Steel Fibre Reinforced Concrete : Application and Performance Highlights · Steel Fibre Reinforced Concrete : Application and Performance Highlights S teel Fibre reinforced concrete

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Steel Fibre Reinforced Concrete : Application and Performance Highlights

Steel Fibre reinforced concrete (SFRC) is defined as concrete made with hydraulic cement containing fine and coarse aggregate and discontinuous discrete fi-

bre. In SFRC, thousands of small fibres are dispersed and distributed randomly in the concrete during mixing, and thus improve concrete properties. SFRC is being increasingly used to improve static and dynamic tensile strength, energy absorbing capacity and better fatigue strength. It is now well established that one of the important properties of steel fi-bre reinforced concrete (SFRC) is its superior resistance to cracking and crack propagation. As a result of this ability to arrest cracks, fibre composites possess increased extensi-bility and tensile strength, both at first crack and at ultimate, particular under flexural loading; and the fibres are able to hold the matrix together even after extensive cracking. The net result of all these is to impart to the fibre composite pro-nounced post – cracking ductility which is unheard of in ordi-

nary concrete. The transformation from a brittle to a ductile type of material would increase substantially the energy ab-sorption characteristics of the fibre composite and its ability to withstand repeatedly applied, shock or impact loading.

Effects of Steel Fibres in Concrete

Fibres (Refer Figure 1(a)) are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibres produced greater impact, abrasion and shatter resistance in concrete. Generally fibres do not increase the flexural strength of con-crete and so cannot replace moment resisting or structural steel reinforcement. Indeed, some fibres actually reduce the strength of concrete. The amount of fibres added to the concrete mix is expressed as a percentage of total volume

Sonjoy Deb, B.Tech, Civil Associate Editor

CONCRETE: SFRC

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of the composite (concrete and fibres), termed volume frac-tion (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fibre length (l) by its diameter (d). fibres with a non circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fibre is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fibre usually segments the flexural strength and the toughness of the matrix. However, fibres which are too long tend to ball in the mix and create workability problems. Some recent research indicated that using fibres in concrete has limited effect on the impact resistance of the materials. This finding is very important since traditionally, people think that the ductility increases when concrete is reinforced with fibres. The results also indicated out that the use of micro fibres offers better impact resistance compared with the longer fibres.

use of steel fibres in concrete generally reduces the slump by about 50 mm. To overcome this and to improve work-ability, it is highly recommended that a superplasticizer be included in the mix. This is especially true for SFRC used for high performance applications. Generally, the ACI Commit-tee Report No. ACI 554 ‘Guide for Specifying, Mixing, Placing and Finishing Steel Fibre Reinforced Concrete’ is followed for the design of SFRC mixes appropriate to specific applica-tions. Refer Figure 2(b) for steel fibre mixed concrete.

Figure 1 (a) : Typical Steel Fibre

The randomly-oriented steel fibres assist in controlling the propagation of micro-cracks present in the matrix, first by improving the overall cracking resistance of matrix itself, and later by bridging across even smaller cracks formed af-ter the application of load on the member, thereby prevent-ing their widening into major cracks (Refer Figure 1 below).

The idea that concrete can be strengthened by fibre in-clusion was first put forward by Porter in 1910, but little prog-ress was made in its development till 1963, when Roumaldi and Batson carried out extensive laboratory investigations and published their classical paper on the subject. Since then, there has been a great wave of interest in and applications of SFRC in many parts of the world. While steel fibres im-prove the compressive strength of concrete only marginally by about 10 to 30%, significant improvement is achieved in several other properties of concrete as listed in Table. Some popular shapes of fibres are given in Figure 2 (a).

Typical Mix Design of SFRC

Typical mix proportions for SFRC will be: cement 325 to 560 kg; water-cement ratio 0.4-0.6; ratio of fine aggregate to total aggregate 0.5-1.0; maximum aggregate size 10mm; air content 6-9%; fibre content 0.5-2.5% by volume of concrete. An appropriate pozzolan may be used as a replacement for a portion of the Portland cement to improve workability fur-ther, and reduce heat of hydration and production cost. The

Figure 1 (b) : Effects of Steel Fibre in Concrete Crack Prevention

Figure 2 (a): Different types of steel fibres Figure 2(b) : Steel Fibre Mixed Concrete

Mechanical properties and strength Aspect of SFRC

One needs to understand that although all the different types of fibres mentioned previously work in improving the properties of concrete in some way; they do so with varying degrees of performance. For example, and not all of them fulfil the requirements on field and lack technical details that a designer needs to assess the fibre performance in the structure. The idea is to have an “engineered fibre” not just any alternative to make the solution work in a manner it is envisioned to in the structure.

The various properties of SFRC’s can be seen in the fol-lowing figures. Relative strength and toughness of the fibre reinforced mortar and concrete can be seen in Figure 3 As the percentage of fibres increases, the strength and toughness of fibre concrete increases. The increase in toughness and the effect of aspect ratio can be seen in Figure 4. The effect of different types of fibres on the uniaxial tensile strength is presented in Figure 5. The variation of compressive strength and the strain is shown in Figure 6. The strain of SFRC corre-sponding to peak compressive strength increases as the vol-ume fraction of fibres increases. As aspect ratio increases, the compressive strength of SFRC also increases marginal-ly. The load vs deflection of SFRC beam subjected to bending is presented in Figure’s 7 and 8. As the load increases, the deflection also increases. However the area under the load – deflection curve also increases substantially depending the type and amount of fibres added.

To summarize the above discussion and to add more, the properties & performance of concrete Improved by addition of Steel Fibres are as mentioned below:-

- Flexural Strength: Flexural bending strength can be in-creased of up to 3 times more compared to conventional concrete.

- Fatigue Resistance: Almost 1 1/2 times increase in fa-tigue strength.

- Impact Resistance: Greater resistance to damage in case of a heavy impact.

- Permeability: The material is less porous.- Abrasion Resistance: More effective composition against

abrasion and spalling.

CONCRETE: SFRC

Figure 3 : Relative strength vs percentage of aligned fibres

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- Shrinkage: Shrinkage cracks can be eliminated.- Corrosion: Corrosion may affect the material but it will

be limited in certain areas.

Applications of SFRC

The applications of SFRC depend on the ingenuity of the designer and builder in taking advantage of its much en-hanced and superior static and dynamic tensile strength, ductility, energy-absorbing characteristics, abrasion resis-tance and fatigue strength. Growing experience and confi-dence by engineers, designers and contractors has led to many new areas of use particularly in precast, cast in-situ, and shotcrete applications. Traditional application where SFRC was initially used as pavements, has now gained wide acceptance in the construction of a number of airport run-ways, heavy-duty and container yard floors in several parts of the world due to savings in cost and superior performance during service.

rial costs of the concrete, and this has tended to limit the use of SFRC to special applications.

The six major areas in which Steel Fibres can be used to achieve hi-strength, durable and economical concrete are:

Overlays

Roads, Airfields, Runways, Container, Movement and Storage Yards, Industrial Floors and Bridges. (Refer Figure 9)

Hydraulic and Marine Structures

Dams, Spillways, Aprons, Boats and Barges, Sea Protec-tion Works.

Defence and Military Structures

Aircrafts Hangers, Missile and Weaponry Storage Struc-tures, Blast Resistant Structure, Ammunition Production and Storage Depots, Underground Shelters etc.

Shotcreting Applications

Tunnel Linings, Domes, Mine Linings, Rock-Slope Stabi-lization, Repaint and Restoration Distresses Concrete Struc-tures etc. (Refer Figure 10)

Special Structures

Machine Foundations, Currency Vaults and Strong Rooms, Impact and Fibre Protective Shells and Lost Forms, Column-Beam Joints in Seismic-Resistant Structures, End Zones of Prestressed Concrete Elements, High Volume Steel Fibre Reinforce Concrete structures made out of SIFCON and CRC (Slurry Infiltrated Fibre Concrete and Compact Reinforced Concrete). (Refer Figure 11 for major structures built with SFRC).

Precast Concrete Elements

SFRC benefits made it a very important technology in precast industry. Fibre material and type have a pronounced effect on shrinkage cracking. Steel fibres are the most ef-fective in inhibiting crack growth. Precast concrete products are susceptible to degradation as a result of sulfate attack, freeze-thaw cycling, alkali-silica reaction, and corrosion of embedded reinforcing bars, if present. In all of these cases, permeability to water plays an important part. Steel Fibre reinforcement improves crack resistance, increases the surface roughness of cracks, and promotes multiple-crack development, thereby signifcantly reducing the permeabil-ity of concrete in service. In case of stresses and stress-in-duced cracks, results have shown that cracks dramatically increase the permeability of plain concrete, while the per-meability of fibre-reinforced concrete remains far below that of plain concrete under service conditions. Corrosion of steel reinforcing bars in precast concrete remains a major concern. Chloride contamination of concrete is usually to blame, and the mechanisms by which chloride ions promote reinforcing bar corrosion in concrete are well understood. As indicated before, fibers decrease water permeability in both stressed and unstressed concrete and, hence, slow the rate of chloride diffusion. The inclusion of steel fibre in con-crete could be a feasible solution for prolonging the life of

The uses of SFRC over the past thirty years have been so varied and so widespread, that it is difficult to categorize them. The most common applications are pavements, tun-nel linings, pavements and slabs, shotcrete and now shot-crete also containing silica fume, airport pavements, bridge deck slab repairs, and so on. There has also been some re-cent experimental work on roller-compacted concrete (RCC) reinforced with steel fibres. The list is endless, apparently limited only by the ingenuity of the engineers involved. The fibres themselves are, unfortunately, relatively expensive; a 1% steel fibre addition will approximately double the mate-

CONCRETE: SFRC

Figure 4 : Toughness and strength in relation to plain concrete

Figure 5 : Uniaxial tensile strength vs strain for different FRCs

Figure 6 : Compressive strength vs strain diagram for SFRC

Figure 7 : Schematic load-deflection curves for fibre composites in bending

Figure 8 : Schematic load-deflec-tion curves for fibre composites in bending

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concrete structures. Refer Figure 12 for some applications of SFRC in Precast industry.

Few Applications of SFRC in India

Fibre reinforced concrete is in use since many years in India, but the structural applications are very much limited. However, its application is picking up in the recent days. Fol-lowing are some of the major projects where large quantities of steel fibres are used.

1. More than 400 tones of Shaktiman Steel Fibres have been used recently in the construction of a road overlay for a project at Mathura (UP).

2. They have also been successfully used at the end an-chorage zones of prestressed concrete girders for re-sisting bursting and spalling forces in bridge projects in Bangalore and Ahmedabad executed by one of the re-puted construction companies.

3. The fibres have also been used for heavy-duty industrial floors.

4. Other projects include Samsonity Factory-Nasik, BIPL Plant-Pune, KRCLMSRDC tunnels, Natha Jakri Hydro Electric Plant, Kol HEP, Baglihar HEP, Chamera HEP, Sala HEP, Ranganadi HEP, Sirsisilam project, Tehri Dam project, Uri Dam Project, etc.

5. Used in many tunnelling projects and for slope stabilisa-tion in India.

The Principal Advantages of SFRC

The principal advantages of SFRC are listed :

- Cost savings of 10% - 30% over conventional concrete flooring systems.

- Reinforcement throughout the section in all directions versus one plane of reinforcement (sometimes in the

sub-grade) in only two directions.- Increased ultimate flexural strength of the concrete

composite and thus thinner sections.- Increased flexural fatigue endurance and again thinner

slabs. - Increased flexural toughness, or the ability to absorb energy. - Increased impact resistance and thus reduced chipping

and joint spalling. - Increased shear strength and thus the ability to transfer

loads across joints in thin sections. - Increased tensile strength and tensile strain capacity

thus allowing increased contraction/construction joint spacing

Figure 9 : Storage Yards / Industrial Floors using SFRC

Figure 10: Tunnel Lining Using SFRC (KRCL-MSRDC Tunnel)

Figure 11 : Major Structures Built With SFRC

CONCRETE: SFRC

Figure 12 : Precast Railway Track, Segmental Tunnel Lining & Sewer Pipes made using Steel Fibre Reinforced Concrete.

Conclusion

Steel fibres have been in prevalence elsewhere in the world for over four decades in various applications. Reduced construction time, simplified reinforcement drawings, no stockyard, enhanced job safety and increased durability and ductility are only some main benefits of SFRC, which are mentioned in that context. The increasing interest in the use of steel fibre reinforcement has created a need for established design and analysis methods. Fibre reinforcement is mainly used in applications such as industrial floors and sprayed concrete, although other application areas exist. Apart from increased load-carrying capacity, one of the main benefits of adding fibres to concrete is the potential reduction in crack width, which depends on the amount of fibres added and positively affects the durability of the finished structure. It is the need of hour to have some special knowledge in place to understand, design and execute this special building mate-rial.

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