Effect of Steel Fibers on the Engineering Performance of Concrete

INTRODUCTION

The main concern about concrete is its brittleness while its strength is being increased. The higher the strength of concrete, the lower is its ductility. This inverse relation between strength and ductility is a serious drawback when using concrete is of concern. A compromise between these two conflicting properties of concrete can be obtained by adding short fibers (Bayramov et al., 2004). Therefore, it becomes a more significant problem to improve the ductility of concrete. The use of steel fiber reinforced concrete has steadily increased during the last three decades. Considerable developments have taken place in the field of steel fiber reinforced concrete as reported by Bentur and Mindess (1990). The field of application of steel fiber reinforced concrete may include highway and airfield pavements, hydraulic structures, tunnel linings etc. Additionally, the composite has potential for many more applications, specially, in the area of structural elements (ACI Committee 544, 1993). The addition of steel fibers significantly improves many of the engineering properties of mortar and concrete, notably impact strength and toughness. Flexural strength, fatigue strength, tensile strength and the ability to resist cracking and spalling are also enhanced (Nataraja et al., 1999).

WORKABILITY AND FLOW ABILITY OF STEEL FIBER CONCRETE MIXES

The use of fibers is known to affect the workability and the flow characteristics of plain concrete essentially. Many researches investigated the effect of the aspect ratio and volume content on the flow ability of concrete (Noor et al., 2006). The higher the aspect ratio is, the fewer fibers could be included to surpass the critical fiber content. For the same fiber content, better workability is achieved at lower aspect ratios (Kareem and Naranyanan, 1983).

A mix design method is proposed for fiber reinforced concrete. First, their assumption was that the most workable concrete would be achieved if its granular skeleton would be optimized. They found that the optimum aggregate proportions were independent of the volume and properties of the paste. After varying the ratio of the sand to aggregate by trial and error, an optimum content of sand was found to achieve the best workability. This ratio depended on the type and amount of fibers. Next, an adjustment of the water, cement and superplasticizer has to be performed to attain the desired workability. Usually, the cement content has to be adjusted if a higher sand content was applied. Because fibers are known to affect the workability of concrete, the question arises whether the fibers are detrimental to the workability of SCC. Steel fibers with a longer shape than aggregate of the same volume have the higher specific surfaces. The degree to which workability decreases depends on the type and content of fibers used, on the matrix in which they are embedded and the properties of the constituents of the matrix on their own. A high content of fibers is difficult to distribute uniformly; a good distribution, however, it’s required to achieve optimum benefits of the fibers (Sedran and Larrad, 1999).

Grumewald and Walraven (2001) investigated an experimental study on the effect of four types of steel fibers at different contents on the workability of two Self Compacted Concrete (SCC) with different compositions. It was found that the flow behavior of fiber reinforced mixtures differs from that of plain SCC. The observations also indicated that when a homogenous distribution was achieved with critical fiber content, the formation of a stiff structure of the granular skeleton will make the flow under concretes’ own weight almost impossible. It was found that a considerable amount of fibers allowed self-compacting behavior. However, the composition of the reference mix would have some influences on the maximum possible fiber content.

THE EFFECT OF STEEL FIBER ON COMPRESSIVE, SPLITTING TENSILE AND FLEXURAL STRENGTH OF CONCRETE

The compressive strength test is considered the most suitable method of evaluating the behavior of SFRC for underground construction at an early age, because in many cases (such as in tunnels) SFRC is mainly subjected to compression (Ding and Wolfgang, 2000). Many researchers believed that steel fibers do not have the significant influence on the compressive behavior of concrete due to the small volume of fibers in concrete mix (Armelin and Helene, 1995). In general, the effect of addition of steel fibers on compressive strength ranges from negligible to marginal and sometimes up to 25% as reported by Balaguru and Shah (1992).

Nagarkar et al. (1987) indicated that the compressive strength, splitting tensile and flexural strengths increase with increasing fibers content. The compressive, splitting tensile and flexural strength increased by 13-40% for fibrous concrete containing steel fibers with aspect ratio of 105 at 0.5% volume fraction. Dawood and Ramli (2010) had investigated the effect of steel fiber content with different percentages of steel fiber from (0-2%) on the flowable mortar. The results indicated that the compressive strength has increased by 21% as the steel fiber fractions was 1.25%. On the other hand, the flexural strength results recorded a significant increase of about 200% by the inclusion of steel fiber up to 1.75%. These results according to the authors are related to the improvement of mechanical bond between the cement paste and the steel fibers when the flow of mortar is adequately applied.

THE EFFECT OF STEEL FIBER ON TOUGHNESS AND IMPACT CAPACITY OF CONCRETE

Toughness is a measure of the ability of the material to absorb energy during deformation estimated using the area under the stress-strain curves (Balaguru and Shah, 1992; Dawood and Ramli, 2009a). The main contribution of steel fibers to concrete can mainly be observed after matrix cracking. If a proper design is made, after the matrix cracking, randomly distributed, short fibers in the matrix arrest microcracks, bridge these cracks, undergo a pull-out process and limit crack propagation (Banthia and Trottier, 1995; Kurihara et al., 2000). ASTM (1997) C-1018 proposed a method for evaluating the toughness indices and JSCE-SF4 (1984) recommends determining the flexural toughness factor.

To design and analyze structures using steel fiber reinforced concrete for compression, the stress-strain behavior of the material in compression is needed. While the compressive strength is used for the strength calculation of the structural components, the stress-strain curve is needed to evaluate the toughness of the material for consideration of ductility (Bayramov et al., 2004).

Nataraja et al. (1999), Banthia and Sappakittipakorn (2007) and Dawood and Ramli (2009b) studied the affect of steel fibers on toughness. It can be observed that the toughness improves with the increasing content of fibers, the reason being the ability of fibers in arresting cracks at both micro-and macro-levels. At micro-level, fibers inhibit the initiation of cracks, while at macro-cracks; fibers provide effective bridging and impart sources of toughness and ductility.