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。     

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。      

CFRP布加固木梁界面粘结应力的试验研究和理论分析

为了研究CFRP布加固木梁的界面粘结应力,对6根CFRP布加固的木梁进行了静力试验,得到了碳纤维布端部的应变分布。由相邻两测点的应变计算出了CFRP布与木材的平均界面粘结剪应力。忽略碳纤维布和木梁的剪切变形,推导了在任意荷载作用下两者之间的界面粘结剪应力和粘结正应力的计算公式,并根据边界条件给出了在两点对称集中荷载作用下公式中的系数,应用该公式计算了试验梁碳纤维布端部的界面粘结剪应力和正应力,结果表明粘结剪应力的计算值与试验值吻合较好,说明该公式是可行的。     

CFRP板加固含Ⅰ型裂纹混凝土的断裂扩展规律

制作两个含Ⅰ型预制裂缝的混凝土试件并进行三点弯曲试验,在其中一个试件底面粘贴碳纤维板进行加固,以CCD相机为观测系统,用数字散斑技术对混凝土试件预制裂缝尖端的扩展特征、裂缝尖端张开位移及应力强度因子等进行分析,研究了碳纤维复合材料(CFRP)板的加固效果。结果表明:外贴CFRP板加固能增加混凝土的抗弯刚度,显著提高混凝土的抗弯承载力,延缓裂缝开展;在相同荷载作用下,外贴CFRP板加固可以减小试件预制裂缝扩展宽度和距离;外CFRP板加固含Ⅰ型预制裂缝混凝土试件的裂缝尖端应力强度因子增加速度小于相同的未粘贴CFRP板混凝土试件。     

重组竹-混凝土界面粘结-滑移本构模型

为研究胶粘剂连接的重组竹-混凝土界面粘结性能及构建粘结-滑移本构模型,对44个重组竹-混凝土粘结试件进行单剪试验,并考虑了粘结长度、重组竹粘结宽度与厚度、混凝土强度及胶层厚度等因素对粘结性能的影响。研究结果表明：在不同影响因素下,试件破坏模式基本相同,均为混凝土表面发生剥离破坏,粘结界面间裂缝从加载端产生并向自由端发展,破坏过程分为弹性阶段、软化阶段和脱粘平台阶段;界面峰值剪应力随重组竹厚度、混凝土强度、胶层厚度增加而增大,随粘结宽度增加而减小。根据试验粘结-滑移曲线,建立了重组竹-混凝土界面粘结-滑移本构模型,与实验结果进行对比,该模型能较好地反映重组竹-混凝土界面剪应力与滑移量间的关系。     

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。     

复杂应力状态对混凝土梁外贴FRP条带抗剪贡献的影响

FRP剥离是外贴FRP抗剪加固混凝土梁主要的破坏模式之一。以往研究中往往简单的将面内剪切试验得到的FRP-混凝土界面粘结滑移关系应用于外贴FRP抗剪加固梁的剥离承载力计算。外贴FRP抗剪加固梁中FRP下的混凝土的应力状态与面内剪切试验情况有较大差别,这对FRP-混凝土界面的力学性能具有较大的影响。因此,以往的方法高估了FRP条带的抗剪贡献。该文研究了混凝土多轴应力状态对FRP-混凝土界面性能的影响,并根据试验研究结果,提出了U形FRP加固混凝土梁中FRP剥离应变的折减系数。与试验结果的对比计算分析表明:使用该折减系数修正后的设计公式更加合理。     

基于ABAQUS的表面内嵌CFRP筋粘结滑移性能数值模拟分析

利用ABAQUS有限元软件,对混凝土、CFRP筋或板条以及粘结剂分别采用不同的单元类型,特别是运用Spring2非线性弹簧单元来模拟表面内嵌CFRP筋或板条混凝土的粘结作用,同时考虑材料各自的本构关系,对表面内嵌CFRP筋混凝土拉拔试件和内嵌CFRP板条加固混凝土梁试件进行有限元模拟,将模拟结果与已有试验结果进行对比验证。结果表明,利用数值模拟的方法可以得到较为准确的拉拔试件的极限值,也可以较为正确地模拟加固梁的受力过程。在验证模拟结果正确的基础上,进一步分析了拉拔试件CFRP筋应力、滑移、粘结应力随不同位置的变化规律以及加固梁CFRP板条粘结区域内应变和粘结应力的分布情况。研究结果表明,对于拉拔试件,不同荷载等级作用下,CFRP筋在粘结区域内不同位置处的应力呈非线性变化,而粘结应力峰值出现在距加载点20mm~40mm位置处,其随位置的变化曲线呈偏态曲线特点。对于表面内嵌CFRP板条加固梁,粘结区域内CFRP应变和粘结应力在梁屈服后激增,且呈非线性变化。     

CFRP加固火灾作用后圆钢管混凝土构件的侧向撞击性能研究

建立了火灾作用后和碳纤维增强复合材料(CFRP)加固受火后圆钢管混凝土构件的侧向撞击数值模型,通过不同试验分别验证了模型的准确性。分析了加固受火后构件的撞击全过程,对比了构件的撞击力、跨中挠度和截面弯矩。对构件的抗撞击承载力和抗弯承载力,塑性变形和吸能能力,以及内力分布与发展进行了分析,并给出构件在撞击荷载作用下跨中最大挠度简化计算公式,最后讨论了CFRP加固方式对受火后构件撞击性能的影响。结果表明:采用CFRP加固受火后构件的撞击力平台值和平均截面弯矩提高,跨中挠度和撞击持续时间明显减小,CFRP加固对构件的抗撞击性能和抗弯能力提升显著;构件的抗撞击承载力、抗弯承载力和吸能能力随着受火时间的增加逐渐降低;构件在跨中产生不同程度的塑性变形,其主要通过形成塑性铰吸收能量;在峰值阶段构件的弯矩和剪力分布形态与相应静态荷载作用时差异明显,但在平台阶段时其分布形态与静态荷载作用时一致;简化计算公式可以很好地计算构件撞击后的跨中最大挠度,CFRP加固方式对受火后构件的抗撞击性能影响明显。     

CFRP筋与海水海砂混凝土粘结性能试验与机制分析

通过碳纤维增强树脂复合材料(CFRP)筋与海水海砂混凝土的中心及偏心拉拔试验,对比了CFRP筋在不同条件下拔出时的粘结应力及滑移值大小,得到完整的CFRP筋/海水海砂混凝土粘结-滑移曲线,同时分析了CFRP筋在混凝土中发生滑移时会产生的若干破坏模式,在此基础上对粘结-滑移曲线不同受力阶段特点进行了分析。同时,采用正交试验的方式,获得了不同因素对CFRP筋/混凝土粘结强度的影响程度。试验表明,CFRP筋/混凝土粘结力中摩擦力的占比较大,且粘结强度最主要的影响因素是混凝土的强度等级。除了传统拉拔试验中会出现的受力筋拉出破坏和混凝土劈裂破坏外,CFRP筋还会由于自身的外形、工艺及力学性能的影响,发生自身破坏。对不同破坏模式、受力筋外形的三种CFRP筋/海水海砂混凝土粘结-滑移曲线,采用多种理论表达式进行对比,最终得到不同条件下CFRP/海水海砂混凝土的粘结-滑移本构关系表达式。     

Interaction between CFRP-repair and initial damage of wide-flange steel beams subjected to three-point bending

This paper presents the interaction between carbon fiber reinforced polymer (CFRP) composites and the level of damage in steel beams. An experimental program is conducted with three different sizes of notches at midspan of the beams to simulate initial damage prior to repair. A three-dimensional finite element analysis (FEA) is conducted to predict experimental behavior. Unlike existing predictive models that assume perfect bond between CFRP and steel substrate, the proposed modeling approach explicitly accounts for the bond–slip behavior of CFRP–steel interface. CFRP-repair improves the load-carrying capacity of damaged beams that have failed by crack propagation across the steel section with wide opening of the notch. For repaired beams; stress concentrations at a damage location result in local debonding of the CFRP sheet, followed by complete debonding failure of the sheet. These failure modes are found to be independent of the level of initial damage (notch depth). CFRP-repair delays crack-formation of the repaired beams; however, such an effect is not significant once a crack propagates towards the upper flange. The level of initial damage influences debonding propagation rates of the CFRP. Two distinct bond–slip responses of the CFRP–steel interface are experimentally observed.     

CFRP筋在RPC中锚固性能的理论分析及试验研究

静载试验详细研究了碳纤维增强塑料CFRP筋在活性粉末混凝土RPC的锚固性能。试验结果表明:对于抗拉强度不大于3000MPa的表面压纹CFRP筋在抗压强度130MPa的RPC中的临界锚固长度为20倍CFRP筋直径;多根压纹CFRP筋的合理筋间距为1倍CFRP筋直径。平均粘结强度及其对应滑移量的公式具有较好的适用性。平均粘结应力与滑移之间的预测曲线与试验曲线吻合较好,验证了提出的粘结滑移本构关系。理论推导了锚固变量沿锚固长度分布的表达式,算例验证了有效性。分析表明:距离自由端约为0.6倍锚固长度位置处的粘结应力等于平均粘结应力。对于压纹CFRP筋,当锚长≤12.5倍筋材直径时,粘结应力沿埋长分布较为均匀,其不均匀性系数在1.02―1.05之间;当埋长>12.5倍且≤20倍筋材直径时,粘结应力沿埋长分布较为不均匀,其不均匀性系数在1.05―1.14之间。     

Fatigue behaviour of the bond interface between carbon fibre‐reinforced polymer sheets and concrete

Externally bonded carbon fibre‐reinforced polymers (CFRPs) have been applied to retrofit and strengthen civil structures. In this study, four‐point bending beams were manufactured and tested to examine the fatigue behaviour of the CFRP–concrete interface. The results indicated that the specimens exhibited debonding failure in the concrete beneath the adhesive layer under static loading. However, when cyclic loads were imposed on the small beams, debonding failure may occur in the adhesive layer. Moreover, fitting expressions were proposed to predict the shear stress–slip relationship between the CFRP sheets and concrete and the flexural strength of the CFRP‐strengthened beams under static loads, and good agreement with the test data was obtained. Finally, a fatigue life prediction model was also presented to capture the fatigue life of the CFRP–concrete interface under cyclic loads. The calculation results showed that the fatigue strength of the CFRP–concrete bond interface was approximately 65% of the ultimate load capacity.     

Pull out of FRP reinforcement from masonry pillars: Experimental and numerical results

In this paper, debonding phenomena between carbon fiber reinforced polymer (CFRP) strips and masonry support were investigated on the basis of single-lap shear tests, considering different dimensions of the bond length. To capture the post-peak response of the CFRP–masonry joint, the slip between the support and the reinforcement strip was controlled using a clip gauge positioned at the end of the reinforcement. The tests were simulated by means of a finite element model able to capture the post-peak snap-back behavior due to the failure process. The numerical model is based on zero-thickness interface elements and on a proper non-linear cohesive law. The comparison between experimental and numerical results was performed in terms of overall response, measured by both the machine stroke and the clip gauge positioned at the free end of the reinforcement. The cases of effective bond length greater and lesser than the minimum anchorage length, suggested by the CNR Italian recommendation, were considered.     

Investigation into bond behaviour of a new CFRP anchorage system for concrete utilising a mechanically strengthened substrate

Failure of CFRP strengthening systems when applied to concrete structures is usually typified by de-lamination of the CFRP from the concrete substrate. To prevent this type of failure, current standards and design guidelines impose strict limitations on the maximum strain level of the composite material which may be utilised in design. Emerging research has shown that anchoring the ends of the CFRP plates or sheets can result in a significantly higher load/stress being reached before de-bonding occurs. This paper investigates a method of anchoring CFRP laminates by utilisation of a mechanical chase cut into the concrete over the anchorage length. Experiments have shown this to be an effective way to improve the strength of the concrete substrate, resulting in higher CFRP bond strength and improvements in maximum elongations, bond stress, slip and load carrying capacity of the CFRP to concrete joint.     

Durability of CFRP-concrete joints under freeze-thaw cycling

The long-term durability of fiber reinforced polymer (FRP) strengthening systems under freeze-thaw cycling is crucial to the safety of structures in cold climates. The durability of the FRP-concrete bond interface under freeze-thaw cycling was investigated in the study reported here, with exposure condition, concrete grade, and number of freeze-thaw cycles as the parameters considered. The behavior of the carbon FRP (CFRP)-concrete bond interface was investigated with single-face shear tests. The results indicate that the bond strength, bond stiffness, interfacial fracture energy, and maximum slip of the joints decrease with increases in the number of freeze-thaw cycles, and they are also affected by the exposure environment. The depth of cracking and effective bond length increase with increases in cycle number, thus affecting bond stiffness and strength. The deterioration of bond strength can be attributed to the damage caused to the concrete by the freeze-thaw cycling.     

Influence of water on bond behavior between CFRP sheet and natural calcareous stones

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (Fiber Reinforced Plastic) are externally applied to a masonry substrate. In the performed research a substrate made by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets has been analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP, the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets and stones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resins before and after their immersion in water, evaluating the effects of this agent on the properties of the materials. The influence of the aging in water on the interface stone-reinforcement was analyzed in terms of bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode of failure. In addition the possibility of calibrating design relationships, taking into account the influence of environmental conditions is discussed. Detailed results on adhesives and composites aged in water have been reported in a previous paper while in the present work the significant decay of the mechanical properties of the stone is specifically investigated. With regard to the conditioning treatment a reduction of the bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bond stress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtained results the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once the properties of the aged materials are considered in the provided relationships.     

Influence of water on bond behavior between CFRP sheet and natural calcareous stones

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (Fiber Reinforced Plastic) are externally applied to a masonry substrate. In the performed research a substrate made by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets has been analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP, the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets and stones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resins before and after their immersion in water, evaluating the effects of this agent on the properties of the materials. The influence of the aging in water on the interface stone-reinforcement was analyzed in terms of bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode of failure. In addition the possibility of calibrating design relationships, taking into account the influence of environmental conditions is discussed. Detailed results on adhesives and composites aged in water have been reported in a previous paper while in the present work the significant decay of the mechanical properties of the stone is specifically investigated. With regard to the conditioning treatment a reduction of the bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bond stress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtained results the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once the properties of the aged materials are considered in the provided relationships.     

Hybrid epoxy-silyl modified polymer adhesives for CFRP sheets bonded to a steel substrate

This paper presents an experimental study examining the interfacial behavior between a steel substrate and carbon fiber reinforced polymer (CFRP) sheets bonded with hybrid epoxy-silyl modified polymer (SMP) adhesives. The epoxy adhesive has high modulus and strength characteristics, while the SMP adhesive possesses a low modulus with permanent elastic nature. The hypothesis tested is that a combination of these two distinct materials can alleviate interfacial stresses along the bond line with maintaining adequate strength. Two types of double-lap tension tests are conducted to evaluate the bond-capacity of the epoxy and SMP adhesives and to study the effect of various hybrid bond schemes. Test results show that the specimens bonded with homogeneous epoxy demonstrate abrupt failure, whereas those with SMP exhibit gradual load-softening at failure. The load-carrying capacity and stiffness of the CFRP–steel interface are not influenced by hybrid bond configurations. The degree of CFRP-debonding is, however, affected by the hybrid bond scheme. Stress transfer from the steel substrate to the CFRP is well maintained along the hybrid bond line with significant local deformability of the interface layer. Analytical models report that shear stresses along the CFRP–steel interface are noticeably mitigated at geometric discontinuities and the proposed hybrid bond technique can be used for structure-level application.