strengthening behavior of reinforced concrete t-beams using external

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Available online at http://arjournal.org APPLIED RESEARCH JOURNAL RESEARCH ARTICLE ISSN: 2423-4796 Applied Research Journal

Vol.2, Issue, 11, pp.433-439, November, 2016

STRENGTHENING BEHAVIOR OF REINFORCED CONCRETE T-BEAMS USING EXTERNAL PRESTRESSED TENDONS AbdulMuttalib I. Said, * Ali Hussein Ali Al-Ahmed and Dhafer M. Al-Fendawy Department of Civil Engineering, College of Engineering, Baghdad University, Iraq.

ARTICLE INFO

ABSTRACT

Article History:

The Experimental investigations of strengthening reinforced concrete Tbeams by external prestressed tendons are presented. The tested beams are partially strengthening span instead of strengthening whole span technique as convention. The test variable is chosen to study the effects of tendons depth levels relative to beam top fiber expressed as depth ratio (eccentricity) which is equal to depth of the strand to height of the section. Therefore, this study has been conducted to show the ability of using partial span strengthening and the effectiveness of increasing depth ratio of the external strands. Four identical reinforced concrete T-beams were tested up to failure. One of these beams (without strengthening) was used as a reference or control beam. While, the other three beams were partially strengthened (only 83% of the effective length) with two tendons. Tested beams were strengthening by fixed tendons in variable level and configuration by jacking at a constant stress equal to 600 MPa. All beams were tested up to failure by applying two concentrated loads at the third points. The test results show the ability of using partially strengthening technique to improve and enhance an existing beam. The important parameters that affect the behavior, such as failure mode, crack pattern, strain profile, and deflection at mid span are evaluated and discussed. Generally, test results shows that the deflection under service load decreases and the load capacity of the strengthened beams increase with increasing in the depth ratio of external strands.

Received: 15, October, 2016 Final Accepted: 27, November, 2016 Published Online: 29, December, 2016 Key words:

Deflection, Experimental, External, Prestressed, Strengthening, T-Beams.

© Copy Right, ARJ, 2016. All rights reserved

1. INTRODUCTION Repairing and strengthening of deteriorated, damaged and substandard infrastructure had become one of the important challenges confronting civil engineers worldwide. Some beams could not fix the anchorage system with ends of beams because of abutment of wall, columns or neighbor structures [1]. So that, this study have been shown the effect and ability of using partially strengthening technique for strengthening. The effect of increase eccentricity or depth ratio of external strands as considered in the present study has been reported herein. The strengthening operation depends on which part of the structure needs to be enhanced. The stresses which cause weakness of some parts of a structure are due to probably bending moment, shear, or in some times due to torsion. So, the strengthening may be for the flexural, diagonal stresses and twist requirements. In the present study, only flexural strengthening is considered. Prestressing is the deliberate creation of permanent internal member in a structure or system in order to improve its performance [2]. External prestressing is possible both to reduce the deflection of the structure and its crack widths under service loads and to increase its load-carrying capacity. All these goals are achieved with a limited intervention that does not overload the structure. Together with the bending moment, the external tendons also impose an axial force. This axial force increases the shear strength of structure. The stress in the external tendons can grow up to the yielding value when the member reaches to ultimate strength [3]. *Corresponding author: Ali Hussein Ali Al-Ahmed, Email: [email protected] Department of civil engineering, University of Baghdad, Baghdad, Iraq.

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2. EXPERIMENTAL PROGRAM 2.1. General The flexural behavior of reinforced concrete (RC) T-beams strengthened with external prestressed tendons is investigated. The objective of the experimental investigation was to examine effectiveness and feasibility of strengthening using external post-tensioning technique to increase flexural capacity and improve serviceability of RC beams. Effectiveness of locally strengthened beams with external prestressed tendons have been investigated in terms of strengthening ratio which equal to length of strengthening region (length of tendon or distance between anchors) divided by length of the beam. Effects of tendons depth levels relative to beam top fiber expressed as depth ratio, which equal to depth of the strand to height of the section. Finally, effect of strand configurations had been investigated also. 2.2. Details of Specimens Four RC T-beam having effective length equal to (3000 mm) and overall length of (3200 mm) were casted with cross sectional dimensions of hf =75 mm, bf=350 mm, bw= 150 mm and h=250 mm. One of them is considered as control beam (Reference beam) without strengthening while; the other three beams were partially strengthened by using two external tendons. The anchor points were fixed far from the ends by 250 mm so that the strengthening span ratio is equal to 0.83 for all strengthened beams. The first beam denoted as AT-1 has depth ratio equal to 0.8h and straight tendons profile as shown in Fig. 1. The second beam denoted as BT-1 has depth ratio equal to 1.0h and straight tendons profile as shown in Fig. 2. Finally, the third one denoted as DT-1 has depth ratio equal to 0.8h at anchors points and 1.2h at mid span so the tendons profile was draped as shown in Fig. 3. Beams reinforcements have been designed according to ACI Code as under-reinforced section i.e. (ρw ˂ρmax) [4]. 2ϕ12 mm deformed bars was used for longitudinal reinforcements in tension with effective depth of 220 mm (510 MPa yield stress). 4ϕ10 mm deformed bars was used for reinforcement in compression with a depth of 30mm (580 MPa yield stress). Stirrups of ϕ8@100 mm deformed bars (540 MPa yield stress) have been used in all beams. Cubic compressive strength of concrete at 28 days was 29.7 MPa for control beam and for AT-1 beam. While the cubic compressive strength of concrete at 28 day are 29.21 and 27.54 MPa for beams BT-1 and DT-1 respectively. 250

3200 2500

a

250

b

100 200

a

deviator

Ø8mm@100mm

AT-1

b

3000

Ø8mm@100mm

4Ø1 0mm

100

4Ø10mm

100 200

200 250

250

100

Strands anchor system

100 230

2 Ø12mm

a-a

Strands

230

2 Ø12mm

b-b

deviator

Figure 1 Geometrical properties of beam AT-1 (All dimensions are in mm). 250

3200 2500

a

250

b

250 150

a deviator

BT-1

b

3000 2000

Ø8mm@100mm

Ø8mm@100mm

4Ø10mm

100

4Ø10mm

100 250

250 250

150

250

150

2 Ø12mm

Strands anchor system

230

a-a

Strands

deviator

2 Ø12mm

230

b-b

Figure 2 Geometrical properties of beam BT-1 (All dimensions are in mm).

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AbdulMuttalib I. Said et. al.

Figure 3 Geometrical properties of beam DT-1 (All dimensions are in mm).

2.3. Jacking Process Two stress-relieved strands were used by fixing into two sides of the web of the strengthening beams. Diameter of strand was 12.7 mm (Grade 1860MPa, 270 ksi). Fixing strands on the T-beams was used by manufactured equipment’s from steel plates in each ends of the two strands. Fixing equipment anchored on two sides of concrete beams by drilling holes and used four steel bolts having diameter of 16 mm. In addition, to ensure the cohesive, epoxy adhesive was used to fill the space between hole and bolts as shown in Fig. 4. To reduce the second order effect on the behavior of external prestressed beams, one deviator was used in mid span of all strengthened beams. Prestressing techniques conducted by install two strands on each side in a symmetrical manner to avoid any lateral deformation, which is avoided by jacking two strands simultaneously by using balance device. ACI limitation [5] for tension stresses in top fiber of beam was most critical during initial jacking and has led to jacking stresses of 1700 MPa, 810 MPa and 600 MPa for Beams AT-1, BT-1 and DT-1 respectively. As jacking stress is not a parameter during this experimental work and it is intended to be similar for strengthening beams, then it has been determined from the lower bound of Beam DT-1 (600 MPa). Fig. 5 shows jacking technique.

Figure 4 Fixing technique.

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Figure 5 Jacking techniques.

2.4. Testing Equipment Equipment such as a hydraulic jack is used for post-tensioning process, and loading frame for applying loads. Strain tools have been used as measurement instruments, feeler gauge for crack width measuring, and finally a dial gauge for recording the deflection at mid span of tested beams. All beams were tested under a static two-point load. A loading rate was of 5 kN/min. The testing frame and load arrangement is as shown in Fig. 6. P additional plate Th. 10 mm steel plate thickness 25 mm

1000

1000

Dial Gauge

1000 support

3000 3200

Figure 6 Testing frame.

3. TEST RESULTS This section will be discussing the results obtained from the experimental work. Main variables were the depth ratio for straight and draped tendons for beams have been strengthening partially along the span by (0.83%). The results have been compared in order to determine the significance of the under consideration variables. 3.1. Failure Mode The mode failure was predicted by measuring concrete strains at mid-span section of beams, fitting with straight lines and drawn for yield and ultimate conditions. Stresses in the ordinary reinforcement were


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calculated as strains at steel level, (ds), multiplied by steel modulus of elasticity (Es) but not greater than yield stress. Table 1 is lists the results of experimental test. Failure load capacity for control beam was equal to 63 kN when ordinary reinforcement has been yielded at section in pure bending moment zone. While the partially strengthening beam, AT-1 and BT-1, are failed at load capacity equal to 112 kN and 133 kN respectively. The failure modes have been formed when plastic hinges generated under points load. Finally, load capacity for strengthening beam, DT-1, which strengthening by used draped strands is equal to 140 kN. In this case failure has been happened when concrete crushed at top fiber of concrete. The results show that an increasing in strands depth ratio led to increases the load capacity of the strengthened beams by 77.8%, 111.1%, and 122.2% as compared to control beam due to increased depth of neutral axis or eccentricity at mid span section. Table 1 Experimental result (failure loads and strain values) Beam Designation

Control beam AT-1 BT-1 DT-1

Depth ratio and strand profile

P (failure)(kN)

---0.8h, S 1.0h, S 0.8h-1.0h, D

Failure mode

% Increasing in ultimate load

ɛc at U (mm/mm)

ɛs at U (mm/mm)

Y Y Y C

---77.8 111.1 122.2

-0.0007 -0.0023 -0.0024 -0.0027

0.0027 0.0027 0.003 0.0022

63 112 133 140

Y: yielding in ordinary reinforcement, C: crushing concrete at top fiber, h: depth of beam, S: straight tendons, D: draped tendons, U: ultimate stage of loading, ɛc: concrete strain at top fiber, ɛs: strain in ordinary bottom reinforcement.

3.2. Load-Deflection Response A reinforced concrete beam subjected to flexure should have adequate stiffness to limit deflections that adversely affect strength or serviceability of structure. Table 24.2.2 of ACI Code [5] limits the maximum permissible deflections to l/180 (Ds2), or l/360 (Ds1) for immediate deflection due to live loads. The deflection measured with a dial gage at mid-span section reflects the behavior of strengthened beam and changes for different depth ratio. Increasing depth ratio for strengthening beams AT-1 and BT-1 or using draped for strengthening beam DT-1 have effects on service deflection load as listed in Table. 2. Deflection decreases with increases in the depth ratio as presented in Fig. 7. This may be due to apply counteract upward load at mid-span section, and this ratio is larger for draped tendons.

Ds1=8.33 mm

140 112 Load KN

Ds2=16.67 mm

168

84 56

Control Beam AT-1

28

BT-1 DT-1

0 0

10

20

30

40

50

Deflection mm Figure 7 Load-mid span deflection curves of tested beams.

Beam Designation Control beam AT-1 BT-1 DT-1

Table 2 Experimental results (service deflection loads) Depth ratio and % Increasing in P1(kN) P2(kN) strand profile P1 ---21 ---49 0.8h, S 70 233.3 98 1.0h, S 77 266.7 105 0.8h-1.0h, D 84 300.0 112

% Increasing in P2 ---100.0 114.3 128.6

S: straight tendons, D: draped tendons, P1: corresponding load to service deflection Ds1, P2: corresponding load to service deflection Ds2

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3.3. Increment of Strand Stresses Strain in strands has been determined by measuring the displacement between two demec points dividing by the distance between them. On the other hand, increment stress in the strands within the elastic range are calculated by (Δfps =Δɛps ×Eps), where Δɛps and Eps represents the increment of strand strain and strands modulus of elasticity respectively. Increments of strands stresses above initial stress (600MPa) are illustrated in Fig. 8. From this figure it could be noticed that the initial portion of the curve is slightly inclined. Curve becomes more and more inclined with crack progress and approaching a horizontal asymptote at ultimate stage of loading. Stresses in the strand, activity of the strand and resulting counteracting moments are proportional to the depth ratio. Increment stresses in strengthening beam DT-1 (draped strands) is greater than that in strengthening beams of AT-1 and BT-1 (straight strands). Increment strand stresses measured experimentally and predicated analytically according to 20.3.2.4.1 of ACI Code [5] are presented in Table 3. 168 140

LOAD KN

112 84 56 AT-1

28

BT-1 DT-1

0 0

200

400

600

800

STRESS of strand MPa

Figure 8 Increment stress in strands after initial stress.

Beam Designation

Table 3 Stresses developed in strands fps=fpe+∆fps ∆fps(MPa) ∆fps(MPa) at Y at U fps at Y fps at U


AT-1 BT-1 DT-1

0.8h, S 1.0h, S 0.8h-1.2h, D

293 466 568

397 590 707

893 1066 1168

996 1190 1307

fps (MPa) ACI Code Table(20.3.2.4.1) 786 818 830

Y: yielding in ordinary reinforcement, U: ultimate stage of loading, h: depth of beam, S: straight tendons, D: draped tendons, fps: Stress in tendons after initial stage, fpe: initial jacking stress (600 MPa).

3.4. Cracks Maximum tolerable crack width depends on structure function and surrounding environmental conditions. ACI Committee 423.3R [6] considers service crack as a reasonable guide to accept crack widths in concrete structures under various exterior exposure, humidity, moist air, and soil conditions. Allowable crack width or service crack is either equal to or less than 0.31mm [6]. Table .4 illustrates the first crack load, service cracks load, and the percentage increasing for all tested beams. The results show that, when the depth ratio increases, as in beams (AT-1, BT-1, and DT-1), the load that causes first cracks will be increased with percent values of 100.0%, 133.3%, and 166.7% respectively as compared to reference beam. Also, the load causes service cracks will be increased but the values of percentage increment less than those values of first cracks. That means the partially strengthening technique will increased strength of the beams and resist the cracks potential in preliminary stages more than the progressive stages as shown in Fig. 9. Table 2 Results of first cracking and service loads Beam Designation

Control beam AT-1 BT-1 DT-1


---0.8h, S 1.0h, S 0.8h-1.0h, D

First cracking load Pcr (kN)

21 42 49 56


Service load Ps (kN)


---100.0 133.3 166.7

56 91 98 105

---62.5 75.0 87.5

Ps : Service load corresponding to crack width of 0.31 mm according to ACI Committee 423.3R [6]


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140

Load KN

112

84

56 Control Beam

28

AT-1 BT-1 DT-1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Crack Width mm Figure 9 Load- crack width curves for tested beams

4. CONCLUSIONS Strengthening part of the RC T-beam span by external prestress was proved to enhance the strength of RC beams. The performance and procedure of strengthening and prestressing of beams was found to be effective and of remarkable effects. Partial span prestressing technique in strengthening can provide a space between the supports and anchors system as a free area for fixing and jacking. Partially strengthening technique enhances the strength of T-section beams when depth ratio increased or used draped tendons instead of straight tendons. Depth ratio represented the eccentricity of the strand position according to neutral axis so the increasing value of depth ratio will increases the eccentricity and increases the ability of the section to resistance load and reduced deflection in addition to narrowed the cracks. The stresses in the strand increase when increasing the depth ratio or using draped tendons .

5. REFERENCES [1] Tan, K.H, Al Farooq, M. A. and Ng, C.K. 2001. Behavior of Simple-Span Reinforced Concrete Beams Locally Strengthened with External Tendons. ACI Structural Journal. 98(2): 174-183. [2] Naaman, A.E. 2012. Prestressed Concrete Analysis and Design Fundamentals. 3rd Edition. Ann Arbor. Michigan. USA: 1176. [3] Harajli, M.H., and Kanj, M. 1992. Service load Deflection of Concrete Members Prestressed with Unbonded Tendons. Journal of Structural Engineering. ASCE. 118(9): 2569-2588. [4] Nilson, A.H., Darwin, D. and Dolan, C.W. 2009. Design of Concrete Structures. 14th Edition. McGrawHill Education: 816 [5] ACI 318-14 and ACI 328R-14. 2014. Building Code Requirements for Structural Concrete and Commentary. American Concrete Institute. USA: 519 [6] ACI Committee 423.3R-05. 2005. Recommendations for Concrete Members Prestressed with Unbonded Tendons. Reported by ACI Committee 423. American Concrete Institute. USA: 21.