无腹筋RC悬臂梁抗剪强度及尺寸效应理论研究

不同于混凝土材料,钢筋混凝土(RC)构件的破坏模式与机制更为复杂,采用混凝土材料尺寸效应理论难以描述构件的尺寸效应行为。我国相关规范中并没有系统考虑构件尺寸及纵筋率对RC梁抗剪承载力的影响。为研究剪跨比及纵筋率对无腹筋RC梁剪切破坏及抗剪强度尺寸效应的影响,采用三维细观数值模拟方法,建立了RC梁剪切破坏力学分析模型,研究了剪跨比及纵筋率对RC梁剪切破坏及抗剪强度尺寸效应的影响机制与规律。研究结果表明,RC梁抗剪强度表现出极为显著的尺寸效应现象;RC梁抗剪强度随剪跨比的增大而减小,随纵筋率的增大而增大;剪跨比较小时,纵筋率对抗剪强度的影响尤为显著。此外,基于Ba■ant材料层次尺寸效应律,提出了考虑剪跨比及纵筋率影响的RC梁抗剪强度尺寸效应理论公式。对比试验结果,验证了所提公式的准确性与合理性。     

冻融循环下预应力CFRP板加固钢筋混凝土梁的耐久性研究

预应力CFRP加固混凝土结构技术由于具有显著优势,越来越多地被应用在桥梁加固中,本文针对冻融循环作用下预应力CFRP板加固钢筋混凝土梁的耐久性能进行了实验研究。通过12片加固梁试件的实验研究了不同次数冻融循环作用下预应力CFRP板加固梁的破坏形态和承载性能,分析了混凝土强度等级、冻融循环次数、CFRP初始应力水平等因素对加固梁耐久性能的影响。实验结果表明:经历冻融循环后试件的开裂荷载和极限承载能力都有了不同程度的下降,冻融侵蚀对CFRP加固混凝土结构产生了明显的不利影响;随着冻融循环次数的增加,加固试件的破坏模式逐渐由混凝土保护层剥离转变为界面剥离的破坏形态;冻融循环作用对预应力加固试件的整体不利影响要大于非预应力试件;混凝土强度为C60的预应力CFRP加固试件在冻融侵蚀作用下的退化要较强度为C30的加固试件显著。     

碳纤维布加固钢筋混凝土梁受力性能的试验研究

通过对8根矩形截面钢筋混凝土梁的静力加载试验,研究了抗弯和抗剪两种加固方式对钢筋混凝土梁的破坏形态和力学性能的影响,对比分析了碳纤维加固率、试验梁的状态、混凝土强度对加固效果的影响。结果表明：经碳纤维抗弯哥口抗剪加固后加圉梁相应的力学性能指标值明显提高;混凝土裂缝扩展得到了有效延缓,加固效果明显;抗弯加固后可显著提高混凝土梁的承载力和延性,但提高程度并不与加固率成正比;预裂粱抗弯加固后承载力和延性得到提高,但与其相同加固参数的梁相比,其承载力和延性有所降低;抗剪加固时混凝土强度越高,加固后混凝土梁的承载力和变形能力提高幅度越大,其中混凝土梁的变形能力受混凝土强度的影响较大。最后,对加固梁的极限承载力进行了理论分析与计算,建立了实用的理论计算公式,计算结果与试验结果吻合较好。     

Experimental study and analysis of RC beams strengthened with CFRP laminates under sustaining load

Six reinforced concrete beams strengthened in flexure using carbon fiber reinforced polymer (CFRP) laminates subjected to different sustaining loads were tested. The main goal of the test is to examine the effects of initial load and load history on the ultimate strength of strengthened reinforced concrete beams by externally bonded CFRP laminates. The main experimental parameters include different levels of sustaining load at the time of strengthening and load history. To explain the experimental results in quantitative terms, a theoretical model for flexural behavior of the strengthened reinforced concrete beam is also developed. Test results in the current study show that sustaining load levels at the time of strengthening have important influence on the ultimate strength of strengthened reinforced concrete beams. If the initial load is basically same, the ultimate strength of reinforced concrete beams strengthened with CFRP laminates is almost same regardless of load history at the time of strengthening.     

Étude de l?adhérence des joncs et des plats composite avec le béton par flexion (beam test)

The Near Surface Mounted (NSM) technique has been used in recent years for the strengthening of reinforced concrete beams. It involves the insertion of strips or rods of polymers reinforced with carbon fiber (CFRP) in the groove made previously in the concrete cover of corresponding surfaces, filled with epoxy adhesive for fixation. In order to characterize the bond behavior of the laminate and rods to concrete, an experimental work of pullout-bending tests was carried out. The pullout force at the laminate and the slip at the free and loaded end were measured. The influences of the concrete strength, the strength of the bond, and bond length on the bonding behavior between the three materials concrete, epoxy adhesive and CFRP were analyzed.     

碳纤维布与钢板复合加固梁剥离破坏研究

通过12根碳纤维布与钢板复合加固钢筋混凝土梁的抗弯性能研究,结果表明复合加固方式能有效地改善被加固构件的受力性能,但常由于复合加固层的剥离可能导致加固效果的降低。复合加固层与被加固构件之间的剥离是由于薄弱截面在剪应力及正应力的集中作用下而产生的,文中对复合加固层与混凝土之间的粘结剪应力及剥离正应力的计算公式分别进行推导,并进一步对碳纤维布与钢板复合加固的剥离机理进行分析,为工程应用提供依据。     

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.     

Process of debonding in RC beams shear-strengthened with FRP U-strips or side strips

Reinforced concrete (RC) beams may be strengthened for shear using externally bonded fiber reinforced polymer (FRP) composites in the form of side bonding, U-jacketing or complete wrapping. The shear failure of almost all RC beams shear-strengthened with side bonded FRP and the majority of those strengthened with FRP U-jackets, is due to debonding of the FRP. The bond behavior between the externally-bonded FRP reinforcement (referred to as FRP strips for simplicity) and the concrete substrate therefore plays a crucial role in the failure process of these beams. Despite extensive research in the past decade, there is still a lack of understanding of how debonding of FRP strips in such a beam propagates and how the debonding process affects its shear behavior. This paper presents an analytical study on the progressive debonding of FRP strips in such strengthened beams. The complete debonding process is modeled and the contribution of the FRP strips to the shear capacity of the beam is quantified. The validity of the analytical solution is verified by comparing its predictions with numerical results from a finite element analysis. This analytical treatment represents a significant step forward in understanding how interaction between FRP strips, steel stirrups and concrete affects the shear resistance of RC beams shear-strengthened with FRP strips.     

钢筋混凝土梁受弯破坏过程的细观数值模拟研究

在细观层次上将混凝土视为由粗骨料、硬化水泥砂浆及界面粘结带组成的三相非均质复合材料,能够较好地模拟混凝土加载时的裂缝扩展过程和分布规律。本文采用细观刚体弹簧元法模拟了钢筋混凝土梁的弯曲受力性能和破坏过程,加载方式采用两点对称加载。数值计算得到了钢筋混凝土梁的破坏形态和荷载一变形曲线,并分析了受力过程中纵向钢筋的应力变化。与试验结果对比表明,细观刚体弹簧元法可以有效模拟钢筋混凝土梁的裂缝开展过程、破坏形态和荷载一变形响应。     

Residual stiffness assessment of structurally failed reinforced concrete structure by dynamic testing and finite element model updating

When an under-reinforced concrete beam structure has been loaded to the point where reinforcing steel on the tension side has yielded, it is deemed to have structurally failed and the full load capacity and stiffness can no longer be developed. When unloaded from the point of failure, the residual stiffness of the structure is difficult to estimate. There is a need to establish the serviceability of the structure and ultimately establish the levels of further loading that can be sustained before total collapse. In this paper we present a method for assessing residual stiffness of such a “failed” reinforced concrete structure, through the application of dynamic testing and finite element (FE) model updating. In an experimental study, failed zones in a beam structure were simulated in a FE model. Through a procedure of sensitivity-based updating using the measured modal properties, the stiffness distribution along the failed beam structure was identified.     

PVC-CFRP管钢筋混凝土柱抗震性能非线性有限元分析

在试验研究基础上,借助有限元分析软件,选取合理材料本构关系,建立PVC-CFRP管钢筋混凝土柱有限元分析模型,验证分析模型的正确性。对试验进行延拓分析,探讨混凝土强度等级、纵筋配筋率、CFRP条带层数及剪跨比等对试件骨架曲线、刚度退化影响规律,分析低周往复荷载作用下PVC-CFRP管钢筋混凝土柱的受力性能,揭示低周往复荷载作用下PVC-CFRP管钢筋混凝土柱的受力工作机理。     

A fracture model of reinforced concrete

Analyzed in this work is the failure mechanism of unidirectionally reinforced concrete under general stress state. The fracture process is described analytically by establishing the relation between loading and damage that involves the constitutive parameters of the reinforced concrete. Taken into account are the stiffnesses of mortar and reinforcement, bond strength of interface and mortar fracture toughness. An estimate on the ultimate strength is made with results given to the shear strength of a concrete slab.     

大气环境下的钢筋混凝土桥梁时变可靠度分析

为了探究钢筋混凝土桥梁抗力退化对桥梁可靠性能的影响,综合考虑一般大气环境下混凝土强度、钢筋强度、混凝土与钢筋黏结性能的降低,建立普通钢筋混凝土桥梁的抗力退化时变模型,分析规范车辆荷载作用的桥梁时变可靠度。研究结果表明,(1)一般大气环境下,桥梁抗力随着服役时间的增加而减小,其中钢筋强度退化对桥梁抗力的影响最大,其次是钢筋与混凝土黏结性能降低,而混凝土强度退化对桥梁抗力的影响相对较小; (2)桥梁可靠指标随着服役时间的增加而减小,密集行车占比越大,桥梁承载能力失效的概率就越大;非平稳车载作用下的桥梁可靠指标小于平稳车载,桥梁需要维修补强的时间比平稳车载提前; (3)多梁式简支梁桥在建造时应该提高边梁的安全储备,在管养时可以采取限载措施进行干预。     

钢筋混凝土结构破坏过程的近场动力学模拟

采用近场动力学(Peridynamics,PD)方法对钢筋混凝土结构破坏过程进行模拟,在"键"型近场动力学模型的基础上,考虑物质点对间的转动以突破泊松比的限制,采用能够描述混凝土材料的拉压异性和断裂特征的损伤模型,引入动态松弛、分级加载、平衡收敛准则和冲击接触等算法,建立了能准确描述钢筋混凝土结构破坏的近场动力学模型。模拟了钢筋混凝土梁在不同荷载作用下破坏的全过程,裂纹扩展路径以及最终破坏形式均与试验结果吻合,为解决工程结构破坏问题提供了一种有效的方法。     

钢筋锈胀引发混凝土保护层开裂破坏的细观数值研究

钢筋锈蚀膨胀引起保护层混凝土开裂是影响钢筋混凝土结构耐久性和服役寿命的重要因素。考虑到混凝土细观结构组成对保护层破坏模式的影响,从细观角度出发,将混凝土看作由骨料、砂浆基质及两者间界面过渡区组成的三相复合材料,建立了描述钢筋锈胀力学行为的混凝土随机骨料模型。采用塑性损伤本构关系模型来表征砂浆基质和过渡区界面的力学行为,假定钢筋均匀锈蚀,对钢筋锈胀引起的混凝土保护层开裂破坏过程进行了细观数值研究。对比了宏观均匀模型与细观非均质模型下获得的保护层破坏模式,探讨了径厚比(c/d)、钢筋位置(中部和角区)及混凝土拉伸强度对保护层破坏模式及保护层胀裂时钢筋锈蚀水平的影响,得到了一些有益结论。     

THE FAILURE MODES OF CONCRETE BEAMS STRENGTHENED WITH NEAR SURFACE MOUNTED FRP BARS

为研究不同参数下表面内嵌纤维筋加固后T 形混凝土梁的破坏模式, 对5 根不同梁端锚固、FRP(fiber reinforced polymer) 筋表面特征和FRP 筋类型的T 形混凝土梁进行受弯性能试验. 结果表明, 无梁端锚固、光圆GFRP (glass fiber reinforced polymer) 筋和CFRP (carbon fiber reinforced polymer) 筋加固梁试件发生粘结破坏. 梁端锚固和FRP 筋表面特征影响加固梁试件的极限载荷, CFRP 筋加固梁试件的屈服载荷和极限载荷较大. 螺纹FRP 筋和有梁端锚固加固梁试件FRP 筋利用率较高. 因此, 有梁端锚固的表面内嵌螺纹GFRP 筋加固是最为有效的加固方式.     

一般大气环境下锈蚀钢筋混凝土梁的时变可靠度分析

根据一般大气环境下锈蚀钢筋混凝土梁中的钢筋截面损失时变模型和抗剪、抗弯承载力退化模型,建立了考虑其随机性的极限状态方程。基于概率守恒原理分析得到满足该极限状态方程的概率密度演化方程,并引入吸收边界条件,提出了梁时变可靠度的计算方法。以受对称集中荷载的锈蚀钢筋混凝土简支梁为例,展示了目标函数的概率密度演化过程,并讨论了配置不同直径箍筋和不同厚度保护层时的时变可靠度。结果表明,在保证梁具有足够初始抗剪强度和抗弯强度的前提下,适当增大保护层厚度和箍筋直径(具有相同配箍率)能延缓混凝土构件力学性能的退化。通过和100万次蒙特卡洛模拟结果的对比可知,选用本文方法能在以付出较小计算代价的前提下,获得计算精度良好的分析结果。     

ESPI技术对外贴纤维混凝土加固承载的实验研究

采用电子散斑干涉技术,对外贴碳纤维加固混凝土梁的外贴材料位移的分布特征,进行了全场实时测量,通过实验获得的散斑干涉条纹图可以得到外贴材料与混凝土梁的粘结传力长度随粘结长度及初始载荷之间的关系;了解用于加固的碳纤维材料的应变分布特点和产生梁侧剥离破坏时的碳纤维表面位移(应变)的演化过程。实验还说明了电子散斑干涉技术不仅可用于位移的测量,而且也可用于结构安全监测和破坏预报。文中给出了对C20D、C25A和C60C侧贴碳纤维板加固在不同载荷作用直到构件破坏前的位移测试及对试件C60C轴线上的剪应力分析结果。     

腹筋对连续深梁剪切破坏影响的有限元分析

我国现行混凝土结构设计规范关于腹筋对深梁抗剪承载力影响的考虑,主要基于简支深梁的试验数据和计算模型,相关规定对于连续深梁的适用性有待验证。腹筋对连续深梁抗剪性能的影响机理十分复杂,系统的试验研究较少,试验结果比较分散。为了弥补试验研究的不足,采用非线性有限元方法模拟钢筋混凝土连续深梁的受力性能。利用已有的试验数据,验证了基于ATENA软件建立的有限元模型,并以此为基础,对以腹筋配筋率和剪跨比为变量的64根连续深梁进行分析,研究腹筋对连续深梁受剪承载力的影响。有限元分析结果与规范公式和压杆-拉杆模型(STM)及最新提出的三参数机动法(3PKT)的比较表明,中国规范高估了水平腹筋对构件受剪承载力的贡献,STM和3PKT方法也存在各自的不足。     

FATIGUE LIFE PREDICTION OF RC BEAMS STRENGTHENED WITH PRESTRESSED CFRP UNDER CYCLIC BENDING LOADS

The application of prestressed carbon reinforced polymer(prestressed CFRP)in reinforced concrete(RC)members can improve the mechanical properties of strengthened structures and strengthening efficiency.This paper proposed a semi-empirical prediction formula of fatigue lives of the RC beams strengthened with prestressed CFRP under bending loads.The formula is established based on the fatigue life prediction method of RC beams and fatigue experimental data of non-prestressed CFRP reinforced beams done before.Fatigue effect coefficient of the formula was confirmed by the fatigue experiments of the RC beams strengthened with prestressed carbon fiber laminate(prestressed CFL)under cyclic bending loads.Fatigue lives of the strengthened beams predicted using the formula agreed well with the experimental data.