Flow characteristics and acceptance criteria of fiber-reinforced self-compacted concrete (FR-SCC)

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Abstract

Self-compacting concrete (SCC) possesses exceptional flowability characteristics in its fresh state. While fibers are specified for their ability to limit concrete shrinkage cracks at early age and to enhance some of the concrete properties, inclusion of such fibers is expected to affect the flowability characteristics of SCC. This study investigates how the inclusion of fibrillated polypropylene fibers with different fiber content and the inclusion of steel fiber types with different aspect ratio and volume content affect the flowability of SCC. The flow characteristics were assessed by considering the slump flow test, V-funnel test and filling box test. It was found that it is quite possible to achieve self-compacting properties while using fiber reinforcement. While the mix composition and fiber type can greatly influence concrete flowability, there exists a maximum fiber content that could be used to produce fiber reinforced self-compacting concrete (FR-SCC). Moreover, our observations were used to develop a new acceptance criterion for FR-SCC.

Highlights

► Investigate the flowability characteristic of SCC including different fibers. ► Study the effect of using different content of fibrillated polypropylene fiber. ► Study the effect of using steel fiber with various aspect ratio and volume content. ► New acceptance criteria for fiber-reinforced self-compacting concrete.

Introduction

The term “self-compacting concrete” (SCC) identifies a category of concrete that can be cast into a framework and fill it completely under its own weight without the need of any type of compacting or external vibration. SCC also has a great resistance to segregation and a high ability to flow around obstacles such as reinforcements or narrow sections. Since its development in the mid 1980s, various investigations have been carried out and this type of concrete has been used in several structures worldwide, culminating in the establishment of international guidelines [1], [2], [3], [4]. SCC offers many benefits and advantages over conventional concrete. Such advantages include an improved concrete quality, fast rate of construction and low overall cost. Most of the hardened concrete properties of SCC are similar to those of conventional concrete, and most of the structural design requirements were reported to be unchanged [5]. Because SCC mixes usually have low water to cement ratio, SCC tends to be stronger, less permeable and eventually more durable compared with normal vibrated concrete. Such characteristics make it possible to produce durable structures when using SCC independent of on-site conditions relating to the quality of labor, casting and compacting systems available.

The inclusion of fibers in SCC will extend its benefits. Fibers bridge cracks and retard their propagation and thus improve the tensile and flexural strength and fracture toughness of hardened concrete. Therefore, the use of fibers may extend the possible fields of application of SCC. However, fibers are also known to affect the workability and flow characteristics of fresh concrete. The degree to which workability decreases depends on the type and content of fibers used and on the matrix and its constituents in which they are embedded. A good fiber distribution, however, is necessary in order to achieve optimum benefits of the fibers. Ultimately, a compromise between acceptable workability of fresh concrete and improved performance of hardened concrete has to be made. Moreover, it is well understood that the reduction of workability due to the addition of fibers is mainly dependent on the type, shape and amount of fibers used. The content of fibers that can be used while the mix is still workable depends entirely on the mixture composition and the type of fiber.

Because fibers are known to affect the workability of concrete, the question arises whether the fibers are detrimental to the workability of SCC and whether it is possible to maintain the high workability of SCC while using fiber reinforcement in the mixture. Recently, several investigations were carried out to examine the ability of incorporating fibers into SCC mixtures [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. It was generally concluded that including fibers will extend the technical benefits and the application possibilities of SCC [6], [9], [13], [15]. Moreover, it was concluded that the flow characteristics of fiber-reinforced self-compacting concrete (FR-SCC) differ from that of plain SCC [6], [10], [14] and the flow ability is impaired when fiber volume and/or aspect ratio increases [16]. Several factors were found to affect the amount of fibers that could be included without affecting the flow characteristics of SCC, such as original mix composition, fiber type, and fiber geometry (i.e. length and aspect ratio) [6], [9], [10], [11], [12], [13], [14]. Other factors were also identified but could not be quantified such as fiber shape and surface roughness [12]. It was also shown that some common testing techniques used for assessing plain SCC might not be suitable for FR-SCC [14]. Also, the flow through restricted space of SCC including fibers is considered more important than just judging its unrestricted flow. This might lead to increasing the free bar spacing compared with plain SCC in order to avoid blocking [6], [11]. Therefore, the inclusion of fibers needs to be carefully optimized and the flow characteristics of the FR-SCC shall be properly evaluated. While all investigations examined the effect of different types of fibers on SCC, there is very limited information about the maximum fiber content to be used without affecting fundamental SCC flow characteristics (flowability, viscosity, filling capacity and passing ability).

This study investigates and quantifies how the inclusion of different types of fibers affects the flow characteristics of self-compacting concrete (SCC). It was also aimed to determine the maximum fiber content that can be used without affecting the flowability of SCC. Workability for FR-SCC was evaluated from all aspects; flowability, viscosity, filling capacity and passing ability through reinforcement (i.e. risk for blocking). Experimental investigations were carried out on several concrete mixes in order to map the dependence of the workability of SCC on mix composition, type and amount of fibers. Two types of fibers were used in this study; fibrillated polypropylene fibers and steel fibers with various aspect ratios. A new acceptance criterion for FR-SCC is suggested. The effect of mix composition on the maximum fiber volume of each fiber type is also identified.

Section snippets

Materials

Cement used in this study was ordinary Portland cement (ASTM Type I). Standard silica fume was used as mineral additive; it had a specific gravity of 2.2. Chemical composition of used cement and silica fume is given in Table 1. The specific surface of the silica fume was 15.2 m2/gm. The coarse aggregate was natural siliceous gravel of nominal size of 19 mm, a specific gravity of 2.63, absorption% of 0.9%, and Los Angeles value of 14.3%. The sand used was natural siliceous sand with fineness

Results and discussions

For polypropylene fibers, the fiber volume is used to compare the fiber effect on the SCC flowability characteristics. For steel fibers a “fiber factor” (Vf * L/D) is used to compare the effect of fiber on SCC flowability. The use of the fiber factor (Vf * L/D) has been adopted by other researchers to examine the combined effect of fiber volume and aspect ratio on concrete characteristics [6]. Besides the quantitative measurements of the different flowability characteristics, the distribution of

Conclusions

Experimental investigations of SCC including fiber reinforcement have showed that it is possible to maintain self-compacting flow characteristics while using fiber reinforcement. The maximum fiber content that can be used without affecting the flowability characteristics of SCC has been determined. It is evident that the mix compositions mainly paste and mortar volume and the fiber type greatly influence the maximum possible fiber content.

The experimental investigations have also shown that the

Acknowledgment

The authors would like to acknowledge the help by Mrs. E. Khattab in performing experiments of reference mixes. The financial support by home institutions for both authors is greatly acknowledged.

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