Flexural strengthening of RC beams with prestressed NSM CFRP rods – Experimental and analytical investigation

Construction and Building Materials 23 (2009) 3292–3300

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Flexural strengthening of RC beams with prestressed NSM CFRP rods – Experimental and analytical investigation Moataz Badawi, Khaled Soudki * Department of Civil and Environmental Engineering, University of Waterloo, 200 University Ave. West, Waterloo, ON, Canada N2L 3G1

a r t i c l e

i n f o

Article history: Received 9 December 2007 Received in revised form 21 February 2009 Accepted 12 March 2009 Available online 17 July 2009 Keywords: Prestressed NSM CFRP rods Analytical modeling

a b s t r a c t The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods was investigated. Four RC beams (254 mm deep by 152 mm wide by 3500 mm long) were tested under monotonic loading. One beam was kept un-strengthened as a control beam. One beam was strengthened with a non-prestressed NSM CFRP rod. Two beams were strengthened with prestressed NSM CFRP rods stressed to 40% and 60% of the rod’s ultimate strength. The test results showed that strengthening with non-prestressed NSM CFRP rod enhanced the flexural response of the beam compared to that of the control beam. A remarkable improvement in the response was obtained when the RC beams were strengthened with prestressed (40% and 60%) NSM CFRP rods. An increase up to 90% in the yield load and a 79% in the ultimate load compared to those of the control beam were obtained. An analytical model was developed using sectional analysis method to predict the flexural response of RC beams strengthened with prestressed NSM CFRP rods. The proposed model showed excellent agreement with the experimental results. Ó 2009 Published by Elsevier Ltd.

1. Introduction Fiber reinforced polymer (FRP) reinforcement has been shown to be an effective method to increase the flexural and shear capacity of reinforced concrete (RC) structures. Advantages of FRP material for strengthening over traditional methods are: high strength to weight ratio, high fatigue strength, non-corroding, and high chemical resistance [2]. FRP strengthening can be applied as externally bonded (EB) FRP reinforcement or near-surface mounted (NSM) FRP reinforcement. In the NSM technique, the FRP reinforcement is epoxy bonded inside grooves pre-cut on the concrete surface. NSM technique was first introduced in the late 1940s [7] using steel reinforcement but was limited due to steel corrosion. Practical strengthening applications of concrete structures with the NSM system are more limited than the EB system. To address this gap, the engineering community carried out recent research on various structural aspects of NSM strengthening [3–5,21]. Numerous research work has been reported on the bond behavior and flexural strengthening of concrete members with non-prestressed NSM FRP reinforcement and very limited research has been carried on shear strengthening with NSM [6,11–13,15,19,20,22,23]. To the author’s knowledge, the use of prestressed NSM FRP reinforcement for strengthening RC structures has been limited to few investigations in the published literature [9,17,18]. The level of pre* Corresponding author. E-mail address: [email protected] (K. Soudki). 0950-0618/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.conbuildmat.2009.03.005

stressing in these studies ranged from 25% to 40% of the ultimate capacity of the CFRP rods. It is worth noting that FRP prestressing allows for a better utilization of the FRP reinforcement and improves the service performance of the strengthened member. The current study aims to further our knowledge on the effects of prestressed FRP as NSM reinforcement for strengthening RC beams. The paper presents the experimental results of RC beams strengthened with NSM prestressed FRP reinforcement and develops an analytical model, based on sectional analysis, to predict the flexural behavior of beams with prestressed NSM FRP reinforcement. 2. Experimental program 2.1. Test matrix Four RC beam specimens were tested. The first beam was the control beam with no strengthening. The second beam was strengthened with non-prestressed (0% prestress level) NSM CFRP rod. The third and fourth beams were strengthened with 40% and 60% prestressed NSM CFRP rod, respectively. 2.2. Specimen configurations Fig. 1 shows a schematic of the specimen geometry and reinforcement details. The beams were designed, to fail in flexure as under reinforced beams, according to the Canadian code [10]. The beam cross-section was 152 mm  254 mm with a beam length of 3500 mm. The beam was reinforced in tension with two 15 M (15 mm diameter) and in compression with two 10 M (11.2 mm diameter) reinforcing bars. A concrete cover of 30 mm was used. Shear reinforcement was provided by using 8 mm smooth stirrups spaced at 75 mm c/c.

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3500 mm Strengthened Beam

Control

NSM Groove

2 No. 10M

Epoxy 25 mm

254 mm

254 mm

CFRP Rod (9.5 mm)

2 No. 15M 152 mm

152 mm

15 mm

Fig. 1. Specimen design.

Clamp Anchor Hydraulic Jack Screw Adjustor

Clamp Anchor Steel Plate Load Cell CFRP Rod Steel C-Section

Steel Plate Clamp Anchor

RC Beam

CFRP Rod

RC Beam Steel Plate

Load Cell

Fig. 2. Prestressing set-up. 2.3. Material properties A single batch of concrete supplied by a local ready mix was used to cast all the specimens. The average 28-day compressive strength of the concrete was 45 ± 2.9 MPa based on testing three 100  200 mm cylinders. The yield strength of the steel reinforcement was 440 ± 4 MPa and its young’s modulus was 190 ± 1.9 GPa based on testing three steel coupons according to [8]. The CFRP rod was Aslan #3 (9.5 mm diameter) with reported properties as: ultimate strength of 1970 MPa, a strain at break of 1.45%, and a modulus of elasticity of 136 GPa. Sikadur 30 two-part epoxy had shear strength of 15 MPa, static modulus of 12,800 MPa, and elongation at break of 1%.

1100 mm. The beams were instrumented with several strain gauges at the midspan section: on the top compressive fiber of the concrete, tension and compression steel reinforcement, and on the CFRP rod. To measure the vertical deflections of the beam, three linear variable differential transducers (LVDT) were used and located at the mid-span, under the loading point, and at the mid-shear span. Fig. 3 shows the test set-up.

TM

2.4. Strengthening procedure The CFRP rod was epoxy bonded inside a single groove pre-cut on the concrete surface on the tension face of the beam. The dimensions of the groove, 15 mm wide  25 mm deep, were chosen based on the recommendations given in Ref. [1]. For the non-prestressed strengthened beams, the groove was filled half way with epoxy, the CFRP rod was placed inside the groove and epoxy was used to completely fill the groove. For the prestressed strengthened beams, the CFRP rod was placed in the groove with no epoxy and prestressed using the prestressing set-up shown in Fig. 2. The prestressing force in the CFRP rod was maintained using mechanical clamp anchors at each end of the beam and then the epoxy was applied to completely fill the groove. The mechanical clamp anchors were left in place for at least 6 days before they were removed from the system. During the prestressing process, the strain gages on the CFRP rod and load cells at the beam ends were monitored using a data acquisition system (DAQ). It is important to note that the prestressing set-up used works for laboratory application. In the field, due to a restriction of accessibility to the beam ends, the prestressing concept presented here could be used with some modifications. A mechanical anchor system shall be designed and attached to the concrete near the ends of the beam. Then the NSM prestressing operation is carried by reacting against the attached mechanical system. 2.5. Loading test set-up All the beams were monotonically loaded using stroke-control in four-point bending up to failure with a loading rate of 1.5 mm/min. The loading points and supports were selected to give a beam span of 3300 mm with a shear span of

3. Experimental results 3.1. Prestressing stresses The prestressing force in the CFRP rod was monitored using strain gauges mounted on the rod and a load cell placed at each end of the beam. The initial prestress in the CFRP rod was 2–5% over the target stress level to overcome any prestress losses at the anchors or creep due to epoxy. The prestressing force ranged from 53 to 55 kN in the 40% prestress and 80–83 kN in the 60% prestress level. The effective prestressing stress in the CFRP rod at mid-span was 788 MPa (40% prestress level) and 1182 MPa (60% prestress level). Very little prestress losses (due to adhesive creep or anchor slip) were measured by the strain gauges on the CFRP rod because the clamped anchors at the rod ends were kept in place during epoxy curing. When the anchors were removed, the prestressing force at the beam ends dropped to zero and gradually increased within 200 mm distance (transfer length) from the beam ends to the effective prestress level at mid-span. The effective prestress at mid-span was almost identical to the initial jacking stress. 3.2. Strain profile Fig. 4 plots the strain profile versus the beam depth using strain readings for the concrete, steel, and CFRP reinforcement. The results are for one strain gage on the concrete and the FRP rod and two strain gauges on the reinforcing bars. For the non-prestressed

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3.3. Load–deflection The load–deflection curves for all beams are shown in Fig. 5. The beams exhibited a tri-linear response characterized by cracking, yielding, and post-yielding stages. 3.3.1. Cracking stage The cracking load is at the onset of the first stiffness change. The cracking load of control beam was 10.20 kN at a deflection of 1.86 mm. The non-prestressed CFRP strengthened beam had a slight increase in the cracking load and a small decrease in the deflection. A remarkable increase in the cracking load approximately 3–4 times was obtained when the beam was strengthened with a 40% and 60% prestressed CFRP rod. 3.3.2. Yield stage The yield load is the load at which the internal steel reinforcement reaches the yield strain. The control beam had a yield load of 55.10 kN with a mid-span deflection of 23.5 mm. The beam strengthened with non-prestressed CFRP rod had an increase of 26% in the yield load over that of the control beam. The yield loads of the 40% and 60% prestressed strengthened beams were 72.4% and 90.6% higher than that of the control beam. 3.3.3. Ultimate stage The ultimate load is at beam failure which occurred by yielding of the tension steel reinforcement followed by concrete crushing for the control and the non-prestressed beams or CFRP rod rupture for the prestressed beams. The ultimate load of the control beam was 64.30 kN with a maximum mid-span deflection of 85.30 mm. The ultimate load of the non-prestressed strengthened beam was 96.50 kN, a 50% increase compared to the control beam, at a mid-span deflection of 65.50 mm. The ultimate load for the 40% prestressed strengthened beam was 115.25 kN representing a

300 Top Concrete

250

Compression Steel

200

10 kN 20 kN 30 kN 40 kN 50 kN 60 kN 70 kN

150

100 50

Tension Steel CFRP Rod

0 -3000

-2000

-1000

0 1000 Strain (micro-strain)

2000

300 Distance from the bottom of the beam (mm)

strengthened beam the results show that the strain profiles are linear (Fig. 4a). The strain compatibility confirms that the CFRP rod was fully bonded and that there is full-composite action and no slip between the CFRP and the concrete. In case of the prestressed strengthened beams, there is slip occurring between the CFRP rod and the epoxy possibly due to shear deformation in the epoxy, resulting in strain incompatibility between the rod and the concrete at the level of the FRP rod (Fig. 4b).

Distance from the bottom of the beam (mm)

Fig. 3. Loading set-up.

250

150

4000

10 kN 20 kN 30 kN 40 kN 50 kN 60 kN 70 kN 80 kN 90 kN 100 kN

100

50

Tension Steel CFRP Rod

-3000

Compression Steel

200

3000

-2000

0 -1000

0

1000

2000

3000

4000

Strain (micro-strain)

Fig. 4. Typical strain profiles during loading.

79.2% increase over that of the control beam or a 20% increase over the non-prestressed beam. For the 60% prestressed strengthened beam, the ultimate load was 112.01 kN, which is 2.6% less than for the 40% prestressed strengthened beam. It should be noted that in the field, most retrofits will be carried on already cracked members and hence the beam response will differ from the laboratory strengthened un-cracked specimens.

M. Badawi, K. Soudki / Construction and Building Materials 23 (2009) 3292–3300

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140 120

40% Prestressed 60% Prestressed 0% Prestressed

Load (kN)

100

Control

80 60 40

Control 0% Prestressed

20

40% Prestressed 60% Prestressed

0 0

20

40

60

80

100

120

Deflction (mm)

Fig. 5. Load–deflection of the tested beams.

Fig. 7. Shear cracks along the CFRP rod.

3.4. Mode of failure Two types of failure modes were observed. The first mode was by concrete crushing at the top fiber of the cross-section after yielding of the tension steel reinforcement. This mode was observed for the control and the beam strengthened with non-prestressed NSM CFRP rods. When a prestressed CFRP rod was used for strengthening the beam, the mode of failure was by rupture in the CFRP rod after yielding of the tension steel reinforcement. Fig. 6 shows the two modes of failure. At onset of failure, shear cracks (V-pattern in the groove) developed on the epoxy (bondlike) and along the NSM groove within the mid-span region and traveled towards the ends of the beam (Fig. 7). 3.5. Ductility The ductility of the beam, defined as the ratio of ultimate deflection to yield deflection, decreases as the prestressing level increases. In comparison to the control beam, the ductility was reduced by 30.6%, 47.2%, and 63.9% for the 0% (non-prestressed), 40% and 60% prestressed strengthened beam, respectively. The reduction in the ductility is possibly due to the increased tension reinforcement ratio (steel and CFRP) and prestressing which leads to less energy dissipation. 3.6. Strain in the concrete top face Fig. 8a plots the load versus concrete compressive strain for the control, and 0%, 40% and 60% prestressed strengthened beams. The strain offset due to prestressing was not measured due to the presence of hair-line cracks at the strain gauge locations. The initial

strain due to prestressing at the top fiber was calculated using prestressed concrete principals and combined with the strains due to loading for the 40% and 60% prestressed strengthened beams (see Fig. 8a). In general, the compressive strains in the control and non-prestressed strengthened beam (failed by concrete crushing) were much higher than those of the prestressed strengthened beams (failed by FRP rupture). The initial tensile strain offset lowered the less compressive strains at failure for the prestressed strengthened beams. Also, these beams exhibited less compression strain at the top fiber of the section than the control beam for a given load. The presence of prestressed CFRP rod resulted in reducing the cracked portion of the beam cross-section and as a result the compression force was distributed on a larger un-cracked cross-sectional area. 3.7. Strain in tension steel reinforcement Fig. 8b plots the load versus strain in the tension reinforcement during loading. The tension steel reinforcement in the control and 0% prestressed strengthened beam had zero strain at zero load. On the other hand, the tension steel reinforcement in the prestressed strengthened beams had initial compression strain before any external load is applied. Prestressing of the CFRP rod induces initial compressive strains in the tension (bottom) steel reinforcement in the beam cross-section. As the external loading is applied onto the beam, the bottom steel reinforcement goes from compression to tensile strains. In the FRP strengthened beams, the tensile force was shared by the steel reinforcement and the CFRP rod which re-

Fig. 6. Modes of failure.

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sulted in higher loads over the control beam. In the prestressed strengthened beams, the initial compressive strains due to prestressing increased the load range required to reach the yield stress in the steel reinforcement and consequently higher loads were attained.

140

120

60% Prestressed

100

80

60

Control 0% Prestressed 40% Prestressed 60% Prestressed

-3500

-3000

20

Control

-2000

-1500

-1000

-500

0 500

0

Strain in Concrete (micro-strain)

a) Load-strain in concrete 120

40% Prestressed

100

3.8. Strain in CFRP reinforcement Fig. 8c shows the applied load versus the strain in the CFRP rod. The initial strain readings in the prestressed CFRP rod are shown at zero load. The initial tensile strains were 5420 le and 8200 le in the 40% and 60% prestressed beams, respectively. As the external loading was applied, the CFRP rods experienced additional tensile strains. It is worth noting that after cracking, the slope of the load–strain curve was similar for the non-prestressed and prestressed beams. Failure of the 0% prestressed (non-prestressed) strengthened beam was by concrete crushing at a maximum CFRP tensile strain of about 11,000 le. On the other hand, the maximum strain measured in the CFRP rod in the prestressed strengthened beams was 13,600 le for the 40% and 60% prestressed beams. The rupture strain for the CFRP rod, as reported by the manufacturer, is 14,500 le.

40

0% Prestressed

-2500

Load (kN)

40% Prestressed

60% Prestressed

Load (kN)

80

4. Analytical model 60

40

The proposed model for the flexural response of NSM FRP prestressed beams is based on the strain compatibility and sectional analysis. Assumptions in the model are as follows:

0% Prestressed Control 0% Prestressed 40% Prestressed 60% Prestressed

20 Control

0 -500

0

500

1000

1500

2000

2500

3000

3500

4000

Strain in Steel (micro-strain)

b) load-strain in tension steel

4.1. Model concept

120

100

0% Prestressed 40% Prestressed 60% Prestressed

0% Prestressed

The concept of the model is based on dividing the beam into a number of elements (sections). These elements fall into: uncracked and cracked regions as shown in Fig. 9. The length of the elements within the cracked zone is set equal to the average flexural spacing. The un-cracked region is analyzed using elements having length equal to the average flexural crack spacing.

80

Load (kN)

 Plane sections remain plane after bending.  Perfect bond exists between the concrete, the steel, and the CFRP rod.  Shear effects are neglected.  Tension stiffening was ignored.

60 40% Prestressed 60% Prestressed

40

4.2. Material models The material models for the concrete, steel and CFRP reinforcement are as follows: Concrete [16]:

20 strains due to prestressing

strains due to loading

0 0

2000

4000

6000

8000

10000

Strain in the CFRP Rod (micro-strain)

c) load-strain in CFRP rod Fig. 8. Load versus strain.

12000

 2 !

14000

fc ¼

fc0

with

2e

eo

 0

eo ¼ 2fEcc

Fig. 9. Model concept.

e eo

pffiffiffiffi Ec ¼ 4500 fc0

ð1Þ

M. Badawi, K. Soudki / Construction and Building Materials 23 (2009) 3292–3300

Steel reinforcement



es Es

es  ey fs ¼ fy þ 0:01ES ðeS  ey Þ es  ey

ð2Þ

CFRP reinforcement

fcfrp ¼ ecfrp Ecfrp

ð3Þ

where fc is the concrete stress corresponding to a given concrete strain (e), fc0 is the specified concrete compressive strength, fs is the steel stress corresponding to a given steel strain (es), fy is the steel yield stress corresponding to the steel yield strain (ey), fcfrp is the CFRP stress corresponding to a given CFRP strain (ecfrp), Ec is the Young’s modulus of concrete, Es is the modulus of steel before yielding (pre-yielding stage), Ecfrp is the Young’s modulus of the CFRP rod, e is the concrete strain corresponding to a given concrete stress (fc), eo is the concrete strain corresponding to the concrete compressive strength, es is the steel strain corresponding to a given steel stress (fs), ey is the steel yield strain corresponding to the steel yield stress (fy) and ecfrp is the CFRP strain corresponding to a given CFRP stress (fcfrp). 4.3. Flexural crack spacing The flexural crack spacing was calculated, based on a Euro-code 2 [14] equation, accounting for the modular ratio of the reinforcement (steel and CFRP) and the location of the neutral axis for the composite section. For a CFRP strengthened beam, flexural crack spacing is calculated as follows:

sm ¼ 50 þ 0:25k1 k2

/ ðqeff Þequiv alent

ð4Þ

As þ nE Afrp ðqeff Þequiv alent ¼ Acef  2:5  b  c:c: Acef ¼ min b  ðh  cÞ=3 Efrp nE ¼ Es

3297

ð5Þ ð6Þ ð7Þ

where As is the area of the tension reinforcement, Acef is the area of concrete in tension, Afrp is the area of the FRP reinforcement, b is the width of the beam cross-section, c is the neutral axis location, c.c. is the concrete cover, Es is Young’s Modulus of the steel reinforcement, Efrp is Young’s Modulus of the CFRP reinforcement, h is the depth of the beam, k1 is the bond coefficient (0.8 for high bond rebar and 1.6 for plain rebar), k2 is the strain distribution coefficient (0.5 for bending and 1.0 for pure tension), nE is Modular ratio of CFRP reinforcement relation to steel, sm is the flexural crack spacing of the RC beam, u is the diameter of the reinforcing bar and qeff is the effective reinforcement ratio. The predicted flexural crack spacing for the control and strengthened beams were 80 mm and 76 mm. The predictions were in a good agreement with the experimental values of 74 mm and 71 mm for the control and strengthened beams, respectively. 4.4. Deflection The mid-span deflection of the beam is estimated based on integration of the curvatures in the un-cracked and cracked sections along one half of the beam length, as follows:

Dmidspan ¼

i¼n X

xi /i sm

i¼1

Fig. 10. Model force equilibrium and strain compatibility.

ð8Þ

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where Dmid-span is the mid-span deflection of the beam, n is the number of element within the half of the beam length, xi is the distance between the support to a given element (i), ui is the curvature at a given element (i) and sm is the average flexural crack spacing. 4.5. Moment capacity The moment capacity of a prestressed strengthened beam is determined by satisfying the force equilibrium and strain compatibility requirements as shown in Fig. 10 and expressed in the following:

abbcfc þ A0s fs0  As fs  Acfrp Ecfrp ðecfrpðpreÞ þ ecfrpðcÞ þ ecfrpðloadÞ Þ ¼ 0

compression stress of concrete, fcfrp is the stress in the CFRP reinforcement, fs0 is the stress in the compression steel reinforcement, fs is the stress in the tension steel reinforcement, M is the beam moment capacity, a is ratio of average stress in the compression stress block to the concrete strength, b is ratio of depth of the compression stress block to the fiber depth of the neutral axis, ec is strain at extreme top of concrete for a given load level, ec0 is corresponding strain in concrete to the concrete compressive strength, eCFRP(pres) is strain in CFRP rod due to initial prestressing force, eCFRP(c) is strain in CFRP rod at decompression in concrete at level of rod and eCFRP(load) is strain in CFRP rod due to loading by compatibility of strain.

ð9Þ

  1 0 M ¼ ab  bcfc c  b þ A0s fs0 ðc  d Þ  As fs ðd  cÞ 2

4.6. Model predictions

 Acfrp Ecfrp ðecfrpðpreÞ þ ecfrpðcÞ þ ecfrpð loadÞ Þðdf  cÞ

ð10Þ

The analytical predictions using the proposed model are presented in this section and compared with experimental results as follows.

With

ð12Þ

4.6.1. Load–deflection Comparisons of the experimental and analytical load–deflection relationships are shown in Fig. 11. A very good agreement between the experimental and predicted results is achieved for all the test beams (control, non-prestressed, 40% prestressed strengthened and 60% prestressed strengthened beam).

where As is the area of the tension steel reinforcement, A0s is the area of the compression steel reinforcement, Acfrp is the area of the CFRP reinforcement, b is beam width, c is depth of neutral axis, fc is the

4.6.2. Concrete strain Fig. 12 shows a comparison of the load–concrete compressive strains for the non-prestressed and 60% prestressed strengthened beams. As seen in Fig. 12, the predicted concrete compressive

 

e 1 ab ¼ c  3 eco ec eco

2ec

eco

120

120 Experimental

110 100

100

90

90

80

Load (kN)

Analytical

70 60 50 Experimental

40

Experimental

110

Analytical

Analytical

80

Experimental

70

Analytical

60 50 40

30

30

20

20

10

10 0

0 0

10

20

30

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60

70

80

90

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110

0

120

10

20

30

Deflction (mm)

40

50

60

70

80

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100

Deflection (mm)

120

120

110

Experimental

110

100

Analytical

100

90

Experimental Analytical

90

Analytical Experimental

80

Load (kN)

6

ð11Þ

 

Load (kN)

b¼

2

 

Load (kN)

4

ec eco

70 60 50

Analytical

80 70 60 50

40

40

30

30

20

20

10

10

Experimental

0

0 0

10

20

30

40

50

60

70

80

90

100

0

10

20

Deflction (mm)

30

40

50

60

Deflction (mm)

Fig. 11. Prediction of the load–deflection.

70

80

90

100

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strains are in good agreement with the measured values. Due to the presence of the shrinkage hair-line cracks at the beam midspan, the tension strain readings in the concrete at the time of prestressing were not recorded.

experimental tension strains in the two bars are compared to the predicted strains. The correlation between the experimental and predicted results for the test beams is within a reasonable agreement.

4.6.3. Tension steel reinforcement strain The comparisons between the experimental and predicted load versus the tension steel reinforcement for the non-prestressed and 60% prestressed strengthened beams are shown in Fig. 13. The

4.6.4. CFRP reinforcement Fig. 14 shows excellent correlation between the analytical and experimental CFRP strains for non-prestressed and 60% prestressed strengthened beams.

120

120 Experimental

Experimental

110

100

100

90

90 Analytical

Analytical

80

80

60 50

70

Load (kN)

70

110

60 50

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40

30

30 20

20 Experimental

10

10

Analytical

0

0 -4000

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Load (kN)

Experimental Analytical

-3000

0

-2500

-2000

-1500

-1000

-500

0

500

Compressive strain in concrete (micro-strain)

Compressove strain in concrete (micro-strain)

Fig. 12. Prediction of the load–compressive strain in concrete.

120

120

110

110 Experimental

Experimental

100

Analytical

Analytical

90

80

80

Load (kN)

90

70 Experimental

60 50

70

Experimental

60 50

40

40

30

30

20

Experimental (right-rebar) Experimental (left-rebar) Analytical

10 0

Experimental (right-rebar)

20

Experimental (left-rebar)l

10

Analytical

0

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-1000

Strain in the tension steel reinforcement (micro-strain)

-500

0

500

1000

1500

2000

2500

3000

3500

Strain in the tension steel reinforcement (micro-strain)

Fig. 13. Prediction of the load–tensile strain in steel reinforcement.

120

120

Experimental Analytical

110

110

100

100

Experimental

90

90

Analytical

80

80

Load (kN)

Load (kN)

Load (kN)

100

70 60 50 Analytical

60 50 40

40 Experimental

30

70

30 Experimental Analytical

20 10

20 10

0

0 0

1500

3000

4500

6000

7500

9000

Strain in CFRP Rod (micro-strain)

(a) Non-prestressed

10500

12000

0

2000

4000

6000

8000

10000

Strain in CFRP Rod (micro-strain)

(b) 60% Prestressed

Fig. 14. Prediction of the load–tensile strain in CFRP reinforcement.

12000

14000

4000

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M. Badawi, K. Soudki / Construction and Building Materials 23 (2009) 3292–3300

The above results confirm the validity of the proposed analytical model to predict the flexural behavior of RC beams strengthened with prestressed NSM CFRP rods. 5. Conclusions  The results of this study illustrate that prestressed NSM CFRP reinforcement provides a strengthening technique that would address serviceability concerns including excessive deflections and cracking in structural members.  The NSM technique is very effective in increasing the flexural capacity of a RC beam. With non-prestressed strengthened RC beam, a slight reduction in the ductility is obtained with respect to that of the control beam.  Prestressed NSM FRP strengthening enhances the service performance of RC beams by reducing the deflection at different load stages.  Prestressing the NSM CFRP rod up to 40% of its capacity almost doubled the flexural load capacity but reduced the ductility to half in comparison to an un-strengthened beam.  Ductility of a NSM CFRP strengthened beam is reduced as the level of prestressing is increased.  The proposed strengthening technique is a step towards utilizing prestressed NSM CFRP rod in practice. More research is required to develop a self reacting system able to prestress the CFRP rod in the field.  The proposed model using sectional analysis and strain compatibility gave a good agreement with the experimental results. Acknowledgment The authors would like to thank all the technicians of the structural laboratory at University of Waterloo, in particular Mr. Ken Bowman and Mr. Douglas Hirst. References [1] ACI 440.2R-08. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. Farmington Hills (MI): American Concrete Institute; 2008. 80 p. [2] ACI 440R-07. Fiber Reinforced Polymer Reinforcement for Concrete Structures. Manual of concrete practice. Farmington Hills (MI): American Concrete Institute; 2007. 100 p.

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