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Pasco -Kennewic k Interci ty Bridge. Segme ntalJy asse mbled p resiressed conc rete cable-stayedbridge.s pans 4 7 981 4 7 ft. Counesy Arvi d Grant and Assoc iates lnc.

2 1 2 P ara mete rs Affecti ng the Q ua lity 1 Co ncrete

Strength and endu rance are two maj or qualities that are partic ularly important in pr estresse d concre te structures. Lon g-term d etrirn ental effects cao ra pidJy red uce the prestress ing fo rces and co uld r esult in un expeeted failur e Hence measures have to be taken ensure strict quali ty co ntro l and qua1ity assurance a l the va rious stages of p roduction

2 1 1 Introd uction

Concrete. particularly hi gh-strength concr ete is a major constituent of l pre stressedconcrete elements. Hence t strength and l ong-term endur ance have to be ac hievedthrough pr oper quality control and quality a ssuran ce a t the production stage . Num erou stexts are ava ilabl e concret e prod uction qualit y co ntrol and code requir ements. Th efollowin g discussion i s intend ed to highlight th e topies directly r elated to conc rete in pr e

stresse d elements and sys tems; itis

assum ed that the reader is aJready famili ar with th efundamentals of concrete and reinfor ced concrete.

1 C ON CRETE

M T R I L S N O S Y S T M S

F O R P R S T R S S IN G



2 3 Compressive Strength Depending on the type of mix the properties of ag-gregate and the time and quality of the curing compressive strengths of concrete can beobtained up to 20 000 psi or more. Commercial production of concrete with ordinary ag

2 3 Properties of Hardened Concrete

The mechanical properties of hardened concrete can be classified into two categories:short term or instantaneous properties and long term properties. The short term prop-erties are strength in compre ssion tension and shear; and stiffness as measured by themodulus of elasticity. The long term properties can be classified in terms of creep andshrinkage. The following subsections present sorne details on these properties.

and construction as well as maintenance. Figure 2.1 shows the various factors that result

in good quality concrete.

Figure2 Principal properties of good concrete.

Large maximum aggregatesizeEfficient gradingMinimum slumpMinimum cement contentOptimal automated plant operationAdmixtures and entrained airQuality assuranceand control

Good quality of pasteLow w ratioOptimal cement contentSound aggregate grading

and vibrationLow air content

Low w ratioProper curingDense homogeneousconcreteHigh strengthWear·resistingaggregateGood surface texture

Appropriate cement typeLow w ratioProper curingAlkal i resistant aggregateSuitable admixtureUseof superplasticizers or

polymers asadmixturesAir entrainment

Resistanceto weardeterioration

Resistanceto weatheringand chemicals

Appropriate cement type:low C A MgO free lime;

low Na 0 and K 0

Chapter 2 Materials and Systems for Prestressing2



2.1.3.2 Te ns ile Strength The tensi le strength of co ncrete is relatively low, A goodappr oxima tion for the tens ile s tr e ngth j , is O·lOf; <fa 2 f; It is more diffic ult to measure tensile sirength tban compr essive s trength beca use o f the gr ipping problems withtesting machines. A number of rn ethods are available for te nsión testing. the most commonly used m ethod being the cylind er spli tting. or Br azi lian , test.

For members subjected bendin g, the value of the modulus of rupture /, ratherthan lhe tensile spli tting strength is used in d esigno The modulu s of rup ture is measured by testing to failu re plain con crete beams 6 in. square in cross sec tion, having a

gregate is usually in tbe range 4.000 to 12.000 psi, wi th the most comm on concretestrengths being in th e 6.000 psi leve l.

The compr essive s trength ~ based on stand ard 6 in by 12 in. cylind ers curedund er standard l aboratory conditions and t ested al a specified rat e of load ing al 28 day sof age. The standard specifications used in lh e United St a tes a re usually taken íromAST M C-39. The strength o f concr ete in the ac tuaJ structure may not be the sarne as thatof be cylinder because of th e diff erence in compaction and curin g co ndit ions.

For a strength te st. th e ACI cod e specifi es using the average of t wo cy lind ers frornthe same sampl e tested at lhe same age, wbicb i s usually 28 days. As for the frequency oítesting, the code specifi es that th e strength of an individual cla ss of concrete can b e consider ed satisfactory i 1) the ave rage of all sets of three co nsecutiv e strengtb tests equalsor exceeds the requir ed ~ and 2) no indi vidu al strength test average of two cylind ers)falls below Lh erequir ed by mo re than 500 p si. Tbe a verage co ncrete strength for whic h

a co ncrete mixtur e mu st be designed shouJd exc eed ¡ by a n amount th at depends on Lheuniform ity of plan t production.

Not e lhat the design ~ should not b e the ave rage cy linder strength , but ratber themin imum conceiva ble cy lind er str ength.

hoto 2.1 Concrete cy lind ers tested O fail ure in compr essio n. Speci rnen A. low- epoxycernen content; spec imen B. higb-e poxy-ce mem coment, Tests by Nawy . Sun, and Sa uer. )

1 Concrete



span of 18 in and loaded at their th ird points ASTM C-78 . The modulu s oí ruptur e hasa higher value than th e tens ile spli tting stre ngth. The AC I specifies a va lue of 7.5 i fori modulu s of rupture of normal-we ight co ncrete.

most cases, lightw eight concrete has a lowe r tensile strength th an does normalweight concrete. The ollowin are lhe code stipul ations fo r lightweigh t concrete:

o to 2.2 Electr ón microscope phoiographs of concrete f rom specímens A and B Lhepreceding photograp h. Tests by Nawy e t al.

hapter Materials and Systems tor Prestressing



2 .1

5

2.1.3.3 hear trength Shear stre ngth is more diffic ult dete rmin e ex perim entaLlythan tb e tests discussed pr evious ly beca use of t he diffi culty n isolating shear f rom orherstresses . Thi s is one of the reasons for the large var iation in shear-strength v alues report ed in th e literatur e, vary ing fro m 20 percent of the compress ive strength in normalloadíng to a considerably higher percentage o f up t o 85 percent of the cornpres sivestrengtb in cases where direct sbear exists in combination w t compr ess ion . Contr ol of astructural d esign by s hear stre ngtb is signifi canl only in rare cases , since shear stresses

rnu st ordin arily be lim ited to continu aUy lower values in order to protect the co ncretefrom fail ure n diago nal tension.

2. l is not spec ified, use a f actor of 0 .75 fo r aLl-lightwe igbt concrete and 0.85 fo rsand-li ghtweig ht concrete. Linear interpo lation may be used Io r mix tures of naturalsand and li ghtw eight fine aggr egate.

L If the splittin g tensile srrength fa is specified,

= 1.09 c :S 7 5 Vf;

Photo 2 .3 Frac ture surfaces n tensiJe spliuin g tests of concretes w itb diffe rent lecontents. Speci mens C l and CIV hav e higher le contento hence mor e bond fai luresthan speci men CVI . Te sts by Naw y et al.

1 Concrete



Since the stress-strain curve shown in Figure 2.4 is curvilinear at a very early stage of itsloadin g history, Young s modulus o f elasticity can be applied only to the tang ent of thecurve at the origin o The initial slope of the tangent to the cur ve is defined as the initialtangent modulus, and it is also pos sible to construct a tangent modulus at any po int of thecurve. The slope of the straight line that connects the origin to a given s tress about 4 f~ determines the secant modulu s of elasticity of concrete . This value , term ed in design calculation the modulus of el sticity satisfies th e practical assumption that strains occurring during loading can be con sidered b asicalIy elastic compl etely recoverable onunloading), and th at any subsequent strain due to the load is r ega rded as cre ep.

The ACI buildi ng code gives the folIowing expressions for calculating the secantmodulus of elasticit y of concrete, Ec

e = w~· vt for 90 W c 1551b /ft3 2.2a)

where W c is the density of concrete in pounds per cubic foot 1 lb /fe 16.02 k g/rrr ) and jis the compressive cylinder strength in psi. For normal-weight concrete,

2 3 MOOULUS OF ELASTICITV ANO CHANGE IN COMPRESSIVE STRENGTH WITH TIME

Knowledge of the stress -strain rel ationship of conc rete is es senti al for de veloping all theanalysis and design t erms and proc edures in concrete structur es. Figure 2.2 sh ows a typical stress-strain curv e obtained from tests using cyl indrical conc rete specimen s loaded inuniaxial compression over several minutes. Th e first portion o f the curve, to about 40percent of the ultim ate strength can essentiall y be consid ered linear for alI practicalpurpo ses. After appro ximately 70 percent of the failure stre ss, the material lo ses a largeportion of its stiffness , thereby incr easing the curvilinearity of the diagram. At ultimateload, cracks parallel to the direction of loading b ecome distinctly visible, and most concrete cylinders except those with very low strengths) suddenly fail shortly th ereafter.Figure 2.3 shows th e stress -strain curv es of concr ete of various strengths report ed by thePortland Cement A ssociation. t can be observed that 1) the low er the strength of concrete, the higher the f ailure strain; 2) the length o f the initial r elatively linea r portion increases with the incr ease in the compressive str ength of concr ete; and 3) th ere is anapparent reduction in ductility with increased strength.

2 2 STRESS STRAIN CURVE OF CONCRETE

Figure 2 2 Typical stress strain curve of concrete

Chapter 2 Materials and Systems tor Prestressing6

Strain, e



sd) SS3JlS

sd) SS3JlS

Eo¡

C.x

oL C

r



where MPa and W c Kg/m ,Today , concrete strength up to 20,000 psi 138 MPa) is easily achieved using a max

imum stone aggregate size of n. 9.5 mm) and pozzolamic cementitious partial replacements for th e cement such as silica fume. Such strengths can b e obtained in the fieldunder strict quality control and quality assurance conditions. For strengths in the range of20,000 to 30,000 138-206 MPa), other constituents such as steel or carbon fibers have tobe added to the mixture. In all these c ases, mixture design has to be mad e by several fieldtrial batch es five or more), modifying the mixture components for the workabilityneeded in concrete placement. Ste el cylinder molds size 4 in. diamet er) x 8 in. lengthhave to be used , applying the appropriate dimensional correction . t is also necessar y to

2.3b) W 1 5

MPa) [3.32~ 6,895 ] 23;0

where psi and W c lb/ftor

2.3a)

W 1 5e psi) = [40 ,000~ 106] 14~

2 3 1 High Strength Concrete

High-strength concrete is termed as such by the ACI 318 Code when the cylinder compressive strength exceeds 6 ,000 psi 41.4 MPa). For concrete having compressivestrengths 6,000 to 12 ,000 psi 42-84 MP a), the expressions for the modulus of concrete Refs . 2.11, 2 .35, 2.38)

2.2.b)c 0 043w1 5~ MPa

or

E 57,000~ psi 4,700~ MPa)

Figure 4 Tangent and secant moduli of concrete

Strain, e




(2.4a)

39

Weight of solid silica fume onl y. Water contained as part o f the emuls ion must be subtract ed from the tot alwater allowed.

Table 2.1 Mixture Proportions for > 18 000 PSI

Superplasticizer

Coarse Fine W. R. Graceaggregate aggregate Dartard Mighty

iin. paving sand Cement Water Silica fume 40 150 lb lb lb lb gal oz/100 lb cement

1872 1165 957 217 13 2.1 9 .81894 1165 956 217 13 2.1 16.4

(1805) (1100) (950) (w/c 0.22) (70 lb)a (6.0) (Up to 24)

Since prestressing is performed in most cases prior to concrete s achieving its 28 daysstrength, it is important to determine the concrete compressive strength at the prestressing stage as well as the concrete modulus at the various stages in the loading history of the element. The general expression for the compressive strength as a function oftime (Ref. 2.18) is

2.3.2 Initial Compressive Strength and Modulus

grind the cylinder ends , then cap them with high strength capping compound for loadtesting, or to apply the load directly to the ground ends of the cylinder or through a removable steel cap with a hard neoprene pad bearing directly on the ground specimenends. Preparation of the cylinders should resemble as closely as possible the field conditions of concrete placement. Mock-up placement of the high-strength concr ete is advisable in order to evaluate the construction procedures and performance of the concrete in

field conditions and to identify potential problems with batching, placement , and testingof the concrete at early ages. Corrective measures should be taken immediately.A good example of the use of high-strength concrete in the range 20,000 psi

(138 MPa) at 56 days and a concrete modulus 7.8 X 106 psi (53.8 x 103 MPa) is theTwo Union Squar e Building, Seattle, Washington (Ref. 2.11,2.38) . Actual typical mixture obtained is listed in Table 2.1, with the design mixture values in parentheses .

A slump of 8 in . with w /c = 0.22 resulted from the mix proportions indicated . A typical compressive vs. age plot for the indicated mixture based on 4 in. x 8 in. cylinder testsis shown in Figure 5

Recent work at Rutgers (Refs. 2.36, 2.37) on high-strength composite constructionhas resulted in considerable enhancement of the ductility of high-strength reinforced

concrete beams. Prestressed concrete prisms of high-strength concrete were used in lieuof the normal mild steel bar reinforcement. The mixture proportions in lb/yd were asshown in Table 2.2 . The mixture was designed for a seven-day compressive strength of12,000 psi (84 MPa) . The ratio of the cementations /fine /coarse aggregate was 1:1.22:2 .06and the slump vari ed between 4 to 6 in . (100-150 mm). The prestressing strands werestress-relieved 270K (1900 MPa) 7 wire Hn. (9 .5 mm) diame ter strands. Figur e 2.6 showsthe cross section of the composite beams. Concrete achieved in sorne of the mixtures a7 day strength of 13,250 psi (91.4 MPa). The tested specimens were instrumented with afiber-optic system developed by the author using Bragg Grating sensors both internallyand externally.

3 Modulus of Elasticity and Change in compressive Strength With Time



1 lb /yd 0.59 Kg/rrr

1100851

Powder LiquidPortland silica fume supercement force- plasticizertype 111 Water 10 ,000 Grace

3) 4) 5) 6)

720 288 180 54

2)1)

Fineaggregate

naturalsand

Coarseaggregate

am

Table 2 .2 Mixture P roportions in Ib /yd For Compos ite Beams f ~ > 13 ,000 PSI

where t 28 days compress ive s trengtht time in daysex factor d epen ding on t ype of cement and c uring co nditio ns

4.00 fo r moist -cured t ype -I cement and 2 .30 for moist -cured typ e-Ill ce ment 1.00 for steam -cured type -I cemen t and 0.70 for steam -cure d type-Ill ce ment

fac tor depen ding on the same param eters for exgivi ng co rresponding va luesof 0.85,0.92,0 .95 , and 0.98, re spec tively

2.4b)

Hence, for a typic al mois t-cured type -I cemen t concre te,

t

4.00 85t t

Figure 2.5 Compressive strength versus age of high -strength concre te.

Age at t est days)

2

2

9

? 8

x

¡;; 7a

.; ; 6~ >

~ 5

Eo 4~

;; 13u

12

11

Chap te r 2 Mater ials and Systems for Prestressing0



J

onQJ

OQ

l C lI

¡¡

OQ~o ~

Q

Q íU oo

OQJJ

~1 ií~o

J

J

§

OQ

eQ

21 ií

S L O L

t ~

l

~ l10



where H is the mean humidi ty n percen t.It has to be pointed o ut tbat th ese ex pressions a re va lid only in general terms, since

the val ue of th e mo dulus o f elasticity affected by factors o ther than l oads, s ucb as mois-ture in th e concrete specimen, the wa ter/cement ratio. the age of the co ncrete. a nd t em-perature. Therefore. for speci al structures suc h as a rches, t unnels , and tanks, the modu lusof elasticity needs to be determin ed from test results.

2 .6b 100 - YI 1.75 2.25 65pper:

The creep ratio t has upp er and l ower limits as follows for prcstressed quality con -

crete:

eJastic strain

2.6c100 - Y = 0.75 0.75 50ower:

ultim are creep slrain Y t

where Y is the creep rati o defined as

2 .6a c

-

n YI

2.5

The effective modulus o f concre te, is

stress

elastic stra in cree p stra inand th e ultim are effective modulus given by

oto 2.4 Scaoning electro n microscopephotograph of concrete frac ture surface. Tests by N aw y. SUD,and Sauer.

Chapter M a ter ía ls and Systems tor Pres tressing



Figure 7 Strain time curve.

Time, t

elasticstrainl

r : :

Immediate elastic strain, Ee 250 1 6 in.jin.Shrinkage strain after 1 year , Esh 500 1 6 in. jin.

Creep strain after 1 year, Ec 750 1 6 in.jin.E t

1,500 10 6 in.jin.

These relative values illustrate that stress-strain relationships for short-term loading lose

their significance and long-term loadings become dominant in their effect on the behavior of a structure .

Figure 2 .8 qualitatively shows, in a three-dimensional model , the three types ofstrain discussed that result from sustained compressive stress and shrinkage . Since creepis time dependent , this model has to be such that its orthogonal axes are deformation,stress, and time .

Numerous tests have indicated that creep deformation is proportional to appliedstress, but the proportionality is valid only for low-stress levels. The upper limit of the relationship cannot be determined accurately, but can vary between 0.2 and 0.5 of the ulti-

Creep or lateral material flow, is the increase in strain with time due to a sustained load .The initial deformation due to load is the elastic strain while the additional strain due tothe same sustained load is the creep strain This practical assumption is quite acceptable,since the initial recorded deformation ineludes few time-dependent effects .

Figure 2.7 illustrates the increase in creep strain with time, and as in the case ofshrinkage, it can be seen that creep rate decreases with time. Creep cannot be observeddirectly and can be determined only by deducting elastic strain and shrinkage strain fromthe total deformation. Although shrinkage and creep are not independent phenomena, itcan be assumed that superposition of strains is valid; hence,

Total strain t elastic strain e creep c shrinkage E sh

An example of the relative numerical values of strain due to the foregoing threefactors for a normal concrete specimen subjected to 900 psi in compression is as follows:

4 CREEP

Limited work exists on the determination of the modulus of elasticity in t ension because the low-tensile strength of concrete is normally disregarded in calculations. t is,however, valid to assume within those limitations that the value of the modulus in tension is equal to that in compression.

4 Creep



Figure 9 a Section parallel to the stress-deformation plane. b Section parallel to the deformation-time plane.

ba

Shrinkage

Time

Static strain

c: .~E

~

Total deformation under astress at a time

mate st rength Thi s range in th e limit of the p roportionalit y is due to the large extentof mic rocracks at about 40 percent of the ultim ate load.

Figure 2.9a show s a section of the three-dimensional model in Figure 2 .8 parallel tothe pl ane contain ing the stress and deformation axes at time t It indicates th at both elastic and creep strains are linearly proportional to the applied stre ss. In a similar mannerFigur e 2.9b illustrat es a section parallel to the plane containing the time and strain axesat a stress f henc e i t shows the familiar relationships of creep with time and shrinkagewith time .

As in the case o f shrinkag e creep is not completely re versible. a spec imen is unloaded after a period under a sustained load an immediate ela stic recovery is obtainedwhich i s less than the strain precip itated on load ing. The instantaneous recovery is followed by a gradual decrease in st rain called r ee p r e overy The extent of the recovery

Figure 8 Three-dimensional model of time-dependent structural behavior .

Chapter 2 Materials and Systems for Prestressing



2 4 2 Rheological Models

Rheological models ar e mechanical devices that portray the general deformat ion behavior and flow of materials under stress. A model is basically composed of elastic springsand ideal dashpots denoting stress, elastic strain, delayed elastic strain , irrecoverablestrain, and time. The springs represent the proportionality between stress and strain, andthe dashpots represent the proportionality of stress to the rate of strain. A spring and adashpot in parallel form a Kelvin unit, and in series they form a Maxwell unit.

Two rheological models will be discussed: the Burgers model and the Ross model.The Burgers model in Figure 2.11 is shown since it can approximately simulate the stressstrain-time behavior of concrete at the limit of proportionality with sorne limitations.This model simulates the instantaneous recov erable strain , the delayed recoverable

As in shrinkage , creep increases the deflection of beams and slabs and causes loss of prestress. In addition, the initial eccentricity of a reinforced concrete column increases withtime due to creep, resulting in the transfer of the compressive load from the concrete tothe steel in the section.

Once the steel yields, additional load has to be carried by the concrete. Consequently, the resisting capacity of the column is reduced and the curvature of the column

increases further, resulting in overstress in the concrete , leading to failure .

2 4 1 Effects of Creep

depends on the ag e of the concrete when loaded , with older concretes presenting higher

creep recoveries, while residual strains or deformations become frozen in the structuralelement see Figure 2.10 .

Creep is closely related to shrinkage , and as a general rule, a concrete that resistsshrinkage also presents a low creep tendency , as both phenomena are related to the hydrated cement paste . Hence, creep is influenced by the composition of the concrete, theenvironmental conditions, and the size of the specimen, but principally creep depends onloading as a function of time.

The composition of a concrete specimen can be essentially defined by the water/cement ratio and water/cementitious ratio when admixtures are used, aggregate and cement types, and aggregate and cement contents . Therefore, like shrinkage, an increase inthe water/cement ratio and in the cement content increases creep. Also, as in shrinkage,

the aggregate induces a restraining effect such that an increase in aggregat e content reduces creep .

Figure 2 1 Creep recovery versus time

Time,

Residual strain

Creep recovery

ci

ti:

f immediate recovery

Unloadingf.1 immediate elastic

deformation~__ 7 -r

Specimen under aconstant load

45 4 Creep



Figure 2 2 Ross model

P t

D

e

or aver age C u 2.35.

2.9)

The ultimate creep co efficient , u is given by

Cu PuEe

wher e unit cre ep coefficient, generally called specific creept; stress inte nsity in the structural membe r corresponding to unit strai n ci

2.8)

where a and b are constants determ inable from t ests.Work by Branson Refs. 2.18 and 2.19) has simplified cr eep evaluation . The addi

tional strain Eeu due to creep can b e defined as

2.7)t

C -a bt

elastic strain in the spr ing, b and the irricoverabl e time-depend ent strains in the dashpots , e and d The w eakness in the model is that it continues to deform at a uniform rateas long as the load i s sustained b y the Maxw ell dashpot-a b ehavior not simil ar to concret e, where creep r eaches a limiting value with time , as shown in F igure 2 .7.

A modification in the form o f the Ross rheological model in Figure 2 .12 c an eliminate th is deficiency. in this model represents the Hookian direct praportionality of

stres s-to-strain elem ent, D is the d ashpot, and and C are th e elastic spring s that cantran smit the applied l oad P t to th e enclosing cyl inder walls by dir ect friction . Since eachcoil h as a defined frictional resist ance , only thos e coils whose resistance equ als the applied lo ad P t are di splaced; the others remain un stressed, symbolizing the irr ecoverabledeform ation in conc rete. As the load continues to increase, it ov ercomes the spr ing resistance o f unit B pulling out the spr ing from the dashpot and signifying failure in a concrete elemento Mor e rigorous model s, such as Roll s, hav e been used to a ssist inpredicting the creep strains. M athematical e xpressions for such predictions c an be ver yrigorou s. One conv enient expression due to Ro ss defines the c reep C under lo ad after atime int erval t as

Figure 2 Burge rs model

e l vi n u n i t

P t _- ...


a xw ell u nit



Ph oto 5 Energy Ce nter. New Orl ean s. Louisiana. C ollnesy. Post-TensioningInstitut e.)

2.14)c 1.27 - 0.0067 H

For greater than 40 p ercent relative humidity , a fu rther multipli er correction fa ctor of

2.13)

2.12)

a) For rnoi st-cured concr ete load ed at an a ge of 7 days o r mor e,

1.25 -0.118

b) For steam-cured concr ete load ed at aD age of 1 to 3 days or more,

1.1 3 0 .095

wbere ís tbe tim e in days and P is tbe time rnultiplier. Stand ard conditi ons as defined b yBran son pertain to concretes o f slump 4 in 10 c m) or less a nd a r elative hurnidity of 40percent.

Whcn condition s are not stand ard, creep correction fac tors have to be applied toEqua tions 2.10 or 2.11 a s follow s:

2.11)P 10 0 .6

or, alternatively,

2.10)

Bran son s model, verifi ed by extensiv e tests, relates the creep coefficient C ¡ at anytim e to the ultimat e creep coefficie nt for standard conditi ons) as

0 .6

C¡ 10 0 .6

4 Creep



Figure 2 3 Shrinkage time curve

Time

Aggregate The aggregate acts to restrain the shrinkage of the cement p aste; hence

concretes with high aggregat e content are le ss vulnerable to shrinkage In additionthe degree of restraint of a given concrete is determined by the properti es of aggre -gates: Those with a high modulus of elasticity or with rou gh surfaces are more resis-tant to the shrinkage processWater/cement ratio The higher the water /cement ratio th e higher the shrinkage ef-fects Figure 2 14 is a typical plot relating a ggregate content to water /cement ratio

3 Sire of the concr ete elemento Both the rate and the total m agnitude of shrinkage de-crease with an inc rease in the volume of th e concrete elem ent Howev er the dura-tion of shrinkage is longer for larger memb ers since more time is needed for dryingto reach the int ernal regions It is possible that 1 year ma y be needed for the drying

Basicall y there are two types of sh rinkage: plastic shrinkage and drying shrinkage Plas-tic shrinkage occurs during the first few hours after placing fre sh concrete in the formsExposed surfaces such as fIoor slab s are more e asily affected by e xposure to dry air be-cause o f their large contact surfac e In such cas es moisture evapora tes fast er from theconcrete surface than it is replaced by the bleed w ater from th e lower layers o f the con-crete el ements Drying shrinkage on the other hand occurs a fter the concrete has al-ready attained its final set and a good portion of th e chemical hydration proc ess in thecement gel has been accomplished

Dry ing shrinkage is the decrease in the volume of a concr ete element wh en it losesmoisture by evapor ation The opposite phenom enon that is volume incre ase throughwater absorption is t ermed swelling In other words shrinkage and swelling represent

water movement out of or into the g el structure o f a concrete sp ecimen due to the differ-ence in humidity or saturation le vels between th e specimen and the surroundings irre-spective of the externalload

Shrinkage is not a completely reversible process a concrete unit is saturated withwater after having fully shrunk it will not expand to its original volume Figure 2 13 re-lates the increase in shrinkage strain Esh with time The rate decreases with t ime sinceolder concretes are more resistant to stress and consequentl y undergo less shrinkagesuch that the shrink age strain becomes almost as ymptotic with time

Several factors aff ect the magnitude of drying shrinkage:

has to be applied in addition to those of Equations 2 12 and 2 13 where relative hu-midity value in percent


2 5 SHRINKAGE



where ESH u = 800 X 10- 6 in./in. if local data are not available.

2.15

a For moist-cured concrete any time t after 7 days,

tE--- E

SH t t SH u

Branson Ref. 2.18 recommends the following relationships for the shrinkage strain as afunction of time for standard conditions of humidity 40 percent :

process to begin at a depth of 10 in. from the exposed surface, and 10 years to beginat 24 in. below the external surface.

Medium ambient conditions. The relative humidity of the medium greatly affectsthe magnitude of shrinkage; the rate of shrinkage is lower at high states of relativehumidity. The environment temperature is another factor, in that shrinkage becomes stabilized at low temperatures.

5. Amount of reinforcement. Reinforced concrete shrinks less than plain concrete; the

relative difference is a function of the reinforcement percentage.6. Admixtures. This effect varies depending on the type of admixture. An accelerator

such as calcium chloride, used to accelerate the hardening and setting of the concrete, increases the shrinkage. Pozzolans can also increase the drying shrinkage,whereas air-entraining agents have little effect.

7. Type of cemento Rapid-hardening cement shrinks somewhat more than other types,while shrinkage-compensating cement minimizes or elimina tes shrinkage crackingif used with restraining reinforcement.

8. Carbonation. Carbonation shrinkage is caused by the reaction between the carbondioxide C0 2 present in the atmosphere and that present in the cement paste. The

amount of the combined shrinkage varies according to the sequence of occurrenceof carbonation and drying processes. both phenomena take place simultaneously,less shrinkage develops. The process of carbonation, however, is dramatically reduced at relative humidities below 50 percent.

Figure 2 14 w ratio and aggregate content effect on shrinkage

Water/cement ratio

Aggregatecontent by

volume percent

~

...., 1200 ,.~

49Shrinkage



Figure 2 .15 Various forms of ASTM approved defarmed bars.

To increase the bond betw een concret e and steel, proj ections caUed deformations

are rolled onto the bar surface as showninFigure 2.15, n accordanc e with AS TM specifications . The deformat ions SbOWD must satisfy ASTM Specification A616 -76 for th e bar sto be acc epted as deform ed. Deform ed wir e has indentations p resse d into th e wir e or barto serve a s defo rmation s. Exc ept for wir e used in spiral r einforc ement in column s, onlydefo rmed bars , deform ed wir es, or wir e fabric mad e from smooth or deforrn ed wir e maybe used n reinforced co ncrete under app roved pr actice .

L Young s moduJus ,

2. Yie ld strengtb , 3. Ultim ate strengtb ,f4. Steel grade designation5. Size or diameter of tbe bar or wir e

Steel reinforcement for concret e consi sts o f bars , wire s, and w elded wir e fabric, all ofwhi ch are man ufactured in accordance with ASTM stand ards. The most importan t properties of reinforcing steel ar e:

(2.17b)

2.17a

kSH = 3.00 - 3 R

2.6 NONPR STR SSING R INFOR M NT

k 1.40 - O.OlOR

b For 80 H s 100 percent ,

a For 40 80 percent ,

For otber than standard humidit y. a correct ion factor h as to be appli ed to Equations 2 .15and 2 .16 as follows:

(2.16)

€ ES H J 55 t S fI u

b For steam -cured concr ete afte r the age o f 1 to 3 days .

Chapt e r 2 Materials and Systems to r Prestre s sin g0



Table 2.3 Reinforcement Grades and Strengths

Minimum yield point or Ultimateyield strength, y strength,

1982 Standard type psi psi

Billet steel A615)

Grade 40 40,000 70,000Grade 60 60 ,000 90,000Axle steel A617)

Grade 40 40,000 70,000Grade 60 60,000 90,000

Low-alloy steel A706) : Grade 60 60,000 80,000

Deformed wireReinforced 75,000 85,000Fabric 70,000 80,000

Smooth wireReinforced 70,000 80,000Fabric 65,000 ,56,000 75,000 ,70,000

Figure 2.16 shows typical stress-strain curves for grades 40, 60, and 75 steels. Thesehave corresponding yield strengths of 40,000 , 60,000, and 75,000 psi 276 , 345, and 517Nzmrrr , respectively) and generally have well-defined yield points. For steels that lack awell-defined yield point, the yield-strength value is taken as the strength correspondingto a unit strain of 0.005 for grades 40 and 60 steels, and 0.0035 for grade 80 steel. The ultimate tensile strengths corresponding to the 40 , 60, and 80 grade steels are 70,000, 90,000,and 100,000 psi 483, 621, and 690 Nzmrrr ), respectively, and sorne steel types are given inTable 2.3. The percent elongation at fracture, which varies with the grade, bar diameter,and manufacturing source, ranges from 4.5 to 12 percent over an 8-in. 203.2-mm) gagelength.

Welded wire fabric is increasingly used for slabs because of the ease of placing thefabric sheets, the control over reinforcement spacing , and the better bond . The fabric reinforcement is made of smooth or deformed wires which run in perpendicular directions

Figure 2.16 Typical stress-strain diagrams for various nonprestressing steels.

Strain [in/in or mm/mm]

0.005 0.015 1

2

400

6

8

51

14

12

1

¡ 8

6

4

2

Nonprestressing Reinforcement



s

oo /

o i

~

C ID

o

N

mm i

Soz

_

c ne

ümc

Se~

~Sem

O

e



Pboto 2 .6 Pr estressed co ncrete Vald ez floating doc k. Designed by ABAM Engineers, bui lt in two pieces in Taco ma, Was hin gto n, then towe d Alaska by deployrnent. Co llnesy ABAME ngineers , Tacoma, Was hingto n.)

2. 7.1 Types of Reinforce me nt

Beca use of tbe high cree p and shrink age losses in conc rete. effective prestressing c an b eachieved by using very high-strength s teels in the range of 270,000 ps i or more 1,862MP a or higher). Such high- stresse d steels a re able 10 counterba lance these losses in th e

7 PRESTRESSING RE INF ORCEMENT

and are welded toge ther at int ersec tions. Table 2.4 p resen ts gco rnetrica l prope riies fo rsome standard wir e reinforceme nt.

For most mild steels . the behavio r is ass umed to be e Jastoplastic and Young s modulus is taken as 29 x 10 6 psi 200 x < MP a). Table 2.3 p resents the re info rcement-gra destrengths, and Table 2.5 presents geo metrical prope rties of the vario us sizes o f bars.

Bar WeightStandard nominal dimen s ion s

designation per toot Diameter , Cross-sectional Perimeternumber lb [in. mm ] area , in.2 in.

3 0.376 0.375 10 ) 0.11 1.178

4 0.668 0.500 13) 0.20 1.5715 1.043 0.625 16) 0.31 1.963

6 1.502 0.750 19 ) 0.44 2.356

7 2.044 0.875 22) 0.60 2 .749

8 2.670 1.000 25) 0.79 3.142

9 3.400 1.128 29) 1.00 3.544 4.303 1.270 32) 1.27 3.990 5.313 1. 410 36) L.56 4.43014 7.65 1 .693 43) 2.25 5 .3218 13.60 2 .257 57) 4.00 7.09

Ta ble 2 5 We igh t, Area , and Per imete r of Ind ividu al Ba rs

537 P rest ressing Reinforcement



Min. tensile strength Min. stress at 1

Nominal psi) extension psi)

diameter in.) Type BA TypeWA Type BA TypeWA

0.192 250,000 212,5000.196 240,000 250,000 204,000 212,5000.250 240,000 240,000 204,000 204,000

0.276 235,000 235,000 199,750 199,750

ource Post- Tensioning Institute

Table 2.6 Wire tor Prestressed oncrete

Stress-relieved wires are cold-drawn single wires conforming to ASTM standard A421;stress-relieved strands conform to ASTM standard A 416. The strands are made fromseven wires by twisting six of them on a pitch of 12- to 16-wire diameter around a slightlylarger, straight control wire. Stress-relieving is done after the wires are woven into thestrand. The geometrical properties of the wires and strands as required by ASTM aregiven in Tables 2.6 and 2.7, respectively.

To maximize the steel area of the 7-wire strand for any nominal diameter, the standard wire can be drawn through a die to form a compacted strand as shown in Figure2.17 b ; this is opposed to the standard 7 wire strand in Figure 2.17 a . ASTM standard A779 requires the minimum strengths and geometrical properties given in Table 2.8.

Figure 2.18 a shows a typical stress-strain diagram for wire and strand prestressingsteels, while Figure 2.18 b shows values relative to those of mild steel.

2.7.2 Stress-Relieved and Low-Relaxation Wires and Strands

Wires or strands that are not stress-relieved, such as the straightened wires or oiltempered wires often used in other countries, exhibit higher relaxation losses than stress

relieved wires or strands. Consequently, it is important to account for the appropriatemagnitude of losses once a determination is made on the type of prestressing steel required.

• Uncoated stress-relieved or low-relaxation wires.• Uncoated stress-relieved strands and low-relaxation strands.• Uncoated high-strength steel bars.

surrounding concrete and have adequate leftover stress levels to sustain the required prestressing force. The magnitude of normal prestress losses can be expected to be in therange of 35,000 to 60,000 psi 241 to 414 MPa . The initial prestress would thus have to bevery high, on the order of 180,000 to 220,000 psi 1,241 to 1,517 MPa . From the aforementioned magnitud e of prestress losses, it can be inferred that normal steels with yieldstrengths 60,000 psi 414 MPa would have little prestressing stress left after losses,

obviating the need for using very high-strength steels for prestressing concrete members.Prestressing reinforcement can be in the form of single wires, strands composed of

several wires twisted to form a single element, and high-strength bars. Three types commonly used in the United States are:




Figure 2.17 Standard and compacted 7-wire prestressing strands. a Standard

strand section. b Compacted strand section.

ba

2.7.3 High-Tensile-Strength Prestressing Bars

High-tensile-strength alloy steel bars for prestressing are either smooth or deformed, and

are available in nominal diameters from

n. (19 mm) to Hin. (35 mm). They must conform to ASTM standard A 722. Cold drawn in order to raise their yield strength, thesebars are stress relieved as well to increase their ductility . Stress relieving is achieved byheating the bar to an appropriate temperature , generally below 500 °C. Though essentially the same stress-relieving process is employed for bars as for strands, the tensilestrength of prestressing bars has to be a minimum of 150,000 psi (1,034 MPa) , with a minimum yield strength of 85 percent of the ultimate strength for smooth bars and 80 percentfor deformed bars.

*100,000psi 689 .5 MPa

0.1 in . 2.54 mm ; 1 in. 645 mm ?

weight: mult. by 1.49to obtain w eight in kg per 1 ,000m.

1,000 lb 4,448 Newton o urce Post-Tensioning Institut e

Nominal weight Minimum loadof strands at 1 extension

lb per 1000 ft * lb

122 7,650197 12,300272 17,000367 23,000490 30 ,600737 45,900

290 19,550390 26,350520 35,100740 49 ,800

Nominal Breaking strength Nominal steeldiameter of of strand area of strandstrand in. min.lb sq in.

GRADE 250

:);(0.250) 9,000 0.036fs(0.313) 14,500 0.058i(0.375) 20,000 0.080 (0.438 27,000 0.108 ( 0.5OO) 36,000 0 .144

HO.600) 54,000 0.216

GRADE 270

i(0.375) 23,000 0.085is(0.438) 31,000 0.115

HO.500) 41,300 0.153

HO .600) 58,600 0.217

Table 2.7 Seven-Wire Standard Strand for Prestressed Concrete

7 Prestressing Reinforcement



Figure 2.18a Stress-strain diagram tor prestressing steel.

Strain

0.07 in/in.06.05.04.03.02.01

1 Elongation

Strand p 27.5 X 10 psiWire p 29.0 10 psi

Bar p 27.0 X 10 psi 1B6.2 X 10 MPa)50

100

Grade 160 alloy bar

.~ 150 i

í

x

200

270

250

Grade 270 strand

Stress relaxation in prestressing steel is the loss of prestress when the wires or strands aresubjected to essentially constant strain. t is identical to creep in concrete, except thatcreep is a ch nge in strain whereas steel relaxation is a loss in steel stress Where t time,

2.7.4 Steel Relaxation

Table 2.9 lists the geometrical properties of the prestressing bas as required byASTM standard A 722, and Figure 2.18 shows a typical stress-strain diagram for suchbars.

*1000 lb 4,448 Newton

Grade 27 ; pu 270,000 psi ult. strength 1,862 MPa)

1 in. 25.4 mm; 1 in. 2 645 mrrr

Nominal weightof strand

per 1,000 ft-Ib

Nominalsteel area

in.

NominalBreaking strengthof strand min. Ib *

Nominaldiameter in.

Table 2.8 Seven-Wire Compacted Strand tor Prestressed Concrete [ASTM A779]


600

8731176

0.174

0.2560.346

47,000

67,44085,430

0.60.7



Nominal NominalBar type* diameter in. steel area in.

Smooth Alloy 0.750 0.442

Steel Grade 0.875 0.601145 or 160 1.000 0.785 ASTMA722) 1.125 0.994

1.250 1.2271.375 1.485

Deformed 0.625 0.280Bars 1.000 0.852

1.250 1.295

*Grade 145;fpu 145,000 psi 1,000 MPa)

Grade 160: t; u 160,000 psi 1,103 MPa)

1 in. 25.4 mm; 1 in. 2 645 mm

Table 2.9 Steel Bars for Prestressed Concrete

2.18)

in hours, after prestressing, the loss of stress due to re1axation in stress-re1ieved wires andstrands can be eva1uated from the expression

Figure 2.18 b Stress-Strain Diagram for Prestressing Steel Strands in Comparison with Mild Steel Bar Reinforcement.

Strain

6 4 2 in./in.

Jo

14c

l l 12~

1

57Prestressing Reinforcement



Protection against corrosion of prestressing steel is more critical than in the case of nonprestressed steel. Such precaution is necessary since the strength of the prestressed concrete element is a function of the prestressing force, which in turn is a function of theprestressing tendon area. Reduction of the prestressing steel area due to corrosion candrastically reduce the nominal moment strength of the prestressed section, which canlead to premature failure of the structural system. In pretensioned members, protectionagainst corrosion is provided by the concrete surrounding the tendon, provided that adequate concrete cover is available. In post-tensioned members, protection can be obtainedby full grouting of the ducts after prestressing is completed or by greasing.

Another form of wire or strand deterioration is stress corrosion which is characterized by the formation of microscopic cracks in the steel which lead to brittleness and fail-

2 7 5 Corros ion and Deterioration of Strands

Figure 2.19 shows the relative relaxation loss for stress-relieved and lowrelaxation steels for 7-wire strands held at constant length at 29.5°C.

2.19og t [ ¡ J i )il t J 0.55

provided that fp / fpy 0.55 and t 0.85 fp u for stress-relieved strands and 0.90 for lowrelaxation strands. Also, t 0.82 fp y immediately after transfer but fp i : : ;0.74 fp u for pretensioned, and 0.70 fp u for post-tensioned, concrete. In general, fp i 0.70 f p u

t is possible to decrease stress relaxation loss by subjecting strands that are initiallystressed to 70 percent of their ultimate strength fp u to temperatures of 200e to 1000e for

an extended time in order to produce a permanent elongation-a process called stabiliza-tion. The prestressing steel thus produced is termed low relaxation steel and has a relaxation stress loss that is 25 percent of that of normal stress-relieved steel.

The expression for stress relaxation in low-relaxation prestressing steels is

Figure 2 9 Relaxation loss vs. time for stress-relieved low-relaxation prestressing steels at 70 percent of the ultimate. Courtesy Post-Tensioning Institute.

Time hours

Low_relaxation ------


1.000.00000.0000.000.000000



a Due to tendon jacking force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.94 py

but not greater than the lesser of 0.80 and the maximum value recom-mended by the manufacturer of prestressing tendons or anchorages.

b Immediately after prestress transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.82 but not greater than 0.74 p u

e Post-tensioning t endons , at anchorages and couplers, immediately aft ertendon anchorag e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.70 p u

12y1j;

6 f~ v :

45 t

2.8.2 Prestressing steel stresses

Tensile stress in prestressing tendons shall not exceed the following:

a Extreme fiber stress in compression due to prestress plus sustained load,where sustained d ead load and live load are a large part of the totalservice load .

b Extreme fiber stress in compression due to prestress plus total load , if thelive load is transient .

e Extreme fiber stress in tension in precompressed tensile zone . d Extreme fib er stress in tension in precompressed tensile zone of memb ers

except two-way slab systems , where analysis based on transformedcracked sections and on bilinear moment-deflection relationships showsthat immediate and long-time deflections comply with the ACI definition

requirements and minimum concrete cover requirements .

Where computed tensile stresses exceed these values, bonded auxiliary r einforcement nonprestressed o r prestressed shall be provid ed in the tensile zone to resist the totalten sile force in concrete computed under the assumption of an uncracked s ection .

Stresses in concrete at service loads aft er allowance for all prestress losses shallnot exceed the following:

6 f~i

V i V i

a Extreme fib er stress in compression .

b Extreme fiber stress in tension except as permitted in e .

e Extreme fiber stress in tension at ends of simply supported member s .

Stress es in concrete immediately after prestress transfer before time-dependent prestress losses shall not exceed the following:

2.8.1 Concrete stresses in Flexure

py = specified yield strength of prestressing tendons, in psit = specified yield strength of nonprestressed r einforcement , in psi p u = specified tensile strength of prestressing tendons , in psi = specified compressive strength of concr ete, in psif~i = compressive strength of concrete at time of initial prestress

Following are definitions of sorne important mathematical terms used in this s ection:

8 CI M XIMUM PERMlssl LE STRESS ES IN CONCRETE NO REINFORCEMENT

ure. This type of reduction in strength can occur only under very high stress and, thoughinfrequent , is difficult to prevent.

Aci Maximum Permissible Stress es in Concrete and Reinforcement



2 9 4 Relative Humidity Values

Figure 2.20 gives the mean annual relative humidity values for all regions in the UnitedStates in percent, to be used for evaluating shrinkage losses in concrete.

e Post-tensioning tendons at anchorage, immediately after tendonanchorage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.70 fp u

fp y 0.85 fp u for low-relaxation,fpy 0.90 f p u

Hence for 270 K tendons used in the book, fp i at transfer 0.70 x 270,000 189,000 psi 1300 MPa is applied for uniformity.

94 fp y : : ;0.80 fp u

82 fp y : : ;0.74 fp u

2 9 3 Prestressing Steel Stresses

a Due to tendon jacking for . b Immediately after prestress transfer .

2 9 2 2 Anchorage Bearing Stresses

Post-tensioned anchorage at service load. . . . . . . . . . . . . . . . . . . . . . . . . . .. 3,000 psi but not to exceed 0.9 f~¡

2 9 2 1 Cracking Stresses Modulus of rupture from tests or if not available.For normal-weight concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5~For sand-lightweight concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3~For all other lightweight concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5~

Tension in other are as is limited by the allowable temporary stresses specified inSection 2.8.1.

Y t3~

0 4 0 ompression .Tension in the precompressed tensile zone

a For members with bonded reinforcement .For severe corrosive exposure conditions, such as coastal are as ..

b For members without bonded reinforcement .

2 9 2 Concrete Stresses at Service Load after Losses

TensionPrecompressed tensile zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. No temporaryallowable stress es are specified.Other Areas

tension areas with no bonded reinforcement 200 psi or 3~Where the calculated tensile stress exceeds this value, bonded reinforcementshall be provided to resist the total tension force in the concrete computed onthe assumption of an uncracked section. The maximum ten sile stress shall not ex-ceed 7.5~

0 60f~i0 5 5 f~ i

2 9 1 Concrete Stresses before Creep and Shrinkage Losses

CompressionPretensioned members .Post-tensioned members .

2 9 AASHTO MAXIMUM PERMISSIBLE STRESSES IN CONCRETE ANO REINFORCEMENT




2 1 1 Pretensioning

Prestressing steel is pretensioned against independent anchorages prior to the placementof concrete around it. Such anchorages are supported by large and stable bulkheads tosupport the exceedingly high concentrated forces applied to the individual tendons. Theterm pretensioning means pretensioning of the prestressing steel not the b eam it

serves. Consequently a pretensioned beam is a prestressed beam in which the prestressing ten don is tensioned prior to casting the section while a post tensioned beam is one inwhich the prestressing tendon is tensioned after the beam has been cast and has achievedthe major portion of its concrete strength. Pretensioning is normally performed at precasting plants where a precasting stressing bed of a long reinforced concrete slab is caston the ground with vertical anchor bulkheads or walls at its ends. The steel strands arestretched and anchored to the vertical walls which are designed to resist the large eccentrie prestressing forces. Prestressing can be accomplished by prestressing individualstrands or al the strands at one jacking operation .

For harped ten don profiles the prestressing bed is provided with hold-down devices as shown in Figure 2.21. Since the bed can be several hundred feet long several pre

cast prestressed elements can be produced in one operation and the exposedprestressing strands between them can be cut after the concrete hardens. Pretensioning

61

PRESTRESSINGSYSTEMS ANO ANCHORAGES

Figure 2 2 Mean annual relative humidity Courtesy Prestressed Concrete Institute

0 Prestressing Systems and Anchorages



2 10 2 Post Tensioning

In post-tensioning, the strands, wires, or bars are tensioned after hardening of the concrete. The strands are placed in the longitudinal ducts within the precast concrete ele-

several elements in a prestressing bed is represented schematically in Figure 2.22, whileharping of tendons in a prestressing bed system is shown in Figure 2.23.

In pretensioning, strands and single wires are ancho red by several patented systems. One of these, a chuck system by Supreme Products, is used for anchoring tendons

in post-tensioning. The gripping mechanism of this system is illustrated in Figure 2.24 c .Other anchorage systems and ductile connections are shown in Figure 2.24 d , e , and f . A prestressing bed for moderately sized pretensioned beams up to 24 ft 7.32 m longwas developed and used by the author in Ref. 2.31 for his continuing work on the behavior of pretensioned and post-tensioned structural systems. Supreme Products anchoragechucks have been used together with the Freyssinet jack, where applicable. Figures 2.25and 2.26 give details of the prestressing bed system also used for post-tensioning developed by Nawy and Potyondy at Rutgers University, while Figure 2.27 shows the dimensional details of the system.

Figure 2 21 Hold down anchor for harping pretensioning tendons. CourtesyPost Tensioning Institute.

anchors

Center holehydraulic jack


Harped strandgroup



Figure 2 23 Harping of tendons in a prestressing bed system

2 10 3 J acking Systems

One of the fund amental compon ents of a prestress ing operation i s the jac king sys tem applied , Le.. the mann er in whicb tbe pre stress ing force is transferred ro the steel tendon s.Such a force is appli ed tbr ougb th e use of h ydrauli c jacks of capacity 10 to 20 tons and astroke from 6 to 48 in dependin g on whether pretensioning or post-tensioning is usedand wh ether indi vidual t endon s are being prestresse d or all th e tendons are beingstresse d simult aneousJ y. the latt er cas e. large-ca pacity jacks are needed, with a strokeof al least 30 in. 762 mm . Of course, the cos t will be higher than sequeotial tensioning .Figure 2.28 s how s a 5OO- toomulti strand jack for simuJt aneous jackin g through a centerhole.

ment. Th e prestress ing force is tran sferr ed tbr ough end anchorages s uch as tbe SupremeProd ucts chucks shown in Figure 2 .24. Th e tendon s of srrands should n be bonded orgrouted p rior t o full pr estres sing.

Figure 2 22 Schemat ic of pretens ioning bed

Precastconcre te e lemen t

Prestress ingb e slab

0 Pres tressing Systems and Anchorages



Figure 2.24 a St ress S trand Anchor , b Monostrand anchor , e SupremeP roduets aneho rageehuek. Courtesy P ost -Tension ing Institute .

e

Cap

b Monostrand ancho r.a Strand anchor.

2.10.4.1 Grout ing m a ter ia lsL Portland Cemento Portland cern ent sbould conf orm one the following specifi-

cations: ASTM C150. T ype 1 n, or m.

2.10.4 Grouting 01Post-Tens ion e d Te ndon s

In order ro pr ovide perman ent protection fo r the post tensioned steel and ro develop abond b etween tbe p restressing steel and th e surroundin g concrete tbe prestressing ductshave to b e filled under press ure witb tb e appr opriat e cernenl grout in an injection p rocess .

Chapter 2 Materials an d S ystems tor Pre st ress ing



Figure 2.24 continued Multiple anchorages , couplers and ductile connectors Courtesy Dywidag S ysems International : d Multiple anchorage , e Coupler , f Dywidag ductile connectors DDC f or d uc tile precast beam-column connec tions in seismic zones . See also details In Figures13. 9 a nd 1 3. 10.

f

e

d

0 Pres tressing Systems and Anchorages



Figu re 2.26 Intermed ia te connect ions betwee n trames of the prestressing sys-tem for cont inuous beams Nawy et a l. .

2.10 .4.2 Ducts

L Fo rming. a Formed Du cts, Ducts fo rmed by sbeath left n place s bould b e of a ty pe that

does not p ermi t the entrance of ce ment paste. They s hould tr ansfer bond

Figu re 2.25 P res tress tens ion ing arrangement Nawy et aL .

Cernent u sed for gro uting should be Ir esh and should no t contain an y lump sor other indic ations of hydration or pac k set.

2. Water. The wa ter used n the gro ut should b e potab le. clean, and free of injuriou squantiti es of substances known to be ha rmful t o portl and c ernent or pr estressingsteel.

3. Admi xtures. Adrnix tures. used, sbould im part the properties of low wa ter content, goo d flow. minimum bleed, and expansi on desired. Their formulationshould contain n o chernicals in quantities that rnay bave a harmful effect the prestress ing steel or cernent. Admixt ures co ntaining chlorides as el in excess of 5

percent b y we igbt of admixtur e, assumin g 1 lb of a dmixtur e per sack of cernent),fluorid es, sulphi tes , or nitra tes sbould o ot be use d. Aluminum p owder of the properfineoess an d quantity, o r any oth er appr ove d gas- evolvin g material whi ch is well

disperse d through th e other admixtur e. may be used to obtain 5 to 10 p ercent unrestrained expans ión of the gro ut see Refs. 2.11 ,2.38) .

Chapter 2 Materials and Systems tor P restress ing6



stress es as required and should retain their shape under the weight of the concrete. Metallic sheaths should be of a ferrous metal, and they may be galvanized.

b Cored Ducts. Cored ducts should be formed with no constrictions which wouldtend to block the passage of grout. AH coring material should be removed.

Figure 2 27 Dimensioning details of the pretensioning or post-tensioning laboratory system used for research at Rutgers Nawy et al. .

b

24 -2

r

- · ~ E t tf . f

1- f

1 _ f _ _

- 1-

a

It 1 X 16 X32t 2 X 16 X32L1 1 2 1 1 2 1 1j f--1 -0 ___ 1 ~1 .---12 -0 ---- ..-H1 > < ~ I ·-1 -0 --1 r-

3 dia. extra strong pipe

Prestressing Systems and Anchorages



2 Grout Op enin gs or Vents AlI du cts should ha ve gr out openings a l both ends Pordrap ed cables ll high point s should h ave a grout vent except wh ere the cable cur -varure is small such as in continuou s slabs Grout vents or drain b oles should b eprovided at lowpoint s i the tendon is to be plac ed stresse d and gr outed in a fre ez-ing climate All grout openings or vents should includ e provisions for pr eventinggrout leakag e

3 Duct Siz e For tendons mad e up of a plurali ty of wires bars or strand s tbe ductarea should be at least twic e tbe net area of the prestressing steel Por t endonsmade up of a single wire bar or strand th e duct d iameter should be at least j in

larger than th e nominal diam eter of tb e wir e bar or strand4 Pla cement of Du cts After th e placement of duct s reioforcement and tormin gare

complete an in spection should b e mad e to locat e possible duct dama ge Ductsshoul d be securely fastened at clo se enough int erval s to avoid displacementduringconcretíng All hole s or openings in th e duct mu st be repaired prior to placem entoíconcrete Grout opening s and v ents must be securely anchor ed to th e duct and to

Figure 2.28 Stresstek Multistrand 500 ton jack. Courtesy P ost Tensio ning In-s titute.

Pho to 7 Pr e str ess c onduit fo r a b rid ge d ec k

Chapter 2 Materials and Systems tor P re stressin g



Figure 9 Prestressing of preload circular tank Courtesy N.A. Legales Pre -load Technology Ine . New York.

Additional d etails and specification s on groutin g are given b y the Post-Te nsionlngInstírute n Ref. 2.29.

2.10.4.3 r out in Process

L Ducts with concrete wall s c ored duct s s hould b e f1ush ed to ensure that lhe concret e is thor oughly w eued.

2. All grout and hi gh-p oint v ent openings should be o peo wh en groutin g start s. Groutshould b e allow ed lO flow from the fina vent after the inlet pip e untiJ any residualflushing water or entrapped air h as been remove d. at wh ich tim e the ve nt shouJd b ecapp ed or otherwise c1o sed. R emainin g ve nts should b e clo sed in sequence in th esame mann er. Th e pumpin g pressure at the tendón inlet shouLd not exceed 250 psig.

3. Grout shou ld be pump ed through th e duct and continuousl y wasted al the outletpipe until n o visible slugs o f wat er or air are e jected. The efflux tim e of th e ejectedgrout should n ot be less tban tb e injeeted grout. T o ensure that th e tendon remain sfilled with grout the outlet andlor inl et should b e closed. Plugs , cap s, or valv es thusrequir ed should n OI be removed or op ened until Lh egrout has seto

4 Wh en on e-way flow of grout cannot b e rn aintained he grout should b e imm ediatel y flushed out of the duct with wat er.

5 n temp eratur es below 32°F, ducts s hould b e kept free of w ater to av oid dama gedue to freezi ng.

6. Tb e temperature of the co ncrete should b e 35 °F or higher fro rn tb e tim e of groutinguntil job- cured 2-in . cubes of gro ut reach a mínimum compr ess ive s trength of psi.

7. Grout should n ot be aboy e 90°F durin g mixing or pumpin g. necessary the mixingwat er should b e cooled.

either m e forro s or m e reinforcin g steel, ro preven t displace ment durin g co ncretepLacin g operati ons.

0 P reslressing Syslems and Anchorages



SELE TED REFEREN ES

1. You cannot have everything. (Each solution has advantages and disadvantages thathave to be tallied and traded off against each other.)

2. You cannot have something for nothing. (One has to pay in one way or another forsomething which is offered as a free gift into the bargain , notwithstanding a solution s being optimal for the prablem.)

3. It is never too late (e.g., to alter a design, to strengthen a structure before it collapses, or to adjust or even change principles previously employed in the Iight of increased knowIedge and experience).

4. There is no progress without considered risk. (While is important to ensure sufficient safety, overconservatism can never lead to an understanding of novel struc

tures.)5. The proof of the pudding is in the eating. (This is in direct connection with the pre

vious principIe indicating the necessity of tests.)6. SimpIicity is always an advantage, but beware of oversimplification. (The latter may

lead to theoretical calculations which are not always correct in practice , or to a failure to cover all conditions.)

7. Do not generalize, but rather qualify the specific circumstances. (Serious rnisunderstandings may be caused by unreserved generaIizations.)

8. The important question is how good, not how cheap an item is. (A cheap pricegiven by an inexperienced contractor usually results in bad work; similarly, cheap,

unproved appliances may have to be replaced.)9. We live and learn. (It is always possible to increase one s knowledge and experi

ence.)10. There is nothing completely new . (Nothing is achieved instantaneously, but only by

step-by-step development.)

The following ten principles are taken fram Abeles (Ref. 2.32) and applicable not only toprestressing concrete but to any endeavor that the engineer is called upon to undertake:

2.12 TEN PRIN IPLES

Circular prestressing involves the development of hoop or hugging compressive stresseson circular or cylindrical containment vessels, including prestressed water tanks andpipes. It is usually accomplished by a wire-wound technique, in which the concrete pipeor tank is wrapped with continuous high-tensile wire tensioned to prescribed design levels. Such tension results in uniform radial compression that prestresses the concretecylinder or core and prevents tensile stresses from developing in the concrete wall sectionunder internal fluid pressure. Figure 2.29 shows a preload circular tank being prestressedby the wire-wrapping process along its height.

2.11 IR UL R PRESTRESSING

hapter Materials and Systems for Prestressing

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