Modifying CFRP–concrete bond characteristics from pull-out testing

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Magazine of Concrete Research, 2015, 67 (13), 707–717

http://dx.doi.org/10.1680/macr.14.00271

Paper 1400271

Received 18/08/2014; revised 16/10/2014; accepted 27/11/2014

Published online ahead of print 22/01/2015

ICE Publishing: All rights reserved

Magazine of Concrete Research

Volume 67 Issue 13

Modifying CFRP–concrete bond

characteristics from pull-out testing

Haddad, Al-Rousan, Ghanma and Nimri

Modifying CFRP–concretebond characteristics frompull-out testingRami Haythem HaddadProfessor, Department of Civil Engineering, Jordan University of Scienceand Technology, Irbid, Jordan

Rajai Al-RousanAssociate Professor, Department of Civil Engineering, Jordan University ofScience and Technology, Irbid, Jordan

Lina GhanmaLecturer, Department of Civil Engineering, American University ofMadaba, Madaba, Jordan

Zaid NimriEngineer, Consolidated Contractors Company Ltd, Amman, Jordan

Most available models for the prediction of bond characteristics between carbon fibre reinforced polymer (CFRP)

composites and concrete are based on data from tests on pull-out specimens, but the use of such characteristics in the

analysis and design of reinforced concrete and CFRP strengthened or repaired beams could yield inaccurate

estimations. The present work aimed to develop empirical models to generate modification factors for bond

characteristics as obtained on small-size pull-out specimens. For this, an experimental programme was designed and

conducted to relate bond characteristics from pull-out tests to those from concrete beam bond specimens. CFRP plates

or sheets were bonded to both types of specimens at length and width ratios of 1/3, 2/3 and 1. The beams were tested

under four-point loading with load measurements acquired against CFRP elongation and its free-end slippage, whereas

pull-out specimens were tested for bond stress against free-end slippage. Using statistical modelling, the bond

characteristics from both types of specimens were correlated to obtain modification factors in terms of the geometric

properties of CFRP composites. The findings indicate that bond length and width ratios have respectively significant

and insignificant impact on bond characteristics, regardless of the bond type specimen employed. The modification

factors reveal that pull-out specimens tend to overestimate bond strength yet underestimate bond slippage at failure.

Notation BL bond length

B W bond width

bf /bc CFRP composite to concrete width ratio

f 9c compressive strength of concrete at 28 d

f t tensile strength of concrete Lf / Lc CFRP composite to concrete length ratio

MFS modification factor for bond slippage

MF modification factor for bond strength

S bond slippage

S max slippage at maximum bond stress

Æ1, Æ2 statistical linear regression factors

W geometric factor of repair to concrete width ratio

L geometric factor of repair to concrete length ratio

ª assumption factor

ªS stress concentration factor for S max

ª stress concentration factor for max

bond stress

max maximum bond stress

IntroductionStructural retrofitting has received increasing interest in recent

years, especially because of progressive deterioration of concrete

structures due to faults in design and construction practices and

the need to add more storeys to existing concrete buildings as a

consequence of dramatic increases in land prices in many main

cities and crucial locations worldwide. The repair or strengthen-

ing of structures requires inexpensive yet efficient strengthening

materials that can be easily attached to concrete members. In the past, steel plates were extensively used to repair or strengthen

structural members to improve or regain their original structural

capacity. Recently, however, the use of steel plates in such

applications has reduced due to their high weight, expensive

initial and maintenance costs, and their vulnerability to corrosion

attack (Ali et al., 2000; Jones et al., 1988; Oehlers and Ali,

1998). The repair of existing concrete elements with concrete

layers reinforced with traditional steel bars, steel or synthetic

fibres, or a combination of both, has been attempted, but their

application remains minimal owing to disadvantages of relatively

high weight, limited strength enhancements and undesirable

modifications to architectural appearance (Haddad and Ashteyate,

2001; Haddad and Smadi, 2004).

Enormous efforts have been made towards finding alternatives to

overcome these disadvantages. Recent works have proven that

fibre reinforced polymer (FRP) composites meet the engineering

707





concrete mixture used to fabricate the various specimens. Themasses per cubic metre of the different ingredients were deter-

mined using the ACI mix design method: the corresponding

values for cement, water, coarse aggregate, fine aggregate and

silica sand were 371, 231, 819, 650 and 123 kg/m3, respectively.

The concrete mixture, with 30 mm slump, achieved the target

compressive strength of 30 MPa.

Mixing of the concrete ingredients was performed in a tilting

type mixer according to ASTM C 685 (ASTM, 2005). Specially

made wooded moulds were used for casting of the beams and

concrete blocks, with inner dimensions of 150 3 200 3 1400 mm

and 150 3 150 3 100 mm, respectively. Concrete was placed in

the moulds in two layers and consolidated using a vibrating table,

before the final surface was smoothed by use of a trowel. The

cast specimens were covered with wet burlap for 24 h before

being placed in a water tank to cure for another 27 d.

Properties and bonding procedure for CFRP composites

CFRP sheets and plates, manufactured by SIKA and BASF,

respectively, were adhered to the concrete specimens using the

correct BASF resins. The tensile strength, tensile modulus of

elasticity and strain at breaking were, respectively, 3900 N/mm2,

230 000 N/mm2 and 1.5% for CFRP sheets and 2800 N/mm2,

165 000 N/mm2 and 0.8% for CFRP plates. The adhesive used to

bond the CFRP plates had a bond strength of 2.5 MPa, whereasthat used to bond the CFRP sheets had a compressive strength of

60 MPa after 7 d of curing. The epoxies used to bond the sheets

and plates to the concrete had mixed density values of 1 .06 and

1.7 g/cm3, respectively. Their mechanical properties, as provided

by the manufacturer (BASF), indicated bond and compressive

strengths in excess of 2.5 MPa and 60 MPa, respectively.

CFRP sheets and plates were bonded to 20 RC beams and plain

concrete blocks at various geometric configurations using the

correct resins; the results from two specimens were averaged and

used as the test value. The concrete areas to be bonded with

CFRP composites were roughened and treated to remove cementlaitance, loose and friable material; dust was removed using a

vacuum cleaner and the areas were treated with an organic solvent

to further clean the surface and reduce moisture content. Scissors

and a cutting machine were used to cut the desired dimensions of

CFRP sheets and plates, respectively. The epoxy resin was

prepared in a mechanical mixer using the correct proportions of

bonding materials and accompanying stiffener. The concrete

surface where the CFRP sheets were to be bonded was coated

with the resin epoxy before the sheets were placed and rolled over

to become saturated with the epoxy; they were then covered with

a layer of epoxy, using half the amount used for the first layer.

The CFRP plates were lightly hammered after being placed on

top of the epoxy layer painted onto the concrete surface.

Application of the CFRP composites to the concrete blocks (i.e.

the pull-out specimens) followed a similar procedure, with bond

lengths reduced to a tenth of those used for the beams. The two

Pull-outforce

CylinderCFRP sheetPlaster

tape

Lc

bc

bf

Frame gripped from hereby testing machine

(b)

Lf

Pull-outforce

CFRP plate

CFRP sheet

Plastertape

Lc

bc

bf

Pull-outforce

(a)

Lf

Concrete block

Figure 2. Schematic illustration showing the configurations of the

two types of pull-out bond test specimens: (a) far-end and

(b) near-end

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parallel faces of the concrete blocks of the far-end specimens

were marked such that the CFRP plates were centred across their

widths (Figure 2(a)). The CFRP plates were then adhered using

the specified epoxy to both faces of the blocks in two stages

during which a constant separation distance between both blockswas maintained using special steel fixtures. Furthermore, the

CFRP plates were anchored with CFRP sheets at one end of the

far-end pull-out specimens to force bond failure at other end. For

the near-end pull-out specimens (Figure 2(b)), the CFRP sheets

were bonded to a single concrete block such that the centre lines

of the sheets on both faces of the block were kept aligned and

parallel.

Load testing

The set-up for the beam bond test is shown in Figure 3. Whilethe beam was subjected to four-point test loading using a

hydraulic jack of 400 kN capacity at a loading rate of about

0.5 kN/s, mid-span deflection, slippage of the CFRP composites

and elongation of the middle portions of the CFRP strips or

sheets was measured using linear variable differential transfor-

Pull-out specimens Beam specimens

BL: mm BW: mm max:a kPa Slip: mm BL: mm BW: mm max: kPa Slip: mm

P-L4-W5 40 50 2800 0.022 P-L40-W5 400 50 430 a 2.00

P-L8-W5 80 50 2680 0.017 P-L80-W5 800 50 520 1.34

P-L12-W5 120 50 2300 0.012 P-L120-W5 1200 50 530 0.90

P-L8-W10 80 100 2880 0.021 P-L80-W10 800 100 640 1.58

P-L8-W15 80 150 2510 0.036 P-L80-W15 800 150 410 2.57

S-L4-W5 40 50 3900 0.130 S-L40-W5 400 50 310 3.40

S-L8-W5 80 50 2270 0.070 S-L80-W5 800 50 370 1.80

S-L12-W5 120 50 1670 0.021 S-L120-W5 1200 50 430 1.75

S-L8-W10 80 100 2400 0.190 S-L80-W10 800 100 420 1

.25S-L8-W15 80 150 2100 0.620 S-L80-W15 800 150 220 5.90

a Average from two test specimens

Table 1. Bond characteristics as obtained from pull-out and beam

specimens

Applied load p

Hydraulic jack

Load cell

Spread beam

Roller

FRP

LVDT(Mid-span deflection)

LVDT(Slippage)

Hinge

400 400 400

12001400

LVDT(FRP strain)

Figure 3. Schematic illustration of RC beam bond test set-up

(dimensions in mm)

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mers (LVDTs), mounted as shown. The different measurementswere acquired using a data acquisition system before being

analysed to obtain bond strength against slip relationships.

The pull-out test set-ups used to determine the bond behaviour

between concrete and CFRP plates or sheets are shown in

Figure 4. The CFRP plates and sheets were pulled out using a

tensile force acting at 0.2 kN/s, with slippage measurements

between CFRP and concrete acquired using two LVDTs

(mounted at the two opposite sides of the block). To ensure

similar stress transfer sharing of the two parallel plates or sheets

under pull-out loading, the top and bottom steel fixtures used in

the test set-up were aligned precisely along the centre lines of

the pull-out specimens, as illustrated in Figure 4. Consequently,

the failure modes of both types of pull-out specimens showed

no signs of twisting of CFRP plates or sheets. A data acquisi-

tion system was used to acquire load, elongation and slippage

measurements.

Results and discussion

Effect of geometric dimensions of CFRP on bond–slip

behaviour

Pull-out specimens

The bond–slip curves for pull-out specimens bonded to CFRP

plates and sheets followed a general non-linear trend behaviour similar to that reported in the literature and depicted in the

typical curves of Figure 5. The ultimate bond strength max and

slippage at failure were obtained for the different specimens and

are listed Table 1.

As can be seen from Table 1, the bond strength is inversely

proportional to the CFRP bond length: the ratio of bond strength at

bonded lengths of 120 mm and 80 mm to that at a bond length of

40 mm were 96% and 82% for CFRP plates and 58% and 43% for

CFRP sheets, respectively. This indicates that the effect of bond

length on residual bond strength is also affected by the type of

CFRP laminate. For a constant width ratio, the bond strengthshowed an increase, while the slippage showed a decrease with

bond length ratio. These behaviours are explained by the shear

stress distribution along the length of the CFRP composites.

According to Tounsi and Benyoucef (2007), vertical deviation

between the maximum and minimum shearing stresses along the

CFRP length is proportional to bond length. Rationally, the average

shearing stress would be the highest for CFRP composites bonded

at a length of 40 mm followed, in sequence, by bond lengths of

80 mm and 120 mm. Chajes et al. (1996) reported that, when

increased beyond a certain value, the bond length would not be fully

utilised in transferring stresses. The effect of CFRP plate bond

width on the bond behaviour can be observed in Table 1. By

increasing the bond width of the CFRP plate, the bond strength is

increased as long as the bond width is smaller than that of concrete.

When the width of the CFRP plate or sheet matches that of the

concrete, stress concentration is developed at the edges and thus

bond strength is reduced, as reported by Subramaniam et al. (2007).

(a)

(b)

Figure 4. (a) Far-end pull-out test set-up for blocks bonded to

CFRP plates. (b) Near-end pull-out test set-up for blocks bonded

to CFRP sheets

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Beam test specimens

The bond–slip curves of the beam specimens followed a similar

trend to those of the pull-out specimens, as shown in the typical

curves presented in Figure 5. The bond characteristics for differ-

ent beam specimens tested are also summarised in Table 1, whichshows that the bond strength is proportional to bond length but

inversely proportional to slippage. The bond strength behaviour

with bond length in beams contradicts that obtained from the

pull-out specimens owing to the presence of cracking in the

beams under loading. For the loading case considered, the highest

cracking intensity was observed in the high moment zone, with

lower flexural cracking intensity towards the end supports. CFRP

plates or sheets with smaller bond lengths would thus be more

detrimentally affected by the presence of cracking. The bond

width in the beams had a similar impact on bond trend behaviour

as that observed in the pull-out specimens.

It should be noted that, in order to study CFRP–concrete bond

behaviour, the different beams bonded to the CFRP composites

must attain close flexural loading capacities. This condition was

satisfied as the difference in load capacities of the various beams

was less than 5%. In addition, all the beams showed similar

failure modes; a typical cracking pattern is shown in Figure 6.The beams showed end interfacial debonding of the CFRP

composites, except for the case of CFRP plates bonded to

concrete along the full span of the beam when debonding

occurred at the plates’ middle third, corresponding to the high

moment zone. The CFRP composites separated from the concrete

through surface peeling of concrete, as shown in Figure 7.

Modelling bond behaviourThe results obtained from testing the 20 RC beams and pull-out

specimens showed that

j the bond–slip curves show an increasing trend behaviour up

to ultimate bond stress

j this portion of the bond–slip curves is completely non-linear,

as shown in Figure 5.

Based on these findings, a model similar to that of Lu et al.

(2005) is proposed to describe the bond–slip behaviour between

CFRP composites and the concrete surface of either beam or

0

0·5

1·0

1·5

2·0

2·5

3·0

0 0·1 0·2 0·3 0·4 0·5 0·6

Bondstress

:MPa

τ

Slip: mm(a)

S-L8-W5 S-L8-W10

S-L8-W15

0

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

0 1 2 3 4 5 6

Bondstress

:MPa

τ

Slip: mm(b)

P-L80-W10

P-L80-W5P-L80-W15

Figure 5. Bond–slip curve from (a) pull-out and (b) beam

specimens, bonded to CFRP sheets at various widths

Figure 6. Cracking pattern of beam P-L80-W5

(a)

(b)

Figure 7. Concrete skin peeling as appeared on surfaces of

(a) CFRP sheets and (b) CFRP plates

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pull-out specimens, with parameters determined using the data presented in Table 1. The model states

¼ max

S

S max

1=2

if S < S max1:

where is the bond stress and S is the corresponding bond

slippage, max is the maximum bond stress and S max is the

corresponding slip. The bond characteristics are expressed as

max

¼ Æ1ª

W

L f

t2:

S max ¼ Æ2ªS W L f t3:

in which f t is the tensile strength of concrete, related to the

compressive strength of concrete ( f 9c) through f t ¼ 0.33( f 9c)1=2

(Reinhardt et al., 1986), and ª and ªS are the stress concentra-

tion factors for max and S max, respectively. The ª factors are

assumed equal to 1 except for specimens having a CFRP width to

concrete ratio of 1 in which here ª was assumed to be 0 .9 and 2

for the computation of max and S max, respectively. W and L aregeometric factors of the repair to concrete width ratio and repair

to concrete length ratio, respectively.

The expressions listed in Table 2 were reached by trial and error.

The formulae were first assumed based on existing literature

before being modified to enhance the predictability of maximum

bond strength and slippage by way of Equations 2 and 3. Three

geometric factors were calculated based on the present results

considering repair to concrete width or repair to concrete length

ratios of 1/3, 2/3 and 3/3. Those, along with assumed and

calculated parameters of Equations 2 and 3, were used to obtain

the parameters Æ1 and Æ2 by statistical linear regression, as

depicted in Figure 8; the results are summarised in Table 2.Except for one set of data, the fitting potential of the linear model

for the present data can be rated as excellent, with R2 values

exceeding 90%.

Predictability of developed models

The results from previously reported pull-out tests were used to

compare the predictability of the proposed model. The predic-

tions of bond strength using the present model with respect to

actual values provided by different researchers (Al-Rousan et

al., 2013; Haddad et al., 2013; Ren, 2003; Takeo et al., 1997;

Tan, 2002; Wu et al., 2001; Zhao et al., 2000) are summarised

in Table 3. The ratio of predicted to actual bond strength varied

from 0.94 to 1.96 and 0.89 to 3.11 for data on CFRP plates

and sheets, respectively. This means that the precision of the

predictions was affected by the data source. Yet, considering the

heterogeneity of the data, it can be said that the proposed model

possesses moderate to high prediction potential of bond

strength.

To examine the proposed models’ predictability further, three

different models from the literature (Lu et al., 2005; Monti et al.,

2003; Neubauer and Rostasy, 1999) were used along with the

present model to predict bond strength based on the present data.

The ratio of predicted to actual bond strength ranged from 1 .00

to 1.72 with relatively high divergence from unity for the modelsof Monti et al. (2003) and Neubauer and Rostasy (1999),

especially when data from the tests on CFRP sheets were used.

The results also indicate that the model proposed by Lu et al.

(2005) shows high to moderate predictability depending on

whether the data used were obtained from tests on CFRP plates

or sheets. The present model, as may be expected, showed the

best predictability potential. It must be noted, however, that the

ratios of predicted to actual bond strength are averages from data

clusters corresponding to different references. It is thus probable

that, for particular data points, the ratio of predicted to actual

bond strength would deviate significantly from the averages

reported here. The coefficients of variation obtained can be

Specimen L W CFRP plates CFRP sheets

Æ1 3 103 Æ2 3 103 Æ1 3 103 Æ1 3 103

Pull-out ln 0.9 (Lf=Lc)

0.9 þ (Lf=Lc)

1=2

þ 8.8 241 1.54 205 6.34

Beam Bond stress, plate: ln 1 þ (Lf=Lc)

2 (Lf=Lc)

1=2

þ 2 2 þ (bf=bc)

4 þ (bf=bc)

1=2

170 10 180 NA NA

Beam Slippage, sheet: e2.205 1 þ (Lf=Lc)

2 (Lf=Lc)

1=2

NA NA 170 10 180

Table 2. Bond stress–slippage model parameters of Equations 2

and 3. Lf / Lc and b f / bc are CFRP composite to concrete length and

width ratios

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0

0·1

0·2

0·3

0·4

0·5

0·6

0 1 2 3 4

τ max:MPa

γτ L w tf

(a)

CFRP plate

τ max 0·17 γτ L w tf

R2

0·99

0

1

2

3

4

0 0·10 0·15 0·20 0·30

Slip:mm

γS L w tf

(b)

0·250·05

CFRP plate

S max S L w tγ f 10·2

R2

0·99

0

0·1

0·2

0·3

0·4

0·5

0·6

0 1 2 3 4

τ max:MPa

γτ L w tf

(c)

CFRP sheet

τ max 0·17 γτ L w tf

R2

0·99

0

1

2

3

4

0 0·10 0·15 0·20 0·30

Slip:mm

γS L w tf

(d)

0·250·05

CFRP sheet

S max S L w tγ f 10·2

R2

1·00

Figure 8. Linear fit of experimental data to obtain Æ factors for

CFRP plates and sheets bonded to concrete beams

Data source Predicted/actual bond strength Coefficient of variation Correlation coefficient

Plates Sheets Plates Sheets Plates Sheets

Takeo et al. (1997) 0.94 0.89 0.237 0.21 0.252 0.252

Tan (2002) 1.29 1.1 0.174 0.166 0. 77 0. 773

Zhao et al. (2000) 1.96 1.67 0.22 0.22 0.766 0.766

Ren (2003) 1.1 0.94 0.24 0.237 0.862 0.862

Al-Rousan et al. (2013) 1.1 1.1 0.14 0.14 0.011 0.011

Haddad et al. (2013) 1.3 1.3 0.152 0.152 0.032 0.032

Wu et al. (2001) NA 3.11 NA 0.44 NA 0.0122

NA, not available

Table 3. Average ratio of predicted bond to test bond strength

using the present model based on data from literature (based on

pull-out tests)

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classified as low to moderate depending on the heterogeneity of the data used.

A lack of literature regarding bond strength or slippage behaviour

between CFRP composites and RC beam elements made it

difficult to examine the predictability of the present models of

Table 2. However, the relatively high fit potential of the present

models to the present data can be considered as a strong indicator

of satisfactory predictability. In addition, the previous discussions

of bond results from beam specimens were rationalised against

the key parameters considered, namely CFRP form (plates or

sheets) and bond length and bond width ratios.

Modification factor for size effect

The ultimate bond strength and slippage as obtained from testing

small pull-out specimens bonded to either CFRP plates of sheets

showed, respectively, higher and lower values compared with those

obtained from testing real bond test specimens of similar repair

configuration and geometric ratios. This can be explained by the

fact that the concrete of the pull-out specimens did not suffer any

significant cracking on loading, contrary to the beams – where

cracks were generated at the tension side of the high moment zone

at relatively low loads before cracks extended to the shear zone

prior to beam failure. Modification factors thus need to be applied to bond characteristics when measuring using pull-out specimens

or estimated from corresponding empirical bond–slip models.

The modification factors for bond strength and slippage applied

to pull-out measurements or relevant bond–slip models are

defined as

MF ¼Beam

Pull-out specimen4:

for bond strength and

MFS ¼S max, Beam

S max, Pull-out specimen5:

for bond slip.

The modification factors are depicted against bond length and

bond width in Figure 9. It can be concluded that the modification

factors for both bond strength and slippage are dependent on the

0

0·05

0·10

0·15

0·20

0·25

0·30

1/3 1/3

1/31/3

2/3 2/3

2/32/3

3/3 3/3

3/33/3

Modificationfactor(MF)

Ratio

CFRP plates

Bond strength

Bonded length effectBonded width effect

MF 0·179 MF 81·22

MF 0·211

MF 0·13 0·012e 2·1 x MF 1·58e

1·004 x

MF 0·15 0·017e 1·9 x

0

0·05

0·10

0·15

0·20

0·25

0·30


Ratio

CFRP sheetsBond strength


0

20

40

60

80

100

120


Ratio

CFRP platesSlip


0

10

20

30


Ratio

CFRP sheetsSlip


MF 19·724

MF 3·97e 1·1 x

Figure 9. Modification factors applied to bond strength and

slippage between CFRP plates or sheets and concrete as

estimated by pull-out-test models

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bond length ratio rather than the bond width ratio. The corre-sponding reduction and magnification in bond strength and

slippage from pull-out specimens were relatively high, reaching

85% and 120 times, respectively. It is evident that the modifica-

tion factors were not significantly affected by either the type or

materials characteristics of the CFRP composites used. It is

important to note that the modification factors reported here may

not be applicable for deteriorating concrete as the degradation

level in bond characteristics would be affected, among many

other factors, by the type of bond test specimen. Further research

is thus needed to generate new modification factors for the bond

characteristics between damaged concrete and CFRP composites.

Summary and conclusionsAn experimental programme was designed and conducted to

relate bond characteristics from tests on pull-out specimens to

those from concrete beam specimens. CFRP plates or sheets were

bonded to both types of specimens at bond length and width

ratios of 1/2, 2/3 and 3/3 of their dimensions. The beams were

tested under four-point loading with load measurements acquired

against CFRP elongation and free-end slippage. The pull-out

specimens were tested for bond stress against free-end slippage.

The bond width of CFRP plates and sheets affected the bond

behaviour of beams and pull-out specimens in the same manner:

the bond strength increased with increasing bond width as longas the bond width was smaller than the width of the concrete.

The models proposed for predicting bond strength and corre-

sponding slippage of pull-out and beam specimens provided a

very good fit to data from the present work and the literature.

The developed formulae for the modification factors were

sensitive to CFRP bond length rather bond width and provide an

acceptable and conservative estimate for bond characteristics of

beams bonded to CFRP composites. The present findings also

revealed that the form and material characteristics of the CFRP

composites used had negligible impact on the modification

factors to be applied to bond characteristics obtained frommeasurements on pull-out specimens or from model predictions.

AcknowledgementThe authors acknowledge the technical and financial support

provided by Jordan University of Science and Technology.

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C o p y r i g h t o f M a g a z i n e o f C o n c r e t e R e s e a r c h i s t h e p r o p e r t y o f T h o m a s T e l f o r d L t d a n d i t s

c o n t e n t m a y n o t b e c o p i e d o r e m a i l e d t o m u l t i p l e s i t e s o r p o s t e d t o a l i s t s e r v w i t h o u t t h e

c o p y r i g h t h o l d e r ' s e x p r e s s w r i t t e n p e r m i s s i o n . H o w e v e r , u s e r s m a y p r i n t , d o w n l o a d , o r e m a i l

a r t i c l e s f o r i n d i v i d u a l u s e .

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