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Masters Theses Student Theses and Dissertations

1952

An investigation of end-anchorage and performance of SR-4 strain An investigation of end-anchorage and performance of SR-4 strain

gages on a prestressed concrete beam gages on a prestressed concrete beam

Howard W. Nunez Jr.

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AN INVESTIGATION OF END-Al~CHORAGE AND

PERFORMANCE OF SR-4 STRAIN GAGES ON

A PRES·l'RESSED CONCRE-TE BEAM

BY

HOViARD \'i • NU.NEZ # JR o

A

THESIS

..

submitted to the faculty of the

SCHOOL OF MINES AND METALLURGY OF 'mE UNIVERSITY OF MISSOURI

in partial fulfillment of the work required for the

Degree of

MASTER OF SCIENCE IN CIVIL EUGINEERING

Rolla~ Missouri.

1952

ACKN0'1!LEDG~ffiNT

I wish to exp~ss my sincere thanks to the many who

have halped ·me, both directly and indirect~y, in this in­

vas tiga tion·.

I am deeply indebted· to Prof •. E. w. Carlton .for. his

continuous advice, encouragement, and guidance in this

work. To Prof o A~ Vo Kilpa.trick I would like to express

11

my thanks for his excellent machine work in making the end­

anchorag~s employed.

My thanks to Mr. K. p. Billner, President of Vacuum

Concrete~. Inc. of Philadelphia, for the model of" the e·nd­

anchorage as well .as for bery helpfu~ advice; to John A.

Roebling's Son·a for their generous gift of .wire used in

this project; to Prof. J. K. Roberts for his photogr~phic

work; and to Mr. J. H. Senne f ·or his help and advice re-

gard~ng SR-4 strain gages. I ~m grateful to Ko Delap~

J. Latham» Go Latham, and R. Hof.fman, all of w~om gave

uilling and able assistance in· the pouring, .. prestressing ·

and testing of the beamo

Acknowledgment • • •

List of Illustrations

Introduction ••

Historical Sketch

• •

•

•

•

•

General Discussion • •

A Discussion of Methods

• •

• •

• •

• •

• •

•

CON'rENTS

• • • • •

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• . - . • • • • • •

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• • • • • • • • •

• • • • • • • • •

• • • • • • • • •

A Discussion of Materials • • • • • • • • • • • • • •

J.~a terials Used in the Beam •

Teat Cylindezts • • • • • • •

• • • • • • • • •

• • • • • • • • • •

• • •

• • •

iii

Page

ii

iv

l

3

7

12

18

28

48

Prestressing • • • • • -· • • • • • • • • • • • • • • • 55

Test Procedure and Results

Calculations • • • • • • •

•

•

• •

• •

Conclusions and Recommendations

Bibliography • • • • • • • • • •

Vita • • • • • • • • • • • • • •

• •

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• • . .. . . • • • • • •

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69

87

90

93

95

Figure

l

2

3

4

5

6-8

9

10

LIST OF ILLUS·r.RATIONS

Prestressed and reinforced contrete beams under load • .. • • • • • ·• • •

. . ~ • • • • • • • • •

Compressive forces applied by pre-tensioning •

Compressive forces applied by post .. tensioning •

E.f£ect of bleeding on concrete strength • • • •

Concrete stress vs. concre .. te strength •••••

1v

]?age ·

1~-.

11

11

27

27

Views of · .forms • • • • . . . . • • • • . .. . . • 36-39 .

View o:f beam end • • • • • • • • • • • • • • • • • 40

Stress-st~ain diagram for wires • • • • • • •• 41

11-12 Location of wirel!J and gaces • • • • • • • • • • 42'

13-14.

15-16

_17

18-20

21

22-24

Collar and plug unit • • Je • • • • •• • • • • • 43-44 ~ -

Applying strain gages to ~~re •• • • • • • • • • 45-46

Strain ga~es applied to concrete beam ••••• 47 .

1~st cyli~ders • • • • • • • • • • • • • • • • 51-53

Stress~strain diagram for concrete • • • • • • 54

pr·estressing unit • ·• • • • • • • • • • • •• •

25-26 . Jacking unit • • • • • • • • • • • • • •

• 61-63

64-65 • • • 27_•:29 ·Los.s ~ prestress • • • . . . . . • • . .. .. • • .• . 66-68

30-32· Beam prior · to testing • • • • .• . • . , · • . • • • • • 7,5-77

· .. 33-35

'36

37

38

·39-40

Crack openings . . •" . • • • • • • • • • • •• • •

Beam at ,maxixnum deflection ·• •••••••••

Beam after remova·l of load • • • ·• • • • • . .• .•

Beam a.fter complete failure • • •. • • • • • • . •

7a~ao .

81

82

s·3'

Fiber s·tresses • • • • • . . .. . .. • •• . • . • 85-86

1

INTRODUCTION

In recent years there has been brought to the attention

of e ·Il6ineers and architects a new medium of' construction--·.;.

pres~ressed concrete. Reinforced concrete has long been re-

cognized as a satisfactory material f'or use in the construe-

tion of buildings, bridges, highways, tanks and other struc-

tures. However, moat : ·concrete· designers will concede that

this material has many drawbacks~· For instance, in a simple

beam of rectangular cross-section, roughly only one half or the concre ·te is relied on to resist compressive forces. i'he

tensile strength of the concrete is considered to be zero, ··

and steel bars or rods must be inserted to carry the tensile ..

stresses. 1nis assumption necessarily increases the dead

loads which must be considered in design.

Prestressed concrete refers to members in which the

concrete is subjected to stress ·before it receives any live

loads. fhis stress ls usually in the form of a compressive

force which acts on the whole cross-sectional area 'of the

member. In this way the entire area of the member can be

used to resist the applied loads.

As in any new field of endeavor. many problems are en-..

countered which must be investigated. In this rield o~ pre-

stressed concrete probably the largest stumbling block is

the method of anchoring the high tensile strength wires

··which are used to induce the compressive stresses in the

concrete.

It is the p~pose of this paper to investigate one me-

2

thod of end-anchorage ,and also to -- investigate the perfor­

mance of SR-4 strain gages applied to both · ,the . prestres·sing

wires and the con ere te.

HIS.:l:ORICAL SKETCH

•]he basic idea o~ pPestressed concrete is the elimina­

tion of' tensile stresses by superimposing compressive

·stresses. One method of _accomplishing ~his. is by stretch­

ing the reinforcement. '.Phis n1uch was known more than six-

ty years ago. In 1886, the first patent on prestressed

concrete was issued to p. H. Jackson of San Francisco. This

patent was for a process of. tightening steel tie-rods in

cast-sGone and concrete arches by means of' turn-buckles and

nuts and bolts. Jackson's scheme was tried under various

3

condi.tions, but with .little or no success. rhe_ neutralizing ·

compre~~ive stres~es could be induc~d, but they· could not be .~:.; ..

maintained inde.I·ini te ly by means of ·the ordinary bar rein-

:forcement then in use.

The ~irst_pe~son to achieve any real degree of' success

in this new method of' construction was R. E·. :pill of' .Alexan­

dria, Nebraska, (l·) who applied for a patent in 1925. Dill

(1) u. s. Progress in Prestressed Concrete, Architectural

Record~ Vol. 110, No. 2, pp. 148-156.

pro~uced mainly posts and slabs by the post-tensioning m~thod.

In order to overcome the major obst~cle encountered by Jack­

son, Dill employed high. tensile s·trength steel wire which .. was coated with a plastic swbstance .to prevent bond with the

concrete, the wires bei~g tensioned a~ter the concrete had

set, thus avoiding "any loss oi .. · the prestress f'orce due to

4

shrinkage of the con ere teo

Since that time, European engineers s.uch as J o Mandl of

Austria, Eugene Freyssinet of' France~ Gustave Magnel of Bel-

gium, Edwald Hoyer of' Germany~ and Colonnetti of Italy have

led the way in making prestressed concrete a revolutionary

and highly successful new medium of constructiono Their

primary work has been done in the field of bridges, or which

the 410-foot, two span, continuous box-girder Sclayn Bridge( 2 )

(2} Schofield, E. R., Prestressed Concrete Used for Boldly

Designed St~uctures in Europe, Civil Engineering, ~ept.

1949, VOlo l9o P• 596o

is a most beautiful and impressive example. Another strik-

ing bridge project is that of the five almost identical pre-

stressed concrete bridges over the Marne Rivero These

bridges, designed and built by Freyssinet, were assembled

£rom precast units and prestressed to form flat-arches span­

ning 243 feet and having a crown depth of' only 37 incheso

Due to the fact that all the spans were the same, the bridges

w~re cons ·tructed on a mass production basis, as many as six-

ty identical pieces being made from each m~~do

Another example of European practice is a one-story

textile mill in Ghent, Bel£ium, which covers 8 1/2 acreso

Here again, mass production methods were employed with the .. necessary one hundred main 5irders, 72 feet in length, and

the six hundred secondary beams, 45 feet in length, being

'•

5

cast ln steel ffiOlds. The whole structure was erected in a

period of one year with no more than seventy workmen engaged

on the job at any one time.

A runway at Orly Airport near Paris, France, constructed

by Freyssinet in 1948, points out the possibilities of pre­

stressed concrete for use in highway and ~irport slab con­

struction. This strip is 1440 feet long, 200 feet wide, and

only 6 inches thick, yet it is claimed to be capable of sup­

porting loads or a conventional runway four times as thick.

The pavement consists of 40-inch square pre-cast concrete

blocks. T.he blocks were installed using 45-degree diagonal

joints, making only transverse prestressing necessary. one

inch vertical rollers help to reduce friction in these joints.

Freyssinet claimed that this runway could support aircraft

three times the size of any then in existence.

Here in America, virtually the only application o£ pre­

stressing up until the last few years has been in the pro­

duction of concrete tanks and cylindrical pipe. T.his process

of circular prestressing is somewhat different from linear

prestressing and has been perfected to a high degree almost

exclusively by American engineers. The reluctance of the Am­

erican engineer to show other than a passing interest in the

European scheme of linear prestressing ~ay be attributed al­

most entirely to the methods involved.

It has been argued that a great dirference exists be-

tween the two continents in regard to the relative importance

of labor and material. European labor is cheap, while ma-

6

terials are in short aupplyo American labor is decidedly

costly, and materials have been, at least up until the pre­

sent t~eD relatively plentifulo For this reason, the Euro­

pean designer is primarily interested in re£~ements o~ de­

sign to conserve material, while. the American designer is

more concerned with meChanical me~ods of construction which

can keep labor requirements to a minimumo

In the last few years numerous organizations have"been

rormed in this country, devoted to the study and manufacture

of prestressed concrete, and, as a result, remarkable ad-

vances have been made toward overcominB some of the prevail-. in£ inertiao Constantly being presented are new methods ~d

new ideas which are in m~ny ways simpler and at the same

t~e better than current European practiceso

GENERAL DISCUSSION

T.he ter.m "prestressingn does not exactly describe what • is do~e to the concrete because it is actually a pre-com-

pressive force which is exerted on the member. By various

means,· .a_t large compressive force is appl-ied to the concrete

prior to its being put to use., a:nd this ... .f~rce is mad.ntaltned

to some degree t.hroughout the life of' the s true. ture. · lhis

pre-compressive force is made so large that.t und.er wo~king

loads, the tensile forces develo_ped only reduce the total

compressive forces and never actually pl~ce the conc~ete in

tension.

7

The essential differenca between th~ various methods ot c

pre-compressing. is in the way _ in which the forces are applied

to the member.· This may be done by pre-tensioning or _by

post-ten~ioning, .the prefix indicating whether the steel is

tensioned be:fore or af'ter the ·concrete has hardened.

In pre-tensioning, the- compressive force is applied to

the concrete member ·through the action o:f bond. Very small,

high strength -steel wires are strung between two abutments, ·

one f'ixed and ·the other movable. The wires traverse the

length of the forms and are held in their proper places by

·holes in the abutments or by spacer blocks. When the mov­

able abutment is rorced away from the other one, a stress in

the neighborhood ot 200,000 psi. can be induced - in the wires.

While the wires are held in this state, concrete is placed _

~to forms which have been set up around them.. When the con­

crete has hardened"sufficiently.; the wires are _released !'rom

8

the a'tutments. When . the wires are- released, they have a

tendency · to snap back to their original _ length;, much like a

rubber band, but the b·ond of the concrete · along ·t;heir rull

length p~events them from doing so. Thus~ the pre-co~pres-. . . . . .

. .

: sive force is transmitted. to the concrete member as a com-

bination ot mechanical bond resistance and friction with

radial compress-ion. See Fig. 2. This method is · particular­

ly s~itable . fo~ operationa involv~ng the manufacture of ·a

larg~ number of similar units, since ·the wires may be ten-·

s·ioned in lengths of fro~ 200 .feet to 1000 :feet. A large

number of uni, ts may then be cast at one time and the wires

cut between each unit to separate them. This will eliminate

the necessity --cif end anchorages and _jacking operations· for· · . . . .. . .

each member.· However·, this method does present _a problem in

the .form of a loss of prestress due to the shrinkage of the

concrete after the wires have· been released.

In post-tensioning, the compressive force is applied· to

the concrete _through bearing plates a.t the ends o.f' tpe· mem~

ber. See Fig. 3. Prior to the pla.c~ng of the concrete,

.~ollow ducts are _placed in the forms. Arter the concrete

has hardened,. these are wlthd.rs.wu, and the ·wires· ~e ·threaded . ' ' ,

through the passages and :f'astehed. to some f'orm of ·:·jac;k. ·· Re-. . . ' . \ , . .

aotin~~_ ag~lnst the'.~$~ds ~.r th~ concrete member, the , J~ck

stresses the wires to around 120,000 psi. Under - this

s~es_sed -condition ~hey are wedged .against an anchoi-age plate, '• .

arid when_ -_-_~e jack ia released., the stress J;n ~11~:-, :w:ire~ 1~

·transferred- to the bear1ng plate. ·and thence to ;tl\8 · con.cre te. ,: .

T.he _purpose of the passages is to keep the wires from

being bonded to ·the concrete. Another method of accomplish­

ing this and~ at the same timeJ eliminating the . expense e.nd

time necessi-tated by the· ducts is to coat the wire.s with as­

phalt or grease or to wrap them in waterproof paper. Where

a number of wires are in the form of a cable, a thin metal

sheath may be employed to surround the entire cable and pre-

-vent bond.

While the post-tensioning method is most adaptable to

individual job conditions~ it also ~as great promise in the

field of mass pr9duced units. O.f special interest are beams

and girders built of pre-cast .building block units. These (

units e.·re very inexpensive, yet very strong, and may be made

in a wide variety · of shapes on standard commercial concrete

block machines. ·fhe wires may then be strung in passages

cast inside the un.its or in channels on the outside.

A number of .the advantages o~ prestressed concrete are

stated herewith:

..

1. Prestressing can make concrete crackless~ which 1s

conducive to greater durability under severe ex-

·posure conditions.

2. ·pres-tressing minimizes deflections .~nd ·reduces the

depth o:f beams and . girders and the thicknes_~ ot

slabs~ thus giving greater headroom.

3. As compared with conventional reinforced concrete~

9

prestx-essed concrete permits substant~al savi~gsin .

· materials: up .~o . as much as ,·75 .per' cent in steel

10

and approx:Imately 25 per cent in concrete.

4. Prestressing results in maxinlum rtgidity under

working loads and maximum flexibili·ty under exces­

sive overloads.

5. Design calculations are considerably quicker and

more accurate.

6. T.he architect can design concrete structures with

much cleaner, slimmer lines.

7. Even under extremely heavy loads~ the member will

return to its original shape as long as the elas­

tic limit of' the s tee 1 has not ·been exceeded.

· ·~ Unloaded -

Loaded

Pre stressed Concrete

1~-LJJ_J-~·1 ' Unloaded

Lodded

ReIn forced Concrete

Fio. Showing Difference Between Prestressed

and Reinforced Concrete Under Load.

Fig. 2 Compressive

Forces Applied By Pre-tensioning.

8 e a r in g P 1~a t e . Collar

Plug

FiQ. 3 Compressive

Forces Applied By Post- tensioning .

11

12

.A. DISCUSSION OF METHODS

FREYSSINET ·METHOD: The Freyssinet method uses a. number

of cables eomoosed o£ from 8 to 18 wires eRch, the wires be-- '

ing separated and spaced by a center core. Bond is pre-

vented between the cable and the concrete either by covering

the cable with a sheath before the concrete is placed or by

pulling the cable through holes formed in the beam by hollow

ducts. The wires of the cable are. locked in place at each

end of the beam by a wire-gripping device which consists of

a cast concrete conical plug and a ca.at concre ·te cylinder

with a tapered hole through its length. Both units are spi·•

rally bound by wire :for added strength. 'I'he cylinder is

cast into the end of the·beam at the time of pouring.

Arter the concrete has hardened sufficiently# all the

wires are gripped around the circumrerence or a unique dou-

ble-actlng hydraulic jack and stressed at about 125,000 psi.

Then a center ram on the jack is activated, and the conical

plug is driven securely into the tapered hole, sec~ing all

-th~ . wires · Wlder load. Mortar is thel'). pumped into the cable

hole to protect the wires aLainst corrosion as well as to

provide -extra strength through bond.

The features of this method are that all wires in ·any

one cable are tensioned at the sam~ t~e# thus reducLng

costs; .that the anchorages do not project at the beam ends;

.. and that the atreBs on the beam is uniform. However, there

1s a disadvantage in s ·tressing all the wires at once because

it is then ~poss1Db to determine whether or not they all

13

have equal stress. . If there is slippage of one wire in the

wedge, the entire job must te redone.

l.f.AGNEL MElliOD: In the Magnel method, which is also a

post-tensioning method, the wires are arranged in a series or

vertical and horizontal rows to form a cable. There may b~

as many as 8 wires per layer and 64 wires in all. '!he grip­

ping device at the end consists of steel plates called sand­

wich plates. These plates are a~?ut 1 l/2-in. x 3 1/2-in. x

6-in., and each bas four wedge-shaped slots. ·Two wires are

placed in each slot, and, when they are stressed the required

amount, a tapered wedge is driven between them by hand. 'r.his

process is repeated for all the wires in the cable, the

plates· being stacked one .. on top of another. These sandwich

plates are later covered over with a cast-in-place extension

to the beam. Grout is . pum.ped around the wires for pr·otec­

tion and to provide bond.

Since only tv.'o wires are tensioned at a time in the

Mae;nel method, there is obviously better control over their

stresses; the required jacks may be simpler and smaller; and,

because a large number of wires are used in each cable, the

total compressive force is very large and quite .suitable for

long span beams. Obvious drawbacks to thi:;J_method are found

in the expense or machining so ·large a nun1ber of' blocks as

are required and in the extra time involved in stressing on­

.ly two wires at a time.

The Magnel method was used· on -Philadelphia's Walnut

Lane Bridge, the first . large prestressed concrete bridge

..

14

in America.

· SHORER SYS1"EM: The Shorer system, based on the bonding

action of the wires, has as its central feature a high

strength steel tube which is used as the .bearing .area for

jacking. •rne prestressing wires are wound about the tube in . .

a helical ~ashion with a very wide pitch, half in one direc-

tion and hal.f in the other. 'They are kept away .from the

tube by spac~r discs along its length. A patented device· a.t

the ends of the tube secures . the wires and enables the jack

to bear against the tube, thus compressing it and at the

same time stretching the wires. When the wires are stretched,

they are .fastened in place, and the jack is removed. The

entire·· unit, wires and tube, can then be set in the form in

the same manner as is standard reinforcement, except that

the end of the tube ·must be accessible when the form ·is re-

moved. In this manner, the steel tube takes the entire com-·

pressive ·load ·. Af'ter the concrete has hardened, the locking

device on the end is removed, and the tube, which has had a

bond-breaker applied to it, is also removed. The wires, be­

ing £ully bonded to the concrete, transfer t~e entire com-,

pressive stress to the concrete. 'lhe hble is then grouted.

BILLNER ME '.rHOD: ·fue unique feature o~. the Billner me­

thod is that, although it is a post-tensioning process, no

end-anChorages are needed.(3) Billner employs high~strength

(3) Billner, K. P., New Prestressing Method Utilizes . Vacuum

Process, Journal of the American Concrete Institute,

Vol~ 22, No. 2, 1950.

' 15

wires of comparatively large diameter which are anchored 1n

the concre·te by means o:f large loops formed on the ends.

All of the wires except these loops are coated with some ma­

terial to prevent bond with the concrete. Before the con­

cr>ete is cast, a thin sheet metal partition is placed at the

mid-span so that after the concrete has been cast and -hae

hardened, the beam will- be in two halves tied together only

by the wires. 'fuese two section~ . are then jacked apart a

predetermined· distance, the separation plate removed, and

the space filled with a high-strength grout. When the _ grout

bas hardened, the jacks are released, and the beam is pre­

stressed by the ef'for-cs o:f the wires to regain their orig­

inal length.

1lhis process has distinc·t advantages in that all of' the

wires are ftressed · as· one, and ·wires 3/8-in. in diameter may

be used~ thus reducing the number of wires required. Also;

the expense ot manu.factur ing and ins te.lling end•anehorages

is eliminated.

ELE.CTRIC MEI'HOD: This tinique method was developed by

K. P. Billner and R. w. Carlson.< 4 > Steel bars coated with

(4) Billner, K. P. and Carlson, R. w., Elsetric Prestressing

o~ Reinforcing Steel, J~~nal of the, American Concrete

Institute. : Vol. 14, No .• 6, 1943.

. . )

a tberim~la.st:k material and having· threaded ends are- placed .

Ln the can~r&te in the . s~e manner as in ordinary re1nforc-

.ing, e~cept that the. -ends- are _"left protrUding. ·After the

concr~te ·bas .harden_ed 1· _a ··· low - voltage 1 high amperage c~ent .

is ·passed through the · bars, -eahsing them .<to ._-hea.t . up. -· Tb.a -· . ; . '*~;:·; tbermopias·tic' material 'melts .upotf::~eating a~d de~:tr_oys any

. . . . .

bond• thus all_owing the_. 1;>a~s to elo:Q.ga~e. Nuts are then.

tigh.tened · aga'inst · t~e ends of the beam, and the current _is

removed. , co~pressive · s -tresses are . ~et up in- the· concrete . by

the contractionoof' the steel upon cooling. '_ This method . .

proyed very ffuccessful in making slabs only two ·inches · thick ·

to be . used as , wa:lls f'9r hous.es. · r

ROEBLING-.ME1'HOD: Altbough· the method developed by John . ..

A· Roebling •s. _Sons Company is not eritj.rely . different f'roin • . .

the Freyssinet or 'Magnel .. methods, it does have a number of.' ..

disting~ishing features whi.ch make it a very sintple and

highly e~fectlve .-·inethod of· pre·s tressing concrete •

1b.e f'irst such f'eature of this metho.d is - the· Us.e. of

pre-s tre tch_ed . .- galvanized . bridge sj:;rand which . has been 'thol'-

oughly develope~ py R~bling. By using this cable wh.ich is . ... ....

mhde up _ o~ a n~ber of' h~gh strength wi~es ~refabr'i ·cated in- ,

. to cable :f'orm~ a large area o.f steel may be se·cured .with one ... '

end-ancho~a.ge ,_ thus .gre.at~y reducing -· the · time . and · labor in-· :. ·'

. '

· .vo~ved iri ... tha~ stev, in sam~ instance• .. e~a~~~ng~ on.ly .. one . . ·c·.a-

. ble t9 d.~ what Ot<h6rwisa liiight; reqti.ir& s~x :oi' 8iiA1/,i~~~e:te " '>

. : .wire_a ... ~ - .

· ..

To _accomp_any these cables .R-oe-bling -u,sea a special . . ' · I,

' :'swaged :f~t~ing s'imilar· to that used . ~n: · ·aircraft · ·c~n.t:roll -~$• · . . . .

bles. ·_ This fitting · ~s · .. ~ sl'1m< cylindr._ieal· terminal-rrom · . J

17

1 1/2-in. to 2 3/4-in. in diameter in one end of which the

cable is fastened by pouring molten zinc around the wires,

much like standard zinc sockets. A threaded. stud is screwed

into the other end, and a · special nut on this thread bears,

against the concrete through a steel bearing plate to hold

the preatressipgiforea.

·This is, of course,a post-tensioning process, and by

the very nature of the anchoraging system any loss of the

prestressing .force over a period of time may be corrected

.simply by res tretching the cable and taking up on the. nut.

A standard poured z~c socket about five inches in dia­

meter is used rar heavier cables, and the development o.f a

~draulic jack wi Gh a hole ·cnrough the center o.f the ram ha·S

greatly simplified the stretching o.f these cables.

18

A DISCUSSION OF :MATERIALS

REINFORCEbmNT: Of . first concern in the matter of pre­

stres_sing is the nature of' the material that is to be used ·

f'or reinf'orcem.ent. Since the primary object of prestressing

is the induction of prel~inary internal stresses in a con-

crete member, it becomes essential to use a steel of conaid-

erably high. strength so that losses of stress in the steel

due to elastic and plastic deformations and shrinkage will

be only a small part of the initial stress, thereby leaving

ample stress to· be transmitted to the concrete.

There are numerous varieties of high. strength steel on· ·

the market today, ranging from ·the fine piano wire which is ··

in common usage in Europe to steel bars 1 l/8-in. in diame-

ter which are used by one concern in England. In -between

these two are wires of varying sizes which are either

stretched singly or coiled into cables and stretched several I

at a time. i'he Ro~bling bridge strand cable is a notable

example of the latter. · It should be noted that wires in the

sl~~s of 0.010-in. to· 0.100-in. are used almost exclusively

in bonded prestressed concrete because it,is difficult to

provide end-anchorages f'or wire of this small size when it

is used in post-tensioned prestressed concrete.

Typical of the wire being used in post-tensioned pre­

stressed concrete is the 0.276-in. diameter cold drawn wire

.;manufactured by the John A. Roebling's Sons Co. 1bis wire,

which would have a very small cross-section for use in ordi-

nary reinforced concrete design, is used both in single

strand f'orm and in coiled cables containing from 3 to 5

wires each. When this wire is·used singly~ the matter of

end-anchorage becomes rather dif£icult~ but when made into

19

cables 1 the use oi' a Roebling swaged terminal aff'ords a very

convenient method of anchorLng the wires. One drawback . of . .

this wire when useu singly is the very large coil curvature

that is ~parted to it. ihis wire is usually delivered in

six-foot diameter coils which have a .free coil curvature of

over twelve feet. 1rh1a makes the wire somewhat unmanageable

for placing in forms.

The effect of creep on the wire used in prestressing is

also of importance. Studies have show.n(5) that there is a

( 5) Gcxifrey 1 H. · J. 1 Steel Wire for· Prestressed Concrete •

Froceedings of the .First United States Conference · on

:Prestressed Concrete, 1951, p. 151.

definite relationship between the creep of cold drawn wire

and the elas ·tic properties o:r the material. lbese studies · . ,

indicate 6 however 6 that the amount of creep is negligible if

the s ·tress is at_ ·or below, the proportional limit of the

wire.

Since the elastic properties of prestressing wire are

of utmost ~portance, it is necessary to specify and control

the stress-strain characterist~··· ; of the wire. ~is charac-.. te~istic may be determined by means . of the modulus me.thod,

in Which a minLmum modu~us of elasticity is req~ired at a

20

specified stress~ or . by means of a permanent strain or or~-

s~t method. Roebling Co. uses what is known as the elonga­

tion method which requires a min~um stress at 0.7 per ce~t

elongation. ;£his math~ el~nates the drawing of stress­

strain curves~ and ma.teri.al meeting this requirement will

have the ne:cessary elastic properties.

In most specifications.~or prestressing wire some ac­

count will be taken of elastic deformation of the wire and

also_ o:r creep ·· and shrinkage. :Ih1s is generally done by

speci.fying an initial and a f'inal pres tress. l'hese f'orces

usually dif"f'er by .from 15~000 psi. to 35~000 psi., the ini-

tial prestress i'orce generally being taken as about 50 to

60 per ·cent. of the ultim~.te strength of" the wire. It has

been .found (S) that under average conditions about 25 per

(6) Dobe~l~ c.~ Prestressed Concrete 1£8nks~ Proceedings of ·

the First United States Conference on Prestressed Con-

crete~ 1951, p. 10.

cent or the above losses occurs within 20 days~ 50 per cent

._within 60 _days~ and 75 per cent within 130 days after pre­

stressing. After that the rate of loss appr.oaches a hori­

zontal straight line condition. For this reason it seems

advisable to employ son~ means of end-anChorage which would

allow a restressing of the wires to compensate for a part of '• this loss.

..

21

CONCHEi~: 'I'he .strength of' concrete specified for pre­

stressed construction is generally higher than that for con-,-

ventional reinforced concrete. Concrete spe-cified :for pre-;.

stressed construction will usually have a minimum 28-day cy-

linder strength o:f 5#000 . psi.# which is probably the top val-

ue for conventional concrete.

There are many factors which contblne to af:tect the

strength or concrete. Some of these are aggregate size and

qua~ity# fineness at• cement# composition of cement# size_ of

the sec ·ci.on# water-cement ratios, admixtures# and curing.

~nile all of these are ~portant, there are some over which

the concrete maker . has little control; however# others such

as the·· water-cement ratio and the method and time of · curing,

both of wnich may be greatly ·varied, can be determined and

controlled by the concrete maker to produce a cer ·tain·

strength concrete.. Ihe exact relationship of these factors

has been ·set f'orth in other literature. (7)

(7) Design and Can trol o:r · Concrete Hixes, Por ·tland Cement

.1'\.asociation.

In general terms, it may be said that . ~he lower the

water-cement ratio, the stronger the concrete. For lYPe I

Por~land cement the 28-aay u1t1mate compressive strength fora

6 gal. per sack water-cenent ratio is about 5500 psi. whereas

for a 4 gal. per sack water--cement ratio the strength is

22

about 7500 psi.(B) .Another way of looking at this same

( 8 ) Ib ia • ~ p • 7 •

.. thing is that with the lowfjr water-cement ratio a given

<

strength. will be reached in a shorter period of time~ which

is very important in prestressed cons"i:~ruction.

An accompanying i'eature of this low water-cement ratio

is a marked decrease in the slump o:f the concrete. Most

conventional concre~~ will have a slump of £rom three to

£ive inches~ whereas the average concrete used for prestr~s~ed

construction will have a slump of i'rom one to two inches. In.

nearly--all in~tances thi.f? low slump will make;, 'thts pla.cing in forms 1 particularly of sections with thin weos and closely

spaced reinfarcement 1 extrem~ly difficult. On the Walnut

Lane Bridge in Philadelphia 1 (9) specifications called i'or

(9) Baxter~ S. S. 1 Construction o£ the Walnut Lane ~ridge 1

Proceedings o1· tb.~ First United States Confer~nce on

Prestre~sed Concrete~ 1951, P• 47.

5400 psi. concrete with a max:Lmum two inch .elump. ihi.s slump .

was maintainea until requests t·rom the 1'1el<1 resulted :l.n in­

creasing the Qlump to three and on~-half inches in order that

the concrete could be more easily placed. The strength of .. the cancrete 1 which had been runnin-g over 7000 pa1. 6 was

materially reduced but $till mainta1nea above 5400 psi.

Vibration may be used very advantageously in placing

concr~tt1 ror prtjs ·trt:ssea work~. It is a most effective

means of' reducing the water content because -of the sti:ff'er

consistencies ~nd harsher mixtures which may be placed by

'this method. In order to be fully effective, however, vi­

bration should be or ·-a high frequency, in the neighborhood

of 6000 v.p.m. Either external or internal vibrators or a

combination o.f the two may be •us~.d.

23

Still another impoltant .factor in the quality of' con­

crete is the effect of bleeding~ which may be defined as the

forcing upward o:f' mixing_ water by the gravitational settle­

ment of' the heavier constituents of fresh concrete. Iests

have shown(lO) that as a result of bleeding there is· a

(10) Kennedy, H. L.~ High Strength Concrete, Proceedings of

the First United States Con£erence on Prestressed Con- ­

crete, 1951, p. 128.

marked dec~ease in the compressive strength of concrete from

the bottom to the top of a relatively deep section. See

Fig. 4. · Because _ of' this reduction o.f strength, the ability

of ccncrete to resist the applied stress in. a conventionally

rein.forced COncrete beam is :frequently Q]DlOBtL ifiVersely pro-

portional to the s ·cresses existing· in ~ccordance with the

theory o:£ planar distribution of stresses, as· sJ:town in

Fig. 5. In other words, due to bleeding the concrete is

weakest where it should be· strongest. It might be mentioned

that advantage may pe. taken of ·this . :fact ·by casting beUUJ o:r

slabs upside down.

P.he age and~ correspondingly~ the strength of the con-

crete at which the prestressing t· ·orce is applied . is of con­

·siderable importance. Some specif'ications allow a somewhat

h1gher ·compressive strength at the transfer ~f' prestress

than at the applica·tion of design loads. Average figure~

f·or these are from 2/3 f~ t ·o 0.8 :f~ at transfer o:f prestress

and. from 0.3 f~ to Oo4 r6 at application of. load~ :f~ being

the 28-day cylinder strength.

One reason for allowing a higher compressive stress

when prestress is f'irst transferred to· the concrete is that

the initial concrete stresses induced at that t~e decrease

considerably as ·the - steel prestress decreasea~ owing to the . ..

efrect of' creep ·and· shrinkage. . .

In conventional reinforced concrete it is customary to

disregard all tensile ~tresses in the concrete, but such an

attitude is not justified in ·the design of·· prestressed con­

crete. One major purpose ot·· prestressing concrete is to

prevent cracking by making the concrete per£orm homogeneous­

ly. '.rhe steel is designed to produce this ef·f'ect. Actually

reduced.· · The once-cracked section performs as i.f it· had

never been cracked, and the prestress is as eff'ective as

ever.

When. prestress · is first transferred to the CO:t:J.crete,

ccmpr~ssion is induced i.~ ·th~·-'.:· bottom fibers. Some designs ·,.A· .

2.~

will requi-re . :the stress· in the top :fibers to .be zero under

this initial loading 1 but t4ere is no har~ in allowing ~ome

tensile s tre.sses as long as they: are kept below . the va.l:.ue . .. . .. .

tha_t causes cracking 1 which. is generally about O.l5· f~~ Al­

lowing tensile -stresses in the top fibers at trans£er of

prestress will generally result in better proportioned sec-.·

tions. As the allowable tensile stress ·is increased at the ·

top, .t~e. centroid· of th~. prestre-ssing wire can be ·moved

. closer ·· to the ~bottom1 t_hus ·becoming more effective in indue-:

ing compressive :·atr·ess.es in ~iihe bottom .fibers.

The term . ":fully prestresse~" re_.fers to prestressing in

whic.h. no _ .. tensf~n· i~ allowed ~n the bottom . :fibers under d_e-

sign load. ·Structures in whl.ch some tension is allowed may . , .. ..... .

be . c··a.l~~d 11 partial~y prestre·s .sed 11 ~ Where safety against . ~ - ~

cracking is o:r greatest impcr tance 1 a bigh degree of pr_e-

.stress s·hould be_ provided. On the otb:er hand~ in structures ·

in·which· cracks at· or near working loads e.r.e not, objectio~-/

able, t ·he desired. cha.·re.cteris.tics can be obtainedl .. · by apply-. . ~ .

ing ·a amall:er· pre.stress force. · V.~er~ .. ·~pp·lic.ab~e:~. · .partial · • ...J

prestressing may result in c.onsi.derable sa'Vings ·· ~n prestres~ . . -~. .

sing labor and· .fittings as wel:-l as. in concrete. ·; The · subject·

of par~i.al prestress~g has been d.iscussed in deta.il .· by

26

- ( ll) P. W. Aoe le s •

( 11) Abeles~ P. w. ~ l.:h.e Principles of' Pres tressed Concrete~

N.Y.# Ungar Publishing Co., 1949.

. c:

• • 0 CD

3420 psi

4050 psi

• ··~~*-----------------------~ ~ .a 4 4280 psi

.c 8 l!il!!!!.!o~!!l-----------------~~ 0

•• %

0

Fig. 4

~4TO psi

1000 2000 3000 4000 eooo

Com·pressiNe Strength - lb. per sq. ln.

E f f e c t of.. b I e e d in g o n uniformity

of concrete st-rength.

ct:=======:f=' ---l•• T Distribution of Stress

on Concrete Section

Ability of Concrete to

Resist Applied Stresses

27

28

FORM: Since the beam itself was to be 6-in. x 8-in. x

10-f't. 6-in.~ it was necessary to make the sides and the

bottom o:r the f'orm rro1n two thicknesses of' 3/8-in. plywood

securely neiled together -since 3/4-in. plywood was not

available in the required lengths. Exterior grade plywood

was used throU£hout. ·Xbe bottom and ends were _grooved to

receive the sides, and bracing was used at the quarter

points to prevent bulging of the sides. Both ends of the

fona were cut out to f'it around the f'ive anchoring collars

w~ich held the wires in place. A piece of' 1/2-in. hal£­

round was nailed to the bottom at one end to f'orm a keyway

in the bottom of' the beam which prevented lateral movement

oi the beam while beLng tested~

\ .

Steel bearing plates 4 1/2-in. x 6-~. x 5/8-in. were

f'as tened to each end of' the t·orm by 5/16-in. bolts through

the f'orm. Each plate was drilled to · allow passage of the

f'ive wires. Since these plates were to remain in tne fin­

ished beam, two 3/8-in. x 4-in. carriage bolts wer~ screwed

into threaded ·holes in the upper eorners o£ each plate and

allowed to extend into the concrete to provide anchorage.

The entire inside o£ the f'orm was painted with several coats

of motor oil to prevent bond or the concrete. See Figs. 6-9.

CONCRETE: l'o. secure the full advanta.ses of' prest11essed

·.concrete requires considerably higher concrete strength than

is customary in ordinary reinforced concrete. TO secure

higher strength copcrete the water-cement ratio was reduced

29

to 4 gal per sack. In order to reduce the curing time to a

minimum, high-early strength cement was used. The mix was

as £ollows, with all materials conforming to A.S.T.M. spec~­

:fications:

Water - (4 gal per sack of cement)

Cement - Type III High-Early

Sand - All passing #4 sieve, F .M.-2.30

Gravel - Crushed lilnestone,3/4 1imax. size

1 Cu. Ft. Yield

11.9 lbs.

33.7 lbs.

4.6.3 lbs.

59.0 lbs.

This mix resulted in a very stiff, dry ·concrete which

had a slump o:f from 1 1/2-in. to 2-in. and was rather di:f:fi-

cult to place. · It was necessary to f"orce the concrete under . .

and around the wires with the fingers as the spacing between ..

the wires was too small to allow adequate rodding. Above the

level of the wires considerable redding was necessary in or-

dar to ·obtain good :fillinc of' the form.

The concrete was mix8d in a small portable electric

mixer of two and one-hal:f cubic feet capacity. Since the ..

total volume of· the concrete required, including eiGht

s tanda.rd 6-in·. x 12-in. cylinders, was ~ive and one-half' cu-. bic :feet, it was necessary to make the concrete in three

batches: two of two cubic feet each and one of one and one-

half cubic feet. Care was taken to keep the batches as

nearly the same as possible, and test cylinders were taken

from each batch.

CURING: The beam was moist cured by placing about

1-in. of sand over the top and sprinkling with water twice a

day. •£he sand was. covered with used cement sacks to help

'•

30

retain th® moisture.. The teat cyli.nders were stored in a

moist closet. Three were tested. at the t·ime o:f a.ppl1ca tion

of the pres tre-ss, and !'our at the time o:r testing of the

beam.

REINFORCE~~NT: Th~ reinforcing wire used in this pro­

ject was furnished by the John A. Roebling's Sons Company or Trenton, New Jersey. Data furnished on this special acid

steel prestressed concrete wi--re is as :follows:

Dia. -- __ .;.. __ --·---- -----------------------0. 276-in·. ·ultimate Strength------------------------240,000 psi. Min. Value. at 0.7% Elongatio·n------------180,000 psi. l41n. Ult. Elongation in lO" Gage Length--4% Design Stress-------------""'!- -------------120,000 psi. Tensioning Stres~------------------------135,000 psi.

An average s·tress ?.train diagram is shown in Fig. 10. ·

Five wires were ·used in the beam, three straight and two

parabolic. A light w~re £raffia was used to keep the two

parabolic wires on the same level as the straight wires at

the mid-section. Each wire was left extending two feet be­

yond one end o.f. the beam to provide for tensioning at a

later date. Bond between th·e wires and the concrete was .. .

prevented by sheathing each wire in 7/16-in. electrical '·

looVt• (Figs. 15 and 16) 'Ibis me tho<l proved very sa tiafac-

tory and much more convenient than painting each wire with

a.ephalt·._or·wrapp1ng ;with waterproof .paper. The. 1nside'd·1am­

eter of the loom was just slightly larger than the diameter

of the wire. The loom was cut and the wire exposed f:or a

length ot, 2 3/4-in •. at each stra:Ln. gage loeat1_on. ibe a.r-

. rangement and numbering of the wires is indic.ated· in·

31 .

Figs. 11 and 12. Due to the. large radius coil curvature or the wires, it· was necessary to apply a· slight initial ten-

. .

·sion. to each wire to straighten it and ·to hold it i.n the

proper place during placing of the concrete •.

END-ANCHORAGE: '!he. model o.f the end-anchorage used on

this beam was furnished by Mr. K. p. B1.llner, President o£

Vacuum Con ere te, Inc. of' Phi lade lph1a. '!he units u~ed were

machined by .Prof. A. v. Ki-lpatri~k o:f the Mechanical Engi­

nee~ing _ Department o:f the Missouri School of' Mines and

Metallurgy, who suggested and incorporated several changes

to increase ·,the gripping. power of the p·lugs and to adapt

them to the s ize wire used.

Each unit . consistec1_ ot: a steel collar 1 ;.;j4-in. squa~e

$lld 3/4~in. thick with a ~pared hole in the cen tel" and a

steel plug l-in. long, · turned to the same taper as the hole

and drilled ~o slip · over the wire. Both collar and plug

-were machined .from cold-rolled ~teal. r.L'he . .four collars for

the parabolic wires were milled to a slant on the b,~ck side

so that the &rxis of the hole · would be pare.lle l to. the axis

of the wire as it emerged .from the bearing plate. · Tile ·plug

was tspped . with a 5/16~1n. x 2•-NF thread ·Cutter to PX"Ovid$

a toothed gr:l.pp:l.ng surfa.ce adjacent to the .wire. Eaeh p·lug

was ~lso alit into q:uar:ter segments in - the mann6r of a CO'l­

.let in order to help ·increa·se the gr:Lpplng pow~p. All plugs

were case hardened and< carburized. by Mr. Gene Langston of .. . .

the Jtetal~urgy Department of the :Missouri SchoQl ·. 9£ ~ines

and MetAllurg-y. lhi.s uni~ is dimensioned in Fig. i3 and,

pictured in : Fig. 14-o ·

SR-4 STRAIN GAGES: A t.o.tal of' twenty-two ·type A-7 SR-4

str~in gages ~as used in the beam: four on the- center ·_wir~,

three on· each of' the other wires~ .~d thre~. on both ~he top

and bpttom of the beam at ·the centerline. Fig. 11 shows the

arrangement and -numbering of' . th~se gages~ Two of' the test

c~linders were also inscrumented by placing two gag~~ on the

sides of e~ch cylinder~ spacing ~he gages ' 180°. apart.

The·ory: _Essentially an SR-4 strairi .gage consists

of' a leng"t?h o:f ·very f'ine special alloy wire in . the n 'eighbor­

hood of' 0.001-in. in diameter arranged to f'orm a grid pat-

tern and bonded to a paper or bakelite base. In use~ the

gage is cemented 'to the .member to be te·s ted and is ·tl}us

strai.ned unif'ormly with the test m~mber in either tension or

compression. As the m_eniber is strained; the cr~>ss-sectional

area of' the gage -wires .change. ~ thus causing ·a change in the

resistance of' the wires.· · This change in resistance may_ be

me_asured wi.th . a_ sensitive · wheatstone ·bridge or by s.pecial .· . instruments such as the Baldwin-Southwark Model K Strain . In-

dicator which is calibrated directly in micro . inches pe_r .

inch and thus gives the· c.orrect ·value o:f the strain when the

bridge is balanced. Strains due to tempe~ature - cha~ges may ·

. be ·e.l~inated by introducing as. a :fixed x-esistance in· one :arm

of' the bridg~ a s~milar type gage known as a c~mpensating·

gage which i .s cemented to the same· type mater.ial and kept

under the flame conditi.ons as the a.c ti ve gage o

Application:-. fue method o:f applying these .- gage.s

33

was substantially the ·same as outlined by J. H. Senne.< 12 )

(12) Senne, J. H·., Investigation of Stress and Crack Distr'-­

but1on in Concrete Slabs .. · Containing Welded Wire Rein-

forcement, Missouri . School of Mines and li~e .tallurgy,

1951. pp. 13-15.

After fir~t sanding the wires to .remove all rust and Sf;ale

and··;;wiping with·;. ajL··swal:i soaked in acetone until no more dirt

came off', the SR-4 strain gages were cemented to the cleaned

areas, using a heavy coat of Duco cement. Each gage was

clamped f'or three hours, using a clamp (Fig. 15) ·devised by .

Mr. Senne, af't.er which time .the clamp was removed and the

cement allowed 't6 air dry for a period or three days. All .

gages were then coated ·with molten cerese wax as the first

step in waterproofing.

waterproofing o.f the gages when placed in concrete is

extremely impor~ant since any intrusion of water to .~the

gages themselves will destroy their .usefu.lness by giving

f'alse indications of resistance on the s ·crain indicator •

. Not only is the exclusion of moisture important; it ·is

equally important that a11· moisture·. a.nd cement solve~t · be

e .liminated .from the gages before wate·rprooi'ing. · _For this

reason, it · is essential that the gages be allowed to dry

thoroughly and to age be.fore waterproo.fing is begun. Heat .. may be· used to hasten this process.·

I . .

After the wax was -'applied, short neoprene-covered ex-

34

tarnal lead wires were soldered to the gage lead wires.

These 1ead wires were later brought out o~ the concrete and

soldered to longer leads which were connected to the bridg~

balancer. Next, the entire · gage area was given two coats of

liquid rll:bber cement, · care being taken that all exposed wire

was covered. Since there··.was to be a. substantial elongation

of: each wire under the prestressing load, a 3/4-in. sponge

_rubber pad 1/4-in. thick was wrapped around the wire just in

·front of each gage., serving as e. cushion to prevent strip­

ping-o~f of the gage. Also, the lead wires were doubled

back on themselves so that they would have some slack to al~

low for movem~nt. Following this, the gages were given

three rairly thick coat~~s of 3M Special Weather-Strip Ad­

hesive. This rubber adhesive covered the entire gage area

and extended ove·r the electrical loom .for a short dis.tance

on both s-ides. F-inally, the entire area was covered with

electrician's rubber tape. One gage was cemented to a s ·hort

length of wire and waj;erproofed by the above proces~ •. 'I'his

gage and wire were placed in a 6-in. x 12-in. cylinder at

the time of casting the concrete and served as a temperature

compensating gage during the testing.

The procedure £or applying the gages to ·the concrete '·'

waa a_ little different. After · the concrete was cured and.

removed f'rom the rorm., the surface~ where the · gages were· to

be applied were ground smooth with a grinding wheel and then '•

brushed with a· wire brush • . ~ex.t, · p~eces of o •. OBO-in~ ·thick

celluloid were cemented to these prepared s·urraces with a .

35

heavy coat o~ Duco cement. 1~is was allowed to dry ror tw~

days, arter which the gages were cemented · to the celluloid

and allowed to dry t·or two more days. The 6ages were the~.

given a coat o~ wax, and the entire areas were painted with

lacqu~r to prevent moisture f·rom entering. Finally, the

lead wires were solde~ed on. ihls same procedure was ~ol­

lowed in applying gates to the test cylinders.(See Fig. 17)

Fig. 6 General view or form, showing bracipg and m~­thod of bringing , out strain gage lead wires. Prestfess·ing wires. are under slight tension.

36

Fig. 7 Interior of· form sho\vinL bec..ring ple. te, ~n­choring bolts, loom-covered wires and, in lower left, SR-4 strain ge.g,e wrapped 1-vi th r_ubber tape. ·

37

:.~-:.ig. 8 Form end before cas tinf_ con ere t.e 1 3ilow:i.nt end-anchorac:.es bearint: agalnst bearing plste.

39

F~g. ·g View o.f beam ehd-1 . showing bear:Lng plate, end-_ anchorages, and keyway.

40

.,

--·--·- -·---·-··,- .. . -~l . , - I .l i ~ . I .. I . 1,. ~ --_ ~· _·: . . ·_: -- 1. . . - i .. . . --

... ~-- ~ . ~.-.· l . . ··I -

. . . --- ! . .. : - --r- - 1 - -. : -~ ~ · -. _ . - f . - -t----~ ~-·-__ .- .. -- .. . .. -~--~~ .. ---.. - · · _.....__~ ___ _:.__ --·-·---'-- ----· ---- ---- ---- - -----

1 ~ ~ ·- --- i I - .

... _ .. ! I L .. i J_ - !

i . 1 ·.:·-.. ... ... -·-·--·- -r----- . 1

.. . :-- - _ ·-· ~ .. --i·-·· I l

i~- . i

i L-~~-~ - -2-0

(

1------ --16&-

c:r .

l . 1- -~ •·.

. --~:~~-¥-~ •. -... --.. -~- - - -L I - ! ~ ~ --- . ·I

. ! . . . ·-- t· -. ~ I

! .

.i

. !· .. i ' ... ! --- 1 -----~ - -+-:::-:---+1_·---

; --:- -! . :1 . . L .. . . l .. , i

- ! --_ ! · ! I · • -~ · ! ' ~ ·· •' t · . ---~----.. -·-·-----__ .. _;_- ·-- -·--·--·-;~- - ~----·--·-t,. :_ ____ ..

' :;

l

- ---- - ------4--.---~ . ·----- ---i--··-- -i i

! I . j

l-1 -! ~ ..

• ' j

~-~--+Y-1R-+- · ------ __]__ ~--- --~~-· · · ·_t - ·- - --··. ___ J _________ t .. ~--- ·~-~- l _____ _ ' a. l, · l t· I I I i I ·- ! . -I .·· .. I 1 . 1.. .. . !.. ; : -~ I- I I I

1

- . ~ ~ , .r·. 1

. ! i .

,_ ' I I ·I . - -l-- . . -

I. I . .

.. ----~-~-- -- --~· -- t ' +---------~-· .... ·---:----. . i .

i

I I )

I . I

l I

' - - ·- ---- • . ....j.- · ··--· .. - .. . ----- -.-+-.: --· ------- - ~ ----- --:-----'----:-~ : .. :

. I

i ' . . -

... , I . . . q-·-· ' - I . -

-- - -- ~- ----~-- : ------ ---1 I .

. -~ . . ~ - ··

I l 1 - . t-­

. - I

:. ! ··· .• .-:: ~~ : - . -. I . - . . . : Fig. 10 1 I .

----=- --- ----------· . - :.....-~-- ·~- -- -- - ------~~---- - -~- -- -- --; _ __:_. _____ ·· -··· _:_ .. (; ___ :_ ___ : __ --·~ ·-1~ ... ··---- -----~---· --·---· -- .;.... - ·.- · -·------+------. ~ . _ ! i . !stress- Str1in c t.it~e t~r Roebling·~ ·•

.. . · - ·!' . . r:276-:iri. Pfestreslil ng rire. i ·I j -- . I - .

-~....:...· -~-1- · __ ; -- ----------·---1 .. .. _. illt.imatL.S1refl.g .. thi=.24.3,CJ.OQ-~i.-· ---· ·-- -- -------·~ .- .: ___ ._ ----~- -- · -- -··--- · j.. .. .. . .. .. ' . \ ' . • .• ' .

d: ~ ! M~lis of !Elastiqity-31xiO ~si. . i . . ; .. -. ·I ! . I . . . -·. ~ - ·l '· .. ~ . t

.! . I - - - ! ! - ~-

; . ! I ------ --- - -·-· · .. ··~.._ _____ ....;. ______ +-___________ ,._ ____ _,_ __ .;-.__..,_ ____ ......, ___________ ~----

~ 2000 40bo &OIOO eopo 1oooo 1,! 12?00 14 ·OO 1s()oo 1a9oo

1'. • StrOin- MiCra iin. per in. ~ · ~ · ! : ;

J .

'· i i

. I

I

--AI -

Fig. II

Fig. 12

I -E-1 ' E-2 --- . -

- D-1 ·--·...........-- - 0·2

-- C-1 =--:1- C-2 ! C-3 .-- - -B-1 - B-2 - B-3

-. . ~ -· - --A2 -A 3

Location ot SR-4 Strain Gages on Prestressing Wires

Wires A, C a E Straight; WirPs 8 aD Parabolic.

Location of Wires

a_t End of Beam. a"

-

~ ~

..

? ~

-~-

B D

@) @::: 7'"'

~ r--

A c E

@ @ @·

~·-s"·-1

Concrete Beam

Bearing Plate

Collar Plug Wire

·------ 0-3 C-4

E-3

. Jackmg End

Wire

E 0 c 8 ·A

42

3.. ~· 'r ·5.. , - -- 1- ---------~~ - - a _ _,; I " 4 ~--l ____ \ 8

~" 8

•

I i I T ------\ T 3" 5" l !1." I~ 8 1 32

1 ------_I __L

/\ Toper for Parabolic Wires

Collar- Cold Rolled Steel

r-1"-· ·~ -- . --.- .. ---~,- -

~-~--------- I , 2 I" I -32

____ ____. _j ___ ,_~-~ I~ II X 24 N F

II I" iri6 Saw Cut

Plug- Case Hardned Steel

I II

Scale; 12

=I"

43

44

-Fig·. 14

Collar .and plug used a:s end~aneho:rage o

Fig. 15 -SR-4 Strain Gage being cemented to wire. under pressure of spa:c.ial cla.mpo Note ga~e lead lllires o

45

Fig. 16 Extern~l lead wires soldered to strain gage:leads and one coat of rubber cement applied. Note electrical loom covering pre­stressing wire.

46

47 -

Fig. 17 SR-4 Strain Gages and ~es Dial on top of beam.: . '!he Ames Dial is at mid-span.

48

TEST · CYLINDERS

· Three.of the standard 6-in. by 12-in. test cylinders

were subJected to compressive testing a~ the time o:f appli.­

cation o:f pr.estress, which corresponded to a concrete age o:f

seventeen days. ·It was noted., when the :forms were· removed.,

·that four of· the cylinders. showed marked honeyco~bing ·, indi­

cating insuf·f'icie.nt rodding at the time_. o.f pouring. The

other three cylinders were very sound and smooth. The cy~

linders were all capped on both ends with plaster of' paris

in the recommended manner. All cylinders were tested on a

200~000-lb~ Tinius-Olsen Universal Tes·ting Machine.

The first cylinder tested.. proved entirely unsa tis:fac-· ·

tory. Apparently the cylinder was inaccura ·tely placed in

the machine, or the load was applied too fast because the

cylinder broke be:fore any load readings could be taken.

Cylinder number two was one of' the two which were in-

strumented with SR-4 strain gages. Extreme care was taken

i~ centering this cylinder, and the load was applied at the

· slowest speed. lfue movement o.f the loading head was stopped

at intervals, and the applied- load and ~orresponding strain

on the two gages we~e recorded. rnis information is incor­

porated in the stress-strain diagram, Fig. 21. This cylin-

· der was ·loaded to the ~ull capacity of ~he testing machine--

200,000-lb., corresponding to a stress of a ·little over .. 7000 psi.--th~ only indications o.f .failu-re beinb a ~mall

flaked-of~ place on the bottom. See Fig. 18. ~ben all the

load was re.cnoved from the cylinder, both gages returned to

49

ze~o, indicating no permanent -·seto

?ne third cylinder tested: _(without SR-4 strain gages·)

watl also loaded to· the_capaci.ty o£.the machlne Vlitbout'com­

plete failure_o .At stresses of'_ . _6220 ps~., 6530 psi., and-

6£60 psi. pronounced cr~shing Y/E::i" noted aroUnd the top and

bottom, t~::ere be~ng considerable honeycombinG at these

points o At a . _load o:f 200,000-;_lb (7070 ps:l.,o . stress) the ma-.

ci-line was shut o:f.f. After the machine had stopped and while

the beam was b~ing brought into rinal exact balance, the cy-

llnder orokeo

·.At the tlnie of t~stinc the beam, the concret~ age being

thirty-one days, the remaining four cylinders were subjected

to compression testingo -Cylinder number :four_ (F:1g_. 19) was

t~e se eond of' the ins truman ted cYlinders o Data. was taken as ~ . ...

on cylil1der number twoo This cylinder showed a definite

crushinG strength o:f 6855 psi_o Cylinder number .five ua.s

1oaded to 190,660-lba--6730 psi.--at which-load :l.t :failed·o

:Pig. 20 shotis this cyl.inder · a:fter .fai:lure o Cylinders .number

s:ix and seven wore loaded to 200,000~1bo--7070:: pO!.o--and

. . - -

showed no indication·or !'allure whatsoevex-.

The -stress-strain dtagram (Fig. 21) shous the average

stresses .f'or cylinders number two and four, the two instru-

:nented c-:.rlinderso . From this· curve the modulus of' elaBticity

w~s calculated by the three recognized methods; ioeo, by the

tangent to the curve at the-origin, by the-tangent to the . ,

curve at the working stress, and by the secant to the curve

n t the v:orking stress.. The results are sho\VD in the .follow-

.. ing table:

Modulus o:r Elas tici ty ~

Tangent a.t origin---.:.----------8.06 ·x 106 psio

. Tangent at 2400 psi.------;...--~-7.98 x 106 psi.

Secant at 2400 . psi.-----------8.15 x 106 ·psi.

Average----:--------~------------s-.06 x 106 p~i~·

50

Figo .18 Cylinder No. 2 after .loading to limit or test­ing machine--7070psi. Small flaked-_of'£ area near bottom was only indication o~ £ailureo Note SR-4 Strain Gage.

51

Fig. 19 Cylinder No. 4 af·ter f'.ailure ·: . s tres.s-6855 ps-1. Note SR--4 Str~in . Gage on· side~

52 ·

Fig. -2G Cylinder No. 5

Breaking strength 190,660 lbs.--6730 psi.

53

55

P?.ES :ffiESSING

T'ae bea.n1 was re::!oved f'ro~ the rorm ai'tet' c"u.ring .for

peven days and was placed on the testing frame.

practically no honeycomt:inr; whats?ever ·on the "team, and_ the

concrete appeared very den se and sound. Strain ~a ~es were ' ........ c

applied io t~e beam and extens~ons were soldered to the lead

wires ar.!.d connec t;ed to the br.id{;e balanci:i13 U..'rli ts. The six-

teen strain gages on the ~-.,i.res v;e:::~e conn ec-te<i t;o an. And.erson

24-switching unit ~6ridge Balancer, and ~i1e six s ·ur1·ace g~ges

on the ou tside oi' t :1e conc1-.ete beam we1-.e conn.ected to a

.Baldwin-Sou·th\'\'E''"rk 12-switch.ing unit Bridge J3£~lancer. Each

of these un:its . w~s in turn connected to a ~aldwin-Southwark

.Model K Strain Indicator which recorded st1-aains. directly in

micro i nches 9e1-a inch. At · this point all the g a [;es wer·e

balanced to record zero.· ·fhis balance was not changed

throughout the testing.

=r-ne testinz f~rar:"le usea was a 1nodl:fic~.tion o:f the i'rame

designed b·y J. Senne .• ( 1 3 ) The modification consisted of

(-13) Ibid., p.24.

lengt11eriilig the frame f"rOL'l a f'ive-f'oot span to a ten-:root ·

span by subs ·l;itut:i!l.g lor.li.er I-bean1s for the sides. The two . .

vertical T-sections and ·l;he cross !-beam \Vere kept at mid-

span. •ffie two ends or the frwue were the same ones used by

Mr. Senne. ?.his Changed the capacity of tLe rrame from

3-ft. x 5-f't. to 3-ft. x 10-ft. Extensions were welded to .

the •£-sections to increase the dis ~ance between the top of

the concrete beaill and the cross-beam.

56

'rhe concrete was pr~stressed by stretching each wire

individually with a jacking unit which consisted essentially

of two Simplex 30-ton hydraulic rams o.f the center-hole

type. One oi' ·these rams was used to pull the .wire~ ·and the

other was used to push the plug home into. the collar when

the desired wire stress was reached. These two units were

positioned bottom-to-bottom by two steel rods bent to ·f'orm

U-bolts and bolted throue:h a 3/8-in. steel plate at the top_

of the _ pushing ram. A hole was bored in the center o.f th~·s

plate so that the 2 1/4-in·. ·dia:~·;1eter piston of the ram could

pass through it. Welded aro~~d this hole was a 4 .1/2-in.

lensth of 2 1/2-~in. pipe which had e. 2 1/2-in. to l-in. pipe

reducer ·welded to it. This reducer was ::na.chined f'lat on the

small end to secure good bearins against the steel collars.

Tt£ reaucer~ pipe, and plate unit served as a stand-of'£ to

hold t'C.e rams away i'ro:m the end of the beam so that the

wires not being stretched could be kept out of the way. A

follower made from a .piece of' 3/4-in. steel rod, drilled

5/16-in. throughout its length~ was turned to a tight fit

and pressed into tl1e center hole of' the pushing ra1n. This

rod se1•ved as an extension of tb.e pis ton of" the pushin2: ram

and acted a~a.ins t the plug to pu~h it l1.orne.

In this way ti:e _ pullin,2: ram was able to ree.c t throagh

· the st.and-of·f u..YJ.it azainst the end of the ·beam itself as the

wire was be ir.:.g stretched and was inde pendent o.f . the p~shing

ram which was used o.nlJ7 for sett;:;..n.:.:_ the plug. Both :raams were

57

connected to pumping units . by 48-in. lengths·_ of flexible

.·pose. tbe pumping un:i t for the pusp~ng ram was a: · s.tar;.dard

Simplex RP-:-6 pmnp designed for use with .the Simplex hydrau- _

· lie rams. The pumping unit . .for . the pulling · ram ~as. adap_ted

from the hydraulic unit of a war surplus aircraft tail

hoist. See Figs. 22-26.

-The wire tensioning process involved the -following

s tepa:

1. A collar and plug unit was slipped over a wire to

bear against the bearing plate in the end of' the

beam ._opposite the jack. ·This plug was driven in

ti~ht until the threads ins.ide the plug gripped the

wire._

2. A collar and plug unit VIas slipped ·aver the same·

wire. to bear aga:Lnst the bearing -plate at the jack

end of the beam. This plug was left loo~~, but

car.e was taken to see that it remained in the hole

-in .the collar.

3. The jacking unit .was then slipped over the wire,

the stand-o:f.f end bearing agains·t the coll~r. 'Ihe

wire passed through the follower rod and the center

hole of both rams.

4. Another collar and plug unit was slipped over the . .

end or the wire to bear ·against the piston of the

.pulling ran1. This plug was also driven tight so

that .it would not slip on the wire.

58

5. A strain reading or 4200 micro inches per inch was ­

set on the- Strain Indicator. · This strain corres-

ponded to a wire stress o.f about 136,000 psi- ~ ·The

switching unit was set to one o:f the sages on the

wire to be stretched.

6. ~ne pulling rfuu was activated, and the -wire was

stretched until the Strain· Indicator returned to ·

zero, -indicating that the wire had reached the_ de-

sired stress.

7. ~nile this load was ·being hel~, the pushing ram was

activated· and the plug d-riven· home as ti ~·"'·tlv as - t..>.u. · "

possible~

8. Pressure· · on the pullins ram was released, th~ collar

and _ plug. at the end removed, and tne whole jacking

\mit ta.ken o:rr. -.

9. This same procedure was repeated on all ~ive wires. "•

The wires were stressed in the ~allowing sequence:

C.,D,B,E,A.

Considerable difficulty was encountered in the process

of prestressing. Almost all o:f the dif'.ficulty was d~e to . . .

the inadequacies in the end-anchorage units. It was .found

that it took several driving blows to set the plug satisfac- -

torily at the end opposite the . jack. Each o.f the plugs at

this end showed some slipping duri-~g the stretching ·of' the

wires. l!i.hen this .. happened, it was _ necessa-ry _to release the

jacks ·and to use a sleage ha..rnmer and a short length of'- steel

rod bored at one end and slipped U.,..._ i:! a.;ainst the plug to -re-

set the plug.

· On two of' the · wi_res - --A and· E-- ~l:1is_ slippine ·proved par-

:ticularly bad·. . .

In both cases . the wire very suddenly slipped

all t_he way through the plug and about one i:q.ch up into the :

-beam. 1-:qe absence -o:f bond made it possible to drive these

wires back through the be~m. an(!. to reset the collar and ·_:plug . . . . ,, . . .

around them-• · ·:rhe sudden and excessive movement o:r · these two

wires inside the beam caused all the· strain gages on botn

· wires to be stripped of'f' and become inactive. \'ihen thes·e ·

two _wires were finally stretched~ _a pressure _ dial _on- the hy­

draulic unit of .the pulling ra.n1 was used as an ·indication of'

the load applied.

On the ·other three wires a total of' :five gages· was lost _

in the :·prestressing process, leaving only f'ive out or six-. . .. : .

.. .

teen st·rain gages · still ac_ t~ve _when the testing was begun.

Ow"ing to th~- · urii'or.eseen difi'iculties encoun~_ered ., . th~

complete proce~s ~.f prestressing -required two days. ,Records . .

of' the ~-~e of: · stressin~ - e~ch wire were kept. Also, s-Grain

readings were taken on both the wires and the concrete for a ...

p 'eriod o:f tw_elve days from the time of• prestressing until

the time of' first testing-~ the purpose being to observe ariy

loss of prestress _ whic_h_ might occur ove:r that short p-eriodo

The results of th.j.s observation are shown on_ Figs. 27·-29 in

which · the average. stre~s in each ·wire is p'lotted .. aga~inst· · . .

·. time. lhe · loss o:f _prestress is . .fi.6ured_ . .from . the time ·that

.. prestressing was completed, because, as may 'b~ seen f'rom · the

graphs:~ the stretching ·ot: each successfve \Vire caused ·a

sharp ~eduction o:f st~~ss . in the wires already tensionedo

The: graphs may be summarized as· follows :

YJire Initial Prestress

Psio

· Final ·Pres_ tres a ·

Mter 12 Days Psi.·o · .

· _Loss In Per Cent P~~stress · . . Loss

Psi ·~

60

A-~--------- .-----G~ges · rnactive~------~~----.•---~--~---

. B

c

D

156,250

.:138, 900

70,700

141,000 .

134,750

64,400

. . 15,150 .

4,150 .·

4,150

E-~~---~--~-----~dages Inactive--~----~--~-----~~----

It should b~ ... noted - ~hat there was con-siderable sl·ipping

of the anchorage on wire D, reault~g in a ~oss o~ 65,300 r•

·psi. ·or 48 per cent from the desired 136,000. psio Several

· attempt~ · ~ere made . to ¢orrect :this loss ·,~ but · a stress ~igher . .

.than the ·lnltial .?0,700t.;psio could not be maintainedo · Ap-_

par~ntly, _the thre&ds · in. the plug had cut into the wir~ in

su.ch a manner · that. the wir·e was ~o·t actually separate ~rom

the plug, and the plug YJas . simply moving with the wire o . ·.

Concrete

'.-::

I" c; 2 2 Pipe

~" Pia tl!

. '

. .. :. ··'

. . _______ ..., ____ _ _______ ,__ _ _._ --

{Pushing Ram)

Simp·lex Hose

Coupll nos

I ' i - _....,_ ---- ---

----~ -...-..-.- ... ---1-1-- __ ._

I : , I :

· I i (Pullin' Ram)

2- 30-ton Simplex Center-Hole Rams

I

.. j ;

.Fig. 22 Diagrammatic Sketc.h of. P·restrt11inQ Unit (Stand-Off Assembly Shown ·in Half- Section)

' '

61

Center-Holt

Pis ton

Collar 8

Fig •. 23 Component parts ot prestressing unit. Left to right: stand-off unit, pushing ram with follower, pullib£ ram.

62

Fig. 24

Prestressing unit assembled.

63

Fig. 2~ Jacking unit. Left to right: Simplex pwnp t ·o activate pushing . ram,- hose connections, prestressing 'unit,. ·hydraulic· pump to act-. ivate pulling ram.

64

F-ig. ·26 Jacking :unit in position for stressing a wire. Strain gage switching units at extreme right.

65

.... I I

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. ;

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t: ·--= .L: I

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l - i ¥ • •

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66

, "1-' •

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67

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68

TEST PROCEDURE AL"'ID RESULTS

The testing of the beam wa·s done in two stages. In the .

first " stage~ ·the load was carried to slightly bey.on~ the

point where cracking occ~red • . The results of strain read­

ings taken.during this loading were tabulated . arid interp~et­

ed prior to- the second stage of loading in which· the beam

was tested to destruction.

The load was applied by an a-ton hydraulic jack locat-•

ed at the mid--span . of the beam. The load was transmitted to

-the beam _ tbro"ugh two 4-1~ • . I-beams whose reac·tion points ·

were eleven inches on either side of the mid-point of . the

beam. :: Two . ~hort lengths .. of 5/8-in •. steel rods served to

transfer the load from the !-beams to the concrete ·beam.

T.his ·arrangement· was decided_upon so that there would' be

pure moment ana·no shear at the center of the beam and so

that cle~ran~e - would be provided for the strain gages locat­

ed on the top surface of the beam at the center. ~e ap­

plied load was measured by means of a loading cell placed

between the top of the ·jack and th.e cross-beam o:f the test·­

ing frame·. All ~oads ·mentioned in . the discuss ion which .fol-

·lows are loads· measured. at the jack.

· This loading cell .was made of a piece of magnesil:lm rod·

2 1/2-in. long with _a di~eter of ~ 3/4-in. and was instru­

mented with four type A-7 s ·t;rain gages •. ,Two measuring gages

were ce.mented on oppos.ite· sides of .the rod, parallel . t~ the .

axis~ ~us preventing ~ny error which might be introduced by

eccentricitie·s in ·loading. Th~se . gages were c·onnected in

70

series.· The other two gages served as- compensating gages

and. we~e · c.emented ·to the rod . 1~.<;>0 apar·t and 90° .from the .

meas·U:~ing gages w:tth the i_r · .axes perpendicular . to -the axis "of'

th~ ·roci. These gages were_ also_ .·connected in· series.-.. · A·ll

·~ages · we-~e __ affixed in the same rr.e.-nner as described .for tho.se

placed on the v1ires, and the loading celt was wrapped with

e'lec .trician 1s plastic tape. The lo.ading cell was then cal~~

br.ated .in the Tinius~olsen testirig _machine, using l<?adi~~

incre~ents _of 250 .pounds up to ~0,000 pounds. A piece o.f

sheet lead w-as- placed at ~a~h .end of the- loading _cell to ,

provide positi_ve· bearing . a6a~nst_ the jack and the cross:-beam~

The :first' s ·tage of loading was. begun by apply~ng loads

·to tbe . }?eam in _increments of 250 pounds. After each incre­

ment of ·load _ was - - -~pp_lied, strain gage readings we~e t~ken on ·

all the.active g~ges, and deflections were read from Ames

dials. Five. o.f th~se dials were f ~xed to an independent

stand and: arranged so that they were evenly --spaced along the ·

top of' the beam;· OJ;le dial bei_ng at the center line. The

Ames . dia'ls . read in thousandths o:r an inch up to one . inch.

The ~oading was continue~ in 250 pound ·in,cr·ements un-ti:-1

a total load o:r 3000 pounds was applied~ All - readings were

· taken., and the load was re le!).sed. Then the zero · l ·oad : read.-­

ings ·. were - · ch~cked _~ These· readings varied .from the - initial

readings .. by from o.· to 10 micro il?-cf:ies for the wires and by

.trom 30 _. t~ 45 micro · ~nches for· the concrete. '!he maximum

·deflections under the .3000 pound load· was 0.091-in.

Lo8.ding was s·~arted ·again· and taken up to 6000 -pounds

71

. .

in 250 .pound increments, .at which point· the loading incre-

ment v:as ~hanged to 500 pounqs and the_ load continued to . .

9000· pounds. .st·rain and ·def'le ction readings . were .taken at --

.each new incretnen t.

The· ~irst visible · crack appeared between 7500 and aboo ·

pounds of load. This crack was - dir~ .ctly ·under one on ·the

points of' application o:f the load ·to the beam. When . the ·

load of' 9000 "pounds was reached, -i~rge cracks h~d opened .un­

der ·b~th load points and near mid-span. S1naller cracks had ,

~ppeared between these. Fig. _34 shows very clearly the size

and extent of the largest cra.ck under a load of 9000 pounds·.:.

~his was the f'irst crack to appear. The max~um width or .. · . .

this ·crack · was _about 1/16-in.~ and ·it extended over ha~f the .

depth of' the beam. The maximum de£lection under the 9000

pound load was 0.344-in.

Ar~er sufricient· readings and _ investigation~ were .made

while . holding the 9000 pound load, the jack ·was released and

the load· returned to· zero. · Again · zero load strain &nd de-

fle_ ~tion read~ngs w~re taken. Variations o.f :from 0 to- 140

micro inches ~rom tl1e· initial strain. readings £or the . wires

and o:f !:rom 15 to 35 . tnicro inches £rom the :initia-l st~ain

readings .for he c oncrej:;e were noted-. No ~·rmanent de:fle·c- ·

·tion \Vas· noted.

Upon removal ·o£ the load all cracks closed- completely •

. fig. 35~ which was taken after removal of the load,- shows

.the same ·-area as Fi.g. 34. Here may be · seen how completely

the largest _ crac~ closed. ~e trace _o:r the crack may be

72.

. .

seen in the rlaked-off wh;i.tewash with w.hich the side of: the

beam was pain ted in order to 1:11ake cracks more vi~·1 ble •

. ·Loading was resumed enq carried to 10.,000 pounds in in­

crements of ·.1000 pounds • Tb.~ same cracks . that had appea,red . ...

before · opened under a load of· 700Q pounds. The width of' the·

largest crack ~as about 3/32-in.~ and it extended 4 7/8-in.

from the bottom .of the beam. The maximum deflection .under

this load was · 0.5418-in • . or 1/410 ·o'! the span. This load . of

lO.,OOq pounds was approxima~e1y 1~9 time~ the ·aesign load of

5300 pounds. ~en the ·load wa~ released, all cracks ag~in

closed, arid the beam. returned to its original position.

The second s ta~e ,or testing, in which the beam was

lo~ded ·t·o·d·estruction, was begun six days later. Load was

applied in increments of 500. pOUndS up to 101 000 pounds., at . .

which po_int the . i~cremen ·t was cha:oged to 250 pounds. :lhe

rirst cr~cks were again observed at 7000 pounds of load.

All ·cracks follov1ed th~ ·samE;) pattern and sppead as be:fore.

The first.failure occurred at 11,500 pounds. ~is was

a f~ilure., not. of the concrete~ but of one of the end-anchor­

ages on wi:r~ c., resulting in the loss of all tension C?·n that

·wire. This~ of course, materially reduced the prestress

force on the beam., causing· considerab~e extr·a deflect;i.on ac·­

· c~panied by a reduction of load down to 9250 pounds. T.he

deflection measured with a ruler (the Ames dials~ having be-

.~ome inactive) just before this anchorage ?~oke was

1 l/16-in. The deflection just after.the break was

l 5/16-in.

73

~ter this faiiure, ~oading was resumed and carried up

to a load or 10,750 pounds and a derlection of 1 13/16-~o~

at which load wir~ B pulled through its end-anchorage.

It was during this stage of loading that·the beam first

began to show sign of crushing. The largest. of the cracks

had spread to within about l-in. of the top, and the con­

crete in this area began to spall and flake o~~- as more l~~d.

was applied. All crushing occurred just to the inside of

one of the steel rods which served as reactions for the I-

beamso Figs. 36, 37 and 38 show the location and extant o~

this crushingo- A~ter the first anchorage failed at 11,500 -

pounds load, it was impossible to reach this load again;

there~ore, the load at wnich the concrete failed could not

definitely be determined.

After wire B pulled through its end-anchorage 1 the load

was released. The tension in the thrae remaining wires

caused the beam ~ to resume its original , posltion a1most com­

pletely. Crufrb~d concrete in the cracks prevented them from

closing entirelyo .< '

> I

The process of applying and releasing load was con-)

tinued ·until wires D and E failed. The concrete _continued

to c~sh slightly in ·the same areao Finally, w~th :~4ly uire l "' ' ' ~!..,.

A s .tj_ll active, the beam was loaded to approx1mately '7000 _

poundao T.he jack was extended to its limit whiqh. resulted

.in a beam def'lecti:on of \ approximQ~Y 5•ino The largest

crack opened .to ap~ut 1-ino ~t this deflectiono When this

load was releasedJ rthe beam still attempted to resume its

74'•

original position, the.re ·.being a per·ma:t:lent deflection ot:

7/16-in • . Most o:f this permanent deflection was due to con-

cret;e ·particles which ha.d·wedged themselves in .the crack

.openings.

Figs 0 39 .and 40 snow a compa~ison bet\Veen fiber stresses

for the top and bottom as calcula. ted . . f'ro1n.- .. t 'he ·des.ign . pre_­

stressing ·force. and: ·as taken :from SR-4. str·ain readings. . The

stresses on the bottom o.f the bea...l'D. are in r ·airly ·close

agreement up . to the polrit at which the beam ·cracked. At

·this load the actual stresses qegan to fall o:f'fo . This was ·

due to the fact that ·cracks occurred on :poth · sidea~ of the·

strain gages~ .thereby isolating them: :from the ·tensile. forces •.

The comp~ess.±ire :l::r~resses-·in the· top fibers .of the beam . show

marked de via ti.on f-ront the calcula:,t ·ea stresses. This may 'be . ~·

explained by the fact that ·the . ~.hart gage length of the

strain gages ·. resulted in the :ve<?ording o:f stress concen.tra­

tions whi:ch were very high rather than av~rage stress dls­

tributio:ria.

Fig •. 30 Beam prior to beginning test. Left to right: Anderson unit for ~11easurlnc load, jack and loading cell on beam, Baldwin-Sou.thwark unit for measuring wire and c.oncrete strains.

75

76

F~g. 31 General view at' beam ·flnd test frame .•

Eig.· 32 . General view of test procedur-e. At left .: applying .load .and. ~ead­ing Ames Dials. At right:·r.ead1ng and recording strain measur.e­ments.

77

Fig. 33 View through .rnagnify i.ng g1a~ s, showing loca­tion of largest crack'under 9000 pound load.

.78

79

-pig. 34 Size .and ·:extent of largest crack opening under load o.f· 9000 pounds.

.. ·

, '

F i . ''5 . 8. v Same area as Fie~ 34, showing closure o~ .crack after removal of load. Note trace of c1-aack in flaked-off whitewash.

80

. ·Fig. 36 . Beam unci.er load after four of five wires had failed. Defle~tion approximately 5-fn. Beam r ·etur:ned nearly to ,·normal after re­moval of this load.

81

82

Fig. 37 Beam afte~, completion or>test and removal of'. load.

Fig. '38 Beam after- . removal of :loading ·assembly, .showl~g location and ex-tent of crushing- zone. ·

. .

MEASUREMENTS TAKEN DURING FI~AL STAGE OF TESTING

Wire and Concrete: Top numbers = ·$tress (psi.), Bottom ·numbers= Strain (micro in. per in.).

Deflection (in.) measured at center line.

Load in kips :

o.o 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 ·5.0 5.5 6.0 6J5 ;.;: 7.0 7.5

8 , .. .,~, 0 I~ 10• ,.,,_,, 1.1, f8o ltf tJ/1 If. O'ftJ 'i 1211 14;110 14,22 6 1~ ~lo 14 .J I.() I~ 31~ 14 .,.,~ I~S4'D :~ "-"" 14;61() Jf ?II J41+ Ji- ,. ....... .Sif f4'J'I f.TS.I 4fS?I. tf4"#(, f , • ..r 4'''- ~,.0 ..,,,2· ''.1 rf 7.!. +711

! c I~ ,J ,. ~~~ 4 .,. IR.~4IO ~~~Sf• ,~,~- I I, ?tJ o ltl, ,.,. I~ 12o ~~/· , "1 () It!, fZII I~ 9111 I-!, tJS'O I~ lltJ I~ 2,111 j~ 3ZtJ 13 .,,. j

), ~ .

·~~· .11ft> 4 .,~~ 4•J.r •... ,, 4 0 1'-r. 4 IJfS 411' .. ,.~.r 4./..r~ ~ ,~,. 4114 . "''" 4ZJ() 'fR. ,, ;~ 1. f~

D ~ t•~ ~ IJ.J_tt ?ISO ~ ·~() '1, l.lo '1.~ I•• ~ ,,. ~z~%o ~z~o ~2-f .. . 2.1.1• 1,Jifl ~ 41• ~ SIJO t ,,,. ·~?I# ~

A ItS'S 2~'-' 22. 'IS - ~~··· 2.4 tJI> 2.J~6 ll.J2• ~330 2 J40 ZJF~ · Z4 11 t, 2Jio z .Jf•6 24/f "11·48 ~4- l.r

• -I,, 'f - /., 'f.J -14 f (, -l.,%4 f 4 14'1 - ?SR. - iS7 - ... ~ ~ /{, 14.1 Jl3 ., ?" S.s'tf ·..r/J' .;~ ..rll S4J ! · Bottom 244 Z-111

,,,, 11'4 Ill f! .r? JD .2 II 41 S'l "' 7# 7% ,,

u .. I,Sif -~ ,,. -1,/, f8 -I~ 14-J ... ~II? -I 'II I -<, D/6 -2, '7~ - ~ ~'11 -2,SIS .... ,2 7~ -2''" --!RIJ - .1.., ,.J -.ftJtJ -~,~-~ & ·TOP· 12i?

, .

'J'' ' 0 If? .211 R.ll 211 2.z,A 2SIJ 2. 'l~ .GI.r .]12 . .JJ' .,,, 44 s•• 4'?s-

Deflection ~.IJDO II I. IJI,.O I I.DJS.r 1. IS'f_,- ,.DIZI ~.II>JtJ 1.124.> ~.14{,1) O.li~ZS' ~./8os 1?./ff() ~.221D 1.2 4J.t IJ • .. . , ,. /1.·2ffS tJ.I:IItJ

- Denote·s Compression ! '

. Load in kips 8.0 8.5 9.0 ·~ 10.0 10.25 . IG ... SO' . 10.75 11.0 0 11.25 II. SO

B /4 110 IS: I•• IS, 41 I 14; ~I'· 1.1; fJ:D l41R" ~~.~,, 14, 'fll ,,/ '~/) J"R'• '~ ... :. .J.J ... ,. 4f.s"A - ¥•44 .$'1+f Sltf, ~·?'1 .r.Jit. .Sf.t/"'1 ss''J .r''.r • 'o! ' ' 0 I~,,. li,~lttfO '1 ~~~- '~l?~· ~~ f41J I~ lllf• I~SIIJ /~?+• ,,, , '0 -- 1

~ c .j 446J 447~ .fi~IJ iiJfO 'J.S' ..,_,_£() fl·&r .r,,IJ .s-o?• f /Ill -D

1; ,,. 1,~ OJ.• ~~·· ~ .. ,, ~-''() ~~~~# ~IS. "'~·IJ ~ ,4. ~~ ~~· I~,,. l

2.rJ.s l-'11 I 'I.·• .2 "'JII 2.81S Z.li!IJ a•.r• 4.s-& .1/.JII ~~~· J .. ,. ,\

• ~tom .Sit .FI/ 4 .,. < ' .~- -~ f a-,, R..r~ RJ(, ~,, I.')/ 2.4S" ~IS

1 'J , .. so. Jl .17 .II_ .lf ..If J.J Jt. &')

! Tip. ... ~271. ·S:If' -~,,, . .., . -tJs f -~~~,, - 8f.J' - 8' 9'! - 9'.r" - 1 ,,., ~~~ ?~0 -II, 2'111 8 'Sf '1.1 I IJ11· .. .,,1 . I "Ol I' .s-9 116'1 II. 7 1~!1 J,IJ• '"''' Deflectio~: b • .II+S ~. 44SS o. 4 f 7S •. ss,. ~,~J4r "· '1(,0 tJ.?afs IJ.'JII,D IJ. ?'2D ~.'11DI /. D/,2.s-

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90

CONCLUSIONS AND RECOM}·:ENDATIONS

The test beam very clearly demonstrated the remarkable

elastic properties which are ·inherent in prestressed concrete

structures. 'r.he absence of cracks under design load.and the

closing of cracks caused · by. greater loads are also chara~~

teristic ·o"f a properly designed pre.stressed concrete beam. . .

It will be not·ed from the design calculations tha.t

cross-sectional are~s of 0.3-sq. in. of reinforcing steel

and 48-~q. in. of concrete were used in this beam t~ carry .a

design load of' 5300 pounds on a 10-ft. span·. Analysis of a .

conventional reinforced concre ·te beam f'or the same load and

span .will show that 0.83-sq. in. of steel and 91-sq. in. of'

concrete would be required. 'rhus prestressing in this one cP instance resulted in a saving of about 36· per cent in steel

and about 53 per cent in ·COncrete. 'rhis saving, wh~le sub-

stantial in itself, could ·have peen made still greater by

using an I cross·-sectiori for the prestressed concrete beam •

. This type cross-section with its thi~ web is recognized as

the best for prestressed design where shear is not impor-~ ..

·tant, but it cannot be used satisfactorily in conventional

reinforced concrete design.

As a whole, .the testing procedure and the .results ob­

tained were quite .satis£actory. The instrumentation of the .. prestressed wires with the type A-7 SR-4 strain gages was ·

very satisfactory; however, it is believed that more effi­

cient ~ethods of protecting the gages may be devised. Near

the ja.ck'ing end of a beam there is a great 1?endency for a

91

gage t6. s t:rip off due to the rather· large ·_elongation o:f _the

wire at that point. The · autbo~ be~l--ieves _that _by .employing

more new_~y developed methods · of- a.ffixing st-rain g~ges ~ .us:~ng

epon resin ~:s - a. cementing · ~t-~·rial, anq by using somewhat . .

sma.lle:r lead· wires it would be po_ssible to e_nclose the _gages . . . ·. · · . . · ·.· .

and :the l~ads . in the elec·trical lo.om completely, bringing

all the lead wires out at the end. This would c-onsiderably

reduce the· time and labor required to waterproof 'the gages .. . · · .·

and wo~ld entirely ~rev.e~t the gages f'rom being· strippe.d

of'f', since they wou·ld · ·al~ -~ove inside the·'loom as the wire ~ ·

is lengthene-d _·under stress •

It is also recommended that a dif'f'erent type strain

gage--possibly a type A-9 with a six _inch gage length--be

used to record concrete strains on the surface of a beam in

plf:lce of th~ ·ty~e _A-7 w~t_h it_s ori"e-q_u~rter lnch gage length.

This short gage length tended to record stress concentra-. . .. . .•·

tions rather thaJi. an aye~~ge distribution of stress. Ibe

extremely high compressive . stresses in· the concrete shown in .. .

Fig. 40 indicate this. It is also recommended that when two

or ·mor·e gages are used to -record the same type ot· strain in

the s.?rae ar~~ on · a · -concrete beam, they be connected in

se~ies~ thereby - giving . a~ average strain reading f'or the . .

area ·with only one setting .. of the strain indica. tor. This . . ~ ..

would als? ·a.pp_ly to ~train gages used on test cylinders.

The pre-stressing unit developed .for this project proved

emin:en-tly~· successful for th_is ·type of' end-anchorage. Only - .

one _chang~ is sugges~ed. If' the pumping unit were of suf-

92

~icien-t s:tze,. arid · -i ·:r the proper type ·or q~ick cross-o·ver

va.lve could be insta.lled in the .hose lines at the· . pump~ it .

would. t~en · }?e possib~.e to op.erat~ ·b()th _rams. · .. with the . _same.·. .. . . . .

puinp. T.h:ts · -v.rotild make the equipnfe:nt as -well as .. th·a :·.pr~stres­

s~ng :operation._ muc·h · ~impler.

The - plug and :col_lar system of' .. end-anchorage us·ed:_ ·.:Ln .. . :. -·

this ·project could : ~e modif'ied· ·in ·several· - ways~ This.: unit

failed before the concrete· beam C?egan . to show· az?.y signs ·of

c~ushi~g. Fai·lure . w.as du~ - to the shearing qf'f' of' - ~he

threads inside the plug. .This threaded area DlUSt be ·auffi- :.

ciently tough· a·nd hard. ~o be able to bite into the pr~s .tres­

sing wi~e which in it~elf . is extremely hard. To assure this

gripping the thre.ads· must be . v_ery . ~harp and preferably qui.te · . .

·fine• Also, i:t is believed that a. much thmner .plug _witll. a .-

much .flatter taper would .ha·ve stronger gripping ·power.

93

BIBLIOGRAPHY

l. · Books:

Abeles, P. w. The Principles of· Prestressed -' concrete. N. Y ~, Ungar Publishing Co., 1949 • .

2. Periodicals:

·Schofield, E. R. Prestressed Concrete Used for·Boldly .pesigned _Strucutres in Europe. Civil Engineering, Sept. 1949·, Vol. 19, p. 596.

u.· S. Progress in Prestressed Concrete. · Architectural Record, Vol. 110, No. 2, pp. 148-156.

Zollman, c. c. -Prestressed Concrete. Construction. The Military Engin~er, Vol. XLIII, No. 291, 1951, PP• 1-6.

3. Publications of Learned Societies:

Eillner, K. P. New Prestressing Jviethod utilizes Vacuum Process. Journal of · the American Concrete Institute, Vol. 22, No. 2, 1950.

Bi1lner, K. p. and Carlson, R. w. Electric Prestres­sing of Re·inforcing s ··t;eel • .. Journal of . the Americal Conerete Iris~itute, _·val. 14, No • . 6, 1943.

4. Published Material:

Baxter, s. ·S. Construction Of the Walnut Lane Bridge. Proceedi~gs of the First United States Conference on Prestressed 9onc~ete~ 1951, p. 41.

Birdsall, B. ·Prestressed Concrete. John A. R.oebling's· Sons Co., ·rrenton, N. J.

Design and Control o.f Concrete Mixes, Portland Cement ·Association. 9th· Edition.

Desi·gn o.f Pres tressed Concrete:, P·ortle.nd Cement Asso­ciation. 1950.

Dobell, c. Prestressed Concrete Tanks. Proceedings o~ the First United .States Conference on Prestressed Con­crete, 1951, P• 10.

Gcxi~rey,- H. J. Steel Wire for Prestressed Concrete. Proceedings of the First United StB.ted Conference on Prestressed Concrete, 1951, p. 151.

94

How To Apply SR-4 Strain Gages, 3ulletin 279-B. Bald­win-Lima-Hamilton- Cornoration, Philadelphia, Pa.

Kennedy, H. L. High.Strength Concreteo Proceedings o£ the First United States Conference bn Prestressed Con­crete, 1951, p~ 128.

5. Unpublished Material:

Senne, J. H. Investigation of Stress and Crack Dis­tribution in Concrete Slab~ Containing Welded Wire Re­

-in:forcement. Thesis, !.Iissouri School o£ Mines and Metallur6y, Rolla, Mo. 1951.

VITA

Howard vv·. Nunez, Jr. was. born on January _27, 1925, in

· Natchez, Mississippi, ~e son of Mr. and v~s. Howard w. Nunez.

95

He received his ebrly educati-on in . Natc}1.ez, graduating

.frorrL ~~atchez Eigh Schoql in ·June· 1943. The same month he

was inducted into the Army. His twenty~nine mont~s of ser•

vice included .four montl"ls with an ASTP unit at Randolph-

Macon College in As_hland, Virginia, and_ n-i~e months of over­

seas service with an engineer combat battalion in the ETO.

He was _ separated f'rom ·the ·Army in November 1945.

In January 1946 he . ~ntered the Alabama 'Polytechnic In­

stitute in Auburn, Alabama, where he received his B.S. in

c.E. in March 1949. In June 1948 he was married to Miss

Dorothy Till of Hammond, Louisiana.

Following graduation he wo·rked as an architectural

dra.f"tsme.n in Natchez until September 1949 when he accepted

a position as Ins .tructor of' Civil E~gineering at the I1~issouri

School o.f Mines and Met~llurgy at Rolla. At the same time

he ·started work on an advanced degree in Civil Engineering.