FRP-strengthened Reinforced Concrete Research Articles

Experimental Study on Intermediate Crack Debonding in FRP-Strengthened RC Flexural Members

A commonly observed failure mode of reinforced concrete (RC) flexural members strengthened with a fibre reinforced polymer (FRP) plate bonded to their tension face involves debonding that initiates at a major flexural or flexural shear crack and propagates towards a plate end, which has often been referred to as intermediate crack (IC) debonding. This paper first presents the results of IC debonding tests conducted on eighteen cantilever slabs in which IC debonding initiated at or near the fixed end, and four simply-supported slabs in three-point bending where IC debonding initiated at the load position. A number of factors are covered by these tests, including the type of FRP material, the width of FRP strip, the length of FRP strip, the slab width, the concrete strength, the longitudinal steel ratio, the concrete cover, and pre-cracking of concrete. The test results are next used to assess the accuracy of four available strength models that can be used to design against IC debonding. Of these four models, Teng et al's (2003) model is shown to provide safe predictions of the test results in most cases but can lead to overly conservative results; the scatter of the predictions is also large. The other three models from recent design guidelines (JSCE, 2001, fib 2001, ACI 440 2002) are not sufficiently safe for use in design. This assessment clearly illustrates the need for much further research to develop a more accurate IC debonding strength model.

Finite element simulation of debonding in FRP-to-concrete bonded joints

The behaviour of reinforced concrete (RC) structures strengthened with externally bonded fibre-reinforced polymer (FRP) reinforcement in the forms of thin plates or sheets is often dominated by debonding of the FRP reinforcement from concrete. As a result, a large number of studies have addressed debonding failures in FRP-strengthened RC structures, with many of them being focussed on understanding the behaviour of simple FRP-to-concrete bonded joints in which an FRP plate/sheet is bonded to a concrete prism and is subject to a tensile force. Despite the many existing studies, there are still substantial uncertainties and difficulties with the finite element modelling of debonding failures due to the complex behaviour of cracked concrete. This paper explores the use of different crack models in modelling and compares their predictions of the debonding behaviour of FRP-to-concrete bonded joints. The results presented in this study show that a non-coaxial rotating angle crack model (RACM) is required to accurately predict this debonding behaviour. Furthermore, the debonding mechanism is also examined using results obtained with a non-co-axial RACM.

Meso-scale finite element model for FRP sheets/plates bonded to concrete

External bonding of fiber-reinforced polymer (FRP) plates or sheets has recently emerged as a popular method for the strengthening of reinforced concrete (RC) structures. The behavior of such FRP-strengthened RC structures is often controlled by the behavior of the interface between FRP and concrete, and this interfacial behavior is commonly studied through a pull test in which an FRP sheet or plate is bonded to a concrete prism and is subject to tension. In this paper, a meso-scale finite element (FE) model implemented with the MSC.MARC program is presented for the simulation of interfacial debonding failures in a pull test. In this model, very small nearly square elements (0.25–0.5 mm in size) are used with the fixed angle crack model (FACM) to capture the development and propagation of cracks in the concrete layer adjacent to the adhesive layer. The effect of element size is taken into account in modeling both the tensile and shear behavior of cracked concrete. Comparisons between the predictions of this model and test results are presented to demonstrate the capability and accuracy of this FE model. The debonding mechanism is also examined using results obtained with the FE model. Finally, a method for the determination of the local bond–slip curve of the FRP-to-concrete interface from the FE results is described.

Ductility and deformability of fibre-reinforced polymer-strengthened reinforced concrete beams

This paper discusses the definition of, and the distinction between, deformability and ductility of reinforced concrete (RC) beams that are strengthened by advanced composites. The study examines the suitability of a new, design-based method for the determination of deformability, as well as an energy-based process, which is found to be suitable for quantifying the ductility levels of fibre-reinforced polymer (FRP)-strengthened RC members. Ten FRP-strengthened RC beams and four slabs have been load-tested to ultimate failure in the current study. The test results, together with the load–deflection data of an additional 26 beams from literature, form the basis of analyses and discussions. The paper concludes that high deformability does not necessarily lead to good ductility, as very brittle failure modes of such beams have been observed in the experimental studies and reported in literature. It was found that a ductility index of between 2·0 and 2·5 reflects 25–33% of elastic energy stored in the strengthened system. This level of elastic energy is considered to be the maximum acceptable in FRP-strengthened concrete flexural elements for ductile behaviour. It was also found that for ductile failure modes the deformability and ductility indices tend to converge, whereas for brittle behaviour the deformability index could be up to 33% higher. The results presented in this paper provide a rationale for the ductility considerations to be incorporated into the development of design equations for FRP strengthening.

Stiffness and ductility of fibre-reinforced polymer-strengthened reinforced concrete members

The use of fibre-reinforced polymer (FRP) composites for repair and strengthening of civil engineering structures is gaining increasing acceptance worldwide and can be considered close to achieving the status of mainstream technology. Especially widespread is the application of externally bonded FRP laminates and bars for flexural strengthening of reinforced concrete (RC) members. Although a vast amount of experimental and theoretical research has been devoted to quantifying the strength of FRP-strengthened beams, less attention has been paid to stiffness and ductility aspects. The aim of this work is to carry out a systematic numerical analysis of the influence of the most relevant parameters on stiffness and ductility of FRP-strengthened RC cross-sections. As far as stiffness is concerned, the substantial advantages related to the use of active (i.e. pre- or post-tensioned) strengthening systems are quantified. For evaluation of ductility, an energy-based index is proposed which allows comparison of cross-sections with different amounts and types of FRP reinforcement accounting implicitly for their different moment–curvature behaviour. The damage accumulation of the cross-section is also addressed by means of an energy-based index and of the Park–Ang damage index.

Behaviour and strength of FRP-strengthened RC structures: a state-of-the-art review

Extensive research has been carried out in recent years on the use of fibre-reinforced polymer (FRP) composites in the strengthening of reinforced concrete (RC) structures. This paper provides a concise review of existing research on the behaviour and strength of FRP-strengthened RC structures, with a strong focus on those studies which contribute directly to the development of strength models. Topics covered include flexural and shear strengthening of beams, flexural strengthening of slabs, and strengthening of columns subject to both static and seismic loads. For each of the topics covered, the methods of strengthening are first explained, followed by a description of the common failure modes. Available strength models are then summarised and discussed.

Shear capacity of FRP-strengthened RC beams: FRP debonding

Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre-reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.

FRP-strengthened RC beams. II: assessment of debonding strength models

Reinforced concrete (RC) beams strengthened in flexure by the bonding of a fiber reinforced polymer (FRP) plate to the tension face are susceptible to brittle debonding failures. Such failures commonly initiate at or near one of the plate ends at a load below that to achieve flexural failure of the plated section. For a successful design of flexural strengthening using FRP composites, it is important to be able to predict such plate end debonding failures. In the first of these two companion papers, 12 plate end debonding strength models have been reviewed and summarized (Engng Struct 24(4) (2002) 385–395). The aim of the present paper is to provide a comprehensive assessment of the strengths and weaknesses of all the 12 models. To this end, a large test database containing the test results of 59 beams reported to have failed by plate end debonding is first presented. This database was carefully constructed from an extensive survey of the published literature. Both statistical and graphical comparisons between test results and the predictions of the debonding strength models are next presented. A new simple debonding strength model which is superior to existing models is also proposed.