experimental evaluation of tendon stress in externally prestressed … · 2018. 10. 10. ·...

VOL. 13, NO. 18, SEPTEMBER 2018 ISSN 1819-6608

ARPN Journal of Engineering and Applied Sciences ©2006-2018 Asian Research Publishing Network (ARPN). All rights reserved.

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4840

EXPERIMENTAL EVALUATION OF TENDON STRESS IN EXTERNALLY

PRESTRESSED COMPOSITE ULTRA HIGH PERFORMANCE

CONCRETE-STEEL GIRDER

Abdul Mutlib I. Said

1 and Larah Riyadh Abdulwahed

2

1Department of Civil Engineering, College of Engineering, University of Baghdad, Baghdad, Republic of Iraq 2University of Baghdad, Baghdad, Iraq

E-Mail: [email protected]

ABSTRACT

Experimental programmed was carried out to investigate the influence of external prestress on composite steel I-

girder decks.This program included fabricating and testing twelve scales down 1/4, were designed according to AASHTO

LRFD 2012 standard specification. Each girder was test as simply supported with span of 3.90m and classified in six

groups. The first and second groups consist of two girders have a concrete compressive strength of 50 MPa. The third and

fourth groups consist of two girders of concrete compressive strength of 70MPa. The fifth and sixth groups consist of two

girders of concrete compressive strength of 90MPa.In all groups; the girder has straight eccentricity and deviator at the mid

span. The applied loaded incrementally up to failure under the action of two point loads for each increment of load. The

prestressing force in strand of diameter (12.7 mm) in the girders of group (1, 3and 5) was (9) Ton (stress equal to 918 MPa

= 0.493 fpu) applied after setting the superimposed dead load on RC deck slab, while in group (2,4and 6) was (7) Ton

(stress equal to 714 MPa = 0.384 fpu). The variables in the experimental investigation were the compressive strength of the

concrete (50, 70, and 90 MPa), level and path of the prestressing force and their paths straight or deviator (80, 120, 160 and

200 mm), with or without deviator and the magnitude of the applied prestressing force (7 Ton and 9Ton). The percentage

increase in stress in external prestress strand from ultimate stress in strand (fpu=1860 MPa) after applying two point loading

were rang from (0.16 to 0.39 fpu of strand).

Keyword: external prestress, composite section, compressive strength.

1. INTRODUCTION

The research work carried out in strengthening or

rehabilitation of existing structures is enormous and covers

various types of elements that are commonly used in

engineering construction [1] and [2]. External prestressing,

initially developed for bridges, is now becoming popular

and applicable for a variety of structural systems [3].

Batchelor and Setya (1971) [4] tested four composite

steel-concrete beams to investigate their general behavior

under static instantaneous and short-term sustained loads.

Three beams were respectively prestressed to 0.7, 0.8 and

0.9 of the specified yield stress of the external prestressing

strands, while the fourth beam was tested without external

prestressing to work as a reference beam. The effects of

the amount of prestressing force, shrinkage, creep and

cracking on the performance of these beams were

investigated. The tests revealed that higher flexural

stiffness and ultimate load capacity were obtained for a

composite steel-concrete beam prestressed with a higher

level of external prestressing force. Chen and Gu (2005)

[5] investigated the ultimate moment and incremental

tendon stress of steel-concrete composite beams

prestressed with external tendons under positive moment.

The load was exerted to the test specimen with a 500 kN

hydraulic jack by a loading beam. It was shown that the

external prestressing increased the yield load and the

ultimate resistance of the composite beams as well as

reducing the deflection at serviceability state. Mousa

(2015)[6] the research is an experimental and theoretical

investigation of using external prestressing technique in

strengthening an existing girder bridge. The experimental

program consists of twelve composite steel-concrete post-

tensioned model girders. Each girder was tested as simply

supported with a span of 3 m and loaded incrementally up

to failure under the action of two point loads. Results of

the experimental investigation showed appreciable

enhancement in the load carrying capacity of the

investigation externally prestressed model girders as

compared to that of the nonprestressed reference girder.

The girders of group one showed percentage increase in

the ultimate load of 7.14%, 36.90% and 45.24% for

prestress force 1 Ton, 3Ton and 5Ton respectively.

2. EXPERIMENTAL PROGRAM

2.1 Manufacturing of the models

The experimental tests were conducted on model

simply supported steel - concrete composite girders. All

models had similar dimensions and in fact they were

selected to be 1/4 scale of the prototype composite bridge

girder, the properties of the model girder section are given

in Table-1 considering three different cases. The

compressive strength 𝑓𝑐′of 50 MPa (Case1), 70 MPa (case

2) and 90 MPa (Case 3). Each model composite test girder

included Steel beam HEA 160 and concrete deck slab of

cross section 300mm x 55mm reinforced with welded wire

fabric WWF (gauge 150mm x 150mm and ϕ 6mm) and

two rows of ϕ 8mm diameter studs (height 40mm) spaced

80mm in transverse direction and the spaced in the

longitudinal direction from the end of the girder are at

50mm at 1200mm, 100mm at 750mm c/c as shown

Figure-1.

mailto:[email protected]




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Table-1. Properties of the model girders equivalent steel section.

Case I - Composite with 𝒇𝒄′=50, n=6

Ad2

(mm4)

d (mm)

C.G to NA

Iᵒ (mm

4)

A×Yb

(mm3)

Yb(mm)

centroid

to bottom

A

(mm2)

Size

(mm) Member

1070259.937

1573403.177

19289091.15

27.26236

-44.2376

-115.738

9720

1203052

9720

212400

61104

6480

147.5

76

4.5

1440

804

1440

160 × 9

6 × 134

160 × 9

section HEA160

Top flange

Web

Bottom flange

9658073.882 59.26236 693229.2 493625 179.5 2750 50 × 55

Slab concrete

equivalent to steel

300/n=50

31590828.15 1915721.2 773609 6434 Summation 𝑌𝑏̅̅̅̅ = 120.2376 mm, I NA= 33506549 mm

4, slab width = 300,n =

𝐸𝑠𝐸𝑐

Case II - Composite with𝒇𝒄′=70, n=5

Ad2

(mm4)

d (mm)

C.G to NA

Iᵒ (mm

4)

A×Yb

(mm3)

Yb(mm)

centroid

to bottom

A

(mm2)

Size

(mm) Member

735192.4764

1922897.641

20876079.07

22.59536

-48.9046

-120.405

9720

1203052

9720

212400

61104

6480

147.5

76

4.5

1440

804

1440

160 × 9

6 × 134

160 × 9

section HEA160

Top flange

Web

Bottom flange

9836156.298 54.59536 831875 592350 179.5 3300 60 × 55

Slab concrete

equivalent to steel

300/n=60

3337032549 2054367 872334 6984 Summation 𝑌�̅�=124.9046 mm, I NA= 35424692 mm

4,

Case III - Composite with𝒇𝒄′=90, n=4.5

Ad2

(mm4)

d (mm)

C.G to NA

Iᵒ (mm

4)

A×Yb

(mm3)

Yb(mm)

centroid

to bottom

A

(mm2)

Size

(mm) Member

568800.568

2142804.826

21830198.18

19.87462

-51.6254

-123.125

9720

1203052

9720

212400

61104

6480

147.5

76

4.5

1440

804

1440

160 × 9

6 × 134

160 × 9

section HEA160

Top flange

Web

Bottom flange

9865925.039 51.87462 924213.1 658100.9 179.5 3666.3 66.6×55

Slab concrete

equivalent to steel

300/n=66.6

34407728.61 2146705.1 938084.9 7350.3 Summation 𝑌�̅�=127.6254mm, INA = 36554434 mm

4




4842

Cross section

(b) Details and distribution of shear connector for Bridge model.

Figure-1. Model steel - concrete composite girder used in experimental tests.

2.2 Concrete mixes Three concrete mixes were designed and prepared

for constructing the reinforced concrete deck slab of the

composite model beams. Mix (1) was high strength

concrete while mix (2) was a high strength concrete and

mix (3) was ultra-high compressive strength concrete [7],

[8] and [9]. The mix properties of the constituents by

weight for these three mixes are listed in Table-2.

Table-2. Properties of the three types of concrete mixes used in deck slab.

Mix Type of

concrete

Mix properties (kg/m3) W/C

ratio

Viscocrete

5930* %

Steel fiber**

%

f'c (MPa)

cylinder

strength Cement Sand silica Water

1 High strength 685 1137 23.5 246.6 0.36 1.5 0.5 50

2 High strength 770 1140 115.5 208 0.27 2.5 0.5 70

3 Ultra-high

strength 925 1000 232 222 0.24 3.5 0.5 90

*percent of cement weight

**percent of mix volume




4843

2.3 Equivalent loads Before the application of the external prestressing

on the girders, an important criterion was satisfied to get

an exact simulation of the model composite girders with

the prototype composite girder. The equivalent loads were

applied on the model to induce the same longitudinal

bottom steel flange stress as that of the full-scale bridge

due to real self-weight and superimposed dead loads.

Concrete block450mm×300mm×100 will use as an

additional mass to satisfy the simulation requirement of

specific gravity loads. The equivalents (self-weight and

superimposed equal to3.2 kN/m) are used as addition

superimposed dead load which gives the same stress with

prototype.

2.4 Estimation prestress force Figure-2 shows a model of composite beam

subjected to externally longitudinal prestressing force

applied at a level 80 and 120 mm below its bottom flange

face represent the case of straight tendon with constant

eccentricity Figure-(2-a) represent the second case when

use the deviator at mid span at a level 160 and 200mm

below its bottom flange. Each of these two sections is of

0.45 m distance away from the nearer support. The

maximum value of the applied prestress force (Pr) in this

case from the allowable stress in concrete at top fiber at

mid span of composite beam which is calculated as

follows; (𝒏 × 𝒇𝒕= − 𝑷𝒓𝑨 + 𝑷𝒓×𝒆×𝑪𝑰 − 𝑴×𝑪𝑰 ). Tensile

stress 𝒇𝒕in the concrete is equal to 0.4√𝑓𝑐′ according to

ACI code [10] was allowed to induce in the concrete.

Eccentricity (e = 80 or 160mm + Yb') which lead that the

prestress force 𝑷𝒓 equal to (9 Ton) and for (e = 120 or

200mm+ Yb') the prestress force is (7 Ton).

a) without deviator b) with deviator

Figure-2. Location of the external prestress tendon.

2.5 Instrumentation and testing procedure

After the superimposed dead load was applied

(through the use of concrete blocks uniformly distributed

on the top of the model along its full length) the two point

loads were then applied on the model girder in successive

increments, up to failure. Thereafter the prestress tendon

was post–tensioned to the specified force required for the

test as shown in Figure-3. The concentrated load was

subjected on the test model girder specimen through a jack

of the testing machine as shown in Figure-4. After the

preparations were finished and the initial readings of the

dial gauges at mid–span and under point load were taken,

the load was applied with a loading increment rate of

about 5 kN.

Figure-3. Prestress post-tension.

Figure-4. Loading system in the testing machine.

3. RESULTS AND DISCUSSIONS

Table-3 gives a full detailed description of the

experimental results for all the tested composite girders

models of the present investigation.




4844

Table-3. Experimental test results of the tested models.

Group Symbol Designation

e @ distance

45cm from

end span

e @mid

span by

deviator

𝒇𝒄′ (MPa)

P(kN) ∆u mid span

(mm)

∆u Under point

load (mm)

1

G1 GP9S-e80 80 80

50

210 25.43 21.44

G2 GP9D-e160 80 160 230 26.53 22.68

2

G3 GP7S-e120 120 120 215 25.88 21067

G4 GP7D-e200 120 200 235 26.94 22.82

3

G5 GP9S-e80 80 80

70

260 29.87 25.81

G6 GP9D-e160 80 160 280 33.54 29.74

4

G7 GP7S-e120 120 120 265 30.19 26.68

G8 GP7D-e200 120 200 285 34.12 30.57

5

G9 GP9S-e80 80 80

90

300 43.04 37.42

G10 GP9D-e160 80 160 320 48.31 40.03

6

G11 GP7S-e120 120 120 305 43.97 35.98

G12 GP7D-e200 120 200 325 49.55 42.69

* P = Total applied on two point load (P = 2V)

G = Girder

P9 = the value of the external prestressing force was 9 Tons

P7= the value of the external prestressing force was 7 Tons

S = Strand of prestress represents as straight line

D =Strand of prestress represents by deviator

3.1 Forces and stresses in strands of group (1and 2)

fc' =50 MPa

This group 1 and 2 consists of the four steel -

concrete composite model girders G1, G2, G3and G4

which were designated as GP9S-e80, GP9D-e160, GP7S-

e120, and GP7D-e200 respectively. Each of which was

subjected to an external prestressing force prior to the

gradual application of two–point loading up to failure.

There are two stage of force generated in strand. The first

stage produced by the initial external prestress force of

(7or 9 Ton) and the second stage is generated when the

deflection occur due to the application of two point load.

Strain in strand has been determined by measuring the

reading of strain gauges due to the applied the loads as

shown in Figure-5. On the other hand, stresses and force in

strand are calculated. The load - force (stress) in strand for

these girders are shown in Figure-6.

Figure-5. Measurement of the strand.




4845

Figure-6. Increment force and stress in the strands after initial external prestress force.

It can be seen from Table-4 the initial force and stress in strand and percentage increases in ultimate force in strand

Table-4. Percentage increase in ultimate force in strand.

Percentage of

increment (Force)

Final stress

MPa

Final force

kN

Initial stress

MPa

Initial force

kN

fc'

Designation Symbol

46% 1339 131.21 918 90

50

GP9S-e80 G1

33% 1219 119.45 918 90 GP9D-e160 G2

69% 1209 118.45 714 70 GP7S-e120 G3

48% 1061 103.94 714 70 GP7D-e200 G4




4846


fc' =70 MPa

This group 3 and 4 consists of the four steel–concrete composite model girders G5, G6, G7and G8









The load – force (stress) in strand for these girders are

shown in Figure-7. It can be seen from Table-5 the initial

force and stress in strand and percentage increases in

ultimate force in strand.





4847


Percentage of increment (Force)

Final stress

MPa

Final force

kN

Initial stress

MPa

Initial force

kN

fc'

Designation Symbol

55% 1421 139.30 918 90

70

GP9S-e80 G5

41% 1299 127.31 918 90 GP9D-e160 G6

82% 1303 127.65 714 70 GP7S-e120 G7

63% 1165 114.13 714 70 GP7D-e200 G8


fc' =90 MPa

This group 5 and 6 consists of the four steel -

concrete composite model girders G9, G10, G11and G12









The load - force (stress) in strand for these girders are

shown in Figure-8. It can be seen from Table- 8 the initial

force and stress in strand and percentage increases in

ultimate force in strand.




4848



Percentage of

increment (Force)

Final stress

MPa

Final force

kN

Initial stress

MPa

Initial force

kN

fc'

Designation Symbol

74% 1597 156.51 918 90

90

GP9S-e80 G9

60% 1470 144.07 918 90 GP9D-e160 G10

102% 1448 141.94 714 70 GP7S-e120 G11

86% 1327 130.03 714 70 GP7D-e200 G12

4. CONCLUSIONS

a) The use of 8mm diameter, 40mm height, studs in two

rows along the full length of each tested simply

supported composite steel-concrete model girder was

found adequate to give full interaction between the

RC deck slab and the steel I-beam. This full

interaction was assured by the facts that no shear slip

was produced in the studs nor vertical separation

between the RC deck slab and the steel I-beam was

observed.

b) In composite steel-concrete modeled girders, no

longitudinal cracks were seen in the RC deck slab

during the whole stages of loading. This is because

the ratio of the projected length of the slab to its

thickness was small enough that prevented any action

in the deck slab.

c) The force and stress in prestress strand are increased

after subject the two point loading. Group 1, 3 and 5

were initial force and stress equal to (90 kN and 918

MPa). The final force in strand in group 1 was

reached (131.21 kN and 119.45kN), in group 3

(139.30kN and 127.31kN) and in group 5 (156.51kN

and 144.07kN).The actual stress in external prestress

strand after applied two point loading were (1339,

1219, 1421, 1299, 1597, 1470, MPa) for girders in

group 1, 3 and 5 respectively. The actual increase in

stress in external prestress strand after applied two

point loading were (421, 301, 503, 381, 679, 552

MPa) for girders in group 1, 3 and 5 respectively.

d) Group 2, 4 and 6 were initial force and stress equal to

(70 kN and 714 MPa). The final force in strand in

group 2 was reached (118.45kN and 103.94kN), in

group 4 (127.65kN and 114.13 kN) and in group 6

(141.94kN and 130.03kN). The actual stress in

external prestress strand after applied two point

loading were (1209, 1061, 1303, 1165, 1448, 1327,

MPa) for girders in group 2, 4 and 6 respectively.

The actual increase in stress in external prestress

strand after applied two point loading were (495, 347,

589, 451, 734, 613 MPa) for girders in group 2, 4 and

6 respectively.

e) The percentage increase in stress in external prestress

strand from ultimate stress in strand (fpu=1860 MPa)

after applied two point loading rang from (0.16 to

0.39 fpu of strand).

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4849

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[9] Graybeal B. and Hartman J. 2002. Ultra High

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[10] ACI 318M-08. 2008. Building Code Requirements for

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