Fibre reinforcement cement and concrete

Fiber reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It

contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers,

glass fibers, synthetic fibers and natural fibers. Within these different fibers that character of fiber reinforced

concrete changes with varying concretes, fiber materials, geometries, distribution, orientation and densities.

Fiber reinforcement in the form of asbestos fibers has been used in the cement industry for nearly a hundred

years. However, during the past twenty years a variety of new organic and inorganic fibers have been introduced

into the construction industry, and the theoretical understanding of their behavior at fiber volumes of between

0.1 and 10% in the composite has been comprehensively developed. The major uses of steel and polypropylene

fibers are in massive concrete structures whereas carbon, cellulose, glass, natural, polypropylene, and polyvinyl

alcohol fibers are used in thin sheet products. The theory and practice of these relatively new bulk markets for

fibers are reviewed, together with examples of the most widely used products.

A fiber-reinforced building material in one embodiment incorporates cellulose fibers that are chemically treated

with a dispersant to impart improved dispersibility to the fibers. The fibers are treated with a dispersant that

deactivates the hydroxyl sites of the fiber surfaces and in some cases, making the fiber surface more

hydrophobic. The dispersant inhibits the hydroxyl groups on the cellulose fiber surface from bonding with

hydroxyl groups of other fibers and from bonding with hydroxyl groups of the same fiber, thereby significantly

reducing inter-fiber and intra-fiber hydrogen bonding. The treated fibers can be readily dispersed and uniformly

distributed throughout a mixture without re-clustering or reclumping once the mechanical mixing action stops.

The chemically treated fibers with improved dispersibility improve the fiber distribution and reinforcing

efficiency, which in turn improves key physical and mechanical properties of the material such as the modulus of

rupture, z-direction tensile strength, and toughness, and surface finishes. With improved fiber reinforcing

efficiency, less dosage of fiber is needed to achieve the required physical and mechanical properties.

Fiber-reinforcement is mainly used in shotcrete, but can also be used in normal concrete. Fiber-reinforced

normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of

construction parts (beams, pillars, foundations etc) either alone or with hand-tied rebar’s.

Concrete reinforced with fibers (which are usually steel, glass or "plastic" fibers) is less expensive than hand-tied

rebar, while still increasing the tensile strength many times. Shape, dimension and length of fiber are important.

A thin and short fiber, for example short hair-shaped glass fiber, will only be effective the first hours after

pouring the concrete (reduces cracking while the concrete is stiffening) but will not increase the concrete tensile

strength. A normal size fiber for European shotcrete (1 mm diameter, 45 mm length—steel or "plastic") will

increase the concrete tensile strength.

Steel is the strongest commonly-available fiber, and come in different lengths (30 to 80 mm in Europe) and

shapes (end-hooks). Steel fibers can only be used on surfaces that can tolerate or avoid corrosion and rust

stains. In some cases, a steel-fiber surface is faced with other materials.

Glass fiber is inexpensive and corrosion-proof, but not as ductile as steel. Recently, spun basalt fiber, long

available in Eastern Europe, has become available in the U.S. and Western Europe. Basalt fiber is stronger and

less expensive than glass, but historically, has not resisted the alkaline environment of Portland cement well

enough to be used as direct reinforcement. New materials use plastic binders to isolate the basalt fiber from the

cement.

The premium fibers are graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weight and

corrosion-proof. Some experimenters have had promising early results with carbon nano tubes, but the material

is still far too expensive for any building.

For statistical calculations there is a new modelling in the book: B.Wietek, Stahlfaserbeton, edited by Vieweg +

Teubner, 2008, ISBN 978-3-8348-0592-8. Here is the possibility to calculate also columns, platforms, beams and

also shear situations which show a better behavior than concrete alone. We hope that a translation of this book

is available soon, that we can reconstruct all the theory which is given.

The behavior of fiber-reinforced cement composite (FRCC) flexural members under large displacement reversals

was experimentally evaluated. Emphasis was placed on estimating the displacement capacity and shear strength

of members constructed with strain-hardening FRCC materials. Two types of fibers were used: Ultrahigh

molecular weight polyethylene fibers and steel hooked fibers in volume fractions ranging between 1.0 and 2.0%.

The primary experimental variables were: (1) fiber type and volume fraction; (2) type of cement-based matrix

(concrete or mortar); (3) average shear stress demand at flexural yielding; and (4) shear resistance provided

through hoops versus total shear demand. All specimens constructed with a strain-hardening FRCC, with or

without web reinforcement, exhibited drift capacities of at least 4.0%. A shear stress level of 0.30√′+c[MPa]

represented a lower bound for which no shear failure occurred in the strain-hardening FRCC test specimens,

regardless of the member inelastic rotation demand. In addition, buckling of longitudinal reinforcement in the

strain-hardening FRCC members without web reinforcement was not observed up to plastic hinge rotations of

4.0%

Concrete lends itself to a variety of innovative designs as a result of its many desirable properties. Not only can it

be cast in diverse shapes; but it also possesses high compressive strength, stiffness, low thermal and electrical

conductivity and low combustibility and toxicity.

Two characteristics, however, have limited its use: it is brittle and weak in tension. Recently, however the

development of fiber-reinforced composites in the plastics and aerospace fields has provided a technical basis

for improving these deficiencies.

This Digest describes the general properties and application of fiber-reinforced concrete used in construction.

The promise of thinner and stronger elements reduced weight and controlled cracking by simply adding a small

amount of fibers is an attractive feature of fiber-reinforced concrete.

Testing Fiber Reinforced Concrete

Role of Fibers

When the loads imposed on concrete approach that for failure cracks will propagate, sometimes rapidly; fibers

in concrete provide a means of arresting the crack growth. Reinforcing steel bars in concrete have: the same

beneficial effect because they act as long continuous fibers. Short discontinuous fibers have the. Advantage,

however, of being uniformly mixed and dispersed throughout the concrete. Fibers are added to a concrete mix

which normally contains cement, water and fine and coarse aggregate. Among the more common fibers used

are steel, glass, asbestos and polypropylene.

Physical and Mechanical Properties of Selected Fibers

Fiber Diameter µm

Specific Gravity Failure Srain

% Modulus of Elasticity, GPa

Tensile Strength, GPa

Steel 5-500 7.8 3-4 200 1-3

Glass 9-15 2.6 2-3.5 80 2-3

Polypropylene 7.5 0.9 20.0 5 0.5

Mica Flakes 0.01-200 2.9 -- 170 0.25

Asbestos 0.02-20 2.5-3.4 2.3 200 3

Carbon 7.5 1.7-2.0 0.5-1.0 300-400 2-3

If the modulus of elasticity of the fiber is high with respect to the modulus of elasticity of the concrete or mortar

binder, the fibers help carry the load, thereby increasing the tensile strength of the material. Increases in the

length: diameter ratio of the fibers usually augments the flexural strength and toughness of the concrete. The

values of this ratio are usually restricted to between 100 and 200 since fibers which are too long tend to "ball" in

the mix and create workability problems.

As a rule, fibers are generally randomly distributed in the concrete; however, processing the concrete so that

the fibers become aligned in the direction of applied stress will result in even greater tensile or flexural

strengths.

Fabrication

Before mixing the concrete, the fiber length, amount and design mix variables are adjusted to prevent the fibers

from balling. Satisfactory reinforced mixes usually contain a mortar volume of about 70 per cent compared with

a mortar volume of about 50 percent for typical unreinforced concrete mixes.

Fiber-reinforced cement boards contain no coarse aggregate. These products are usually made by spraying

mortar and chopped fiber simultaneously. Mortar with a high water: cement ratio is used to facilitate spraying.

Other application methods include simple casting, which is less versatile than spraying, and press moulding,

which results in lower effective water: cement ratio, thus producing a stronger product.

Chemical admixtures are added to fiber-reinforced concrete mixes primarily to increase the workability of the

mix. In North America, air-entraining agents and water-reducing admixtures are usually added to mixes with a

fine aggregate content of 50 per cent or more.

Superplasticizers1, when added to fiber-reinforced concrete, can lower water: cement ratios, and improve the

strength, volumetric stability and handling characteristics of the wet mix.

Fiber Reinforcement for Concrete

Fiber reinforcement protects concrete. It minimizes cracking as the concrete dries and it helps hold the concrete

together if cracks develop after the concrete has hardened. Fibers are also available for decorative concrete

Fibre

Various Applications

A proliferation of new developments in fiber-reinforced concrete technology has greatly extended the range of

applications

Application of Various Fibers in Cement Products

Fiber Type Application

Glass Precast panels, curtain wall facings, sewer pipe, thin concrete shell roofs, wall plaster for concrete block.

Steel Cellular concrete roofing units, pavement overlays, bridge decks, refractoriness, concrete pipe, airport runways, pressure vessels, blast-resistant structures, tunnel linings, ship-hull construction.

Polypropylene, nylon

Foundation piles, prestressed piles, facing panels, flotation units for walkways and moorings in marinas, road-patching material, heavyweight coatings for underwater pipe.

Asbestos Sheet, pipe, boards, fireproofing and insulating materials, sewer pipes, corrugated and flat roofing sheets, wall lining.

Carbon Corrugated units for floor construction, single and double curvature membrane structures, boat hulls, scaffold boards.

Mica Flakes Partially replace asbestos in cement boards, concrete pipe, repair materials.

Fibers also generally reduce creep strain, which is defined as the time-dependent deformation of concrete under a constant stress. For instance, steel-fiber-reinforced concrete can have tensile creep values 50 to 60 per cent of those for normal concrete. Compressive creep values, however, may be only 10 to 20 per cent of those for normal concrete.

Shrinkage of concrete, which is caused by the withdrawal of water from concrete during drying, is lessened by fibers. Shrinkage of glass-fiber-reinforced concrete is decreased by up to 35 per cent with the addition of 1.5 per cent by volume of fibers.

Other properties of concrete, such as compressive strength and modulus of elasticity, are not included in the tables since they are affected to a much lesser degree by his presence of fibers.

Environmental Factors

The resistance of fiber-reinforced concrete to environmental factors such as frost action depends on the quality

of the concrete matrix material and should not differ substantially from that of conventional concrete. Fibers

can be effective, however, in reducing frost damage because of their crack-arresting properties. Care should be

taken to ensure that an adequate amount of entrained air is incorporated in the mix to provide additional

resistance to freezing and salt corrosion.

Other environmental problems such as acid attack, sulphate attack and alkali-aggregate reaction are generally

not augmented by the presence of fibers unless there is a chemical reaction between the fiber and the concrete.

Effect of fibers in concrete

Fibers are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also

lower the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater

impact, abrasion and shatter resistance in concrete. Generally fibers do not increase the flexural strength of

concrete, so it can not replace moment resisting or structural steel reinforcement. Some fibers reduce the

strength of concrete.

The amount of fibers added to a concrete mix is measured as a percentage of the total volume of the composite

(concrete and fibers) termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is

calculated by dividing fiber length (l) by its diameter (d). Fibers with a non-circular cross section use an

equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fiber 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 fiber usually segments the flexural strength and toughness of the matrix.

However, fibers which are too long tend to "ball" in the mix and create workability problems.

Some recent research indicated that using fibers in concrete has limited effect on the impact resistance of

concrete materials [1 & 2]. This finding is very important since traditionally people think the ductility increases

when concrete reinforced with fibers. The results also pointed out that the micro fibers is better in impact

resistance compared with the longer fibers.

The High Speed 1 tunnel linings incorporated concrete containing 1 kg/m³ of polypropylene fibers, of diameter

18 & 32 μm, giving the benefits noted below

Benefits

Polypropylene fibers can:

Improve mix cohesion, improving pumpability over long distances

Improve freeze-thaw resistance

Improve resistance to explosive spalling in case of a severe fire

Improve impact resistance

Increase resistance to plastic

Some developments in fiber reinforced concrete

The newly developed FRC named Engineered Cementitious Composite (ECC) is 500 times more resistant to cracking and 40 percent lighter than traditional concrete. ECC can sustain strain-hardening up to several percent strain, resulting in a material ductility of at least two orders of magnitude higher when compared to normal concrete or standard fiber reinforced concrete. ECC also has unique cracking behavior. When loaded to beyond the elastic range, ECC maintains crack width to below 100 µm, even when deformed to several percent tensile strains.

Recent studies performed on a high-performance fiber-reinforced concrete in a bridge deck found that adding fibers provided residual strength and controlled cracking. There were fewer and narrower cracks in the FRC even though the FRC had more shrinkage than the control. Residual strength is directly proportional to the fiber content.

A new kind of natural fiber reinforced concrete (NFRC) made of cellulose fibers processed from genetically modified slash pine trees is giving good results. The cellulose fibers are longer and greater in diameter than other timber sources. Some studies were performed using waste carpet fibers in concrete as an environmentally friendly use of recycled carpet waste. A carpet typically consists of two layers of backing (usually fabric from polypropylene tape yarns), joined by CaCO3 filled styrene-butadiene latex rubber (SBR), and face fibers (majority being nylon 6 and nylon 66 textured yarns). Such nylon and polypropylene fibers can be used for concrete reinforcement.

For statically calculations there is a new modelling in the book: B.Wietek, Stahlfaserbeton, edited by Vieweg + Teubner, 2008, ISBN 978-3-8348-0592-8. Here is the possibility to calculate also columns, platforms, beams and also shear situations which show a better behavior than concrete allone. We hope that a translation of this book is available soon, that we can reconstruct all the theory which is given.

Conclusion

Innovations in engineering design, which often establish the need for new building materials, have made fiber-

reinforced cements very popular. The possibility of increased tensile strength and impact resistance offers

potential reductions in the weight and thickness of building components and should also cut down on damage

resulting from shipping and handling.

Although ASTM C440-74a describes the use of asbestos-cement and related products, there are, at this time, no

general ASTM standards for fiber-reinforced cement, mortar and concrete. Until these standards become

available, it will be necessary to rely on the experience and judgment of both the designer and the fiber

manufacturer. The onus is thus on the designer to be aware of the limitations presently inherent in some of

these composites, particularly the durability of glass-fiber-reinforced products.

Reference

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd223_e.html

http://en.wikipedia.org/wiki/Fiber_reinforced_concrete

www.google.com (for searching)