,..

Prestressed Concrete Bridge Members

Progress Report 12

STATIC TESTS ON PRESTRESSED CONCRETE BEAMS

. USING 7/16" STRANDS

by

Louis J. Deb1y

(Not For Publication)

'. . ~.,

This work has been carried out as apart of an investigation sponsored bythe following:

American Steel & Wire Div., U.S. SteelConcrete Products Company of·AmepicaLehigh UniversityPennsylvania State Highway Departmen~

Reinforced Con9rete Research CouncilJohn A. Roebling's Sons CorporationU.S. Bureau of Public Roads

Fritz Engi~eering LaboratoryDepartment ofClvi1 ·Engineering

Lehigh·UniversityBethlehem, Pennsylvania

September 1, 1955Fritz Laboratory Report 223.12

(Not for Publication)

TAB L E o F CON TEN T S--------

I. INTRODUCTION

II. DESIGN AND DESCR+PTION OF BEAMS

A. Concrete MixesB. Concrete Properties

1. Compressive Strengths(a) Release(b) Static Testing

C. Description of Beams and Mode of Loading

D. Design Stresses Based on BPR Criteria

III. TESTING PROCEDURE

A~ JackingB. Release

1. Instrumentation2. Procedure

C• Static Tes ts

1. Instrumentation2. Procedure

IV. RESULTS OF TESTS

~. At Release

1. Slip2. Strains in Concrete from Whittemore Readings

B. Static Tests

1

1

12

223

3

3

4

45

55

6

66

6

6

67

8

.1. Load vs. Deflection Curves 82. Estimate of Prestress From Test Data 83. Computation of Ec Values From Deflection

Curves and Cylinder Tests . 9.4. Computation of Ultimate Load Using BPR Criteria 9

C. Slip

D. Crack Pattern

V. CONCLUSIONS

9

10

11

Concrete Strain Distribution Beam 2-A IConcrete Strain Distribution Beam 2-B IConcrete Strain Distribution Beam 2-A II

Concrete Strain Distribution Beam 2-B II

Concrete Strain Distributi"on Beam 2-A III

Concrete Strain Distribution Beam 2-B III"Lo~d Deflection Curve for First Cycle

Load Deflection Curve (Beams 2-A I and2-B I)Load Deflection Curve (Beams 2-AIT and 2-BTI)Load Deflection Curve (Beams 2-A III and 2-B III)Close-up of Jacking EngPrestressing Frame & Beams Before Release

Beams After Test Showing Cracking PatternStatic Test Set-upSlip Measurement During Static Test

LIS T 0 F FIGURES- - - - ~

Page

131415

16

17

18" 19

20

21222323

24

24

25

I.

II.

III.

IV.

V.

VI.

VII.

LIS T 0 F TAB L E S

Batch Weights In Pounds Per Cubic Yard

Stresses at Release Compared to Ultimate CylinderStrengths at Prestressing BPR.

Comparison of Losses in Steel Stresses as Calculatedby BPR Criteria

Slip Me~surements at Release

Prestress Forces & Cracking Loads

Modulus of Elasticity

Ultimate Load

2

4

4

7

8

9

9

A C K N 0 w- LED GEM E N T S----------------, '

,

This report concerns part of an investigation being carried

out under the auspices of the Institute of Research of Lehigh Uni-/

versity. The work is under the guidance of the Lehigh Prestressed

Concrete Committee which is composed of the following members:

Mr. T. Germundsson, Reinforced Concrete Research CouncilMr. L.A. Porter, Pennsylvania Department of HighwaysMr~ J.L. Stinson, U.S. Bureau of Public RoadsMr. N. VanEenam, U.S. Bu1eau of Public RoadsMr. S.L. Selvaggio, Conc~ete Products Co~ of AmericaMr. F.S. Burtch, John A.: Roebling's Sons CorporationMr. H.K. Preston, John Ai. Roebling's Sons CorporationMr. W.O. Everling, American Steel & Wire Div., U.S. SteelProf. W.J. Eney, Lehigh University

The overall program of prestressed concrete research at Le-

high University is under the directorship of Dr. C.E. Ekberg, Jr.,

Assistant Professor of Civil Engineering. Mr. A. Smislova, formerly

Assistant Professor of Civil Engineering served as. assistant projectII

'-:7 director and had direct responsibili ty for the bond portion of the

program before terminating his association with Lehigh.

Assistance in the pouring and testing of the beams was given

by Mr. A.M. Locsin, Research Fellow; Mr. D.I. Sherman and Mr. F.L.

Tor~es, Research Assistants.

,, -1

I. I N T ROD U C T ION

"The beams described in this report constitute the second

series of tests which form a phase of pilot tests aimed to determine

the bond characteristics of 7/16" strand. Th~ first series were

tested with third-point loading and are described in Progress Report

No.ll. This second series consists of eight beams tested with center-

point loading, but otherwise the testing was similar to that in the-

first series.

The beams are designated as follows: the first number indi~

cates the series; in this case, second; the letter A or B designates

the strand manufacturer; the third number (Roman Numerals) design-

ates the mix used except the zero (0) series which are the non-

prestressed beams. The casting date follows.

Although strands A and B have a nominal diameter of 7/16 in.

they represent different steel areas and hence different steel to

concrete ratios.

. A. Concre te Mixes

o F-,.- B E A M S._.- _._._,

The aggregates used for the beams were obtained from local

suppliers who also supplied the aggregates used for the beams of

the first series. The sand had a fineness modulus of 2.54 and the

crushed stone, (one-half inch maxi~um size) a fineness modulus of

5.45. Curves consisting of sieve opening size versus percentage

passing were plotted for the aggregates and compared to the Fulle f

and EMPA curves •. It was found that 38 percent sand based on weight

of total aggregate gave a curve lying in between the Fuller and

EMPA curves. Trial batches were mixed and cylinders poured1n or-

der to ascertain the mix proportions and required strengths.

-2

High early strength cement was used for all the mixes except

Mix III where Type I cement was used with plastiment. Plastiment is

an admixture to increase workability and gives high early strength

concrete.

TABLE 1. Batch Weights In Pounds Per Cubic Yard'

Mix 0 Mix I Mix II Mix III

Cement 611 536 752 940Type III Type III Type III Type I

Water 352 334 375 394

Sand 1170 1190 1120 1110-

Stone 1910 1940. 1820 1670.--

Adm. PIas timent 5.0 ,

B. Concrete Properties

1. Compressive Strengths

(a) Release

The mixes were designed to yield the following cylinder

strengths (fci) at release of 3500 psi, 5500 psi, and 7500 psi for

mixes I, II, and III respectively. It was not possible in all cases

to test the cylinders on the scheduled date so that the following

strengths resulted: 4180 psi, 6340 psi and 8300 psi.

At the time the beams were poured two cylinders were taken

from the batches which surrounded the strands and one cylinder

taken from each of the following batches. The cylinders used to

determine the strength at release were those taken from the batches.'

which surrounded the strands ....,..,

All cylinders were capped in a Tinius Olsen Capping Machine

using Carbo-Vitrobond manufactured by Atlas Mineral Products Co.

-3..... '

(b) Static Testing

The cylinders were tested usually on the day following t1).E:?:,

beam test. The average of the cylinder values is shown in Table VII.

It will be noticed that in Table VII the cylinders for beams

2-A III and 2-B III have a lower value at test than at release. The

original caps had been defective and the cylinders were recapped.

Upon examination of the cylinders after the destruction test, it was

found that the new caps were also defective and cau~ed the lower

strength.

C. Description of Beams and Mode of Loading

The beams were cast 12 in. wide by 6 in. deep and 12 ft. long.

Eac'h beam contained 2 strands of the same manufacturer, the center ­

of the steel being ~ inc~es from the bottom and spaced 2 inches from

the edge; this resulted in a horizontal spacing of 8 inches.

The beams were tested on supports 11'-6" on centers wi th a

hydraulic testing machine applying load at midspan.

D. Design Stresses Based on BPR Criteria

The 12 in. by 6 in. beam section gives a calculated zero

stress at the extreme, top fiber due to the prestressing force ex.,..

cluding dead weight of the beam. The calculated stresses in the

bottom fibers are shown in Table II for, 2 cases; first the instant

of release, secondly, after los~es have taken place. Table III

principally gives the initial mea~ured prestressing forces and the

calculated prestressing forces after losses.

-4

T~LE 110 Stresses at Release Compared to UltimateCylinder Strengths at Prestressing BPR.

fc!i ....-"

Age at Gylinder %of fb %ofRelease Strength fb f&i After f6i

DAYS Beam Noo at Release Losses,

7 2-A I 4120 1008 24.4 870 21.1

7 2-B I 4244 1030 24.3 894 21.

10 2-A II 6500 850 13 01 730 11.2

10 2-B II 6180 878 1402 760 12.3

26 2-A III 8640 672 7 08 570 6.6

26 2-B III 7970 766 9.6 660 8.3

"

TABLE III. Comparison of Losses in Steel Stressesas Calculated by BPR Criteria

Strand A Ultimate Strength 27,800 Ibs.Strand B .Ultimate Strength 27,800 Ibs.

Ini tial 70 of Prestressing 'fa of ' 'fa ofBeam No. Prestress- Ultimate Force After Ultimate Losses

ing,Force Losses BPR

2-A I 37,800 68 32,740 60.7 13.4

,2-B I 37,800 70 32,760 58.9 13.3

2-A II 37,800 68 32,740 60 07 13.4

2-B II 37,800 70 32,760 5809 13.3

2-A III 37,800 68 32,740 60.7 '13.4

2-B III 37,800 70 32,760 58.9 13.3

"

Ao Jacking'

In order to have the same tension in the strands prior to

mechanical jacking the following procedure was followed: After the

cables had been placed in the jacking frame, each cable was ten­

sioned individually to a small load (about 3000 Ibs.) using a

-5

hydraulic jack at the end opposite to the jacking end. The strand­

vise was then adjusted up against the jacking frame, the hydraulic

jack released and the reading noted. By tensioning the two inside

strands before the outside ones, it was possible to start the mech­

anical jacking with almost the same tensions in all of the cables.

The strands were then stretched to approximately one-half of the

final load and left that way for a few hours until the pouring was

about to start at which time they were tensioned up to th~ required

load. At time of pouring the tension in the str~nds was approximately

18,900 Ibs. with the maximum deviation between strands recorded as

1200 Ibs.

B. Release

1. Instrumentation

The purpose of the instrumentation was first to measure the

slip of the strands'into the concrete, and secondly to measure the

change of strain in the concrete along the vertical sides of the beams,

at the level of the strand.

For measuring slip, 2 Ames Dials were attached to a bracket

which was clamped to each strand. See Figs. 11 and 12. The plungers

of the dials rested up against the smooth face of steel plates which

were attached to the end of the beam by means of sealing wax.

For strain measurement, Whittemore plugs were attached to the

sides of the beam two inches from the ends and ten inches on center.

Thus it was possible to measure strains on 10 in. gage lengths at the

level of the strands from one end of the beam to the other.

2. Procedure

The jacks were released gradually and the slip recorded. Read­

ings on the Whittemore gage plugs were taken after release and again

after the beam was on supports placed 11 ft. 6 in. on centers.

-6

c. Static Tests

10 Instrumentation

The instrumentation for the static tests was for the purpose

of determining if there was any end slip of the strands and for det-

ermining the relationship between load and deflection of the beams.

The measurement of end slip of the strand was instrumented in

the same way as for the slip measurement at time of release. See

.Fig. 15.

The measurement of deflections was accomplished by means of a

single Ames Dial. supported underneath the beam at midspan. In order

to eliminate any errors due to deformation of the supports and other

causes, the Ames Dial was suspended from a small steel angle which was

mounted on dowels projecting out from one side of the beam at mid-

depth over each support. See Fig. 14.

2. Procedure

The load was applied in three cycles. The first cycle, in

increments of 500 lbs. was taken one incre~nt beyond the cracking

load. The load was removed and then applied in 1000 lb. increments

up to the maximum load in the first cycle; then in 500 lb. incre-

ments to approximately one and a half times the cracking load. After

removal of load~ the third cycle of loading consisted of 2000 lb. in­

crements up to the maximum previous load, and from then on in 500 lb.

increments to ultimate.

IV. RES U L T SO F T EST S

A. At Release

1. Slip

The slip readings recorded in Table IV are the average readings

-7

from the two Ames Dials on each strand minus the amount of strain in

the strand between the clamp which holds the Ames Dials and the end

of the beam.

TABLE IV. Slip Measurements at Release

Cylinder Age Slip in.xlO""'"Beam No. Strength in Jacking Anchor Average

at Release Days Strand End End Values

2-A I 4120 7 North 55.4 75.465~8South 51.4 80.9

2-B I 4244 7 North 65.5 63.0 64.7South 63.8 66.3

2-A II 6500 10North 39.0 44.0

41.1South 40.7 40.7

2-B II 6180 10North 55.5 52.

53.9South 60.5 47.5

2-A III 8640 26 North 50.1 52.1 53.7South 42.5 70.5

2-B III 7970 26 North 68.0 58.0 59.9South 65.2 48.2

2. Strains in Concrete from Whittemore Readings

The concrete strain distribution resulting from the Vfuitte-

more readings ~re shown in Figs. 1 to 6. These values represent the

average of the readings which '..vere taken on both sides of the beam.

'rhe plotted values of strain indicate the average strain between two

Whittemore Gage plugs, ten ~nches apart and are plotted to scale-

horizontally at the midpoints between plugs. The first horizontal

space is net to scale. The first set of readings was taken immed-

iately before the release of the prestress. Ehe second set Immed-

iately after- re.lease with the beams 3till resting on the forms. The

.beams Ne.re ther: lifted and placed 'In 3upports 1],.' - 6" on center. rp.e

time ['eq1~lired to take all of the~3e r.>eadings was approx1mAtely twc and

a half horu~s. "Rea.ding~.we~e also taker3.t longer intervals in order

to see the effect of plastic flow in the beams.

These figures also.give an indication of the anchorage length

of the strand. As the cur,re flattens, the strain becomes constant

-8

indica ting that the t.otal pres tressing force is developed. Dis­

crepancies in the curve may be attributed to p'ersonal error in the

manipulation of the gage although the general shape of the curves

are satisfactory.

B. Static Tes ts

1. Load vs. Deflection Curves

The load deflection curve for the first cycle is shown in

Fig. ? to a large horizontal scale for the 6 beams tested. The

permanent set after the first cycle, which included loading beyond

the cracking load, is compared for the tested beams.

Figs. 8,9,10 compare the load "deflection chal'>acteristics of

the A and B series for all the cycles up to ultimate load. The beams

continued recovering after removal of the loads and were allowed to

do so until the readings on the Ames Dial (.001") became constant at

which time the deflection was recorded.

2. Estimate of Prestress From Test Data

Using the observed cracking load cancelling the tensile strength

of the concrete (established from the modulus of rupture tests) to-

gether with the moments produced by Dead Load and Live Load, it is

possible to arrive at an experimental value of the prestressing force.

The calculated values are given in Table V. See Appendix for typical

calculation.

TABLE V. Prestress Forces & Cracking Loads

Prestress Prestress Test Obsvd. Compo Obs.Ckng.LoadBeam No. Force Calcu- Force From if alues Crkng. Crkng. Computed

lated From BPR Criteria BPR Load Load Cracking. Test Data Criteria Load

2-A I 40,700* 32,740 1.24 3620 3170 .142-B I 43,600* 32,760 1.33 3790 3170 1.192-A II 40,000* 32,740 1.22 3500 3080 1.132-B II 37,700 32,760 1.15 3360 3080 1.092-A III 41,500* 32,740 1.27 3780 3280 1.152-B III 47,700* 32,760 1.45 4000 3140 1.27'f: . .Calculated values result in hlgher prestresslng loads than the orig­inal tension force in the strand.

-':.:1

3. Computations of Ec Values from Deflection Curvesand Cylinder Tests .

Values of Young's Modulus are computed by using the average

deflection values below the cracking load and from test cylinders.

The Ec tabulated are the values obtained from the third cycle of

loading.

TABLE VI. Modulus of Elasticity

Ec Computed From Ec Computed Ec Deflection Values.Beam No. Deflection Values From Cylind- Ec Cylinder Tests I

er Tests

2-A 0 5.76 x 10 6 4.96 x 10 6 1.162-B 0 4.70 5.24 .898

2-A I 5.13 ,4.42 1.162-B I 5.13 4.46 1.15

2-A II 5 .. 89 4.69 1.252-B II 5.86 4.59 1.282-A III 6.01 4.81 1.25

2-B III 5 .. 78 5.52 1.05

4. Computation of Ultimate Load Using BPR Criteria

Table VII compares the ultimate load to the computed ultimate

load using BPR Criteria.

TABLE VII. Ultimate Load

Age Cylinder Ultimate ComputedI Ul timate

Beam No. at Strength Load Ult. Load ComputedTest At Test From BPR Ultimate

Criteria

2-A 0 32 6876 5000 5820 .86 -2-B 0 31 6545 5720 6040 .95

2-A I 32 5530 5350 5420 .99

2-B I 32 5540 5780 5560 1.04

2-A II 31 7430- 5920 5840 1.01

2-B II 32 I 7590 6120 6040 1.012-A III 29 7800 5910 5820 1.02

2-B III 29 6970 6190 6060 1.02

C. Slip

No slip was recorded on any of the eight beams at ultimate load.,

-10

D. Crack Pattern

The beams were whitewashed prior to testing so that the cracks

would be more detectable. Tne length of the cracks together with the

value of the load were marked on the beam. Later the cracks were

painted for photographic purposes. See Figure 13.

The spacing and sequence of the cracks were noted during the

test. The cracks are quite uniformly spaced and occurred "alternately

on either side of the centerline. The cracks were spaced appro~i­

mately 11 inches.

V •..

( a) Anchorage Length

CON C L U S ION S

-11

The curves representing the relative strains in the concrete

at and after release are shown in Figs. 1 to 6. It can be seen from

these curves that the anchorage length in the majority of cases is

approximately 27 ins. or 62 diameters for the particular time at

which these readings were taken.

(b) Effect of Concrete Strengths

The anchorage length does not vary appreciably over the range

of concre~e strengths~ namely 4120 psi to 8640 psi, used in these

tests. It can be seen from Fig. 7 that there is a slight increase

in beam stiffness with increasing concrete strengths. Table VI

shows an increase in Youngs Modulus of approximately 16% based on the

deflection in the prestressed beams.

(c) Prestress Forces

'rhe prestress forces, calculated on the basis of cracking load

and modulus of rupture, are compared to the prest~ess forces yalcu-

lated from BPR Criteria in Table V. The experimental values result

in higher values than given by the Criteria. Evidently the flexural

strength of the rupture modulus specimens was somewhat lower than the

flexural strength of the actual test members.

(d) Cracki~g & Ultimate Loads

Table V compares the observed cracking loads to the calculated

ones. The observed cracking loads are higher than the computed one~

and again the difference between the modulus rupture tests and actual

beam values is evident.

-12

CONCLUSIONS' (continued)

The actuai ultimate load compares very favorably with the

calculated ones o See Table VII.

(e) Slip

The cables did not slip at ultimate load.

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Fig o 11 Close-up of Jacking End

Fig o 12 Prestressing Frame & Beams Before Release

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Fig. 13 Beams After Test Showing Cracking Pattern

Fig. 14 Static Test Set-up

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Pig. 15 Slip I.1easurement During Static Test

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