Stress-strain Model For FRP-confined Concrete Research Articles

Confinement mechanism and modeling of basalt TRM confined concrete: Effect of mortar matrix grade

Textile reinforced mortar (TRM) confinement significantly enhances the compressive strength and ultimate axial strain of concrete. This study mainly investigated the influence of mortar strength on the stress-strain behavior of TRM-confined concrete to advance understanding of its confinement mechanism. We conducted 48 compression tests on concrete columns, comparing those without jackets to those confined in basalt TRM (BTRM). Variables included the number of textile layers (0 to 4) and mortar matrix strengths (from low-grade ‘M1′ to high-grade ‘M3′). Results indicated that a critical confinement stiffness ratio of 0.024 is essential for strain hardening behavior with marked strain capacity enhancement. Low-grade mortar diminished the effective confinement stiffness of BTRM jackets, manifesting as either a steeper descending plateau in the strain-softening stress-strain curve or a more gradual ascending plateau in the strain-hardening curve. High-strength mortar significantly reduced both the hoop rupture strain of BTRM jackets and the ultimate axial strain of the confined concrete. This study integrated the identified BTRM confinement mechanism into an existing stress-strain model for FRP-confined concrete, enhancing its predictive accuracy for BTRM-confined concrete. Specially, a coefficient, termed km, was introduced to quantify the mortar grade's impact on confinement stiffness in the confining pressure equation. Predictive results closely matched experimental data, especially in replicating plateau characteristics of the stress-strain curve.

FRP spiral strip-confined concrete columns: Stress-strain behavior and size effect

It is well-known that the hoop rupture strain of fiber-reinforced polymer (FRP) jacket in an FRP-confined concrete column is commonly much lower than that obtained from coupon tests. To improve the strain efficiency of the FRP jacket, FRP spiral strip wrapping scheme has been proposed. In the present study, totally 24 circular column specimens [including 16 FRP spiral strip-confined concrete (FSSCC) column specimens] of four different sizes, with a diameter ranging from 100 mm to 400 mm, were tested under uniaxial compression. The particle image velocimetry (PIV) technique was adopted to obtain the hoop and axial strains as well as the strain distributions of the test specimens. The test results showed that the FRP hoop rupture strains measured by the PIV technique in the FSSCC columns are close to that from the coupon tests. The size effect in terms of compressive strength and stress-strain behavior is obvious in the test FSSCC columns. An existing stress-strain model for FRP-confined concrete was then modified to predict the test results. Finally, a field application of the FRP spiral strip wrapping technique completed recently was also briefly introduced.

Behavior and modelling of FRP-concrete-steel hybrid double-skin tubular columns under repeated unloading/reloading cycles

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs) consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. Extensive research has been conducted on the monotonic behavior of hybrid DSTCs, but only a limited number of studies have examined their behavior under cyclic axial compression. In particular, no systematic experimental study has been conducted on hybrid DSTCs under repeated unloading/reloading cycles. This paper first presents a systematic experimental study on hybrid DSTCs under two types of loading schemes: (a) a single unloading/reloading cycle to evaluate the relationship between the unloading strain and the plastic strain; (b) repeated unloading/reloading cycles to investigate the effect of loading history. In the present study, hybrid DSTCs were prepared using filament-wound FRP tubes of a relatively large scale (up to 300 mm in diameter), and the experimental program covered a wide range of concrete strengths (from 40.9 MPa to 104.4 MPa). The systematic experimental study with extensive instrumentation allowed the cyclic stress-strain behavior of the concrete in hybrid DSTCs to be examined in detail, clarifying the cumulative effect of the loading history on both the stress deterioration and the plastic strain of the concrete. This paper also presents a detailed comparison between the test results and the predictions of an existing cyclic stress-strain model for FRP-confined concrete in solid columns, in terms of the repeated unloading/reloading cycles. The cyclic stress-strain model is shown to provide reasonably accurate predictions, and thus can be used together with a monotonic stress-stress model for the concrete in hybrid DSTCs to predict the complete cyclic stress-strain curve of such concrete.

Behavior of large-scale FRP-confined rectangular RC columns under eccentric compression

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.

Advanced stress-strain model for FRP-confined concrete in square columns

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.

Data-driven ultimate conditions prediction and stress-strain model for FRP-confined concrete

Fiber Reinforced Polymer has been widely used in the retrofit of existing structures and the construction of new structures. The ultimate conditions and stress–strain model of FRP-confined composites are critical to structural design and prediction of structural response, especially under extreme loads such as earthquakes. In this paper, a data-driven neural network prediction model for ultimate conditions and stress–strain constitutive relation of FRP-confined concrete is proposed, and the validity and accuracy of the model are verified. A uniaxial compression database containing 169 FRP-confined normal concrete cylinders is collected from the open literature, and the quality of the database is examined and evaluated in detail. Based on the feed forward neural network technology, a prediction model for the ultimate conditions of FRP-confined normal concrete cylinders is established. Configurations and hyper-parameters of the network are carefully analyzed, and the optimal model is used for prediction and comparison. Besides, a uniaxial stress–strain model for FRP-confined concrete is established using a neural network with a recursive structure. The prediction accuracy of the proposed model is proven to be superior to the existing design-oriented models. The data-driven neural network prediction models developed in this paper can provide a rapid prediction and design for FRP-confined composites.

A stress-path dependent stress-strain model for FRP-confined concrete

High-strength concrete (HSC) has higher strength-to-weight ratio and stiffness than normal-strength concrete (NSC). Therefore, the use of HSC can decrease the construction and demolition waste and embodied carbon content of structural members that enhances the urban sustainability. However, HSC is more brittle than NSC. To further push up the maximum concrete strength limit in practical construction, confining concrete by fibre-reinforced polymer (FRP) has been advocated to restore ductility. Compared with using hollow-steel tube as confinement, FRP has lighter weight, higher tensile strength, better corrosion resistance, and is more durable and flexible. Nevertheless, it is up to now a difficult task to predict accurately the uni-axial stress-strain behaviour of FRP-confined concrete since the effect of confining stress, concrete strength, hoop and axial strains are inter-related and need to be determined simultaneously. Herein, to better understand and simulate the behaviour of FRP-confined concrete, a stress-strain model has been developed, which consists of the following three main components: (1) A hoop strain equation elaborated from the authors’ previous study on steel-confined concrete columns for application to FRP-confined concrete; (2) A modified confined concrete model considering stress-path of confining stress (or history of hoop strain); (3) Interaction between FRP and concrete. The model was verified based on 321 test results obtained from the literature, the design application of the which to a broad range of FRP-confined concrete structures is thus ensured.

Stress-Strain Model for FRP-Confined Concrete in Eccentrically Loaded Circular Columns

AbstractExtensive research has been conducted on FRP (fiber-reinforced polymers)-confined concrete columns under concentric compression, leading to many stress-strain models for such concrete. Thes...

Behavior of Concrete-Filled FRP Tubes under Cyclic Axial Compression

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an attractive form of hybrid compression members incorporating FRP. CFFTs have several advantages over traditional column forms, including their excellent corrosion resistance and ductility. Much research has been conducted on CFFTs over recent years, but no systematic experimental study has been concerned with the cyclic axial compressive behavior of CFFTs with a filament-wound FRP tube; such studies are needed for the development of a cyclic stress-strain model for the concrete in CFFTs. This paper therefore presents an experimental study on the behavior of circular CFFTs under cyclic axial compression. The experimental program included the strength of concrete as a key variable so that it also provides a much needed supplement to the very limited existing research on the cyclic compressive behavior of FRP-confined high-strength concrete (HSC). The test results are compared with a monotonic stress-strain model and a cyclic stress-strain model for FRP-confined concrete, both of which have been based on test databases that are limited to concrete confined with an FRP wrap and include only a small number of tests for HSC. The test results show that the cyclic axial stress-strain behavior of concrete in CFFTs is generally similar to that of concrete confined by an FRP wrap. The test results also show that the monotonic stress-strain model perform reasonably well for HSC in CFFTs, but revisions to the cyclic stress-strain model are needed before it can provide accurate predictions for HSC in cyclically loaded CFFTs.

Analysis and behaviour of FRP-confined short concrete columns subjected to eccentric loading

Fibre-reinforced polymer (FRP) composites were widely utilized in civil engineering structures as the retrofit of reinforced concrete (RC) columns. To design FRP jackets safely and economically, the behaviour of such columns should be predicted first. This paper is concerned with the analysis and behaviour of FRP-confined RC circular and rectangular short columns subjected to eccentric loading. A simple design-oriented stress-strain model for FRP-confined concrete in a section analysis was first proposed. The accuracy was then proved by two test data. Following that, a parametric study including amount of FRP confinement, FRP strain capacity, unconfined concrete strength and shape of column section is provided. Some conclusions were obtained at the end of the paper. The work here will provide a comprehensive understanding of the behaviour of FRP-confined concrete columns. The simplicity of the model also enables a simple equivalent stress block to be developed for direct use in practical design.

Design-Oriented Stress-Strain Model for FRP-Confined Concrete in Rectangular Columns

The behavior of fiber reinforced polymer (FRP)-confined concrete in circular columns has been extensively studied, but much less is known about concrete in FRP-confined rectangular columns in which the concrete is nonuniformly confined and the effectiveness of confinement is much reduced. This paper presents a simple design-oriented stress-strain model for FRP-confined concrete in rectangular columns. To this end, existing theoretical models are first reviewed. A database of existing test results collected by the authors is next presented, which is then augmented by a new set of results from tests conducted by the authors. The augmented test database is next employed to assess the existing theoretical models. Finally, a stress-strain model is presented, which is an extension of a recent design-oriented stress-strain model developed for concrete uniformly-confined with FRP based on test results of circular concrete specimens. The proposed stress-strain model is shown to provide satisfactory predictions of the stress-strain curves obtained from the authors' new tests.