|Antonius||Department of Civil Engineering, Faculty of Engineering, Universitas Islam Sultan Agung, Semarang 50112, Indonesia|
|Purwanto||Department of Civil Engineering, Faculty of Engineering, Universitas Diponegoro, Jl. Prof. Soedarto, SH Tembalang, Semarang 50275, Indonesia|
|Primasiwi Harprastanti||Department of Civil Engineering, Faculty of Engineering, Universitas Diponegoro, Jl. Prof. Soedarto, SH Tembalang, Semarang 50275, Indonesia|
This paper presents an experimental study of eight reinforced concrete beams and aims to evaluate the flexural behavior and ductility of burned beams at various temperatures. The beam specimens contained steel fibers comprising 0.5% of the volume of the concrete. They were divided into two types, with the flexural region consisting of a single reinforcement system (without compressive reinforcement) and double reinforcement bars. Each type of beam was controlled (not burned), and burned at 300°C, 600°C and 900°C. The beam testing employed a four-point loading system (pure bending in the test region) and worked monotonically. The experimental results show that the performance of beams with the double reinforcement system to the resilient load was better than with a single reinforcement system beam. This occurred at normal temperatures and up to high temperatures. The longitudinal reinforcement installed inside the beam was very well protected and only lost yield stress of 17% of the initial stress, even though the beam was burned at high temperatures. The analysis of the flexural beam capacity using the stress-strain model of steel fibers at elevated temperatures shows that the results differed little from the flexural capacity of the experimental results.
Beam; Ductility; Flexure strength; Steel fiber; Temperatures
Steel fibrous concrete has been a popular material for more than three decades because of its excellent mechanical properties, such as shrinkage and a relatively low creep, high toughness and non-corrosive nature. The inter-conjugated matrix bonds in steel-reinforced concrete play a major role in maintaining the compactness of the constituents, so the steel-fiber concrete has excellent cracking strength (Jansson et al., 2012; Iqbal et al., 2015; Dhinakaran et al., 2016; Janani & Santhi, 2018). Steel fibers also play an important role in significantly increasing the ductility of concrete (Antonius, 2015; Han et al., 2015). Therefore, this material has high energy absorption properties compared to normal concrete, so it is useful when used as a structural material for earthquake resistance. However, steel fibrous concrete has a low flowable property (Madandoust et al., 2015); this can be solved by adding a Suplasticizer or Viscocrete in certain doses to improve workability.
Kodur (2014) explains that concrete material will experience quality degradation if burned at a high temperature. This can be seen from the existence of cracks and spalling in the concrete that are easy to occur, which will consequently cause the effect of plastic deformation of the steel reinforcement, which will trigger the loss of structural capacity. Moreover, Raouffard & Nishiyama (2016) found that the loss of load bearing capacity and the stiffness of normal concrete beams were 30% and 50%, respectively when exposed to fire. Evaluation of steel fiber-reinforced concrete material at high temperatures was conducted by Antonius et al. (2014). In order to develop a research database of fire-resistant steel fibrous concrete, various researches of steel fiber-reinforced concrete burned at a temperature of 800°C have been conducted (Shaikh & Taweel, 2015; Petrus et al., 2016). The results of this study essentially revealed that steel fiber in concrete content at a high temperature was relatively effective in reducing plastic shrinkage and spalling. Harshavardhan & BalaMurugan (2016) attempted to apply fibrous concrete to the structure of a nuclear building by making steel-fiber concrete with a high density. The constitutive equation of steel fibrous concrete at high temperatures has also been proposed by Blesak et al. (2016). Furthermore, in the field of confined steel fibrous concrete, Zaidi et al. (2016) also proposed a model of confinement of the material in the post-burn phase.
In brief, based on the research development described above, further research into the structural element of steel fibrous concrete is an interesting field of study; that is, the element of the fire resistance of the fiber reinforced concrete beam structure element up to 900oC. It is very important to experimentally study the behavior of the flexure and ductility of the beam, as this will provide more complete beam performance information from normal to high temperatures. Compared to normal concrete, which has gone through advanced research development and has been thoroughly explored (Jang et al., 2009; Antonius et al., 2017), research on reinforced fiber concrete, or more specifically reinforced concrete beams at normal to high temperatures (± 900oC) given a pure bending load is still rare.
This study was conducted with the main objective of comprehensively understanding the behavior of the flexure and ductility of steel fiber reinforced concrete in normal to high temperatures. The experimental program was conducted by making a beam, with the parameters reviewed including temperature (normal temperature, 300°C, 600°C and 900°C) and the longitudinal reinforcement effect (with and without compressive reinforcement in the flexural region). The steel fiber used was limited to 0.5% of the volume of concrete. The load applied to the beam was pure bending. The research is very useful as a basis for modeling the behavior of the bending and ductility of steel fiber reinforced concrete beams. Based on the results of the study, prediction of the effect of fire incidents on the behavior of steel fibrous concrete structures can be made. In addition, the results of the study are also expected to contribute to the development of design standards, such as the Indonesian National Standard (2013) for concrete structures.
The specimens of steel fiber reinforced concrete beams behave purely and did not experience shear failure. This shows the role of steel fibers in developing significant flexural deformations. The collapse pattern applies to both single-reinforcement system specimens and double reinforcement systems. The higher the temperature, the lower the decrease in the flexural beam capacity of the double reinforcement system than that of the single-beam reinforcement density system. Compared with the moment in each type of beam, the percentage of loss of flexural capacity of the beam without compressive pressure is higher. The beam without compressive reinforcement lost its ductility faster, by almost 50% (from normal temperature to a temperature of 300oC). But the ductility degradation of the beam with the compressive reinforcement was not significant (from normal up to a temperature of 600oC). The longitudinal reinforcement embedded in the steel fibrous concrete was highly protected against high temperatures, whereas the initial yield stress only lost a maximum of 17% of the yield stress at 900°C. The prediction of flexural capacity using a constitutive model of steel fibrous concrete at various temperatures is very good and relatively close to the experimental flexural capacity.
The experimental program presented in this paper is the second year of the results of the Competitive Research Program (Hibah Bersaing), Contract No. 002/006.2/PP/SP/2013, funded by the Directorate General of Higher Education, Ministry of Education and Culture, Republic of Indonesia, and conducted at the Structural Laboratory, Department of Civil Engineering, Semarang State University. The support received for this research is gratefully acknowledged.
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