Evaluation of the experimental curvature ductility of RC beams externally strengthened with CFRP bands
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.
References (27)
- et al.
Debonding of FRP in bending: simplified model and experimental validation
Constr Build Mater
(2007) - et al.
Effect of GM patterns on ductility and debonding control of FRP sheets in RC strengthened beams
Constr Build Mater
(2015) - et al.
Flexural ductility fundamental mechanisms governing all RC members in particular FRP RC
Constr Build Mater
(2013) - et al.
Intermediate crack induced debonding in RC beams and slabs
Constr Build Mater
(2003) - et al.
Structural effectiveness of FRP materials in strengthening RC beams
Eng Struct
(2015) - ACI Committee 440, “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete...
- et al.
Mechanical properties of glass fiber reinforced polymers members for structural applications
Mater Res
(2015) - et al.
Sisal/carbon fibre reinforced hybrid composites: tensile, flexural and chemical resistance properties
J Polym Environ
(2010) - Breña Sergio F, Bramblett Regan M, Benouaich Michaël A, Wood Sharon L, Kreger Michael E. “Use of carbon fiber...
- Al-Shawaf A, Al-Mahaidi R, Zhao XL. “Study on bond characteristics of CFRP/steel double lap shear joints at subzero...