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

International Journal of Civil, Mechanical and Energy Science (IJCMES) [Vol-2, Issue-1, Jan-Feb, 2016]
Infogain Publication (Infogainpublication.com) ISSN : 2455-5304
www.ijcmes.com Page | 36
Behavior of RC Beams Retrofitted/Strengthened
With External Post-Tension System
Gouda Ghanem1
, Sayed Abd El-Bakey2
, Tarek Ali3
, Sameh Yehia4
1
Prof. of Strength & Properties of Materials, Faculty of Engineering, Helwan University, Cairo, Egypt
Dean of Higher Institute of Engineering, Shorouk Academy, Cairo, Egypt
2
Prof. of Strength & Properties of Materials, Housing & Building National Research Centre, Cairo, Egypt
3
Prof. of Strength & Properties of Materials, Faculty of Engineering, Helwan University, Cairo, Egypt
4
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, GFRP
bars 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 built
in 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 of
prestressing 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 different
percentage (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

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

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 angles
on 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 special
epoxy 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

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 determine
deflection 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)

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)

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 of
Hydraulic 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 to
its 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)

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 theoretical
and 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 load
of 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 bars
at 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 of
tested 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.

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.
[2] 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
Tendons in Post-Tensioning", PTI Journal, Vol.1,
No. 3, pp. 22-29.
[4] V. J. FERRARI and J. B. DE HANAI,(2011),"
Influence of steel fibers on structural behavior of
beams strengthened with CFRP".
[5] Taiping Tang and Hamid
Saadatmanesh,(2005),"Analytical and Experimental
Studies of Fiber-Reinforced Polymer-Strengthened
Concrete Beams Under Impact Loading".
[6] Kyoung-Kyu Choi and Hong-Gun Park,(2010),"
Evaluation of Inelastic Deformation Capacity of
Beams Subjected to Cyclic Loading".
[7] Egyptian Code of Practice [ECP], 2007.

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