Direct tensile self-sensing and fracture energy of steel-fiber-reinforced concretes
Introduction
There is a great demand to enhance the structural resistance of civil infrastructures and to prevent their catastrophic disasters during service life under extreme loads or bad environment, e.g., impact and blast, earthquake, marine corrosion … One of promising construction materials for the demand in enhancing the structural resistance and preventing their catastrophic disasters is steel-fiber-reinforced concretes (SFRCs). This is owing to their high mechanical resistance, high energy absorption capacity and durability based on their work-hardening responses accompanied by multiple microcracks [[1], [2], [3], [4]]. These superior properties can be achieved with suitable type and content of discrete fiber added in the mixture [5]. The embedded fibers in a SFRCs contribute to crack bridging and provide resistance to a crack localization and then result a high toughness [6] which plays a role as a cohesive capacity of material components, i.e., high energy absorption capacity of SFRCs. Specially, the shape of tensile softening part beyond the post crack of SFRCs meaningfully results their flexural behaviors, i.e, determing deflection-softening or deflection–hardening performance [7]. Hence, fracture energy is considered as the key parameter to evaluate the energy absorption capacity [8] as well as to define the mechanical response of SFRCs [9].
In addition, SFRCs have been classified as a smart construction materials with their self-sensing abilities [10]. This smart property of SFRCs can be applied for structural health monitoring (SHM) that is one of the most important works in management and maintenance of a construction during service life [11,12]. The traditional SHM technique has largely utilized the real-time data acquisition using embedded or attached sensors in recording displacement, strain and temperature of structures. Nevertheless, this method has exposed some limitations such as low strain-sensing capacity, low durability and expensive cost [13]. The new SHM technique suggested in this study is using self-sensing construction materials, i.e., materials that can sense strain/stress and damage/cracks by measuring the electrical resistivity of these materials under loading. This approach has recently attracted much interest from many researchers since the suggested approach can overcome the disadvantages of using attached or embedded sensors.
The self-sensing abilities of normal concrete and mortar dopted with carbon fibers have been early discovered since the 1990s by Chen and Chung [14,15]. After that, Chung [16] and Wen et al. [17] explored the self-sensing abilities of fiber-reinforced concretes. Freshly, the damage-sensing abilities of strain-hardeing cement based materials were also found with various composites, e.g., engineered cementitious composites (Ranade et al. [18]), carbon black engineered cementitious composites (Li et al. [19]), high-performance fiber-reinforced concretes (Nguyen et al. [20,22], Song et al. [21]) and ultra-high-performance fiber-reinforced concretes (Kim et al. [23]). Generally, the self-sensing capacity of cement-based materials has been classified into self strain-sensing and self damage-sensing, those acted in linear-elastic zone and nonlinear-inelastic zone, respectively. In the previous research [20], the authors conducted an experimental test to investigate the electromechanical responses of SFRCs with sixs different types of steel fibers and found that the SFRCs with twisted fibers produced not only the highest damage-sensing capability but also the best mechanical resistance. Next, the authors investigated the influence of fiber content on damage-sensing capacity of SFRCs [21]. Nonetheless, these two investigations did not focused on their strain-sensing behaviors acting in elastic zone. This is really an information gap because linear-elastic behaviors of a structural member has commonly occurred in long-time service, e.g., the prestressed beams usually behave in elasticity owing to sufficiently compressive-prestressing force that can control cracking and deflection. Therefore, the electro-tensile behaviors of SFRCs in the elastic zone should be thoroughly understood; in addition, enhancing self-sensing of SFRCs should be also studied.
The situation has motivated the investigation performed in this paper, which focuses on the tensile strain-sensing capabilities and fracture energy of SFRCs based on the test data of this study and two previous studies [20,21]. Fig. 1 shows the layout of this investigation that is aimed to develop the self-sensing SFRCs accompanied with their high mechanical resistances. The specific objectives are 1) to investigate the electro-tensile behavior of the plain SFRCs, 2) to evaluate the sensitivity of various fiber type on the electro-tensile behavior SFRCs, 3) to explore the influence of the fiber contents on the electro-tensile behavior of SFRCs, 4) to achieve an enhancement of self-sensing capacity of SFRCs by adding carbon black (CB) or ground granulated blast furnace slag (GGBS) in the mixture, and 5) to evaluate the fracture energy of the investigated SFRCs.
Section snippets
Self-sensing ability of a strain-hardening cement-based material
The self-sensing property of a strain-hardening cement-based materials has been classified into strain-sensing and damage-sensing which performed in the elastic and strain-hardning zone, respectively. Fig. 2 shows the typical electro-tensile behavior of a strain-hardening SFRCs. In this figure, the dashed curve and the solid curve describe the tensile response and the electrical resistivity response, respectively. The stress and its corresponding strain at the first crack are named as and
Experiments
As shown the layout of the investigation in Fig. 1, the experimental test in this study was conducted for objectives 1 and 4, i.e., the electro-tensile behavior of plain mortar SFRC and the effects of adding carbon black (CB) or ground granulated blast furnace slag (GGBS). In objective 4, M1 was named for the control mixture added neither CB nor GGBS. M2 and M3 were named for the mixtures added CB and GGBS, respectively. The M2 was used an amount of the CB 1% of the cement by weight, while the
Electro-tensile behavior of the plain mortar SFRCs
Conducting a direct tensile test for a plain mortar material is really hard owing to its poor tensile resistance. However, in this study, the authors tried to experiment for the plain mortar SFRC with careful action. Fig. 6 shows the electro-tensile response of plain mortar SFRC using matrix M1. At failure point, both the tensile stress-strain responses and the electrical resistivity-strain responses exhibited brittle failures with sudden descent (stress-strain responses) or sudden ascent
Conclusions
The testing results provided useful information on the self-sensing capacities and fracture energy of SFRCs. The following observations and conclusions could be drawn from this research:
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The plain mortar matrix of SFRCs exhibited the strain-softening behaviors and no self-sensing ability.
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The first gauge factor, considering all strain-hardening SFRCs, was observed to be significantly greater than the post gauge factor, about 2.3–6 times.
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In the comparison of six fiber types with same content of
Acknowledgments
The research was funded by Ho Chi Minh city University of Technology and Education (No. T2019-78TĐ), and, it was further supported by the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the MSIT(2019R1A4A1021702). The opinions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors.
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