Bond of FRP Reinforcement in Concrete - a Challenge

The bond of ordinary steel reinforcement in concrete depends on many factors, such as the pullout resistance, the geometry of a concrete member, the placement of a bar in the member cross section, the cover splitting, the confinement caused by concrete and the surrounding reinforcement, the order of bond-crack appearance, and the bond-stress distribution along the bond length. The bond of FRP reinforcement depends on even a greater number of factors. Moreover, the types of FRP bars are numerous. Their surface is weaker than that of steel bars and may fracture by bond forces. The surface of FRP bars is softer and does not create as high local stress concentrations in bond contact points to concrete as the harder steel bars do. This fact often delays the appearance of cover splitting cracks along the bars. However, the load necessary for developing the crack pattern of ultimate splitting failure in concrete is then very dependent on whether the bar surface is glossy or rough. The FRP reinforcement can also be used for external shear and/or flexural strengthening of existing members. For this application, FRP bars are placed in grooves cut on the surface of the member to be strengthened and are fixed there with a cement mortar or epoxy paste. In such an application, the performance of bond between the FRP rod and the mortar or resin and then between the mortar or resin and concrete is critical for the effectiveness of the technique. The presence of two interfaces increases the number of parameters needed to characterize the global joint behavior and introduces new possible failure modes. The fundament for the bond resistance estimation should be an accepted bond philosophy linked to appropriate models. A system of bond tests should provide necessary coefficients for the models.     

Experimental Study on the Flexural Ductility of BFRP Bar Concrete Beams With Bamboo Fiber and Steel Wire Mesh北大核心CSCD

To study the effects of bamboo fiber and steel wire mesh on the flexural ductility of basalt fiber reinforced polymer（BFRP）bar concrete beams, 7 BFRP bar concrete beams with bamboo fiber and steel wire mesh were tested with different bamboo fiber lengths (0 mm, 30 mm and 45 mm) and different steel wire mesh layout ranges (0, 1/2 maximum bending moment point layout and full beam length layout). The flexural failure tests of the 7 beams were carried out, and the initial crack loads, the crack developments, the ultimate loads and the deformations were detected. The effects of the fiber length and the wire mesh layout range on the crack resistance and the deformation resistance of the specimens were analyzed based on the test data. With the function model, the equivalent yield points of the 7 test beams were obtained, and their ductility coefficients were calculated. The results show that, the addition of bamboo fiber and steel wire mesh increases the cracking loads of BFRP bar concrete beams by 12%~68%, decreases the crack spacings and the crack length development speed, reduces the test beam deformation under the same load, and increases the ductility coefficient by 1.58%~31.75%. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.     

Research in the Limiting Efficiency of Plastic Deformation of Multilayer Tension (Compression) Bars

A method is proposed for calculating the limiting loads of multilayer bars in plastic deformation. The regularities of changes in the efficiency factor (EF) of plastic deformation in relation to the elastic moduli of the materials of which the bars are composed, the stress which causes the material plasticity, and variations in the cross-sectional areas of layers are examined. Mathematical expressions are derived which allow one to easily calculate the EF for arbitrary values of variable parameters and to find the limiting values of EF when one of the parameters is changed. The equivalent tension diagram for a multilayer bar is obtained in relation to the strength and stiffness of materials and their number and cross-sectional areas.     

Long-Term Durability of Glass-Fiber-Reinforced Composites under Operation in Pulp and Reactant Pipelines

Composite pipes 215 mm in diameter with a 6-mm wall, 3-mm functional layer, and ±60° glass-fiber-reinforced plastic (GFRP) layers were examined by means of microstructural analysis and mechanical testing. Three types of pipes were considered: unused, after a five-year operation at a pressure of 0.6 MPa and temperature of 70°C, and after a five-year operation at 0.6 MPa and 20°C. The GFRP load-carrying layer and the interfaces were investigated by a metallographic optical microscope, a computerized microscanning equipment, and a software image processing package. The initial and accumulated damages in the microstructure of the composite and interfaces were examined. The mechanical properties of the pipes after a long-term operation were examined on two-layer specimens in three-point bending. The stiffness and strength characteristics were measured in the axial and circumferential directions under tensile and compressive loads at elevated temperatures. Composite pipes, 8 m in length and 215 mm in diameter, were simulated numerically as shells of revolution. The shell model was corrected by a refined Timoshenko theory and a dual-modulus temperature-dependent model of stiffness for the multilayer composite structure. The strength margin of the composite pipes was determined based on real strength properties.     

Behavior of Adhesively Bonded Concrete-CFRP Joints at Low and High Temperatures

This paper presents an evaluation of the results obtained from an initial study carried out at the IBMB, Technical University of Braunschweig, on the influence of temperature on adhesively bonded plate-concrete joint systems. The results of a theoretical model are also presented. The type of specimen used in this study is a three-point bending beam. The concrete specimens were prismatic in form, 100 × 100 × 700 mm without an internal steel reinforcement, strengthened at the intradoss with two types of UD CFRP lamina 20 mm wide and 590 mm long of thickness 1.4 mm for the E = 300 GPa type and 1.24 mm for the E = 175 GPa type. The plate-bonded specimens were designed to produce bending failure under a load, by not bonding the lamina and the concrete around the midspan of the specimens. The specimens were tested to failure at –100°, –30°, and +40°C after an approximately homogeneous temperature distribution within the concrete specimen has been reached. For comparison only, specimens of the same type were tested to failure at room temperature. The results obtained showed varying behavior of the bonded plate-concrete joint depending on variations in temperature. Furthermore, they showed different failure mechanisms. Greater reductions in the ultimate bond force were recorded for the test specimens strengthened with lamina having a high elastic modulus. A reasonable correlation was found between the experimental and theoretical results.     

Nonlinear finite element analysis and modeling of a precast hybrid beam–column connection subjected to cyclic loads

In this work, a detailed three-dimensional (3D) nonlinear finite element model is developed to study the response and predict the behavior of precast hybrid beam–column connection subjected to cyclic loads that was tested at the National Institute of Standards and Technology (NIST) laboratory. The precast joint is modeled using 3D solid elements and surface-to-surface contact elements between the beam/column faces and interface grout in the vicinity of the connection. The model takes into account the pre-tension effect in the post-tensioning strand and the nonlinear material behavior of concrete. The model response is compared with experimental test results and yielded good agreement at all stages of loading. Fracture of the mild-steel bars resulted in the failure of the connection. In order to predict this failure mode, stress and strain fields in the mild-steel bars at the beam–column interface were generated from the analyzed model. Such fields of stresses and strains are hard to measure in experimental testing. In addition, the magnitude of the force developed in the post-tensioning steel tendon was also monitored and it was observed that it did not yield during the entire loading history. Successful finite element modeling will provide a practical and economical tool to investigate the behavior of such connections.     

HB-FRP加固混凝土结构的粘结滑移统一模型

为了进一步简化HB-FRP(hybrid bonding FRP)加固技术的粘结滑移模型, 并基于先期研究的HB-FRP粘结滑移分区模型开展研究.在假定HB-FRP加固技术的粘结滑移统一模型表达式的基础上,推导了钢扣件部位的粘结应力分布系数.将HB-FRP加固作用分为普通FRP粘结性能和钢扣件产生的粘结性能两部分,依据能量方法,推导了FRP张拉力与滑移量的表达式.基于理论分析和数值求解,研究了界面滑移量的分布特征.基于模型试验测试结果,研究了粘结滑移统一模型中的待定系数表达式.研究结果表明:建立的HB-FRP加固混凝土结构的粘结滑移统一模型能有效预测加固界面的剥离承载力及有效粘结长度.     

Bond-slip behavior of CFRP plate-concrete interface

The paper deals with evaluation of the bond performance between a CFRP plate and concrete with respect to various compressive strengths of concrete and bond lengths of the CFRP plate as parameters. To consider stress conditions in the tensile zone of reinforced concrete (RC) structures, double-lap axial tension tests were conducted for eight specimens with CFRP plates bonded to concrete prisms. In addition, a simple linear bond-slip model for the CFRP plate/concrete joints, developed from the bond tests, was used. To verify the model proposed, a total of seven RC beams were strengthened with CFRP plates and tested in flexure employing various bond lengths, strengthening methods, and numbers of CFRP plates. A nonlinear finite-element analysis, with the bond–slip model incorporated in the DIANA program, was performed for the strengthened RC beams. Also, the results of flexural test and analytical predictions are found to be in close agreement in terms of yield and ultimate loads and ductility.     

Anchorage strength models for end-debonding predictions in RC beams strengthened with FRP composites

The increase in the flexural capacity of RC beams obtained by externally bonding FRP composites to their tension side is often limited by the premature and brittle debonding of the external reinforcement. An in-depth understanding of this complex failure mechanism, however, has not yet been achieved. With specific regard to end-debonding failure modes, extensive experimental observations reported in the literature highlight the important distinction, often neglected in strength models proposed by researchers, between the peel-off and rip-off end-debonding types of failure. The peel-off failure is generally characterized by a failure plane located within the first few millimetres of the concrete cover, whilst the rip-off failure penetrates deeper into the concrete cover and propagates along the tensile steel reinforcement. A new rip-off strength model is described in this paper. The model proposed is based on the Chen and Teng peel-off model and relies upon additional theoretical considerations. The influence of the amount of the internal tensile steel reinforcement and the effective anchorage length of FRP are considered and discussed. The validity of the new model is analyzed further through comparisons with test results, findings of a numerical investigation, and a parametric study. The new rip-off strength model is assessed against a database comprising results from 62 beams tested by various researchers and is shown to yield less conservative results. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 3, pp. 373–388, May–June, 2008.     

High-order accurate equations describing vibrations of thin bars

A method for deriving one-dimensional wave propagation equations in thin inhomogeneous anisotropic bars based on the mathematical homogenization theory for periodic media is used to obtain equations governing the longitudinal and transverse vibrations of a homogeneous circular bar. The equations are derived up to O(ε⁸) terms and take into account variable body forces and surface loads. Here, ε is the ratio of the bar’s typical thickness to the typical wavelength.     

Stochasic stability of viscoewtic bars

This paper is devoted to the investigation of stability of viscoelsstic bars compressed by stochastic forces at infinite time interval, The problem of the bar buckling is considered in dynamic statement. Some sufficient conditions of mean square stability of viscoelastic bars are derived for arbitrary relaxation measure and different types of the end fixing     

An alternative approach to characterize time series data: Case study on Malaysian rainfall data

This paper focuses on the characterization of time series rainfall data to understand the behavior in the Malaysian rainfall data. An analysis of the rainfall behavior of different time periods is also conducted. The rainfall data is rounded. A bar with a width of 12 pixels is drawn for each day in the data. The length of the bars drawn is equal to the corresponding rainfall value. Each bar is separated by a space of 2 pixels. Bars for data from different year are stored in different rows. Morphological opening is performed on the barcode image obtained, using line kernels of increasing length. Connected component-labeling algorithm is implemented on the resulting images to identify the individual bar codes and to compute the number of bars remaining after the opening process. A daily rainfall record for the duration of 30 years, obtained from the Melaka Meteorological Station, is analyzed. The results provide characterization of behavior in daily rainfall data.     

Partially connected composite beams

Analytical solutions to some boundary value problems for a steel-concrete composite beam with deformable shear connectors in the elastic range are presented. The obtained results show that the distribution of slip on the bond interface between the concrete slab and the steel girder is not a monotone function, even in the case of uniform loading on the composite beam. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

具有凸缘加劲肋圆孔的应力分析

本文讨论无限平面内具有凸缘加劲肋圆孔的应力分析问题。所谓凸缘加劲肋系指孔周用型钢或其他形状的构件加劲,进行应力分析时难以将其视为板的一部分来处理的加劲肋。文中讨论了两种荷载情形:一为薄板在无限远点处应力σ_X(∞),σ_Y(∞)及τ_XY(∞)的作用;另一为薄板受线性应力的作用。分析方法是:将加劲肋视为圆形杆件,把加劲肋与薄板间相互作用之径向力q₀(θ)及切向力t₀(θ)表示成三角级数,分别求出加劲肋轴线之位移与具有圆孔薄板孔周之位移,利用加劲肋与薄板孔周变形一致的变形协调条件,确定径向力q₀(θ)及切向力t₀(θ),从而得到加劲肋及薄板之位移和内力的算式。     

芳纶纤维平纹织布在爆炸载荷下的动态响应与失效行为的数值分析

为了研究柔性纤维织布的抗爆性能,通过数值模拟的方法对芳纶纤维织布在爆炸冲击下的响应与失效行为进行了分析。对国产芳纶纤维织布H1000D-AP220进行了力学性能试验,建立了柔性平纹织布的本构模型和爆炸冲击数值分析模型,对不同厚度不同铺层角度织布进行了爆炸冲击数值分析,获得了织布在不同爆炸载荷下的动态响应和失效模式。结果表明,织布在爆炸冲击载荷下主要表现为中心撕裂破孔和简支边界处拉伸撕裂2种典型的失效模式,并伴有明显褶皱,宽度方向出现织布向内侧收缩翻转现象;相比于中间层织布,迎爆面和背爆面吸能较多;在计算分析中改变了织布的层叠角度,获得了更好的抗爆效果。     

The Method of Three-Point Bending in Testing Thin High-Strength Reinforced Plastics at Large Deflections

A method is presented for determining the flexural strength of unidirectional composites from three-point bending tests at large deflections. An analytical model is proposed for calculating the flexural stress in testing thin bars in the case of large deflections. The model takes into account the changes in the support reactions at bar ends and in the span of the bar caused by its deflection. In the model offered, the influence of transverse shear and the friction at supports are neglected. The problem is solved in elliptic integrals of the first and second kind. The results obtained are compared with experimental tension data. The method elaborated for calculating flexural stresses has an obvious advantage over the conventional engineering procedure, because the calculation accuracy of the stresses increases considerably in the case of large deflections. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 5, pp. 691–704, September–October, 2005.     

Influence of various types of elastic foundation on the divergence and flutter of Ziegler's model structure

Ziegler's model structure (consisting of two rigid massless bars connected by a frictionless joint, with two rotational springs under follower force) is utilized in order to illustrate the influence of various types of elastic foundation on the buckling loads. In particular, continuous rotary or translational (Winkler) foundations are studied, whereas in the case of discrete foundation two translational springs are introduced in addition to the rotary springs.On leave from the Technion, Haifa, Israel.     

Elastic blade root connection in wind turbine using flexible elements from composites

The aim of an elastic blade root connection with a hub is to decrease loads in the blade root section during wind gusts. Two designs of connection were considered: for the load reduction on the blade in operating regime and for stopped blade unloading under storm wind. In the first case two versions of joint were discussed: the first one — with hinges and U-shaped composite beams, the second one — formed with straight beams oriented in different directions. Both joints have low torsional stiffness in wind direction and much higher stiffnesses around two other axes. Formulas for angular stiffnesses and the methods of obtaining the nonlinear behavior of the joint are presented. The objective of the flexible spar was to allow the blades to bend back out of the wind to reduce loads when the wind turbine was stationary in storm conditions. Calculations supported the feasibility of such a design. With a low torsional stiffness, spar (which can be rigidly connected to the blade) acts as a pitching beam for turbine control. A compound spar design consisting of pultruded bars clamped through specified distance was proposed. Torsional stiffnesses of different types of spars with equal specified bending rigidity were compared.Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 3, pp. 388–400, May–June, 1996.