An important application of fiber-reinforced polymer (FRP) composites is to provide confinement to reinforced concrete (RC) columns to enhance their load-carrying capacity. However, this application is generally restricted to short columns as existing design guidelines do not contain provisions for the design of FRP jackets for slender columns. This situation has been due to both the scarcity of test data and the lack of rigorous theoretical studies into the behavior of slender FRP-confined RC columns. This paper presents a theoretical model for slender FRP-confined circular RC columns based on the numerical integration method; Lam and Teng’s stress–strain model is employed to describe the behavior of FRP-confined concrete in the column. Predictions from the theoretical column model are compared with existing test results, which demonstrates that the theoretical model is reasonably accurate in reproducing the experimental results of FRP-confined circular RC columns. These comparisons also demonstrate the need to conduct careful tests on large-scale columns to eliminate some uncertainties associated with the existing test data to enable a more conclusive verification of the proposed theoretical column model.
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Recent research at the University of Queensland (UQ) has led to the development of a new type of structures called “hybrid fibre reinforced polymer (FRP)-timber (HFT) structures”. In HFT structures, FRP is combined with timber veneers to create high-performance, lightweight, easy-to-construct structural members. These HFT members utilize the orthotropic properties of both timber and FRP in a complementary combination to produce much improved composite properties and to maximise the load-carrying capacity of members for a given amount of material. While preliminary experimental work has demonstrated the potential of HFT sections as high-performance sustainable structural components, much more work is needed to better understand the behaviour of HFT structures. This paper presents the results of an experimental study into the local buckling behaviour of HFT thin-walled Cee section short columns. The experimental programme consisted of fifteen HFT column specimens, including all-timber columns and three different types of HFT columns. The test results are presented and discussed. HFT Cee section short columns carried significantly higher axial loads than the corresponding all-timber columns. The ultimate load-to-weight ratio of the HFT sections is shown to be comparable to or significantly higher than that of cold-formed thin-walled steel Cee sections.
A novel method to recycle concrete is to crush demolition concrete into large pieces and then to directly mix the resulting recycled concrete lumps (RCLs) with fresh concrete to produce a new kind of recycled concrete referred to as “compound concrete”. This method avoids the complexity of recycling concrete into aggregates and enables the achievement of a higher recycling ratio and a lower recycling cost. However, due to the large sizes of RCLs and the weak interfaces between fresh concrete and RCLs, compound concrete is much more heterogeneous than normal concrete. A new technique has recently been explored to improve the properties of such compound concrete, in which the compound concrete is provided with a substantial amount of confinement from an external fiber-reinforced polymer (FRP) confining tube. This paper presents the results of an experimental program of axial compression tests on compound concrete-filled FRP tubular columns in which the FRP tubes were prefabricated using the wet lay-up method with fibers only in the hoop direction. The tubes had a negligible axial stiffness, which allows the stress-strain behavior of FRP-confined compound concrete to be clearly revealed. The test results show that, when a significant level of FRP confinement is provided, the behavior of FRP-confined compound concrete is similar to that of FRP-confined normal concrete with a strength equal to that of the fresh concrete. An existing stress-strain model previously developed for FRP-confined normal concrete is evaluated in the paper using the test results of FRP-confined compound concrete.
Over the past decade, fiber-reinforced polymer (FRP) composites have gained wide acceptance as a new generation of structural materials for civil engineering applications due to their unique advantages including their high strength to weight ratio and excellent corrosion resistance. In particular,many possibilities of using FRP in concrete construction have been explored, including the strengthening of existing concrete structures with bonded FRP reinforcement, the use of FRP reinforcing/pre-stressing bars in new concrete structures, and the combination of FRP shapes with concrete to arrive at hybrid FRP-concrete members such as concrete-filled FRP tubular columns. More recently, the use of FRP in steel structures has received much attention. Because both FRP and steel are capable of resisting high tensile stresses, they do not complement each other as well as do FRP and concrete. As a result, the potential for the beneficial use of FRP in steel structures is less than that in concrete structures. This paper first presents a critical discussion of applications where the use of FRP with steel presents significant advantages and then provides a summary of recent research at The Hong Kong Polytechnic University exploring the use of FRP to enhance the performance of steel structures.
The use of steel plates or externally bonded fibre-reinforced polymer laminates for flexural strengthening of concrete, masonry, timber or metallic structures is a technique that has become very popular. The effectiveness of this technique hinges heavily on the performance of the bond between the strengthening plate and the substrate, which has been the subject of many existing studies. In particular, the interfacial stresses between a beam and a soffit plate within the linear elastic range have been addressed by numerous analytical investigations. Surprisingly, none of these investigations has examined interfacial stresses in members with a curved soffit, despite that such members are often found in practice. This paper presents an analytical model for the interfacial stresses between a curved member of uniform section size and a thin plate bonded to its soffit. The governing differential equations for the interfacial shear and normal stresses are formulated and then solved with appropriate simplifying assumptions. Two numerical examples are presented to illustrate the effect of the curvature of the member on the interfacial stress distributions in a simply supported curved beam for the two cases of a point load and a uniformly distributed load. The analytical solution is verified by comparing its predictions with those from a finite element model.
The compressive behavior of fiber-reinforced polymer (FRP)-confined concrete columns with a noncircular cross section has been investigated through extensive experimental, analytical, and numerical research, but a unified theoretical/numerical approach that can accurately predict both their section-average behavior and local concrete behavior is not yet available. In noncircular columns under axial compression, the concrete is typically under a nonuniform stress state of three-dimensional (3D) compression, with the lateral compressive stresses being the reactive stresses from the confining device (i.e., passive confinement). The authors of the present paper recently developed a plasticity constitutive model for concrete under general 3D compressive stresses, which possesses a potential surface with an evolutionary deviatoric trace that can accurately capture the results of existing compression tests of concrete cubes under nonuniform, passive confinement. This paper explores the application and capability of this evolutionary potential-surface trace (EPT) plasticity constitutive model in finite-element (FE) analysis of FRP-confined square, rectangular, and elliptical plain-concrete columns under concentric compression. The section-average behavior of all the selected noncircular columns predicted by these FE analyses was close to the existing experimental data. The numerical results obtained with the EPT plasticity constitutive model were then examined in detail to achieve an improved understanding of local concrete behavior in FRP-confined noncircular columns.
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Fiber-reinforced polymer (FRP) jacketing has become an attractive technique for strengthening/retrofitting reinforced concrete (RC) columns. Extensive research has been conducted on FRP-confined rectangular columns under axial compression, leading to a significant number of stress-strain models for FRP-confined concrete in these columns. However, most of these models have been developed based on test results of small-scale columns, so their applicability to large FRP-confined rectangular RC columns has yet to be properly validated. To this end, the present paper first presents the test results of an experimental study consisting of nine large-scale rectangular RC columns, including eight FRP-confined RC columns and one RC column without FRP jacketing as the control specimen, tested under axial compression. The experimental program examined the sectional corner radius and the FRP jacket thickness as the key test variables. Five representative design-oriented stress-strain models for FRP-confined concrete in rectangular columns, identified from critical reviews of the existing literature, are then assessed using the test results to examine their validity for these large-scale columns.
In the FRP strengthening of steel structures, cohesion failure in the adhesive is the preferred mode of debonding failure at FRP-to-steel interfaces so that the design theory can be established based on the properties of the adhesive. In this paper, results from a systematic experimental study are presented to exam-ine the effects of steel surface treatment and adhesive properties on the adhesion strength between steel and adhesive. The test results show that adhesion failure can be avoided if the steel surface is grit-blasted prior to bonding and the treated surface can be characterised using three key surface parameters.
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Extensive research has been conducted on the behavior of fiber reinforced polymer (FRP)-confined concrete in both circular and rectangular concrete columns. In the former columns, the stress-strain behavior of FRP-confined concrete is now well understood and can be closely predicted, but the same cannot be said about rectangular columns. This paper presents a new attempt at understanding and modeling the confinement mechanism in square columns as a special case of rectangular columns, leading to a new stress-strain model. The salient features of the new model include a more rigorous definition of the effective confinement area and a corner hoop strain-axial strain relationship based on advanced finite element results as well as a more reliable definition of the ultimate condition. The proposed model is analogous in approach to analysis-oriented stress-strain models for FRP-confined concrete in circular columns and represents a more advanced and robust method for modeling the stress-strain behavior of FRP-confined concrete in square columns than the existing empirically-based stress-strain models. The approach is also easily extendable to FRP-confined concrete in rectangular columns. The proposed model is shown to be accurate and perform better than the existing stress-strain models of the same type in predicting existing test results.
