Behavior of RC Beams Retrofitted/Strengthened With External Post-Tension System

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International Journal of Civil, Mechanical and Energy Science (IJCMES) [Vol-2, Issue-, Jan-!e", 2#$%

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Behavior of RC Beams Retrofitted/Strengthened

With External Post-Tension SystemGouda Ghanem

1, Sayed Abd El-Bakey

2, Tarek Ali

3, Sameh Yehia

4

1Prof. of Strength & Properties of Materials, Faculty of Engineering, Helwan University, Cairo, Egypt

Dean of Higher Institute of Engineering, Shorouk Academy, Cairo, Egypt2Prof. of Strength & Properties of Materials, Housing & Building National Research Centre, Cairo, Egypt

3Prof. of Strength & Properties of Materials, Faculty of Engineering, Helwan University, Cairo, Egypt4 Assistant Professor, Higher Institute of Engineering, El-Shorouk Academy, Cairo, Egypt

Abstract— This paper presents a study on the flexural

behavior of strengthened RC beams using external post-

tensioning technique under the effect of cyclic loads. Post

tensioning techniques is a new method to improve the

behavior of cracked and sound beams. This new

technique was used in this research to improve the

behavior of cracked and un-cracked beams. The study

consists of two stages, the first stage is an experimental

program which is carried out in lap to test casted beams,

and the second stage is a theoretical program which was

carried out to verify the results of experimental program.

The behavior of RC beams in different levels of cracks

was studied, crack pattern was observed and failure type

was recorded. Comparisons between the behaviors of

different RC beams were performed. The experimental

study included using of prestressing steel bars, GFRPbars and the effect of different percentage of shear

reinforcement was also taken into consideration.

Specimens were tested under the effect of cyclic load.

Finally the simulations of tested beams were modeled in

finite element software (ANSYS) to verify the results of

experimental work compared to theoretical analysis.

Keywords— Flexural Behavior, Strengthened RC Beam,

Post-Tensioning Technique, Cyclic Loads.

I.

INTRODUCTION

Nowadays, some of the concrete structures those are builtin the past years were inadequate to carry service loads.

This insufficient load carrying capacity has been resulted

from poor maintenance, increasing in legal load limit,

insufficient reinforcement, excessive deflections,

structural damages or steel corrosion, which leads to

cracks. Post-Tensioning techniques are one of a number

of methods used to improve the behavior of beams and

repair it to carry additional loads and enhance

serviceability limits; also new materials are developed to

enhance the performance of structural elements. Among

these materials, FRP are used as reinforcement bars for

different elements and can be used as surface treatment

technique. Cyclic loads have a critical effect on structural

element. It usually causes failure of structural elements at

early load stages. Considering these factors, the aim and

objective of this research were pointed out.

II.

HEADINGS

2.1. OBJECTIVE:

The scope of this research focused on the behavior of

failure mechanism of RC Beams retrofitted/strengthened

using outside steel and GFRP bars under effect of cyclic

loads. The study includes experimental and theoretical

work to verify the results with each other. Beams were

manufactured in a way to include the purposed

parameters, which are stated as follow:

a) Effect of using post tensioning technique on beams

behavior.

b)

Study the effect of using different reinforcement ofprestressing bars.

Study the effect of using different percentage of shear

reinforcement.

3.1. EXPERIMENTAL WORK PROGRAM:

Eight beams specimens were prepared with constant

percentage of steel reinforcement (2Y12 Bottom / 2Y10

Upper). GFRP bars are used with different percentage of

reinforcements (2Y10, 2Y12 and 2Y16) for external

prestressing bars were included. In additional to, beam

specimen with (2Y12) steel prestressing external bars.

The stirrups are mild steel and were used in differentpercentage (5R8/m – R8/m and 10R8/m). Constant

parameters, like compressive strength of concrete

(Fcu) = 250 kg/cm2, volume fraction of GFRP bars equal

0.6, cross-section of the beam specimen is 15 x 30 cm,

length of 230 cm and clear span equal to 210 cm were

selected. A trial beam specimen (not included in eight

beam specimens) firstly was casted to try our system and

to ensure the system performance. The outcome of testing

the trial beam was very beneficiation in directing the test

beams to the appropriate procedures. See Table (1) which

is shows the details of beam specimens. Also, See


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Figure (1) for typical details of beam specimen's

workshop drawings.

Table (1): Details of Beam Specimens

*Shear Reinforcement May Be Varies According to Beam

Specimen Code

**Prestressing Bars Installed Externally According to

Beam Specimen Code

Fig.1: Typical Details for Beam Specimen

(Dimensions in mm)

3.2. MANUFACTURING PROCEDURES OF

SPECIMENS:

Mixing process started and the time of mixing was 2

minutes. Casting specimens were made according to the

traditional process stated in code of practice ECP, see

Figures (2), (3), (4), (5), (6) and (7) which represents

specimens manufacturing.

Fig.2: Final Setup for Strain Gauge and Steel Cage

Fig.3: Steel Reinforcement Cage in Steel Form

Fig. 4: Specimen during Compacting

Fig.5: Final Casted Specimens

Fig.6: Specimen after Removing the Molds


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Fig.7: Curing of Specimens

3.3. INSTALLATION OF PRESTRESSING

SYSTEM:

Installation of prestressing system was carried in three

stages. The first stage is to mark four points on beam side

to install two angles for each side. The second stage is to

drill the marked points to pin four rivets to fix two angleson the two side of beam. The third stage is the final stage

in which the prestressing bar was installed in place on the

sides of beam. The following Figure (8) to Figure (11)

show the installing process.

Fig.8: Marked Points

Fig.9: Drilling Process

Fig.10: Drilled Points

Fig.11: Steel Angle Installation

After the four angles were installed, (Two Angles for

Each Side). The prestressing bar was glued with specialepoxy to steel hollow grips and finally fixed into angles

by nuts. A Strain gauge was fixed on the prestressing bar

to measure strain in bar to adjust the prestressing force.

By controlling rotation of the nut, the prestressing force

could be generated. It should be mention that prestressing

force was generated after loading beam specimens at level

of crack approximately 50% of the ultimate load.

Figure (12) and (13) present the installed angles on trial

beam specimen. Notable that trial beam specimen are

eight angles each of them are fixed back to back but other

beam specimens with four angles only.

Fig. 12: Strain Gauge Glued and Fixation Nuts


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Fig.13: Prestressing System on GFRP Prestressing Bar

3.4. TEST SETUP:

Beam specimens were tested using steel frame. Hydraulic

jack of 100-ton capacity, the load was measured using a

50 ton load cell. Also strain meter recorded stain in main

reinforcement and two LVDT used to determinedeflection of specimen at middle and middle third of

beam specimen, Figure (14) shows the details of the test

setup.

Fig. 14: Final Setup

3.5. TESTING STAGES:

After preparing and installing test setup for beam

specimens. Specimens were carried by crane to the main

frame to start the process of testing and the testing

process started. The rate of loading and testing process

was controlled by computer to reach certain load at

approximately 50% of ultimate load of control specimen

(A). Loading controlled by one unit of computer

(Hydraulic Jack). Deflection of beam measured at middle,

middle third of clear span of tested beam specimen and

strain in main steel bar was recorded. Strain in

prestressing external bars were recorded with strain

meter. Cracks were observed, detected and marked with

marker pen. Specimens tested as hinged-roller beam

(Simply Supported Beam). Tested beams are subjected to

effect of cyclic loads to reach certain degree of crack

approximately 50% of ultimate load of control specimen

(A) “This Level of Damage was Stacked for all Program”

according to the behavior of the control beam specimen

(A). After reaching the proposed load, the applied loads

were released, so that, the beam is carrying its own

weight only then prestressing system installed and

external prestressing bars was subjected to level of tensile

stress changes with respect to the bar diameter of

prestressing bar and applied with respect to strain in bar

Then the beam was reloaded under cyclic load until

failure. All other tested beams were tested successively.

4.1. RESULTS, ANALYSIS AND DISCUSSIONS:

The ultimate load of beam specimens tested in the

experimental work presented as follow in the shown

Table (2), which also, represents the total details of each

beam specimen and the ultimate Load of it. Figures from

(15) to (22) show the relationship between load and

middle deflection for tested specimens.

Table (2): Ultimate Load of Tested Specimens

Fig. 15: Relationship between Load and Middle

Deflection for Specimen (A)

Fig.16: Relationship between Load and Middle Deflection

for Specimen (B)


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Fig.17: Relationship between Load and Middle Deflection

for Specimen (C)


Deflection for Specimen (D)


Deflection for Specimen (E)


Deflection for Specimen (F)


Deflection for Specimen (G)


Deflection for Specimen (H)


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Loading process started at initial load equal zero then

cyclic loads were applied to specimen by two

concentrated loads. Specimen subjected to cyclic load up

to failure. Loading cycles approximately equal 14 cycles.

Hydraulic jack and loading process ended after the load of

specimen recorded negative values, which mean a huge

steep descending happened in relationship between

deflection and load. At the end of testing, the specimen

reached to failure and ultimate Load recorded as shown

above in Table (2). In all other specimens except

specimen (H), the level of crack taken at 50% of ultimate

load for control specimen (A) and that equal at

approximately 4.74 ton the system will be install but level

of crack in specimen (H) taken at zero% of ultimate load

for control specimen (A). The system of prestressing is

installed after releasing existing loads to zero. Also, it

seem that by increasing load (Downward Process ofHydraulic Jack) the deflection at midpoint of the

specimen increased. After releasing load (Upward Process

of The Hydraulic Jack) the specimen obtain its stiffness

and deflection reduced. The specimen in the first cycle

has stiffness more than other cycles because the specimen

in second cycle started with residual deflection in

comparison to first cycle and so on. Last cycles have a

crack width more than earlier as observed from crack

growth and propagation of crack pattern. Last cycles give

approximately the same ultimate load but more

deflections recorded, that mean the specimen reached toits critical state and failed. One can note that deflection at

middle third of specimen less than middle point of

specimen in all stages with ratio depends on specimen

type and that clear from the intervals between cycles of

deflection curve at middle third of specimen and middle

of specimen. Figure (23) to show the mode of failure at

crack pattern.

(A) Control

(B) With Steel Prestressing Bars - 2Y12

(C) With GFRP Prestressing Bars - 2Y10

(D) With GFRP Prestressing Bars - 2Y12

(E) With GFRP Prestressing Bars - 2Y16

(F) With GFRP Prestressing Bars - 2Y12 + Change in

Shear Reinforcement - R8/15 cm

(G) With GFRP Prestressing Bars - 2Y12 + Change in

Shear Reinforcement - R8/10 cm

(H) With GFRP Prestressing Bars + Strengthened at

Cracking Load Level Equal Zero

Fig. 23: Crack Pattern for Different Beam Specimens

(A),(B),(C),(D),(E),(F),(G) and (H)


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5.1. COMPUTER MODELING:

This section of discussion showed the comparison

between experimental and theoretical results. The analysis

was mode using the computer ANSYS program. The

difference between results obtained and found to be in

acceptance range. Figure (24), shows the relationships

between theoretical and experimental results in specified

parameter of ultimate load. The relationship gives general

view about ultimate load in theoretical and experimental

results.

Specimen Code, Ultimate Load (ton)

Fig. 24: Relationship between Experimental and

Theoretical Results in Parameter of Ultimate Load

Figure (25), shows the relationship between theoreticaland experimental results in specified parameter of middle

deflection in each specimen. It is seem from the figure

that the model estimate load deflection results by

accuracy about 90%.

Specimen Type, Middle Deflection (mm)

Fig. 25: Relationship between Experimental and

Theoretical Results in Parameter of Middle Deflection

III.

CONCLUSION

Based on the test results presented herein, the following

conclusions are drawn:

1-The post-tensioning techniques enhanced the

performance of cracked beams can restore and enhance

their capacities. At cracking load level equal 50% of

ultimate load of non-strengthened beam, the ultimate load

of strengthened beams with steel prestressing bars were

more than ultimate load of non-strengthened beam by 7%.

2-The post-tensioning technique using prestressing GFRP

bars recovered the value of ultimate load of non-

strengthened beam then gained ultimate capacity load

over that of non-strengthened beam by a range of 5 to

21%. The percentage of increasing load capacity depends

on level of stress in prestressing bars, by increasing level

of stress, the percentage of ultimate load increased. The

recorded percentage based on installing prestressing

system at cracking load level equal 50% of ultimate loadof non-strengthened beam.

3-Increasing shear reinforcement (stirrups) showed a little

significant effect on the behavior of studied beams. The

value of ultimate load of studied beams differs in the

range of 3%. This percentage was too small to be

effective but during testing of theses beams, by increasing

shear reinforcements (stirrups), the crack width reduced

for studied beams in the maximum shear zone.

4-The cracking load level for strengthened beams has a

significant effect on ultimate load of studied beams. The

beams strengthened with external GFRP prestressing barsat cracking load level equal zero% of ultimate load of

non-strengthened beam, gave ultimate load more than

beams strengthened at cracking load level equal 50% of

ultimate load of non-strengthened beam by 23%. It was

also noted that, beams strengthened at cracking load level

equal zero% of ultimate load of non-strengthened beam,

gave ultimate load more than non-strengthened beam by

36%. These calculated percentages were collected at the

same prestressing level.

5-Failure mode of beams with prestressing bars

characterized by banded cracks initiated at middle third oftested beams (pure bending moment zone). The cracks

propagated to nearby the top of strengthened beams. By

increasing load, the diagonal tension cracks appeared.

There is no yield or rupture observed for the external

prestressing bars for all the beams studied. It was also

noted that, the prestressing bars didn't reach their full

capacity of ultimate strength of bar. In general, the failure

happened firstly in concrete then the strain in main steel

reinforcement increased and lead to excessive cracks.


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REFERENCES

[1] Islam M. El-Habbal, Hany A. Abdalla and Ashraf H.

El-Zanaty, (2003),"STRENGTHENING OF

REINFORCED CONCRETE BEAMS USING

EXTERNAL PRESTRESSING". Tenth International

Colloquium on Structural and Geotechnical

Engineering, April 22-24, 2003, Ain Shams

University, Cairo, Egypt.

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K.-S. Choi, Y.-C. You, Y.-H. Park, J.-S. Park and K.-

H. Kim,(2005),"Behavior of RC Beams Strengthened

with Externally Post-Tensioning CFRP Strips".

[3]

A.Elrefai, J. West, and K. Soudki,(2003),"FRP

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V. J. FERRARI and J. B. DE HANAI,(2011),"

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Studies of Fiber-Reinforced Polymer-Strengthened

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[6] Kyoung-Kyu Choi and Hong-Gun Park,(2010),"

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Behavior of RC Beams Retrofitted/Strengthened With External Post-Tension System

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