筋拔出粘结试验,试验变量为FRP筋种类、FRP筋直径、混凝土强度和粘结长度,拔出试验记录了拔出荷载、FRP筋加载端滑移、破坏现象。FRP筋破坏形式是表面脱落和筋断裂,而混凝土未出现明显破坏。随着FRP筋的种类、直径、粘结长度不同,粘结强度变化范围是7.96~41.80MPa,粘结强度随着粘结长度、直径增加而减少,与混凝土强度无关。     

碳纤维增强塑料布与混凝土基层粘结行为研究

通过自行设计的试验方法,研究了30组粘贴有碳纤维增强塑料布(CFRP布)的混凝土试件粘结界面的破坏形态、粘结应力和碳纤维增强塑料布应变分布特性以及粘结破坏的发展过程.结果表明,粘结应力由碳纤维增强塑料布的加载端向自由端逐渐传递,破坏瞬时发生,破坏形式以剥离破坏为主;存在一个有效粘结长度,约为100 mm.研究结果说明自行设计的粘结试验方法有一定可行性,可为测定CFRP布-混凝土界面粘结强度指标提供新的思路.