Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.     

Limiting shear creep of epoxy adhesive at the FRP–concrete interface using multi-walled carbon nanotubes

Fiber reinforced polymer (FRP) composites are widely used in structural strengthening and retrofitting due to their high strength-to-weight ratio and non-corrosive properties. However, one of the recently recognized drawbacks of common FRP strengthening systems is the relatively high shear creep deformation of epoxy adhesives when FRP sheets are used to strengthen concrete structures against sustained loads. On the other hand, carbon nanotubes (CNTs) are reported to provide significant enhancement to various mechanical properties when used in epoxy adhesives. This enhancement is attributed to the extraordinary mechanical properties of the CNTs and their ability to bond to epoxy. In this article, we report the results of experimental and analytical investigations conducted to examine shear creep behavior of multi-walled carbon nanotubes (MWCNTs) reinforced epoxy nanocomposite used at the FRP–concrete interface. Double shear tests were performed on FRP sheets bonded to concrete blocks with MWCNTs reinforced epoxy nanocomposite. Various levels of pristine and functionalized MWCNTs by weight were examined including 0.1%, 0.5%, 1.0% and 1.5%. The viscoelastic behavior of MWCNTs reinforced epoxy nanocomposite was simulated with rheological models and the models' parameters were extracted and discussed. The results show the ability of MWCNTs to significantly reduce creep compliance of epoxy at the FRP–concrete interface making it a viable solution if FRP is used to strengthen concrete structures subjected to sustained stress.     

Tensile creep of a structural epoxy adhesive: Experimental and analytical characterization

Epoxy adhesives are nowadays being extensively used in Civil Engineering applications, mostly in the scope of the rehabilitation of reinforced concrete (RC) structures. In this context, epoxy adhesives are used to provide adequate stress transference from fibre reinforced polymers (FRP) to the surrounding concrete substrate. Most recently, the possibility of using prestressed FRPs bonded with these epoxy adhesives is also being explored in order to maximize the potentialities of this strengthening approach. In this context, the understanding of the long term behaviour of the involved materials becomes essential. Even when non-prestressed FRPs are used a certain amount of stress is permanently applied on the adhesive interface during the serviceability conditions of the strengthened structure, and the creep of the adhesive may cause a continuous variation in the deformational response of the element. In this context, this paper presents a study aiming to experimentally characterize the tensile creep behaviour of an epoxy-based adhesive currently used in the strengthening of concrete structures with carbon FRP (CFRP) systems. To analytically describe the tensile creep behaviour, the modified Burgers model was fitted to the experimental creep curves, and the obtained results revealed that this model is capable of predicting with very good accuracy the long term behaviour of this material up to a sustained stress level of 60% of the adhesive׳s tensile strength.     

A Theoretical Improved Adhesively Bonded Bi-material Beam Model for FRP–RC Hybrid Beams

In this paper we present an improved bi-material beam theory with adhesive interface, which has been applied to the study of the interfacial behavior in a concrete beam reinforced by an externally bonded fibre reinforced polymer (FRP) plate. The work explicitly considers the interfacial slip effect on the structural performance by including the effect of adherend shear deformations. This new method needs only one differential equation to determine both shear and normal interfacial stress whereas the others solutions in the literature need two differential equations. Compared with previously published analytical results, this one improves the accuracy of predicting the interfacial stresses and the solution is in a closed form. This research is helpful in the understanding of the mechanical behavior of the interface and design of FRP–reinforced concrete (RC) hybrid beams.     

Theoretical and finite element studies of interfacial stresses in reinforced concrete beams strengthened by externally FRP laminates plate

The paper presents the results of an analytical and numerical solution for interfacial stresses in carbon fiber reinforced plastic (CFRP)–reinforced concrete (RC) hybrid beams studied by the finite element method. The analytical analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. The adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. In numerical analysis, the mesh sensitivity test shows that the finite element results for interfacial stresses are not sensitive to the finite element mesh. The finite element analysis then is used to calculate the interfacial stress distribution and evaluate the effect of the structural parameters on the interfacial behavior. It is shown that both the normal and shear stresses at the interface are influenced by the material and geometry parameters of the composite beam. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions. We can conclude that this research is helpful for the understanding the mechanical behavior of the interface and design of the FRP–RC hybrid structures.     

Interfacial stresses in FRP-plated RC beams: Effect of adherend shear deformations

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre reinforced polymer (FRP) plates to their tensile faces. An important failure mode of such plated beams is the debonding of the FRP plates from the concrete due to high level of stress concentration in the adhesive at the ends of the FRP plate. This paper presents an improved solution for interfacial stresses in a concrete beam bonded with the FRP plate by including the effect of the adherend shear deformations. The analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions.     

On the Effect of Time-Dependent Deformations on the Interface Behaviour of RC Beams Strengthened by FRP Plates

In this paper, the effect of time-dependent deformations (such as shrinkage and creep) on the interfacial stresses between a concrete beam and a fibre reinforced polymer plate is presented. The analysis given here involves a closed-form solution for such stresses and includes creep and shrinkage effects. The adherend shear deformations have been included in the present theoretical analysis by assuming a parabolic shear stress through the thickness of both concrete beam and fibre reinforced polymer panel. Contrary to some existing studies, the assumption that both the concrete beam and the fibre reinforced polymer panel have the same curvature is not used in this investigation. The influence of creep and shrinkage effect relative to the time of the casting and the time of the loading of the beams is taken into account. Numerical examples of a typical concrete beam strengthened with an externally bonded fibre reinforced polymer plate are discussed with the emphasis on the shear and normal stresses at the edge of the plate.     

芳纶纤维加固钢筋混凝土梁锚固长度的简化计算方法

根据芳纶纤维(Aramid Fiber Reinforced Plastic,简称 AFRP)补强加固钢筋混凝土梁的粘结破坏的试验结果,分析AFRP加固钢筋混凝土抗弯构件粘结界面的剪应力的分布规律,即在纤维截断点处存在较高的应力集中,随着离截断点距离的增大剪应力分布逐渐趋于均匀.粘结锚固长度不足和过高的应力集中是造成AFRP加固钢筋混凝土构件早期破坏的主要原因.采用"齿"状块体力学计算模型和混凝土裂缝理论推导了AFRP加固钢筋混凝土梁所需要的有效锚固长度,并通过修正得出了AFRP加固钢筋混凝土受弯构件最小锚固长度的简化计算公式,提出了AFRP的容许应变值和避免AFRP早期破坏应采用的措施,可供AFRP加固工程设计和施工参考.     

FRP与混凝土界面粘结性能的试验研究

纤维(FRP)与混凝土的粘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粘结性能进行了试验研究。分析了FRP应变、局部粘结剪应力发展规律以及沿粘结长度在各级荷载下的分布规律。考察了混凝土强度和FRP粘结长度对粘结强度等粘结性能的影响。验证了FRP有效粘结长度,探讨了有效粘结长度的影响因素,计算得到了局部粘结剪应力滑移关系曲线。通过对试验结果的统计回归分析,提出了局部粘结剪应力滑移本构关系模型以及有效粘结长度计算公式,分析结果与试验结果都吻合较好,可供实际加固改造工程应用及完善相应规范的编制参考。     

基于十字交叉法的陶瓷胶粘剂剪切蠕变性能研究

本研究设计了“十字交叉法”陶瓷胶粘剂剪切蠕变试验装置,选取刚性环氧树脂及柔性硅酮结构胶进行剪切蠕变试验,研究了环境温度、剪切应力、粘结面积等因素对胶粘剂剪切蠕变的影响,通过模型拟合对胶粘剂的剪切蠕变行为进行了分析和预测,探究了两种胶粘剂的蠕变破坏模式。结果表明:采用十字交叉法能够准确便捷地测试陶瓷胶粘剂的蠕变性能。增大胶粘层柔性、提高环境温度、增大剪切应力都会加速蠕变的发展,但粘结面积对蠕变速率无明显影响。刚性环氧树脂胶粘剂试样的蠕变失效形式为粘结层内聚破坏及界面脱粘,符合时间硬化模型;柔性硅酮结构胶试样失效形式为粘结层内聚破坏,符合Burgers模型。     

混凝土梁嵌粘混合FRP筋弯曲性能试验研究

分别在钢筋混凝土简支梁和连续梁的受弯区、负弯矩区表层嵌粘不同FRP筋材进行加固,进行单调加载弯曲试验,研究不同初始荷载、FRP筋种类对简支梁、连续梁嵌粘FRP筋后的特征荷载、应变、变形等的影响程度,记录加载过程中梁体裂缝扩展情况。结果表明:不同FRP筋混合嵌入加固混凝土梁,可显著提高简支梁、连续梁的抗弯承载力,改善使用阶段性能,与单一FRP筋材嵌粘加固相比,裂缝分布特征相似,延性更佳;在初始荷载未完全卸除时,混凝土梁表层嵌粘不同FRP筋的加固效果略逊于无初始荷载作用下的加固梁,不过效果仍比较理想,建议实际工程中尽可能卸除外部荷载后再进行嵌粘混合FRP筋加固。     

Effect of shear deformation on interfacial stresses in plated beams subjected to arbitrary loading

Bonding a fibre reinforced polymer (FRP) composite or metallic plate to the soffit of a reinforced concrete (RC), timber or metallic beam can significantly increase its strength and other aspects of structural performance. These hybrid beams are often found to fail due to premature debonding of the plate from the original beam in a brittle manner. This has led to the development of many analytical solutions over the last two decades to quantify the interfacial shear and normal stresses between the adherends. The adherends are subjected to axial, bending and shear deformations. However, most analytical solutions have neglected the influence of shear deformation of the adherends. For the few solutions which consider this effect in an approximate manner, their applicability is limited to one or two specific load cases. This paper presents a general analytical solution for the interfacial stresses in plated beams under an arbitrary loading with the shear deformation of the adherends duly considered. The shear stress distribution is assumed to be parabolic through the depth of the adherends in predicting the interfacial shear stress and Timoshenko's beam theory is adopted in predicting interfacial normal stress to account for the shear deformation. The solution is applicable to a beam of arbitrary prismatic cross-section bonded symmetrically or asymmetrically with a thin or thick plate, both having linear elastic material properties. The effect of shear deformation is illustrated through an example beam. The influence of material and geometric parameters of the adherends and adhesive on the interfacial stress concentrations at the plate end is discussed.     

Investigation of the creep behavior of graphene oxide nanoplatelet-reinforced adhesively bonded joints

In this paper, the effect of adding graphene oxide nano-platelets (GONPs) into the adhesive layer was investigated on the creep behavior of adhesively bonded joints. The neat and GONP-reinforced adhesive joints were manufactured and tested under creep loading with different stress and temperature levels. 0.1?wt% GONPs revealed the highest improvement on the adhesive joint creep behavior amongst the studied weight percentages. Furthermore, the effect of GONPs on the creep behavior of adhesive joints was more significant at higher temperatures. It was found that adding 0.1?wt% of GONPs into the adhesive layer imposed reductions of 21%, 31% and 34% in the elastic shear strains and reductions of 24%, 31% and 37% in the creep shear strains of SLJs under testing temperatures of 30, 40 and 50?°C, respectively. The Burgers rheological model was employed for simulating the creep behavior of the neat and GONP-reinforced adhesive joints. The Burgers model parameters were obtained as functions of testing temperature, creep shear stress and GONP weight percentage using a response surface methodology. Reasonable agreement was obtained between the modeled and experimental creep behaviors of the adhesive joints.     

An Investigation of Interfacial Stresses in Reinforced Concrete Beams Using FRP Laminates

Reinforcement of reinforced concrete (RC) beams against bending through utilization of bonded fibre-reinforced plastic (FRP) laminates has been accepted as an effective method of strengthening. In this study, the effects of FRP reinforcement over the parameters of interfacial stresses in reinforced concrete beams were examined both experimentally and numerically. Essentially, the main goal of the study was to investigate quantitatively the behaviour of the RC beams strengthened with adhesively bonded FRP. In order to achieve this goal, an experimental study was initially carried out. Afterwards, the ANSYS® WB finite element program was employed to model and analyze the RC beams externally bonded to FRP. The obtained results are expected to demonstrate the main characteristics of interfacial stress distributions inside beams strengthened with FRP. The evaluation of interfacial stresses provides the basis for understanding the main characteristics in such beams and for developing suitable design rules.     

Influence of elevated temperatures on epoxy adhesive used in CFRP strengthening systems for civil engineering applications

This paper presents experimental investigations about the influence of elevated temperatures on the mechanical behaviour of an epoxy adhesive typically used in carbon fibre reinforced polymer (CFRP) strengthening systems and numerical investigations about the influence of changes underwent by the adhesive on the response of bonded joints between CFRP strips and concrete. The experiments included shear and tensile tests at elevated temperatures (up to 120 °C) on a commercial epoxy adhesive. In both types of tests, the mechanical response of the adhesive at different temperatures was assessed, namely in terms of stress vs. strain curves, stiffness, strength and failure modes. The results obtained highlighted the considerable reduction of both shear and tensile properties with increasing temperatures: at 70 °C the shear and tensile strengths are both reduced to around 15% of the corresponding ambient temperature strengths, while the tensile and shear moduli can be considered negligible. Analytical formulae were fit to the test data, describing the reduction with temperature of the adhesive's tensile and shear properties. In the numerical investigations, three-dimensional finite element models were developed to simulate previous double-lap shear tests performed on concrete blocks strengthened with CFRP strips according to either the externally bonded reinforcement (EBR) or the near surface mounted (NSM) techniques, using the epoxy adhesive characterized in the present study. Two distinct modelling strategies were adopted for the concrete-CFRP bond in order to assess the relative importance of the adhesive distortion and interfacial slippage at the concrete-adhesive-CFRP interfaces in the overall slip between concrete and CFRP: (i) to explicitly simulate the adhesive, considering the mechanical properties determined in the tests and assuming a perfect bond at all interfaces; and, alternatively, (ii) to simulate the CFRP-concrete interaction by means of global bilinear bond-slip laws for different temperatures. Comparison between numerical results and test data allowed quantifying the relative importance of the adhesive distortion and of the interfacial slippage at the bonded interfaces as a function of temperature, providing a better understanding of the contribution of these two mechanisms to the CFRP-concrete bond at elevated temperature. While the former effect is the most relevant at ambient temperature, with elevated temperature the interfacial slippage at the bonded interfaces becomes the most relevant mechanism.     

An experimental study on the retrofitting effects of reinforced concrete columns damaged by rebar corrosion strengthened with carbon fiber sheets

This paper presents the results of an experiment study on the structural behavior of reinforced concrete (RC) columns damaged by rebar corrosion and the retrofitting effects of damaged RC columns strengthened with carbon fiber sheets (CFS). In the experiment, a cyclic horizontal loading test was carried out using RC columns damaged by different degrees of rebar corrosion and strengthened with CFS. As a result, it was revealed that the deterioration of their structural behavior was mainly caused by the decline in the confining effect due to the falling off of concrete cover and the reduction of mechanical properties of corrosion rebar. In addition, the test proved that shear strengthening using CFS is an very effective retrofit technique that prevents bond splitting cracks and shear cracks from growing and improves the ductility of RC columns with corroded rebars due to the confining effect of CFS.     

Bond quality at the FRP–wood interface using wood-laminating adhesives

Fibre-reinforced polymer (FRP)-strengthened glulam would be a more economically viable product if a single adhesive type could be utilised at all the bonded interfaces. This paper describes a test programme that examines the hygrothermal compliance of five commercial wood-laminating adhesives when bonding commercially viable FRP materials to wood. It was seen that the integrity of the bond depended not only on adhesive type but also on the FRP type under consideration. For one of the FRP types, moisture-cycled FRP–wood bonded specimens obtained high wood failure percentages and good shear strength results that compared well with non-moisture-cycled FRP–wood specimens, non-moisture-cycled wood–wood bonded specimens and solid control specimens taken from the same board. This encouraging result suggests an alternative to the expensive structural epoxy adhesives, which are generally accepted as the appropriate adhesive in FRP-strengthened glulam.     

纤维布与混凝土之间粘结力的试验方法研究

本文在文献研究的基础上,设计了一种简单易行的试验装置,通过直剪试验测定复合纤维布与混凝土之间的粘结强度,得出各测点离端部距离-应变关系曲线,进而绘出复合纤维布中沿粘结方向的粘结应力分布,以确定有效粘结长度.所得初步结论为梁抗弯、抗剪加固的粘结提供设计参数.     

复合纤维抗剪加固掺粉煤灰混凝土外伸梁试验研究

通过对比试验方法,分别研究了外掺粉煤灰混凝土外伸梁在完好状态、损坏状态、不持荷状态和持荷状态下,采用复合纤维对梁斜截面进行抗剪加固的受力性能和破坏特征.在试验结果对比分析的基础上,对复合纤维抗剪加固机理进行了分析.研究结果表明,复合纤维加固能改善抗剪破坏的脆性,明显提高外伸梁的抗剪承载能力,增加外伸梁刚度,降低挠度.