Structural performance of corroded RC beams repaired with CFRP sheets

Composite Structures 92 (2010) 1931–1938

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Structural performance of corroded RC beams repaired with CFRP sheets A.H. Al-Saidy a,*, A.S. Al-Harthy a, K.S. Al-Jabri a, M. Abdul-Halim b, N.M. Al-Shidi c a

Department of Civil and Architectural Engineering, Sultan Qaboos University, Oman Department of Civil Engineering, Jordan University of Science and Technology, Jordan c Al-Muhandis Engineering Consultancy, Muscat, Oman b

a r t i c l e

i n f o

Article history: Available online 13 January 2010 Keywords: Strengthening Rehabilitation Retrofitting Corrosion Advanced composite materials CFRP sheets

a b s t r a c t Corrosion of reinforcement is a serious problem and is the main cause of concrete structures deterioration costing millions of dollars even though the majority of such structures are at the early age of their expected service life. This paper presents the experimental results of damaged/repaired reinforced concrete beams. The experimental program consisted of reinforced concrete rectangular beam specimens exposed to accelerated corrosion. The corrosion rate was varied between 5% and 15% which represents loss in cross-sectional area of the steel reinforcement in the tension side. Corroded beams were repaired by bonding carbon fiber reinforced polymer (CFRP) sheets to the tension side to restore the strength loss due to corrosion. Different strengthening schemes were used to repair the damaged beams. Test results showed detrimental effect of corrosion on strength as well as the bond between steel reinforcement and the surrounding concrete. Corroded beams showed lower stiffness and strength than control (uncorroded) beams. However, strength of damaged beams due to corrosion was restored to the undamaged state when strengthened with CFRP sheets. On the other hand, the ultimate deflection of strengthened beams was less than ultimate deflection of un-strengthened beams. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Many structures in harsh environments have experienced unacceptable loss in serviceability or safety much earlier than anticipated, due to the corrosion of reinforcing steel and thus need rehabilitation, strengthening or replacement. Corrosion presents a problem for reinforced concrete (RC) structures for two reasons. First, as steel corrodes, there is a corresponding loss in cross-sectional area. Secondly, the corrosion products occupy a large volume than the original steel, which exerts substantial tensile forces on the surrounding concrete and causes it to crack and spall off. The steel corrosion can also lead to loss of structural bond between the reinforcement and concrete causing failure of a heavily corroded RC member [1–4]. This implies that if corrosion cracking can be prevented or delayed, a certain degree of structural strength may be maintained in a corroding RC member. The use of fiber reinforced polymers (FRP) to retrofit and rehabilitate concrete structures is rapidly becoming a popular new technology. The advantages of using composite materials for retrofitting, as, opposed to steel plate bonding or jacketing, are that FRP are light weight, have a high tensile strength, and posses a high resistance to acids and bases making them essentially non-corrosive. The use of externally bonded carbon fiber reinforced polymers (CFRP) sheets

* Corresponding author. Tel.: +968 24141340; fax: +968 24141331. E-mail address: [email protected] (A.H. Al-Saidy). 0263-8223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2010.01.001

for flexure and shear strengthening of reinforced concrete structures has been investigated extensively [5–10]. A common mode of failure in externally bonded FRP is the bond failure at the interface between the FRP and the concrete surface. This mode of failure lead to the development of a new strengthening technique which is known as near surface mounted (NSM) rods or strips [11]. In NSM method, a groove is made in the concrete at the tension face of the beam and the rod is inserted and bonded with epoxy to the concrete. The surface of the bonded area in NSM is larger compared to that of externally bonded FRP sheets. Mechanically fastened FRP (MF-FRP) strips were also used for strengthening of RC beams to overcome the bond mode of failure [12]. The externally bonded FRP sheets or plates are commonly used in the passive state, but a more efficient way is to pre-stress the FRP sheets (active state) before bonding to the concrete surface [13]. By pre-stressing the FRP, the material is used in an efficient way that not only contributes to increase the live load capacity, but also resist part of the dead load on the strengthened member. Although the effects of external FRP strengthening of RC beams on increasing shear and flexure capacity vary significantly, all reported studies showed increase in the capacity of strengthened beams. The percentage of increase depends on the type of specimen tested, the orientation and type of CFRP reinforcement and applied loading. Few studies were reported in the literature [14–18] related to strengthening of corroded RC members. A laboratory study carried out by Soudki et al. [14–16] in which the overall program included 16 small-scale reinforced concrete beams (100  150  1200 mm)

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and 20 larger-scale beams (152  254  3200 mm). The specimens were exposed to different corrosion levels (minor at 5%, moderate at 10% and severe at 15% mass loss) by means of a constant impressed current. The strengthening scheme used in the small-scale beams consisted of CFRP flexural laminate bonded to the tension face, with the fiber orientation in the longitudinal direction. Then, the beams were followed by transverse laminates bonded to the tension face and up each side of the beam, with the fiber orientation in the transverse direction. In the large-scale beams, FRP sheets were applied using two repair schemes. The first scheme involved wrapping the specimen intermittently with U-shaped glass (GFRP) strips around the tension face and the sides. The second scheme involved flexural strengthening of the corroded specimen by externally bonding carbon (CFRP) sheet to the tension face of the specimen and then wrapping the specimen with U-shaped GFRP sheets. All strengthened beams exhibited increased stiffness over un-strengthened specimens and marked an increase in the yield and ultimate strength. Bonacci and Maalej [17] carried out an experimental program to provide a realistic assessment of the potential of using FRP materials in the repair and strengthening of reinforced concrete flexural members exposed to a corrosive environment. A total of seven specimens (270  400  4350 mm) were tested. Four of the seven RC beams were reinforced externally with one or two layers of CFRP composite. Some specimens were tested under monotonic loading and other specimens were tested under sustained loading. CFRP external reinforcement increased beam load carrying capacities by 10–35% and reduced deflection by 10–32% with respect to the control specimen. The results showed that the use of FRP sheets for strengthening corroded reinforced concrete beams is an efficient technique that can maintain structural integrity and enhance the behavior of such beams. In a recent study by Deng et al. [18], aramid fiber reinforced polymer (AFRP) sheets were used to strengthen corroded con-

crete beams with different degrees of corrosion (minor: reinforcement mass loss is 2.0%, medium: reinforcement mass loss is 6.0%). In all strengthened beams it was possible to improve the cracking load, yield load and ultimate load in comparison with the un-strengthened corroded beams under same degrees of corrosion. The cracking load, yield load and ultimate load of minor corroded RC beam strengthened with AFRP sheets was respectively increased by 20%, 27% and 60%. The ductility (deformation) of the strengthened beams was also improved through significant increase in the ultimate deflection of strengthened beams in comparison with the un-strengthened corroded beams. With the limited number of studies of corroded RC beams strengthened with FRP, there is a need for further investigation. In most of the previous studies, the corrosion was limited to the middle third of the beam with minor to mild rate of corrosion. Different strengthening schemes need also to be investigated using other available FRP materials. This paper presents the experimental results of damaged/repaired reinforced concrete beams. The experimental program consisted of reinforced concrete rectangular beam specimens exposed to accelerated corrosion. The corrosion rate was varied between 5% (mild) and 15% (severe) which represents loss in cross-sectional area of the steel reinforcement in the tension side. Damaged beams were repaired using three different schemes by bonding CFRP sheets to the tension side to restore the strength loss due to corrosion. 2. Experimental study 2.1. Specimen details The experimental program consisted of 10 reinforced concrete rectangular beam specimens. Six of the beam specimens were strengthened with CFRP sheets using three different strengthening

Fig. 1. Beam specimen reinforcement configuration.

A.H. Al-Saidy et al. / Composite Structures 92 (2010) 1931–1938 Table 1 Summary of beam specimens. Specimen

Corrosion %

No. of CFRP sheets

Strengthening scheme

C0 C5 M5S1 M5S2 C10 M10S2 C15 M15S2 M15S2-2L M15S3

0 5 5 5 10 10 15 15 15 15

None None 1 Long 1 Long None 1 Long None 1 Long 2 Long 1 Long

None None Scheme Scheme None Scheme None Scheme Scheme Scheme

1 2 2 2 2 3

schemes. The tensile reinforcement of six of the strengthened beams and three of the un-strengthened beams were corroded to various degrees by means of an impressed electrical current. To accelerate the corrosion process for the tension steel reinforcement, salt (NaCl) was added to the mixer before concrete casting so that 3% chloride ions, by weight of cement, were uniformly distributed along the bottom third of each specimen to be corroded.

1933

Following the corrosion phase, the beam specimens were tested to failure in flexure, in four-point bending. The reinforcement details of the specimens are shown in Fig. 1. Each specimen was 2500 mm long, 170 mm wide and 300 mm deep rectangular cross-section and reinforced by two 10-mm-diameter (hot rolled high yield reinforcement steel bars) bottom longitudinal deformed reinforcing bars, two 6-mm-diameter stainless steel (type 316) top longitudinal plain reinforcing bars, and 8-mm-diameter epoxy coated deformed reinforcing stirrups spaced at 100 mm. A typical clear cover of 30 mm was used all around the stirrups. The specimen description is summarized in Table 1. Specimens C0, C5, C10 and C15 represent control specimens that were subjected to monotonic loading and not strengthened with CFRP sheets. The number describes the level of corrosion which ranges from 0% (no corrosion) to 15%. Specimens M5S1 and M5S2 were subjected to 5% mass loss (corrosion) and strengthened with CFRP sheet using Scheme 1 (S1) and Scheme 2 (S2), respectively. All beams were strengthened with one CFRP layer except M15S2-2L was strengthened with two layers (2L) using Scheme 2 (S2). The definition of Schemes 1, 2 and 3 are described in the following section.

Fig. 2. Schematic of strengthening schemes: (a) Scheme 1, (b) Scheme 2 and (c) Scheme 3.

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2.3. Material properties

Table 2 Concrete mix constituents. Constituent

Mix 1 (per m3)

Mix 2 (per m3)

Ordinary Portland cement (kg) Washed sand (kg) Fine sand (kg) 10 mm crushed aggregate (kg) Total free water (L) Free water–cement ratio Admixture–Pozzolith 390 N (L) Air content (assumed) (%) Temperature (°C) NaCl (salt) (kg)

390 825 145 920 196.8 0.5 3 1.5 30 0

390 825 145 920 196.8 0.5 3 1.5 30 11.7

The concrete was obtained from a local ready mix company. The concrete mix constituents are presented in Table 2. Mix 2 was used to cast the bottom third of the beam, while Mix 1 was used for the remaining portion of the beam. Salt was added to Mix 2 in proportion of 3% by weight of cement. The concrete of Mix 1 had 28 day cylinder strength of 45 MPa with a slump of 75 mm, while the salted concrete (Mix 2) had 28 day cylinder strength of 39 MPa with a slump of 75 mm. Three 10 mm diameter reinforcing bars from batch materials were tested to determine the material properties. The average yield strength of the 10 mm diameter reinforcing bars was measured to be 617 MPa and the modulus of elasticity was 200 GPa. The yield strength of the 8 mm diameter epoxy coated deformed reinforcing stirrups was 250 MPa. The CFRP was used in the form of a unidirectional sheet with dry thickness of 0.117 mm [19]. A two part epoxy [20] was used to bond the CFRP sheet to the concrete surface. One coat of epoxy was first applied to the prepared concrete surface, then the CFRP sheet was attached to the epoxy coated concrete and a second coat of epoxy was applied on top of the CFRP sheet. The average thickness of the CFRP laminate (sheet + epoxy) was measured to be around 0.4 mm. Material properties of the CFRP laminate are presented in Table 3.

Table 3 Material properties for sheet composite (laminate). Ef (tensile modulus) (GPa) eRupture (strain) (%) Width of CFRP sheet (mm) Thickness of CFRP (mm) Ff (ultimate strength) (MPa)

84.0 1.25 150.0 0.400 1050.0

2.2. Strengthening schemes Six specimens were strengthened with CFRP sheets. Three different strengthening schemes were used. In Scheme 1, the CFRP sheet size was 150 mm wide and 2300 mm long, with fibers oriented parallel to the specimen axis (the longitudinal direction) was bonded to the tensile face of the beam specimen, as shown in Fig. 2a. Scheme 2 is similar to Scheme 1 but in this scheme additional U-shaped CFRP sheets were applied around the cross-section, with their fibers oriented in the transverse direction (i.e., perpendicular to the specimen axis). These sheets were placed at centerline of the point load so that they covered the specimen tension face and ran up both sides to a height of 200 mm and width of 150 mm. The purpose of these U-shaped straps was to anchor and prevent the tension face CFRP sheet from prematurely debonding off the concrete surface, as shown in Fig. 2b. In Scheme 3, the CFRP sheet was 2300 mm long and 300 mm wide, with fibers oriented parallel to the specimen axis (the longitudinal direction). The CFRP was wrapped around both sides of the beam specimen to a height of 65 mm from the bottom face as shown in Fig. 2c.

2.4. Accelerated corrosion process Accelerated corrosion was carried out by impressing an electric current through the main longitudinal bottom reinforcing bars of about 323 mA, which corresponds to approximate current density of 207 lA/cm2. This current density was obtained by dividing the total impressed current by the surface area of the portion of the steel reinforcement cage that is in the salted concrete (specimen’s bottom third). The stainless steel reinforcing bar with the length of 2560 mm located at the side of each specimen acted as the cathode for this corrosion process, whereas the tension reinforcing bars acted as the anode. A DC power supply was used to provide the current desired. During construction of the specimens, the tension reinforcing bars were extended about 60 mm out of each specimen from one end to facilitate connection to the power supply. The specimens and stainless steel bars were connected in parallel to the power supply. Specimens were placed in a fiberglass tank with size of 3000 mm long, 2500 mm wide and 1000 mm high which contained salted water (3% of salt). Specimens were placed on

Rebar (Anode) Water Tank 300 mm 890 mm

10 mm 100 mm Power Supply Square Wood 100 mm thick Stainless –steel Bar (Cathode) Fig. 3. Schematic of accelerated corrosion setup.

3% Salted Water

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100 mm high square wooden section. The level of salted water was 110 mm, which submerged 10 mm of the bottom of the beam specimen. To obtain theoretical mass loss of 5% in tension reinforcement rebar of specimen, a constant current of 323 mA was applied to each specimen for a period of 19 days. Therefore, a constant current of 323 mA was applied to each specimen for a period of 38 days, to obtain theoretical mass loss of 10% in tension reinforcement rebar of specimen. Likewise, to obtain theoretical mass loss of 15% in tension reinforcement rebar of specimen, a constant current of 323 mA was applied in each specimen for a period of 57 days. Fig. 3 shows schematic diagram of accelerated corrosion setup. The corrosion current was monitored daily, and any drift was corrected. To estimate the approximate mass loss associated with this corrosion process, Faraday’s law was used [14,21]. Impressed current calculations are presented in Table 4.

Table 5 Experimental load–mid-span deflection results for beam specimens. Beam designation

Pc (KN)

Py (KN)

Pu (KN)

Dc (mm)

Dy (mm)

Du (mm)

Mode of failure

C0

21.0

51.0

58.0

0.93

8.4

39.5

C5

24.0

43.5

50.0

1.70

8.0

48.3

C10 C15 M5S1

25.0 23.4 27.7

43.6 35.6 53.9

51.1 41.6 67.0

1.2 0.95 1.0

8.3 5.6 8.1

44.0 23.4 19.9

M5S2 M10S2 M15S2

26.0 25.0 28.1

54.1 50.9 43.8

71.8 63.4 56.9

1.5 1.3 1.5

9.2 9.1 7.0

24.5 18.7 17.2

M15S2-2L

25.9

52.1

68.5

1.3

7.8

15.6

M15S3

21.9

50.6

69.9

1.3

9.5

22.0

Flexural failure Flexural failure Steel rupture Steel rupture Debonding of CFRP CFRP rupture CFRP rupture Debonding of CFRP Debonding of CFRP CFRP rupture

2.5. Load setup Following the corrosion period, all specimens were loaded in four-point loading (see Fig. 1). The load was applied using a 250 kN hydraulic actuator through a spreader steel beam to the specimen. Each specimen spanned 2400 mm and was loaded symmetrically about its centerline at two points 600 mm apart. A linear variable displacement transducer (LVDT) with a range capacity of 100 mm was used to measure the span displacement of the test specimens. Extensometer of gauge length of 200 mm and range capacity of ±5 mm was used to measure the side strain of concrete. Two 60 mm long electrical strain gauges were used to measure the strain in the concrete top surface while eight 6 mm electrical strain gauges were distributed along the centerline of CFRP sheet in the direction of fiber for specimens strengthened using Scheme 1. In specimens strengthened using Scheme 2, two additional strain gauges were used on the U-shaped straps at the point of load to monitor the strains induced in the side CFRP U-shaped straps. Instrumentations of specimens strengthened with Scheme 3 were similar to Scheme 1 specimens, but in Scheme 3 five additional strain gauges were placed to the side of wrapped CFRP sheet. 3. Discussion of test results The load–mid-span deflection results for beam specimens are summarized in Table 5. In the table, Pc, Py, and Pu are the cracking load, yielding load, and ultimate load, respectively; Dc, Dy, and Du are cracking deflection, yielding deflection, and ultimate deflection, respectively.

3.1. Effect of corrosion Two longitudinal corrosion cracks were noted on the specimen before testing, each extending parallel to the length of the specimen. They were located on the bottom of the specimen, under the rebar and rust staining was noted along these cracks. Generally, the corrosion cracks proceeded from the rebar to the soffit of the beam. No corrosion cracking was observed on the sides of the beam. Fig. 4 presents the load–deflection curves for these specimens. In general, the yield and ultimate strength decreased when the cross-section of reinforcement decreased, while the ultimate deflection was increased when the degree of corrosion increased. It is observed that beam with 10% (C10) corrosion has similar or slightly higher ultimate load at failure than the beam with 5% corrosion (C5) though the expected failure load to be lower that the one with 5% corrosion due to the loss of cross-sectional area. Other researchers [3] also observed a similar behavior in which it was concluded that an enhancement of anchorage capacity, believed to be associated principally with increased radial stresses on the bar–concrete interface, was able to offset loss of bar section. However, Beam C15 failed by steel rupture in a brittle manner since the reinforcement ratio was reduced by corrosion to a level close to the minimum ratio required by ACI Code [22]. The provided area of steel in the uncorroded beam (Specimen C0) was 157 mm2 compared to the minimum area required by ACI Code of 125 mm2. It is also noted that the loss in strength of these specimens was not linearly proportional to the percentage mass loss of steel.

Table 4 Impressed current calculations.

Current Knowing that 1 A h consume 1.04 g of Iron using Faraday’s law [11,18] Amphere hour required for the mass loss No. of hours required with current of 323 mA (h) No. of days Applied current (mA) Current density (lA/cm2)

2480.0

2480.0

2480.0

10.0 78.6 2.0 389,714 155,886 3063 5.0 153.2

10.0 78.6 2.0 389,714 155,886 3063 10.0 306.3

10.0 78.6 2.0 389,714 155,886 3063 15.0 459.5

70 60

C0 C10 C5

50

Load (KN)

Length of exposed tensile steel in concrete (mm) Diameter of rebar (mm) Cross-sectional area of rebar (mm2) No. of rebars Volume of tensile steel (mm3) Surface area (mm2) Mass of steel in salted concrete (g) Percentage of mass loss (%) Mass loss (g)

C15

40 30 20 10

147.3 456.4

294.5 912.7

441.8 1369.1

19.0 323.0 207.2

38.0 323.0 207.2

57.0 323.0 207.2

0 0

10

20

30

40

50

Mid-Span Deflection (mm) Fig. 4. Load–deflection curves of control beam specimens.

60

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3.2. Specimens with 5% corrosion

80

M5S2

70

M5S1

Load (KN)

The Specimens M5S1 and M5S2 were strengthened using Schemes 1 and 2 and subjected to monotonic loading, whereas Specimens C0 and C5 are Control Beams. The load–deflection curves for these beams are presented in Fig. 5. Yield and ultimate strengths of the strengthened specimens increased over the corresponding un-strengthened Specimen C5. Yield strength of Specimens M5S1 and M5S2 were 23.9% and 24.3%, respectively, greater than the yield strength of Specimen C5. Ultimate strength of Specimens M5S1 and M5S2 were 34.2% and 43.8%, respectively, greater than the ultimate strength of Specimen C5. Specimen M5S1 failed due to debonding of the CFRP sheet, while Specimen M5S2 failed due to rupture of CFRP sheet. This is attributed to the anchorage provided by the U-straps in Specimen M5S2, which forced CFRP rupture instead of CFRP debonding failure observed in Spec-

C0

60

C5

50 40 30 20 10 0 0

10

20

30

40

50

60

Mid-Span Deflection (mm) Fig. 5. Load–deflection curves of 5% corrosion specimens.

Fig. 6. Mode of failure in Specimens M5S1 and M5S2: (a) CFRP sheet debonding in M5S1 and (b) CFRP sheet rupture in M5S2.

a 0.009 0.008

Strain

0.007

27.7 46.3 53.9 61.3 67.0

0.006 0.005 0.004 0.003 0.002 0.001 0 -0.001 -1500

-1000

-500

0

500

1000

1500

Location of Strain Gauges from Center(mm)

b Fig. 7. (a) Strain gauges locations in the CFRP sheet and the U-shaped straps and (b) longitudinal strain in the CFRP sheet in beam M5S1 at different loading stages.

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3.3. Specimens with 10% corrosion The load–deflection curves for Specimens C0, C10, M10S2 are presented in Fig. 9. Specimen M10S2 strengthened using Scheme 2, whereas Specimens C0 and C10 are Control Beams. Ultimate strengths of the strengthened specimen increased slightly over the corresponding un-strengthened Specimen C10. Yield strength of Specimen M10S2 was 16.6% greater than the yield strength of Specimen C10. Ultimate strength of Specimen M10S2 was 24.0% greater than the ultimate strength of Specimen C10. Ultimate deflection of M10S2 was 58% less than the ultimate deflection of Specimen C10. Failure of M10S2 was due to CFRP sheet rupture.

3.4. Specimens with 15% corrosion Specimens M15S2 and M15S2-2L were strengthened using Scheme 2; however, M15S2-2L was strengthened using two layers of CFRP sheet. Specimen M15S3 was strengthened using Scheme 3 and Specimens C0 and C15 are Control Beams. It should be noted here that the cross-sectional area of the CFRP sheet in M15S3 and M15S2-2L are equal. Load–deflection curves for these specimens are presented in Fig. 10. The ultimate strength of Specimens M15S2, M15S2-2L and M15S3 were 36.7%, 64.7% and 69.9%, respectively, greater than the ultimate strength of Specimen C15. Ultimate deflection of Specimens M15S2, M15S2-2L

80 M15S2-2L

70

Load (KN)

imen M5S1 (see Fig. 6). The effect of using U-straps is observed on the higher strength and deflection measured in Specimen M5S2 compared to M5S1. Ultimate deflection of Specimens M5S1 and M5S2 were 59% and 49.2%, respectively, less than the ultimate deflection of Specimen C5. The strain along the CFRP sheet recorded at different loading stages is shown in Fig. 7. It is observed that the strain is symmetric about the centerline of the beam. As expected, the middle part of the CFRP sheets (region of plastic hinge) is subjected to the highest strain which increased significantly in the CFRP sheet beyond the yield load of the internal steel reinforcement which was around 53 kN. The strain in the U-straps is presented in Fig. 8 indicating the contribution of the straps on anchoring the CFRP sheet from debonding. As a matter of fact the effect of the straps was more noticeable in beams with multi-layers of CFRP as in beam M15S2-2L and in beams with higher corrosion rate as can be seen in Fig. 8. It is also noted that the U-shaped straps become more effective once the cracking load, Pc is reached.

60

M15S3 C0

M15S2

50 C15

40 30 20 10 0 0

10

20

30

40

50

Mid-Span Deflection (mm) Fig. 10. Load–deflection curves of 15% corrosion specimens.

80

80

Load (KN)

Load (KN)

M10S2 M15S2

60

M10S2

60

M5S2

70

M5S2

70

M15S2 50 40 30

C0

50 40 30 20

20

10

10

0

0

0

0

0.0001

0.0002

0.0003

0.0004

10

0.0005

20

30

40

50

Mid-Span Deflection (mm)

Strain

a

Fig. 8. Load vs. strain in U-shaped straps.

60 M10S2

M5S2

50

Load (KN)

M10S2

60

Load (KN)

C0 M15S2

70 C0 C10

50

40 30 20

40 30

10

20 0

10

0

2

4

6

8

10

12

14

Mid-Span Deflection (mm)

0 0

10

20

30

40

50

b

Mid-Span Deflection (mm) Fig. 9. Load–deflection curves of 10% corrosion specimens.

Fig. 11. (a) Load–deflection curves of strengthened beam specimens at various level of corrosion and (b) zoomed part for deflection range 0–13 mm.

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and M15S3 were 26.6%, 33.1% and 5.9%, respectively, less than the ultimate deflection of Specimen C15. As noted earlier, the area of CFRP sheet used for strengthening of Specimens M15S2-2L and M15S3 was the same. It is noted that using two layers of CFRP in M15S-2L increased the stiffness; however, the beam exhibited less ductility when compared to M15S3 which exhibited more ductile (large deflection) behavior before failure. In M15S3 the CFRP sheet was bonded to a larger surface area compared to the surface area used to bond two layers of CFRP sheet in M15S2-2L. Therefore, the layout of CFRP reinforcement affects the behavior of the strengthened specimen, even if the total amount of used CFRP sheet is equal. 3.5. Specimens of strengthened beams at various level of corrosion Fig. 11 compares the experimental results of strengthened beams M5S2, M10S2 and M15S2 having different corrosion levels with the undamaged Control Beam C0. It is noted from Fig. 11 that all strengthened specimens recovered the original strength except Specimen M15S2. Ultimate strength of Specimens M5S2 and M10S2 were 23.9% and 9.3%, respectively, greater than the ultimate strength of Specimen C0, while the ultimate strength of Specimen M15S2 was 2% less than the ultimate strength of Specimen C0 which was mainly due to premature debonding failure of Specimen M15S2. In addition, the ductility of all strengthened specimens was less than the control beam. Ultimate deflection of Specimens M5S2, M10S2 and M15S2 were, 38%, 52.8% and 56.6%, respectively, less than the ultimate deflection of Specimen C0. 4. Conclusions Based on the experimental results, the following conclusions are drawn:  The use of CFRP sheets for strengthening corroded RC beams is capable of maintaining the structural integrity and increasing the ultimate strength of these beams to a level above the ultimate strength of the control beam.  The use of CFRP sheets for strengthening RC beams decreased the ultimate deflection of these beams to a level below the control beam.  The layout of CFRP reinforcement was more important than the total amount used in the efficiency of strengthening corrosion damaged beams as observed in beams M15S2-2L and M15S3.  The use of transverse straps (U-shaped CFPR sheet) anchored the flexural CFRP sheet and prevented any delamination at concentrated loads. The U-shaped straps are more effective in beams with higher rate of corrosion.

Acknowledgements This research was funded by Sultan Qaboos University through an internal grant (Grant No. IG/ENG/CAED/04/03) which is highly acknowledged. References [1] ACI Committee 222. Corrosion of metals in concrete. Detroit (Michigan): ACI 222R-96, American Concrete Institute; 1996. p. 29. [2] Mangat PS, Elgarf MS. Flexural strength of concrete beams with corroding reinforcement. ACI Struct J 1999;96(1):149–58. [3] Cairns J, Plizzari G, Du Y, Law DW, Franzoni C. Mechanical properties of corrosion damaged reinforcement. ACI Mater J 2005;102(4):256–64. [4] Shannag MJ, Al-Ateek SA. Flexural behavior of strengthened concrete beams with corroding reinforcement. Construct Build Mater 2006;20:834–40. [5] Priestley MN, Seible F. Repair of column using fiber glass/epoxy jacket and epoxy injection. Report #93-04, SEQAD Consulting Engineers; 1993. [6] Saadatmanesh M, Ehsani M, Li M. Strength and ductility of concrete columns externally reinforced with fiber composite straps. ACI Struct J 1994;91(4): 443–6. [7] Xiao Y. Shear retrofit of reinforced concrete bridge piers using prefabricated composites. In: Composites in infrastructure, proceedings of the second international conference on composites in infrastructure, Tucson, Arizona; 1998. p. 221–33. [8] Chaallal O, Nollet MJ, Saleh K. Use of CFRP strips for flexure and shear strengthening of RC members. In: Composites in infrastructure, proceedings of the second international conference on composites in infrastructure, Tucson, Arizona; 1998. p. 249–59. [9] Hutchinson R, Abdelrahman A, Rizkalla S. Shear strengthening using CFRP sheets for prestressed concrete bridge girders in Manitoba, Canada. In: Composites in infrastructure, proceedings of the second international conference on composites in infrastructure, Tucson, Arizona; 1998. p. 261–75. [10] Pham H, Al-Mahaidi R. Experimental investigation into flexural retrofitting of reinforced concrete bridge beams using FRP composites. Compos Struct 2004;66(1–4):617–25. [11] Capozucca R. Static and dynamic response of damaged RC beams strengthened with NSM CFRP rods. Compos Struct 2009;91(3):237–48. [12] Bank LC, Arora D. Analysis of RC beams strengthened with mechanically fastened FRP (MF-FRP) strips. Compos Struct 2007;79(2):180–91. [13] Xue W, Tan Y, Zeng L. Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates. Compos Struct 2010;92(3): 612–22. [14] Soudki KA, Sherwood T, Masoud S. FRP repair of corrosion-damaged reinforced concrete beams. Report, Department of Civil Engineering, University of Waterloo, Waterloo, Canada, 2000. [15] Soudki KA, Rteil AA, Al-Hammoud R, Topper TH. Fatigue strength of fibrereinforced-polymer-repaired beams subjected to mild corrosion. Can J Civ Eng 2007;34(3):414–21. [16] El Maaddawy T, Soudki K, Topper T. Performance evaluation of carbon fiberreinforced polymer-repaired beams under corrosive environmental conditions. ACI Struct J 2007;104(1):3–11. [17] Bonacci JF, Maalej M. Externally bonded FRP for rehabilitation of corrosion damaged concrete beams. ACI Struct J 2000;97(5):703–11. [18] Deng Z, Li J, Lin H. Experimental study on flexural performance of corroded RC beams strengthened with AFRP sheets. Key Eng Mater 2009;405–406:343–9. [19] MBT-MBrace™ Fibre. ‘‘Product Data Sheet – MBT-MBrace CF 240”. [20] MBT-MBrace™ Resin Systems. ‘‘Product Data Sheet – MBT-MBrace Saturant”. [21] Jones DA. Principles and prevention of corrosion. New York: MacMillan Publishing Company; 1992. [22] ACI Committee 318. Building code requirements for structural concrete and commentary. Detroit (Michigan): American Concrete Institute; 2005.