Modeling in-plane and out-of-plane displacement fields in pull-off tests on FRP strips

The present paper deals with modeling FRP strips bonded to concrete blocks and tested in pull-off. The investigation starts from the experimental observations obtained by means of an optical image-correlation measurement system which is able to monitor the 3D displacement components of a fine mesh of points on the surface of both the FRP strip and concrete block. Thus, refined measurements of both in-plane and out-of-plane displacements of the FRP strips are available.A brief overview of the key contributions available in the scientific literature on modeling the bond behavior of FRP strips glued to a concrete substrate is firstly proposed. Then, a novel model based on simulating the behavior of the FRP strip as a Bernoulli beam on a layer of springs is formulated. It is aimed at determining the 2D displacement field of the FRP strip during a pull-off test up to debonding which actually occurs in a mixed fracture mode. The model is firstly formulated within the linear range by assuming elastic behavior for the above mentioned springs. The nonlinear behavior due to the cracking of concrete beneath the adhesive interface is then introduced for simulating the above mentioned experimental results. In particular, a bilinear relationship is assumed between interface slips and shear stresses, as is generally accepted within the scientific literature. Furthermore, a damage model is considered for reducing the stiffness of the transverse springs and simulating the crack propagation at the adhesive-concrete interface. Although this is a simplified way of modeling the nonlinear behavior of concrete in shear/tension, it results in rather accurate simulations of the available experimental results. In fact, it can simulate accurately the overall behavior observed in three experimental tests on specimens characterized by significantly different mechanical properties of the strip. Since the model assumes a small set of mechanical parameters for describing the mechanical behavior of the adhesive FRP-concrete interface and results in a reasonable small set of equations, it can be easily employed for identifying the above mentioned mechanical behavior indirectly. Other numerical models already available in the scientific literature (especially those based on the theory of finite elements) for simulating the 2D displacement field in the debonding stage are generally based on much more equations and require a much higher computational effort which makes impractical their use in an indirect identification procedure like the one presented in this paper. In fact, one of the main results of this study consists in determining the distribution of normal (peeling) stresses throughout the adhesive-to-concrete interface. This stress component (Mode I) can neither be directly measured during the tests nor determined by the theoretical models usually adopted for simulating the fracturing behavior of FRP-to-concrete joints in the so-called fracture process in “mode II”. Finally, it was found that the shear stresses are significantly higher than the peeling ones and control the crack propagation process.     

连续纤维在房屋建筑结构加固中的研究与应用

就当前研究的混凝土结构梁、板、柱、节点及墙体等采用FRP进行加固的补强方式进行了回顾，介绍了其应用状况，与其他加固技术相比，是一项有广阔市场前景的技术。     

An investigation of the composite action of an FRP/concrete prismatic beam

A novel composite/concrete beam has been fabricated to use two component materials, fibre-reinforced polymer composite and concrete, to their best advantage. The concrete and composite are wholly under compressive and tensile strains, respectively, together with any shear strain components, except where under direct compression at the supports. The shear transfer at the interface between these two components requires investigation and this paper describes six techniques used to develop this connection to give the most efficient shear transfer in order for the whole beam to act compositely. The six methods are used independently of one another and include the use of indents in the vertical walls of the permanent composite shuttering retaining the concrete, bonding the concrete to the permanent shuttering, forming the shear connection by using bolts through both the concrete and permanent shuttering, two methods using a resin injection to fill a pre-formed gap between the concrete and permanent shuttering and finally using an adhesive compatible with freshly made concrete. It was concluded that the most appropriate method of ensuring complete composite action between the two component materials was to bond the cured concrete to the permanent shuttering along the two vertical sides. However, this is not a practical option as it assumes that the concrete is removed from the mould after it has cured and then bonded back into the permanent shuttering. It is suggested that the best practical method is to use the adhesive compatible with freshly made concrete which still gave an adequate level of composite action.     

FRP土钉主要性能的试验研究

介绍一种新型FRP土钉的主要特点。对FRP筋体抗拉强度和黏结强度及FRP土钉的承载力分别进行室内试验和现场拉拔试验研究,描述并分析其破坏特征。试验表明:FRP筋体抗拉强度高达550MPa,在筋材设计时,可采用修正的混合定律公式进行预测,FRP筋在抗拉强度试验中表现为脆性破坏,树脂基体破碎先于纤维拉断,断裂截面呈参差不齐的毛刷状;FRP筋体与水泥砂浆的黏结强度取决于砂浆体的抗剪强度,黏结应力沿杆长非均匀分布,且FRP筋的最大黏结应力较钢筋更为集中;FRP土钉的变形主要是弹性变形,对于较短的FRP土钉,其承载力大小取决于土钉表面与土体间的抗剪强度,其极限抗拔承载力高于同直径的HRB335钢筋的屈服抗拉强度10%以上,满足工程应用的需要。故FRP土钉可广泛应用于基坑和边坡支护,特别是污染土、盐渍土等腐蚀环境下的土钉支护工程。     

外贴纤维与混凝土结合面的粘结滑移关系

本文在外贴纤维复合材料(FRP)与混凝土结合面双剪试验中,采用电子散斑干涉技术(ESPI)对FRP-混凝土结合面的变形场进行了测试,重点研究了结合面的粘结滑移关系。试验结果表明,FRP-混凝土结合面的粘结滑移关系曲线的发展过程由非线性上升段和不稳定下降段两部分组成,峰值应力与混凝土强度有关,达到应力峰值时的滑移和极限滑移受混凝土强度和FRP的形式(板或布)等的影响不大。通过结合面滑移刚度衰减规律的分析,本文提出了FRP-混凝土结合面粘结滑移本构关系的基本模式,该模式基于FRP-混凝土结合面的初始(弹性)滑移刚度,力学概念明确。     

FRP与混凝土界面黏结-滑移本构关系的试验研究

FRP与混凝土的黏结性能是外贴纤维增强聚合物加固钢筋混凝土结构技术的关键问题。采用修正梁模型,对9个外贴FRP条带加固混凝土受弯构件的黏结性能进行了试验研究,考察混凝土强度和FRP黏结长度对黏结性能的影响,分析了FRP应变以及局部黏结剪应力发展规律以及沿黏结长度在各级荷载下的分布规律,计算得到了局部黏结剪应力-滑移关系曲线。通过对试验结果的统计回归分析,提出3种不同复杂程度的局部黏结剪应力-滑移本构关系模型,3种本构关系模型与试验结果都吻合较好,可供实际加固改造工程应用以及完善相应规范的编制参考。     

预应力FRP筋混凝土梁受弯性能试验研究

对5根以FRP筋为有粘结预应力筋、以普通钢筋为非预应力筋的预应力混凝土梁受弯性能进行试验研究。根据试验结果对预应力FRP筋混凝土梁受力过程、破坏模式及裂缝情况进行了较为详细的研究。测定了梁的极限承载力、变形发展及裂缝分布等情况。试验研究结果表明：与预应钢绞线混凝土梁相比，预应力CFRP筋混凝土梁具有很好的变形能力而极限承载力相差不多，但是预应力AFRP筋混凝土梁承载力较低。通过计算得到的极限承载力、挠度与试验结果吻合较好。     

FPR-混凝土粘结界面的规律

根据试验数据,研究了FRP与混凝土粘结的非线性模型II界面规律。所提出的界面规律基于差分方程,并考虑了高剪应力下粘性剂和混凝土保护层的非线性作用。模型中的参数均在拉-拉剥离试验下校准,同时采用了不同荷载水平、不同粘结长度、沿FRP板方向的最大传递荷载与应变。通过有限粘结长度决定的最大传递荷载的估计值来确定界面规律中的断裂能,同时根据应变确定剪切-滑移关系。并通过一系列文献的试验结果验证参数优化程序。在粘结-滑移模型中采用所提出的界面定律进行数值模拟计算,得出粘结区域中FRP的应变、剪应力和滑移与试验结果很吻合。     

Experimental study on bond behavior between FRP and concrete

In this paper, two kinds of tests were designed to investigate and study the bond behavior between fiber-reinforced-plastic (FRP) and concrete. By newly designed double-shear test setup, the strain development and distribution in FRP plate was acquired. At the same time, the direct shear bond distribution was given and the effective bond length was suggested. Through a bending test on simply supported beam with an extended end, the strain and stress distribution of FRP plate in the negative moment zone were studied. Based on the test results, conclusions including the maximum effective bond length, the mean bond stress and the development length for flexural bond in the negative moment zone were presented. The effective bond length is about 100 mm and the mean bond stress is about 1.287 MPa. The development length for flexural bond in the negative moment zone is about 300 mm.     

组合FRP技术改善混凝土矩形柱的抗剪性能试验研究

进行利用植筋及FRP(纤维增强复合材料)布加固钢筋混凝土矩形柱以提高抗剪性能增强延性的新型加固技术研究.通过5个钢筋混凝土方柱的模型试验,分别研究植GFRP(玻璃纤维)筋、横向包裹CFRP(碳纤维)布及两者组合的加固技术对提高混凝土矩形柱抗剪性能的作用,分析讨论不同加固方法对混凝土矩形柱位移延性系数提高、刚度退化及能量耗散能力的影响.试验结果表明,采用植GFRP筋和包裹CFRP布的组合加固技术较前两种加固方法能更加有效地改善钢筋混凝土矩形柱的抗剪性能.     

FRP筋与混凝土粘结滑移性能的试验研究

通过对埋入混凝土一定深度的螺纹FRP筋的对称拉拔试验,研究了FRP筋与混凝土间界面的粘结滑移及界面应力传递机理,分析了自FRP筋加载端至自由端界面粘结滑移的发展、传递过程,探讨了FRP筋沿埋入深度方向的界面剪应力变化及界面粘结滑移的破坏特征。本文研究结果表明,FRP筋与混凝土界面剪应力及FRP筋滑移自FRP筋加载端至自由端逐步传递,滑移量逐渐减小;FRP筋混凝土与钢筋混凝土滑移破坏有着本质的区别,钢筋混凝土产生滑移时主要是混凝土撕裂和压碎,而FRP筋混凝土滑移破坏是以筋肋削弱或剪切破坏为主要特征。     

外包增强纤维布加固钢筋混凝土柱试验研究

分析了外包增强纤维布加固钢筋混凝土柱作用机理,介绍了工程中纤维布加固工序。通过试验,结果表明,使用纤维布加固钢筋混凝土柱,能提高桥墩的延性,具有较好的实用价值。     

FRP复合材料与混凝土的粘结强度试验研究

FRP复合材料与混凝土的粘结强度是混凝土结构外粘FRP板材这一加固新技术的重要研究课题。本文首先采用FRP板-混凝土块搭接接头的单剪试验方法,考虑FRP板的搭接长度、支座高度变化、试件对中偏位、FRP板与混凝土块的宽度比等因素,对FRP与混凝土的粘结强度进行了深入研究。然后利用试验结果与最近提出的陈-滕粘结强度公式作了比较研究。研究结果表明:(1)粘结破坏有FRP剥离破坏和混凝土拉剪破坏两种形式;(2)FRP板的搭接长度、试件对中偏位、FRP板与混凝土块的宽度比等因素对粘结强度有显著影响,支座高度变化也有一定影响;(3)粘结强度不小于“有效粘结长度”时,陈-滕粘结强度公式具有较高精度。     

Analytical model for the torsional behaviour of reinforced concrete beams retrofitted with FRP materials

An analytical approach for the prediction of the torsional response of reinforced concrete (RC) beams strengthened with fibre-reinforced-polymer (FRP) materials is described. The analysis method employs the combination of two different theoretical models: a smeared crack model for plain concrete in torsion for the elastic till the first cracking response and a properly modified truss model for the post-cracking response. The proposed method addresses the contribution of the externally bonded FRP materials to the torsional capacity of RC beams using specially developed equations in the well-known truss theory and utilizes softened and FRP-confined concrete stress–strain relationships. Detailed verification of this methodology is achieved through extensive comparisons between analytically predicted behaviour curves and experimentally obtained ones. The experimental data comprise a series of 12 tests in pure torsion and an additional database of experimental information for 24 specimens compiled from works around the world. These comparisons demonstrated that the proposed model is capable of adequately describing the full torsional response and of predicting with satisfactory accuracy the torsional moment at cracking and the ultimate torque capacity of FRP strengthened RC beams. Applications of the developed method as an assessment tool to strengthened beams with U-shaped FRP configurations and some first design examples that demonstrate the contribution of the FRP materials on the torsional response are also included.     

谈包裹预应力筋波纹管的固定技术改进

介绍了预应力混凝土的应用背景及性能要求,分析了有粘结预应力筋波纹管传统固定方式的不足之处,就波纹管的固定方式提出了改进措施,可供同类工程施工参考借鉴。     

套筒灌胶式FRP筋材锚固体系的试验研究

在国外几种较为成熟的FRP筋材锚固方式的基础上,对基于我国现有张拉设备的套筒灌胶式FRP筋材锚固体系进行了试验研究,对影响该体系锚固性能的主要因素进行了分析,并归纳了该锚固体系操作的主要过程及优、缺点,从而开发出安全、经济、实用的锚固体系。     

纤维增强塑料加固混凝土结构研究

纤维片材加固作为一种新兴的结构加固技术,由于其材料优异的力学性能及施工简便等特点,迅速成为各国加固研究领域的热点.中国在纤维加固方面的研究已基本涵盖了其所涉及的所有领域,取得了大量的研究成果,并在一定程度上指导着工程实践.但是由于纤维加固结构的复杂性,目前所取得的研究成果还不能解决结构加固的所有问题.对中国近几年来纤维加固所取得的研究成果进行了总结,同时也指出了在该领域研究中存在的问题及今后的发展方向.     

Experimental study of bonded steel concrete composite structures

This paper deals with the experimental analysis of the mechanical behaviour of bonded steel-concrete composite structures. The steel girder and the concrete slab are assembled by adhesives. The effect of the main parameters, such as the adhesive nature and the irregular thickness of the adhesive joint, on mechanical performance and ultimate load is studied. Two adhesives are used in this work. The results show that the connection between the steel girder and the concrete slab ensured by epoxy adhesive is perfect and without any slip in the steel-concrete interface. In the case of the composite beam interface ensured by polyurethane, the connection is flexible. The influence of the variation in adhesive thickness, 2 mm in the transverse direction and 4 mm in the longitudinal direction, on the mechanical behaviour and ultimate force is not important. The first cracks in the concrete slab appear in the transverse direction. The experimental results show that it is possible to realise a steel concrete composite structure bonded by adhesive.     

带蒙皮FRP格栅增强混凝土板受弯理论分析

提出了一种全新的FRP-混凝土的组合形式(带蒙皮FRP格栅增强混凝板),与之前的FRP格栅增强混凝土板件不同,该类型板件的最大特点在于FRP格栅下侧增设有FRP蒙皮,因此在浇筑时,该蒙皮既能充当浇筑混凝土时的模板,又能参与成型后板件的受力;预测了该类型板件可能出现的三种破坏模式,并且推导出了该类型板件在不同破坏模式下板件的抗弯极限承载力公式。     

FRP筋混凝土结构的发展

归纳了FRP材料的优点,分析了混凝土结构目前存在的问题,探讨了FRP筋混凝土结构的发展现状,对FRP混凝土结构的前景进行了展望,以促进FRP筋混凝土结构的研究,推广FRP筋混凝土结构的应用。