Strengthening and repair of RC beams with fiber reinforced concrete

The use of a jacket made of fiber reinforced concrete with tensile hardening behavior for strengthening RC beams is investigated by means of full-scale tests on 4.55 m long beams. A 40 mm jacket of this material was directly applied to the beam surface. Both the strengthening and the repair of RC beams were studied. In particular, in the latter case the beam was initially damaged and eventually repaired. A numerical analysis is also performed in order to better understand the reinforcement behavior. The experimental and numerical results show the effectiveness of the proposed technique both at ultimate and serviceability limit states.     

Flexural behaviour of RC beams in fibre reinforced concrete

Even though a number of research studies have demonstrated the effectiveness of Fibre Reinforced Concrete (FRC) in improving the structural response of RC members under different loading conditions, some concerns recently arose on the sectional ductility under flexure which can be reduced under specific conditions. In fact, fibres do not significantly increase the ultimate moment of RC members and, with rather tough FRC and low strain-hardening ratio of the longitudinal rebars, the rotation capacity can substantially decrease owing to a cracking localization at ultimate limit state.This paper focuses on this topic with a number of experimental results on full-scale FRC beams tested under flexure.Experimental results evidence that fibres, when provided in sufficient amount, are able to move the beam failure from concrete crushing to steel rupture. Under certain circumstances, the overall ductility, measured in terms of displacements, may decrease.On the other hand, in all cases the addition of fibres determines a stiffer and in general enhanced post-cracking behaviour in service conditions.     

Tension-stiffening model for FRP-strenghened RC concrete two-way slabs

The main focus of this paper is to present a tension-stiffening model that is suitable for finite element analysis (FEA) aimed at investigating the effect of FRP strengthening on the tensile behaviour of concrete slabs. Available experimental results of the FRP-strengthened reinforced concrete slabs are used to calibrate the finite element model based on the ultimate load carrying capacity of the two-way slabs. The proposed tension-stiffening model is implemented into the constitutive concrete model defined in a general-purpose finite element code. Reinforced concrete behaviour in tension can signifcantly be changed due to strengthening. An overall increase in the post-peak stiffness based on the tensile stress-strain relationship is observed. A simplified bilinear model is introduced to define the behaviour of the FRP-strengthened concrete in tension. An expression of the fracture energy density is introduced to define the area under the concrete tensile stress-strain relationship. The tensile stress-strain relationship of concrete is referred to as the tension-stiffening model. It is shown numerically that the ultimate load capacity of two-way slab specimens is sensitive to the fracture energy density. Hence, a distinction has to be made between the definitions of the tension-stiffening model of FRP-strengthened and un-strengthened concrete. This distinction is the focus of this paper.

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Résumé L'objectif principal de cet article est de présenter un modèle approprié de raidissement en traction pour l'analyse d'éléments finis (AEF). L'analyse est destinée pour l'étude de l'effet du renforcement en polymère renforcé de fibres (PRF) sur le comportement en traction des dalles en béton armé. Les résultats expérimentaux des dalles renforcées sont employés pour calibrer le modèle d'éléments finis basés sur la capacité ultime des dalles bidirectionnelle. Le modèle de raidissement en traction proposé, est appliqué dans un code général d'élément finis. Le comportement du béton armé renforcé en traction peut être changé d'une manière significative due au renforcement. On observe une augmentation en tension de la rigidité du poteau crête basée sur la relation contrainte-déformation. Un modèle bilinéaire simplifié est présenté pour définir le comportement du béton renforcé de PRF en traction. Une expression de la densité d'énergie de rupture est présentée pour définir l'aire sous la courbe contrainte-déformation. La relation contrainte-déformation du béton en tension est appelée modèle raidissement en traction. On montre numériquement que la capacité de la charge ultime de spécimens bi-directionnels de dalle est affectée par densité d'énergie de rupture. Par conséquent, une distinction doit être faite entre les définitions du modèle raidissement en traction du béton renforcé de PRF est béton non renforcé. Cette distinction est l'objet de cet article.

     

Parametric study for concrete cover separation failure of retrofitted SNSM strengthened RC beams

ABSTRACT

The side near-surface mounted (SNSM) method is a new flexural strengthening method for reinforced concrete (RC) beams which was proposed to allow near-surface mounted (NSM) strengthening to be applied on beams with small width. As a relatively new strengthening method, further studies are needed to determine the effects of strengthening parameters on the flexural performance of RC beams. In response to that, this paper presents a parametric study on the concrete cover separation failure of SNSM strengthened beams using a simulation method based on the moment-rotation (M/θ) approach.     

FRP-OFBG智能复合筋及其在加筋混凝土梁中的应用

结合纤维增强聚合物(FRP)的强度特性和光纤光栅(OFBG)的感知特性,研制开发出了FRP-OFBG智能复合筋,研究了它的力学、微观结构、感知等特性。通过FRP-OF-BG加筋混凝土梁静载试验,监测了FRP筋的应变和混凝土开裂,研究了FRP筋与混凝土的滑移和混凝土梁的应变分布。结果表明,FRP-OFBG智能复合筋克服了光纤光栅在混凝土中埋设的工艺难题,是集感知和受力、功能材料和结构材料于一体的新型土木工程材料,既可以方便地作为混凝土结构的内部传感元件,也可以作为结构受力筋,具有很好的应用前景。     

Influence of long-term chloride diffusion in concrete and the resulting corrosion of reinforcement on the serviceability of RC beams

This paper presents the chloride penetration and the effect of chloride ingress on the serviceability of reinforced concrete (RC) beams. A series of experimental studies were carried out on beams with various corroded ages up to 28 years. The chloride content in different locations were tested during various periods. Different states influencing the serviceability of the corroded beams were investigated, including the maximum width of the corrosion-induced cracks of the concrete cover, the mid-span deflection of the beams under the service load and their load-bearing capacity. Based on the results available from this programme, the service life of corroded beams was predicted by the corrosion process of the reinforcement. The results showed that the chloride corrosion could significantly deteriorate the serviceability of the beams. The current criteria concerning the chloride content at the level of the reinforcement of the concrete beams and the maximum width of the corrosion-induced cracks appear to be very conservative.     

Application of a self-compacting ultra-high-performance fibre-reinforced concrete to retrofit RC beams subjected to repeated loading

The aim of this paper is to describe the performance of reinforced concrete (RC) beams retrofitted with a self-compacting ultra-high-performance fibre-reinforced concrete (UHPFRC) under three-point bend cyclic loading. It is found that retrofitting the RC beams with a thin UHPFRC strip on the tension face increases their endurance limit under a non-zero mean stress cyclic loading from approximately 40% to approximately 60% of their static three-point flexural strength. Moreover, the retrofitted beams behave as a composite structure, with no delamination of the retrofit strip being observed in any of the fatigue tests.     

A discrete spectral model for intermediate crack debonding in FRP-strengthened RC beams

One of the common failure modes of reinforced concrete (RC) beams strengthened in flexure with a bonded fibre-reinforced polymer (FRP) is intermediate crack (IC) debonding, which is originated at a critical section in the vicinity of flexural cracks and propagates to a plate end. Despite considerable research over the last years, few reliable and simplified IC debonding strength models have been developed. This paper firstly presents a one-dimensional model based on the discrete crack approach for concrete and the spectral element method for the numerical simulation of the IC debonding process. The progressive formation of flexural cracks and subsequent concrete–FRP interfacial debonding is formulated by the introduction of a new element able to represent both phenomena simultaneously without perturbing the numerical procedure. Furthermore, with the proposed model, high frequency dynamic response for these kinds of structures can also be obtained in a very simple and non-expensive way, which makes this procedure very useful as a tool for diagnoses and detection of debonding in its initial stage by monitoring the change in local dynamic characteristics.     

Hybrid fiber influence on strength and toughness of RC beams

In this paper, a newly developed method of evaluating the shear toughness of fiber reinforced RC beams in terms of its post-peak nominal average equivalent shear strength was investigated. This method is simple and the advantages are significant.     

The effect of CFRP and CFRP/concrete interface models when modelling retrofitted RC beams with FEM

Concrete structures retrofitted with fibre reinforced plastic (FRP) applications have become widespread in the last decade due to the economic benefit from it. This paper presents a finite element analysis which is validated against laboratory tests of eight beams. All beams had the same rectangular cross-section geometry and were loaded under four point bending, but differed in the length of the carbon fibre reinforced plastic (CFRP) plate. The commercial numerical analysis tool Abaqus was used, and different material models were evaluated with respect to their ability to describe the behaviour of the beams. Linear elastic isotropic and orthotropic models were used for the CFRP and a perfect bond model and a cohesive bond model was used for the concrete–CFRP interface. A plastic damage model was used for the concrete. The analyses results show good agreement with the experimental data regarding load–displacement response, crack pattern and debonding failure mode when the cohesive bond model is used. The perfect bond model failed to capture the softening behaviour of the beams. There is no significant difference between the elastic isotropic and orthotropic models for the CFRP.     

Numerical simulation of rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load

The aim of this paper is to simulate the rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load and also, to ascertain the influence of design parameters such as the elastic modulus, thickness and length of the FRP plate on the debonding performance. In order to simplify the simulation, some basic equilibrium equations are formulated and then the stresses of the rebar and FRP plate are numerically solved, and stress intensity factor is avoided in the simulation by fundamentals of fracture mechanics because of its complexity around the crack tip of bi-material interface. With the combination of finite element method and difference approximation, authors program the degradation model of coefficient of friction, debond criterion, propagation law and loop of load process into a commercial finite element code to investigate the fatigue debonding. The relationships between the debond length as well as other fatigue parameters and number of cyclic load are obtained and discussed.     

Degradation of bond between FRP and RC beams

Beams and slabs externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce the expected durability of the system. Bond degradation is a frequent cause of premature failure of structural elements and environmental conditions are known to relate to such failures. The study shows the effects of cycles of salt fog, temperature and moisture as well as immersion in salt water on the bending response of beams externally reinforced with GFRP or CFRP, especially on bond between FRP reinforcement and concrete. Temperature cycles (−10 °C; 10 °C) and moisture cycles were associated with failure in the concrete substrate, while salt fog cycles originated failure at the interface concrete–adhesive. Immersion in salt water and salt fog caused considerable degradation of bond between the GFRP strips and concrete. However, immersion did not lower the load carrying capacity of beams, unlike temperature cycles (−10 °C; 10 °C) that caused considerable loss. No significant differences were detected on the behavior of the systems strengthened with GFRP and CFRP, perhaps because the design of the tests impeded failure of the fibres.     

A viscoelastic continuum damage model and its application to uniaxial behavior of asphalt concrete

An existing viscoelastic constitutive model which accounts for the effects of rate-dependent damage growth is described and applied successfully to characterize the uniaxial stress, constant strain rate behavior of asphalt concrete. The special case of an elastic continuum damage model with multiaxial loading, which is based upon thermodynamics of irreversible processes with internal state variables, is first reviewed and then it is shown how this model has been extended to a corresponding viscoelastic damage model through the use of an elastic-viscoelastic correspondence principle. The general mathematical model is next specialized to uniaxial loading. A rate-type evolution law, similar in form to a crack growth law for a viscoelastic medium, is adopted for describing the damage growth within the body. Results from laboratory tests of uniaxial specimens under axial tension at different strain rates are then shown to be consistent with the theory. The discussion of data analysis describes the specific procedure used here to obtain the material parameters in the constitutive model for uniaxial loading and how the method may be generalized for multiaxial loading.     

Failure diagrams of FRP strengthened RC beams

Amongst various methods developed for strengthening and rehabilitation of reinforced concrete (RC) beams, external bonding of fibre reinforced plastic (FRP) strips to the beam has been widely accepted as an effective and convenient method. The experimental research on FRP strengthened RC beams has shown five most common modes, including (i) rupture of FRP strips; (ii) compression failure after yielding of steel; (iii) compression failure before yielding of steel; (iv) delamination of FRP strips due to crack; and (v) concrete cover separation. In this paper, a failure diagram is established to show the relationship and the transfer tendency among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with FRP thickness and the distance from the end of FRP strips to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is mostly likely to occur. The predictions based on the present failure diagram are compared to 33 experimental data from the literature and good agreement on failure mode and ultimate load has been obtained. Some discussion and recommendation for practical design are given.     

FRP-strengthened RC beams under sustained loads and weathering

Glass FRP-strengthened RC beams were subjected to sustained loads and placed for different periods outdoors, indoors, and in chambers that accelerate the effects of outdoor tropical weathering by a factor of six. Beams subjected to outdoor weathering had up to 18% larger crack widths and 16% larger deflections compared to those kept indoors at the end of 1 and 2¾ years, respectively. The increase in deflections and crack widths was lesser for beams with a higher FRP reinforcement ratio. The residual flexural strength and ductility of the beams decreased with longer weathering periods. Also, the failure mode of the beams changed from concrete crushing to FRP rupture, indicating a deterioration in the mechanical properties of the FRP laminates. Analytical methods which account for material degradation in concrete and FRP laminates are presented and found to predict the long-term flexural characteristics of the beams well.     

Genetic algorithm model for shear capacity of RC beams reinforced with externally bonded FRP

A variety of on-site construction applications using FRP materials have been realized worldwide. However, this technology is currently at a stage where its future widespread implementation and competitiveness will depend on the development of reliable design guidelines based on sound engineering principles. This paper presents simple, yet improved, equations to calculate the shear capacity of FRP bonded-reinforced concrete beams based on the genetic algorithms (GAs) approach applied to 212 experimental data points available in the open literature. The performance of the proposed equations was compared to that of commonly used shear design methods, namely the ACI 440, Eurocode (EC2), the Matthys Model, Colotti model and the ISIS Canada guidelines. Results demonstrate that the proposed equations better agree with the available experimental data than the existing models investigated. Moreover, a sensitivity analysis was carried out to investigate the effect of the shear span-to-depth ratio on the shear capacity contributed by concrete, the ultimate effective strain in FRP sheets, and the ultimate effective stress in transverse rebars. Results indicate that the shear span-to-depth ratio has a significant effect on the shear behaviour of FRP bonded-reinforced concrete beams.     

Probabilistic analysis of the cracking of RC beams

Three beams of rectangular cross-section and having same cross-sectional dimensions were tested in two-point bending, over an effective span of 4.2 m. All three beams contained steel only in the tension zone and the tension steel was distributed in three different ways. A deterministic analysis of strains, crack spacings and crack widths for all three beams is carried out and the results are compared with the respective experimental values. A probabilistic analysis of strains, crack spacings and crack widths is performed for all the three beams at different stages of loading.     

Strengthening of RC beams with epoxy-bonded fibre-composite materials

Strengthening of concrete beams with externally bonded fibre-reinforced plastic (FRP) materials appears to be a feasible way of increasing the load-carrying capacity and stiffness characteristics of existing structures. FRP-strengthened concrete beams can fail in several ways when loaded in bending. The following collapse mechanisms are identified and analysed in this study: steel yield-FRP rupture, steel yield-concrete crushing, compressive failure, and debonding. Here we obtain equations describing each failure mechanism using the strain compatibility method, concepts of fracture mechanics and a simple model for the FRP peeling-off debonding mechanism due to the development of shear cracks. We then produce diagrams showing the beam designs for which each failure mechanism is dominant, examine the effect of FRP sheets on the ductility and stiffness of strengthened components, and give results of four-point bending tests confirming our analysis. The analytical results obtained can be used in establishing an FRP selection procedure for external strengthening of reinforced concrete members with lightweight and durable materials.     

Impact behaviour of concrete beams

An instrumented impact machine was used to carry out impact tests on concrete beams, 100×125 mm in cross-section and 1400 mm long. The simply supported beams were struck at their midpoints by a 345 kg mass impact hammer, dropped from various heights. The instrumentation included strain gauges mounted on the striking end of the hammer, strain gauges mounted on one support anvil, and three accelerometers placed at various locations along the beam. The data were collected using a 5-channel data acquisition system. Normal strength, high strength, and fibre reinforced concrete beams were tested. In general, it was found that the properties of concrete under the high stress rates associated with impact loading could not be predicted from conventional static tests.     

Development and application of innovative triaxially braided ductile FRP fabric for strengthening concrete beams

In this paper, an innovative triaxially braided ductile fiber reinforced polymer (FRP) fabric has been developed for strengthening reinforced concrete beams. The fabric was designed to potentially avoid most of the drawbacks experienced by currently available FRP strengthening systems. The ideal characteristics of a strengthening material for reinforced concrete beams were first investigated. Then, an analytical study was conducted to investigate the undulation effect of diagonal yarns on their loading behavior. Based on these investigations, the fabric was designed, manufactured, and its mechanical properties were experimentally evaluated. The effectiveness of the fabric as a strengthening material for reinforced concrete beams has been investigated by testing four reinforced concrete beams strengthened in flexure and shear using the new fabric. The test results showed that the fabric met its design objectives.