Evaluation of Effective Prestress Force in 28-Year-Old Prestressed Concrete Bridge Beams

78

Stephen Pessiki, Ph.D. Associate Professor

Department of Civil and Environmental Engineering

Lehigh University Bethlehem, Pennsylvania

Mark Kaczinski, P.E. Research Engineer Center for Advanced Technology for Large Structural Systems Lehigh University Bethlehem, Pennsylvania

Herbert H. Wescott, P.E. Senior Engineer

L S Transit Systems, Inc. Ph iladelphia, Pennsylvania

Over the past four decades, a large number of

composite prestressed concrete beam/slab bridges

have been built in North America. To eva luate the

load rating of these bridges, an assumption must

be made co ncern ing the existing effective

prestress force. This assumption is difficult to

make because the effective prestress force is

influenced by several time-dependent phenomena

such as shrinkage and creep of the concrete and

relaxation of the prestressing strands. This paper

presents the results of an experimenta l study to

determine the effective prestress force in two full­

sca le prestressed concrete bridge beams that were

removed from a bridge structure after a period of

28 years in service. An average prestress loss of 7 8 percent was determined for the two specimens. This loss value is approximately 60 percent of the

loss predicted according to design specifications.

The use of prestressed concrete beams in highway bridges was first introduced in the United States in 1950 with the construction of the Walnut Lane

Memorial Bridge in Philadelphia, Pennsylvania. Over the past four decades, a large number of composite prestressed concrete beam/slab bridges have been built in North Amer­ica. Many of these structures have been in service for more than 25 years.

To evaluate the load rating of these bridges, an assump­tion must be made concerning the existing effective pre­stress force. This assumption is difficult to make because the effective prestress force is influenced by several time-

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dependent phenomena such as shrink­age and creep of the concrete and re­laxation of the prestressing strands.

This paper presents the results of an experimental study that was recently completed at the Center for Advanced Technology for Large Structural Sys­tems (ATLSS) at Lehigh University to determine the effective prestress force in two prestressed concrete 1-beams after approximately 28 years of service (Kaczinski, Wescott and Pessiki, 1994).' The two beams tested were Pennsylva­nia Department of Transportation (PennDOT) 24 x 60 in. (610 x 1524 mm) prestressed concrete I-beams with a span of 89 ft (27 . I m) and overall length of 90 ft 5 in. (27 .6 m).

The beams were removed from ser­vice on the dual seven-span Shenango River Bridge on Interstate 80 in Mer­cer County, Pennsylvania, and tested in the 5000 kip (22.2 MN) capacity universal testing machine at Lehigh University's Fritz Engineering Labo­ratory. The first specimen was a Type J interior beam from Span 3 and was marked 3-J. The second specimen was also a Type J interior beam from Span 3 of the adjacent structure and was marked 4-J.

Load tests were performed on each specimen to obtain the decompression load. Three independent techniques, using visual observation, strain gauges, and displacement transducers, were used to determine the decom­pression load in the bottom fiber of each beam. An average prestress loss of 60 percent was determined for the two specimens. These loss values are approximately 60 percent of the loss values predicted according to the AASHTO (1992)2 and PennDOT De­sign Manual 4 (1993)' specifications. Finally, once the decompression loads were determined , the beams were loaded to failure .

EXPERIMENTAL PROGRAM Presented below are details of the

beam specimens, test setup, test proce­dure and instrumentation.

Beam Details

Beam details are given in Fig. l. Each beam had the same nominal di­mensions and layout of non prestressed

November-December 1996

8" 8" 8"

24"

CENTER PORTION OF BEAM

24"

£ OF BEARINGS

4spa. @ 4"

4spa. @ 2"

Fig. 1. Beam details and center portion of beam and at centerline of bearings. Note: 1 in.= 25.4 mm.

and prestressed reinforcement. As shown in Fig. I , each beam was rein­forced with a total of 50 7/16 in. (11 mm) diameter strands. Of these, 36 strands were used in a straight pro­file, and 14 strands were harped at hold-down points located 14 ft (4 m) from each side of midspan.

Each beam was designed to act in service in composite action with an 8 in. (203 mm) thick concrete slab that spanned the 7 ft 11 5/s in. (2.4 m) center-to-center spacing of the girders in the bridge. The slab was removed during demolition of the bridge and removal of the girders. Because the primary purpose of the tests was the determination of prestress losses, each beam was tested without the slab present.

Test Setup

Both beam spec im ens were ar­ranged in the testing machine in the same manner. The instrumentation layout was also similar for both beams with the exception of the strain gauging at crack locations. Each beam was positioned in the 5000 kip (22.2 MN) capacity universal test ma­chine in a three-point loading config-

uration, as shown in Fig. 2. To ac­commodate translation and rotation at the reaction ends, the beams were supported at both ends on pedestals with roller pins and base plates.

Load was applied to the specimens from the test machine through a fixed roller and a full width distribution plate. Two steel frames were posi­tioned 11 ft 3 in. (3.4 m) from each side of the beam centerline to restrain the beam in the event of a sudden fail­ure. As an additional safety precau­tion, the top of each end of each beam was secured with cables to prevent the beam from toppling off the end bear­ing pedestal. Fig. 3 is a photograph of the test setup.

Before conducting any tests, a thor­ough inspection was made of the beams to identify any unusual condi­tions. No existing cracks or other signs of distress were noticed and both beams appeared to be in excellent con­dition prior to testing. In addition, each beam was surveyed prior to test­ing to measure the camber of the beam un~~,r its self-weight. The results of this -measurement indicated a positive midspan camber of 1.31 in . (33.3 mm) for Beam 3-J and 1.56 in. (39.6 mm) for Beam 4-J.

79

APPLIED LOAD

rPENNOOT 24" X 60" ~ BRACING ( TYP. l P/S I-BEAM

5' o" I I i i

I I I

n I I E ~

1 1 I 3" I I. 1 1 I

44 1 6" 44'

89' 0"

Fig. 2. Test configuration . Note: 1 ft = 0.305 m; 1 in. = 25.4 mm.

Test Procedure and Instrumentation

Each beam was tested in three sepa­rate phases. In the first phase of test­ing, load was applied to create and lo­cate a series of flexural cracks to instrument with strain gauges and dis­placement transducers. The second phase of testing was used to deterrrUne the decompression load in each beam, based on strain and displacement mea­surements of crack openings for the cracks identified and instrumented in the first phase of testing. Phase 3 of

Fig. 3. Test setup.

80

each test involved overload to failure. Note that the test setup (see Fig. 2) was unchanged through all phases of testing. The specifics of each test phase are described below.

Cracking Load Test - The meth­ods used to determine the decompres­sion load required that the beams be loaded to cause flexural cracking, and that the locations of these cracks be marked so they could be located and instrumented after load was removed from the beams. Before loading the beams to cause cracking, the beams were instrumented with eight 2 in. (51 mm) gauge length bonded metal foil strain gauges, as shown in Fig. 4. Four gauges were placed on each side to measure the strain distribution through the depth of the member. The strain gauges were placed on a section of the beam at a distance 1.5 times the beam depth or 90 in. (2.3 m) east or west of rrUdspan.

The beams were loaded at a rate of approximately 6 kips (26.7 kN) per minute in incremental steps to allow for visual inspection for cracks in the bottom flange near rrUdspan. When in­creasing to any new load level above 80 kips (356 kN), a 5 kip (22.2 kN) in­spection interval was typically used. For both beams, initial cracking was visually observed for the first time at a midspan load of approximately 145 kips (645 kN). The load was then in­creased to 155 kips (689 kN) and held constant while all cracks were identi­fied and marked.

A total of six load cycles, with peri­odic pauses to inspect for cracks, were

I u

~ 3"

6"

then conducted between 50 and 155 kips (222 and 689 kN) to ensure that all cracks were identified in the midspan region. After completion of the sixth load cycle, the beams were completely unloaded and then pre­pared for the decompression load test.

Decompression Load Test - Each beam was repeatedly loaded and un­loaded in a quasistatic manner in an attempt to determine the decompres­sion load for the bottom fiber of each beam. Three methods were used to de­termine the decompression load during each load cycle: (1) visual ob­servation of crack opening; (2) mea­surements of crack opening using dis­placement transducers (linear variable differential transformers, or L VDT) ; and (3) measurements of crack open­ing using strain gauges.

Several cracks located near midspan of each beam were instrumented with strain gauges as shown in Figs. 5 and 6. The instrumented cracks were de­noted as Cracks A, B, and C on Beam 3-J and Cracks A, Bl , B2, Cl , C2, D and E on Beam 4-J. The strain gauges were mounted on the surface of the concrete beams adjacent to the indi­cated crack and measured decompres­sion strains in the extreme fiber of the specimen as load was applied.

Typically, the gauges were mounted on the bottom surface of the beam ; however, at Cracks A and C 1 on Beam 4-J, the gauges were mounted on the side of the bottom flange as shown in Figs. 6(a) and (b) . Two strain gauges were applied at each lo­cation, one on each side of the crack

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't STRAIN GAUGES I i A~ ,·

i BEAM

I I I I ~ I

I I I I

::: -?- I I -?-0

I I <.0

j' I I T I I

l I

I I I

l A~ i. 90"

BEAM ELE VATION

1- · - N & S GAUGE #1

1- · - · - · N & S GAUGE#2

( s)

1- · - - - · N & S GAUGE#3

: ... :. ... . ~ .. . ·. ~ .. · ~ . : .. .

1- · - N & S GAUGE #4

SEC T ION A-A

Fig. 4. Locations of stra in gauges to measure strai n d istri but ion through depth of member. Note: 1 in . = 25. 4 mm.

and aligned along the longitudinal axis of the beam.

The decompression load was fo und by examining load vs. strain curves for each strain gauge pair. These curves typically exhibited a bilinear response. In the first stage of thi s bilinear re­sponse, an increase in load was ac­companied by a proportional increase in tensile strain (i.e., reduction in pre­compress io n). Durin g th e seco nd stage, an increase in applied load was no longer accompanied by a propor­tional increase in strain , as load was no longer transferred across the crack

November-December 1996

at the beam surface. The load at the transition fro m the firs t stage to the second stage was taken as the decom­pression load.

As noted in Fi g. 5 , stra in gauges with gauge lengths of 0 .25, 0 .5 and 2.0 in. (6.4 , 12.7 and 5 1 mm) were used to determine what effect, if any, gauge length had on measuring de­compression load during the test of Beam 3-J . It was fo und th at gauge length had no effect on the determina­tion of decompression load. For any particular crack that was instrumented, two gauges of different gauge lengths

gave similar results for the measured decompression load. Therefore, all of the crack strain gauges installed on Beam 4-J were 0.25 in . (6.4 mm) long.

Meas ure me nts of crack ope nin g with di splacement transducers were also used to determine the decompres­sion load. For Beam 3-J, a ±0.25 in . (±6.4 mm) stroke linear variable dif­fe re nti a l transfo rmer (L VDT) di s­pl acement tra nsducer was moun ted across a crack on the side of the bot­tom flange. The crack selected was the first noticed during the initial cracking phase and was also the largest crack

81

ct. CRACK 11 B II ____J I

t_ BEAM

I

I . . .J. I ,N

' GAUGE 2. OE~ 0

GAUGE 'j ..

z.ow~ 11

GAUGE 0. 5E I I ( E l

~ GAUGE 0.25W~ I ---

GAUGE I L GAUGE 0.25E .

~+----------;---~1

(a)

13/{ ~6 5/16 11+J---18 9/16 11 ------.I' ct. CRACK 11 C II~ ( s) ct. CRACK II A II~

BEAM

1 II L VDT

( E l

LVDT

BOTTOM OF BEAM L VDT MOUNTING ( TYP. l

CRACK

(b) NORTH ELEVATION OF BEAM

Fig. 5. Crack instrumentation on Beam 3-J: (a) beam bottom; (b) north elevation. Notation used in the fi gure is as follows: GAUGE 0.5E is a 0.5 in. (12.7 mm) gauge length strain gauge on the east side of the crack. Note: 1 in . = 25.4 mm.

and the one closest to midspan of the beam. The L VDT was mounted across the crack as shown in Fig. 5(b). A ±1 in. (±25.4 mrn) stroke L VDT was also mounted on the side of the bottom flange to measure crack openings dur­ing the ultimate load test.

For Beam 4-J , two ±0.25 in. (±6.4 mm) stroke L VDTs were mounted across cracks on the sides of the bot­tom flange. Cracks A and C L were in­strumented with L VDTs as shown in Figs. 6(b) and (c).

In a manner similar to the strain gauge method, the decompression load

82

was determined by studying the load vs. crack opening measurement plot. This plot typically showed an increas­ing amount of crack opening displace­ment per unit load after the crack began to open . The load that corresponds to this change in displacement rate was taken as the decompression load.

In each cycle of load , the beams were loaded at a rate of approximately 6 kips (26.7 kN) per minute to a maxi­mum midspan load of about 130 kips (578 kN). All transducers were moni­tored using a computer-based data ac­quisition system with samples being

saved at 10 kip (44.5 kN) or 5 kip (22.2 kN) intervals . A LO kip (44.5 kN) sampling interval was used when the midspan load was less than 50 kips (222 kN) and the 5 kip (22.2 kN) in­terval was used for loads between 50 and 130 kips (222 and 578 kN) . The beams were then unloaded to zero kips for the completion of Cycle 1 of the decompression study. Decompression Cycles 2, 3 and 4 were conducted fol­lowing similar procedures.

In addition to the instrumentation used to determine the decompression load, additional displacement trans-

PCI JOURNAL

t CRACK " E II

I

} BEAM ~CRACK " B1"

.t---•----- 3 2 1/{ --------,fl~- 1 6 3/ { ---,!'

! ! ~r---GAU GE E 1 tt-----r i~GAUGE W I GAUGE E1j I

I ' ' I GAUGE w ~ ~~ ~N ~tfGAUGE E S

~ - ~-~ .L·- · -·-·-r · - - - - - - - - - - - - --j~G~u~~-~- -f · ---1- -~ --~ ;:=- ::: j GAUGE E~ j =-

L i ~GAUGE E I j GAUGE W .

~ j ! GAUGE W j j I

(a)

( E l

(b)

(WJ

BOTTOM OF BEAM

(c)

L+l i i i

I I ->1'-+-- 1 4 II----+

I I I I I <t CRACK "C2 II~ i t_ CRACK

1'-l ---21 "----,tl 1 1 ~CRACK "o• <Sl

} BEAM

I I II j ['t /4 L VDT

·-r- · '[~ STRAIN GAUGES

t-+-2 " <t_ CRAC K "C1"

NORTH ELEVATION OF BEAM

4"

} BEAM

._.I_<t1/ 4" LYDT I

·- - '[~STRAIN GAUG~S

.f'----- 2 0. ---,!'

~ CRACK "A"

SOUTH ELEVATION OF BEAM

( w)

( E l

"B2"

<Wl

Fig. 6 . Crack instrumentation on Beam 4-j : (a) beam bottom; (b) north elevation; and (c) south elevation. Note: 1 in . = 25.4 mm.

November-December 1996 83

ducers were mounted on each side of the bottom flange at midspan to mea­sure midspan deflections. The average value of these two measurements was used in presentin g a ll deflection results.

Ultimate Strength Test- Upon completion of the decompression load test, each beam was loaded to failure using the same instrumentation and load configuration described above. For the purpose of this test program, failure was specified by PennDOT as the occurrence of any one of the fol­lowing events: (1 ) crack opening of 0.75 in. (19 mm) in the bottom of the beam; (2) deflection of the beam equal to L/150, where L = span length; or (3) failure of any one prestressing strand.

The initial rate of loading was ap­proximately 6 kip s (26 .7 kN) per minute, and data were sampled at 5 to 10 kip (22.2 to 44.5 kN) intervals up to approximately 165 kips (734 kN). As yielding and large deflections began to occur, data were samp led and saved at intervals of midspan de­flections of 0.1 in. (2.5 mm) for Beam 3-J and 0.2 in. (5.1 mm) for Beam 4-J. Both beams were continuously loaded to 236 and 238 kips (1 048 and 1057 kN), respective ly, until fai lure oc­curred due to a midspan deflection of L/150 or 7 .12 in. (181 mm) . After reaching this failure condition, the beams were unloaded to approximately 175 kips (778 kN) to mark the loca­tions of cracks on the beam surface.

Although significant flex ural shear cracking was visible on each beam, no sign of impending collapse was evi­dent and the deci sion was made to continue testing the specimens to fai l­ure . Load was again applied to the beam until peak resistances of 251 and 254 kips ( 1115 and 1128 kN) were reached for Beam 3-J and Beam 4-J, respectively.

Material Property Tests - After completion of the ultimate load tests, several 4 in. (102 mm) diameter 8 in. (203 mm) long core samples were re­moved from each beam to determine concrete material properties. The cores were removed from each beam web adjacent to its intersection with the top flan ge in uncracked areas. For each beam, five cores were taken for com­pressive strength tests, and three of the

84

c :.::; c "6

400 r-----.-----,-----,-----,-----,-----~1-----, I

Cr. A-2.0W 300 ---

Cr. A·2.0E

: (a) I I I I I I

I I I I I ~--------~-------~---- ----L-------J--------1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

: : : _.-~----...... I I I I

-~ 200 .s Cr. A-0.25W

--E-

I I -+ - - - -----1----1 I I I I I

I I - -+--------~--------+ --------

1 I I I I I I I I

c ·~ .... en

Cr. A·0 .25E 1 c__--,-__ _j i

I I I I I I

I I I I

100 I I I -1----------+--------1--- ------4--- - - ---

1 I I I I I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I I I I

0 ~~--~------~----~~------~~ ----~~~----~~ ----~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

400 -~--------------~----~----------~---,

300 Cr. B-0.5E

I I

I I I I I

--~--------~-------J ________ l...... .. ~--~f-~-~-~-----1 I I I I I I I I I I I

c: t t t l "=: Cr. B-2.0W 1 1 1 1

-~ t : t 0 I : : :

.~ 200 - --------~-------~--------~-- --~--------t--------~-------E l : : t l l

- 1 I I I I I I I I I I I I I I I I I ! I I I I I I I I I I I

c -~ ....

I I I I I I I I I

(/)

, oo - --------~---- -1--------r-------~--- - - ---+-------+-------, I I I I I I I I I I I

I I I I I

i : i I i lbl I I I I I I I I I I I I I I I I I I I I

0 ~-----+------~~ ----~~------+~ ------~~ ----~~----~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

400 -r---~-----,1 ----,-----~----~--~----~ I I I (c) I I I I I

300 _ _ Cr. C-0.5E _J--------~---- --- ~--------~--------l- -------1 I I I I I l I I I I I I I I I I I I I I I I I I I I I I I Cr. C-0.5W I I I I I

I I I I .5 e 1 : : :

.!::! 200 . --------1--------~--------~-------~---E i i i i

- I I I c

-~ .... en

I I I I I I I I I I I I I I I I I I I

I I I I I I

I I I , 00 --------1--------~ -- -------,- -------+-------~-------1 I I I I I I

1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

0 .=~--~------~----~----~~ ------~~----~~----~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

Fig. 7. Load vs. strain for decompress ion load Cycle 1 fo r Beam 3-j : (a) Crack A; (b) Crack B; and (c) Crack C. Note: 1 kip= 4.448 kN .

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400 --r-----~--~~--~----~----~----~1 ----~ I I : (a) I I I

I I I I I 320 ---~--------r-------~--------r-------,--------Cr. A - E : : : : :

I I I I I -- I I I I I I I I I

C : : : t

~ 240 - --· c_c_r._A~--w------' ___ ~-------+-------4--- ---+------~--------~ : I : : : u : I

-.E : :

I I I

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w : : I I I

80 - --------~---1 I

0 --~. ~--~--~----~--~----~--~--~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

400 -~----~----~----~----~----~----~~-----, I

~ .S 6

---~----1--------f-------j·----··--!-------l---:'' __ l;j I I I I I t I I I I I I I I I I I I I I I I I

.!:! 200 -.s t I I I I

-------~-------~--------~-------4------ -~-------1 I I I I I I I I I I I

c ·~ +J (/)

c ~ .s 6

I I I I I I I I I I I I I I I I I I I I I I

-------r-------~---1 I 100

I I I I I

0 -~. ~--~--~----~--~--~----~--~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

400 -.---~----~--~----~--~----~~ ---, I I

: (c ) I I I I I

300 - - Cr. D- E --~--------t-------+-------t--------~--------- 1 I I I I I I I I I I I I I I

Cr. O - W I I I I I I I I I I I I I I I

'---.-I -- : : : t : I I I I I

.!:i 200 -.s I I I I

-------~-------~--------~-------~--' I I I I I I I I I

c -~ +J (/)

I I I I I I I I I I I I I I I I I

1 00 - -------~------- I I I I I I

0 --~. ~--+---~----~--~----~----~--~ 0 20 40 60 80 100 120 140

Midspan Load (kips)

Fig. 8. Load vs. strain for decompression load Cycle 1 for Beam 4-J: (a) Crack A; (b) Crack B1 ; and (c) Crack D. Note: 1 kip= 4.448 kN .

November-December 1996

five cores for Beam 4-J were instru­mented to determine the modulus of elasticity of the concrete. Cores were also taken from Beam 4-J for split cylinder tests .

Compress ion tests conducted fol­lowing ASTM C394 test procedures res ul ted in compressive strengths of 8760 psi (60.4 MPa) for Beam 3-J and 8180 psi (56.4 MPa) for Beam 4-J . Shop drawings for the beams specified a 28-day design compressive strength of 5100 psi (35.2 MPa). The average compressive strength of 8440 psi (58.2 MPa) from the cores is 65 percent greater than the original de s ign strength. Modul us data taken during the compression testing of three cores from Beam 4-J indicated an average compressive modulus of elasticity of 4 ,945 ,000 ps i (34100 MPa). This value is I I percent greater than that predicted by the equation Ec = 57 ,ooo .ft: (psi) for normal weight concrete.

Split cy linder tests were conducted on five cores from Beam 4-J. These tests were conducted fo Uowing ASTM C4964 test procedures and resul ts indi­cate an average split cyli nder tensile strength of 760 psi (5.2 MPa). This value is 8.3 .[1Z (based on the com­pressive strength of the cores in psi) , which is consistent with the values of 6 to 8.[1Z given in theACI 318 Code.5

RESULTS AND OBSERVATIONS

Discussed below are observations on cracking and decompression loads, prestress losses and ultimate strength tests.

Cracking Loads

The first crack observed on Beam 3-J was located on the north side of the beam at an applied load of I 45 kips (645 kN) . Two cracks on each side of Beam 4-J were first observed at a load of 148 kips (658 kN). During the add itional cycles, cracks could be seen opening at applied loads of 110 to 115 kips ( 489 to 511 kN). The fact that the first cracks were not observed until approximately 145 kips (645 kN) during the first cycle suggests that the beams had remained uncracked during their 28 years of service.

85

Decompression loads Table 1. Summary of decompression loads for Beam 3-j .

Decompression loads (kips) Plots of applied load vs . strain for decompression Cycle 1 are presented in Fig. 7 for strain gauges installed ad­jacent to cracks on Beam 3-J. Fig. 8 plots selected pairs of results for Beam 4-J (results from other gauges on Beam 4-J are presented in Kaczinski etal. ').

Crack Gauge ~ Cycle 1 Cycle2 Cycle3 Cycle4 Average

In general, the strain gauges exhib­ited the predicted bilinear behavior and could, therefore, be used to deter­mine the decompression load. This was accomplished by fitting straight lines to the two distinct portions of the load-strain response, and the in­tersection of these lines was consid­ered the decompression load. An ex­ample of this technique is illustrated in Fig. 9.

In some cases, the change in slope was not well defined and extrapolat­ing the decompression load became more difficult. A summary of results for each strain gauge location and cycle of loading are presented in Ta­bles 1 and 2 for Beams 3-J and 4-J, respectively.

The decompression loads obtained from the strain gauge measurements were found to be repeatable and to vary by no more than 3 to 5 percent between load cycles for each strain gauge loca-

c ~ .!: 6

300 -

.!:i 200

.§ c -~

(i)

A 2.0W 84

A 2.0E 85

A 0.2SW 78

A 0.2SE 76 I B 0.2SE 92

B 2.0W 92 -

c O.SE 99

c O.SW 99

- LVDT 90 I Note: I kip = 4.448 kN.

tion. As noted earlier, the decompres­sion loads also appeared to be indepen­dent of strain gauge size. However, as was expected, the actual strains mea­sured at the decompression load are in­fluenced by strain gauge size.

Also included in Tables 1 and 2 are the decompression load results mea­sured by the 0.25 in. (6.4 mm) LVDTs mounted across cracks on the side of the bottom flange. These results were obtained by locating the point on the midspan load vs . crack opening dis­placement plot that deviates from lin­ear elastic behavior. The results of crack opening measurements at Crack

0 20 40 60 80 100 120 140 Midspan Load (kips)

Fig. 9. Example of determination of decompression load from the load-strain data. Resu lts shown here are for Beam 3-), Strain Gauge 2.0E at Crack A, Load Cycle 1. Note: 1 kip= 4.448 kN.

86

84

83

78

78

92

93

96

96

89

84 84 84 - -

83 83 84

78 78 78

78 78 78 I

92 92 92 ~.

93 93 93 --

96 98 97

97 97 97 -

113 113 -

A during Load Cycle 3 for Beam 4-J are presented in Fig. 10.

The distribution of strain through the beam depth [90 in. (2.3 m) west of midspan] is presented in Fig. 11 for loads of 50, 100 and 150 kips (222, 445 , and 667 kN) on Beam 3-J. Using these results, the neutral axis is found at approximately 27.5 in. (698 mm) from the bottom of the beam. This value is in close agreement with the theoretical value of 28.3 in. (719 mm) for an uncracked section .

Table 3 summarizes the average de­compression loads found by visual in­spection of crack opening (performed by unaided eye) and measurements of crack opening using strain gauges and displacement transducers. In this study it was found that the decompression loads obtained from the displacement transducer measurements were not as repeatable as the decompression loads obtained from the strain gauge mea­surements. This is shown in Tables land 2.

In addition, the decompression loads from the di splacement tran sducer measurements were generally higher than the decompression loads obtained from strain gauge measurements. Sim­ilarly, the decompression loads deter­mined from visual observation of crack opening were higher than those obtained from the strain gauge mea­surements.

In summary , the average decom­pression load values obtained from the strain gauge measurements were deemed the most consistent and reli­able, and were thus used in the loss calculations described below.

PCI JOURNAL

Table 2. Su mmary of decompress ion loads for Beam 4-j .

Decompression loads (kips) ' T

Crack Gauge Cycle I Cyde2 Cycle 3 Cycle4 Average

A East 92 I 93 90 90 91 - - ~

A West

I

97 I 92 90 91 92 - -- - - --

A LVDT - - 90 91 91 -

Bl East I

90 92 90 90 91

Bl West 88 88 87 89 88 - -

B2 East 104 106 105 105 105

B2 West 104 I l l 101 106 106 1- I

I -

Cl East 94 93 95 95 94 . - ..l C l West 96 91 93 95 94

Cl LVDT - - 11 3 117 115 -

C2 East - 80 82 81 ·K-= C2 West 85 85 84 83

D East 89 90 90 89 -- -

D West 89 83 85 85 I-

I -+- -

E East 93 92 I 92 92

j 1-· -

E West 96 96 I 96 96

Note: I kip= 4.448 kN.

150 -~------~----~------~------~------~

~ 100 -g "0

"' 0 ..J c

"' c. C/)

"0 ~ 50 -

I I I I I I I

I I I -----------+-----------t·----------t-----------: : I I I I

I I I I I I I I I I I I I I I I t I I I I t I I I I I I I I I I I 1 I I I I I I --------+-----------+-----------+-----------+-----------1 I I I I 1 I I I I I I I I I I I I I I I I I I I I I t I I I I I I I I t I I I I I I I I I I I I I I I I I I I

0 -~. ------~------~------+-------~----~ 0 0 .002 0 .004 0 .006

Crack Opening (in.) 0.008 0 .0 1

92

96

Fig. 10. Disp lacement measurement of crack opening fo r Beam 4-J, Crack A, Load Cycle 3. Note: 1 kip= 4.448 kN ; 1 in . = 25 .4 mm.

Table 3. Summary of average decompress ion loads found by v isual inspection, stra in gauge measurements and d isp lacement measurements.

Average decompression load (kips)

Specimen Visual inspection Strain gauge Displacement transducer --

Beam 3-J 110-11 5 88 I -

Beam 4-J 110-115 92 I 103

Note: I kip= 4.448 kN.

November-December 1996

Prestress Losses

The average decompression load at each crack location, the known beam section properties, and a simple elastic analysis were used to compute the ef­fect ive pres tressi ng fo rce in eac h beam. An example of the calculations is g iven in the Appendix. Using the decompression load values determined from the strain gauge measurements, average pres tress losses of 18. 1 and 17.2 percent were obtained for Beams 3-J and 4-J, respecti vely. Thus, the av­erage prestress loss for both beams is about 18 percent.

Calculations were performed to deter­mine the loss of prestress predicted by the AASHTO and PennDOT des ign specifications. These calculations, pre­sented in detail by Kaczinski et a!. ,' consist of both methods (Modi f ied Bureau of Public Roads and Lehigh M ethod) a ll owed in the PennDOT Design Manual Part 4 (1993) as well as the procedure suggested in the AASHTO Bridge Specifications (1992).

The results of these calculations pre­dict prestress losses of 29, 32 and 33 percent for the Modi fied Bureau of Public Roads, Lehigh, and AASHTO methods, respecti vely. Thus, the aver­age measured prestress loss of 18 per­cent is approx imately 60 percent of that predicted by each of the three de­sign code procedures. This difference between measured and code-predicted values is not unu sual and has bee n fo und by other researchers (Rabbat 1984,6 Shenoy and Frantz 199F) over the past decade.

Ult imate Strength Tests

Plots of load vs. midspan deflection for the ul ti mate load tests for Beams 3-J and 4-J are presented in Figs. 12 and 13, respectively. Beam 3-J was tested to an ultimate load of 251 kips (1115 kN). Failure occurred by crush­ing of the concrete in the compression zone under the load point. At the ulti ­mate load, the midspan deflection was 9.42 in . (239 mm), which corresponds to L/114, and crack opening displace­ments of less than 0.13 in. (3.3 mm) were measured by both the 0.25 and 1 in . (6.4 and 25 .4 mm) LVDTs. In­clined cracks in the web were spaced at approximately 12 in. (305 mm) and at midspan extended to within 12 in .

87

I I I

---; 50 - ______ j _____ _ .~ : - I QJ I

C> : ;ij 40 ______ j_ ______ l _______ L

u:: 1 t I E t 1 1 Q I I I

::: ~ : : I

.il 30 - ---1 N.A. @ 27.5 in . r ~----.-r----E I ' I I

E : : : I

@50 kip

-+-

@ 100 kip

-e-

~ 20 ------+------+------~-------t------~-- - --: (.) I I I I I I

~ ! I ! l l Ci) I I I I I

0 1 0 ------+------+------~-------t------~----- -t--1 I I I I I I I I I I I I I I I I I I I I I I I I I I

0 +. ----~~----~~----~~----~~----~~~--~----~--~ -500 -400 -300 -200 -100 0 100 200 300

Strain (micro-in./in.)

Fig. 11 . Distribution of stra in through the cross section for Beam 3-j at loads of 50, 100 and 150 kips (222, 445 and 667 kN ). Note: 1 kip= 4.448 kN; 1 in . = 25.4 mm.

300 -·r-----,~,-----.------.------.------1,-----, I I I I I I I I I I I I I

250 - ---------t---------f---------t---------t- __ L ________ _ I I I I : I I I I I I I I I I I I I I I

~ 200 - ---------+------- I --t---------t---------;g l : :

I I I I I I

I I I I I I

-- -·-------- --L-- -------- ..__--------1 I I I I I I I I I I I I I I

I I I I I I I I

--+---------+---------.f-----------L----------1 I I I I I I I I I I I I I I I I I I I

I I I I I I I I I I

------ ---------+---------+---------r---------+----------1 I I I I I I t I I t I I t I t t I I I t I I t t t t 1 t I

0 -~. --~~~------~~ ------~~ ------~~ ----~~----~ 0 2 4 6 8 10 12

Midspan Def lection (in .)

Fi g. 12. Beam 3-J load vs. midspan deflection for loading to failure. Note: 1 kip= 4.448 kN; 1 in . = 25.4 mm.

300 -r-----~----~------~-----;! -----;------, :

I t I I

250 - ---------+---------+---------1---------J--1 I I t t I I I 1 1 I I I

: : : : 8. 200 - ---------+---------I I ------4---- ----g : : ~ ""C

"' I I I I I I t I 1 I

I I I I .3 150 _________ ,.. _________ -+----------!----------+---- ----1 I 1 I I

c: : l 1 : : ~ : 1 J : I Cl) I I I I I

~ 1 00 - ----- ---+---------+---------1---------1---------1---- ----1 I I I I I I I I I I I I 1 I I I I I I 1 I I I I

50 - -- ------+---------1---------1---------1---------~---- ----1 1 I I I I I 1 I I I I I I I I I I I I I I I I I

0 ~. ------~~ ----~~------~~ ------~~ ------~~ --L-~ 0 2 4 6 8 10 12

Midspan Deflection (in.)

Fig. 13. Beam 4-j load vs. midspan deflection for loading to failure. Note: 1 kip= 4.448 kN ; 1 in .= 25.4 mm.

88

(305 mm) of the top of the beam be­fore becoming nearly horizontal.

Beam 4-J was tested to an ultimate load of 254 kips (1128 kN) and a mid span deflection of 9.5 in . (241 mm) , which corresponds to L/112 . After reaching thi s peak resistance, loading was continued until a midspan deflection of 10 in . (254 mm) was measured. At the 10 in. (254 rnm) de­flection , the di splacement transducers were out of range and the load was re­duced to 180 kips (801 kN) to remove these instruments.

Upon reloading, the head travel of the testing machine was used to mea­sure midspan deflections. At a deflec­tion of 10.9 in. (277 rnm) or L/90, and a load of 242 kips (1075 kN) , Beam 4-J failed due to a full-depth diagonal tension crack near midspan. Prior to failure, cracks in the beam web had extended to within 12 in. (305 rnm) of the top flange and concrete in the top flange began spalling in a manner sim­ilar to the failw·e of Beam 3-J.

At fai lure, crack opening displace­ments of 0.08 and 0.10 in. (2.0 and 2.5 mm) were measured by the 0 .25 in. (6.4 mm) LVDTs on Cracks A and Cl. A detailed examination of the failed region revealed that none of the prestressing strands had failed; how­ever, several stirrups were fractured.

Based on the concrete compressive strength determined from coring and the experimentally determined loss values, the predicted shear and mo­ment capacities of the non-composite 24 x 60 in . (6 10 x 1524 mm) pre­stressed beam using AASHTO proce­dures are 162 kips (720 kN) and 6029 kip-ft (8 175 kN-m), re spectively. These values are in close agreement with the average maximum shear force and bending moment of 166 kips (737 kN) and 6492 kip-ft (8803 kN-m) ap­plied to each beam (including both self-weight and applied load).

In considering the ultimate strength of each beam, it is noted that neither beam was reinforced for shear for a concentrated load at midspan, and that the beams were tested without the cast-in-place slabs that would have provided a larger compression flange. The focus of this study, however, was the determination of prestress losses and not an evaluation of strength.

PCI JOURNAL

CONCLUSIONS The following conclusions are based

on the results of this study: 1. A visual inspection of each beam

in the laboratory revealed members that were in excellent physical condi­tion with no indication that cracking had occurred in service . Testing seemed to confirm that each beam had remained uncracked in service.

2. Based on the test results, an aver­age prestress loss of 18 percent was determined for the two specimens. Predicted prestress losses of 29, 32 and 33 percent were computed by the Modified Bureau of Public Roads, Lehigh, and AASHTO methods, re­spectively. Thus, the experimentally determined losses were less than the predicted losses. Specifically, the av­erage experimentally determined pre­stress is approximately 60 percent of that predicted by each of the three de­sign code procedures.

3. Three independent techniques were used to experimentally determine decompression load in each beam: (a) visual observation of crack opening; (b) measurements of crack opening using displacement transducers; and (c) measurements of crack opening using strain gauges. The use of strain gauges seemed to produce the most re-

peatable and reliable results. In using this method, it is suggested that a min­imum of three to five cracks be instru­mented to account for the scatter that was observed.

4. In this study, it was found that determining the decompression load by visually observing crack reopening will generally provide unconservative results. The minimum load at which crack opening was visually observed was 110 kips (489 kN), which would correspond to a prestress loss of ap­proximately 3 percent. This overesti­mation of decompression load would result in lower than actual prestress losses and unconservative predictions of flexural capacity.

ACKNOWLEDGMENT The work reported in this paper was

supported by the Pennsylvania Depart­ment of Transportation (PennDOT). The authors gratefully acknowledge the support of PennDOT and the assistance provided by several individuals in the conduct of this work: Jennifer Murphy, Frank Stokes, and Charles Hittinger. Any opinions, findings, and conclu­sions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsor or the individuals acknowledged above.

REFERENCES I. Kaczinski, M. R., Wescott, H. H., and

Pessiki, S., "Decompression and Ulti­mate Load Tests of 28-Year-Old Pre­stressed Concrete Bridge Beams," ATLSS Report No. 94 .CTI03 3. 1, Center for Advanced Technology for Large Structural Systems, September 1994, 56 pp.

2. AASHTO, Standard Specifications for Highway Bridges, Fifteenth Edition, American Association of State High­way and Transportation Officials, Washington, D.C., 1992.

3. PennDOT, "Design Manual Part 4 -Structures," Pennsylvania Department of Transportation , Harrisburg, PA, August 1993.

4. ASTM, Annual Book of ASTM Stan­dards, American Society for Testing and Material s, Philadelphia , PA, 1996.

5. ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-95 )," American Concrete Institute, Farmington Hills, MI , 1995.

6. Rabbat, B. G., "25-Year-Old Pre­stressed Concrete Bridge Girders Tested," PCI JOURNAL, V. 29, No. 1, January-February 1984, pp. 177-179.

7. Shenoy, C. V ., and Frantz, G. C., "Structural Tests of 27-Year-Old Pre­stressed Concrete Bridge Beams," PCI JOURNAL, V. 36, No. 5, September­October 1991, pp. 80-90.

APPENDIX- DETERMINATION OF EFFECTIVE PRESTRESS FORCE AND PRESTRESS LOSS

This numerical design example shows how the effective prestress force and prestress loss can be calculated. The example is based on the data from Beam 3-J, Strain Gauge 2.0E, Crack A, Load Cycle 1 (see Fig. 9). Pdec = 85 kips (378 kN) =applied load

at decompression (see Fig. 9) x = 42.953 ft (13.1 m) = distance

from crack position to nearest support [see Fig. 5(a)]

wd1 = 0.883 kips per ft (1240 kg/m) = uniform self-weight of beam

A = 848 in.2 (0.55 m2) = cross-sec­

tional area of beam lx = 355800 in.4 (148.1 X 109 mm4

)

= strong axis moment of inertia e = 22.7 in. (576 mm) = eccentric­

ity of prestress force r = 20.5 in. (520 mm) = radius of

gyration

November-December 1996

Yr = 28.3 in. (719 mm) = distance from centroid to crack location. For cracks on the bottom sur­face of the beam, this value is 28.3 in. (719 mm)

L = 89ft (27.1 m) =beam span

Beam self-weight moment at the po­sition along the span where the crack is located, Md1:

Mdl = 'lz[wd1x(L-x)] = 873 kip-ft (1183 kN-m)

(A1)

Moment at the position along the span where the crack is located due to applied decompression load Pdec:

Mdec = 1/z(Pdec X) = 1826 kip-ft (2476 kN-m)

(A2)

The total moment at the position along the span where the crack is lo­cated, M 10ta(

Mtotal = (Mdl + Mdec) = 2699 kip-ft (3660 kN-m)

(A3)

For an effective prestress force of Pe, the stress at the crack location, f, is computed as:

f= -(P /A)[ I + (ey1)1r] + (M1y1)1lx (A4)

At decompression, f = 0, and Eq. (A4) is solved for the effective pre­stress force P, = 863 kips (3838 kN). The initial prestress force P; was 1085 kips (4826 kN), so the percent loss is:

Prestress Loss= [(P;- Pe)IP;]lOO = 20.5 percent

89