Fibre Concrete ( Sfrc & Pfc )

Fibre Concrete ( SFRC & PFC ) SAB 2112

Introduction

1.0 Introduction

The properties of unreinforced cement and concrete have been well document and an

understanding of the basic principles of concrete technology has been assumed in writing the

text. One of the problems of a cement-based matrix is the inherently brittle type of failure which

occurs under tensile stress systems or impact loading and in the construction industry, a major

reason for the growing interest in the performance of fibres in cement based materials is the

desire to increase the toughness or tensile properties of the basic matrix.

Emphasis on energy conservation has also stimulated interest in methods of replacing

materials such as cast iron, glass-reinforced plastics, and bituminous materials by the use of

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fibre cement and fibre concrete. By replacing this existing materials, we able to satisfy the

performance requirements of these various interests, adequate material properties must be

achieved in the fibre composite and the main objective of engineers in attempting to modify the

properties of cement or concrete are to improve the tensile or flexural strength, to improve the

impact strength, to control cracking and the mode of failure by means of post-cracking ductility,

and to change the rheology or flow characteristic of the material in the fresh state. ( D.J Hannat,

1978 )

It is now well established that one of the important properties of steel fibre reinforced

concrete ( SRFC ) is its superior resistance to cracking and crack propagation. As a result of

this ability to arrest cracks, fibre composites posses increased extensibility and tensile strength,

both at first crack and ultimate, particular under flexural loading and the fibres are able to hold

the matrix together even after extensive cracking.

Fibre reinforced concrete ( FRC ) may be defined as a composite materials with Portland

cement, aggregate, and fibres. Plain, unreinforced concrete is a brittle material with low tensile

strength and a low strain capacity. The role of fibre is to brigde across the cracks that develop

provides some post cracking “ ductility ˮ . if the fibres are sufficiently strong, sufficient bonded to

materials, and permit fibre reinforced concrete to carry significant stresses over a relatively large

strain capacity in the post cracking stage.

There are many ways of increasing the strength of the concrete. The real function of

concrete is to increase the toughness of the concrete under any type of loading. That is the

fibres tend to increase the strain and peak load and provide a great deal of energy absorption in

post-peak portion between the load versus deflection curve. When the reinforcement is in the

form of short discrete fibres, they act as rigid inclusions in the concrete.

The strength of fiber-reinforced concrete are not significantly greater than same mixes

were we used without the fibers. The resulting concretes, are together and have the greater

resistance to cracking and higher impact resistance. The use of fibers has increased the

versatility of concrete by reducing its brittleness. The improvement obtained in the toughness of

concrete by adding fibers is dependent on the fiber’s aspect ratio that is length and diameter

and most typically the aspect ratios used vary from about 25 up to as mush as 150, with 100

being about an average value. ( Jack C. McCormac , 2005 )

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Properties of Materials

2.1 Properties of Fibres and Matrices

The main factors controlling the theoretical performance of the composite materials are

the physical properties of fibers and the matrix and the strength of the bond between two.

The modulus of elasticity of the fiber is generally less than five times that the matrix and this

combined with the low fiber volume fraction means that the modulus of composite is not

greatly different from that of the matrix.

The fibres can be divided into two main groups , that are cellulose, nylon and

polypropylene with lower moduli while the other group with high moduli such as asbestos,

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glass, steel, carbon, and Kevlar, which is a form of aromatic polyamide introduced by

DuPont. The last two are included for the sake of completeness but high cost seem to rule

them out for major engineering applications.

The low modulus of fibers are generally subject to relatively high creep which

means that if they are used to support permanent high stresses an a cracked composite,

considerable elongations or deflections may occur over a period of time. They are more

likely to be used in situations where the matrix is expected to be uncracked but overloads

such as handling stresses, impacts or wind loads are significant.

Another problem with low moduli fibers are they have large values of Poisson’s ratio

means that if stretch along their axis, they contract sideways much more than the other

fibers. This will leads to a high lateral tensile stress at the fiber matrix interface which is

likely to cause a short aligned fiber to debond and pull out. Other devices such as woven

meshes or fibrillated fibers may therefore be necessary to give efficient composites.

Even the high modulus fibers may require mechanical bonding to avoid pull out unless

the specific area is very large. This steel fibers are commonly produced with varying cross-

sectional to provide anchorage and glass fiber bundles may be penetrated with cement

hydration products to give an effective mechanical bond after a period of time.

The maximum particles of size also important because its affects the fiber

distribution and the quantity of fibers which can be included in the composites. Actually, the

average particles size of cement paste before hydration is between 10 and 30 microns

where the mortar is considered to contains aggregate particles up to 5 mm maximum size.

Concrete to be intend to be used in conjunction with fibers should not have particles greater

than 20 mm and not greater than 10 mm otherwise uniform fiber distribution becomes

difficult to achieve.

To avoid shrinkage and surface problems in finished products it is advisable to

use at least 50 percent by volume of mineral filler, which may be aggregate or could include

pulverized fuel ash, or limestone dust. If the inert filler consists of a large volume of coarse

aggregate, the volumes of the fibers which can be included will be limited that will turn limit

the tensile strength and ductility of the composite.

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Strength of the matrix is affected by the free water or cement ratio and this

parameter also has a lesser effect on the modulus. The value of fiber density are important

and enable the relationship between fiber volume which is required for the theoretical

treatment and the fiber weight.

The commercial viability of fiber composites is critically dependent on the costs of the

fibers which controlling influence on the cost of the product because the matrix is so cheap.

A commercial decision on existing product can be profitably replace with one made from

fibre cement or fibre concrete may depend on using the cheapest and suitabale fibre to fulfill

the strength and durability equipments.

Unfortunately, it is not possible to give specific prices because costs of fibres are

subject to rapid change depending on demand and energy costs in their production. The

cost may have be taken regarding saving in labour or transport costs compared with the

cost of equivalent products in timber, steel, aluminium, plastic, or reinforced concrete before

these new materials are use in large quantities.

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Steel Fibre Reinforced Concrete

3.1 Steel Fibres

The uses of steel fibre have mainly been in conjunction with concrete or mortar

and has been placed on the production of the thin sheets products which form the bulk of

the asbestos cement or glass fibre cement industries. Steel is most commonly used

materials for the fibres. Fibresteel is used generally for high flexural fatigue strength, high

wear resistance requirements, precast applications and so on. Steel fibres come in many

shapes and sizes each with different performance levels. To make a true comparison

between different products and ensure the correct selection is made, it is recommended that

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each potential supplier be asked to supply accredited test results for their products in

accordance with one of the internationally recognised standards.

Picture 1 : Steel Fibre

3.2 Mix Design for Steel Fibre Reinforced Concrete ( 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. For steel fibre reinforced shotcrete, different considerations apply, with

most mix designs being arrived at empirically. A particular fibre type, orientation and

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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, in practice it is very

difficult to achieve a uniform mix if the aspect ratio is greater than about 100.

3.3 Technology for Producing Steel Fibre Reinforced Precast Concrete ( SFRC )

SFRC can, in general, be produced using conventional concrete practice, though

there are obviously some important differences. The basic problem is to introduce a

sufficient volume of uniformly dispersed to achieve the desired improvements in

mechanical behaviour, while retaining sufficient workability in the fresh mix to permit

proper mixing, placing and finishing. The performance of the hardened concrete is

enhanced more by fibres with a higher aspect ratio, since this improves the fibre-matrix

bond. On the other hand, a high aspect ratio adversely affects the workability of the fresh

mix. In general, the problems of both workability and uniform distribution increase with

increasing fibre length and volume.

One of the chief difficulties in obtaining a uniform fibre distribution is the tendency

for steel fibres to ball or clump together. Clumping may be caused by a number of

factors:

I. The fibres may already be clumped together before they are added to the

mix; normal mixing action will not break down these clumps.

II. Fibres may be added too quickly to allow them to disperse in the mixer.

III. Too high a volume of fibres may be added.

IV. The mixer itself may be too worn or inefficient to disperse the fibres.

V. Introducing the fibres to the mixer before the other concrete ingredients will

cause them to clump together.

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In view of this, care must be taken in the mixing procedures. Most commonly,

when using a transit mix truck or revolving drum mixer, the fibres should be added last to

the wet concrete. The concrete alone, typically, should have a slump of 50-75 mm

greater than the desired slump of the SFRC. Of course, the fibres should be added free

of clumps, usually by first passing them through an appropriate screen. Once the fibres

are all in the mixer, about 30-40 revolutions at mixing speed should properly disperse

the fibres. Alternatively, the fibres may be added to the fine aggregate on a conveyor

belt during the addition of aggregate to the concrete mix. The use of collated fibres held

together by a water-soluble sizing which dissolves during mixing largely eliminates the

problem of clumping.

SFRC can be placed adequately using normal concrete equipment. It appears to

be very stiff because the fibres tend to inhibit flow; however when vibrated, the material

will flow readily into the forms. It should be noted that water should be added to SFRC

mixes to improve the workability only with great care, since above a w/c ratio of about

0.5, additional water may increase the slump of the SFRC without increasing its

workability and place ability under vibration. The finishing operations with SFRC are

essentially the same as for ordinary concrete, thought perhaps more care must be taken

regarding workmanship.

3.4 Structural Use of Steel Fibre Reinforced Precast Concrete ( 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 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.

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For beams containing both fibres and continuous reinforcing bars, the situation is

complex, since the fibres act in two ways:

I. 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

II. They improve the bond between the matrix and the reinforcing bars by

inhibiting the 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.

3.5 Applications of Steel Fibre Reinforced Precast Concrete ( 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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Picture 2 :Steel Fibre Reinforced Concrete

Picture 3 : Steel Fibre Reinforced Precast Concrete

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Polypropylene Fibre Concrete

4.1 Polypropylene Fibre

Today it is very common to add polypropylene fibres into concrete for

strengthening concrete and for protection of concrete against micro cracks. Most common

count for this application is a PP with a relatively short cut of 12 mm. Other cuts used but

less common are 6, 18 and 24 mm. The function of the PP fibre mixed into concrete is not

to replace the steel but to avoid the creation of micro cracks in the concrete. Fibres are

coated with spinning oil to improve wetting, improve dispersion within the cement paste,

increase the extent of contact and improve bond to the hardened concrete.

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The fibres are manufactured in a continuous process by extrusion of polypropylene.

The extruded material is heated, stretched to improve tensile strength, coated with spinning

oil and cut to the required lengths. The manufacturing process includes control checks on

heating and cooling temperatures, operating speeds and pressures, stretch ratio and quality

of cut and stretching. Quality assurance checks are conducted on spinning oil content,

moisture content, weight and denier.

The fibres are packed in measured quantities in dispersible paper bags, suitable for

1 m3 of concrete. The bagged fibres are delivered in cardboard boxes. Boxes of fibres must

be stored on a clean surface, in dry conditions under cover and away from the possibility of

damage. Each box bears the manufacturer's and product name, batch number.

Picture 4 : Polypropylene Fibre – Mesh

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Picture 5 : Polypropylene Fibre

4.2 Applications of Polypropylene Fibre Concrete ( PFC )

Most of this Polypropylene Fibre Concrete ( PFC ) are used for our needed in

construction nowdays. There are many uses like :

I. Most small builder, cas sales dan DIY applications

II. Internal floor-slabs ( retail stores,warehouses )

III. External slabs ( driveways )

IV. Agricultural applications

V. Roads, pavements, kerbs

VI. Shortcrete : thin section walling

VII. Overlays, patch repair

VIII. Water retaining structures, marine applications

IX. Safety applications like strongrooms

X. Deep lift walls

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4.3 Mix Design for Polypropylene Fibre Concrete ( PFC )

To use polypropylene fibers this is basically all the concrete worker must know.

Concrete mix design does not have to be altered, and no special equipment or slump

modifications are required, even for pumping or shotcreting. Only two things must be

determined: how much fiber to add and what length of fiber to use. Polypropylene fibers

are manufactured in small bundles that look like miniature sheaves of straw.

The bundles are fibrillated, which means they are made of many small fibers.

During the mixing operation,the movement of aggregate shears these bundles into

smaller bundles and individual fibers. Fibrillated fibers reportedly do not cling together or

“ball.” One manufacturer also makes a single-filament fiber for use in prepackaged dry

mortar mixes. These fibers are 1⁄4 and 1⁄2 inch long. They are not added to

alreadymixed concrete. As already described, adding the fiber is simple one

premeasured bag of fibers for every cubic yard of concrete.

Fibers are packaged in the amount recommended by the manufacturer for use in

1 cubic yard of concrete. Recommendations vary. One manufacturer recommends 3

pounds of fiber per cubic yard of concrete (or 0.2 percent by volume) for most

applications; other manufacturers recommend about 11⁄2 pounds per cubic yard (or

about 0.1percent fiber by volume) .

If the jobsite is less than a 30-minute drive from the concrete plant, the fibers can

be added at the plant along with the cement and aggregate. If the jobsite is farther than

a 30-minute drive, the fibers should be added at the site. When used in high-slump

mixes or cellular lightweight concrete mixes or when mixed in turbine mixers, additional

mixing time may be necessary.

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4.4 The Effects on Concrete

ASTM Committee C09.03.04 and ACI Committee 544 are both now in the

process of developing standard tests for measuring the properties of fiber reinforced

concretes. Until such tests are standardized, reported results will vary depending on the

test method used. Some of these present tests may not accurately describe the behavior

of fiber reinforced concretes in field installations. Based on the research that has been

done, though, some of the effects polypropylene fibers have on the properties of

concrete are described here. As noted, testing research is ongoing.

4.5 Effects on The Properties of Hardened Concrete

In hardened concrete, polypropylene fibers act as crack arresters. Like any

secondary reinforcement, the fibers tend to stop cracks from propagating by holding the

concrete together so cracks cannot spread wider or grow longer. However, since

polypropylene fibers are distributed throughout the concrete, they are effective close to

where cracks start at the aggregate paste interface. The fibers thus form a sort of three-

dimensional reinforcement that distributes tensile stresses more evenly throughout the

concre t e. According to tests now beingreviewed by ASTM C09.03.04, this three

dimensional reinforcement results in improved impact resistance, particularly in the

impact resistance to ultimate failure. Flexural strength and tensile strength are also

improved, each about 10 percent. The compressive strength of concrete that has

undergone its potential drying shrinkage is about the same for concrete with or without

Polypropylene fibers. The ductility of concrete is said to be improved too because of

polypropylene’s low modulus of elasticity.

4.6 Features and Benefits

The addition of fibres helps to maximize the intrinsic early strength of the

concrete. For the future, we can improve the concrete’s resistance to plastic shrinkage

cracking. Then, it is inhibits formation of micro-cracks due to dimensional change and it

can reduces sedimentation.

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The benefit of this PFC are it can reduced frequency of plastic cracking. That is

why this PFC is suitable for safety application like strongrooms. Other than that, it can

improved durability and reduced permeability and also can cut the construction cost.

Finally, this PFC will decrease risk of plastic settlement cracking over re-bar.

Picture 6 : Polypropylene Fibre Concrete

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Conclusion

Nowdays, there are many new and modern technologies in upgrading all

construction materials. The main purpose are want to increase the strength and make it

as high workability product. As long as it can save the environment, we can use it. By

replacing this existing materials, we able to satisfy the performance requirements of

these various interests, adequate material properties must be achieved in the fibre

composite and the main objective of engineers in attempting to modify the properties of

cement or concrete are to improve the tensile or flexural strength, to improve the impact

strength, to control cracking and the mode of failure by means of post-cracking ductility,

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and to change the rheology or flow characteristic of the material in the fresh state. ( D.J

Hannat, 1978 )

Fiber-reinforced concrete used in structural applications requires characteristic

material properties that can be easily incorporated into existing design procedures. This

paper investigates the postcracking response of reinforced concrete tension members

made with both plain and steel fiber-reinforced concrete ( SFRC ). Loading was either

monotonic or cyclic, and shrinkage effects are included in analysis of the member

response. Tension-stiffening results are used to determine the average tensile response

of concrete after cracking, and an expression is developed to predict this smeared

behavior as a material property for cracked SFRC, as well as to estimate crack

spacings. Specimens with steel fibers exhibited increased tension stiffening and smaller

crack spacings, which both contributed to a reduction in crack widths. The postcracking

tensile strength of fiber concrete at the cracks is the determining factor affecting

behavior and is a fundamental material property used to predict tension stiffening and

crack behavior for conventionally reinforced SFRC. The uniaxial strength of SFRC

immediately after cracking governs serviceability behavior, while the postcracking

strength at larger deformations governs strength design and is responsible for tension

stiffening after yielding of the reinforcement. Cyclic loading did not have a significant

effect on either tension stiffening or crack width control for the specimens tested.

Rational use of fiber-reinforced concrete ( FRC ) in structural applications

requires a consistent approach to design that incorporates material properties that are

relevant to the types of analysis typically used for reinforced concrete. The main

advantage of using fibers in oncrete is to improve the postcracking response,and for this

reason the postcracking tensile strength is a material property most appropriate for

design with FRC.

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REFERENCES

o D.J Hannat (1978),Fibre Cement and Fibre Concrete, John Wiley & Sons, Inc.

o George C. Hoff ( 1984 ), Fiber Reinforced Concrete International Symposium,

American Concrete Institute.

o Jack C. McCormac, Jack K. Nelson ( 2005 ), Design of Reinforce Concrete Sixth

Edition, John Wiley & Sons, Inc.

o K. H. Tan and M. A. Mansur, “ Shear Transfer in Reinforced Fiber Concrete ” - Journal of Material in Civil © ASCE Vol. 2,No. 4, November, 1990

o Nguyen Van CHANH, “ Steel Fiber Reinforced Concrete ”- Faculty of Civil Engineering, Ho Chi Minh City University of Technology.

O Peter H.Bischoff ( 2003 ), “ Tension Stiffening and Cracking of Steel Fiber-Reinforced Concrete ”- Journal of Material in Civil © ASCE / MARCH/APRIL 2003.

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Fibre Concrete ( Sfrc & Pfc )