GRADE 500C REINFORCEMENTGUARANTEED DUCTILITY

GRADE 500C REINFORCEMENTGUARANTEED DUCTILITY

CI/SfB

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Uniclass P227

September 2005

CELSA

INTRODUCTION

DUCTILITY• Introduction to ductility• The need for ductility in

reinforcing steels• Defining the ductility of

reinforcing steels• Comparison of ductility

according to steel grade

CELSA DUKTIL 500CREINFORCING STEELS• Introduction• Tensile properties• Bend performance• Fatigue performance• B0nd performance• Weldability• Mechanical capacities

CELSA DUKTIL 500CREINFORCING BARS• Introduction• Rib geometry• Identification• Labelling• Test certification

CELSA DUKTIL 500CREINFORCING COILS• Introduction• Rib geometry• Identification• Labelling• Test certification • Advantages of using

coiled Celsa Duktil 500C

INDEXINDEX

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APPLICATIONS FORCELSA DUKTIL 500C • Applications for Celsa Duktil

500C special ductility steel• Advantages of using 500C

high ductility steels

APPENDIX • Why we need ductility in

reinforced concrete structures.By Professor Andrew Beeby,University of Leeds

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CELSA DUCTILITY: GUARANTEE OF SAFETYCELSA DUCTILITY: GUARANTEE OF SAFETY

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INTRODUCTION

When referring to reinforced concrete,ductility is often regarded as a complexconcept. However, put simply, it refers to abuilding’s ability to dissipate energy anddeform without brittle or sudden failure.

Regarding the steel bars used for concretereinforcement (rebar), ductility can bedefined as the ability of the bar to deformplastically without fracture, whilstmaintaining its strength. This concept canbe directly related to the non-linearbehaviour of structures in which ductilityplays an important role.

For many years engineers have recognisedthat continuous construction (i.e. staticallyindeterminant structures) achieves fargreater load carrying capacity than similarsimple structures (single span – staticallydeterminant). This is due to a continuousstructure’s ability to transfer potentialoverloading stresses to other less worked regions of the structure via rotation and deformation.

This behaviour can lead to significantdeformation for only a small increase inload. The design of structural elementsperforming in this way goes far beyond therealms of linear elastic analysis.

Much testing has been carried out tounderstand this plastic performance better.Not only did this prove that the responsewas very different to that theoreticallypredicted, but it led to the development ofa factor for flow or plasticity. In design, thisfactor for flow and plasticity is adopted inthe form of moment redistribution.

In 1965 the basic principles of steel’s non-linear behaviour were published in CEBBulletin No 52. Today the subject is againbeing studied to produce plastic designcalculations based on better understanding,increased computing power and reviseddesign standards. These standards haveincorporated indirect methods of plasticcalculation based on the redistribution ofmoments obtained from elastic analysis.

It should never be forgotten that the ability to take a reinforced concreteelement beyond linear elastic analysis is almost entirely due to the ductility of the steel reinforcement.

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All structures need ductility, as well asstrength. In reinforced concrete, it is thereinforcing steel, which provides thestructural element with ductility. Ductility of the element is its ability to fail bydeflection or extensive cracking in anoverload situation, without suddencatastrophic collapse.

Recent developments in design codes, suchas Eurocode 2 (BS EN 1992-1-1:2004) haverecognised the importance of the ductility ofthe reinforcing steel. New parameters for thespecification of steel ductility have beenintroduced, in both Eurocode 2 and inmaterial standards.

Eurocode 2 recognises three differentreinforcing steel classes (A, B and C), based on ductility. Design rules nowrecognise the importance of steel ductilityand provide designers with the means ofusing the additional ductility provided byhigh ductility steels.

The British Standard for reinforcing steels(BS 4449), which will be used inconjunction with the new EuropeanStandard (BS EN 10080), has been revisedto reflect the three ductility classes ofEurocode 2. The characteristic yield strengthhas also been increased from 460 MPa to500 MPa. The three grades introduced in BS 4449:2005 are B500A, B500B and B500C.

This brochure explains the importance ofductility in reinforced concrete structures,and the importance of ductile reinforcingsteels. This is dealt with in some detail inthe Appendix, written by Professor AndrewW. Beeby of Leeds University.

Celsa Steel (UK) Ltd, on the basis ofextensive experience in group companies in Spain, has developed new steels for the UK market that fully meet all of therequirements of BS 4449 Grade B500C. This steel is called CELSA DUKTIL 500C.

The brochure describes CELSA DUKTIL 500Csteels, their manufacture and properties.

CELSA DUKTIL 500C steels, new to the UKmarket, will give designers the ability to usehigh ductility steels, gaining the full designbenefits of Eurocode 2, producing structureswith added robustness and safety.

INTRODUCTION TO DUCTILITY

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THE NEED FOR DUCTILITY IN REINFORCING STEELS

Strength is a characteristic of steel requiredin calculations, and recognised in standardsand design codes, because steel plays anintegral role in the mechanical behaviour ofreinforced concrete.

Standards define strength requirements by means of two parameters: the elasticlimit or yield strength (which is used in the steel grade designation) and theultimate tensile strength.

Strength is a necessary requirement but it is not sufficient in itself to define thebehaviour of reinforcing steel in concrete.There is also a requirement for ductility.Concrete is a brittle material, withoutductility, and cannot be used for structuralapplications without reinforcement.

The ductility within a structure, somethingwhich concrete does not have, is providedby the steel reinforcement. Therefore thesteel must be sufficiently ductile so thatevery reinforced concrete section, includingstructural elements, has the capacity todeform by an adequate amount.

Initially concrete was considered as thelimiting factor for plastic rotation because ofits low level of ductility. In general it wasconsidered that the capacity for plasticrotation was independent of the type ofsteel used, because it was assumed that theconcrete was the limiting factor. The steelwas assumed to have sufficient ductility sothat it did not limit rotation.

This presumption was made because at that time steel had comparatively lowstrength and high ductility owing to itschemical composition and the manufacturingprocesses employed. Later steels withhigher strength and lower ductility wereintroduced, such as cold rolled wire(Grade A material).

The ductility of a steel is its capacity to deform under load without

fracture once the yield strength has been exceeded

Further studies have shown that the level ofductility of the steel influences and limitsthe rotation of plastic hinges.

In addition to the strength and ductilityrequirements, reinforced concrete needssufficient bonding between steel andconcrete to ensure the composite worksefficiently and cracking is controlled.

Ductility is a property of reinforcing steelthat is essential in those applications inwhich structures are subject to unexpectedforces (seismic, dynamic, impact etc). Inthese situations, in which redistribution of moments is envisaged, it is not possible to calculate the loadings preciselyowing to the nature of the loads or because of lack of knowledge about theeffects on the structure.

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REDISTRIBUTION OF MOMENTS ACCORDING TO EUROCODE 2

ELASTIC BEHAVIOUR

0.80M: low ductility steel0.70M: high ductility steel

M’ + 0.20M (or 0.30M)M‘

EC-2

In the case of a reinforced concrete structuresubject to earthquakes, its behaviour isclosely related to the ductility of itsreinforcing steel. In this situation it isessential to know the capacity of thestructure to react to exceptional loads whenthe elastic limit of the steel is exceeded.

Similarly, in those cases where forces andloads are difficult to quantify, it is desirableto design structures that resist loads thatgreatly exceed the generally accepted valuesused for design calculation without collapsedue to deformation or major cracking.

If the structure is brittle, collapseoccurs without prior warning,

showing virtually no deformation and without cracking

A ductile structure close to collapseshows its condition through

pronounced deformation and cracking

One of the benefits of ductility is the abilityto redistribute loads, dissipating the appliedenergy to flexible elements such as beams.This property allows a more efficient use ofconcrete and steel because the moststressed zones can transfer load to lessstressed areas.

The redistribution of moments is the ability totransfer negative moments to positive onesand vice versa and is recognised in designcodes (ACI, Eurocode 2, CEB-FIP).

Significant redistribution of energy can onlybe achieved if the steel has high ductility.

Eurocode 2 allows the redistribution ofmoments as a function of the ductility classof the steel.

For ductility class A, redistribution is limitedto 20%, whilst for classes B and C, up to30% redistribution is allowed.

Eurocode 2 also allows plastic analysis forbeams, frames and slabs where reinforcingsteels of classes B and C are used. This isnot allowed where class A steels are used.

Particular attention needs to be given to thespecification of reinforcing steels for use inseismic design situations. In this case, thereversal of loading from tension tocompression has a particularly destructiveeffect. Test methods are available whichsimulate seismic actions by subjectingreinforcing steel samples to cyclic loadingunder hysteresis loops.

The behaviour of reinforcing steel undersuch cyclic loading conditions is stronglydependent on ductility. Celsa Duktil 500C iscompatible with Eurocode 8, for the designof seismic structures.

HYSTERESIS LOOP

DEFINING THE DUCTILITY OF REINFORCING STEEL

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The behaviour of steel is characterised byits stress-strain relationship; the amount ofdeformation corresponding to an imposed load.

If a typical stress-strain relationship isanalysed, there are two basic parts to the curve:

elastic region

In this region the strain (deformation) islinearly proportional to the applied load untilthe yield strength is reached. The elastic strainis recovered if the load is removed.

plastic region

Once the yield strength is exceeded,deformation is no longer proportional to load.Small increments of load produce larger plasticdeformation, which cannot be reversed onunloading. Deformation continues withincreasing load until the maximum (ultimate)load is reached (the maximum of the curve).Beyond this point, the load decreases as thecross section reduces in a local area beforefracture (necking).

In hot rolled steels the identification of the elastic limit (yield strength) is very clear,owing to the existence of a yield pointelongation. This is the flat portion of thecurve which marks the change from elasticto plastic behaviour.

In contrast, the stress-strain curves for coldrolled steels lack a defined yield point,making it more difficult to establish theelastic limit. In this case codes and standardsassume that the yield strength corresponds tothe value of tensile stress required to producea permanent deformation of 0.2%; the 0.2%

proof stress. The determination of this valuefrom the stress-strain curve is made byconstructing a line parallel to the linear elasticportion with an offset of 0.2%. Theintersection of this line with stress-straincurve gives the 0.2% proof stress.

Ductility in steel has traditionally beendefined using two parameters obtained fromthe stress-strain σ−ε diagram.

Ultimate tensile strength / yield strengthratio (ft/fy) This is the ratio of the breaking stress orultimate tensile stress (ft) with the yieldstrength (fy). It should be noted that ft andfy are the terms used in reinforced concretenomenclature whilst the equivalent terms insteel standards are Rm and Re.

STRESS-STRAIN GRAPH FOR A HOT ROLLED STEEL – B500C

σf t

fy

εε elastic region ε plastic region

ε

σf t

fy

0,2%

εelastic region

εplastic region

STRESS-STRAIN GRAPH FOR A COLD ROLLED STEEL – B500A

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Elongation to fracture on a 5D gauge length (A5)Traditionally this parameter has indicatedthe deformation capacity of steel. It is thevalue of the extension of a bar up to thepoint of fracture, measured over an initialgauge length of 5 times the bar diameter. Insome specifications the initial gauge lengthis 10 times the bar diameter (called A10).

The A5 parameter is calculated frommeasurements made on the fracturedsample. When the tensile test is carried out on the steel sample, and after fracture,the two halves of the test sample arebutted together and the increase in lengthover the initial gauge length of 5 bardiameters is measured.

new length – original length A5 =

original lengthx 100%

The A5 parameter was used to specifyductility of reinforcing steels in previousrevisions of BS 4449.

Eurocode 2 and BS 4449:2005 now useanother parameter to define ductilityinstead of A5. This is called the totalelongation at maximum force or uniformelongation εu. In steel industry standards,this parameter is designated Agt.

εu or Agt is defined as the uniform extension to maximum load or thepercentage elongation which corresponds to the maximum stress on the stress-strain curve.

Uniform elongation is measured from thestress-strain graph by taking a horizontaltangent at the maximum of the stress-straincurve. The stress at this point is the ultimatetensile strength (ft or Rm) whilst the strain isthe uniform elongation (εu or Agt).

Eurocode 2 specifies three ductility classes(A, B and C) with characteristic values of thetwo parameters, ft/fy (or Rm/Re) and εu (orAgt), which must be met simultaneously.

Nowadays it is possible to quantify ductilityusing a single parameter. Steels can begraded according to this characteristic whichis based on the concept of steels ofequivalent ductility.

The ductility of steel is related to the areaunder the stress-strain curve. This arearepresents the capacity of a steel to deformplastically up to its breaking point.

This area represents the stress and straincapacity of the material after reaching itselastic limit and is a measure of the energyabsorbed in plastic deformation. It is termedthe plastic energy factor.

DETERMINATION OF Agt (εu)

σf t

fy

A (εu )0.2% εgt

PLASTIC ENERGY FACTOR

f t

fy

A (εu)0.2% ε

PLASTICENERGYFACTOR

gt?

σ

STEEL DUCTILITY CLASSES IN EUROCODE 2

Class Characteristic values

ft/fy εu

A ≥ 1.05 ≥ 2.5%

B ≥ 1.05 ≥ 5.0%

C ≥ 1.05 < 1.35 ≥ 7.5%

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The plastic energy factor, which is derivedfrom the area below the stress-strain curve,allows the comparison of levels of ductilityfor different steels at the same yieldstrength level.

As shown in the diagrams, there are largedifferences in the ductilities of cold rolledsteel Grade A, hot rolled steel Grade B andhigh ductility hot rolled steel Grade C.

The plastic energy factor is not currentlyrecognised in codes or standards.

PLASTIC ENERGY FACTOR FOR COLD ROLLED STEEL PLASTIC ENERGY FACTOR FOR HIGH DUCTILITY STEEL

PLASTIC ENERGY FACTOR FOR HOT ROLLED STEELThe plastic energy factor of a hot

rolled steel Grade C is greater thanthat of a hot rolled steel Grade Bwhich is much greater than for a

cold rolled steel Grade A

σ

0.2% εεu(500C)

fy

f t

σ

0.2% εu(500A)

ε

f tfy

σ

0.2% εu(500B)

ε

f t

fy

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COMPARISON OF DUCTILITY

The British Standard for reinforcing steels,BS 4449, has been revised during 2005, toincorporate the following major changes:

• three grades have been introduced,aligned with the three ductility classes of Eurocode 2;

• the characteristic yield strength has beenincreased from 460 MPa to 500MPa;

• the standard has been aligned with therequirements of the European Standard,BS EN 10080.

The three grades in BS 4449:2005 have thefollowing characteristic properties;

B500A fy ≥ 500MPa (ft/fy) ≥ 1.05 εu ≥ 2.5%

This grade is normally cold rolled steelproduced by cold rolling of a plain hotrolled rod to produce ribbed wire. This isgenerally used in the production of weldedfabric and is considered low ductility.

B500B fy ≥ 500MPa (ft/fy) ≥ 1.08 εu ≥ 5.0%

This grade is hot rolled steel in which the ribbed bar shape is formed in a hotrolling process. This grade is considerednormal ductility.

B500C fy ≥ 500MPa (ft/fy) ≥ 1.15 to 1.3 εu ≥ 7.5%

This grade is also hot rolled steel but usingprocesses designed to retain more ductility.This grade is considered high ductility.

In addition to the above tensile propertiesall grades are required to meet commonrequirements for fatigue, bendability,weldability, bond and tolerances.

ACCORDING TO STEEL GRADE

TYPE OF STEEL DUCTILITY

Grade B500A Low

Grade B500B Normal

Grade B500C High

f t

f y1.15

f t

f y1.05

f t

f y1.08

σ

f t (500A-500B)fy (500A)

fy (500B, 500C)

0.2% A(B500A)

ε

f t (500C)

gt A(B500B)

gt A(B500C)

gt

COMPARISON OF STRESS-STRAIN CURVES FOR STEEL GRADES B500C, B500B AND B500A

The greater the values of ft/fy (orRm/Re) and εu (or Agt) the greater

the ductility of the steel

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CELSA DUKTIL 500CREINFORCING STEELSCELSA DUKTIL 500C REINFORCING STEELS

INTRODUCTION

Celsa Duktil 500C is a ribbed reinforcing steelfor use in reinforced concrete. All products are manufactured by electric arc steelmaking,continuous casting and hot rolling processes.

In electric arc steelmaking a 100% scrapcharge is melted by means of a high currentelectrical discharge from carbon electrodes.The molten steel is then tapped into a ladlebefore casting into water cooled coppermoulds where the steel solidifies as it iscontinuously withdrawn from the mould.This produces a solid billet which istransferred to the bar mill where it isreheated in a furnace to a high uniformtemperature in order to give the steelsuitable plasticity for rolling.

In the hot rolling process the billet ispassed through a series of rolling standswhere its section is continuously reduced tothe required dimensions. All the parametersaffecting the rolling process, such astemperature, rolling speed and volume ofcooling water, are constantly and carefullycontrolled. The rib pattern and identificationmarks are rolled on to the product in thefinal rolling stand using a specially notchedpair of rolls. After this final rolling stand, thebar is subjected to the Quench and SelfTemper (QST) accelerated cooling process inwhich the mechanical properties are achieved.

Celsa Duktil 500C complies with all thespecifications and ductility characteristicsrequired by BS 4449 Grade B500C. Theparameters of ductility of this grade of steel,shown here, meet the requirements for high ductility (class C) steel as defined in Eurocode 2. This means that Celsa Duktil 500Cis appropriate for all those situations in whichthe benefits of high ductility are required.

High ductility is a characteristic of steel forreinforced concrete which is highly desirablein most cases and essential in certainsituations, for example where structuresare subject to seismic demands.

TENSILE PROPERTIES

Celsa Duktil 500C is a high ductilityreinforcing steel:

• 500 indicates the value of thecharacteristic yield strength, expressed inMPa (or N/mm2);

• C indicates the steel’s very high ductilitycharacteristics.

The tensile properties of Celsa Duktil 500C are:

* These values are important because if the steel is overstrength, thatis its actual strength exceeds the nominal by a significant amount,the formation of plastic hinges during deformation is inhibited.

CHARACTERISTIC TENSILE PROPERTIES - CELSA DUKTIL 500C

Yield strength (fy) 500 MPa

Stress ratio: Tensile strength/Yield strength (ft/fy) 1.15 to 1.35 (*)

Uniform elongation (εu) ≥ 7.5%

Relationship fy (actual)/fy (nominal) ≤ 1.30*

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BEND PERFORMANCE

Celsa Duktil 500C meets the bendrequirements of BS 4449:2005 GradeB500C. This is defined by a rebend test,with the following mandrel diameters:

d ≤ 16mm 4d

d > 16mm 7d

FATIGUE PERFORMANCE

Celsa Duktil 500C steels meets the fatiguerequirements of BS 4449:2005 Grade B500C.Test are carried out on full section barsusing a sinusoidal tensile load as follows:

All tests are required to survive for 5 millionstress cycles under these loading conditions.

The fatigue performance of Celsa Duktil 500Cmeets the requirements of the NationalAnnex of Eurocode 2, and the requirementsof BS 5400 Pt 4 (concrete bridges).

The procedure is carried out at roomtemperature with a frequency of up t0 200Hz.

The test is continued until 5 million stresscycles have been reached or until thefracture of the test sample.

BOND PERFORMANCE

Celsa Duktil 500C meets the bondrequirements of BS 4449:2005 GradeB500C, defined by characteristic relative rib area (fr) as follows:

6 < d ≤ 12 fr ≥ 0.040

d > 12 fr ≥ 0.056

Reinforcing steels with these values ofrelative rib area are deemed to meet thebond requirements for design according to Eurocode 2.

WELDABILITY

Because of its carefully controlled chemicalcomposition, Celsa Duktil 500C has a carbonequivalent below 0.50%, when calculatedaccording to the following formula:

%Ceq = %C +%Mn

+%Cr + %Mo + %V

+%Ni + %Cu

6 5 15

This meets the requirements of BS 4449:2005 and BS EN 10080 for the definition of a weldable steel.

Celsa Duktil 500C steels can be weldedusing all common welding processes.Reference should be made to BS 7123 forguidance on welding of reinforcing steels.

1 stress cycleσ

σmax

t

σ

σmin

TYPICAL FATIGUE TEST GRAPHONE STRESS CYCLE

Celsa Duktil 500C grade meets the

requirements related to fatigue resistance

required by British Standards

SIZE(mm) MAX STRESS (MPa) MIN STRESS (MPa) No. CYCLES

≤ 16

20

25

32

40

50

250

231

212

200

187.5

187.5

50

46

42

40

37.5

37.5

5 million

5 million

5 million

5 million

5 million

5 million

TABLE OF SECTIONS ANDMECHANICAL CAPACITIESTABLE OF SECTIONS ANDMECHANICAL CAPACITIES

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500C

NOMINAL DIAMETER

mm

CROSS SECTIONmm2

NOMINALWEIGHT

kg/m 100 mm 150 mm 200 mm 250 mm 300 mm

STEEL SECTION IN mm2/m AS A FUNCTION OF BAR SPACING

8 50.3 0.395 503 335 251 201 168

10 78.5 0.617 785 524 393 314 262

12 113.1 0.888 1131 754 565 452 377

16 201.1 1.578 2011 1340 1005 804 670

20 314.2 2.47 3142 2094 1571 1257 1047

25 490.9 3.85 4909 3272 2454 1963 1636

32 804.2 6.31 8042 5362 4021 3217 2681

40 1256.6 9.86 12566 8378 6283 5027 4189

NOMINAL DIAMETER

mm

NOMINAL WEIGHT

kg/m 1 2 3 4 5 6 7 8 9 10

STEEL SECTION IN mm2 ACCORDING TO NUMBER OF BARS

8 0.395 50.3 101 151 201 251 302 352 402 452 503

10 0.617 78.5 157 236 314 393 471 550 628 707 785

12 0.888 113 226 339 452 565 679 792 905 1018 1131

16 1.578 201 402 603 804 1005 1206 1407 1608 1810 2011

20 2.47 314 628 942 1257 1571 1885 2199 2513 2827 3142

25 3.85 491 982 1473 1963 2454 2945 3436 3927 4418 4909

32 6.31 804 1608 2413 3217 4021 4825 5630 6434 7238 8042

40 9.86 1257 2513 3770 5027 6283 7540 8796 10053 11310 12566

NOMINAL DIAMETER

mm

NOMINAL WEIGHT

kg/m 1 2 3 4 5 6 7 8 9 10

MECHANICAL CAPACITY IN kN ACCORDING TO NUMBER OF BARS (Tensile loading) - γs = 1.15

8 0.395 21.85 43.71 65.56 87.42 109.27 131.13 152.98 174.84 196.69 218.55

10 0.617 34.15 68.30 102.44 136.59 170.74 204.89 239.03 273.18 307.33 341.48

12 0.888 49.17 98.35 147.52 196.69 245.86 295.04 344.21 393.38 442.55 491.73

16 1.578 87.42 174.84 262.25 349.67 437.09 524.51 611.93 699.35 786.76 874.18

20 2.470 136.59 273.18 409.77 546.36 682.95 819.55 956.14 1092.73 1229.32 1365.91

25 3.850 213.42 426.85 640.27 853.69 1067.12 1280.54 1493.96 1707.39 1920.81 2134.23

32 6.310 349.67 699.35 1049.02 1398.69 1748.36 2098.04 2447.71 2797.38 3147.06 3496.73

40 9.86 546.36 1092.73 1639.09 2185.46 2731.82 3278.18 3824.55 4370.91 4917.28 5463.64

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CELSA DUKTIL 500CREINFORCING BARSCELSA DUKTIL 500CREINFORCING BARS

IDENTIFICATION MARKS OF CELSA DUKTIL 500C REINFORCING BAR

COUNTRY NO.5 MANUFACTURE NO.12 MARK INDICATES 500 GRADE

INTRODUCTION

Celsa Duktil 500C reinforcing bars aresupplied in sizes from 8mm to 40mm, inlengths from 6m to 18m, and in standardbundle weights of 2 tonnes.

Nominal bar sizes are 8, 10, 12, 16, 20, 25, 32 and 40mm.

Section tolerances on Celsa Duktil 500Cbars meet the requirements of BS 4449:2005 as follows:

RIB GEOMETRY

The rib pattern of Celsa Duktil 500C barscomplies with that for BS 4449:2005. Thebar contains two rows of ribs, one on eachside of the bar, arranged in a herringbonepattern. In each row of ribs, alternating ribsare at low or high angles to the axis of thebar. For improved bending consistency, thebar contains no significant longitudinal ribs.

IDENTIFICATION

The country of origin and works number areidentified by rolled on marks betweennormal transverse ribs. All Celsa Duktil 500Cbar is identified with country number 5, andworks number 12, as shown below. Thedash at the end of the mark identifies thebar as being 500 grade.

LABELLING

All bundles of Celsa Duktil 500C bars arelabelled with steel tallies giving thefollowing information:

• Celsa name and logo

• Bundle number

• Cast number

• Bundle weight

• Material grade

• Date of rolling

TEST CERTIFICATION

All despatches of Celsa Duktil 500C bars are accompanied with a full mill testcertificate, including the results of chemicalanalysis, and all the mechanical testsaccording to BS 4449:2005.

BAR DIAMETER TOLERANCE (weight/m)

≤ 8mm ±6.0%

>8mm ±4.5%

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INTRODUCTION

In recent years the process of bending and tying steel for reinforced concrete has advanced through the greater use ofautomatic bending machines that not onlyreduce cost but improve the quality of theshapes produced.

The use of such new technology has created a need for coiled reinforcing steelwith characteristics adapted for use withthis type of machinery. Celsa Duktil 500C is the solution.

Celsa Duktil 500C reinforcing coils aresupplied in sizes from 8mm to 16mm instandard coil weights of 2 tonnes.

Nominal sizes are 8, 10, 12 and 16mm.

Section tolerances on Celsa Duktil 500C bars meet the requirements of BS 4449:2005 as follows.

RIB GEOMETRY

The rib pattern for Celsa Duktil 500C barscomplies with that for BS 4449:2005 gradeB500C. Seen in cross section, coils of CelsaDuktil 500C have four sides with roundedcorners. There is a row of transverse ribs oneach face so that the overall shape isapproximately circular. This shape isdesigned to give consistent bendingthrough automatic machines.

IDENTIFICATION

The country of origin and works number areidentified by rolled on marks betweennormal transverse ribs. All Celsa Duktil 500C

coil is identified with country number 5, andworks number 12, as shown below. Thedash at the end of mark identifies the baras being 500 grade.

LABELLING

All Celsa Duktil 500C coils are labelled withsteel tallies giving the following information:

• Celsa name and logo

• Bundle number

• Cast number

• Bundle weight

• Material grade

• Date of rolling

TEST CERTIFICATION

All despatches of Celsa Duktil 500C coil areaccompanied with a full mill test certificate,including the results of all mechanical testsaccording to BS 4449:2005.

IDENTIFICATION MARKS OF CELSA DUKTIL 500C REINFORCING COIL

CROSS SECTIONCOUNTRY NO.5 MANUFACTURE NO.12 MARK INDICATES 500 GRADE

BAR DIAMETER TOLERANCE (weight/m)

≤ 8mm ±6.0%

>8mm ±4.5%

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ADVANTAGES OF USING COILEDCELSA DUKTIL 500C

There are considerable advantages in usingcoiled Celsa Duktil 500C:

• geometry with four ribbed faces. Thisgives more consistent bending indifferent directions. In straightening thereis a reduced tendency for the material totwist. When bending there is a reducedtendency for the material to twist out ofplane. As a consequence closed formssuch as links are produced with lessdistortion and twisting of the legs;

• bending machines can be less powerful;

• reduced wear on the machine rolls;

• there is less damage or distortion of theribs during straightening and bendingbecause of the outside envelope of theshape being approximately circular;

• using coils of Celsa Duktil 500C givesexcellent results with machines used for the shaping of reinforcement;

• coiled Celsa Duktil 500C allows the creation of complex shapes for concrete reinforcement.

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APPLICATIONS FORCELSA DUKTIL 500CAPPLICATIONS FOR CELSA DUKTIL 500C

APPLICATIONS FOR CELSADUKTIL 500C SPECIALDUCTILITY STEEL

Celsa Duktil 500C should be used inapplications where high ductility is eitherdesirable or essential. These include:

Structures where calculations have beenworked using non-linear methods andstructures that allow limited energydissipation. Structures need to have steelreinforcement with sufficient ductility toguarantee the required capacity for thedistribution of applied forces.

Structures subject to forces that are difficultto quantify because of the nature of thoseforces or due to lack of knowledge aboutthose forces or their effects on thestructure, such as:

• dynamic loading;• explosions;• sudden impact;• compressive and tensile forces; • unforeseen or accidental overloading.

The use of high ductility steel is necessaryin buildings where catastrophic failure mustbe prevented.

Structures in which there is a high risk offire. If by the action of fire, a section of aductile structure forms plastic hinges anddeforms, the adjacent sections adapt anddelay the collapse of the remaining structurerather than collapsing catastrophically.

As long as a structure has a robust design,the greater the ductility of the steel, thegreater the distribution of loads. This willlead to an increase in the formation of plasticjoints, rather than a failure mechanism.

Structures in which a change of use isforeseen and older structures that requirerestoration. In such situations a limitedredistribution of loads can be achieved withthe purpose of achieving maximum benefitfrom the steel’s ductility. Restoration andrefurbishment can be carried out to maximisethe capacity of the original structure toabsorb different load conditions.

Seismic Design. When subjected to seismicforces, the manner in which the structurereacts is highly dependent on the ductilityof the steel used in its construction. Undersuch stresses the behaviour of the structureand its ability to distribute energy arefundamental because the elastic region ofthe steel is exceeded.

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Ductility is desirable in all cases andessential in some applications

ADVANTAGES OF USING 500CHIGH DUCTILITY STEEL

Ductility is a necessary characteristic ofsteels used to reinforce concrete. It is verydesirable in all cases and essential in someapplications. Celsa Duktil 500C has greaterductility than steels of the same strength of grades 500A and 500B and providesthese advantages:

• allows the formation of plastic hinges in astressed structure that have sufficientrotational capacity to deform and dissipatethe applied load until the collapsemechanism of the structure is reached;

• provides the best solution for structuressubject to earthquakes owing to highdissipation of energy;

• allows the application of non-lineardesign calculations that allow forredistribution of moments and consequentoptimum use of the steel reinforcement;

• in well designed statically non-determinantstructures, high ductility steels facilitatethe distribution of loads.

The use of high ductility steel prevents thesudden catastrophic collapse of structures,reducing loss of life and other damage.

With the availability of new technology, theCelsa Group has been able to develop anew type of steel, Celsa Duktil 500C, thatprovides the benefits required.

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WHY WE NEED DUCTILITY IN REINFORCED CONCRETESTRUCTURES

Professor Andrew W. BeebyUniversity of LeedsPublished in May 2004

Our understanding of the influence ofductility on structural design has increasedconsiderably beyond that available when,for example, BS 8110 (1) was drafted. WhenEurocode 2 (2) is finally published for use(probably sometime in 2004), it will includea classification of the ductility ofreinforcement. This classification will definethe nature of the design processes whichmay be used with a particular class ofreinforcement. The classification is as follows.

Class A: (ft/fy)k ≥ 1.05 εuk ≥ 2.5%

Class B: (ft/fy)k ≥ 1.08 εuk ≥ 5%

Class C: (ft/fy)k = 1.15-1.35 εuk ≥ 7.5%

where (ft/fy)k is the characteristic value ofthe ratio of the ultimate tensile strength (ft)to the yield strength (fy) and εuk is thecharacteristic value of the strain atmaximum stress (also sometimes calleduniform elongation). These parameters areillustrated in Figure 1. In the past, BS 4449(3) only defined the elongation to failure asa measure of ductility but in the mostrecent amendment (1997), two ductilityclasses are defined: 460A and 460B. Inaddition to the elongation to failure, theseclasses have the same ductilityrequirements as Class A and Class B definedin the Eurocode and set out above. A recentpaper by Franks (4) sets out the currentsituation in more detail.

Class A reinforcement can only be used inEC2 design, where moment redistribution isless than 20% and may not be used whereany plastic method of analysis has been

employed. Class B reinforcement may beused with plastic methods of analysis andfor amounts of redistribution up to 30%.Class C reinforcement can be usedinterchangeably with Class B, and is arequirement where there are seismicconditions. Current production methodsnormally result in reinforcement of Class Bor C for bars of above 16mm diameter.However, production methods for smallerbars, which are mostly produced via a coilroute, may lead to reinforcement of Class Aor, occasionally, below. CELSA Steel (UK),who have taken over the plant onceoperated by ASW, and are now the UK’slargest manufacturer of reinforcing steels,have developed a method of using theTempcore process for coil, and expect tosupply Class C reinforcement in all sizes witha characteristic yield strength of 500MPa.

These changes, in code provisions andmanufacturing processes, make it anappropriate moment to re-examine ductilityand consider what its benefits are. Ductilityis generally recognised as being a ‘goodthing’ but the rationale for quantifying itsbenefits only exists in very limited areas,notably the relationship between ductilityand the amount of redistribution permittedin analysis for the ultimate limit state. Sincethis last item has been the subject of muchresearch, it will be considered first.

APPENDIXAPPENDIX

fPlastic strainSTRESS

STRAIN

t

fy

εu

Ultimate tensile strain

Ultimate tensile strengthYeild or proof

strength, fy

FIGURE 1. TYPICAL STRESS-STRAIN CURVE FOR REINFORCEMENTSHOWING PARAMETERS DEFINING DUCTILITY

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Ductility and redistributionThe basic issues are illustrated for the caseof a uniformly loaded fixed ended beam inFigure 2. Under low loads, the beam willbehave more or less elastically until a loadis reached which just causes thereinforcement at the supports to yield. Thebeam does not collapse under this load butthe moment at the support cannot increaseabove the yield moment.

Further loading results in a more rapidincrease in the moment at mid-span until aload is reached which will cause yield at thispoint. Once yield has been reached at bothsupports and at mid-span, a mechanism willhave formed and no further load can becarried. Study of the deflected shape of thebeam at ultimate load will show that thesupport section has to rotate through anangle θ under constant moment. In practice,this rotation occurs over a finite lengthwithin which extensive cracking will develop.This region is called a ‘plastic hinge’.

In a beam such as that shown in Figure 2,the full ultimate load can only be reached ifthe support sections (the plastic hinges) aresufficiently ductile to be able toaccommodate a rotation of θ. Reinforcedconcrete sections are not necessarily

sufficiently ductile and a means has to befound to ensure that the required rotationdoes not exceed the rotation capacity of the section.

The analytical device commonly used toensure this is ‘redistribution’. Analysis underthe ultimate loads is carried out elastically.The support moments can then be reducedand the mid-span moment increased so that the resulting bending moment diagramremains in equilibrium with the loads. It canbe shown that the percentage reduction inthe support moment is proportional to therequired rotation of the support section.

Extensive research has shown that arelationship exists between the ultimaterotation capacity of a section and the ratio of the neutral axis depth to theeffective depth (x/d). Practical rules canconveniently be devised giving arelationship between percentageredistribution and x/d. Both BS 8110 and the Eurocode include such relationships.

In the Eurocode, the design relationshipsvary depending on the ductility of thereinforcement. The Eurocode redistributionrules for concrete with a cube strengthbelow 60MPa are shown in Figure 3.

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fSTRESS

STRAIN

t

fy Moment of resistance at support

Moment of resistance in span

Moments at first yield at support

Moments under ultimate load

θ

FIGURE 2. DEVELOPMENT OF MOMENTS IN A DUCTILE FIXED ENDEDBEAM SUBJECTED TO A UNIFORMLY DISTRIBUTED LOAD

00

5

10

15

20

25

30

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0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5(x/d)

% RED

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EC2 Class A EC2 Class B and CBS 8110

FIGURE 3. DESIGN RELATIONSHIP BETWEEN REDISTRIBUTION AND X/D FORVARIOUS CLASSES OF REINFORCEMENT GIVEN IN EUROCODE 2 AND BS 8110

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APPENDIX

Ductility and plastic methods of designA basic assumption of plastic methods ofanalysis, either of a structure or morelocally, is that the materials are infinitelyductile. While infinite ductility is not actuallyrequired, plastic methods provide no checkon how much ductility is actually needed inany particular case.

If limitations in the ductility of the materialmight invalidate the calculation then eithersome additional, independent, check on theductility is required or plastic methods mustbe avoided.

In practice, we may not actually be awarethat a method we are using is plastic. Theclassic form of plastic analysis used inreinforced concrete is Yield Line Analysis forslabs. Probably few of us actually use thismethod directly and, where we do, BS 8110provides some limited guidance to avoid anexcessive ductility requirement.

The average user of BS 8110 may, however,not be aware that the moment coefficientsgiven for two-way spanning slabs have beenderived from yield line analysis. The averagecode user may also not be aware that manyof the methods used for section design, asopposed to analysis, are also plastic. Strut-and-tie models or truss analogymethods are intrinsically plastic as theyassume that it is possible for the internalforces to redistribute themselves from anelastic situation to the arbitrarily chosenarrangement of struts and ties. It can beargued that design for shear is plastic.

Design of sections for flexure is, moreclearly, plastic, though with a check on thestrain capacity of the concrete and,indirectly, through neutral axis limits, on thestrain in the reinforcement. We are thusmore dependent on plastic methods thanwe may generally be aware. We generallyrely on the formulation of the methods inthe code or on detailing rules to ensure thatthe ductility is adequate. It is probably true,

however, that even the experts on many ofthese methods could not define reliablywhat ductility is actually required and henceat what stage the ductility of reinforcementcould become insufficient.

Ductility and RobustnessThis is the most difficult issue to deal with.We all agree that structures should berobust but what we actually mean by this israther obscure. The nearest we come to adefinition of robustness is the principle thatthe damage suffered by a structure as theresult of an unforeseen event should not bedisproportionate to the cause. This does notgive the designer much help in designing hisstructure as, unfortunately, unlike strength,we have not yet developed any means ofquantifying the concept of robustness.

Despite this problem of quantification, thebasic requirements for a robust design arefairly clear. If part of a structure is damagedby some accident, for example, a gasexplosion, then the structure should hangtogether and not completely collapse. Weachieve this by tying the members togetherso that, even in a damaged state, thestructure does not fall apart. Codes (eg. BS 8110) provide arbitrary rules for thelocation and strength of the ties. What isnot made clear is that, for the structure tohang together after being damaged, the tiesmust also be ductile. In the event of anaccident such as an explosion, a non-ductile

Plastic deformation(ductility)

Deformation at collapse

ENERGY ABSORBED

ULTIMATELOAD

DEFORMATION

FIGURE 4. SCHEMATIC ULTIMATE REQUIREMENTS FOR LOAD AND ENERGY ABSORBTION

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tie would simply rupture during theaccident. What level of ductility is requiredhas, as far as is known, not beeninvestigated. Beeby, in a paper in theStructural Engineer (5) put forward apossible basis for the quantification ofrobustness. The concept arose from theperception that a structure needs to becapable of withstanding two quite differentand independent sets of actions:

a. the structure should be able to support adefined ultimate load and;

b. the structure should be able to absorb,without collapse, a defined energy input.

It was suggested in (5) that the requiredenergy absorbtion should be a function ofthe volume of the member or members orstructure considered. The basic concept isillustrated in Figure 4. This concept has yetto be elaborated into a full design proposalbut does seem to provide a logical frameworkfor a rational and quantitative treatment ofrobustness. It will be seen that it alsodefines the ductility requirements related torobustness of all parts of a structure.

DiscussionThis paper has tried to illustrate theimportance of ductility in the design ofreinforced concrete for the ultimate limitstate. Unfortunately, when assessing therelative economy or efficiency of a design ora particular material, our current methods ofdesign can only take account of thoseaspects of behaviour which can bequantified. This does not mean that aspectsof behaviour which have not beenquantified are less important; it may simplymean that the science has not developed toa level where quantification is possible. Thisis the case for many of the aspects ofductility discussed in this paper and, inparticular, the issue of robustness. It is only

in the case of the analysis of framed

structures that detailed quantitative

requirements for ductility are currently

defined. In all other situations, it is simply

assumed that sufficient ductility exists if

the reinforcement meets the current

specifications. Unfortunately, this means

that the benefits of specifying reinforcement

with a higher ductility, though they clearly

exist in terms of a reduction in risk, are not

quantifiable and are therefore less likely to

be considered. We are constantly developing

designs which are more sensitive; possibly

with greater, but unknown and unquantified,

requirements for ductility. In this

circumstance, one can only feel that the

greater the ductility of the materials being

used, the greater confidence one can have

in the overall performance of the structure,

under whatever foreseen or unforeseen

circumstances may occur.

References(1) BS 8110: Structural Use of Concrete

Part 1: 1997: Code of practice for design

and construction.

(2) CEN. prEN 1992-1-1 (Final draft):

Eurocode 2: Design of concrete

structures – Part 1: General Rules and

rules for buildings. April 2003.

(3) BS 4449:1997 Carbon steel bars for the

reinforcement of concrete. British

Standards Institution, 1997.

(4) Franks, A., Reinforcing steels: processes

and properties. Concrete, June 2003.

(5) Beeby, A. W. Safety of structures and a

new approach to robustness.

The Structural Engineer, Vol. 77 No.4,

16th February 1999.

APPENDIXAPPENDIX

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