7 ijcmes.pdf

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

1Prof. of Strength & Properties of Materials, Faculty of Engineering, Helwan University, Cairo, Egypt Dean of Higher Institute of Engineering, Shorouk Academy, Cairo, Egypt

2Prof. of Strength & Properties of Materials, Housing & Building National Research Centre, Cairo, Egypt 3Prof. 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. KeywordsFlexural 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)

Fig. 18: Relationship between Load and Middle Deflection for Specimen (D)

Fig. 19: Relationship between Load and Middle Deflection for Specimen (E)

Fig. 20: Relationship between Load and Middle Deflection for Specimen (F)

Fig. 21: Relationship between Load and Middle Deflection for Specimen (G)

Fig. 22: Relationship between Load and Middle 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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