Steel fiber reinforced concret

Steel Fiber Reinforced Concrete (SFRC)

Introduction

Over the last years experimental testing on concrete has proven that steel fiber reinforcement provides concrete many benefits, such as:

  • Flexural strength
  • Fatigue resistance
  • Impact resistance
  • Elimination of shrinkage cracks
  • Abrasion resistance
  • Permeability

Though the structural behavior of reinforced concrete beams with steel fibers has been thoroughly investigated for isostatic beams, limited information on the behavior of SFRC beams is provided for continuous (hyperstatic) beams (fig 1). This may sound strange as hyperstatic structures are everywhere from continuous beams of cargo piers to continuous slabs and from non-sway multi storey buildings to the Eiffel tower. Most of the times hyperstatic structures are constructed to put up with heavy loads and a prominent failure probability may be assessed as subjective predicted degree of dangerous event occurrence which cannot be observed frequently, prescribed by Kudzys, Kliukas & Kudsys[1].

Focusing on research

So far, the amount of the accomplished experiments is not big enough to exclude safe results. As the potential damage of a structure should be limited reducing the hazards which their members are to sustain mainly during construction, the lab results should be more thorough. On the other hand, failure may not only occur by the human factor and poor engineering design but also from imperfect recommendations and directions in design codes and standards. It is our goal here, at the University of Luxembourg to ameliorate, via experiments, the formulas to produce fiber reinforced concrete with or without steel bar reinforcement. As a fact, steel fiber reinforced concrete has already been introduced to the University's Research Unit in Engineering Science and many steps forward have already been made. Now is the time to proceed on further thorough investigation on the properties of SFRC with, or not, the admixture of steel bars.

In-situ reinforced concrete beams are often continuous over several spans. These are easier to construct than a series of simply-supported beams, and avoid problems of movement between the ends of such beams.

Broaden testing proposal

A well worth prospect is to test differently formulated types of hyperstatic concrete beams. The idea is to produce specimens of plain concrete, steel bar reinforced concrete, steel fiber reinforced concrete and finally a mixture of steel fibers and steel bars. The latter will be thoroughly investigated and different percentages of fibers and bars (diameters) will be as well added. The results will prove the different properties of the mixture and propose the best utilization for each one.

The specimens' size does not need to be really big, although some bigger specimens could be useful, as we are more interested in quantity. The more series of tests we make the safer the results we get. A 10x10x90cm specimen with a span of 80cm will be efficient for testing as proposed by Hofstetter, Peer, Niederwanger & Frische[2]. The specimen will be loaded to the hydraulic pressure machine and an increasing load will be affecting the matrix. Displacement and total load capacity will be monitored.

Interesting would also be the way fibers disperse thought the matrix. Since steel fibers reinforce in three dimensions throughout the entire matrix, they restrain micro-cracking and act as tiny reinforcing bars. The earlier a crack is intercepted and its' growth inhibited, the less the chance it will develop into a major problem. The dilemma lays in the interference between steel bars and steel fibers. Questions like the following rise:

  • What will be the increase on chemical additives (stabilizer, plasticizer) in order to keep the mixtures workability at a high level?
  • Will the bars tend to keep the fibers from dispersing homogenously?
  • If two different types of fibers are used in the same total percentage, will the total capacity load and displacement result be the same?
  • How much does the diameter and length of the fiber affect the properties of the concrete?

These are only a few questions that are looking for an answer during our experiments. Technical difficulties and failures will also be noticed and appreciated in order to provide us with experience and understanding of the SFRC behaviour.

References

  1. Kudzys, A., Kliukas, R. & Kudzys, A. (2007) On design feature of propped and unpropped hyperstatic structures, Lithuania: Journal of Civil Engineering and Management.
  2. Hofstetter, G., Peer, B., Niedeiwanger, G. & Fritsche G. (2005) Numerical simulation of ultimate load tests on fibre reinforced beams, Barcelona: VIII International Conference on Computational Plasticity.

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