Understanding the Steelin Prestressing

by Walter Podolny, Jr.*

SYNOPSISOn the premise that most of the people who design and fabricate pre-

stressed structures think of the prestressing strand and wire only as a 250 ksior 270 ksi engineering material, this paper discusses the properties of thematerial, how it is manufactured, and some of the work that is being donein the steel industry to improve this product.

INTRODUCTION

From the time that reinforced con-crete, with its inherent weakness intension, was first invented, engineershave attempted to tension the steelreinforcement in such a manner thatthe concrete would be placed in astate of compression, greater thanany tensile stress that would be im-posed by dead or live load. Undersome circumstances a residual ten-sion stress in the concrete may beallowed within code limitations.

Several structures were con-structed utilizing this concept; how-ever, only mild reinforcing steel wasavailable at the time. These struc-tures at first behaved according topredictions, but because of the rela-tively low amount of prestress forcethat could be induced in the mildsteel, they lost their properties dueto the creep and shrinkage of theconcrete and creep of the reinforc-ing steel.

In 1928, E. Freyssinet of France,who is generally credited with themodern development of prestressed

Design Engineer, Marketing TechnicalServices, United States Steel Corp., Pitts-burgh, Pa.

concrete, began using high-strengthsteel wires for prestressing.

The first linear structure to bebuilt in this country was a bridgein Madison County, Tennessee, in1950, which was followed shortlythereafter by the famed 160-ft. spanWalnut Lane Bridge in Philadelphia.

MANUFACTURING PROCESSES

High strength wire is made fromhigh carbon steel of the followingrange of analysis:

Carbon 0.72-0.93%Manganese 0.40-1.10%Phosphorus 0.040% max.Sulfur 0.050% max.Silicon 0.10-0.35%

The steel is rolled into rods ofsuitable diameters. The rod thengoes through a process called patent-ing which is a heat treatment ap-plied to the rods to obtain a uni-form metallurgical structure whichcombines high tensile strength withhigh ductility and thus imparts tothe rod the ability to withstand thedrafting required to produce thedesired wire size and the greatlyenhanced tensile and yield strength.

54 PCI Journal

In drafting or drawing the rodthrough tapered dies, the shape ofthe die, Fig. 1, is very critical. Thereare four distinct zones in the die.The first zone, on the entering sideof the die, is somewhat larger indiameter than the rod to be drawn;its purpose is to afford room for thedie lubricant that adheres to therod. This is called the bell lengthand entering angle and graduallytapers into the second zone. Thesecond zone is called the entrancecone length and consists of the ap-proach length and the reducinglength which is the section wheremost of the actual reduction takesplace. The next zone is called thebearing length and it may have avery slight angle of taper. The exitzone or back relief is in the form of

a countersinking of the back part ofthe hole. This is done as a strength-ening to prevent the circular edge ofthe hole from breaking away(')*.

Fig. 2 shows a 6 Draft ContinuousDrawing Machine that is one of thelargest made. Its major advantageis that it makes possible more uni-form mechanical properties. The ma-chine is equipped with the mostefficient cooling to attain strength,toughness and ductility. When thedrawing process is completed, thewire has very high tensile strength.

In drawing the wire through thetapered dies to reduce its diameterand increase its tensile and yieldstrength by the cold work done toit, the outer surface is squeezed and

° Numbers refer to references at end ofarticle.

SCHEMATIC CROSS SECT OWNOT T® SCALE

CARBROE

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Fig. l—Wire Drawing Die

October, 1967 55

outside wires. Hence, if slipping isto be prevented, it is very importantthat the outer wires contact thecenter wire very tightly. This is ac-complished by using a central wirewith a diameter about 4 per centgreater than the other 6 wires. Whenthese wires are stretched, they tendto straighten out. Inasmuch as theypass from side to side of the centralwire, in attempting to assume astraight line shape they press on thecenter wire, gripping it very tightlyand preventing it from slipping rela-tive to the rest of the strand. Thisconfiguration also provides thestrand with a high and closely con-trolled modulus of elasticity. Theminimum difference in diameters be-tween central and outside wires isvery closely controlled.

Special stranding techniques, in-cluding both pre-forming and post-forming operations, ensure strong,tough strand. The strander works intandem with the straightening andstress-relieving equipment and thisassures a high degree of toughnessand elasticity in the strand. Thestraighter the strand, the easier it isto place and tension, and less likelythat it will become tangled on thejob.

The wire passes through a seriesof straighteners before and afterstranding, then the strand is stress-relieved by the electrical inductionstress-relieving process.

Lengths of wire or strand from

Fig. 3-7-Wire Strand

each production run are cut off atcritical check point stages and putthrough appropriate destructive testsin the laboratory.

A final check is one in which atensile-testing machine pulls thestrand until it breaks, and then re-cords the breaking load. This con-firms that the strand or wire will.meet the requirements of the pre-stressing industry.

STRESS-STRAIN CURVE

Fig. 4 illustrates a typical stress-strain curve which can be applied,in a very general way, to all pre-stressing steel; there are no unitsfor either stress or strain shown. Asstress increases from zero, the strainwill also increase proportionally.This is represented by the first por-tion of the curve which is a straightline up to the proportional limit in-dicated by point B. Beyond the pro-portional limit, strain begins to in-crease more rapidly than stress, andthe stress-strain relationship becomesa curve with increasing curvatureuntil the ultimate strength, or pointC, is reached.

An important factor to observe isthat there is no yield point as thereis in the curve for structural carbonsteel; at least, there is no yield pointbefore the ultimate strength isreached. Instead, between the pro-portional limit, point B, and the ulti-mate strength, point C, the elonga-tion of the steel takes place in aregularly increasing manner, with-out any point where strain increasessuddenly without a correspondingincrease in stress. However, theidea of yield point strength is sowell established in thinking of car-bon steel members that an arbitrarymeans of fixing a "yield strength",for comparing high strength steelsof the sort used for prestressing andfor use in design equations. has been

October. 1967 57

Fig. 4—Typical Stress-Strain Curve

Na

N

Wcc

F'1^fJ,

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UNIT STQA VA (IN./IN.)

established. This is generally calledthe 0.2 per cent offset yield strength.ASTM A416 and A421 require thatthe yield strength be measured bythe 1.0 per cent extension underload method.

If a straight line EF is drawnparallel to the initial straight-lineportion of the curve and at a hori-zontal distance to the right of itequal to a strain 0.002 in./in., whichis the same as 0.2 per cent, the pointE, where that line intersects thestress-strain curve, is called the yieldstrength.

By the extension under the loadmethod a vertical line is drawn atthe point where the total strain H, ortotal extension, is 0.01 in./ in.; thepoint G where this line intersectsthe stress-strain curve is called theyield strength.

It should be noted that the yieldstrength as determined by the 0.2per cent offset method will be atleast 85 per cent of the ultimatetensile strength for stress-relievedwire and strand. ASTM requiresthat the 1.0 per cent extension underload be a minimum of 80 per cent

58 PCI Journal

of the minimum breaking strengthof stress-relieved wire and a mini-mum of 85 per cent of the the mini-mum breaking strength of stress-relieved strand.

It should be borne in mind thatthis so-called yield strength hasno real significance as far as theability of the steel to carry stress isconcerned. It is purely an arbitrarypoint, and might have been takenat 0.10 per cent or 0.30 per centoffset or 0.7 per cent extension underload. This arbitrary yield strengthhas caused the erroneous impressionthat there is a definite critical stressbeyond which one should not go indesign. Yielding begins at rather lowstresses and the degree of yieldingprogresses at a faster rate as stressincreases. This increased yielding isa very gradual phenomenon and thebest that can be said is that there isnot any critical point we can definesuch that there is danger in goingbeyond or absolute safety below.However, the various yield strengthmethods have found a place in spec-ifications and are used in establish-ing working stresses.

MECHANICAL PROPERTIES

The process of stress-relieving hadbeen mentioned previously as caus-ing some important changes in themechanical properties of the wire.One change is that of ultimate ten-sile strength. The ultimate tensilestrength is slightly reduced, but be-cause the stress-relieving tempera-ture is not high and the wire onlyreaches a temperature of about600°F, the reduction in strength is inthe order of about 5 per cent. How-ever, the ductility is greatly in-creased, as there is a 4 per cent mini-mum elongation for wire and a 3.5per cent minimum elongation forstrand. These are specification mini-mum values. Average values are ap-

proximately 7 to 8 per cent (10-in.gage length for wire and 24-in, gagelength for strand). The proportionallimit will be much higher than forthe non-stress-relieved wire, whilethe deviation from the initial straightline portion of the stress-strain curvewill be more gradual. As a result,the yield strength as determined bythe 0.2 per cent offset method will behigher; it will be at least 85 per centof the ultimate tensile strength.

Typical stress-strain curves areshown in Fig. 5 for both as-drawnand stress-relieved wire. The firstpart of the curves follow the samestraight line; this would mean thatthe two wires have the same initialmodulus of elasticity, approximately29,000,000 psi.

Because the efficiency of a strandis always less than 100 per cent,the tensile strength of the strand isalways less than the tensile strengthof the wires composing it. Efficiencyof a strand is defined as the percent-age ratio of measured breakingstrength of a strand to the aggregatestrength of all individual wirestested separately. If a tensile testis made on a piece of strand and thestress-strain curves for the non-stress-relieved and the stress-relievedstrand are compared, Fig. 6, an in-teresting observation can be made inthat the initial straight line portionsof the curves do not coincide; theas-drawn strand is below the stress-relieved and indicates a lower modu-lus of elasticity. When strand isloaded, a considerable construction-al stretch occurs due to the tendencyof the helical wires to straighten andthe resulting compaction of thestrand. For a given stress, a greaterelongation is obtained than with asolid wire, and this gives a lowermodulus of elasticity. For stress-re-lieved strand the initial modulus of

October, 1967. 59

Mi

NWdr)

rJ

UNIT STAN

Fig. 5—Stress-Strain Curve—Wire

elasticity has been found to be ap-proximately 28,200,000 psi.

CREEP

When cold drawn wire sustainsa high tensile stress, it creeps. Creepis defined as the continuing elonga-tion of the wire under constant load,that is, for a constant load the wirewill increase its length as time prog-resses.

The amount of creep increaseslinearly as the logarithm of timeas indicated in Fig. 7 for as-drawnwire loaded to different stress levels.This means that the extension of thewire, or creep, progressively de-creases. Stating it in a different man-ner, one could say that it wouldtake one hundred years to double

the creep which is observed in thefirst month(2}

Stress-relieving makes a pro-nounced change in the creep charac-teristics of a wire. If properly made,stress-relieved wire will show noappreciable creep when stressed toabout 50 per cent of its ultimatetensile strength. Beyond 50 per centcreep begins to become noticeable,Fig. 8, and gradually increases asthe stress approaches ultimatestrength(2)

The rate of creep is no longer alogarithmic one; at a stress as low as60 per cent of ultimate the rate ofcreep is increasing against the log-arithm of time. In reality, the rateof the creep has decreased verymuch, compared to as-drawn wirewhen measured in terms of actual

60 PCI Journal

CREEPIN..1IN.

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Fig. 7—Creep—As-Drawn Wire

October, 1967 61

CFIN

TIME IN "0Q FkS

Fig. 8—Creep--Stress-Relieved Wire

time. The actual rule governing therate of creep of stress-relieved hightensile wire is more complicatedthan a straight logarithmic relation.ship.

RELAXATION

Because of the logarithmic timenature of creep phenomenon in wireand a doubt as to its validity whenapplied to conditions prevailing inprestressed concrete, a more validapproach to the creep behavior ofprestressing steels for the designeris the measurement of stress relax-ation when the wire is held at con-stant length or strain. As might beexpected, relaxation, like creep, fol-lows a substantially straight logarith-mic law at normal stress and temper-ature.

A logarithmic progression has twolimits, zero and infinity. For all prac-

tical purposes, in prestressed con-crete, 1000 hours seems to be apractical measure of an end point.If one considers that the shrinkage,creep and elastic shortening of theconcrete itself will reduce ratherquickly the initial tension in thesteel, and the approximate logarith-mic nature of the steel relaxation,nothing very important can be ex-pected to happen after 1000 hours.

In comparing the 1000-hour val-ues of stress relaxation for the as-drawn and the stress-relieved wires,Fig. 9, it can be seen that the line forthe relaxation values of the as-drawnwire progresses to large values atlow initial stresses. In looking at thecurve for the stress-relieved wire,it is evident that relaxation, by com-parison, is very small up to valuesof about 55 per cent of ultimate

62 PCI Journal

JTSCoLO DRa.y.IN°foc %M,4 UT5

OO.6T 0.98 223 ksi•0.80 0.10 253

0.84 0.72 2Go

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LOSS IM iOOO HOURS

Fig. 9—Relaxation of Several High-Strength 0.196-in. Diameter Wires at Various Stress Levels

strength. Beyond this point the rateof relaxation increases gradually, un-til at about 70 per cent the relaxa-tion of both wires is about equal.Beyond 70 per cent the relaxation ofthe as-drawn wire is actually lessthan that of the stress-relievedwire (2)

DESIGN STRESSES

At this point, one might ask whatworking or initial stress should beused for design. A conclusion couldbe made that there is no point ofstress where the behavior of thesteel changes radically or even mark-edly. Stress-relieved wire could beused, at least for linear structures, ata level of design stress less than55 per cent of the ultimate. How-ever, there are thousands of struc-tures in which initial values larger

than 55 per cent have been em-ployed. Unlike other materials ofconstruction, prestressing is a sys-tem, and all factors have to be con-sidered in determining a level ofsafety, including design, shape, ma-terials and workmanship. Safe work-ing stresses cannot be determinedfor any one element by itself.

The design working stresses in thesteel are fixed at 0.6 of the ultimatetensile strength or at 0.8 of the yieldstrength, whichever is lower.

Initial prestress loads of 60 to 70per cent of ultimate strength arecommonly used in practice. Here,stress-relieved wire with its lowerrelaxation and its larger coil diam-eter are important advantages.One code had issued design criteriawhich permitted temporary initialstresses as high as 80 per cent of

October, 1967 63

ultimate tensile strength. SomeEuropean practices allow initialstressing as high as 90 to 95 percent of ultimate tensile strength. Noexception can be taken to this, pro-vided the designer is aware of theincreased stress relaxation, as indi-cated in Fig. 9. It is not intended tosuggest that stresses higher than arenow considered safe should be rec-ommended. The data presented hereis only for informative purposes. Onemust remember that the ultimatetensile strength referred to in theseillustrations is the actual ultimatestrength of the specimen beingtested. The specifications by whichprestressing steel is purchased callfor minimum guaranteed ultimatetensile strength. Actual tensilestrength may be as much as 15 percent greater than specified. There-fore, anyone who assumes to beworking at a 90 per cent level mayactually be at an 80 to 85 per centlevel which might cause some con-fusion if not thoroughly understood.

CORROSION

Corrosion is the term used todesignate the deterioration of a met-al by the surrounding environment.Moisture is a necessary requirementfor the corrosion process to proceedat ordinary temperature. The sever-ity of corrosion is influenced bythe presence of dissolved salts suchas chlorides and sulfates that maybe present with the moisture. Car-bon steel can be attacked and de-stroyed by corrosion from a numberof sources. As an illustration, whencarbon steel is exposed to an indus-trial atmosphere, the presence ofnitric, hydrochloric or sulfuric acids,or phosphates and ammonium salts,such as occur around fertilizer plants,can cause severe corrosion. It isthus evident that corrosion is a func-tion of the environment. What ef-

fect a particular environment willhave on a specific metal can onlybe determined by exposing thatmetal to the environment.

Calcium chloride has the abilityto stimulate pitting of high-strengthsteel wires. The use of calcium chlo-ride in concrete can seriously affectthe performance of prestressingwires or strand. In a Russian investi-gation it was demonstrated that theinclusion of two per cent calciumchloride, by weight of cement, inspecimens of prestressed concretegirders, resulted in failure before fulldesign static load was reached at theend of one year. In the absence ofcalcium chloride, the members with-stood the design load as well as theexpected failure load.

When prestressing wire is held inoutdoor storage for short intervalsof time, little loss in mechanicalproperties will occur if the environ-ment is one in which only generalcorrosion occurs. For example, testsconducted in Great Britain on 0.200-in. diameter degreased wire exposedduring seasonally bad weathershowed that the reductions in tensilestrength recorded after periods ofone, two and three months were1.01, 1.52 and 2.10 per cent respec-tively ( 4 ) ,

If the wire is to be stored forany length of time, some meansshould be taken to protect it. Forexample, recent developments in thefield of vapor phase corrosion in-hibitors have resulted in their beingincorporated into specialized wrap-ping papers. Protection of this typeis effective in preventing damaginglocalized pitting. Currently, a packis available such that the strandcoil is spirally wrapped with cor-rosion inhibiting paper and thenlocked into a shipping stand afterwhich the entire pack is coveredwith waterproof reinforced paper.

64 PCI Journal

Concrete of appropriate composi-tion and thickness is completely pro-tective of prestressing steel by vir-tue of the environment it creates.However, if a non-dense compositionis selected, if an open-structured,porous aggregate is employed, ifcalcium chloride is used to acceler-ate setting time, or if the externalenvironment is likely to be wet forextensive periods, corrosion may oc-cur.

When it is known that a pre-stressed concrete structure will besubject to frequent contact withmoisture, then steps should be takento prevent water seepage throughpressure or capillary forces by select-ing a formulation that results ina dense, impervious concrete. If nec-essary, consider the incorporation ofa damp-proofing agent, such as butylstearate or mineral oil, and employa low water-to-cement ratio andavoid the use of calcium chlor-ide (10 ). If it is desirable to obtainother properties through the use ofadditives, information should besought from the additive supplierconcerning the effect to be expectedon the prestressing wire.

ELEVATED TEMPERATURE

Elevated temperatures reduce thetensile strength of wire and strand.There is a loss in strength of strandof about 10 per cent at 400° F, a50 per cent loss at 800° F. Part ofthe loss at elevated temperatures isrecoverable after cooling as shownby the following table of the be-havior of the steel strand(5):

Tempera- Loss of Recoveryture Prestress of Loss

Deg. F. %400 10 70600 15 50700 23 30

Very little has been published onthe effect of low temperatures onsteel wire. In the tensile strengthrange of 200-220 ksi, steel wireshows a slight increase in strengthand elastic properties as the temper-ature falls. Sub-zero temperatures onthe Fahrenheit scale would be re-quired to produce a 5 per cent in-crease in tensile strength. As a ruleof thumb, it may be useful to remem-ber that a change in temperature of1°C will produce a change in stressof a fixed wire in the order of 240to 280 psi(4).

CONCLUSION

The best quality of steel availableis used in the manufacture of pre-stressing wire and strand, and theutmost control is used during itsmanufacture. It cannot be empha-sized strongly enough that the careand handling of this material iscritically important. Damage to wirecan have serious consequences.

When flame cutting wires at theend of a member, it is importantto prevent molten metal from com-ing into contact with any other wire.The globule of metal can producean alteration in the strength andductility of the wire or strand bychanging the structure of the steel.

Some properties of this materialrequire further investigation. Therehave been many definitions of anideal wire; however, the ideal isseldom possible for economic orother practical reasons.

Hopefully, the material in thispaper has conveyed to the reader abetter understanding and respectfor the steel material that is thebackbone of the prestressed concreteindustry.

October, 1967 65

REFERENCES

1. "The Making, Shaping and Treating ofSteel"—United States Steel Corpora-tion.

2. Evening, W. 0., "Steel Wire for Pre-stressed Concrete," First National Pre-stressed Concrete Short Course, Oc-tober 10-12, 1955, Maritime Base,St. Petersburg, Florida.

3. Spare, Gordon T., "Prestressing Wires—Stress-Relaxation and . Stress-Corro-sion Up to Date," Wire and WireProducts, December 1954.

4. "Wire for Prestressed Concrete"—Brit-ish Ropes Limited, Doncaster, Eng-land.

5. Abrams, M. S. and Cruz, C. R., "TheBehavior at High Temperature of SteelStrand for Prestressed Concrete," PCAResearch Department Bulletin 134.

6. Monfore, G. E. and Verbeck, G. J.,"Corrosion of Prestressed Wire in Con-crete," Journal of the American Con-crete Institute, Proceedings Vol, 57,No. 5, Nov. 1960, pp. 491-515.

7. Evening, W. 0., "Prestressing SteelUnder High Stresses," World Confer-ence on Prestressed Concrete, July 29-August 2, 1957, San Francisco, Cal-ifornia.

8. Spare, Gordon T., "Creep and Relaxa-tion of High Strength Steel Wires atRoom Temperature," Wire and WireProducts, October 1952.

9. Lin, T. Y., "Design of Prestressed Con-crete Structures," John Wiley & Sons,Inc.

10. LaLonde, W. S., Jr., and Janes, M. F.,"Concrete Engineering Handbook,"McGraw-Hill Book Company, 1961,Chapter I, "Materials for ReinforcedConcrete" by F. G. Lehman.

Discussion of this paper is invited. Please forward your discussion to PCI Headquartersby January 1 to permit publication in the April 1968 issue of the PCI JOURNAL.

66 PCI Journal