Fiber Concorete

Islamic University Of GazaFaculty of EngineeringCivil Engineering DepartmentConcrete Technology ECIV 2341

Research Name

FIBER REINFORCED CONCRETE

SUBMITTED BY:

Haya Ramy Baker 220122837 Aya Mohammed Abu-Fayyad 220123191Tamara Bashier Nabhan 220122154

SUBMITTED TO:

Dr.Maamon Alqdra

2014-2015

Introduction:-

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Concrete is weak in tension and has a brittle character. The concept of using fibers to improve the characteristics of construction materials is very old. Early applicationsinclude addition of straw to mud bricks, horse hair to reinforce plaster and asbestosto reinforce pottery. Use of continuous reinforcement in concrete (reinforced concrete) increases strength and ductility, but requires careful placement and skill. Alternatively, introduction of fibers in discrete form in plain or reinforced concrete may provide a better solution. The modern development of fiber reinforcedconcrete (FRC) started in the early sixties. Addition of fibers to concrete makes ita homogeneous and isotropic material. When concrete cracks, the randomlyoriented fibers start functioning, arrest crack formation and propagation, and thusimprove strength and ductility. Deterioration of concrete structures due to steel corrosion is a matter of considerable concern since the repairing of these structures proved to be a costly process. Repair and rehabilitation of the civil structures needs an enduring repair material. The ideal durable repair material should have low shrinkage, good thermal expansion, and substantial modulus of elasticity, high tensile strength, improved fatigue and impact resistance. Reinforcing the concrete structures with fibers such as polypropylene is one of the possible ways to provide all the criteria of the durable repair material. This type of reinforcement is called Fiber Reinforcement of Concrete Structures. There is an increasing worldwide interest in utilizing fiber reinforced concrete structures for civil infrastructure applications. The bonding between the fibers and the concrete has to be good and the plastic has to withstand the changing environment of freeze and thaw as well as a high PH of 12.5 and a low of PH 6.5 when Saturated with sodium chloride. With these brand new materials, little is known about the effect of fiber percentage on fracture properties under hot and cold conditions and when saturated with seawater. This information is necessary to be able to study the freeze-thaw durability of the fiber reinforced concrete structures under different environmental conditions and also in the marine environment.

History:- 1900s, asbestos fibers were used in concrete. In the 1950s, the concept of composite materials came into being and fiber-reinforced concrete was one of the topics of interest. Once the health risks associated with asbestos were discovered, there was a need to find a replacement for the substance in concrete and other building materials. By the 1960s, steel, glass (GFRC), and synthetic fibers such as polypropylene fibers were used in concrete. Research into new fiber-reinforced concretes continues today.

Fiber Types :- Fibers are produced from different materials in various shapes and sizes. Typicalfiber materials are:-Steel Fibers Straight, crimped, twisted, hooked, ringed, and paddled ends. Diameter rangefrom 0.25 to 0.76 mm.Glass Fibers Straight. Diameter ranges from 0.005 to 0.015mm (may be bonded together toform elements with diameters of0.13 to 1.3mm).

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Natural Organic and Mineral Fibers Wood, asbestos, cotton, bamboo, and rockwool. They come in wide range ofsizes.Polypropylene Fibers Plain, twisted, fibrillated, and with buttoned ends.Other Synthetic Fibers Kevlar, nylon, and polyester. Diameter ranges from 0.02 to 0.38mm.A convenient parameter describing a fiber is its aspect ratio (L/D), defined as thefiber length divided by an equivalent fiber diameter. Typical aspect ratio ranges fromabout 30 to 150 for length of 6 to 75mm.

Mixture Compositions and Placing :- Mixing of FRC can be accomplished by many methods. The mix should have a uniform dispersion of the fibers in order to prevent segregation or balling of the fibersduring mixing. Most balling occurs during the fiber addition process. Increase ofaspect ratio, volume percentage of fiber, and size and quantity of coarse aggregatewill intensify the balling tendencies and decrease the workability. To coat the largesurface area of the fibers with paste, experience indicated that a water cement ratiobetween 0.4 and 0.6, and minimum cement content of400 kg/m3 are required. Compared to conventional concrete, fiber reinforced concrete mixes are generallycharacterized by higher cement factor, higher fine aggregate content and smallersize coarse aggregate. A fiber mix generally requires more vibration to consolidate the mix. External vibration is preferable to prevent fiber segregation. Metal trowels, tube floats, androtating power floats can be used to finish the surface.

Mechanical Properties of FRC Addition of fibers to concrete influences its mechanical properties which significantly depend on the type and percentage of fiber. Fibers with end anchorage

and high aspect ratio were found to have improved effectiveness. It was shown that for the same length and diameter, crimped-end fibers can achieve the same properties as straight fibers using 40 percent less fibers. In determining the mechanical proper ties of FRC, the same equipment and procedure as used for conventional concrete can also be used. Below are cited some properties of FRC determined by different researchers.

Compressive Strength The presence of fibers may alter the failure mode of cylinders, but the fiber effectwill be minor on the improvement of compressive strength values (0 to 15 percent).Modulus of Elasticity Modulus of elasticity of FRC increases slightly with an increase in the fibers content. It was found that for each 1 percent increase in fiber content by volume there is an increase of 3 percent in the modulus of elasticity.

Flexure The flexural strength was reported to be increased by 2.5 times using 4 percentfibers.

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Toughness For FRC, toughness is about 10 to 40 times that of plain concrete.Splitting Tensile Strength The presence of 3 percent fiber by volume was reported to increase the splittingtensile strength of mortar about 2.5 times that of the unreinforced one.Fatigue Strength The addition of fibers increases fatigue strength of about 90 percent and 70 percentof the static strength at 2 x 106 cycles for non-reverse and full reversal of loading, respectively.Impact Resistance The impact strength for fibrous concrete is generally 5 to 10 times that of plain concrete depending on the volume of fiber.Corrosion of Steel Fibers A l0-year exposure of steel fibrous mortar to outdoor weathering in an industrialatmosphere showed no adverse effect on the strength properties. Corrosion wasfound to be confined only to fibers actually exposed on the surface. Steel fibrousmortar continuously immerse in seawater for 10 years exhibited a 15 percent losscompared to 40 percent strength decrease of plain mortar.

Structural Behavior of FRC Fibers combined with reinforcing bars in structural members will be widely used inthe future. The following are some of the structural behaviourI6:Flexure The use of fibers in reinforced concrete flexure members increases ductility, tensile strength, moment capacity, and stiffness. The fibers improve crack control andpreserve post cracking structural integrity of members.Torsion The use of fibers eliminates the sudden failure characteristic of plain concretebeams. It increases stiffness, torsional strength, ductility, rotational capacity, andthe number of cracks with less crack width.

Shear Addition of fibers increases shear capacity of reinforced concrete beams up to 100percent. Addition of randomly distributed fibers increases shear-friction strength,the first crack strength, and ultimate strength.Column The increase of fiber content slightly increases the ductility of axially loaded specimen. The use of fibers helps in reducing the explosive type failure for columns.High Strength Concrete Fibers increases the ductility of high strength concrete. The use of high strengthconcrete and steel produces slender members. Fiber addition will help in controllingcracks and deflections.Cracking and Deflection Tests have shown that fiber reinforcement effectively controls cracking and deflection, in addition to strength improvement. In conventionally reinforced concretebeams, fiber addition increases stiffness, and reduces deflection.

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Background of Fiber Reinforced Concrete :- Portland cement concrete is considered to be a relatively brittle material. When subjected to tensile stresses, non-reinforced concrete will crack and fail. Since mid-1800's steel reinforcing has been used to overcome this problem. As a composite system, the reinforcing steel is assumed to carry all tensile loads. The problem with employing steel in concrete is that over time steel corrodes due to the ingress of chloride ions. In the northeast, where sodium chloride de-icing salts are commonly used and a large amount of coastal area exists, chlorides are readily available for penetration into concrete to promote corrosion, which favors the formation of rust. Rust has a volume between four to ten times of iron, which dissolves to form it. The volume expansion produces large tensile stresses in the concrete, which initiates cracks and results in concrete spalling from the surface. Although some measures are available to reduce corrosion of steel in concrete such as corrosion inhibitive admixtures and coatings, a better and permanent solution may be replace the steel with a reinforcement that is less environmentally sensitive. More recently micro fibers, such as those used in traditional composite materials have been introduced into the concrete mixture to increase its toughness, or ability to resist crack growth. FRC is Portland cement concrete reinforced with more or less randomly distributed fibers. In FRC, thousands of small fibers are dispersed and distributed randomly in the concrete during mixing, and thus improve concrete properties in all directions. Fibers help to improve the post peak ductility performance, pre-crack tensile strength, fatigue strength, impact strength and eliminate temperature and shrinkage cracks. Several different types of fibers have been used to reinforce the cement-based matrices. The choice of fibers varies from synthetic organic materials such as polypropylene or carbon, synthetic inorganic such as steel or glass, natural organic such as cellulose or sisal to natural inorganic asbestos. Currently the commercial products are reinforced with steel, glass, polyester and polypropylene fibers. The selection of the type of fibers is guided by the properties of the fibers such as diameter, specific gravity, young’s modulus, tensile strength etc. The extent of these fibers affects the properties of the cement matrix.

Tensile Load versus Deformation for Plain and Fiber Reinforced Concrete

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Reinforcement Mechanisms :- Concrete carries flaws and micro-cracks both in the material and at the interfaces even before an external load is applied. These defects and micro-cracks emanate from excess water, bleeding, plastic settlement, thermal and shrinkage strains and stress concentrations imposed by external restraints. Under an applied load, distributed micro-cracks propagate coalesce and align themselves to produce macro-cracks. When loads are further increased, conditions of critical crack growth are attained at the tips of the macro-cracks and unstable and catastrophic failure is precipitated. The micro and macro-fracturing processes described above can be favorably modified by adding short, randomly distributed fibers of various suitable materials. Fibers not only suppress the formation of cracks, but also abate their propagation and growth. Soon after placement, evaporation of the mix water and the autogenously process of concrete hydration create shrinkage strains in concrete. If restrained, this contraction can cause stresses far in excess of those needed to cause cracking. In spite of every effort, plastic shrinkage cracking remains a serious concern, particularly in large surface area placements like slabs on grade, thin surface repairs, patching and concrete linings. With large surface areas, fibers engage water in the mix and reduce bleeding and segregation. The result is that there is less water available for evaporation and less overall free shrinkage. When combined with post-crack bridging capability of fibers, fibers reduce crack widths and cracks areas when concrete is retrained. In the hardened state, when fibers are properly bonded, they interact with the matrix at the level of micro-cracks and effectively bridge these cracks thereby providing stress transfer media that delays their coalescence and unstable growth. If the fiber volume fraction is sufficiently high, this may result in an increase in the tensile strength of the matrix. Indeed, for some high volume fraction fiber composite a notable increase in the tensile/flexural strength over and above the plain matrix has been reported. Once the tensile capacity of the composite is reached, and coalescence and conversion of micro-cracks to macro-cracks has occurred, fibers, depending on their length and bonding characteristics continue to restrain crack opening and crack growth by effectively bridging across macro-cracks. This post-peak macro-crack bridging is the primary reinforcement mechanism in the majority of commercial fiber reinforced concrete composites. Based on the discussion above, it emerges that fiber-reinforced cementitious composites can be classified into two broad categories: normal performance (or conventional) fiber-reinforced cementitious composites and high-performance fiber-reinforced cementitious composites. In FRCs with low to medium volume fraction of fibers, fibers do not enhance the tensile/flexural strength of the composite and benefits of fiber reinforcement are limited to energy absorption or ' toughness 'enhancement in the post-cracking regime only. For high performance fiber reinforced composites, on the other hand, with a high fiber dosage, benefits of fiber reinforcement are noted in an increased tensile strength, strain-hardening response before localization and enhanced 'toughness' beyond crack localization.

Applications of FRC:- The uniform dispersion of fibers throughout the concrete mix provides isotropicproperties not common to conventionally reinforced concrete. The applications offibers in concrete industries depend on the designer and builder in taking advantage

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of the static and dynamic characteristics of this new material. The main area of FRCapplications are:Tunnel Lining and Slope Stabilization Steel fiber reinforced shortcrete (SFRS) are being used to line underground openings and rock slope stabilization. It eliminates the need for mesh reinforcement andscaffolding.

Blast Resistant Structures When plain concrete slabs are reinforced conventionally, tests showed thatthere is no reduction of fragment velocities or number of fragments under blast andshock waves. Similarly, reinforced slabs of fibrous concrete, however, showed 20percent reduction in velocities, and over 80 percent in fragmentations.Thin Shell, Walls, Pipes, and Manholes Fibrous concrete permits the use of thinner flat and curved structural elements.Steel fibrous shortcrete is used in the construction of hemispherical domes using theinflated membrane process. Glass fiber reinforced cement or concrete (GFRC), made by the spray-up process, have been used to construct wall panels. Steel andglass fibers addition in concrete pipes and manholes improves strength, reducesthickness, and diminishes handling damages.

Dams and Hydraulic Structure FRC is being used for the construction and repair of dams and other hydraulicstructures to provide resistance to cavitation and severe errosion caused by the impact of large waterborne debris.

Other Applications These include machine tool frames, lighting poles, water and oil tanks and concrete repairs.

Special type of FRC : STEEL FIBER REINFORCED CONCRETE It is now well established that one of the important properties of steel fiber reinforced concrete (SFRC) is its superior resistance to cracking and crack propagation. As a result of this ability to arrest cracks, fiber composites possess increased extensibility and tensile strength, both at first crack and at ultimate, particular under flexural loading; and the fibers are able to hold the matrix together even after extensive cracking. The net result of all these is to impart to the fiber composite pronounced post – cracking ductility which is unheard of in ordinary concrete. The transformation from a brittle to a ductile type of material would increase substantially the energy absorption characteristics of the fiber composite and its ability to withstand repeatedly applied, shock or impact loading.

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 who 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 the

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below figure:

Stress-Strain curves in compression for SFRC

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 much smaller, ranging from as little as no increase in some instances to perhaps 60%, with many investigations indicating intermediate values, as shown in the below figure. 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.

Influence of fibre content on tensile strength

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

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with steel 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 Concrete Institute. 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 inhibiting the growth of cracks emanating from 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.

Experimental moment versus deflection curves for SFRC beams

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Application of SFRC 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, 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.

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SO:-

Conclusion :- Based on the test of one hundred and ninety five specimens made with the available local materials, the following conclusions can be derived:1. No workability problem was encountered for the use of hooked fibers up to 1.5percent in the concrete mix. The straight fibers produce balling at high fiber contentand require special handling procedure.2. Use of fiber produces more closely spaced cracks and reduces crack width. Fibers bridge cracks to resist deformation.3. Fiber addition improves ductility of concrete and its post-cracking load-carrying capacity.4. The mechanical properties of FRC are much improved by the use of hookedfibers than straight fibers, the optimum volume content being 1.5 percent. While fibers addition does not increase the compressive strength, the use of 1.5 percent fiberincrease the flexure strength by 67 percent, the splitting tensile strength by 57 percent, and the impact strength 25 times.5. The toughness index of FRC is increased up to 20 folds (for 1.5 percent hookedfiber content) indicating excellent energy absorbing capacity6. FRC controls cracking and deformation under impact load much better thanplain concrete and increased the impact strength 25 times.

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Photos:-

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REFERENCES

[I] Ramualdi, J.P. and Batson, G.B., The Mechanics of Crack Arrest in Concrete, Journal of the Engineering Mechanics Division, ASCE, 89:147-168 (June, 1983).[2J ACE Committee 544, State-of-the-Art Report on Fiber Reinforced Concrete, ACI Concrete International, 4(5): 9-30 (May, 1982).[3] Naaman, A.E., Fiber Reinforcement for Concrete, ACI Concrete International, 7(3): 21-25 (March,1985).[4] ACI Committee 544, Measurement of Properties of Fiber Reinforced Concrete, (ACI 544.2R-78),American Concrete Institute, Detroit, 7 p. (1978).[5] P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials.[6] Colin D. Johnston, “Fiber reinforced cements and concretes” Advances in concrete technology volume 3 – Gordon and Breach Science publishes – 2001. [7] Perumalsamy N. Balaguru, Sarendra P. Shah, ‘‘Fiber reinforced cement composites’’, Mc Graw Hill International Editions 1992. [8] Arnon Bentur & Sidney Mindess, ‘‘ Fibre reinforced cementitious composites’’ Elsevier applied science London and Newyork 1990. [9] 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.

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Contents:-

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Objects PagesIntroduction 2History 2Fiber types 2Mixture Compositions and Placing

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Mechanical Properties of FRC 3

Background of Fiber Reinforced Concrete

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Reinforcement Mechanisms 6Applications of FRC 7STEEL FIBER REINFORCED CONCRETE

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STATIC MECHANICAL PROPERTIES

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STRUCTURAL USE OF SFRC 9Application of SFRC 10

Conclusion 11

Photos 12-13References 14