No abstract is provided for this article.
No abstract is provided for this article.
Extensive research has been conducted on FRP (fiber-reinforced polymers)-confined concrete columns under concentric compression, leading to many stress-strain models for such concrete. These concentric-loading (CL) stress-strain models have generally been used in the analysis of both concentrically and eccentrically loaded columns. Existing tests, however, have shown that eccentrically loaded FRP-confined concrete columns exhibit some behavioral aspects that cannot be closely predicted using a CL stress-strain model. This paper presents an in-depth investigation into this problem using an advanced three-dimensional (3D) finite element (FE) approach. The stress-strain response of concrete is shown to vary significantly over the section, and the direct use of a single CL stress-strain model for the entire section in the analysis of eccentrically loaded columns may lead to significant errors in the prediction of ultimate displacement/curvature. A stress-strain model for the confined concrete at the extreme compression fiber of the section is also shown to provide a relatively simple and much more accurate option for predicting the ultimate displacement/curvature of eccentrically loaded columns. Based on this conclusion, a so-called eccentricity-dependent (EccD) stress-strain model is proposed based on a comprehensive parametric study using the FE approach. The proposed model can be directly used in a section analysis or a theoretical column model and is proven to provide much more accurate predictions of the ultimate displacement/curvature of test columns than existing CL stress-strain models.
External confinement of concrete using fiber reinforced polymer FFRP) composites has emerged as a popular method for retrofitting of existing reinforced concrete columns for increased strength and ductility. The paper examines the axial stress-strain behavior of FRP confined concrete, understanding of which is necessary for the design of FRP retrofitting measures for columns, particularly when the column is subject to combined bending and axial load and when seismic ductility is of concern. Presented is a new, accurate stress-strain model for concrete with a sufficient amount of confinement from FRP which leads to significant compression strength enhancement. The model is based on a careful interpretation of large test databases gathered from extensive surveys of the existing literature.
Fiber-reinforced polymer (FRP) waste is becoming an environmental concern due to the widespread use and non-biodegradable nature of FRP composites. Cutting FRP waste into short-length randomly distributed reinforcing bars (referred to as “needles” hereafter) as a substitute for part of the coarse aggregate in concrete has been suggested as a possible solution to FRP waste recycling. This paper presents to the authors’ best knowledge the first reported experimental investigation into the effect of GFRP needles as coarse aggregate partial replacement in concrete on the shear behavior of large-scale reinforced concrete (RC) beams. A total of 10 RC beams without steel stirrups in the critical half were tested under four-point bending. The volume replacement ratio of coarse aggregate and the surface type of GFRP needles were chosen as the test parameters. All test beams failed in shear in a brittle manner with their ductility being slightly enhanced by the partial replacement of coarse aggregate using GFRP needles. An enhancement of 8–10% in the load-carrying capacity was observed in beams with helically wrapped needles, while beams with smooth needles showed a slight reduction in the load-carrying capacity. The presence of GFRP needles increased the amount of total energy absorbed by the RC beams by about 33–40%.
No abstract is provided for this article.
Intermediate crack debonding (i.e., IC debonding) is a common failure mode of reinforced concrete (RC) beams flexurally strengthened with a fiber-reinforced polymer (FRP) plate. Due to IC debonding failure, the tensile strength of the FRP plate cannot be fully utilized. While a number of options to suppress this failure mode have been explored, the easiest option is still to install U-jackets of an appropriate layout outside the FRP plate. This paper presents the results of an experimental study into the effectiveness of such U-jacketing on delaying or suppressing IC debonding failure, an issue which has not yet been systematically investigated so far. Eight large-scale RC beams were tested in the present study to investigate the effects of different forms of FRP U-jacketing on IC debonding failure. The test results indicated that vertical FRP U-jackets have a rather limited effect on IC debonding failure. Moreover, strong vertical FRP U-jackets may cause significant out-plan bending in the soffit plate during the deformation process of the beam, leading to the premature rupture of the FRP soffit plate. The tests also showed that inclined U-jacketing is much more effective than vertical U-jacketing, particularly when inclined U-jacketing is provided in a low moment region.
No abstract is provided for this article.
Existing studies have shown that fiber-reinforced polymer (FRP) jacketing/wrapping is highly effective in strengthening circular concrete columns but is much less effective for rectangular/square concrete columns due to the existence of flat sides and sharp corners in the latter. Rounding the corners can enhance the effectiveness of FRP confinement, but its benefit is limited, particularly for large-scale columns, because the corner radius is limited by the presence of internal steel reinforcement. An alternative strengthening technique therefore has been proposed by some researchers, in which the flat sides of a rectangular/square section are modified into slightly curved sides (i.e., curvilinearization) before FRP jacketing/wrapping. This section curvilinearization (SC) method increases the section size by only a limited extent but can substantially enhance the effectiveness of FRP confinement for rectangular/square columns. However, only very limited research has been conducted on curvilinearized rectangular/square columns (CRCs or CSCs), and the few existing studies are limited in scope and were conducted using small specimens. This paper presents the results of a systematic experimental program on the behavior of CSCs, in which 16 small-scale and 10 large-scale FRP-confined square concrete columns with or without section curvilinearization were tested under axial compression. A comparison of the results for columns of the two different sizes indicates that the size effect is very limited in these FRP-confined CSCs. The accuracy of two existing stress–strain models for FRP-confined concrete in CRCs was evaluated using the test results, revealing the need for the development of an improved stress–strain model for use in design.
A large amount of research has been conducted on recycled aggregate concrete (RAC) due to its social, environmental, and economic significance. However, the in situ application of RAC has so far been mainly limited to nonstructural purposes, as the performance of RAC, in both the short and long term, is inferior to its normal concrete counterpart. Existing research has shown that the performance of concrete in compression members can be significantly enhanced through external confinement using steel tubes and fiber-reinforced polymer (FRP) tubes/wraps. Some recent research has examined the behavior of steel tubes filled with RAC, but the research on the behavior of RAC confined with FRP has been rather limited. Research is therefore needed to better understand the stress-strain behavior of and develop a reliable stress-strain model for FRP-confined RAC to facilitate the design of members with FRP-confined RAC. This paper presents the results of the first systematic experimental study on the axial compressive behavior of FRP-confined RAC in which 18 FRP-confined RAC cylinders were tested. The effects of the replacement ratio of coarse aggregate (ratio between the mass of recycled coarse aggregate to the total mass of coarse aggregate) and degree of FRP confinement are investigated. Both the axial and lateral strain responses of the specimens are examined in detail. The test results show that specimens with a replacement ratio of 20% behave similarly to that of normal concrete, but specimens with a replacement ratio of 100% exhibit a lower strength and a different stress-strain response. The applicability of two existing stress-strain models for FRP-confined normal concrete to FRP-confined RAC is also examined.
Extensive research has been conducted on the replacement of steel rebars with fibre-reinforced polymer rebars to eliminate the steel corrosion problem in conventional steel bar–reinforced concrete ...
Numerical modelling and design of cold-formed stainless steel members require accurate knowledge of the material stress-strain behavior over wide ranges of tensile and compressive strains. While many stress-strain models for stainless steels have been developed, when both tensile and compressive behaviors are considered, they are either only capable of accurate predictions over limited strain ranges or defined by rather complex mathematical expressions. This paper presents a new stress-strain model for stainless steel alloys. The proposed stress-strain model, while expressed using simple mathematical expressions, can accurately predict both tensile and compressive full-range stress-strain curves. The proposed stress-strain model is defined using the three basic Ramberg-Osgood parameters and based on a careful interpretation of a large experimental database covering wide ranges of strains. While the proposed stress-strain model is based on the staged approach similar to the existing stress-strain models, a novel method is used to define the second stage strain hardening exponent. Observing the fact that the strain hardening exponent tends to vary with the stress level, the second stage strain hardening exponent is defined in the proposed model as a function of stress level. For use in this model, the best existing equations for predicting the nominal ultimate stress and the corresponding strain (i.e., nominal ultimate strain) respectively are selected through an assessment of existing equations with experimental data. Comparisons between predictions from the proposed model and experimental stress-strain curves for three common structural classes of stainless steel alloys are presented. These comparisons demonstrate clearly the better accuracy of the proposed model over the existing full-range stress-strain models
Fiber-reinforced polymer (FRP) composites have become widely accepted in the strengthening or seismic retrofitting of reinforced concrete (RC) columns in practice. FRP-confined rectangular concrete columns under concentric axial compression have been extensively studied, leading to many stress-strain models (i.e., concentric-loading stress-strain models). Although RC columns in practical structures are commonly subjected to combined axial compression and bending (i.e., eccentric compression), existing research on eccentrically-loaded FRP-confined rectangular RC columns has been much more limited. More specifically, the limited research available has generally been concerned with small-scale RC columns, and the applicability of existing concentric-loading stress-strain models for FRP-confined concrete in the analysis of large-scale eccentrically-loaded rectangular RC columns has not been properly clarified. This paper presents the results of an experimental study including eight large-scale FRP-confined rectangular RC columns tested under eccentric compression. The following key test variables were carefully examined in the experimental program: the load eccentricity, the direction of bending, and the FRP jacket thickness. A theoretical column model is then presented for predicting the responses of the test columns. It is shown that the direct use of a concentric-loading stress-strain model for FRP-confined concrete in the column model leads to significant errors in predicting the ultimate deformation of the test columns.
The direct use of large pieces of crushed demolition concrete (referred to as recycled concrete lumps or RCLs) for mixing with fresh concrete to create a new kind of recycled concrete (referred to as compound concrete), has obvious advantages in terms of recycling efficiency, cost-effectiveness and maximum recycling ratio compared with the recycling of concrete as aggregates. Existing research has revealed certain performance concerns with such compound concrete, including reductions in strength and durability, due to the presence of RCLs. The confinement of compound concrete with an external fiber-reinforced polymer (FRP) confining tube has recently been explored as an effective technique to improve its mechanical properties and durability. This paper presents the results of the first ever experimental study on compound concrete filled FRP tubular (CCFFT) columns aimed at the understanding and modelling of the cyclic stress–strain behavior of FRP-confined compound concrete. The effects of RCL mix ratio, FRP tube thickness, and loading scheme are examined. A monotonic stress–strain model and two cyclic stress–strain models previously developed for FRP-confined normal concrete are used to predict the test results. It is shown that the inclusion of RCLs has a marginal effect on the cyclic stress–strain behavior of FRP-confined concrete.
