Elsevier

Engineering Structures

Volume 225, 15 December 2020, 111292
Engineering Structures

Evaluation of post-heating flexural behavior of steel fiber-reinforced high-strength concrete beams reinforced with FRP bars: Experimental and analytical results

https://doi.org/10.1016/j.engstruct.2020.111292Get rights and content

Highlights

Abstract

Despite many advantages provided by fiber-reinforced polymer (FRP) bars, their low resistance against fire has been an important barrier for their widespread application in reinforced concrete (RC) members. Studying the post-fire performance of concrete structures reinforced with FRP bars can be beneficial in understanding the structural safety and estimating the serviceability conditions after a fire incident. Since little information is available in the literature in this regard, especially for high-strength concrete (HSC) and fiber-reinforced concrete (FRC), this research attempted to investigate the flexural behavior of HSC beams reinforced with glass fiber-reinforced polymer (GFRP) bar and steel fibers after exposure to elevated temperatures. The variables under consideration consisted of the applied temperature (20, 250, 400, and 600 °C), volume ratio of steel fibers (0 and 1%), and reinforcement ratio (0.314 and 0.872%). Following the quasi-static four-point flexural test on the heated and non-heated beam specimens, various parameters involving the load-carrying capacity, load-deflection relationship, cracking pattern in terms of the number and width of cracks at service and ultimate loads, and ductility of the beams were examined. It was found that exposing the beams to 250 °C reduced their load-carrying capacity negligibly and exposing them to 400 °C increased their residual load-carrying capacity notably while exposing them to 600 °C led to severe degradation in their flexural capacity. Further, the effectiveness of steel fibers on the behavior of beams with the lower reinforcement ratio was not significant; however, as the reinforcement ratio of the beams and applied temperature increased, steel fibers properly demonstrated their ability in improving the load-carrying capacity and reducing deflection at service load. By evaluating prediction relationships provided by codes and other researchers against the experimental results observed in this research, ACI 440.1R-06 and ACI 440.1R-15 codes were found to be able to predict deflection under service conditions with proper accuracy. Ultimately, for capturing the load-deflection relationship of heated and non-heated beam specimens, an analytical model was developed using the sectional analysis method, which was able to predict the experimental results appropriately.

Introduction

Until several decades ago, steel bars had practically been the only option for the reinforcement of concrete structures. However, one major weakness of steel, namely its low corrosion resistance, especially in aggressive marine environments, bridges, and parking garages exposed to deicing salts, leads to the deterioration of concrete structures, rendering them not serviceable. Various techniques have been proposed to deal with this problem, among which fiber-reinforced polymer (FRP) bar has been presented as a viable alternative for steel bars [1], [2], [3]. FRP bars are beneficial in many respects due to their high tensile strength, low weight, noncorrosive and nonmagnetic nature, high fatigue endurance, and low electrical and thermal conductivity. Hence, these features make FRP bars a viable alternative for common steel bars used in reinforced concrete (RC) structures in cases where the above-mentioned features are required from the reinforcing bars [2].

Despite the known advantages of FRP bars, their use in reinforcing flexural members including beams and slabs is accompanied by some concerns. The elastic modulus of FRP bars, especially the Glass FRP (GFRP) type, is much less than that of steel bars, which in turn leads to lower post-cracking flexural stiffness and higher crack width in FRP-RC beams in comparison with steel reinforced concrete (SRC) beams per the same reinforcement ratio [4], [5]. Further, due to the linear elastic behavior of FRP bars, failure in flexural FRP-RC members will be of a brittle nature unlike SRC members [6]. Numerous empirical, theoretical, and numerical studies have been conducted in the past two decades to understand the flexural behavior of concrete members reinforced with FRP bars [7], [8]. Kassem et al. [9] took into account the flexural behavior of 24 concrete beams reinforced with FRP bars. All beams failed by concrete crushing, and it was also observed that as the reinforcement ratio of CFRP bars increased, the flexural capacity did not experience a significant increase, such that when the reinforcement ratio increased by 100%, the peak load only improved by 16%. However, this level of load improvement was obtained via increasing the reinforcement ratio by 33% in the beams reinforced with GFRP bars. El-nemer [10] addressed the flexural behavior of GFRP-RC beams made with normal-strength concrete (NSC) and high-strength concrete (HSC). It was reported that using HSC increased the cracking moment, post-cracking stiffness, and load-bearing capacity while reducing the crack width and deflection. Moreover, it was found that in the beams reinforced with GFRP bars with a sand-coated surface, developed cracks were greater in number and smaller in width in comparison with the beams reinforced with GFRP bars with a helically wrapped surface, which was attributed to the superior flexural bonding characteristics of the sand-coated GFRP bars. In addition, Goldston et al. [11] investigated the flexural behavior of GFRP-RC beams made with HSC and ultra-high-strength concrete (UHSC). It was reported that the effect of increasing the concrete strength on the load-carrying capacity was evident only in over-reinforced beams. Further, the over-reinforced beams demonstrated a pseudo-ductile behavior relative to the under-reinforced beams that showed a sudden brittle failure. In the under-reinforced beams, increasing the concrete strength had no impact on post-cracking flexural stiffness, load-carrying capacity, mid-span deflection, and energy-absorbing capacity.

Although it is recommended that FRP bars be employed in combination with HSC, so that the high-strength property of these bars could be optimally employed, the brittleness of HSC limits the overall deformability of flexural members [12]. The approach of enhancing concrete properties through the use of fibers is an attractive solution to overcome issues associated with the ductility and deformability of FRP-RC members [13]. It is well-known that steel fibers are highly effective in resisting deformation during all loading phases, inhibiting the growth and opening of cracks, and enhancing inelastic deformations, ultimate flexural strength, and shear capacity of members [14], [15]. Issa et al. [16] focused on the contribution of different fiber types to the flexural behavior and ductility of FRP-RC beams and observed that among the various fibers types used (propylene, glass, and steel), adding steel fibers had the highest effectiveness in increasing the ductility of the RC beams reinforced with FRP bars. Yang et al. [17] also studied the impact of steel and propylene fibers used separately on the flexural behavior of HSC beams reinforced with FRP bars. The GFRP-RC beams with steel fibers and those with synthetic fibers demonstrated a higher first cracking load, post-cracking flexural stiffness, and flexural strength, as well as inelastic deformations and ductile behavior at failure in comparison with the fiberless beams. Nevertheless, by adding fibers, no enhancement was observed in the ductility of CFRP-RC beams which failed by FRP bar rupture. In their study, Zhu et al. [18] explored the flexural behavior of FRP-RC beams containing steel fibers in the tension zone of section and concluded that even though adding steel fibers in the tension zone is effective in limiting deflection and the large crack width in these beams, leading to economic benefits, to obtain a high ductility, steel fibers must be added to the entire depth of a member.

The structural FRP applications have been limited mainly to bridges and external applications, where fire-resistance considerations are not of particular interest. Apart from cost and ductility aspects, one major barrier facing the use of FRP-RC members in multi-story buildings, parking garages, and industrial structures is the rapid and severe loss of bond, strength, and stiffness of FRPs at elevated temperatures. Before FRPs could be safely utilized in buildings as reinforcement, the performance of these materials under elevated temperatures must first be examined and the mechanical integrity and safety of a fire-damaged structure with FRP reinforcement as well as its serviceability and repairability must be determined.

The deterioration of the mechanical and bond properties of FRP bars in concrete structures during a fire incident leads to unserviceable deflections, loss of tensile reinforcement, and finally structural collapse [19]. Although with an increase in the exposure temperature of FRP-RC members, the FRP bars embedded in concrete do not burn due to the lack of oxygen, their resin will soften, and beyond the glass transition temperature, Tg, (93–120 °C) the elastic modulus of this resin experiences a significant reduction, leading to a reduced stress transferability from concrete to fibers and consequently a considerable reduction in bond strength [6]. When the temperature in FRP composites exceeds the resin decomposition temperature, Td, (300–400 °C) the resin completely loses the ability to transfer loads between fibers and the bond with concrete is lost, leading to the final collapse of the structure [20].

Most efforts for describing the fire-resistance characteristics of FRP-RC members have been concentrated on their load-carrying performance when exposed to elevated temperatures during fire; however, their residual strength characteristics after their exposure to fire and subsequent cooling have not been properly addressed. Gooranorimi et al. [21] investigated the residual flexural strength of GFRP-RC slabs with two bar types with different surface conditions after exposure to elevated temperatures in a furnace for 2 h, where the maximum surface temperature of the bar reached 115 °C. Following exposure to elevated temperatures, the slabs reinforced with helically wrapped sand-coated GFRP bars showed a 10% increase and the slabs reinforced with deformed ribbed GFRP bars showed a 10% decrease in the ultimate load-carrying capacity relative to the control slabs. Based on these observations, the authors concluded that the surface characteristics of GFRP bars might affect the structural behavior of GFRP-RC members after exposure to fire. Irshidat [22] addressed the post-fire behavior of CFRP-RC and GFRP-RC beams. No noticeable reduction was observed after exposure to temperatures of up to 300 °C; however, after exposure to 500 °C, degradation in the flexural strength was considerable. In addition, the CFRP-RC beams were more sensitive to temperature relative to the GFRP-RC beams with in terms of the load-carrying capacity. In a study on the flexural behavior of beams reinforced with GFRP bars after exposure to 500 °C, Hamad et al. [23] observed that the flexural strength of the GFRP-RC beam after exposure to the elevated temperature for 90 min declined by 79%, while this decline in the SRC beam was just 9%. Moreover, it was found that the ultimate load obtained from formulas presented in ACI 440.1, assuming perfect FRP bar-concrete bond, was greater relative to the measured ultimate load.

No study so far has explored the flexural behavior of GFRP-RC beams with steel fiber-reinforced concrete (SFRC) after exposure to elevated temperatures. In this research, high-strength concrete (HSC) beams reinforced with GFRP bars with and without steel fibers were fabricated and subjected to the four-point bending test after experiencing temperatures of 20, 250, 400, and 600 °C. The volume ratio of steel fibers, reinforcement ratio, and applied temperature were the variables under consideration, whose effect on the load–deflection behavior, cracking pattern, and ductility of the beam specimens was investigated. Further, the obtained empirical results under service conditions, namely crack width and deflection, were compared with the predictions of codes and other researchers. Finally, an analytical model for predicting the load–deflection relationship of the heated and non-heated beam specimens was developed using the cross-sectional analysis method.

Section snippets

Research significance

Most studies conducted on the effect of fire or elevated temperatures on FRP-RC members have been concentrated on investigating the fire resistance rate. However, studying the behavior of FRP-RC members after exposure to fire is of particular interest in terms of understating their performance under service conditions and finding out which one of the two options of strengthening or demolishing and reconstructing buildings and industrial structures that have not collapsed after the fire incident

Materials, mix proportions, and mechanical tests

Table 1 summarizes the details of mix designs. In all mix designs, Type I Portland cement and silica fume replacing 10% of cement weight were employed as the cementitious materials. Crushed stone with a maximum particle size of 9.5 mm and crushed sand with a maximum particle size of 4.75 mm were used as the coarse and fine aggregate, respectively. The water-to-cementitious materials (binder) ratio (W/B) was taken as 0.3 to reach a high strength. In line with research by others [24] proving

Mechanical properties

Table 5 lists the mean results obtained from the compression test and the direct tension test. It is seen that at 250 °C slight changes occur in the compressive strength of the PC and SFRC concretes compared with the non-heated concretes; an observation which is in line with research by others [32], [33]. However, tensile strength declined by more than 30% after exposure to 250 °C, suggesting a higher sensitivity of tensile strength to temperature relative to compressive strength. When the

Prediction of deflection

Given a lower elastic modulus of GFRP bars relative to steel bars, the design of concrete beams reinforced with GFRP bars is usually controlled by serviceability limit state requirements. Hence, a method is required to calculate the expected deflections in GFRP-RC members with proper accuracy. In general, the effective moment of inertia approach and the moment-curvature approach are employed to determine the deflection of FRP-RC beams.

In RC members under flexure, when concrete in the tension

Conclusion

Although the use of GFRP bars in structures subjected to aggressive environmental conditions is a common practice nowadays, more research is required to facilitate its application in ordinary buildings due to a number of inherent defects of this material such as low fire resistance. In this study, the flexural behavior of fiber-reinforced high-strength concrete (HSC) beams reinforced with GFRP bars after exposure to elevated temperatures was explored experimentally and analytically. The

CRediT authorship contribution statement

Hamed Jafarzadeh: Conceptualization, Investigation, Software, Methodology, Formal analysis, Resources, Writing - original draft. Mahdi Nematzadeh: Conceptualization, Investigation, Methodology, Formal analysis, Writing - original draft, Project administration, Supervision.

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.

References (60)

  • H. Zhu et al.

    Flexural behavior of partially fiber-reinforced high-strength concrete beams reinforced with FRP bars

    Constr Build Mater

    (2018)
  • M. Saafi

    Effect of fire on FRP reinforced concrete members

    Compos Struct

    (2002)
  • A.M.T. Hassan et al.

    Experimental test methods to determine the uniaxial tensile and compressive behaviour of ultra high performance fibre reinforced concrete (UHPFRC)

    Constr Build Mater

    (2012)
  • A. Karimi et al.

    Axial compressive performance of steel tube columns filled with steel fiber-reinforced high strength concrete containing tire aggregate after exposure to high temperatures

    Eng Struct

    (2020)
  • A. Baradaran-Nasiri et al.

    The effect of elevated temperatures on the mechanical properties of concrete with fine recycled refractory brick aggregate and aluminate cement

    Constr Build Mater

    (2017)
  • M. Nematzadeh et al.

    Post-fire compressive strength of recycled PET aggregate concrete reinforced with steel fibers: Optimization and prediction via RSM and GEP

    Constr Build Mater

    (2020)
  • B. Chen et al.

    Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures

    Cem Concr Res

    (2004)
  • M. Nematzadeh et al.

    Compressive performance of steel fiber-reinforced rubberized concrete core detached from heated CFST

    Constr Build Mater

    (2020)
  • J. Dashti et al.

    Compressive and direct tensile behavior of concrete containing Forta-Ferro fiber and calcium aluminate cement subjected to sulfuric acid attack with optimized design

    Constr Build Mater

    (2020)
  • Y.C. Wang et al.

    An experimental study of the mechanical properties of fibre reinforced polymer (FRP) and steel reinforcing bars at elevated temperatures

    Compos Struct

    (2007)
  • H. Hajiloo et al.

    Mechanical properties of GFRP reinforcing bars at high temperatures

    Constr Build Mater

    (2018)
  • R.J. Hamad et al.

    Mechanical properties and bond characteristics of different fiber reinforced polymer rebars at elevated temperatures

    Constr Build Mater

    (2017)
  • C. Barris et al.

    An experimental study of the flexural behaviour of GFRP RC beams and comparison with prediction models

    Compos Struct

    (2009)
  • D.Y. Yoo et al.

    Structural performance of ultra-high-performance concrete beams with different steel fibers

    Eng Struct

    (2015)
  • D.Y. Yoo et al.

    Flexural behavior of ultra-high-performance fiber-reinforced concrete beams reinforced with GFRP and steel rebars

    Eng Struct

    (2016)
  • ACI (American Concrete Institute). (2007). Report on fiber-reinforced polymer (FRP) reinforcement for concrete...
  • M. Al Rifai et al.

    Durability of basalt FRP reinforcing bars in alkaline solution and moist concrete environments

    Constr Build Mater

    (2020)
  • Abed, F., El Refai, A., & Abdalla, S. (2019, August). Experimental and finite element investigation of the shear...
  • ACI Committee 440. (2015). Guide for the Design and Construction of Structural Concrete Reinforced with...
  • C. Kassem et al.

    Evaluation of flexural behavior and serviceability performance of concrete beams reinforced with FRP bars

    J Compos Constr

    (2011)
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