wirand technical note

8/2/2019 Wirand Technical Note

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Steel fibers

for concretereinforcemen

TECHNICAL NOTE


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WIRAND Technical Note pag. 1/10

Fiber reinforcement:an ancient technique

industrialized inthe 60s

The presence offibers improves thewithdrawal cracking

resistance andconfers ductility

The inhibition ofwithdrawal crackingphenomena confers

a better statisticstability to the tensile

strength

FIBER REINFORCED CONCRETE

The idea to reinforce with resistant man-made fibrous materials but of elevatedbrittleness gets lost in the night of the times; in ancient Egypt straw was added to

the argillaceous paste with which they manufactured bricks giving them greaterbending resistance and therefore better handling after sun-baking.

Other historical examples of fiber reinforcement exist: plaster reinforced withhorsehair, or with straw in the poorer building trade, to avoid unaesthetic

withdrawal crackss, in reinforced plaster false-ceilings, and concrete mix fiberreinforced with asbestos etc.The scientific approach to the fiber reinforcement problem is recent. The first

studies on the use of steel and glass fibers in concrete go back to the 50s and inthe 60s studies are found of concrete being reinforced with synthetics fibers.

Even though studies and above all direct experimentation continues today, thebehavior and field application of fiber reinforced concrete in relation to the basematerial of the fibers are now very well framed.

The presence of fiber, forms, in any case, a micro reinforcement of extremeeffectiveness in regards to the withdrawal crackingping phenomenon; the

mechanical properties of the fibers material influences considerably its ductility.

Using high resistance fibers, its possible to make concretes that even thoughmaintaining almost unchanged their compression strength property (Rck) theypresent a fairly good ductility.Moreover, fibers of high mechanical resistance allow to manufacture highly tough

concretes, a characteristic that make them particularly able for the realization ofprefabricated manufactured products, of moderate thickness

DUCTILITY: A RESOURCE IN THE STRUCTURAL FIELD

Ordinary concrete in respect to tensile stress presents a rigid-elastic behaviour,

that is, once a certain stress threshold (elastic limit) is reached there is an almostinstantaneous fracture. Moreover its difficult to identify the tensile strength; even

though its linked to the compression strength property (1/10. 1/20 Rck) it presentsan excessive statistic distribution within a structural element mass. Above all in

non reinforced manufactured materials, because of the withdrawal, micro cracksor even micro fractures can occur within the masses that are quite visible whichinevitably compromise the ability to absorb locally the tensile stress.

This is the reason why in the theory of reinforced concrete, the tensile strength ofthe mix is assumed equal = 0. The addition of a suitable fibrous reinforcement tothe concrete, inhibiting therefore the withdrawal related phenomenon, confers to it

discreet stability statistics of the tensile strength.Moreover, should a fracture be induced to a fiber reinforced manufactured

material, the fibers present in the concrete mix guarantee, through a sort of seameffect (see fig. 1), the ability to absorb tensile stress

Fig. 1 Qualitative tensile distribution in fiber reinforcement


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WIRAND Technical Note pag.2/10

Macroscopically this behaviour is ductile. In fact ductility is defined as the ability ofa material to adsorb a notable creeping at constant load when the same has

reached the yielding point, which is an intrinsic properties of every material.Usually, because of operative difficulties, direct tensile tests are not carried out onconcretes. Appraisals of tensile strength and ductility are carried out indirectly by

means of bending tests.Fig. 2 illustrates the possible answers in terms of load-deflection curves;

obtainable by testing prismatic concrete samples (beams) exposed to bending,with different contents of fibers reinforcement.

Ductility:the capacity to

achieve significantskids at a constant

load.

Fig. 2 Qualitative diagrams of load-deflection obtainable by bending tests

Into the elastic phase the addition of fiber doesnt modify the mechanicalbehaviour. Different behaviours can occur beyond point A, more often referred toas first crack strength point.

Curve I represent the behaviour of ordinary concrete beams: being the material ofa rigid-elastic type and the structure isostatic, once the first crack strength load is

reached, there is an immediate collapse.Curve II outlines a much slower collapse and moreover the ability to still absorb,

though light, a load after the first crack strength point.Curve III is typical of a ductile material with a constant load yielding and thereforecan be used for the manufacturing of some structural elements. This material

allows to resolve some engineering problematics, in some cases with low costs.

The only difficulty is in the calculus approach in the project. The structuralresources related to the ductility can only be taken advantage of with an elasto-plastic approach.


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The checks of the acceptable tensile strength are substituted by assessments in

regards to ultimate limit state load, that is, to a particular load related to theassessed structure, taking into account the ductility, depending on the degree of

hyper static nature of the same.In a model of acceptable tensile strength the load that induces, in the sectionthat is mainly stressed, a tension equal to the acceptable one, is the collapse load

(in this case the collapse is conventional). The safety factor of the load referred tothe real collapse is unknown; we know only the safety factor of the a.t.s.

An elasto-plastic approach needs a careful study of the structure. Lets consideran isostatic beam subject to a centred load. (fig. 3)

Fig. 3 Load-deflection curves. The structural behaviourr to ductile material elements

The shaping of fibers

is fundamental to thestrength of fiber

reinforced concrete.

If the element is realized with a material characterized by an appreciable ductile

phase, once the elastic limit load has been reached (Pe), the structure will notcollapse; its actually able to accept a further incremented load, with plasticity

behaviour in strained area, until it reaches the load collapse point. (Pu)Its clear that once the design load has been assigned, the safety factor of the

structure is equal to FS=Pu/Pe.

Influence of longitudinal geometry of fibers on the ductility of fiber reinforced mixes

The ductile effect deriving from the addition of fibers to the ordinary mixes is themacroscopic effect of mechanical actions that reciprocally swap fibers and cement

matrix.

In particular the pull-out modalities considerably influence the ductility results offiber reinforced concrete. For the metallic fibers i ts possible to have three differenttypes of pull-out (fig. 4).

The first diagram (A) contains longitudinal non-deformed and generally shortfibers: once a certain load is reached we have the split of the fiber from the

cement matrix and the consequent pull-out.In shaped fibers, the behaviours shown in diagrams (B) and (C) are more realistic.In the first case (B) once the splitting phase has progressively been reached, a

pull-out phase follows with an acceptable friction.


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Fig.4 Pull-out tests: indicating diagrams related to the longitudinal fibers shaping

Much more effective is the behaviour in case (C): there is a first phase, OA, of

perfect contact followed by a phase of progressive splitting before the pull-out.(AB)

As in the FIBRARMEDI L fibres behaviour this last passage is due, to a difficult pullout obtainable with an effective shaping of the fiber. Even in this case, follows a

pull out phase with an acceptable friction.

Its clear that the ductility of the fiber reinforced mix strongly feel the effects of the

particular fibers used.Elastic modules and breaking load of the base material of the fiber, adhesion

compatibility of the fiber base material with the concrete, longitudinal geometry ofthe fiber (the ability of anchorage to the mix) - cross-sectional geometry of the

fiber (section), amount of fiber added to the mix, arrangement and bearing of thefibers within the mix are some of the factors that influence interaction of fiber concrete mix.

Even though the above listed items are important, it remains of fundamental

importance the inflexibility and the elevated tensile strength of the fiber basematerial: steel fibers therefore are the best available product in the market forstructural applications. In application, which require only the contrast of the

withdrawal cracking in the first concrete setting phases, are used synthetic ornatural materials, with low tensile strength.

Several types of metallic fibers exist: the type of steel and therefore the cracking

strength, the geometry, both longitudinal or cross-sectional, the productionprocesses are some of the properties that differentiate the various market

products.

Influence of cross-sectional geometry of fibers on ductility of fiber

reinforced mixesThe geometry of a longitudinal (outline) or cross-sectional (section) fiber is aproperty of fundamental importance when fibers in high resistance materials are

used.

thanks to theelevated elastic

modules and

breaking load, steelis the best solution

for fiber

reinforcement


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The following analytical considerations turn out are more clear: by indicating Lc as

the critical length of a fiber, or the the minimal length necessary to displayssimultaneously its (pull-out) and breaking, the equilibrium is:

( )24

2

c

lr

LD

D=

where

r = tensile strengthl = concrete steel frictional strengthD = diameter of the fiber, for non-circular sections meaning an equivalent

diameter.

The circular sectionimproves thefiber-concrete

adhesion

By indicating with L the length of a fiber and working out the above formula we canobtain the following

l

r

D

L

2

The ratio L/Dis technically indicated as Aspect ratioFor unshaped round fibers (linear) we usually have l=2MPa; if the cracking stress

is 1200 Mpa follows L/D300, that is the excessive slenderness.An effective shape allows us to use, in the above written formulas, an equivalent

value of l major and therefore to use fibers with a ratio aspect much more

contained.

The equivalent value of ll also depends on the shape of the cross-sectional

section. The pull-out of a fiber is due to the lost of adhesion between concreteand steel. Consider for example two fibers (fig.5) one of circular section and theother of rectangular one.

Its known that when a steel sample is subject to an axial traction strain it enduresa lengthening in the direction of the stress with a subsequent contraction in the

orthogonal planes.

Fig. 5 shows in an extreme form, the contractions of two types of fiber.

The structural implications are considerable. In case of circular section fiber as thecontraction increases a loss of adhesion is reached simultaneously on the entire

perimeter. In case of rectangular section fibers the loss occurs quite quickly on theshorter sides.

Such circumstance is expressed in an instant transfer (jerk effect) of load to thelong sides and therefore a pull-out before having reached the ultimate resistance.The circular section, thanks to the symmetry, is the one that is able to maximize

the use of adhesion to the concrete mix.

Fig. 5 Lateral contractions of fibers of different geometrical sections


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Dosage and ductility

Its already been said that the presence of fiber in the matrix of a mix, produces amacroscopic ductile behaviour. Its obvious that whichever is the nature of the

fibers, such result is given by the mechanical action that microscopically takes

place between the fibers and the mix. Its therefore obvious that the ductility offiber reinforced concretes, depends not only on the properties of the intrinsic fiber,but also on the actual quantity that has been introduced in the mix.

Fig. 6 qualitatively shows how the amount of fiber used contributes to the ductility.

Fig. 6 Ductility increasing with addition of fiber (dosage)

The course of the diagram can be divided in three sections. The first one showsan almost null contribution. As a matter of fact by dispersing a few fibers in themix, their distance doesnt allow any interaction; no behavioural alteration, to the

fiber reinforced concrete, will therefore be noticed.

By increasing the number of fiber in the mix, or rather reducing the dimensions ofthe volume of influence of each fiber, layouts of acceptable fiber static overlay(second section), and therefore the possibility of mutual interaction are reached

macroscopical results: ductile crack. In such section, even small fluctuations to thedosage will produce substantial ductility changes.

Finally, over a certain quantity, the ever-increasing contribution turns out to beinsignificant.

Besides, high quantities can produce mixing difficulties and therefore the need toadopt ad hoc devices that are not always harmless in regards to the final strength

of the mix.

The dosage of fibersinfluences the

ductility increment,over a certain limit it

turns out to beinsignificant but cansometimes producemixing difficulties.


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Fields of employment of fiber reinforced concrete

The employment of fiber reinforced concrete is constantly increasing. Besides theconsolidated applications there are several studies under way on new

employment possibilities of fiber reinforced concrete, especially for metal fibers,

which are the only ones capable of structural interaction with the cement matrix.Studies and researches are under way to show how such micro-reinforcements,combined with traditional reinforcement, allow to improve the behaviour of somebuilding materials, in static but most of all in dynamic fields in regards to bending,

torsion and shear stress.

Another research trend of extreme interest is that of high strength concretes(Rck>120Mpa). One of the problems that high strength concrete presents is the

elevated brittleness and therefore the necessity to adopt rather high coefficients ofsafety, achieving excessive weakening of the maximum admitted stress. This isthe reason why is necessary to study various measures that are able to give such

cements ductility and so reducing the coefficients of safety. A possible solutioncould be the addition of steel fibers, information is not yet available as studies and

tests are still under way.

Currently we use fibrous reinforcements, besides particular structural applications,

in the in-plant manufacturing (prefabrication) of thin finishing elements.

Thin elements (Sheet type)For thin bidimensional elements a traditional reinforcement with a weldedmesh usually seems to be ineffective. The limited arm of the inner brace

becomes, most of the times, an inefficient module of strength. Furthermore,the reduction of the section close to the reinforcements leads to a widespread

withdrawal cracking during the drying process. A fibrous reinforcement,particularly a metallic one, allows not only to avoid the withdrawal related

phenomena but also to reach the minimum bending strength requirements.

Concave Elements (Shell type)For very thin and curved elements, be they of single or double curvature, acertain strength in the wet mix is usually required, its usually obtained by

using large quantities of fibers (5% in volume). Because of economic andtechnical reasons (mixing difficulties) non-metallic fibers (glass or syntheticfibers of various nature) are used in these applications

Structural applicationsMetallic fibers are the ones mainly used in structural fields. The ductility givenby the fibers to the concrete, allows the replacement of the ( welded mesh )traditional reinforement. Such replacement allows, besides economic issues,

the convenience to face with greater efficiency and security some problems of

geotechnical nature. One of the main examples of metallic fibers applicationsis the pre-lining with sprayed concrete of centred tunnels.

The traditional procedure sees the insertion of welded mesh between twocenterings and subsequently the spraying of concrete. Once the spraying hasbeen completed the mesh should be approximately in the center of the thickness

and be able in this way to react, even though with an halved inner couple brace,both in positive and negative moments. In reality its extremely difficult to arrange

the meshes correctly between the centerings and moreover in some casesbecause of the peculiarity of the rocks its necessary to spray the concreteimmediately after excavation. The use of steel fiber reinforced concrete,

completely compatible with the spraying pumps, allows us to face the problem


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Another field of Civil Engineering where structural purpose fibers are widely usedis hard paving one (airports, roads, industrial pavements). Heavy and above all

static loads in bituminous mixes, could lead to unacceptable stress; that is why weuse steel fiber reinforced concrete slabs.

Such structural elements, usually of wide surface, present emphasized withdrawalphenomena, the use of metallic fibers highly reduces this phenomena, eliminating

the risk of cracks. By choosing or dosing the fibrous reinforcement properly itspossible to substitute the slow traditional reinforcement with economic andoperational advantages.

Steel fibers are widely used in repairs and maintenance fields. A thin layer of fiber

reinforced concrete usually presents an even stronger toughness than a thickerone realized with the same mix.

In fact if the length of the fiber is more than the thickness of the layer we have,during the spraying and the trimming, a positioning (trend) of the fibers in thesame level of the layer. Cracks spreading in such layer is slowed down and, for

dynamic loads, it has to be repeated over and over again.

The suitability of this material in repairs to tanks, dams, caisson rafters, bridgeslabs and mine or tunnel linings are therefore more than clear.

Durability

The durability issue in steel fiber reinforced concrete has been subject of manystudies. Attention was given most of all to the corrosion of the fibers and in

particular to the ones in proximity of the surface. In fact since the fibers alreadyhave a rather restrained diameter, a possible reduction due to corrosion wouldconsiderably reduce the strength of the fibers and therefore the properties of the

concrete.Studies and tests have highlighted the excellent behaviour in time of steel fiber

reinforced concrete even in particularly harsh environments such as marine ones

or in difficult conditions such as freeze-thaw.In studies regarding fiber reinforced concretes behavioural changes due to

corrosion of the fibers, it has emerged that a moderate degree of corrosion,increasing for grime effect the pull out strength of the fibers, is not always

transformed in loss of mechanical propertiess of the concrete. As of today itopenly appears that steel fiber reinforced concrete thanks to a more compact and

micro-crack free structure have in time, a much more stable behaviour.


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concrete. Transactions of the Japan Concrete Institute, 6 (1984)103-10

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and Lightweight Concrete, 5 (1983) 49-53.Knoblauch, H., Manufacture and properties of steel fiber reinforced concrete and mortar with increasing

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Hannant, D.J. & Edgington, J., Durability of steel fiber concrete. In Fiber Reinforced Cement and Concrete,

Proceedings, RILEM Symposium, The Construction Press, UK 1975, pp. 159-69Mangat, P.S. & Gurusamy, K., Steel fiber reinforced concrete for marine applications. In Proceedings 4th

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Banthia, N. & Foy, C., Marine curing of steel fiber composites. J.Materials in Civil Engineering. ASCE 1(1989)86-96

Halvorsen, G.T. Kesler, C.E. Robinson, A.R. & Stout, J.A., Durability and physical properties of steel fiber

reinforced concrete. Report No. DOT-TST 76T-21, US Department of Transportation, Federal RailroadAdministration, Washington, DC, 1976, 73 pp.Hannant, D.J., Additional data on fiber corrosion in cracked beams and theoretical treatment of the effect of

fiber corrosion on beam load capacity. In fiber Reinforced Cement and Concrete, Proceedings RILEM

Symposium, The Construction Press, UK 1975, pp. 533-8Mangat, P.S. & Gurusamy, K., Corrosion resistance of steel fibres in concrete under marine exposure.

Cement and Concrete Research, 18 (1988) 44-54

Kosa, K. & Naaman, A.E., Corrosion of steel fiber reinforced concrete. ACI Matrials J. (in press).G. A. Plizzari, S. Cangiano, and N. Cere - Postpeak Behavior of Fiber-Reinforced Concrete under Cyclic

Tensile Loads ACI Materials Journal/March-April 2000;

M. Di Prisco, F. Iorio, G. A. Plizzari - High Performance Steel Fibre Reinforced Concrete prestressed roofelements Bochum 2003;

G. A. Plizzari, S. Cangiano, R. Cucitore - A new proposal for a standard test method on fibre reinforced

concrete International Workshop on Structural applications of steel fibre reinforced concrete - Milano2000;

G. A. Plizzari, A. Meda - A new design approach for SFRC slabs on grade based on fracture mechanics ACI Structural Journal Accepted paper;

G. A. Plizzari, A. Meda, B. Belletti, R. Cerioni - Experimental and numerical analyses of SFRC slabs on

grade Sixth RILEM Symposium on FRC - BEFIB 2004;G. A. Plizzari, A. Meda, L. Sorelli, B. Rossi - Experimental investigations on slabs on grade: steel fibers vs.

conventional reinforcement Sixth RILEM Symposium on FRC - BEFIB 2004;G. A. Plizzari, A. Meda, L. Sorelli, B. Rossi - Fracture mechanics for SFRC Pavement - FIB-CEB

Symposium Avignon 2004;

B. Belletti, R. Cerioni, G. A. Plizzari - Fracture in SFRC slabs on grade Sixth RILEM Symposium on FRC

BEFIB 2004;G. A. Plizzari, L. Cominoli, G. Perri, R. Perri - Revestimientos de tneles en concreto reforzado con fibras

metlicas - XVIII Seminario Venezolano de Geotecnia Caracas 2004


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