behaviour rc tbeam under cyclic load

8/7/2019 behaviour RC Tbeam under cyclic load

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O R I G I N A L A R T I C L E

Behavior of RC T-section beams strengthened with CFRP

strips, subjected to cyclic load

H. Murat Tanarslan Sinan Altin

Received: 2 January 2008 / Accepted: 19 May 2009

RILEM 2009

Abstract This paper presents results of an experi-

mental investigation on T-section reinforced concrete

(RC) beams strengthened with externally bonded

carbon fiber-reinforced polymer (CFRP) strips. Spec-

imens, one of which was the control specimen and the

remaining six were the shear deficient test specimens,

were tested under cyclic load to investigate the effect

of CFRP strips on behavior and strength. Five shear

deficient specimens were strengthened with side

bonded and U-jacketed CFRP strips, and remaining

one tested with its virgin condition without strength-ening. The type and arrangement of CFRP strips and

the anchorage used to fasten the strips to the concrete

are the variables of this experimental work. The main

objective was to analyze the behavior and failure

modes of T-section RC beams strengthened in shear

with externally bonded CFRP strips. According to test

results premature debonding was the dominant failure

mode of externally strengthened RC beams so the

effect of anchorage usage on behavior and strength

was also investigated. To verify the reliability of shear

design equations and guidelines, experimental resultswere compared with all common guidelines and

published design equations. This comparison and

validation of guidelines is one of the main objectives

of this work. The test results confirmed that all CFRP

arrangements differ from CFRP strip width and

arrangement, improved the strength and behavior of

the specimens in different level significantly.

Keywords RC beam Strengthening Shear CFRP Cyclic load

List of symbolsa Shear span

d Effective height of the cross section

fc Compressive strength of concrete

L Length of the beam

Vexp Experimental shear forces of specimens

Vcal Calculated shear forces of specimens

eCU Maximum strain of concrete

/ Diameter of reinforcement

Conversion factors

1 mm 0.039 in1 mm2 0.00152 in2

1 kN 0.2248 kips

1 MPa 145 psi

1 Introduction

Many existing reinforced concrete (RC) members are

deficient in strength and in need of strengthening.

H. M. Tanarslan (&)

Department of Civil Engineering, Dokuz Eylul

University, Buca, Izmir 35160, Turkey

e-mail: [email protected]

S. Altin

Department of Civil Engineering, Gazi University,

Ankara, Turkey

Materials and Structures

DOI 10.1617/s11527-009-9509-8


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Construction faults or poor construction applications,

changes in standards, and reduction or total loss of

shear or flexural reinforcements due to corrosion are

some of the factors that cause deficiency. In order to

take the full advantage of ductility and hinder sudden

failure, it is desirable that flexure behavior governs

the ultimate strength rather than the shear. Shearfailure is brittle in nature and comes without adequate

warning. Therefore strengthening of shear deficient

structures is essential to prevent unwanted failure.

Strengthening can be in the form of rehabilitation of

structural members, repairing the damaged structures

or retrofitting the seismic deficiencies. Using exter-

nally bonded carbon fiber-reinforced polymer

(CFRP) for strengthening has become popular in

recent years due to its mechanical advantages. CFRP

sheets are both cost effective and durable, they are

tailorable, non-magnetic, have excellent fatiguebehavior, ease of handling and installation, minimal

disruption to the structure function, good corrosion

resistance and high strength to weight ratio.

The effect of CFRP for flexural strengthening has

been studied both experimentally and analytically [1,

2]. But there is still need to study the problem of shear

because of the interacted parameters; static scheme,

shear span-to-depth ratio (a/d), concrete strength,

interaction of internal reinforcements with strength-

ening material. Many of the earlier studies were

concerned with the proof of CFRP for shear strength-ening [35]. These studies indicated that using CFRP

for shear strengthening is an effective method that

improves the members strength and/or stiffness [6

9]. In these studies also the overall behavior and

failure modes of CFRP strengthened structures were

investigated [1, 1016]. After proving the effect of

CFRP as strengthening material, researchers pursued

to improve the usage of CFRP. The developed

technique has to be more economical and easy usage

than wrapping. Using CFRP as strips cover in demand

features. After determination of the new technique,using CFRP as strips, first the efficiency and than the

performance as shear reinforcement was investigated

[17]. Afterwards the behavior and failure modes were

observed. Nevertheless, monotonic loading was

applied in all these studies and performance of CFRP

strips was not investigated under earthquakes. Thus,

previous studies have to be supported by new studies

in which cyclic loads, which act like earthquake to the

structure, were exposed.

The bond between the CFRP and the concrete is

crucial in guaranteeing the effectiveness of strengthen-

ing when the structure is strengthened externally.

Previous tests subjected to monotonic loading indicated

that premature failure due to debonding was the major

problem for strengthened RC beams. Therefore, appro-

priate considerations must be given to hinder debond-ing. Particularly to prevent debonding researchers used

conventional anchorages. Actually, in literature very

limited amount of studies were encountered about

developed anchorages [18]. Therefore a new anchorage

detail was developed for the experimental program.

Then the performance of developed anchorages was

tested to investigate if it can advance the behavior of

CFRP strengthened RC beam.

This paper presents results of an experimental

study conducted on shear strengthening of RC

T-section beams with CFRP strips under cyclic loads.All seven beams except the flexural reference had no

internal shear reinforcements. Shear deficient speci-

mens were strengthened by using U-jacketed, and side

bonded CFRP strips while one of the shear deficient

specimen was tested with its virgin condition to serve

as a reference to shear deficiency. Developed mechan-

ical anchorage was applied to the U-jacketed speci-

men. The objective of this work was to investigate the

shear performance of strengthened specimens and to

determine the dominant factors that affect the behav-

ior and failure modes of the strengthened T-sectionRC beams under cyclic load. The parameters were

selected as to bear to the objective; (a) CFRP

distribution (b) CFRP orientation and (c) anchorage

usage. To evaluate enhancement of test results, on the

behavior, strength, stiffness and failure mode, were

compared with the control beams and then with each

other. In addition, to verify the reliability of shear

design equations and codes, experimental results were

compared with all common guidelines and published

design equations. This comparison and validation of

codes is one of the main objectives of this work.

2 Experimental program

2.1 Specimens and material properties

Seven T-section RC beams with various CFRP

schemes were manufactured and tested under cyclic

load in the experimental program. The cross-sectional



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procedure includes surface preparation, application ofpriming adhesive layer and bonding of the CFRP

sheets. First, the outer weak surface of the concrete

was removed with sand blasting. Afterwards, loose

particles on the surface of the specimens were

cleaned with compressed air. Once the surface has

been prepared for bonding, the epoxy resin was

prepared in accordance with manufacturers direc-

tions. The pores appearing on the concrete surface

was filled with pre-processed epoxy. Then epoxy

primer was coated at the designated places whereCFRP strips are going to be placed. Later on CFRP

sheets were placed on the coated epoxy primer and

constant pressure was applied on the sheet surface by

a roller to guarantee impregnation of the sheets by the

resin. Then another layer of epoxy was put on top of

the fabric and the extreme resin was cleaned. All

applications were performed at room temperature.

Specimen was cured for at least 15 days under

laboratory conditions before testing. The same

Specimens with w =50 mm (Specimen-7), with Anchorage

Specimens with w =100 mm (Specimen-5)

100

12

00

50x50x5 Steel Plates

50x50x5 L Shape Steel Plates 10 mm Dia. Bolt

1200

400 1675

50 Sf

Sf

50 Sf

Specimens with w =50 mm (Specimen-3, Specimen-4,

f

f

f

V

V

V

1200

285

360

75

285

360

75

28

5

75

3

60

Dimensions in mm.

400 1675

400 1675

Specimen-6)

ADETAIL

3020

25

25

12

12

10Steel bolt

CFRP

50x50x5Steel plate

Fig. 2 CFRP strip arrangements for specimen beams and anchorage detail



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strengthening procedure was carried out for all

strengthened specimens.

2.3 Experimental setup

A schematic view of experimental set-up and the

arrangement of the measurement devices are shownin Fig. 3. All specimens (cantilever beams) were

supported to the strong wall by the help of two

45.0 mm diameter high strength steel mounting rods.

To perform cyclic load to the specimen, a loading

column was designed with hinges by the beams free

end. Loading column contained two hinges, a load

cell and a hydraulic jack. The capacity of the

hydraulic jack was 1000.0 kN while the load cells

capacity was 600.0 kN. Load was applied in cycles of

loading and unloading. Load cycles were selected as

they will help to evaluate the flexure and shear crackspropagations and their affect to behavior. Same

loading cycles were applied to all specimens at the

initial state. After couple of cycles in elastic region,

flexural and shear cracks were occurred. After the

appearance of these cracks, specimens behavior

Table 3 Properties of CFRP and resin

Properties of CFRP

and resin

Remarks

Fiber orientation 0 (Unidirectional)

Construction Warp: Carbon fibers (99% of total areal

weight), Weft: Thermoplastic heat-set

fibers (1% of total areal weight)

Areal weight 220 10 g/m2

Fiber density 1.78 g/cm3

Fabric design

thickness

0.12 mm (based on total carbon content)

Tensile strength of

fibers

4,100 N/mm2

(nominal)

Tensile E modulus

of fibers

231,000 N/mm2

(nominal)

Strain at break of

fibers

1.7% (nominal)

Resin Two component (A and B)Resin mixture ratio A/B = 4/1 (Weight)

Resin tensile

strength

30 N/mm2

Tensile E modulus

of resin

3,800 N/mm2

Rigid Floor

LVDT

Rigid

Wall

Dimensions in mm.

Hinge

Hinge

Load Cell

Hydraulic Jack

Strain Gauge

Fig. 3 Test setup and

instrumentation



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changed because of their distinct shear load carrying

capacities. Loading was increased up to yield load of

flexural reinforcements or until the fall of the

specimen. For each increment of load, the strains

from strain gauges and vertical displacements from

LVDTs were recorded by means of an automatic

data acquisition system. Four linear variable differ-ential transformers (LVDTs) were used to monitor

displacements. The LVTDs are located at the end of

the beam for maximum displacement, under the rigid

support to calculate the undesired displacement and

finally on the rigid support to calculate the rotation.

Measurement of strain is evident to designate the

contribution of CFRP strips to shear capacity. Strain

gauges also help to determine the shear cracks before

propagation by the help of increase at strains. As the

main aim was to designate the contribution of CFRP

strips, eight strain gauges were attached at the sectionmid-height along the fiber direction. The number and

places of strain gauges were designated with consid-

eration where the shear cracks are expected to be

developed, between 150.0 and 1000.0 mm apart from

the beams support [19].

3 Experimental results and evaluations

3.1 Observed behavior and failure modes

Experimental results are summarized in Table 4. All

the major cracks were visually examined at the

experimental program. Control specimen showed

ductile flexural behavior as expected. Large dis-

placements were developed, and specimen reached

at an ultimate load value that was 11% greater than

the yield load. Also a plastic hinge was developed at

the maximum moment region. Specimen-2, which

was fabricated to designate the shear deficiency, was

failed in shear. As the load reached to 39.5 kN,

three main shear cracks were developed in shearspan and the beam failed in shear as can be seen

from Fig. 4.

First crack always appeared as a flexural crack for

all specimens. A linear behavior was observed since

then. Due to the load increments, the initially

developed flexure cracks were advanced through the

sides and caused shear cracks between the CFRP

strips. Besides, shear cracks were also developed at

the unstrengthened part of the specimen, between Table4

Experimentalresults

Specimen#

Ultimate

load

(Vu

)(kN)

Failuredisp.

(du

)(mm)

Initia

lstiffness

(kN/mm)

Stiffnessat

ultimateload

(kN/mm)

Increaseinultimate

displacementafter

strengthening(d

u,

n/d

u,

1)

Increasein

ultimate

loadafter

strengthening

(Vu,

n/V

u,

1)

Failuremode

atultimate

Specimen-1

Control

96.9

2

70.6

0

6.56

2.9

6

Flexure

Specimen-2

Control

39.5

2

8.7

0

6.15

3.6

2

Shear

Specimen-3

Sidebonding

61.6

3

17.7

5

6.33

3.3

3

2.0

4

1.5

6

Shear(debonding)

Specimen-4

Sidebonding

62.9

4

14.5

6

6.41

3.3

0

1.6

7

1.5

9

Shear(debonding)

Specimen-5

Sidebonding

68.4

8

17.7

3

6.70

3.0

1

2.0

4

1.7

3

Shear(debonding)

Specimen-6

U-jacketing

60.0

1

16.2

0

6.38

3.1

0

1.8

6

1.5

2

Shear(debonding)

Specimen-7

U-jacketing

80.6

7

29.8

9

6.89

2.9

3

3.4

4

2.0

4

Shear(rupture)

Forwardloadingstepwasconsideredforultimateloadandultimatedisplacement



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CFRP strips. These cracks then propagated towards

the CFRP strips and advanced along the interfacialconcrete. As the interfacial concrete started to

weaken, the bonds strength is reduced. Afterwards

the CFRP strips were separated from the concrete

surface. The same behavior was examined for all the

specimens that were failed because of debonding.

The failure mechanism of the debonded specimens

was presented in Fig. 5.

Because of debonding, all specimens except

Specimen-7 showed the same behavior until

50.0 kN load level. At about 20.0 kN, flexural cracks

appeared at the flange. Then, in the interval of 25.0and 35.0 kN, shear cracks were occurred at the

unstrengthened part, between the CFRP strips. After

exceeding 35.0 kN load level, shear cracks started to

develop faster, widen, propagate and undertake to the

body of the beam. This behavior was continued until

50.0 kN load level. After exceeding 50.0 kN load

level, dissimilar ultimate load levels were observed

due to the distinct load carrying capacities. However

the behavior and failure modes were exactly the

same. First some strips were separated from concrete

as can be seen from Fig. 6 (with a layer of concrete

adherent to them), then main shear crack waspropagated which is related to debonding. Hereafter,

specimens lost their load carrying capacities and

failed in shear consequently.

The behavior and failure mode of Specimen-7 is

totally different from formerly indicated specimens.

Specimen-7 strengthened in a manner similar to that of

Specimen-6. The main difference from the Specimen-

6 was the anchorage usage. The anchorage usage was

improved the behavior, therefore initial shear crack

propagation was delayed up to 80% when compared

with the specimens without anchorage. As the loadreached up to 80.7 kN and the deflection went up to

29.9 mm, 14th and 15th strips were compelled and

right after ruptured due to the stresses that were exceed

the limit that they can resist. As the strengthened part

lost its resistance against shear, a diagonal shear crack

was propagated abruptly at the decayed section as can

be seen from Fig. 7. The crack was advanced through

the top end of 10th strip and separated 11th, 12th and

13th strips from the concrete. Anchorage delayed the

strips from splitting at lower loads and also prevented

main shear crack propagation due to debonding.According to test results, the developed anchorage

worked so fine that U-jacketing with anchorage

denoted the best performance up to then.

Fig. 5 Failure mode, debonding

Fig. 4 Failure mode of Specimen-1




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3.2 Evaluation of the test results

To acquire strengthening effect, all strengthened

beams were compared with the control beams and

then with each other. Comparisons of specimens were

done by using response envelopes. Response enve-

lopes were drown by connecting peak points of

loading cycles and presented on Fig. 8. CFRP

reinforced specimens exhibited significantly higher

load-carrying capacity than that of the unstrength-

ened specimen, Specimen-2.To evaluate the contribution of strengthening

material strain activity of the strengthened specimens

was also evaluated. It must be point out that the strain

values, reported herein are not necessarily the max-

imum values. They are strictly related to the location

of the strain-gauges with respect to that of the shear

cracks. Actually, to be more realistic the largest strain

of the specimens is submitted here in Figs. 9 and 10.

It is obvious that maximum strain values that were

obtained from the specimens will give essential

information about the contribution of strengtheningmaterial to the shear resistance. When the measured

maximum strain values approach to the ultimate


-100

-80

-60

-40

-20

0

20

40

60

80

100

-40 -30 -20 -10 0 10 20 30 40

Displacement, mm

Shearforce,

kN

1

2

3

4

5

6

7

Specimen #

Fig. 8 Response envelopes

of specimen beams



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tensile strain, 0.017, the expected contribution to the

shear resistance is definitely achieved. Furthermore,

according to ACI-440 committee report, the effective

strain is the maximum strain that can be obtained in

the CFRP system at the ultimate load stage and is

governed by the failure mode of the CFRP system.

Loss of aggregate interlock of the concrete has been

observed to occur less than the ultimate tensile strain.To preclude this mode of failure, ACI-440 committee

report limited the maximum strain and proposes to use

0.004 mm/mm as maximum strain for design. Besides

if the measured strains are below the ACI-440

committee reports expected value that indicates the

expected contribution could not be obtained from the

strengthening material. With respect to that, when the

strain activity of the debonded specimens was

evaluated, it was observed that strain values were

definitely below the ACI-440 committee reports

expected value. Debonding, which cause prematurefailure, hinders to obtain the expected contribution

from the strengthening material.

The concrete strength of the test specimens plays

a great role especially when internal shear reinforce-

ments were omitted. Therefore while fabricating the

specimens extra care was conducted to achieve

similarity in concrete strength. Notwithstanding a

distinction with an interval of 6% less and 8% more

according to reference beam was materialized. While

-100

-80

-60

-40

-20

0

20

40

60

80

100

0,000 0,001 0,002 0,003 0,004

Strain, mm/mm

She

arforce,

kN

-100

-80

-60

-40

-20

0

20

40

60

80

100

0,000 0,001 0,002 0,003 0,004

Strain, mm/mm

Sh

earforce,

kN

-100

-80

-60

-40

-20

0

20

40

60

80

100

0,000 0,001 0,002 0,003 0,004

Strain, mm/mm

Shearforce,

kN

-100

-80

-60

-40

-20

0

20

40

60

80

100

0,000 0,001 0,002 0,003 0,004

Strain, mm/mm

Shearforce,

kN

Specimen-3

Specimen-5Specimen-6

Specimen-4

5060506050

Strain in-5 is maximum

795

5060506050


245


985


1090

(b)(a)

(d)(c)

Fig. 9 Load-maximum CFRP strain curves of Specimen-3, 4, 5 and 6

-100

-80

-60

-40

-20

0

20

40

60

80

100

0,000 0,001 0,002 0,003 0,004 0,005 0,006 0,007 0,008

Strain, mm/mm

Shearforce,

kN

Specimen-7

745

305030

5030

50


Fig. 10 Load-maximum CFRP strain curves of Specimen-7



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evaluating the below comparisons the effect of

concrete strength was not separately considered but

it must be point out that it will affect the evaluations.

However the comparisons will still give enough

knowledge to the researchers about the effect of

selected variables to the behavior. Specimen-3 was

strengthened with 50.0 mm wide CFRP strips, whichwere spaced at 60.0 mm, showed 57% less strength

than Specimen-1 and 56% more strength than Spec-

imen-2. Debonding was also affected the strain

behavior. The largest strain value of Specimen-3

was 0.002 mm/mm. That corresponds only to 50% of

the ACI-440 [20] committee reports expected value.

The recorded CFRP strain was indicated that the

failure was occurred at an average effective stress

level below the nominal strength due to debonding.

Specimen-4 showed 54% less and 59% more

strength than the flexural control specimen andunstrengthened shear deficient specimen, respec-

tively. The maximum vertical strain at the time of

failure was 0.0031 mm/mm (i.e. 78% of the ultimate

strain). By increasing the amount of strengthened area

(decreasing the spacing of CFRP strips from 60.0 to

30.0), an increase of 55% at contribution to the shear

resistance was obtained according to Specimen-3.

However debonding was still governing the behavior.

Debonding hindered to achieve the expected contri-

bution to the shear resistance as can be seen from the

recorded maximum CFRP strain value.Specimen-5 had reached 68.5 kN load level in

forward loading step but failed in that cycle while

backward loading. Specimen showed 73% more

strength than Specimen-2 and 42% less strength than

Specimen-1. The maximum localized CFRP strain

was in the order of 0.0036 mm/mm, which corre-

sponds to 89% of the ACI-440 committee reports

expected value. CFRP strips were contributed well to

the shear resistance, as be seen from the strain level.

Ultimate strain almost reached the ACI-440 commit-

tee reports expected value. However, debondingfrustrated to get the accurate contribution from the

strengthening material.

Specimen-6 was strengthened with 50.0 mm width

U-jacketed CFRP strips which were spaced at

60.0 mm. While planning the strengthening scheme

of Specimen-6, the main aim was to prevent

debonding by the help of orientation. According to

studies, debonding is rarely faced in which

specimens strengthened with U-jacketed CFRP [3].

However, Specimen-6 was still associated with a

rapid, sudden, and unstable separation of the bonded

CFRP strips at the upper ends. Specimen showed

62% less strength than Specimen-1 and 52% more

strength than Specimen-2. The maximum localized

CFRP strain was in the order of 0.0016 mm/mm,which corresponds to 40% of the ACI-440 committee

reports expected value. U-jacketed specimens

behavior was not obtained as planned due to

debonding of the CFRP at the upper ends. If

debonding at the upper ends could be prevented a

better utilization can be obtained from CFRP and

consequently a higher increase in shear capacity

could be obtained.

Specimen-7 was strengthened with U-jacketed

CFRP strips with end anchors. End anchors showed

significant performance for preventing CFRP frompeel off. By preventing peel of CFRP strips was

subjected to higher loads and therefore some CFRP

strips were ruptured. Actually, specimen almost

reached its ductile behavior if we compare with the

yield load of Specimen-1. Specimen-7 showed an

increase in capacity of 104% over reference. Spec-

imen-7 also measured the largest strain up to then,

which was 50% larger than ACI-440 committee

reports expected value, 0.006 mm/mm. Ultimate

load level and strain behavior clearly proved the real

effect of CFRP to shear capacity when debondingwas prevented.

In order to evaluate the contribution of CFRP

strips to shear capacity, initial and ultimate load

stiffness of the test specimens were also evaluated.

Initial stiffness were calculated by using the slope of

the lines that was connecting to origin and the load,

at which first flexural crack was occurred. When the

initial stiffness of CFRP strengthened specimens

were observed, it was seen that the initial stiffness

were shifted in an interval of 3% and 12% more than

that of the shear deficient reference specimen. If thestrengthening material provides less crack propaga-

tions until the ultimate load was reached, minor

decrease at ultimate load stiffness will be material-

ized. However as it was also visually examined,

many cracks were occurred at the shear span until the

fall of the specimens. Therefore the stiffness was

decreased to up to 25% less than that of the reference

specimen.



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4 Comparison of test results and design equations

The use of CFRP composites for shear strengthening

is a widely preferred method in recent years.

Therefore, almost all design standards have propa-

gated provisions to calculate the contribution of

CFRP. Additionally, due to lack of design standards,several researchers have also developed new analyt-

ical models to predict the nominal shear resistance for

CFRP.

In general, truss analogy was considered to find the

contribution of CFRP to shear capacity which is

similar to determine the contribution of steel shear

reinforcement. While predicting the CFRP contribu-

tion, tensile strength of CFRP, strain distribution in the

CFRP along the shear crack and the shear crack angle,

were considered as the effective parameters. Accord-

ing to these models all CFRP strips, intersected by themain shear crack, are assumed to contribute the same

average stress. The differences from one method to

other lies on how much effective stress developed at

ultimate state. Furthermore, shear crack angle gener-

ally assumed to be 45 in all provisions.

To predict the shear resistance due to CFRP,

Concrete Society [21] and ACI 440 are based on

work of Khalifa et al. [22], and fib [23] and Canadian

Standards Association (CSA-S806-02) [24] are based

on the work of Triantafillou [11]. Apart from them,

Chen and Teng [25, 26] developed a strip model,combined with the shear friction approach, based on

the bond mechanism observed from the tests. The

model used empirical expressions based on curve-

fitting of test results to define the effective stress of

CFRP. Analytical model made also clear distinction

between rupture failure and debonding, and devel-

oped two separate models.

For all these methods, total shear resistance of a

strengthened RC section is found as the sum of the

three components.

Vn Vc Vs Vf 1

where Vc is the contribution of concrete, Vs is the

contribution of internal steel shear reinforcement andfinally Vf is the contribution of CFRP at Eq. 1. In this

study only the concrete was contributed to shear force

carrying capacities because specimens do not include

internal shear reinforcements.

The analytical shear contributions of known models

and design standards were compared with the test

results and presented in Table 5. As can be seen from

Table 5, shear resistance due to CFRP, which were

calculated by CSA-S806-02, performed well with the

experiments except Specimen-7. As the predicted

shear resistances of all codes were compared, Chenand Tengs method produced the closest result to the

experimental results for specimen with anchorage.

Analytical shear resistance of the strengthened

specimens with respect to ACI-440 was found in the

interval of 5% and 11% less than the experimental

results. Concrete Societys analytical results were not

well-matched to the experimental values. Neverthe-

less, analytical shear load carrying capacities for fib

TG9.3 were denoted the biggest difference with the

range of 24% and 41% from experimental results. A

significant deviation between experimental andguidelines values for all predictions were observed

for Specimen-7 except Chen and Tengs proposal. To

prevent premature failure, anchorage was applied.

Due to the measures, specimen reached its maximum

shear capacity. The positive influence of anchorages

to shear capacity was not included in the ana-

lytical equations that were suggested by guidelines.

Table 5 Comparison of experimental and analytical results

Specimen # Calculated strengths Experimentalstrengths (kN)

Experimental/calculated

ACI 440

(kN)

fib

(kN)

Concrete

Society (kN)

CSA

(kN)

Chen and

Teng (kN)

Specimen-3 Side bonding 55.9 49.7 62.7 62.3 68.6 61.63 1.10 1.24 0.98 0.99 0.91



Specimen-6 U-jacketing 56.9 47.4 81.8 58.71 69.8 60.01 1.05 1.27 0.73 1.02 0.86

Specimen-7 U-jacketing 64.5 53.0 100.8 65.9 78.2 80.67 1.25 1.52 0.80 1.22 1.03

CSA Canadian Standards Association, CSA-S806-02



13/14

Therefore, the experimental results and analytical

provisions were not brought out close results. Con-

sequently, the proposed design equations can conser-

vatively predict the experimental test results. But the

influence of the ratio a/d and interaction of internal

shear reinforcement to shear capacity due to CFRP

was not included in the guidelines. The proposeddesign results will be more realistic if the parameters

that affect behavior have captured by the guidelines.

5 Conclusions

In this study, the shear performance of T-section RC

beams with shear deficiencies (without stirrups)

strengthened with different configurations of CFRP

strips were investigated. The test results indicated

that the shear strengthening effectiveness with CFRPstrips on the RC beams varies in function of the

spacing of CFRP strips, CFRP strips widths, strip

orientation and anchorage usage. But all in all an

increase in strength was seen in every specimen to

which CFRP applied, regardless of CFRP application.

The arrangement of CFRP strips was among the

effective parameters directing the strength of the

specimens. Experimental results of the Specimen-

3 and Specimen-4 showed that the ultimate

strength increased 3% when the spacing of CFRPstrips was decreased from 60.0 mm to 30.0 mm.

Specimen-4s ultimate shear capacity was

62.9 kN, corresponding to an increase of 59%

over the control beam. By increasing CFRP strips

width from 50.0 to 100.0 mm, a gain of 14% over

Specimen-4 was obtained. Increasing the strength-

ened area on the shear span delayed the initial

shear cracks propagations and a better utilization

was obtained from the strengthening material.

The effect of CFRP orientation to shear capacity

can be evaluated by investigating the behavior ofSpecimen-3 and Specimen-6. Specimen-3s ulti-

mate shear capacity was 61.6 kN that corresponds

to an increase of 56% over the control beam.

Although it is expected to obtain a better contri-

bution from Specimen-6, strengthened by U-

jacketed CFRP, only an increase of 56% over

the control beam was obtained.

Side-bonded and U-jacketed specimen beams,

without anchorage, were collapsed with brittle

shear failure because of debonding. In addition,

the recorded CFRP strain was also indicated that

the failure was occurred at an average effective

stress level below the nominal strength due to

debonding. This is one of the main problems of

CFRP strengthened RC structures. To overcome

debonding, a new mechanical anchorage wasdeveloped in the experimental program. New

developed mechanical anchorage was behaved

efficient under cyclic load. Although there were

no shear reinforcements in the specimens that tied

the beam web and flange together, top anchorages

prevented the separation of beam flange and web.

Top anchorages were also prevented peeling of

the CFRP strips from concrete. In addition, the

function of the end-anchors was to prevent the

premature peeling of the CFRP strips and it was

enormously prosperous under cyclic load. Anchorage application increased the ultimate

strength by 52% according to experimental results

of the Specimen-6 and Specimen-7. It also

prevented debonding at initial state and changed

the failure mode from debonding to rupture. It is

obvious that anchorage usage is the dominant

parameter to achieve the required strength and

behavior from the shear strengthened RC beam.

Shear resistance due to CFRP, which were

calculated by CSA-S806-02, performed well with

the experiments except Specimen-7. As thepredicted shear resistances of all codes were

compared, Chen and Tengs method produced the

closest result to the experimental results for

anchoraged specimen. All analytical results from

guidelines indicated that the used expression to

estimate the contribution of CFRP strips to the

shear capacity is acceptable.

The initial and ultimate load stiffness of Speci-

mens was up to 12% and up to 19% which more

and less than that of the shear deficient control

specimen, respectively. As the strength of spec-imen was increased specimens initial stiffness

was also increases. However, because of the

propagated cracks at the beam web, ultimate load

stiffnesss were decreased when compared with

the unstrengthened specimen, Specimen-2.

The tests performed in the presented series should

be the starting point for designating the behavior and

strength of strengthened RC beams with CFRP strips



14/14

under cyclic loads. It is hoped this study also helps to

understand the shear mechanism of a RC beam

strengthened with externally bonded CFRP strips.

Still further tests and more in depth study are needed

for confirming the degree of effectiveness of each

orientation and arrangement.

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