Steel Fiber Reinforced Concrete (SFRC) Slab: Design and Considerations

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Steel-fiber-reinforced concrete is a state-of-the-art composite material made of hydraulic cements, fine or coarse aggregate, and a dispersion of discontinuous steel fibers.  It may contain pozzolans and additives commonly used with conventional concrete.   The amount of fiber in concrete mixes typically ranges from 0.5 percent to 2.0 percent by volume, although smaller amounts have been used successfully in reduction of plastic- and drying-shrinkage cracking. The main effect of adding steel fibers in concrete, that is also the main advantage of SFRC and the most useful regarding design of hyperstatic construction (like slab on ground), is its post-crack behavior or toughness of SFRC.

Following are important steps in a SFRC floor project process that people involved should understand, be careful and follow in order to work out a performance based floor:

1) Preparation and Design phase

  • Understanding of SFRC and Evaluation of steel fibers and SFRC performance
  • Slab on ground design using toughness value
  • Specifications based on toughness value design

2) Construction phase

  • Pre-construction meetings
  • Utilization of SFRC (concrete mix, fibers mixing and control, use of superplasticizer, etc)
  • Quality control and testing
  •  Surface finishing

 Steel Fibers and Performance of SFRC for Slab on grade Structures

The performance of a type of steel fibers can be determinate by these three parameters :

  • The tensile strength
  • The anchorage (type and efficiency)
  • The length to diameter ratio : Aspect Ratio fiber geometry

The performance of a steel fiber in concrete will depend of each these parameters but also of their interaction. For instance, a fiber with high tensile strength steel but with a bad anchorage in concrete will probably not perform as the steel tensile strength could permit. These 3 parameters will give a toughness value at a certain dosage.

However, for different dosages (amount / volume of concrete), the toughness value for a specific steel fiber will vary. So, the performance of a SFRC will depend of:

  • The type of fiber (L/D ratio, tensile strength and anchorage)
  • The dosage of steel fibers per volume
aspect ratio performance

Toughness of SFRC :

The main effect of adding steel fibers in concrete, that is also the main advantage of SFRC and the most useful regarding design of hyperstatic construction (like slab on ground), is its post-crack behaviour or toughness of SFRC. Steel fibres in concrete start acting when the first crack appears and have the ability to absorb and redistribute the loads (or energy), so that the SFRC will still be able to bear loads even after the formation of cracks. In fact, SFRC has a ductile behaviour or toughness and therefore, that surplus of flexural capacity from the plastic phase (post-crack ductility) can be used for design of structure when deformation must be controlled like slabs or for structures where deformations controlled the design like underground linings. It is the reason why, for the same thickness, a SFRC slab on ground can support higher loads than a conventional concrete slab.



Concrete Society Report TR34- Concrete industrial ground floors

In the Appendix F “Thickness design of ground-supported slabs usingfiber concrete.” of the TR34 Report of the Concrete Society (1995), we can find a method for designing SFRC slab on ground. The method is based on the work of Meyerhof and the yield line theory as an alternative to the Westergaard approach. The Meyerhof design method used for SFRC is based ultimate bearing capacity is estimated on the assumption of a rigid plastic slab resting on an elastic subgrade.

As we have seen before, the toughness of SFRC concrete will be effective after the first crack, so in the plastic phase. With a SFRC slab on ground in the plastic phase, a plastic hinge is formed under load. Due to the post-crack behaviour of SFRC, the slab still can take bending moments. The higher the toughness value is, the higher the residual strength is in the plastic phase. The yield line theory calculates the collapse load as a function of the sum of the two maximum bending moments:


MD : Bending moment for design

M0 : Bending moment that can be taken with the concrete in the elastic phase

MF : Bending moment that can be taken with the steel fibers in the plastic phase

In the yield phase of the SFRC, it is the interaction of the positive and negative moments as a function of the thickness of the slab and the sub-base reaction that determines the final design of the slab. The design flexural stress is then calculated using the following formula:


Fd: Design flexural stress

Fct: Concrete design flexural stress

Re3 : Toughness value or residual strength factor (can be R10,50 )

This explains why the stresses may seem high when compared to those used with the elastic method of Westergaard. Therefore, when using those two formulas in accordance with Meyerhof/Yield line theory design equations, you can find the slab thickness and, at the same time, the toughness value required. Then, you can choose the dosage and type of steel fibre required that will be the best economical and technical option from the Re3 tables to evaluate and compare the performance of steel fibres. (JSCE-SF4) 


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