Shear strengthening of pre-damaged reinforced concrete beams with carbon fiber reinforced polymer sheet strips

This paper presents the results of an experimental investigation on the response of pre-damaged reinforced concrete (RC) beam strengthened in shear using applied-epoxy unidirectional carbon fiber reinforced polymer (CFRP) sheet. The reasearch included four test rectangular simply supported RC beams in shear capacity. One is the control beam, two RC beams are damaged to a predetermined degree from ultimate shear capacity of the control beam, and the last beam is left without pre-damaged and then strengthened with using externally bonded carbon fiber reinforced polymer to upgrade their shear capacity. We focused on the damage degree to beams during strengthening, therefore, only the beams with side- bonded CFRPs strips and horizontal anchored strips were used. The results show the feasibility of using CFRPs to restore or increase the load-carrying capacity in the shear of damaged RC beams. The failure mode of all the CFRP-strengthened beams is debonding of CFRP vertical strips. Two prediction available models in ACI-440 and fib European code were compared with the experimental results.

Investigating Some Parameters Affecting Flexural Behavior of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Laminate

In this paper,the influence of some important parameters affecting the flexural behavior of reinforced concrete beams strengthened with one layer of carbon fiber reinforced polymer(CFRP)laminate has been studied.For this purpose,six reinforced concrete beams were cast and tested in the laboratory.Based on the obtained data,when CFRP laminate is applied to the tension face,too close to the steel rebar,the flexural strength of the strengthened beam is reduced.In general,the performance of the beam strengthened with one wide CFRP strip is better than that strengthened with two equivalent narrow strips.Ultimate load capacity of each strengthened beam was calculated based on the method given by the ACI 440.2R and compared with the test one.It is concluded that,to avoid the steel rebar-CFRP laminate interaction effect,the CFRP laminate depth-to-the effective depth ratio(df/d)should not be smaller than about 1.17.

Flexural rigidity evolvement laws of reinforced concrete beams strengthened with carbon fiber laminates

Extensive research has shown that externally bonded carbon fiber reinforced polymer (CFRP) laminates are particularly suitable for improving the fatigue behavior of reinforced concrete (RC) beams. This paper presents the research on flexural rigidity evolvement laws by testing 14 simple-supported RC beams strengthened with carbon fiber laminates (CFL) under cyclic load, and 2 under monotone load as a reference. The cyclic load tests revealed the peak load applied onto the surface of a supported RC beam strengthened with CFL is linear to the logarithm of its fatigue life, and the flexural rigidity evolvement undergoes three distinct phases: a rapid decrease from the start to about 5% of the fatigue life; an even development from 5% to about 99% of the fatigue life; and a succedent rapid decrease to failure. When the ratio of fatigue cycles to the fatigue life is within 0.05 to 0.99, the flexural rigidity varies linearly with the ratio. The peak load does not affect the flexural rigidity evolvement if it is not high enough to make the main reinforcements yield. The dependences of the flexural rigidity of specimens formed in the same group upon their fatigue cycles normalized by fatigue life are almost coincident. This implies the flexural rigidity may be a material parameter independent of the stress level. These relationships of flexural rigidity to fatigue cycles, and fatigue life may be able to provide some hints for fatigue design and fatigue life evaluation of RC member strengthened with CFL; nevertheless the findings still need verifying by more experiments.

Stiffness degradation-based damage model for RC members and structures using fiber-beam elements

To meet the demand for an accurate and highly efficient damage model with a distinct physical meaning for performance-based earthquake engineering applications, a stiffness degradation-based damage model for reinforced concrete （RC） members and structures was developed using fiber beam-column elements. In this model, damage indices for concrete and steel fibers were defined by the degradation of the initial reloading modulus and the low-cycle fatigue law. Then, section, member, story and structure damage was evaluated by the degradation of the sectional bending stiffness, rod-end bending stiffness, story lateral stiffness and structure lateral stiffness, respectively. The damage model was realized in Matlab by reading in the outputs of OpenSees. The application of the damage model to RC columns and a RC frame indicates that the damage model is capable of accurately predicting the magnitude, position, and evolutionary process of damage, and estimating stow damage more precisely than inter-story drift. Additionally, the damage model establishes a close connection between damage indices at various levels without introducing weighting coefficients or force-displacement relationships. The development of the model has perfected the damage assessment function of OpenSees, laying a solid foundation for damage estimation at various levels of a large-scale structure subjected to seismic loading.

CFRP及EWSS复合加固震损双层高架桥框架式桥墩恢复力模型研究

为研究碳纤维布(Carbon Fiber Reinforced Polymer,CFRP)及外包型钢(Externally Wrapped Steel Section,EWSS)复合加固震损双层高架桥框架式桥墩的恢复力模型,对1榀原型对比试件、1榀无预损加固试件和2榀遭受不同地震损伤加固试件进行了低周往复加载破坏试验,获取了滞回曲线并提取骨架曲线,分析其滞回特性,提出一种弹性段和强化段为双折线、下降段为指数函数且考虑初始损伤的骨架曲线模型;采用试验数据回归拟合方法,定量描述了试件滞回曲线卸载刚度的退化规律,考虑了同级加载承载力退化和定点指向特征,建立了恢复力模型。研究结果表明:复合加固试件滞回曲线捏缩现象明显,呈倒S型,各滞回环分别相交于骨架曲线上正向、负向荷载为屈服荷载0.25倍的点,峰值荷载后EWSS产生包辛格效应;所提出的骨架曲线模型对中度震损和重度震损加固试件下降段指数函数的参数k建议取值分别为3.6和3.4;所建立的骨架曲线模型和恢复力模型计算结果与试验实测结果吻合较好,可为该类结构弹塑性地震反应分析提供依据。

碳纤维板加固H型钢梁抗剥离夹具研制及应用试验

外贴碳纤维板是钢梁抗弯加固的常用方法,但其端部易因界面应力集中导致剥离破坏,严重影响高性能材料的有效利用,特别是结构加固后的安全服役.为此开发出一种适用于H型钢梁的简易夹具——C形槽板夹,以加强碳纤维板的端部锚固.通过多根带夹碳纤维板加固梁的静力加载试验,验证了此夹具的可靠性,并考察了碳纤维板伸入剪跨段长度与剪跨段长度之比及锚固方式(纯粘、端锚和混锚)对加固效果的影响.研究发现端锚加固不同于纯粘,只要梁的控制截面仍在夹具之间,碳纤维板的极限应力、加固效果及利用效率均随其长度缩短而提高,当碳纤维板长度分别为600、750 mm时,前者极限应变和承载力比后者分别高27.3%和8.1%.由于钢梁表面处理质量较差,混锚加固梁均发生突然剥离,后期退化为端锚加固梁,因而相比端锚加固梁加固效果改善不大,需要提高表面处理质量再做类似研究.尽管如此,相比纯粘加固梁,混锚加固梁采用C形槽板夹后剥离破坏被延迟,抗弯性能大为改善.试验过程中还用压电阻抗法对界面剥离情况进行了检测,结果符合实际.

剪力滞对CFRP板-钢梁加固界面应力的影响

为了分析工字钢梁弯曲变形产生的剪力滞效应对黏结层法向应力和切应力的影响,基于虚功原理的能量变分法建立考虑剪力滞的碳纤维增强复合材料(CFRP)加固梁黏结应力计算理论方程.通过与试验和有限元数值模拟结果的对比,验证了理论方程的正确性.开展不同弯矩作用下的工字型钢梁黏结层界面切应力和法向应力参数分析.结果表明,剪力滞引起的翼缘板纵向位移沿横向3次抛物线分布的假定是合理的.剪力滞效应对CFRP加固工字钢梁弯曲变形下的端部界面应力影响不可忽略,影响程度随着截面弯矩和翼缘板宽度的增加而增大.

钢结构梁柱加固节点框架损伤演化过程数值模拟分析

为研究不同因素对钢结构梁柱加固节点框架演化过程损伤的影响,选择4个钢结构梁柱加固节点框架试件作为实验对象,在其他条件不变基础上,改变试件轴压比、钢盖板厚度以及混凝土强度,用ABAQUS软件对钢结构梁柱加固节点框架进行模拟,观察损伤演化过程的应力云图情况,并对屈服位移与峰值位移进行统计。结果表明:轴压比以及钢盖板厚度对受力性能影响不大,混凝土强度对受力性能影响较大,并且随着混凝土强度的增加,钢结构梁柱加固节点框架的承载力也在增加,在节点框架设计时,应适当增加混凝土强度,提高节点框架承载力。

CFRP布对先张法预应力Bulb-T梁抗弯性能影响研究

为研究碳纤维增强复合材料(CFRP)布对先张法预应力Bulb-T梁抗弯性能的影响,采用有限元软件ABAQUS建立了三维实体模型,并进行了四点弯曲试验数值模拟,分析了在不同CFRP布加固方式下Bulb-T梁损伤过程及受力性能。结果表明:先张法预应力Bulb-T梁模拟值与试验结果误差较小,建立的模型可以较准确地模拟足尺预应力钢混梁的破坏形态、预拱度、荷载-位移曲线;CFRP加固能够显著提升梁体的抗弯刚度及开裂荷载,延缓开裂时间、抑制裂缝的扩展,其中,下翼缘板底部粘贴CFRP布可在失效前对跨中裂缝密集区起到有效控制,弯剪区包裹加固减小了梁体加载点及垫块之间“八”字形裂缝的展开;下翼缘板底部粘贴CFRP布对抗弯承载力的提升效果更加显著,且极限承载力和刚度随着加固层数的增加而增大。

不同加固方式对RC框架抗连续倒塌性能的影响研究

为研究不同加固方式对钢筋混凝土框架结构抗连续倒塌能力的影响,建立钢筋混凝土框架有限元模型进行对比分析。用五种不同加固方式分别加固,并模拟结构中柱遭遇偶然荷载后失效情况,对其进行非线性静力分析。ABAQUS模拟结果表明:加腋梁有利于节点区塑性铰的转移,使得框架在梁机制阶段的承载能力明显提升;粘贴碳纤维加固法、粘贴玻璃纤维加固法和增大梁截面加固法在悬索阶段对承载能力有所提升;增大柱截面加固法对抗连续倒塌能力提升很小。考虑材料成本及实际施工情况,各加固方式综合效益由高到低依次为:加腋梁加固法、粘贴玻璃纤维布加固法、粘贴碳纤维布加固法、增大梁截面法加固法、增大柱截面法加固法。

交通工程长条形地下结构平面框架计算问题探讨

交通工程长条形地下结构具有荷载大、恒载占总荷载比例高、构件截面尺寸及配筋量大、通常采用平面框架计算模型的特点,结合现行设计规范要求,通过典型工程算例对比对这类模型中杆件计算长度取值、节点刚域处理及结构正截面配筋计算中压应力考虑原则对计算结果的影响进行探讨。进而对这一类地下结构的设计计算提出优化建议,提高结构设计经济性及施工便利性,提升交通工程建设碳效益。

Experimental study and analysis on fatigue stiffness of RC beams strengthened with CFRP and steel plate

The objective of this work is to investigate the fatigue behavior of reinforced concrete(RC) beams strengthened with externally bonded carbon fiber reinforced polymer(CFRP) and steel plate. An experimental investigation and theoretical analysis were made on the law of deflection development and stiffness degradation, as well as the influence of fatigue load ranges. Test results indicate that the law of three-stage change under fatigue loading is followed by both midspan deflection and permanent deflection, which also have positive correlation with fatigue load amplitude. Fatigue stiffness of composite strengthened beams degrades gradually with the increasing of number of cycles. Based on the experimental results, a theoretical model by effective moment of inertia method is developed for calculating the sectional stiffness of such composite strengthened beams under fatigue loading, and the calculated results are in good agreement with the experimental results.

Strengthening an in-service reinforcement concrete bridge with prestressed CFRP bars

Carbon fiber reinforced polymer (CFRP) bars were prestressed for the structural strengthening of 8 T-shaped rein-forced concrete (RC) beams of a 21-year-old bridge in China. The ultimate bearing capacity of the existing bridge after retrofit was discussed on the basis of concrete structures theory. The flexural strengths of RC beams strengthened with CFRP bars were controlled by the failure of concrete in compression and a prestressing method was applied in the retrofit. The field construction processes of strengthening with CFRP bars-including grouting cracks, cutting groove, grouting epoxy and embedding CFRP bars, surface treating, banding with the U-type CFRP sheets, releasing external prestressed steel tendons-were introduced in detail. In order to evaluate the effectiveness of this strengthening method, field tests using vehicles as live load were applied before and after the retrofit. The test results of deflection and concrete strain of the T-shaped beams with and without strengthening show that the capacity of the repaired bridge, including the bending strength and stiffness, is enhanced. The measurements of crack width also indicate that this strengthening method can enhance the durability of bridges. Therefore, the proposed strengthening technology is feasible and effective.

3种加锚方式下2种碳纤维布加固钢筋砼梁抗剪承载力计算的比较

按照《碳纤维片材加固混凝土结构技术规程》与《混凝土结构加固设计规范》分别对胶锚或钢板锚、环形箍及自锁式、加织物压条一般式3种U形箍加锚方式加固后的梁斜截面承载力进行计算。结果发现,2种计算方式得出胶锚或钢板锚加锚方式的承载力增加值差异最小,加织物压条一般式的差异最大。2个工程抗剪加固应用算例的验证结果与计算结果一致。由此可知,在相同加固要求条件下,采用胶锚或钢板锚加锚方式的加固成本最低。

装配式混凝土建筑梁柱节点加固方法研究

建筑梁柱节点是受力最多的部位,外力破坏下最先发生破坏,对此提出装配式混凝土建筑梁柱节点加固方法.使用芳纶纤维布实现梁柱节点加固,按照加固层数与加固方式不同制备4种试件,利用千斤顶向试件施加荷载模拟外力破坏,测试各个试件的承载力,同时构建有限元模型计算并绘制试件的滞回曲线、骨架曲线与延性变化.试验结果表明,缠绕两层芳纶纤维布并在节点位置二次加固的试件C具有良好的承载力,从滞回曲线与骨架曲线变化效果可以确定该试件能够承载较为严重的外力破坏,同时该试件延性系数提高率达到21.92%,加固性能良好.

锚固方法和预应力水平对CFRP板加固钢筋混凝土梁抗弯性能的影响

为了研究端部锚固方法和预应力水平对碳纤维复合材料(CFRP)板加固钢筋混凝土(RC)梁抗弯性能的影响,进行了6根大尺寸T梁抗弯试验,对失效模式、荷载-挠度曲线、特征荷载、CFRP板强度利用率及延性等指标进行分析。结果表明:锚固方法对RC梁的极限荷载有显著影响,但对开裂荷载和屈服荷载基本没有影响;当预应力水平从0提高到0.5,失效模式从混凝土压碎转变为CFRP拉断,开裂荷载和屈服荷载比未加固试件分别提高了75.0%~237.5%和13.6%~50.9%;极限荷载在混凝土压碎模式下随预应力水平的提高而提高,但在CFRP拉断模式下受预应力水平的影响很小;可靠的端部锚固可提高极限荷载下CFRP的利用率,但施加预应力能明显提高整个受力阶段CFRP的利用率。

CFRP加固损伤钢筋混凝土梁抗冲击性能数值研究

为探究碳纤维增强复合材料(carbon fiber reinforced plastics,CFRP)加固损伤钢筋混凝土(reinforced concrete,RC)梁的抗冲击性能,利用有限元分析软件LS-DYNA,对不同CFRP加固方式下损伤梁的动态响应进行了数值模拟研究,并与物理试验结果进行对比验证.对加固梁的破坏模式、裂缝扩展规律和动态时程响应进行分析.结果表明,U型CFRP加固梁能显著抑制剪切斜裂缝的发展,大幅减小裂缝开展范围;在第1次冲击时,相较于未加固梁,平直型、U型和复合型CFRP加固梁的峰值冲击力分别提高了15.4%、16.6%和13.1%,跨中峰值位移分别降低了4.8%、8.3%和10.1%;在第2次冲击时,U型和复合型CFRP加固梁仍表现出较好的抗冲击性能.研究结果可为CFRP加固损伤RC梁的优化设计提供参考.

P-TRC板加固梁抗弯性能的有限元模拟分析

提出了一种内嵌预应力纤维织物增强混凝土板(P-TRC板)加固混凝土梁的方法,为了减少现场湿作业,板梁之间采用射钉连接。通过有限元建模分析,对比既有试验数据,验证了混凝土本构与纤维织物本构的准确性和数值方法的可靠性。在此基础上,分析了预应力水平、混凝土强度、纵筋配筋率和P-TRC板配网率等因素对加固梁抗弯性能的影响。有限元分析结果表明:纵筋配筋率与P-TRC板配网率的提高降低了织物的利用率;加固梁的屈服荷载随预应力和配网率的提高分别增加了7.2%~22.3%和5.4%~22.4%;极限荷载随预应力水平和配网率的提高分别增加了11.6%~21.7%和12.3%~34.3%。使用射钉将P-TRC板与梁底部连接,此种施工方法快捷方便,契合了建筑工业化的趋势。

锚固碳纤维布加固低强度钢筋混凝土梁有限元分析

为研究不同加固方式下低强度钢筋混凝土梁的受力性能及裂缝开展情况,本文通过有限元软件建立了钢筋混凝土梁的三维有限元模型。通过与试验数据的对比分析,得到的模拟结果和试验结果吻合较好,较准确地反映出混凝土梁的受力特征和受力过程,验证了数值模型的合理性;通过仿真分析进一步验证,相较于碳纤维U形箍锚固梁,钢板U形箍锚固梁可以更好地抑制梁底碳纤维布与混凝土的剥离,从而提高加固效果。

碳纤维网格加固梁抗弯性能试验研究

为促进碳纤维网格这种新型碳纤维产品在桥梁工程中的应用,以桥梁规范中的适筋梁理论为基础设计试件,采用实际工程的碳纤维网格加固方案分别对未开裂的和开裂的钢筋混凝土梁试件进行加固,开展碳纤维网格加固梁正截面破坏试验研究,并与未加固梁试验结果进行对比分析。结果表明:在试验荷载作用下未加固梁、无损伤加固梁和有损伤加固梁均发生了正截面破坏;无损伤加固梁和有损伤加固梁开裂荷载相当,但裂缝特点不同;加载过程中无损伤加固梁的变形小于有损伤加固梁和未加固梁的;荷载较小时,有损伤加固梁与未加固梁跨中挠度相当,随着荷载的增加,有损伤加固梁的变形开始小于未加固梁;无损伤加固梁和有损伤加固梁的抗弯承载力较未加固梁分别提高了13.6%和16.7%;卸载时,加固梁的变形出现了明显的回弹现象;加载过程中未出现界面剥离问题。研究结果可为评估碳纤维网格在桥梁工程中的加固效果提供参考。