外贴CFRP加固混凝土梁裂缝间CFRP与混凝土界面粘结性能研究

碳纤维(CFRP)布与混凝土界面的粘结性能是影响CFRP加固构件性能的关键因素,为了更好地理解其对构件性能的影响,进行了外贴CFRP加固混凝土梁纯弯段内CFRP-混凝土界面粘结性能的研究,建立了研究相邻裂缝间CFRP-混凝土界面粘结滑移的解析模型。该模型不仅可以给出粘结界面上滑移和剪应力的分布,还可以给出外贴CFRP加固混凝土梁中相邻裂缝间各材料的应力分布,进而进行构件开裂分析等。将裂缝间CFRP应变实测分布与模型计算结果进行了比较,结果表明:由该模型导出的CFRP应变分布与试验结果吻合较好。     

CFRP加固钢板的粘结界面剥离破坏

II型界面破坏是碳纤维增强树脂复合材料(CFRP)加固钢板常见的破坏方式之一。为揭示CFRP加固钢板粘结界面破坏的力学机制,开展了单剪试验和双剪试验分别研究了CFRP-钢板界面力学性能及破坏过程,并采用数字图像相关技术(DIC)对CFRP的轴向应变分布进行监测。对比两个试验的破坏模式发现,双剪试件的粘结界面主要发生II型破坏,界面破坏的主要力学原因是剪应力;而存在偏心加载的单剪试件,粘结界面上的剪应力和偏心加载引起的弯矩共同作用,使粘结界面发生I/II型混合模式失效。在II型破坏模式下,不同粘结长度的极限荷载及粘结滑移值随着粘结长度的增大而增大,但当粘结长度超过有效粘结长度后,极限荷载及极限滑移值基本保持不变。而在所讨论的偏心加载引起的界面I/II型混合破坏模式下,不同粘结长度的极限荷载基本不变。基于试验数据得到的双线性粘结-滑移关系建立了有限元模型,对CFRP加固钢板的II型界面粘结破坏行为进行分析,数值模拟结果与试验结果吻合较好。      

碳纤维布加固混凝土结构疲劳特性试验研究

首次提出了界面剪切疲劳试验的新方法,借此研究了疲劳荷载作用下的碳纤维与混凝土粘结界面间的抗疲劳性能。并基于弹性理论,得出剪切变形条件下与弯曲变形条件下的碳纤维与混凝土界面间粘结剪应力的计算公式是相同的结论。因此用界面剪切疲劳试验代替弯曲疲劳试验去考察碳纤维与混凝土界面间粘结强度的疲劳性具有更高的精度和效率。同时试验结果表明:在应力比R=0.2条件下,碳纤维布与混凝土界面间的极限粘结疲劳强度约是其静粘结强度的64%,可见在疲劳荷载的作用下,碳纤维布与混凝土界面间的粘结强度将下降,因此在碳纤维加固的混凝土梁的设计中,应重视疲劳荷载导致的界面粘结强度降低的问题,并且设计时应对粘结剪切强度赋予一定的安全系数。     

FRP加固金属拉伸构件的性能分析

与传统的金属结构加固方法相比,粘贴纤维增强复合材料(FRP)加固具有明显的优势。FRP与金属构件之间的粘结应力会影响加固效果。根据平衡微分方程,对拉伸金属构件对称粘贴FRP加固后的复合构件进行应力分析,得到了剪应力沿复合构件厚度方向的分布,推导出FRP与金属构件之间的粘结应力,得到了FRP有效粘结长度的计算公式。通过碳纤维布双面加固钢板的静力拉伸试验,考察了碳纤维布与钢板之间的粘结应力分布,并对碳纤维布的有效粘结长度进行了分析。理论公式的计算结果与试验结果吻合较好,表明计算方法具有较高的精度。     

CFRP片材-工程水泥基复合材料-混凝土复合界面单面剪切试验研究

为解决碳纤维增强树脂复合材料(CFRP)片材加固混凝土结构时CFRP片材易过早剥离及工程水泥基复合材料(ECC)加固混凝土结构极限承载力提高不足等问题,采用CFRP片材-ECC-混凝土复合界面,以同时发挥CFRP片材高抗拉强度和ECC多缝开裂及耐久性较好的优势。设计21个单面剪切试件并进行单面剪切试验,研究不同ECC厚度和混凝土/ECC强度对复合界面承载力、应变分布及粘结滑移曲线等影响规律。试验结果表明：设置ECC层的单面剪切试件破坏模式均为CFRP片材和ECC界面间的剥离破坏,有效延缓了CFRP片材的剥离,并可以有效地传递界面剪应力。与无ECC层的试件相比,设置ECC层试件的极限承载力增加了27.3%～59.6%。基于陆新征等提出的极限承载力计算模型,提出了考虑ECC厚度的复合界面单面剪切试件的极限承载力预测模型,计算值与试验值相吻合。采用不同粘结滑移模型对试验数据进行分析,对比结果表明：Ferracuti等提出的模型考虑的影响因素较全面且模型的拟合结果较好。     

盐溶液干湿循环对CFRP-混凝土界面粘结性能的影响

为了考察盐溶液干湿循环条件对碳纤维增强复合材料（Carbon fiber reinforced polymer,CFRP）-混凝土界面粘结性能的影响,本文采用5%的NaCl溶液来模拟海水环境,经过不同次数的干湿循环后,利用单面剪切试验对48个试件界面性能的变化情况进行了研究,分析了上述环境对界面破坏形式、界面承载力及界面剪应力等参数的影响。结果表明：在盐溶液干湿循环作用下,界面粘结性能发生显著地退化,具体表现为,随着干湿循环次数的增加,界面承载力会不断降低,且下降程度与混凝土强度和CFRP的粘贴尺寸有关,界面剪应力在不同环境下的分布具有相似性,即荷载的不断增加会使剪应力逐渐由加载端向自由端传递,但在传递过程中,有效传递区域的长度不会发生改变。     

碳纤维布加固RC梁中粘结性能的非线性有限元分析

碳纤维布加固钢筋混凝土(RC)梁中,碳纤维布与梁底混凝土的剥离破坏使碳纤维布的强度不能得到充分发挥。分析碳纤维布与梁底混凝土的粘结应力,是研究碳纤维布加固剥离破坏承载力的基础问题。根据4根碳纤维布加固RC梁的试验研究结果,采用商业有限元程序MSC.Marc建立有限元模型,进行了非线性计算分析。通过分离总粘结应力中的局部粘结应力,得到粘结延伸长度范围内的锚固粘结应力分布,并结合试验数据对其分布规律进行了研究。根据分析和试验结果,引入了“有效锚固粘结长度”和“锚固粘结应力”的概念,给出了极限荷载下锚固粘结应力的计算建议。     

冻融循环对CFRP-烧结粘土砖界面粘结性能影响

为考察冻融循环对碳纤维增强聚合物复合材料(CFRP)-烧结粘土砖界面粘结性能的影响,通过模拟自然冻融环境,在试件经过不同次数的冻融循环后对其进行单面剪切试验。结果表明:在冻融循环作用下,CFRP-烧结粘土砖试件界面粘结性能发生了显著的退化,即随着冻融循环次数的增加,界面承载力和剪应力不断降低;界面剪应力在不同冻融次数下的分布具有相似性,均表现为随着荷载的增加剪应力逐渐由加载端向自由端传递,在传递过程中,有效传递长度变化不显著。在已有界面理论的基础上,根据试验提出了考虑冻融循环时间的界面粘结-滑移模型,通过对比分析,该模型能够很好地反映冻融循环作用下界面粘结性能退化规律。      

环境温差下FRP-混凝土界面粘结行为分析

为明确环境温差对纤维增强聚合物(FRP)加固混凝土构件的界面粘结性能的影响,基于粘结界面的双参数内聚力指数模型,建立了FRP-混凝土粘结结点在温差作用下的界面微分平衡方程,采用边界条件叠加的方法,解析推导了界面相对滑移、界面剪应力和FRP应力-应变分布计算公式。基于解析理论模型,提出了FRP-混凝土界面最大承载温差的计算方法,分析了粘结长度、温差变化和粘结层数对界面粘结性能的影响。结果表明:该文推导出的理论公式计算结果与界面试验结果吻合良好,建立的解析理论模型能够较好地预测温差作用下FRP-混凝土界面粘结行为;界面最大承载温差随粘结长度的增加存在上限值,且由于界面粘结性能的退化,FRP温度应力的最大值出现在达到界面最大承载温差之前;界面剪应力集中在粘结端部区域,受温差变化和FRP粘结层数影响较大,且当环境温差进入胶黏剂玻璃化转变区域后影响最为明显。该结论可用于强日照和高温车间等大温差环境下桥梁和建筑加固构件的温度应力分析和界面承载力评估。     

CFRP布端绕双杆自锁加固混凝土窄梁抗弯试验

为了抵抗粘贴碳纤维增强聚合物基复合材料（CFRP）加固钢筋混凝土结构中常见的剥离破坏,发明了将CFRP布端部以特定方式绕平行双杆实现自锁的方法。鉴于窄梁截面宽度有限,提出将CFRP布贴梁受拉底面布置后,用安装在梁侧面的双L形端锚装置固定双杆,形成侧锚底贴加固方案。完成了5根混凝土强度较低的矩形截面梁四点弯曲试验,其中4根采用上述锚固方式抗弯加固,检验了锚固效果,考察了CFRP布宽度及其沿全长与梁底面是否粘结对加固效果的影响。试验结果表明,采用本文方法进行加固后,端部剥离得以避免,中部剥离即使发生,或在无粘结加固梁受力后期,CFRP布仍能承担较大拉力,因此,极限荷载较对比梁有明显提高。比较而言,CFRP布与梁底有粘结时加固效果更好,CFRP布宽度加大也对提高承载力有益。     

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.     

Near surface mounted CFRP laminates for shear strengthening of concrete beams

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.     

CFRP布加固方案对RC梁剪切性能及尺寸效应影响分析EI北大核心CSCD

采用同时考虑混凝土材料非均质性、钢筋与混凝土之间的相互作用以及CFRP布与混凝土之间的相互作用影响的三维细观数值模拟方法,建立了CFRP布加固RC梁剪切破坏力学分析模型。在验证了细观数值方法合理性的基础上,设计并建立了12根CFRP布加固RC梁细观模型,探究相同CFRP配纤率(用布量)前提下,不同CFRP布加固方案对单调荷载作用下RC梁的剪切性能及尺寸效应的影响。结果表明:CFRP布应变分布与裂缝位置紧密相关,越靠近裂缝位置的CFRP布应变越大,提供的抗剪贡献越多;在CFRP配纤率一致的前提下,CFRP布宽度大厚度小的加固方案优于CFRP布厚度大宽度小的加固方案;CFRP布U型加固RC梁剪切强度存在尺寸效应现象,但相同CFRP配纤率下,不同CFRP布加固方案对名义抗剪强度尺寸效应的影响较小,可以忽略。     

Interfacial shear stress concentration in FRP-strengthened beams

This paper reports the results of an experimental programme designed to study the interfacial shear stress concentration at the plate curtailment of reinforced concrete (RC) beams strengthened in flexure with externally bonded carbon fibre-reinforced polymer (CFRP). Specifically, the study looks at the relationship between the CFRP plate thickness and the interfacial shear stress concentration at the plate curtailment, the failure modes of the CFRP-strengthened beams as well as the efficiency of the CFRP external reinforcing system. Comparing the experimental results with existing models' predictions is another objective of this study. The experimental programme included five RC beams 115 mm×150 mm in cross-section and 1500 mm in length. Four of the RC beams were reinforced externally with CFRP plates of different thicknesses. Tests in this study showed that the thickness of CFRP plate affects not only the load-carrying and deflection capacities of the strengthened beam, but also the shear stress concentration at the CFRP/concrete interface and the beam failure mode.     

Coupled flexural–shear retrofitting of RC beams using CFRP straps

This paper reports on an experimental investigation carried out to investigate the coupling of shear–flexural strengthening of RC beams. The presence of shear straps to enhance shear strength has the dual benefit of delaying debonding of CFRP sheets used for flexural strengthening. Six RC beams were tested; having various combinations of CFRP sheets and straps in addition to an unstrengthened beam as control test. The instrumentations used in these tests cover the strain measurements in different CFRP layers and located along the span, in addition to the slip occurring between the concrete and the CFRP sheets. Test results and observations showed that a significant improvement in the beam strength is gained due to the coupling of CFRP straps and sheets. Further, a more ductile behaviour is obtained as the debonding failure is prevented.     

Efficacy of CFRP-based techniques for the flexural and shear strengthening of concrete beams

Near surface mounted (NSM) and externally bonded reinforcement (EBR) strengthening techniques are based on the use of carbon fiber reinforced polymer (CFRP) materials and have been used for the structural rehabilitation of concrete structures. In the present work, the efficacies of the NSM and EBR techniques for the flexural and shear strengthening of reinforced concrete beams are compared carrying out two experimental groups of tests. For the flexural strengthening, the efficacy of applying CFRP laminates according to NSM is compared to those resulting from applying CFRP laminates and wet lay-up CFRP sheets according to EBR technique. The influences of the equivalent reinforcement ratio (steel and laminates) and spacing of the laminates on the efficiency of the NSM technique for the flexural strengthening is also investigated. A numerical strategy is implemented to analyze the applicability of the FRP effective strain concept, proposed by ACI and fib in the design of FRP systems for the flexural strengthening. To assess the efficacy of the NSM technique for the shear strengthening of concrete beams, four beam series of distinct depth and longitudinal tensile steel reinforcement ratio are tested. Each series is composed of one beam without any shear reinforcement and one beam using the following shear reinforcing systems: conventional steel stirrups; strips of wet lay-up CFRP sheet of U configuration applied according to EBR technique; and laminates of CFRP embedded into vertical or inclined (45°) pre-cut slits on the concrete cover of the beam lateral faces, according to the NSM technique. Using the obtained experimental results, the performance of the analytical formulations proposed by ACI, fib and Italian guidelines is appraised.     

Performance of steel beams strengthened with CFRP laminate – Part 2: FE analyses

A new method for repairing and strengthening steel is under development and consists of using CFRP (carbon-fibre-reinforced-polymer) laminates bonded to the steel substrate. Research on this method has been conducted by a few research groups in recent years. The idea is to let the CFRP laminate carry a large part of the stresses and thereby reduce the load on the steel, which may have had its capacity lowered due to deterioration or fatigue. The present paper presents the results of FE analyses of steel beams strengthened with bonded CFRP laminates. The interfacial shear and peeling stresses that appear in the bond line between the steel and CFRP laminate are studied in both the elastic and plastic phase of the steel beam. Comparisons with the results obtained from laboratory tests conducted on steel beams strengthened with bonded CFRP laminates show that the behaviour of the strengthened beams can be captured using FE analyses. The distribution of the shear and peeling stresses near the end of the bond line were obtained from the FE analyses, together with the interfacial stresses that develop near beam mid-span due to the yielding of the steel. These stresses may exceed the capacity of the adhesive and cause debonding in this region.     

预应力FRP加固混凝土结构技术研究与应用

介绍了笔者进行的预应力芳纶纤维布和碳纤维筋加固混凝土结构的一些主要研究成果,内容包括:预应力芳纶纤维布永久锚具的开发;预应力芳纶纤维布的应力松弛损失研究;预应力芳纶纤维布加固混凝土梁的受弯、受剪性能研究;温度对芳纶纤维布配套粘结材料的力学性能影响研究;体外预应力碳纤维筋局部加固混凝土梁的力学性能研究;碳纤维筋预应力粗纤维混凝土梁的抗震性能研究;预应力纤维布加固混凝土结构的工程应用等。     

预应力高性能混杂纤维增强布加固HPC梁的DNLC单元研究

对于不同混杂配比的预应力高性能混杂纤维增强(HFRP)布,建立高性能HFRP布加固高性能混凝土(HPC)梁的双重非线性分层组合(DNLC)单元模型。根据实体退化单元理论,采用初应力等参层单元对高性能HFRP布的体外预应力效应进行模拟,同时正确地考虑HPC梁的材料非线性效应以及结构的几何非线性,验证DNLC单元分析模型的正确性,并对HPC梁的开裂荷载、普通钢筋应力、高性能HFRP布应力重分布等进行研究。结合试验资料分析表明,预应力碳/玻璃纤维混杂(CFRP/GFRP)布加固HPC梁的理论结果与试验数据吻合程度良好,采用等参层单元有效地模拟高性能HFRP布的预应力作用,及所推导的DNLC单元正确性。预应力GFRP纤维布加固的开裂荷载等均较低,预应力CFRP纤维布加固的开裂荷载等有所提高,但剩余强度过大。HPC梁开裂荷载前预应力高性能HFRP布应力发展缓慢,屈服荷载后,其发展迅速直至结构失效。