Preparation and Reinforcement Adaptability of Jute Fiber Reinforced Magnesium Phosphate Cement Based Composite Materials

To improve the brittleness characteristics of magnesium phosphate cement-based materials(MPC)and to promote its promotion and application in the field of structural reinforcement and repair,this study aimed to increase the toughness of MPC by adding jute fiber,explore the effects of different amounts of jute fiber on the working and mechanical properties of MPC,and prepare jute fiber reinforced magnesium phosphate cement-based materials(JFRMPC)to reinforce damaged beams.The improvement effect of beam performance before and after reinforcement was compared,and the strengthening and toughening mechanisms of jute fiber on MPC were explored through microscopic analysis.The experimental results show that,as the content of jute fiber(JF)increases,the fluidity and setting time of MPC decrease continuously;When the content of jute fiber is 0.8%,the compressive strength,flexural strength,and bonding strength of MPC at 28 days reach their maximum values,which are increased by 18.0%,20.5%,and 22.6%compared to those of M0,respectively.The beam strengthened with JFRMPC can withstand greater deformation,with a deflection of 2.3 times that of the unreinforced beam at failure.The strain of the steel bar is greatly reduced,and the initial crack and failure loads of the reinforced beam are increased by 192.1%and 16.1%,respectively,compared to those of the unreinforced beam.The JF added to the MPC matrix dissipates energy through tensile fracture and debonding pull-out,slowing down stress concentration and inhibiting the free development of cracks in the matrix,enabling JFRMPC to exhibit higher strength and better toughness.The JF does not cause the hydration of MPC to generate new compounds but reduces the amount of hydration products generated.

Electrical resistance stability of high content carbon fiber reinforced cement composite

The influences of curing time, the content of free evaporable water in cement paste, environmental temperature, and alternative heating and cooling on the electrical resistance of high content carbon fiber reinforced cement (CFRC) paste are studied by experiments with specimens of Portland cement 42.5 with 10 mm PAN-based carbon fiber and methylcellulose. Experimental results indicate that the electrical resistance of CFRC increases relatively by 24% within a hydration time of 90 d and almost keeps constant after 14 d, changes hardly with the mass loss of free evaporable water in the concrete dried at 50 °C, increases relatively by 4% when ambient temperature decreases from 15 °C to ?20 °C, and decreases relatively by 13% with temperature increasing by 88 °C. It is suggested that the electric resistance of the CFRC is stable, which is testified by the stable power output obtained by electrifying the CFRC slab with a given voltage. This implies that such kind of high content carbon fiber reinforced cement composite is potentially a desirable electrothermal material for airfield runways and road surfaces deicing.

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.

弯-剪-扭复合受力CFRP布加固RC梁抗扭性能细观数值分析

为了研究弯-剪-扭复合受力条件下碳纤维增强复合材料(carbon fiber reinforced polymer, CFRP)布加固钢筋混凝土(reinforced concrete, RC)梁的抗扭性能,建立体现混凝土材料非均质性、钢筋-混凝土黏结滑移关系和CFRP布-混凝土相互作用关系的细观数值分析模型。由于缺乏CFRP布加固RC梁在弯-剪-扭复合受力作用下失效破坏的典型物理试验,采用复合对比验证法对数值模型的合理性进行验证,即分别与单独受剪、单独受扭以及受弯-扭复合作用下RC梁和CFRP布加固RC梁的试验结果进行对比验证。进而,基于建立的细观数值分析模型探究了配纤率和扭弯比对CFRP布加固RC梁在弯-剪-扭复合受力作用下抗扭力学性能的影响。结果表明:1)采用CFRP布加固能够显著提高RC梁的峰值扭矩;2)随配纤率增加,RC梁损伤分布范围逐渐变大,CFRP布应变逐渐减小,梁整体峰值扭矩增大,但其增大幅度减小;3)随扭弯比增加,RC梁裂缝分布愈加集中,裂缝与水平方向夹角逐渐减小,梁整体峰值扭矩增大,CFRP布应变减小。建立的考虑弯-剪-扭复合受力作用的数值模拟方法可为后续研究复杂应力状态下CFRP布加固RC梁尺寸效应行为等奠定基础。

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.

钢纤维聚丙烯复材加固的建筑梁柱节点抗裂性能研究

为提升建筑结构整体抗断裂性能,建筑建设时将纤维增强水泥基复合材料应用在建筑梁柱节点的关键受力位置。采用纤维增强水泥基复合材料(FRC)加固建筑梁柱节点,模拟实际工况下的受力情况,记录断裂形貌与断裂后的微观结构变化,定量评估不同纤维掺量下的抗裂性能。试验结果表明,0.8%钢纤维掺和1.2%聚丙烯纤维增强水泥基复合材料,加固建筑梁柱节点后,裂缝数量最少,且裂缝面积最小,抗裂性能最佳,其余钢纤维与聚丙烯纤维配比的试件也能一定程度提升梁柱节点抗裂性能。

基于桥梁改造的新旧混凝土结构粘接加固复材改性研究

试验制备了一种聚乙烯醇(PVA)纤维水泥基复合材料(ECC),并将该材料与碳纤维布(CFRP)结合,对新旧钢筋混凝土梁进行复合加固处理,研究材料性能。结果表明,制备的ECC材料具备较好综合性能,粘接效果和抗裂效果良好,可以与CFRP结合,对桥梁混凝土进行粘接加固;相对于普通钢筋混凝土梁,经过ECC/CFRP复合加固后,钢筋混凝土梁的刚度提高,极限荷载明显提升;随着ECC/CFRP复合加固层厚度的增加、CFRP长度的增加,粘接加固效果越好,极限荷载最大值分别达到30.17、29.73 kN。ECC/CFRP复合加固可以提高旧危桥梁混凝土的性能,粘接加固效果较好。

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。

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.

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。

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。

HEC分散剂和纤维掺量对短切碳纤维水泥基材料压敏性的影响

为提高碳纤维水泥基复合材料(CFRCC)的压敏性,研究了羟乙基纤维素(HEC)对CFRCC中短切碳纤维分散特征的影响,采用灵敏度系数法分别探究了不同HEC掺量和短切碳纤维掺量对CFRCC压敏性的影响规律,并根据电化学阻抗法和可蒸发水含量法分析了相应CFRCC的孔结构特征,然后采用SEM测试技术对CFRCC基体中短切碳纤维分散状况以及纤维与基体界面特征进行了深入观察。结果表明:HEC分散剂在CFRCC中的掺量为胶凝材料总质量的0.4%~0.6%时,可以使CFRCC具有良好力学性能的条件下发挥更好的压敏性;CFRCC基体水胶比在0.35时,短切碳纤维的分散效果最好。在相同水胶比条件下,随着短切碳纤维掺量的增加,CFRCC抗折强度均有不同程度的提高,而其抗压强度则逐渐降低。当短切碳纤维体积掺量为0.2%时,短切碳纤维在CFRCC基体中无互相搭接和成簇现象;综合考虑压敏性和力学性能,CFRCC中短切碳纤维的适宜掺量范围为0.2%~0.25%。

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。

碳纤维编织网增强磷酸镁水泥砂浆界面黏结性能研究

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