Framework for the design and analysis of steel fiber reinforced self-compacting concrete structures

https://doi.org/10.1016/j.conbuildmat.2012.04.135Get rights and content

Abstract

This research focuses on the structural applications of steel fiber reinforced concrete (SFRC). Various fibers are first studied, in order to choose the most suitable type in accordance with the specific application. A steel fiber reinforced self-compacting concrete (SFRSCC) mix, designed for improved performance, was used to cast a 3-m-high, 6-m-long and 0.15-m-thick wall. The experimental tests performed on 380 specimens cut from the wall involved mechanical and non-destructive testing (NDT) procedures. Finally, a relationship between magnetic and mechanical properties was established, in order to predict softening and hardening in the flexural behavior of the composite.

Highlights

► Real-life SFRC structure (3 m-high, 6 m-long wall) is analyzed. ► An industrial application case study is proposed. ► Merging SFRC and SCC, a suitable fiber orientation is provided. ► A flexural hardening response is obtained with a low fiber dosage. ► Magnetic methods provide valuable prediction of mechanical properties.

Introduction

The use of steel fibers for reinforcing concrete is, at present, generally limited to pavement and tunnel linings. However, many studies have shown that they are suitable for the design of high-performance fiber-reinforced cementitious composites (HPFRCC) [1] or even, engineered cementitious composites (ECC) [2]. Resistance to these products coupled with the difficulties of analyzing fiber distribution and orientation on-site has hindered their entry onto the market.

Different techniques are referred to in the literature to control fiber orientation and content: from the simplest form of manual counting to complex computed tomography (CT) [3], [4]. Various image analysis [5] and X-ray methods [6] have been applied, as well as new non-destructive testing (NDT) methods. Among the latter, we may highlight alternate current impedance spectroscopy [7], [8], waveguide antennas [9], electrical resistivity measurements [10], open-ended coaxial probe reflectometry methods [11] and magnetic methods [12], [13].

Most researchers focus on the study of molded specimens [14], [15], which are not representative of real casted structures. They do, however, provide an understanding of the different parameters such as wall effects and the orientation factor [16], [17]. At present, there is a huge lack of experience in real-scale structures, with few references in the literature [18], [19], [20], [21].

The selection of the materials is one of the first variables to determine. Several researchers have performed pull-out tests on individual fibers embedded in concrete matrices to study their pull-out mechanisms and them to the macroscopical mechanical properties of the composite [22], [23]. However, there is no agreement on which fiber type might be the most suitable for each application.

Even though information about the influence of matrix strength, fiber geometry, orientation and embedded length in the debonding process and pull-out mechanism of individual fibers can be obtained, it is not suitable for design purposes in structural elements. The randomness of the position and inclination of fibers in a real case, requires regarding the material as a composite, characterized as such.

The addition of fibers to a concrete mix is known to decrease its workability [24]. This has led to new customized mix designs that consider the fibers as new slender aggregates. The aspect ratio of the fibers, grain size and coarse aggregate volume must be limited, in order to avoid fiber balling [5]. Some researchers have verified the suitability of the synergy between steel fiber reinforced concrete and self-compacting concrete technologies [25], [26].

The proposed case study analyzes cylindrical retaining walls. Having neither complicated shapes nor congested reinforcement, these walls are quite unlike two-way slabs. Their rebar quantity is oversized for crack control purposes and their low tensile stress means that the rebars may be completely replaced by fibers. This combined with the elimination of vibration can lead to increase the speed and improve the economic efficiency of the construction process. Also labor resources can be optimized and proceed in a better environment with less noise and risks.

Beyond the controlled environment of the laboratory and small-scale studies, this research establishes relevant design and control procedures under real-life building conditions on-site. A large wall made of steel fiber reinforced self-compacting concrete (SFRSCC) is casted. The performance of the tank can be assessed characterizing the behavior on multiple specimens, cut from the wall, in ultimate and serviceability limit states. Mechanical properties will be checked by means of compressive, bending and Barcelona tests, while watertightness tests will be held for service conditions. The individual behavior of each specimen was determined and correlated with the global behavior of the element. This approach provides a methodological framework, in order to select the different fiber types and concrete mixtures, test the mechanical properties of the material and provide a reliable method to control fiber distribution and orientation.

Section snippets

Materials and mix design

The test fibers shown in Table 1 are typically employed in modern construction processes. Large thick fibers were used in view of the intended structural application.

The test program performed 10 pull outs on each fiber type. The fibers were suspended from a foam core board and embedded in mortar. As fibers tend to slip away from the shortest embedded length [27], this was limited to no more than half of the fiber length. The embedded length of the fibers in the test is therefore an average

Experimental program

The mock-up wall was designed also having in mind the subsequent stage of specimen extraction. Mechanical properties were also studied by means of compression (UNE 83507) and Barcelona tests [36] in cubic specimens and a bending test in prismatic specimens (UNE-EN 14651). For one in two rows, three-point bending tests where carried out on the prismatic specimens. The specimens were notched and tested through crack width control. The serviceability of the concrete for structural applications was

Non-destructive testing

At first sight, the preferential orientation of the fibers may be observed and an intuitive idea is gained of the different distributions throughout the wall. A light segregation of aggregates occurred due to the priming water of the pump. Nevertheless, this helped obtain a proper calibration curve at different densities within the same element. As shown in Fig. 6, the fibers tended to orient in horizontal directions along the length of the wall, according to the mass flow. Fewer fibers were

Conclusions

The paper has presented a methodological framework for the design of SFRSCC and its analysis in a structural application of the industrial sector. Fiber reinforced and self-compacting technologies are merged for optimum performance in circular retaining walls. Once structural behavior is known by means of classical structural analysis tools, most suitable fiber type and mix design are chosen. The structural safety is verified testing specimens cut from a real-scale wall, unlike other studies

Acknowledgements

The authors gratefully acknowledge the support obtained from the Spanish Ministry of Science and Innovation and the Regional Government of Biscay through MIVES IV ref: BIA 2010-20789-C04-04 and BIRGAITEK 7-12-TK-2009-10 grants, respectively. The first author also gratefully acknowledges the participation of Tecnalia, ArcelorMittal-Wire Solutions and Financiera y Minera (Italcementi Group) in the experimental phase and shows his appreciation to Antonio Aguado, Josep Maria Torrents, Ana Blanco

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