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