Elsevier

Structures

Volume 26, August 2020, Pages 1010-1020
Structures

Evaluation of the experimental curvature ductility of RC beams externally strengthened with CFRP bands

https://doi.org/10.1016/j.istruc.2020.04.030Get rights and content

Highlights

  • Seventeen natural scale RC specimens (beams) externally strengthened with CFRP bands were tested to obtain the bending failure modes.

  • The effect of steel reinforcement ratio and FRP axial rigidity on experimental curvature ductility is showed.

  • An approach for the construction of experimental bending moment-curvature diagrams using LVDT’s deflection data is presented.

  • An analytical expression to predict the failure mode and curvature ductility of the RC beams tested is described.

  • A practical method to obtain a well-estimated value of curvature ductility is discussed.

Abstract

This paper presents an experimental research about bending response of a set of reinforced concrete beams strengthened with external CFRP (RCB-SCFRPs). The aim of the research was to make an evaluation of curvature ductility using experimental data and to generate simplified expressions to estimate the ductility of RCB-SCFRP. In addition, the experimental behavior and the resulting failure modes on bending are also studied by experimental tests. Seventeen natural-scale RCB-SCFRPs with different configurations of reinforced steel bars and FRP (Fiber-reinforced polymer) bands from three manufacturers are designed and loaded under monotonic four points bending tests. In fact, three groups of RCB-SCFRPs were designed in order to evaluate the effect of FRP longitudinal ratio and the external transverse FRP reinforcement (Groups I and II) and to obtain the failure bending modes described in ACI 440.2R [1] (Group III).

Flexural capacity and curvature ductility are some of the most important and well-known parameters for analysis and design of traditional RC beams. The calculation of these same parameters in the application of RCB-SCCFRP is not straightforward to compute. Because of this difficulty, the present research focuses on the study of bending behavior and evaluating ductility in terms of FRP axial rigidity, and the steel and CFRP reinforcement ratio.

For this purpose, the experimental bending moment-curvature (M-Ф) diagrams were constructed using a proposed approach based on geometric compatibility. The effect of the transverse FRP reinforcement such as U-clamps is not completely clear, but in most cases, the RCB-SCFRPs with transverse reinforcement showed the best performance in final curvature and a greater ductility. Test results indicate that the RCB-SCFRP bending behavior was critically influenced by the FRP properties. Additionally, the FRP axial rigidity had a critically adverse effect on the curvature ductility of the RCB-SCFRPs tested.

Furthermore, several expressions based on internal forces equilibrium for predicting the failure bending mode and curvature ductility of RCB-SCFRPs are proposed, which are consistent with experimental data.

Finally, a simplified method is proposed to obtain a well-estimated value of curvature ductility considering FRP axial rigidity and steel reinforcement ratio, which could be very helpful to the researcher and designer engineer.

Introduction

The application of fiber-reinforced polymer (FRP) in retrofitting and rehabilitation of existing reinforced concrete infrastructure has been increasing in the last two decades because the owners needing to improve their existing infrastructure and meet the current design codes. For example, FRP is applied in the retrofit of existing buildings, such as houses, offices, industrial buildings, bridges, tanks, chimneys, and other kinds of infrastructures.

The use of FRP is preferred because of their high resistance property against chemical attack and chloride ion compared with steel bars [2], [3]. Similarly, FRP material has low density, which can provide a high strength-to-weight ratio [4], the increment of mass is significantly less than other traditional materials. Although FRP has some limitations, such as a purely elastic-brittle material [5], that means it doesn’t provide any capacity ductile.

The external strengthening with FRP in reinforced concrete beams has shown significant increase in bending and shear strength, also it has demonstrated an increase of the load at initial cracking over control specimens without external FRP [6]. The behavior of a pre-cracked specimen and then strengthened is not significantly different than a strengthened specimen without any initial loaded or pre-cracked [7], [8], [9]. The axial rigidity of the adhesive material plays a critical role in the debonding failure of reinforced concrete beams strengthened with FRP bands (RCB-SCFRPs) [6], [10].

Several researches demonstrate the efficiency in increasing the bending final load for reinforced concrete beams externally strengthened with FRP bands [7], [11], [12]. However, a more brittle behavior also has been exhibited in comparison with similar beams without FRP strengthening [11], [12], [13]. This aspect constitutes a serious limitation in earthquake-resistant structures where the seismic energy must be dissipated by the inelastic behavior of the materials of the structural system, and which manifests itself through steel yielding and concrete cracking [14]. This study is strongly motivated in the evaluation of the bending behavior of RCB-SCFRPs and in identifying which are the relationship between its main properties such as cross-sectional properties (internal and external reinforcement ratios) with failure modes, and curvature ductility.

Reinforced concrete beams externally strengthened with FRP exhibit different failure modes according to the order in which the failure of materials that compose them appear [1], [15]. Some of them are remarkably ductile and others more brittle because of the curvature ductility of the beam is provided almost exclusively by the reinforcing steel. The failure mode is strongly marked by the FRP's elastic modulus [16].

In this study, the curvature ductility and behavior of the strengthened specimens are evaluated. Based on the experimental results, the effect of steel reinforcement and FRP strengthening on the behavior of the strengthened specimens are also evaluated. Additionally, an interesting discussion about how FRP bands axial rigidity, which was expressed as FRP tension modulus per unit width, influences on the results of ultimate curvature and curvature ductility of RC beams is presented. Finally, several expressions based on internal forces equilibrium for predicting the bending failure mode and curvature ductility of RCB-SCFRPs are proposed.

Section snippets

Experimental program

Seventeen natural-scale specimens RCB-SCFRPs with CFRP bands from three manufacturers were designed and subjected under monotonic four-point loading tests up to failure at the CISMID’s laboratory facilities. The experimental data was measured in four points of each specimen until reaching the failure. Furthermore, they were categorized into three groups in order to meet the aims of this study. Variations of the longitudinal and transverse external reinforcement were made in Groups I and II,

Analysis of the specimens

Simple analytical expressions were proposed in order to evaluate the expected failure mode and the corresponding ductility of curvature of the tested specimens

Results

The results about the failure modes obtained from the laboratory tests, as well as the ductility values obtained by the corresponding proposed method are presented in this section.

Comparison of the experimental M-Ф diagrams

M-Ф diagrams for each beam had been constructed using the proposed method in Section 3.2.1. These diagrams were grouped as shown in Fig. 4, Fig. 5, Fig. 6, Fig. 7. In general, the diagrams have a characteristic shape marked by three regions: an uncracked elastic region, a cracked elastic region and an inelastic region where reinforcing steel yielded. Between the last two zones, an irregular zone is appreciated, which is presumed to be due to the hardening of the reinforcing steel.

Illustrative example

The following examples show how the simplified method proposed can be used to rapidly estimate the resulting curvature ductility after strengthen an RC beam with CFRP bands.

For two reinforced concrete beam cross-sections with a specific steel reinforcement ratio, the CFRP options that meet the sufficient flexural strength capacity must be selected in order to obtain the best ductility performance.

The methodology is relatively simple to follow. First step, the CFRP tension modulus per unit width

Conclusions

The effect of steel reinforcement ratio and CFRP bands’ axial rigidity on the behavior of a RCB-SCFRP is evaluated in this study. For this purpose, seventeen natural-scale RCB-SCFRPs with different configurations of reinforced steel bars and CFRP bands from three manufacturers were tested. The results obtained in this study are summarized as follows:

  • The failure modes by bending of RCB-SCFRPs were obtained and its behavior was described.

  • The curvature ductility of the RCB-SCFRPs tested were

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors want to thank the support provided by the Universidad Nacional de Ingenieria (Lima, Peru) and CISMID laboratory in developing this investigation. Also, the important contribution received from the company Top Consult Ingenieria SAC, Peru, is gratefully acknowledged.

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