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Guidance for the Design of Steel-Fibre-Reinforced Concrete by The Concrete Society | PDF Free Download.
Concrete is strong in compression but weak in tension. In structural applications, this is overcome by providing steel reinforcing bars to carry the tensile forces once the concrete has cracked or by prestressing the concrete so that it remains largely in compression under load.
In reinforced concrete, the tensile failure strain of the concrete is significantly lower than the yield strain of the steel reinforcement and the concrete cracks before any significant load is transferred to the steel.
In addition to providing sufficient area of steel to carry the tensile stresses at the ultimate load that is to be applied to the member, the reinforcement is detailed so as to limit the width of the cracks under serviceability conditions to specified levels.
In some applications, a nominal or minimum amount of steel is provided to prevent uncontrolled crack development or to prevent failure in the event of an accidental overload. In such cases crack widths cannot be predicted accurately.
Steel fibers mixed into the concrete can provide an alternative to the provision of conventional steel bars or welded fabric in some applications.
The concept has been in existence for many years (the first patent was applied for in 1874) and it has been used in a limited range of applications: among the first major uses was the patching of bomb craters in runways during World War II.
However, it was during the 1970s that commercial use of this material began to gather momentum, particularly in Europe, Japan, and the USA.
Today, industrial floors and pavements are major applications for steel-fiber-reinforced concrete. In the United Kingdom, several million square meters of steel-fiber-reinforced slabs have been installed over the past ten years, both for ground-supported and pile-supported floors.
Other major applications for fiber-reinforced concrete include external paved areas, sprayed concrete, composite slabs on steel decking, and precast elements.
Fibers are often used to replace the nominal conventional steel fabric in ground-bearing slabs. Increasingly, steel fibers are being used in suspended ground floor slabs on piles to replace much, and in many cases all, of the reinforcement.
Savings in the cost of supplying and fixing the conventional welded fabric reinforcement that is replaced can offset the extra cost of adding fibers to the concrete. There may also be health and safety benefits resulting from the reduced handling of reinforcement.
In addition, problems caused by the displacement of conventional steel in the depth of the slab are avoided. Although steel fibers are widely used in the UK and elsewhere, clear information is still lacking about the nature, use, and properties of fiber-reinforced concrete.
This document is intended to provide an introduction to this type of reinforcement, with guidelines on design and application.
Although steel-fiber-reinforced concrete (SFRC) has been used in the UK and elsewhere for a number of years, there are no agreed design approaches for many of the current applications.
This differs from conventional reinforced concrete using steel bars or welded fabric, which has been covered by national and international design codes for many years.
One example is the design of pile-supported floors, which are widely used for industrial buildings, warehouses, and similar applications, for which various fiber manufacturers and specialist contractors have produced guidelines.
RILEM (the International Union of Laboratories and Experts in Construction Materials, Systems, and Structures) published a design method for steel fibers combined with reinforcement in 2003(').
This used the draft Eurocode 2, ENV 1992-1-V2), as a framework but modified it to reflect the behavior of fiber-reinforced concrete observed in beam tests.
In the Netherlands, CUR (the Centre for Civil Engineering Research and Codes) is preparing recommendations for SFRC industrial floors on piles(3); this guide will be restricted to applications beneath which there will be neither occupancy nor crawl spaces.
This restriction is presumably aimed at limiting the risk to life from a failure. It should be noted that in some cases failures of such floors could still lead to a risk to life due to the collapse of supporting structures or fittings.
The choice of design method should take into account this risk. This Technical Report summarises the wide range of current applications for SFRC, including ground-supported and pile-supported slabs, sprayed concrete, composite slabs on steel decking, and precast units.
The Report also considers practical aspects such as production and quality control. Where possible it presents a detailed review of the design methodologies currently in use, with the aim of promoting an understanding of the technical issues involved.
Normally, it is not possible to give definitive design guidelines but the information provided will allow the designers to exercise judgment in this area of evolving technology. In general, the concrete in these applications has a fiber content of around 40kg/m3.
Elevated suspended slabs with the dosage in the region of 100kg/m3 have been built but elements with such dosages are outside the scope of this Report, as are ultra-high performance concreted4) which are highly specialized materials that may have a dosage of 150kg/m3 or more.
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