bridon construction products

High quality prestressing wire & strand for construction product applications

Construction Products

Drawing from a background of long standing experience and

technology, Bridon is an acknowledged world leader in the

design, manufacture, development and supply of prestressing

wire & strand to meet the needs of the construction industry.

Specialist prestressing wire& strand solutions for the

construction industry

02

Green Bridge, Australia

BRIDON Construction Products

Bridon - the world’sleading specialist inthe manufacture of wireand rope solutions forthe most demandingapplications, deliveringreassurance throughunrivalled experience.

ISO 9001

Bridon operates quality managementsystems which comply with the

requirements of EN ISO 9001:2000.These systems are assessedand registered by accredited

certification bodies.

ISO 14001

Bridon operates environmentalmanagement systems which, whererequired by legislation or risk, comply

with the requirements of EN ISO14001:2004 and are assessed and

registered by accreditedcertification bodies.

03

All statements, technical information and recommendationscontained herein are believed to be reliable, but no guarantee isgiven as to their accuracy and/or completeness. The user mustdetermine the suitability of the product for his own particularpurpose, either alone or in combination with other products andshall assume all risk and liability in connection therewith.

Whilst every attempt has been made to ensure accuracy in thecontent of the tables, the information contained in this cataloguedoes not form part of any contract.

ContentsIntroduction ...................................... 2-3

Stay Cable Strand for Stay Cables ......4

Bridge Wire ..........................................5

Prestressing Strand ..........................6-7

Prestressing Wire..................................8

Packaging ........................................9-10

Technical Data ..............................11-15


04

Recent years have seen the rapid development of stayedstructures and in particular the stay cable bridge.

Modern stay cable techniques allow installation ofcomplete stay cables and at the same time offer the abilityto replace individual stays if necessary.

Fabrication and installation of stay cables using 7-wirestrand have many similarities with post-tensioning.However, the demands exerted on the strand by a staycable system are somewhat different and more onerousthan is the case with traditional post-tensioningapplications. In particular, the high stress variationsimposed by bridge loadings on stay cables demand thatthe strand has a significantly improved fatigueperformance in addition to the usual characteristics ofprestressing stand. The strand must also have goodresistance to multi-axial stresses and to stress corrosion.

In order to achieve these high performance requirements,Bridon has developed a 7-wire strand specifically for staycables with properties which are significantly enhancedover those of conventional strand.

By the use of special steel grades and manufacturingprocesses Bridon is able to offer a product which meets theunique requirements of stay cable designs and therecommendations of the American Post Tensioning Institutionand the FIB.

Stay Cable Strand can be supplied either ungalvanised orgalvanised and in a fully blocked and sheathed conditiontailored to the designer’s particular requirements.

The strand complies fully with most well known National andInternational standards for bright and galvanised prestressingsteels and also with the recommendations of the AmericanPost Tensioning Institute and the FIB for stay cables.

Length Measurement

As the processes employed on the site erection of staycables have become more refined and more demanding,so too has the need to supply Stay Cable Strand inaccurately marked lengths.

Bridon has developed a length measurement systemwhich measures and makes strand to an accuracy of +/-0.01%. As well as minimising waste on site, this allows thestay cable installer to greatly reduce the time and costsinvolved in erecting each cable.

Corrosion Protection

By their very nature stay cables are exposed to theseverities of the elements and corrosion protection istherefore of fundamental importance in the design andperformance of stay cable strand.

This can be achieved by two methods or a combination ofthe two:

Galvanised Strand

By using the hot dip galvanising process, stay cable strandcan be supplied with minimum zinc weights rangingbetween 190g/m2 and 350g/m2. Zinc weights can beadjusted to suit customer’s specific requirements whilst stillmaintaining the strand’s characteristics and superiorfatigue performance.

Where a wedge and barrel type gripping system is to beused then it is normal for a drawn galvanised strand to besupplied. By this process the wire is galvanised before thefinal wiredrawing operation resulting in a smooth finish onthe strand which offers adequate corrosion protection withminimum interference to the gripping mechanism.

Blocked and Sheathed Strand

In this case the strand is coated in a highly corrosion-resistant grease or wax conforming to therecommendations of both the American Post TensioningInstitute and the FIB, by the continuous hot extrusionmethod, in either high density polyethylene orpolypropylene to a minimum radial thickness of between1.00mm and 2.00mm, once again conforming to therecommendations of both the American Post TensioningInstitute and the FIB.

Special tests to confirm the integrity of the blocking mediumand the impact resistance of the sheath are performed onequipment specifically developed by Bridon.

Today’s stay cable structures require strand with increasingly advanced properties. Bridon’s 7-wire Stay CableStrand has been designed to meet the particular needs of these complex structures.

Dee Estuary Flintshire - Stay Cable Bridge, UK

Uddevalla - Stay Cable Bridge, Sweden

Chartist - Stay Cable Bridge, UK

Stay Cable Strand for Stay Cables


05

Bridge Wire

From the initial design consultation to the final product installation Bridon provides its customers with thecomplete bespoke service.

Bridon has a large and modern facility to producegalvanised bridge wire for suspension cables, either foraerial spinning or for parallel wire bundles made on site.Typically such wire is in sizes between 5mm and 7mm indiameter with tensile ranges of between 1570 and1860 N/mm2 and a minimum zinc coat weight of 300g/m2.In addition, this demanding application requires a highlevel of ductility and is assured by appropriate adhesionand continuity test.

Dedicated development work alongside one of the world’sleading steel making groups have resulted in a new rangeof 1860 N/mm2 and 1960 N/mm2 strength wires, which arenow commercially available.

Bridon galvanised bridge wire can be supplied in coils ofbetween 250kg and 2000kg which are suitable for therapid pay-off systems employed in aerial spinning or forlaying into parallel wire bundles.

Jiangyin Suspension Bridge, PR China

Main Cable Wire for the Second Tacoma Suspension Bridge, USA

Jiangyin Suspension Bridge, PR China


Prestressing Strand

06

Product Range

Bridon was the first manufacturer of prestressing strand inthe UK, and is now the foremost producer both in productrange and volume, in the country.

From the early years of prestressing the pattern of demandhas been changing and Bridon has adapted anddeveloped its product range to meet these changes:DYFORM® Strand, a compacted strand was a Bridoninvention; the greasing and simultaneous extrusion of aplastic sheath over strand for use in unbonded tendonswas pioneered by Bridon and now Bridon is leading theway in developing and supplying higher strength strand.All of Bridon’s prestressing product range is produced withvery low relaxation properties making it suitable for use inall prestressing and post-tensioning applications. Thecombination of Bridon’s unique product range and provenproduct quality in performance in projects worldwide hasmade Bridon 7-Wire Prestressing Stand the recognisedleader in its field.

Bridon 7-Wire Prestressing Strand is produced in threetypes: Standard, Super and DYFORM® in sizes rangingfrom 8.00mm to 18.50mm diameter. Bridon can meet therequirements of all the main international specifications forprestressing stands, the most common of which are BS5896, ASTM A416 and Pr EN 10138.

(More detailed information is provided in the ‘TechnicalData’ section of this brochure.)

Pioneered by Bridon in the UK, prestressing strand has been developed and adapted to meet the changes ofan increasingly demanding market. The unique product range and very low relaxation properties of allBRIDON’s prestressing strand makes it suitable for thecomplete scope of prestressing applications.

Concrete Gravity Oil Storage Base for South Arne Platform, UK

Lifting of the structural steel elements in the Athens OlympicStadium using 18mm Dyform Prestressing Strand

Royals Building, London - Post Tensioned Building, UK


07

Prestressing Strand

Applications

Bridon 7-Wire Strand is used as a prestressing element innumerous applications:-

Factory Fabricated concrete elements including sleepers,bridge beams, sign gantries, flooring, lintels, stairways andterracing. In general standard strand is used in theseapplications.

Post Tensioning, both internal and external, of bridges,car parks, stadiums, silos, buildings and major concretestructures such as power stations, oil platforms and LNGtanks. The properties of Superstrand lend themselves touse in this field but increasingly the demands of cost andengineering solutions are generating the use of DYFORM®

Strand in post-tensioning. There has also been a movetowards the use of greased and plastic coated strand asthe advantages of this product in unbonded and externalpost-tensioning solutions has been more widelyunderstood.

Heavy Lifts demand a product which has a very highbreaking load in relation to its diameter, a property which isintrinsic to DYFORM® Strand.

Galvanised Round Wire Strand

Strand made from galvanised wires can be supplied strictlyin accordance with the physical requirements of thespecifications NFA35 - 035, ASTM A475 and EN 10337standard and super grades. Electrolytic couples betweenthe galvanised wires and other material such as ducting,reinforcement and anchorages must be preventedotherwise corrosion can be accelerated. Bridon technicalpersonnel are available to give advice in this developingproduct area.

A1 Haddington to Dunbar Expressway, UK

Salford Quay, Manchester - Post Tensioned Building, UK


Prestressing Wire

08

The principal use of Bridon Prestressing Wire is instressing concrete products, primarily flooring units, lintelsand beams which are extensively used in housebuilding,and small scale commercial and industrial buildings.

The standard specifications for wire have many commonrequirements, and those most frequently used are BS5896, Pr EN10138 and ASTM A421. However, Bridon’stechnical expertise and experience lends itself to themanufacture of Prestressing Wire to a variety of standardsaccording to customers’ individual needs. These can beindividually discussed.

Prestressing Wire can be manufactured with a plain finish,or with one of two patterned indents commonly known as“Belgium” and “Triple”. See page 11 for technical details.

Whilst specifications for Prestressing Wire are often generic, Bridon has the technical expertise to meet theexacting requirements of individual customers.

House floor “T” Beams awaiting despatch

Installation of long span prestressed concrete flooring for industrial application

Wire drawing prior to heat treatment


09

Packaging

Bridon has made considerable investment in packaging to ensure that products are supplied with end usage inmind. All products are packaged to ensure maximum stability in transit, protection in storage and ease of useon site.

Standard coil

Polyweave wrapped coil

UK - Pentapak dispenser

Bripak dispenser

Wooden reel used for the supply ofplastic coated strand

The standard package for strand, bridge wire andprestressing wire is a coil. The final presentation of the coilvaries considerably between each product, and themethod of dispensing individual packs also determinespack presentation:-

Prestressing Strand

Coils are layer wound for stability and ease of use and canbe manufactured to over 7000kg each, but shipping andhandling facilities normally restrict coil weights to the range2000/3000kg, except when supplied in a Pentapak.Individual coils can be supplied blocked, palletised andexport wrapped if required. Various methods ofdispensing are available according to need.

Plastic coated strand can be presented wound on awooden reel to provide extra protection in transit, on siteand in use, or in roughwound and strapped coils.

Stay Cable Strand

Bridon has specifically designed wooden reels toaccommodate Stay Cable Strand. These afford maximumstability in transit, combined with durability, and totalprotection for the strand while still remaining of a size andweight which allow ease of handling on site. Reelscontaining up to 3500kg of strand can be supplied.Each reel can be individually stencilled for ease of identityin use.

Bridge Wire

Bridon’s experience in this field, and considerableinvestment in packaging equipment, allows the provision ofa flexible package suitable for the defined individual needsof the methods employed during cable construction.Coil weights are tailored to each client’sparticular requirements.

The coils are mechanically wound on specially developedblocks which control weight, outside and inside diameter,cast and lift; and are then individually wrapped by machineto provide a package which is totally suited to the rigoursof export shipment, and on-site storage.

Prestressing Wire

The wire is supplied in large diameter prestraightened coilsof up to 2500kg each. Coils can also be export wrapped,and are commonly supplied in weights of approximately1000kg for this purpose.


Packaging

10

precautions may be necessary depending uponclimatic conditions and any atmospheric contamination.Space heating may be necessary in some months. Insome countries additional enclosure as described in (3)may be necessary. Where tendons are to be in theducts for long periods before grouting, it is usuallynecessary to protect them against corrosion using awater-soluble oil. Experience shows that suchprotective oil is flushed through or taken up in theprocess of grouting without any significant effect onsubsequent bond.

Recommendations against damage

Prestressing strand is stressed initially to a highpercentage of the ultimate strength and it is essential thatthe steel is not weakened by mechanical or heat damageon site.

1 Always pay out the strand from the centre of the coil byuse of a dispenser or from the outside of the coil whenplaced on a turntable, equipped with a brake to preventover-run of the strand.

2 Do not allow traffic to run over lengths of strand orallow any other mechanical damage, scraping orabrasion which might significantly reduce thesteel section.

3 When cutting strand on site use abrasive disc cutters. Ifcircumstances make the use of flame cutting necessaryit is very important that any adjacent strand be shieldedfrom the heat and from possible flying globules ofmolten metal.

Strand which has previously been heated by an oxy-acetylene flame for no more than 5 seconds may haveits strength reduced by 30% and it would fail insubsequent tensioning.

Globules of molten metal leave little or no evidence ofsurface damage, but in fact a semi-circular zone ofbrittle quenched steel structure is produced below thesurface, at which point failure can be expected whenthe strand is tensioned. It naturally follows that any sitewelding or cutting operations using electric arc or flameshould not be carried out near prestressing strand.

It must be emphasised that prestressing steel is verydifferent from ordinary reinforcement in properties, mannerof use and expected performance. Failure to observesimple precautions against damage and severe corrosionhas been known to have unfortunate consequences duringconstruction. Except for the special cases of oiled,greased and plastic covered tendons, strands are supplieddry and the surface of the steel will slowly develop asmooth film of rust which is not deleterious. If however,there is an adverse environment or the steel has to bestored on site for some time, either in coils or tendonlengths outside or in ducts, then consideration should begiven to some form of protection to avoid the risk ofgeneral severe corrosion or localised pitting.

The method of storage and subsequent handling shouldbe such that contamination, mechanical or heat damage isprevented.

The best possible protection should be givencommensurate with economic cost. This will generally bemet if the type of storage is sufficient to ensure that thecondition of the steel, when taken out of store is no worsethan the condition that will develop as a consequence ofthe practical procedures used during construction.

Corrosion is insignificant at relative humidities of 70% orless, although it can occur at lower humidities if there isatmospheric pollution. Procedures should be followed tominimise the possibility of condensation of moisture onthe steel.

Recommendations on storage

1 If possible unload strand under cover. Do not allow thecoils to be contaminated by ground conditions.

2 For short term storage outside, stack coils off theground on timber supports covered with plasticsheeting. The area should be well drained. Cover withwaterproof sheet supported on a frame over the stackso that there is no contact, and fasten down below thebottom of the coils but leave a gap at ground level.

With these precautions circulation of air within the stackwill be possible, minimising the possibility ofcondensation of moisture, and any condensation onthe underside of the cover will not be in contact withthe steel. Thick clear polythene sheeting has theadvantage of allowing superficial inspection of thecondition of the stack without removing the cover.

3 For very long term storage outside, in addition to theabove precautions, it is desirable that each coil strandis enclosed with a small bag of vapour phase inhibitorin a polythene bag or wrapping and sealed, forexample by folding over the bag or top edge of thewrapping tightly and fastening down. Alternatively, thebundles or coils may be stacked off the ground on aheavy duty waterproof sheet and small bags of vapourphase inhibitor distributed in the stack which is thencovered as described in (2) but with the bottom of thecover fastened down to the ground sheet to make thewhole enclosure as airtight as possible. The conditionof the strand should be checked periodically and thevapour phase inhibitor renewed as necessary.

4 If available, a dry, clean building provides the beststorage, but for very long term storage additional

Storage and handling on site

DO dry store

DO protect from impact

DO NOT expose to the elements

DO use appropriate lifting beam whentransporting the strand

DO NOT use a sling or otherlifting equipment

DO use correct cutters to remove straps

DO NOT cut straps until strand isloaded into the dispenser

DO load coil with front-side label tofront of dispenser to avoid incorrectpay-off of strand

DO use appropriate dispenser forstrand length, diameter and weight

Use of an incorrect dispenser couldresult in strand deformation


11

Technical Data

mm N/mm2 mm2 g/m kN kN

Nominaldiameter

X-sect.area(Sn)

Mass(M)

Breakingload(Fm)

Specifiedcharacteristic values

TypicalvaluesNominal Tensioning loads @

0.1%Proof load

(Fp0.1)

Load 1%elong’n

(Rt1.0)65%

of GUTS70%

of GUTS75%

of GUTS80%

of GUTS

Tensilestrength

(Rm)

kN kN kN kN kN

77665554.544

300.7300.7221.0221.0153.1153.1153.1124.298.498.4

Dimensions and properties of single wires for prestressed concreteto prEN10138

GUTS=Grade Characteristic Breakload (Fm)Maximum relaxation after 1000 hours for % characteristic breaking load: 60% = 1%, 70% = 2.5%, 80% = 4.5%

60.464.347.350.130.832.734.724.821.022.3

53.273.954.444.127.128.830.529.724.219.6

51.354.740.242.626.227.829.521.117.919.0

39.341.830.732.620.021.322.616.113.714.5

42.345.033.135.121.622.924.317.414.715.6

45.348.235.537.623.124.526.018.615.816.7

48.351.437.840.124.626.227.819.816.817.8

38.538.528.328.319.619.619.615.912.612.6

1570167016701770157016701770162016701770

TypeNominaldiametermm

Nominal values only Specifiedcharacteristic values

Typicalvalues

Breakingload

(Fm) kN

0.1%Proof load(Fp 0.1)kN

Load at 1%elongation(Ft 1.0)kN

Tensilestrength(Rm)N/mm2

Steel areamm2

Mass

kg/m m/1000kg

standard

super

Dyform/compacted

Dimensions and properties of prestressing strandto prEN10138

139.0150.0140.093.069.753.0150.0100.075.054.838.0223.0165.0112.0

1.0901.1701.0950.7260.5480.4081.1700.7810.5820.4320.2981.7401.2900.875

91785591313771825245185512801718231533565757751143

232.0265.0248.0164.6123.492.0279.0186.0139.5102.070.0380.0300.0209.0

204.0228.0213.0141.6106.079.1240.0160.0119.887.961.0327.0258.0238.0

204233218145109812461641239062334264184

16701770177017701770177018601860186018601860170018201860

15.215.715.212.511.09.315.712.911.39.68.018.015.212.7

Property ConventionalPrestressing Strand BRIDON Stay Cable Strand

Fatigue to 2 x 106 cyclesStress rangeMaximum load

FIP deflected tensile testMaximum reduction in load (D)

FIP stress corrosionMinimum lifetime to failureMean lifetime to failure

200N/mm2

70%

28%

1.5 hours4.0 hours

Plain & galvanised300N/mm2

45%

Plain & galvanised20%

galvanised only4 hours12 hours

A comparison of the respective properties of conventional andStay Cable Strand

From leading edge product development to quality assurance, Bridon ensures that customers experience thefull benefit of the technical expertise on offer. All factories operate under a complete inspection and qualitycontrol system which covers every stage of the process, and is fully approved by independent organisation.


Technical Data

12

Tolerances

Diameter 12.5mm and larger :+0.4 – 0.2mm11.0mm and smaller + 0.3 – 0.15mmSteel area and mass + 4% - 2%Elongation is not less than 3.5% at fracture on a gaugelength of 500mm minimumRelaxation: all strands supplied with low relaxationcharacteristics (Class 2).

Other Strands

9.3mm/indented – made from indented wires for use insleepers.15.4mm/250kN minimum breaking load.15.5mm/261kN minimum breaking load.

Greased and plastics covered strand

The Bridon protection process is in three stages:

• application of a protective fluid which penetrates to thecentre wire of the strand

• coating of the strand with a highly corrosion-resistantspecial grease

• continuous hot extrusion of a high density polypropyleneor polyethylene sheath over the strand.

The grease gives long term protection and lubrication forease of movement when stressing the strand. It is water free,typically 130 C drop point, retains workability down to -30 C,is highly corrosion resistant and completely non-hazardous.

The polypropylene or polyethylene sheath is a premiumgrade copolymer, stabilised with finely divided anddispersed carbon black providing the following significantadvantages:

• very high stability in respect of U.V. degradation• high dimensional stability at elevated temperatures• excellent impact resistance even at sub-zerotemperatures

• high-rigidity – will not collapse in service• excellent crack resistance• excellent chemical resistance• high resistance to abrasion and creep• easily repairable on site should damage occur• complies fully with FIB recommendations.

The nominal radial thickness of the sheath can varybetween 0.75mm and 3.00mm depending on strand sizeand use. Laser measurement devices ensure consistencyof sheath thickness. Although the plastic cover is able towithstand some abuse, care is necessary when placingand stressing tendons to minimise damage.

Quality Assurance

Bridon factories operate under a complete inspection andquality control system detailed in a Quality Manual. Thisdefines all the controls at every stage from rod to finishedproduct and includes procedures for control of auxiliarysite services, calibration of recording equipment andinternal audit of the system in line with ISO 9001.

The Quality control system is investigated and approvedby a number of independent organisations, includingindependent checking of mechanical properties andtesting procedures.

ISO 9001 approval is an integral part of Bridon’s totalquality strategy and is independently certified by thetechnical approval bodies of BSI and CARES.

Transmission Lengths

Tests have been reported on the bond characteristics ortransmission lengths of Bridon round wire strand andDYFORM® Strand. Tests were made with prestressedbeams approximately 3m long and 34 x 18cm section anda concrete strength at transfer in the range of 56 to 65n/mm2. The initial stress in the strands was 75% of thespecified characteristic strength.

Transmission length determinations were made in themanner described in earlier Research Reports of theCement & Concrete Association and the average resultsare shown (below) graphically in comparison with earlierC. & C.A. results using round wire strand and initial stressof 70% of specified strength.

Because of the smoother surface configuration ofDYFORM® Strand and its greater initial force comparedwith round wire strand, a larger transmission length for100% transfer of stress is to be expected. The relationshipis approximately proportional, for standard grade roundwire strand the transmission length is about 30 stranddiameters and for DYFORM® Strand about 45 stranddiameters.

From leading edge product development to quality assurance, Bridon ensures that customers experience thefull benefit of the technical expertise on offer. All factories operate under a complete inspection and qualitycontrol system which covers every stage of the process, and is fully approved by independent organisation.

Single wires


13

Technical Data

Load Extension

A typical load extension graph is shown to the right.

The graphs are recorded autographically up to the fracture ofthe test piece using a non-contact extensometer, whichoperates by photo-electric cells following the movement ofilluminated white gauge marks on the test piece as the latterextends under increasing load in the tensile testing machine.Electrical impulses for load and extension are fed into therecorder. In the elastic and early plastic range of extension ahigh magnification is used for extension. In the later stage,up to fracture, the extension recording is switched back to alower magnification to accommodate the total extension on anormal size graph paper. The two magnifications are shownas percentage extensions on the graph grid.

Effect of Deflection on Strand Properties

From time to time tests have been made to note the effectof various forms of deflection on the properties of roundwire strands and DYFORM® Strands.

Deflection at Pins and Saddles

In deflection from a straight line through a triangular threepoint system, if the ratio of pin/strand diameter is verysmall, there is a significant reduction in the strength of thestrand, but the effect can be reduced if the angle ofdeflection is small. Where the deflection is round a saddleof significantly larger diameter than the strand, the loss instrength is small and the larger the ratio of diameters, thelarger the possible angle of deflection without loss ofstrength.

Strand deflected over an adjacent strand

Lengths of DYFORM® Strand stressed diagonally overanother piece of the same diameter strand, with an angleof deflection of 3º at the cross-over point had strengths of99 to 100% of the normal actual strength, but in two out ofthree tests the wire failures occurred at the deflection point.

Strand deflected through 1800

Tests have been made on strands deflected roundcapstans through an angle of 1800. In loading test tofailure, the breaking load was over 90% of the actualstrand strength where the capstan diameter was 8 x stranddiameter and over 96% where the capstan diameter was16 x strand diameter. In an examination of the effect ofbending on properties, strands have been fixed in the

Deflector/stranddiameter Angle of deflection Strength % of normal Strength % of normal

1.0x1.4x

1.75x10.0x12.0x50.0x

3º10º5º10º5º5º10º

99 - 100*7688 - 9070 - 8895 - 9995 - 98100

SimpleTriangularTriangularTriangularTriangularTriangularTriangular

curved position for 7 days under a load of 70% of thebreaking load and then tested after release in the straightcondition. Where the capstan diameter was 10 x stranddiameter, breaking load had not been affected, but 0.1%and 0.2% proof loads were reduced by about 5%. Withcapstan diameters of 120 and 150 x strand diametersubsequent tests did not show any adverse affect onany properties.

Cyclic loading at a point of deflection

DYFORM® Strand deflected through an angle of 10º over apin diameter 1.5 times the strand diameter has enduredwithout failure 300 oscillations of load between 6.4% and63.9% of actual strand strength, at a rate of about 3oscillations/minute. See table below.

*Strand deflected over an adjacent strand


Technical Data

14

At the end of the test the strand was examined. There wasslight marking where there had been contact with the pinand there was a permanent bend in the strand.

Wire and strand properties at cryogenic temperatures

The behaviour of cold drawn carbon steel at very lowtemperatures is generally satisfactory, except that there is areduction in constriction at failure.

Tests on BRIDON products have been made at between -60º C and -190º C with test pieces and prestressing gripscompletely contained within the cold zone. Testing carriedout at Munich University.

Control tests were made at normal temperature. Thepattern of behaviour confirms the evidence of otherinvestigators and a summary of information on ultimatestrength of 0.2% proof stress is given in the graph,expressed as percentage change from the normaltemperature values. The 0.01% and 0.1% proof stressvalues show similar increases as the test temperature falls.Resistance to fatigue does not seem to be much affectedand, as might be expected, relaxation losses are negligibleat low temperatures.

The reduction in ductility is variable. In wire tests at -60º Cthe constriction at the tensile fracture is 5% to 14% lessthan the normal temperature result and 20% to 60% less at-190º C. The elongation, measured after fracture in250mm, shows a loss of 30% to 50%. At -190º C strandelongation, measured to fracture in 600mm, shows asimilar loss, but in tests using prestressing wire and strandgrips there is a greater tendency for fracture to occur at thegrip at the low temperatures and some of the apparentloss in elongation can be attributed to breaks at grips. Forexample DYFORM® Strand tested with testing machinewedges at normal temperature had an elongation of 7%,but in prestressing strand grips failure occurred at the gripand in consequence the elongation was about 35% lowerat -190º C the result was still 4.5% i.e. there was no furtherloss in elongation at the low temperature.

Relaxation

BRIDON has had a specially equipped relaxation testinglaboratory for many years and tests are regularly made.The graphs summarise much of the work and the notespick out some practical points.

Test method

The procedure is in accordance with national andinternational specifications. Loss of load is recordedindirectly by change of strain in a calibrated deflectionbeam. In elevated temperature tests the test piece isheated to the required temperature in the unloadedcondition and maintained at temperature for at least 20hours to establish stability throughout the test system,before the initial load is applied.

Relaxation loss is plotted against time on logarithmicscales. A linear relationship develops on the graph after afew hours on the time scale and this linearity has beenproved in tests on normal relaxation wire and on lowrelaxation strands for periods of over 11 years (100,000hours). Although tests of 1000 hours duration are usual,tests of 200 hours or only 72 hours are sufficient for manypurposes and the plots can be extrapolated to 1000 hours.

Practical interpretation of test results

Laboratory tests are carried out under arbitrary standardconditions of loading at a constant temperature (20º C)and test length. The initial load, for quality controlpurposes, is related to the actual strength of the product.

In construction, initial loads are related on specifiedcharacteristic strength, loading rates may be faster,temperature will vary, creep of concrete and other effectswill permit reduction in the initial load in the early stages.During the life of the structure parts may be subject tosignificant rises in temperature, for example in bridgedecks and in pressure vessels.

Effect of diameter

The size of the product is not significant in relaxationbehaviour; it is the type of final treatment of the 7-wirestrand, for example, which determines whether the producthas normal or low relaxation characteristics.

Effect of loading rate

A fast loading rate in practice will cause short termdifference in relaxation, for example with low relaxationstrand the 1000 hour loss of load will be about 0.3% higherif the load is applied in 3 minutes, compared with a longtime of 10 minutes, but over a very long period this initialeffect is insignificant.

Effect of magnitude of load

The strand average 1000 hour relaxation losses aspercentage of initial load are of the order of figures shownin the table.

When making comparisons it should be remembered thatthe percentage losses are related to different percentageinitial loads, for example in terms of force the 1.0% lossfrom 60% of the actual strength is only half the loss of1.5% from 75% of actual strength. Obviously a small lossof force due to other construction factors in the very earlystages would not have much effect on the subsequentrelaxation of low relaxation strand. In units of force the1000 hour loss from 65% is 88% of the loss from 70%actual strength.

Initial loadActual strength

Low rotation % loss

Normal relaxation % loss

75%

1.6

7.6

70%

1.1

5.6

65%

1.04

4.4

60%

1.0

3.4


15

Technical Data

With normal relaxation strand, the corresponding losswould be about 75% of the loss expected from 70% of theactual strength.

The fact remains that the loss for low relaxation strand isonly about one fifth of the loss for normal relaxation strand.

Effect of temperature

Where a considerable part of the working life will be attemperatures higher than 30º C low relaxation strand ispreferred. The low relaxation product should not be usedat temperatures higher than 80º C unless the workingloads are significantly reduced.

The limited evidence of two stage tests at 20º C and 60º Cindicates that the total long term behaviour will be littledifferent from the relaxation occurring entirely at theelevated temperature.

Estimation of long term relaxation

As indicated under test methods linearity of the relaxationgraph on log-log scales has been proved up to 100,000hours and there is no reason to suppose that therelationship will change in the next part cycle on the log-time scale up to 500,000 hours.

In tests on strand at 20º C and an initial load of 70% ofstrand strength, the evidence from short time tests is thatthe 500,000 hour (57 years) relaxation will average 1.8 x1000 hours loss for the low relaxation product and about 3x 1000 hours loss for normal relaxation strand.

Extrapolations from 10,000 and 100,000 hours tests showa 500,0000 hours loss of 1.5 x 1000 hours loss for lowrelaxation strand and 2.6 x 1000 hours loss for normalrelaxation strand.

In practice these factors may be reduced to take intoaccount other factors which reduce the initial load.

Pictures courtesy of: Structural Systems (UK) Limited. Structural Systems (Northern) P/L, Australia. Laing O’Rourke, UK.Balvac Limited, UK. Freyssinet UK Limited. Caerphilly County Borough Council. Mexboro Concrete Limited. Litecast Limited,Fagioli PSC Limited UK. Tacoma Narrows Constructors, USA.


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