Assortment of Concrete Reinforcement with Steel Fiber Material a Review

a Sreenivasa Hassan Jayaram and b DivyaSreenivasa Na Manager-Projects, Jones Lang LaSalle India, Bangalore, Karnataka

b Senior Design Engineer, L&T ECC Division, India, Bangalore, Karnatakasreenivasaiisc@gmail.com

Introduction

Lot of tests has been conducted to improve the tensile property of concrete members by conventional reinforced steel bars and also by applying restraining techniques. Although these methods increase the tensile strength of concrete members, the inherent tensile strength of concrete is not increased. When the concrete is loaded the micro cracks called ‘first crack’ develops. This leads to inelastic deformation of concrete. It has been found that addition of small closely spaced and uniformly dispersed fibers having very small diameter and length would act as crack arrestor and improves the dynamic and static properties. This type of concrete is called fiber reinforced concrete. It defined as a composite material consisting of concrete and discontinuous, discrete, uniformly dispersed fibers. In which, Steel Fibre is one of the most commonly used fibers. Generally round fibers, of diameter varying from 0.25 to 0.75mm are used. Steel fibers make significant improvements in flexural impact and fatigue strength of concrete. The weak matrix in concrete, when reinforced with steel fibres, uniformly distributed across its entire mass, gets strengthened enormously, thereby rendering the matrix to behave as a composite material with properties significantly different from conventional concrete. Because of the vast improvements achieved by the addition of fibers to concrete, there are several applications where Fibers Reinforced Concrete (FRC) can be intelligently and beneficially used. These fibers have already been used in many large projects involving the construction of industrial floors, pavements, highway-overlays, etc. in India. The principal fibers in common commercial use for Civil Engineering applications include steel (SFRC/SFRS), glass, carbon and aramid. These fibers are also used in the production of continuous fibers and are used as a replacement to reinforcing steel. High percentages of steel fibers are used extensively in pavements and in tunneling. This invention uses Slurry Infiltrated Fiber Concrete (SIFCON). Fibers in the form of mat are also being used in the development of high performance structural composite. Continuous fiber-mat high performance fiber reinforced concrete (HPFRCs) called Slurry Infiltrated Mat Concrete (SIMCON) is used in the production of High performance concrete. Use of basalt fibers are picking up in western countries. Steel fibers are also used in the production new

generation concretes such as Reactive Powder Concrete (RPC), Ductal and Compact Reinforcing Concrete (CRC). Properties and applications of SFRC and some of these newgeneration fiber concrete materials are discussed.

Steel fiber types

The types of steel fibers are defined by ASTM A820:

∑ Type I: cold-drawn wire∑ Type II; cut sheet∑ Type III: melt-extracted∑ Type IV: mill cut∑ Type V: modified cold-drawn wire

Type I fibers have tensile strength from 145,000 to 445,000 psi, while Types II, III, IV, and V have tensile strength as low as 50,000 psi. Fiber shapes range from round wires with deformed ends in different diameters (Type I), rectangular or square rod shapes with dimples (Type II), triangular cross-section and twisted (Type V), or crescent cross-section and corrugated (Type V), as well as other shapes. They also come in different lengths, ranging from 1/4 inch to more than 2 inches. Michael Carter, manager of key accounts for Propel (Fiber mesh), Chattanooga, Tenn., says there is a tradeoff with length. Longer fibers tend to perform better but they can be more difficult to blend and mix well into concrete. To solve this problem, manufacturers often bundle fibers using water-soluble glue to achieve better dispersion in concrete during mixing.

You also can gage fiber effectiveness by aspect ratio—the length divided by the diameter. The higher the aspect ratio the better the performance. Longer fibers have higher aspect ratios. Use aspect ratios to compare fibers of equal length.

Some manufacturers blend steel fibers with polymer plastic macro and micro fibers in order to get a synergistic effect.

Classification according to volume fraction∑ Low volume fraction (<1%)∑ Moderate volume fraction (between 1 and 2%)∑ High volume fraction (greater than 2)

Low volume fraction

The fibers are used to reduce shrinkage cracking. These fibers are used in slabs and pavements that have large exposed surface leading to high shrinkage crack. Disperse fibers offer various advantages of steel bars and wire mesh to reduce shrinkage cracks:(a) The fibers are uniformly distributed in three-dimensions making an efficient loaddistribution(b) The fibers are less sensitive to corrosion than the reinforcing steel bars,(c) The fibers can reduce the labor cost of placing the bars and wire mesh.

Moderate volume fraction

The presence of fibers at this volume fraction increases the modulus of rupture, fracture toughness, and impact resistance. These composite are used in construction methods such as shot Crete and in structures that require energy absorption capability, improved capacity against delamination, spelling, and fatigue.

High volume fraction

The fibers used at this level lead to strain hardening of the composites. Because of thisimproved behavior, these composites are often referred as high-performance fiber-reinforced composites (HPFRC). In the last decade, even better composites were developed and are referred as ultra-high-performance fiber reinforced concretes (UHPFRC).

Toughening Mechanism

The composite will carry increasing loads after the first cracking of the matrix if the pull-out resistance of the fibers at the first crack is greater than the load at first cracking;

∑ At the cracked section, the matrix does not resist any tension and the fibers carry the entire load taken by the composite.

∑ With an increasing load on the composite, the fibers will tend to transfer the additional stress to the matrix through bond stresses. This process of multiple cracking will continue until either fibers fail or the accumulated local debodingwill lead to fiber pull-out .

Total Energy

∑ According to the report by ACI Committee 554 the total energy absorbed in fiber debonding as measured by the area under the load-deflection curve before complete separation of a beam is at least 10 to 40 times higher for fiber-reinforced concrete than for plain concrete.

∑ The magnitude of improvement in toughness is strongly influenced by fiber concentration and resistance of fibers to pull-out which, other factors, such as shape or surface texture.

Optimization Process

∑ From a material and structural point of view, there is a delicate balance in optimizing the bond between the fiber and the matrix.

∑ If the fibers have a weak bond with the matrix, they can slip out at low loads and do not contribute very much to bridge the cracks. In this situation, the fibers do not increase the toughness of the system.

∑ If the bond with the matrix is too strong, many of the fibers may break before they dissipate energy by sliding out. In this case, the fibers behave as non-active inclusions leading to only marginal improvement in the mechanical properties.

Role of Fiber Size

A. To bridge the large number of micro cracks in the composite under load and to avoid large strain localization it is necessary to have a large number of short fibers. The uniform distribution of short fibers can increase the strength and ductility of the composite.

B. Long fibers are needed to bridge discrete macro cracks at higher loads; however the volume fraction of long fibers can be much smaller than the volume fraction of short fibers. The presence of long fibers significantly reduces the workabilityof the mix.

Elastic modulus, creep, and drying shrinkage

∑ Inclusion of steel fibers in concrete has little effect on the modulus of elasticity, drying shrinkage, and compressive creep.

∑ Tensile creep is reduced slightly, but flexural creep can be substantially reduced when very stiff carbon fibers are used.

∑ However, in most studies, because of the low volume the fibers simply acted as rigid inclusions in the matrix, without producing much effect on the dimensional stability of the composite

Durability

∑ When well compacted and cured, concretes containing steel fibers seem to possess excellent durability as long as fibers remain protected by the cement paste.

∑ In most environments, especially those containing chloride, surface rusting is inevitable but the fibers in the interior usually remain uncorroded.

∑ Long-term tests of steel-fiber concrete durability at the Battelle Laboratories in Columbus, Ohio, showed minimum corrosion of fibers and no adverse effect after 7 years of exposure to deicing salt

Glass Fibers

∑ Ordinary glass fiber cannot be used in Portland cement mortars or concretes because of chemical attack by the alkaline cement paste.

∑ Zirconia and other alkali-resistant glass fibers possess better durability to alkalineenvironments, but even these are reported to show a gradual deterioration with time.

∑ Similarly, most natural fibers, such as cotton and wool, and many synthetic polymers suffer from lack of durability to the alkaline environment of the portland cement paste.

Ultra-High-Performance Fiber-Reinforced Composites

There is a new generation of high performance fiber-reinforced composites. In many of these materials the strength, toughness, and durability are significantly improved.

Compact Reinforced Composites (CRC)∑ Researchers in Denmark created Compact Reinforced Composites using metal

fibers, 6 mm long and 0.15 mm in diameter, and volume fractions in the range of 5 to 10 %.

∑ High frequency vibration is needed to obtain adequate compaction. These short fibers increase the tensile strength and toughness of the material.

∑ The increase of strength is greater than the increase in ductility, therefore the structural design of large beams and slabs requires that a higher amount of reinforcing bars be used to take advantage of the composite. The short fibers are an efficient mechanism of crack control around the reinforcing bars.

∑ The final cost of the structure will be much higher than if the structure would be made by traditional methods, therefore the use of compact reinforcedcomposites is mainly justified when the structure requires special behavior, such as high impact resistance or very high mechanical properties.

Reactive Powder Concrete (RPC)∑ Investigators in France by adding metal fibers, 13mm long and 0.15 mm in

diameter, with a maximum volume fraction of 2.5%.

∑ This composite uses fibers that are twice as long as the compact reinforced composites therefore, because of workability limitations, cannot incorporate the same volume fraction of fibers.

∑ The smaller volume fraction results in a smaller increase in the tensile strength of the concrete. Commercial versions of this product have further improved the strength of the matrix, chemically treated the surface of the fiber, and addedmicrofibers.

Slurry-Infiltrated-fibered concrete (SIFCON)∑ The processing of this composite consists in placing the fibers in a formwork and

then infiltrating a high w/c ratio mortar slurry to coat the fibers.∑ Compressive and tensile strengths up to 120 MPa and 40 MPa, respectively have

been obtained. Modulus of rupture up 90 MPa and shear strength up to 28 MPa have been also reported.

∑ In direct tension along the direction of the fibers, the material shows a very ductile response. This composite has been used in pavements slabs, and repair

Engineered Cementitious Composite (EEC)∑ The ultra-high-ductility of this composite, 3-7%, was obtained by optimizing the

interactions between fiber, matrix and its interface.∑ Mathematical models were developed so that a small volume fraction of 2% was

able to provide the large ductility.∑ The material has a very high stain capacity and toughness and controlled crack

propagation The manufacturing of ECC can be done by normal casting or by extrusion.

∑ By using an optimum amount of super plasticizer and non-ionic polymer with steric action, it was possible to obtain self-compacting casting.

∑ Experimental results with extruded pipes indicate that the system has a plastic yielding behavior instead of the typical brittle fracture exhibited when plain concrete is used.

Multiscale-Scale Fiber-Reinforced Concrete (MSFRC)}∑ Researchers the Laboratoire Central des Ponts et Chaussees (France) proposed

to combine short and long fibers to increase the tensile strength, the bearing capacity, and the ductility).

∑ With this blend, good workability was achieved with fiber volume fractions up to 7%.

∑ One typical combination of fibers is 5% straight drawn steel fibers, 5-mm long and 0.25 mm in diameter, and 2% hooked-end drawn steel fibers, 25-mm long and 0.3 mm in diameter.

Mix Design of SFRC As with any other type of concrete, the mix proportions for SFRC depend upon the requirements for a particular job, in terms of strength, workability, and so on. Several

procedures for proportioning SFRC mixes are available, which emphasize the workability of the resulting mix. However, there are some considerations that are particular to SFRC. In general, SFRC mixes contain higher cement contents and higher ratios of fine to coarse aggregate than do ordinary concretes, and so the mix design procedures the apply to conventional concrete may not be entirely applicable to SFRC. Commonly, to reduce the quantity of cement, up to 35% of the cement may be replaced with fly ash. In addition, to improve the workability of higher fibre volume mixes, water reducing admixtures and, in particular, superlasticizers are often used, in conjunction with air entrainment. The range of proportions for normal weight SFRC is shown in table. For steel fibre reinforced shotcrete, different considerations apply, with most mix designs being arrived at empirically. Typical mix designs for steel fibre shotcrete are given in table. A particular fibre type, orientation and percentage of fibers, the workability of the mix decreased as the size and quantity of aggregate particles greater than 5 mm increased; the presence of aggregate particles less than 5 mm in size had little effect on the compacting characteristics of the mix. Figure 1 shows the effects of maximum aggregate size on workability. The second factor which has a major effect on workability is the aspect ratio (l/d) of the fibres. The workability decreases with increasing aspect ratio, as shown in figure 2, in practice it is very difficult to achieve a uniform mix if the aspect ratio is greater than about 100.

Static Mechanical Properties

Compressive Strength

Fibres do little to enhance the static compressive strength of concrete, with increases in strength ranging from essentially nil to perhaps 25%. Even in members which contain conventional reinforcement in addition to the steel fibres, the fibres have little effect on compressive strength. However, the fibres do substantially increase the post-cracking ductility, or energy absorption of the material.

This is shown graphically in the compressive stress-strain curves of SFRC in figure 3

Tensile Strength

Fibres aligned in the direction of the tensile stress may bring about very large increases in direct tensile strength, as high as 133% for 5% of smooth, straight steel fibres. However, for more or less randomly distributed fibres, the increase in strength is muchsmaller, ranging from as little as no increase in some instances to perhaps 60%, with many investigations indicating intermediate values, as shown in figure 4. Splitting-tension test of SFRC show similar result. Thus, adding fibres merely to increase the direct tensile strength is probably not worthwhile. However, as in compression, steel fibres do lead to major increases in the post-cracking behaviour or toughness of the composites.

Flexural Strength

Steel fibres are generally found to have aggregate much greater effect on the flexural strength of SFRC than on either the compressive or tensile strength, with increases of more than 100% having been reported. The increases in flexural strength is particularly sensitive, not only to the fibre volume, but also to the aspect ratio of the fibres, with higher aspect ratio leading to larger strength increases. Figure 5 describes the fibre effect in terms of the combined parameter Wl/d, where l/d is the aspect ratio and W is the weight percent of fibres. It should be noted that for Wl/d > 600, the mix characteristics tended to be quite unsatisfactory. Deformed fibres show the same types of increases at lower volumes, because of their improved bond characteristics.

As was indicated previously, fibres are added to concrete not to improve the strength, but primarily to improve the toughness, or energy absorption capacity. Commonly, the flexural toughness is defined as the area under the complete load-deflection curve in flexure; this is sometimes referred to as the total energy to fracture. Alternatively, the toughness may be defined as the area under the load-deflection curve out to some particular deflection, or out to the point at which the load has fallen back to some fixed percentage of the peak load. Probably the most commonly used measure of toughness is the toughness index proposed by Johnston and incorporated into ASTM C1018. As is the case with flexural strength, flexural toughness also increases at the parameter Wl/d increases, as show in figure 6. The load-deflection curves for different types and volumes of steel fibres can vary enormously, as was shown previously in figure 7. For all of the empirical measures of toughness, fibres with better bond characteristics (i.e. deformed fibres, or fibres with greater aspect ratio) give higher toughness values than do smooth, straight fibres at the same volume concentrations.

Structural use of SFRC As recommended by ACI Committee 544, ‘when used in structural applications, steel fibre reinforced concrete should only be used in a supplementary role to inhibit cracking, to improve resistance to impact or dynamic loading, and to resist material disintegration. In structural members where flexural or tensile loads will occur ….. the reinforcing steel must be capable of supporting the total tensile load’. Thus, while there are a number of techniques for predicting the strength of beams reinforced only withsteel fibres, there are no predictive equations for large SFRC beams, since these would be expected to contain conventional reinforcing bars as well. An extensive guide to design considerations for SFRC has recently been published by the American ConcreteInstitute. In this section, the use of SFRC will be discussed primarily in structural members which also contain conventional reinforcement. For beams containing both fibres and continuous reinforcing bars, the situation is complex, since the fibres act in two ways: (1) They permit the tensile strength of the SFRC to be used in design, because the matrix will no longer lose its load-carrying capacity at first crack; and

(2) They improve the bond between the matrix and the reinforcing bars by inhibitingthe growth of cracks emanating form the deformations (lugs) on the bars. However, it is the improved tensile strength of SFRC that is mostly considered in the beam analysis, since the improvements in bond strength are much more difficult to quantify. Steel fibres have been shown to increase the ultimate moment and ultimate deflection of conventionally reinforced beams; the higher the tensile stress due to the fibres, the higher the ultimate moment.

Crack controlJoints in floors, as necessary as they are, typically deteriorate first, costing owners money for repairs as a floor ages. How much steel fiber is added to a concrete mix depends on the objectives: cost savings, increased joint spacing, or structural improvement. Steel fiber dosages can be as light as 8 pounds to as much as 200 pounds per cubic yard. Increasing the percentage of fibers in a mix allows specifiers to increase the distance between joints. Floors are reinforced to control cracking between sawcuts using the ACI joint spacing guidelines, or fully reinforced for no sawcut joints between construction joints. These are the same guidelines ACI maintains for reinforced floors.Fibers are sometimes listed as a percentage of concrete volume. So for instance, 66 pounds of fiber per cubic yard is about 0.5% by volume. A 1% fiber addition is approximately 132 pounds.

Application of SFRCThe 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, tunnel linings, pavements and slabs, shotcrete and now shotcrete also containing silica fume, airport pavements, bridge deck slab repairs, and so on. There has also been some recent 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 material costs of the concrete, and this has tended to limit the use of SFRC to special applications.The largest application for steel fiber reinforced concrete is floor slab construction, although its use as a replacement for or complement to structural reinforcement in other applications is growing fast. Steel fiber floor/slab applications can save money when compared to other reinforcing systems. In addition, joint spacing can be increased and they can be used as a replacement for structural reinforcement in some cases. In some ways, the role polymer macro fibers and steel fibers play in concrete is similar. Each product can be used to extend joint width in floor slabs and each can reduce curling. Both types of fibers can be mixed successfully in concrete at high dosage rates without interfering with placing and finishing conditions, and they can both be pumped successfully. Steel fibers, however, have other advantages.

Reference and Acknowledgement

[1] ASTM C1018 – 89, Standard Test Method for Flexural Toughness and First Crack Strength of Fibre Reinforced Concrete (Using Beam with Third – Point Loading), 1991 Book of ASTM Standards, Part 04.02, American Society for Testing and Materials, Philadelphia, pp.507 – 513.

[2] JCI Standards for Test Methods of Fibre Reinforced Concrete, Method of Test for Flexural Strength and Flexural Toughness of Fibre Reinforced Concrete (Standard SF4), Japan Concrete Institute, 1983, pp. 45 – 51.

[3] C.H. Henager , “Steel fibrous shotcrete”. A summary of the State – of – the art concrete Int. : Design and construction 1981.

[4] J. Endgington, D.J. Hannant & R.I.T. Williams, “Steel fiber reinforced concrete” Current paper CP 69/74 Building research establishment Garston Watford 1974.

[5] C.D. Johnston, “Steel fiber reinforced mortar and concrete”, A review of mechanical properties. In fiber reinforced concrete ACI – SP 44 – Detroit 1974.

[6] C.D. Johnston, “Definition and measurement of flexural toughness parameters for fiber reinforced concrete” Cem. Concr. Agg. 1982.

[7] Nataraja, M. C., Dhang, N and Gupta, A. P (1999)., ‘Statistical Variations in Impact Resistance of Steel Fiber Reinforced Concrete Subjected to Drop Weight Test’, Cement and Concrete Research, Pergoman press, USA, Vol. 29, No. 7, 1999, pp. 989-995.

[8] Nataraja, M. C., Dhang, N and Gupta, A. P (1999). ‘Stress-strain Curves for Steel Fiber Reinforced Concrete in Compression’, Cement and Concrete Composites, UK, Vol. 21, No. 5/6, 1999, pp. 383-390.

[9] Nataraja, M. C., Dhang, N and Gupta, A. P (2000)., ‘Toughness Characterisation of Steel Fiber Reinforced Concrete by JSCE Approach’, Cement and Concrete Research, Pergoman press, USA, Vol. 30, No. 4, 2000, pp. 593-597.

[10] Nataraja, M. C., Dhang, N and Gupta, A. P (2000)., ‘A Study on the Behaviour of Steel Fiber Reinforced Subjected to Splitting Test’, Asian Journal of Civil Engineering, Teheran, Iran, Vol. 1, No. 1, Jan. 2000, pp. 1-11.

[11] Nataraja, M. C., Dhang, N and Gupta, A. P (2001). ‘Splitting Tensile Strength of Steel Fiber Reinforced Concrete’, Indian Concrete Journal, Vol. 75, No. 4, April 2001, pp. 287-290.

[12] Nataraja, M. C., ‘FIBER REINFORCED CONCRETE- BEHAVIOUR PROPERTIES AND APPLICATION’, Professor of Civil Engineering, Sri Jayachamarajendra College of engineering, Mysore-570 006

[13] Nguyen Van CHANH ., ‘ STEEL FIBER REINFORCED CONCRETE’ Dr.Eng. Deputy Dean, Faculty of Civil Engineering, Ho Chi Minh City University of Technology,

[14] P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials