Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.     

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.     

Effects of corrosion on linear and nonlinear elastic properties of reinforced concrete

Cracks induced by rebar corrosion are among the most important causes of performance deterioration in reinforced concrete elements. Even though several methods have been developed to detect the amount and location of corrosion-induced cracks, the sensitivity to small closed cracks located well below the concrete surface is still an issue to be faced. In this paper, we show the high sensitivity of nonlinear ultrasonic measurements performed using the Scaling Subtraction Method to point out that, under certain conditions, nonlinear indicators might prove to be very efficient and suitable to identify small variations in the element microstructure, such as those due to corrosion crack initiation.     

氨基醇类阻锈剂的阻锈机理

在分析氨基醇类阻锈剂对混凝土和钢筋作用行为的基础上,给出了氨基醇类阻锈剂的阻锈机理,即一方面阻锈剂通过在钢筋表面形成保护膜,隔离侵蚀性离子到达钢筋表面,另一方面通过和混凝土中的水化产物反应生成沉淀,提高混凝土密实度。两者相辅相成,共同起到延缓钢筋腐蚀或降低钢筋腐蚀速率的作用。氨基醇类阻锈剂在碳化混凝土中的阻锈效果微乎其微,在氯盐环境中依然有效,但当混凝土中氯离子浓度或腐蚀速率较高时,其阻锈作用大幅降低。由于阻锈剂的成分较为复杂,其成膜机制等还需进一步的研究。     

用电化学阻抗谱与X射线CT研究混凝土中钢筋的腐蚀行为

通过电化学阻抗谱（electrochemical impedance spectroscopy,EIS）与X射线断层扫描技术（x-ray computed tomography,X-CT）,研究了电迁移氯盐加速混凝土内钢筋的腐蚀行为及钢筋一混凝土界面区的腐蚀特征。结果表明：电迁移15d后钢筋已经开始腐蚀,电迁移60d后...     

Characterisation of bending cracks in R/FRC using image analysis

In this study, experiments were conducted to induce bending cracks of specific surface crack width to reinforced concrete beams made of plain concrete (RC) and reinforced concrete beams made of fibre reinforced concrete (R/FRC). After injecting and impregnating the cracks with dyed epoxy resin, image processing and analysis were employed to investigate the internal crack morphology. Several crack features including crack width (accumulated, effective and maximum), branching and tortuosity were defined and quantified. The results showed that in addition to arrested crack development, the presence of fibres yielded a distinctive change in the internal crack pattern, including increased branching and tortuosity, both of which have positive implications regarding concrete permeation. Likewise, specimens with fibres exhibited reduced maximum individual crack widths near the rebar, potentially increasing the ability of autogenous crack healing and reducing the risk of corrosion initiation.     

Chloride-induced corrosion products of steel in cracked-concrete subjected to different loading conditions

This project focused on examining the composition and distribution of chloride-induced corrosion products at the rebar-concrete interfaces and on crack surfaces in reinforced ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) subjected to different loading conditions. The results indicated that, regardless of the type of loading, there was a larger distribution of corrosion products along the rebar surface in the HPC than in the OPCC. Also, dynamic loading caused a greater detachment of the aggregate-paste bond in OPCC than static loading. The opening and closing of the cracks in salt solution under dynamic load forced corrosion products to flow from the rebar-concrete interface into the cracks in both OPCC and HPC. As a result, corrosion products diffused from the crack into the cement paste in the dynamically loaded OPCC but remained in the cracks in the dynamically loaded HPC, where they induced branched cracks. The mill-scale on the rebar was also evaluated before and after being embedded in concrete and was observed to be porous allowing ingress of species from the cement paste.     

Repair technique of pre-cracked reinforced concrete (RC) beams with transverse openings strengthened with steel plate under sustained load

This paper presents a repairing technique of the steel plate that effectively strengthen RC members, control both the failure modes and the stress distribution around the beam chords and enhance the serviceability of pre-cracked RC beams with openings. Furthermore, to evaluate the usage of a temporary pre-stressed force that was applied on the RC beam so as to close all of the presence shear cracks before the steel plate application. Two un-strengthened control beams, five beams that were pre-cracked before the application of the steel plate, and one beam that was strengthened without pre-cracking were tested. Cracking was first induced, followed by repair using various strengthening technique, and then the beams were tested to failure. The advantages of the shear strengthening of RC beams that were obtained using this technique are based on the closing of all the existing cracks around the openings via epoxy injection. The results show that both the pre-existing damage level and the steel plate strengthening have a significant effect on strengthening effectiveness and failure mode at the ultimate strength. An analytical equation for predicting the shear strength of pre-damaged RC beams with circular openings was then proposed, and the obtained results later compared with those from the tests, results were in accordance.     

PELE侵彻破坏钢筋混凝土靶的数值计算与试验

采用ANSYS/LS-DYNA 3D软件,对横向效应增强型侵彻体(PELE)侵彻破坏钢筋混凝土靶进行数值计算,得到PELE弹对钢筋混凝土靶侵彻破坏效应及开坑直径、崩落直径、通孔直径、钢筋断裂等参数。为验证数值计算结果,利用火炮进行了试验,试验结果与计算结果吻合良好,并利用实验结果进一步分析了高速撞击条件下PELE弹侵彻破坏钢筋混凝土的能力、PELE的横向效能、弹体变形和破坏等问题。     

Investigation of Bond Behavior Between Glass Fiber Composite Reinforcements and Concrete

Worldwide there is a need for the renewal of infrastructure because of age, deterioration, misuse, lack of repair, use of improper materials and techniques, and even due to changing needs. The corrosion of steel reinforcing bars is one of the major causes of degradation of concrete structures. Composites are increasingly being considered as a potential replacement for steel rebar, but there exists a critical lack of fundamental data and understanding of their behavior in relationship to traditional civil engineering materials such as concrete. This paper presents a preliminary investigation on the bond and pullout behavior of glass fiber reinforced composite reinforcement in concrete. The pullout behavior of the system was studied using a concentric pullout test and results are compared with theoretical predictions. The study is aimed at the development of a fundamental understanding of bond behavior and materials-configuration interrelationships needed to further the use of composites as reinforcements for application in areas where corrosion and electromagnetic interference make the use of steel reinforcement inefficient.     

外贴预应力玻璃钢板加固混凝土梁试验研究

本文针对外贴复合材料加固混凝土梁存在的不足,将玻璃纤维材料(GFRP)先施加预应力,再将其外贴到混凝土梁受拉面以提高混凝土梁的承载能力.针对GFRP板横向抗剪切挤压强度低的缺点,研究对GFRP板的预应力施加方法,在此基础上完成了9根不同预应力度,不同混凝土强度等级混凝土梁的抗弯试验;同时完成了4根预应力GFRP板在梁侧立面不同粘贴方式抗剪试验研究.试验结果表明,外贴预应力GFRP板加固混凝土梁可大幅度提高混凝土梁的开裂弯矩、极限承载力及GFRP板的强度利用率,改善梁的裂缝开展情况及提高梁的抗变形能力.     

Cover cracking of reinforced concrete due to rebar corrosion induced by chloride penetration

Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts.This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation.Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one.     

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.     

Effect of Polyolefin Fibers and Supplementary Cementitious Materials on Corrosion Behavior of Cement-Based Composites

The present study evaluated the corrosion behavior of cement-based composites containing polyolefin fibers and supplementary cementitious materials by the impressed voltage corrosion test and the impressed current corrosion test. Material variables included the addition of polyolefin fiber (0, 0.4, 0.8 and 1.6 %), the dosage of ultrafine slag (40 and 60 %), the dosage of fumed silica (2 and 5 %) and mixed cementitious materials (35 % slag plus 5 % fumed silica and 55 % slag plus 5 % fumed silica). The test results indicate that a small amount of polyolefin fibers extends the time that a chlorine ion reaches to the surface of the rebar and decreases the corrosion rate of rebar. Nevertheless, the fiber is not very useful for the corrosion resistance of steel when the chlorine ions reach the surface of the rebar and corrode it. In addition, the inclusion of ultrafine slag or fumed silica can arrest the rebar corrosion and improve the compactness through pozzolanic reactivity and pore filling effects. According to the corrosion behavior and economy performance of the concrete structure, the combination of 55 % ultrafine slag, 5 % fumed silica and 0.8 % polyolefin fibers provides the best corrosion resistance of the tested cement-based composites.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

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.     

Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

心墙沥青混凝土静三轴试验的剪胀性研究

沥青混凝土作为悬浮密实型结构,其剪切特性受密度及应力水平的影响。沥青混凝土的剪胀性将会对其内摩擦角,凝聚力,峰值强度及渗透系数有很大的影响。通过静三轴试验后孔隙率的变化情况表明,随着围压的增加,沥青混凝土的剪胀性变得明显,孔隙率增加明显;骨料在围压为2MPa时,骨料并不会发生破碎,通过测定试样的孔隙率得出,增加的孔隙率均为产生的空隙。     

Effect of corrosion on the bond between concrete and steel rebar

The effect of corrosion on the bond between concrete and steel rebar was studied by measuring both bond strength and contact electrical resistivity. Corrosion of steel rebar in concrete immersed in saturated Ca(OH)₂ solution was found to cause the bond strength to increase, and the contact resistivity increased until 5 weeks of corrosion. Further corrosion caused the bond strength to decrease, while the contact resistivity continued to increase.