8/3/2019 Concrete Basics-Guide to Steel Fibres

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Concrete BasicsConcrete BasicsConcrete BasicsConcrete Basics Guide To Steel FibresGuide To Steel FibresGuide To Steel FibresGuide To Steel Fibres

Comparison of slab on grade floor options from different suppliers

INTRODUCTION

Scancem Materials development of theuse of steel fibres for slabs on grade overthe last ten years has proven theperformance, construction advantagesand cost savings of high performancesteel fibres. This has led to growth indemand and more recently a large rangeof fibres being promoted.

Whilst competition is good for theconsumer it is a concern if the customer

is not getting what he expects or needs.Scancem Materials have established theirtrack record using high performancefibres and a toughness based designapproach. Most of the new fibres on themarket are low performance (hence lowcost/kg) and suppliers are following lessconservative design approaches in orderto be competitive. This is leading to slabfailures, which is reflecting poorly on thesteel fibre industry.

The purpose of this note is to provide asimple guide to specifiers on how to:

differentiate between lowperformance and high performance

fibres

compare the performance ofdifferent SFRC floor options provideby different manufacturers.

Fibres can vary in performance quitedramatically and so fibre dosage and slabthickness should reflect this. Fibregeometry, strength, deformations andtheir ability to be evenly distributedthrough the concrete all have a bearingon the load carrying capacity of SteelFibre Reinforced Concrete (SFRC)

SPECIFICATION OFSFRCSLABS ON

GRADE

There is no Australian standard coveringthe design of SFRC slabs on grade. Theonly guidance comes from TechnicalReport 34 (TR34) from the ConcreteSociety (UK). It requires that Re3 values(% of flexural strength at 3mm deflectionof a standard beam test) shall be such asto assure the stress in the slab isacceptable for the applied stress. This

applied stress should combinetemperature, shrinkage and applied loadinduced stresses. TR34 gives guidance onone load case but the designer is left todetermine how to handle other loadcases.

Scancems design guide fully details howto handle each load case and can be usedto calculate the stress and required Re3.

CHARACTERISTICS OFSTEELFIBRES

There are four properties of steel fibrethat are important.

1. Aspect Ratio (L/D)

All other things being equal performanceincreases as the aspect ratio(length/diameter) of a fibre increases.To increase the aspect ratio of a fibre youmust increase the length while decreasingthe diameter. By doing this you areincreasing the number of fibres per kg.

All else being equal twice the dosage of aloose fibres with aspect ratio of 40 wouldbe required to give the same performance

as a fibre with an aspect ration of 80. This has particular relevance as loosefibres (most suppliers) have to have anaspect ratio of under 50 to avoid ballingwhile Scancem commonly use a collated80 aspect ratio fibre (see 4).

2. Fibre Anchorage Details

In order for the SFRC to continue tocarry load and deform plastically aftercracking has occurred, it is essential that

the fibre is sufficiently anchoredin the concrete matrix to enable thefull tensile strength of the fibres to be

harnessed

fibres pull through as the ultimatefibre capacity is reached. This meansthe full capacity of the fibre isachieved over high deformationsgiving high energy absorption and thecharacteristic ductility required toprevent brittle failure. This is the basisfor TR34s specification of Re3values.

In terms of fibre geometry the followingis important:

Small changes to the shape of ahooked end anchorage can mean thedifference between high and lowtoughness

Continuously deformed fibres,enlarged end fibres and mill cut fibre will bond over a short length of thefibre rather than pull through and amore brittle failure will result. Fullscale tests show that this type offailure is not consistent with increasedload carrying capacity even at highdosages. Manufacturers of these typesof fibres typically do not quote Re3 values as they are low. More typicallythey quote I5, I10, and I30 vales whichcorrespond to the more brittle failuremode.

3. Physical Properties of Steel

Fibres

To maintain the ductility of SFRC andensure the reliability of the plasticdeformation, it is imperative that the well-anchored fibres do not break.Breaking fibres equates to a brittle failuremode (this is the main problem with

fibres continuously deformed). Toprevent breakage steel fibres should bemanufactured with sufficient tensilestrength to ensure the ultimate failuremode is pullout rather than breakage.Fibres manufactured from hard drawnsteel wire make available tensile strengthsin excess of 1000MPa.

Sleet sheet steel fibres are available fromsome supplier are particularly cheap/kgbut their Re3 performance is generallyvery low.

4. FIBRE PACKAGING

High aspect ratio fibres are generallyhighly efficient in structural termshowever, they tend to ball when mixed inconcrete unless packaged correctly.Fibres can be glued together in strips ofabout 30-50 fibres with a water-solubleglue. High aspect ratio fibres whencollated in this way can be added to themix almost like an extra aggregate and noballing will occur. The aspect ratio of thebundle is less and the water-soluble glueguarantees that the fibres are uniformlydistributed in the mix. Fibres are added

Products For Concrete

8/3/2019 Concrete Basics-Guide to Steel Fibres

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into the truck in degradable bags for easeof handling. Low aspect ratio, loosefibres tend to be packaged in boxes. Theycan ball (even though low aspect ratio)and dispersion problems can occour ifnot handled and mixed with care.

SFRCSLAB ONGRADEDESIGN

Based on the fibre properties shown intable 1 (properties shown are frommanufacturers information) we can do acomparative design. The design consistof checking the fibre dosage to achievethe required moment capacity andchecking that the fibre dosage is adequateto achieve load transfer between fibres.

Re3 Requirement

Re3 is a toughness coefficient defined insection 3.2.6 of Technical Report 34(TR34) from the Concrete Society (UK)and is used in Appendix F of the same

publication to determine the momentcarrying capacity of a SFRC slab ongrade.

Re3 is calculated as the average loadcarrying capacity offered after cracking(due to the presence of fibrereinforcement) divided by the flexuraltensile strength of the uncrackedconcrete matrix the ratio is typicallyexpressed as a percentage. TR34 statesthe Re3 value used should be greater than30%. Under this critical value the slab will behave as an unreinforced or plainconcrete slab and should be designed

using the Westergard formulae.Example

If we want to calculate the fibre dosagefor a 150mm thick slab, 32MPa concrete with a moment capacity (Mu ) of 7.80kN/m we know:

Mu = Re3ft x bd2/6 of a SFRC section

Therefore Re3 (ductility required fromfibres and is different for different fibres)

= Mu x 6/bd2ft

= 7.8 x 6 x 106/150 x 150 x 103 x 4.73

= 0.44 (ie ductility factor required)

Comparative Design

17kg/m3 Scanfibre CHO80/60NB(high tensile wire, hooked end fibrewith 80 aspect ratio)

20kg/m3 of Scanfibre CHO65/60NB(high tensile wire, hooked end fibrewith 65 aspect ratio)

30kg/m3 of Qubix UW10/50 fibre (50aspect ratio wire fibre with continuousdeformations. Estimate based onaspect ratio and design. Manufacturersonly quote I5, I10, and I30)

40kg/m3 of Qubix CR50 fibre (slit sheetfibre. Estimate based on type of fibre.No performance values quoted bymanufacturer)

40kg/m3 of BHP EE25HT fibre(enlarged end high tensile fibre)

75 kg/m3 of BHP EE18mm fibre(enlarged end fibre)

Minimum Reinforcement

European publications alsorecommendation that, irrespective of theRe3 requirements the minimum steelfibre dosage permitted shall be thatrequired to give a maximum avaeragespacing factor of 0.45 times the nominalfibre length calculated using the spacingtheory of McKee. This is to ensureadequate overlap between fibres totransfer load. The McKee formula canbe broken down to min. dosage = 67,658/ (Aspect ratio)2

This has been used to calculate theminimum fibre dosage for load transferin table2.

Final Dosage Requirements for

various fibres.

The final dosages required for the variousoptions is found from the maximum of

moment capacity or load transferrequirements and are given in table 2

CONCLUSIONS

Fibre dosage varies according to theperformance of the particular fibrechosen. Merely specifying a dosage willnot take into account the individualperformance of the fibre. At a lowdosage a particular fibre may meet theductility requirements as determined via abeam test, however the dosage may notbe enough to provide the continuousreinforcement required for load transferas determined by McKee.

It is therefore important for designers toagree the toughness design (even whenundertaken by suppliers) and to specifythat the SFRC slabs shall:

1. achieve a performancerequirement for load carrying bystating the required Re3 value

2. comply with minimum dosagerequirements of McKee

In this way the design requirements ofload capacity and load transfer will beequal for all fibre suppliers and the mostcost competitive supplier will win theproject.

Table 1: Fibre Properties

Fibre Properties ScanfibreCHD80/60NB

ScanfibreCHO65/60NB

BHPEE25HT QUBIX CR50

Length 60mm 60mm 25mm 50mmDiameter 0.75mm 0.92mm 0.55mm??mm Aspect Ratio 80 65 45 ??

Tensile strength 1200Mpa 1200Mpa 800Mpa 750MPa Anchorage Hooked ends +

indentHooked ends Enlarged ends Undulating

Collation Collate Collate Loose fibres Loose fibresFibre type Drawn wire Drawn wire Cut sheet Shit SleetPackaging Degradable bags Degradable bags Boxes BoxesRe3 value 44% at 15kgs 44% at 20kgs 38% at 35kgs ??Min. Dose 11kg/m3 16kg/m3 35kg/m3 ??kg/m3

Table 2 Fibre Dosages For Structural Design Of SlabOn Grade using Toughness Design Method.

Fibre Type ForMomentCapacity

For LoadTransfer

FinalDosage

ScanfibreCHO80/60NB

17kg/m3 11kg/m3 17kg/m3

ScanfibreCHO65/60NB

20kg/m3 16kg/m3 20kg/m3

Qubix UW10/50 30kg/m3 27kg 30kg/m3BHP EE18mm 75kg/m3 67kg 75kg/m3

BHP EE25HT 40kg/m3

34kg/m3

40kg/m3

Qubix CR50 40kg/m3 ?kg/m3 40kg/m3

The information given is based on knowledge and performance of the material Every precaution is taken in the manufacture of the product and the responsibility is limited to the quality of supplies, withno guaranty of results in the field as Scancem Materials has no control over site conditions or execution of works

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