Composite Structures 95 (2013) 719–727

Contents lists available at SciVerse ScienceDirect

Composite Structures

journal homepage: www.elsevier .com/locate /compstruct

Bond strength of near surface-mounted FRP plate for retrofit of concrete structures

Soo-Yeon Seo a,⇑, Luciano Feo b, David Hui c

a Department of Architectural Engineering, Korea National University of Transportation, Chungju, South Koreab Department of Civil Engineering, University of Salerno, Fiscano (SA), Italyc Department of Mechanical Engineering, University of New Orleans, New Orleans, LA, USA

a r t i c l e i n f o

Article history:Available online 30 August 2012

Keywords:A. Externally Bonded RetrofitB. Near Surface-Mounted RetrofitC. FRP plateD. Bond testE. Bond lengthF. Shear key

0263-8223/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compstruct.2012.08.038

⇑ Corresponding author. Tel.: +82 43 841 5205; faxE-mail address: syseo@ut.ac.kr (S.-Y. Seo).

a b s t r a c t

Recently, Fiber Reinforced Polymer (FRP) has been widely applied in retrofit of concrete member owing toits ease construction and high structural capacity compared to other materials. Externally Bonded Retrofit(EBR) using FRP sheet or plate is most popular method for flexural retrofit of concrete member. Instrengthened concrete member using this method, however, problems such as dismantling of FRP werefounded after retrofit. This causes a strength reduction of the retrofitted member. To resolve this problem,Near Surface-Mounted Retrofit (NSMR) method was developed and has been being studied.

In this paper, bond capacities of EBR and NSMR using FRP plate were studied experimentally andresults were compared each other. Also the variation of bond strength of NSM FRP plate was evaluatedaccording to bonded length as well as number of shear key.

From the test results, it was found that the member strengthened by NSMR had almost 1.5 times higherbond strength than that by EBR. The contribution of shear key was not observed when the split failure ofconcrete governed.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Retrofit of concrete member in use of Fiber Reinforced Polymer(FRP) depends on what kind of FRP as retrofit material and how toretrofit. The most general method is external retrofit by attachingor wrapping FRP shaped sheet or plate, to concrete surface withepoxy. Up to now, many structures have been retrofitted withthe aforementioned method. But the problems like drop-out inepoxy parts have been reported in many cases due to insufficientdevelopment of bond strength between FRP and concrete. Thisphenomenon is sometimes attributable to the issues of mainte-nance since the FRP retrofitted to concrete surface is greatly ex-posed to the effects from the external environment. In particular,the characteristic of epoxy, which is being vulnerable to fire risk,makes the effect of retrofit almost gone when the structure is ex-posed to high temperature so that additional fire-proof treatmentis required.

To improve such problems, studies on Near Surface-MountedRetrofit (NSMR) methods have been substantially activated re-cently [1–11]. In this method, FRP bar is inserted into slit formedin cover concrete and bonded with concrete using epoxy mortars.Even though it requires additional works of slit formation in con-crete, it is effective in reducing the environmental effects afore-mentioned since FRP is not exposed on.

ll rights reserved.

: +82 43 841 5200.

The excellence of NSMR method is well identified in the studyof Nami [4] of a bending test conducted for high-way bridges. Fromthe test, NSMR using bar-type FRP in concrete cover showed tensilefailure of FRP with strong bond capacity while Externally BondedRetrofit (EBR) using FRP plate to surface peeled off from concreteshowing bond failure between cover concrete and FRP plate. Andthe NSMR method was more effective with contribution rate of29% than the EBR method with 17%.

According to the bending test results of T beams retrofitted withNSM FRP bars by DeLorenzis [5], however, the final failure wasdecided by bond failure of FRP bar even though the retrofit effectincreases to maximum 44%.

EI-Hacha and Rizkalia [6] verified and reported that NSMR hasfar higher retrofit capacity compare to EBR and in particular, NSMRmethod using FRP plate embedding into slit in cover concrete ismost effective from bending tests in use of surface bond retrofitwith same axis rigidity, FRP plate and FRP bar.

Yost [7] verified that NSMR enables the substantial improve-ment of strength and rigidity but causes the reduction of displace-ment ductility and energy ductility from a bending test forreinforced concrete member with a variable of retrofit amount ofNSM FRP plate and longitudinal reinforcement ratio.

These studies well illustrate the retrofit efficiency of NSMRcompare to EBR; but suggest the ultimate bond failure of the firstmethod and the necessity of studies to prevent such a problem.

To tell the existence of bond failure in NSMR, it is desirable todesign proper bond length to prevent bond failure based on

400 140 400

150(200,300)

FRP sheet

FRP plate

FRP plate

Shear key (φ20)

FRP sheet

FRP plate FRP sheet

(a) EBR specimen

(b) NSMR specimen with one layer

(c) NSMR specimen with three layers

Fig. 2. Specimen configuration.

720 S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727

understanding on bond features of retrofitted FRP and concrete.The objective of this study is to assess the anchorage effectsdepending on bond length, shear key in concrete for NSM retrofitby making a laboratory verification so that ultimately, evaluate de-sign bond strength by comparing previous formulae.

2. Experimental program

2.1. Design and production of specimen

Variables for experiment are retrofit method, bond length, shearkey and number of FRP. As retrofit methods, EBR and NSMRembedding FRP plate in slit of cover concrete were considered asshown in Fig. 1. In order to find variation of bond strength due tobond length, specimens with different bond length such as150 mm, 200 mm and 300 mm were designed.

Fig. 2 and Table 1 represent specimen configuration and speci-men features list, respectively. £30 mm pipe was inserted at thecenter of concrete blocks of 200 mm � 200 mm � 400 mm. Thispipe is to pull each block in opposite direction using tension boltspenetrating it during tests. The concrete used for specimen isready-mix concrete with 28 days strength of 24 MPa.

FRP plate used for the test is carbon fiber with thickness of1.2 mm and width of 50 mm and for EBR, one side of it is bondedto surface of concrete blocks while for NSMR, the FRP plate of50 mm in width is divided into three with 16 mm in width andthose divided are layered into one to be bonded and embeddedinto slit with width of 7.1 mm and depth of 20 mm. For specimenswith three layers, each divided FRP is embedded in concrete withkeeping space of 50 mm.

The cross section areas of retrofit materials are ensured as sim-ilar to examine the differences in bond capacity by retrofitmethods.

In the case with shear key, the shear key of 20 mm in diameterand 17 mm in depth was made by using concrete drill. When therewas only one shear key, it was installed at 50 mm location from theload direction and with two shear keys, the installation is made atan interval of 65 mm (bond length 150 mm), 90 mm (bond length200 mm) and 140 mm (bond length 300 mm). To prevent the pre-mature failure of non-measurement parts prior to that of measure-ment parts, the bond length was set long enough at 400 mm fornon-measurement part. In addition, FRP sheet of 300 mm in widthwas additionally retrofitted to non-measurement parts. Photo 1represents specimens after retrofit.

The material properties for specimens are illustrated in Table 2and 3. As for FRP retrofit material, Carbodur-Plate S512/80 of Sika

Fig. 1. Concept of EBR and

was used and bonding resin was two mixing liquid resin in linewith certain mixing ratio after over 7 days of curing at 20 �C.

2.2. Test method

The test set up and associated specimen details are illustrated inPhoto 2. One of two concrete blocks connected by FRP was pulledto opposite direction by using £25 mm tension bolt penetratingcenter of the concrete block until failure. Rollers were installedbottom of each concrete block as well as loading steel block in or-der to minimize friction between base and the concrete blocks.Small frame, which was made of angles, covering concrete blockis installed for each concrete block to hang Linear Volumetric Dif-ferential Transducer (LVDT) to measure the opening displacement.Nut shaped hemisphere was connected to the part of each concreteblock of tension bolt for hinge condition.

NSMR using FRP plate.

Table 1Specimen list.

Specimen Bond length (mm) Retrofit type FRP Shear key

Thickness (mm) Width (mm) Layer

E150 150 EBR 1.2 50 No.E200 200E300 300

N150-1 150 NSMR No.N200-1 200N300-1 300N150-1-1S 150 3.6 16 1 1N200-1-1S 200N300-1-1S 300N150-1-2S 150 2N200-1-2S 200N150-3 150 No.N200-3 200N300-3 300N150-3-1S 150 1.2 16 3 1 per each FRPN200-3-1S 200N300-3-1S 300N150-3-2S 150 2 per each FRPN200-3-2S 200N300-3-2S 300

Table 2Material properties of FRP-Plate.

Type Thickness(mm)

Tensile strength(MPa)

Modulus ofelasticity (MPa)

Carbodur-plateS512/80 1.2 2800 160,000

Table 3Material properties of resin.

Type Compressivestrength(MPa)

Tensilestrength(MPa)

Shearstrength(MPa)

Modulus ofelasticity(MPa)

Sikadur� -30 70 28 18 128,000

S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727 721

Strain gages are bonded at a 30 mm interval; five for specimenof 150 mm in bond length; seven for 200 mm; and ten for 300 mm.As for NSMR specimens, FRP is embedded after pre-bonding ofgages to FRP surface.

2.3. Test result and analysis

2.3.1. Failure shapeWhile EBR specimens failed showing peeling off of FRP at low

load, NSMR specimens showed the phenomenon of concrete split-

Photo 1. Specimen

ting within the embedded depth of FRP as shown in Photo 3. There-fore, wider failure area was measured in NSMR specimens. Fromthis, using NSMR method, it is possible to improve anchoragecapacity due to increased bond area between concrete and FRP.

There were not any big differences of failure shape regarding tothe existence and number of shear key in NSMR specimens. This isbecause NSMR specimens already have a full bond capacity of FRPso that additional shear keys are not able to contribute. In EBRspecimens, the failure pattern was similar to each other regardlessof the bonded length. On the contrary, in NSMR specimens, thelonger bond length it had, the wider failure region in concrete itshowed.

2.3.2. Load–slip curveLoad–slip relation of specimens was compared each other cor-

responding to the test parameters. From Figs. 3–5 representload–slip curve of EBR and NSMR specimens with one-layer andthree- layer for each bond length. NSMR specimen with one-layerFRP has smaller bond area than EBR specimen even if their area ofFRP is same; but has over two times higher bond strength verifyingthe fact that the embedded retrofit brings about high strength. Alsoeven if same amount of FRP was embedded in concrete, NSMRspecimen with three-layer showed higher bond strength thanone-layer.

Load–slip curves of specimens retrofitted by NSMR with differ-ent bond length are given in Fig. 6. The longer bond length results

after retrofit.

E150

Tension bolt

Roller Roller

Oil jack

Photo 2. Test set up.

Photo 3. Typical failure shapes of specimens.

0

40

80

120

160

0 1 2 3 4

Loa

d (k

N)

Displacement (mm)

E150

N-150-3

N150-1

Pδ

Fig. 3. Load–displacement curve of specimen with bond length of 150 mm.

0

40

80

120

160

0 1 2 3 4

Loa

d (k

N)

Displacement (mm)

E200

N-200-3

N200-1

Pδ

Fig. 4. Load–displacement curve of specimen with bond length of 200 mm.

722 S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727

in the increase of strength but the stiffness variation was not clear.The increase of strength was not linearly proportional to bondlength.

As for the stiffness changes by each series, NSMR specimen re-cords higher bond stiffness in general compared to EBR specimen.It is considered that the bond area of NSMR specimen is larger thanthat of EBR specimen so that stiffness gets higher.

As stated in the part of failure shape, the case of EBR shows thephenomenon of peel off in the bonded area regardless of the exis-tence of shear key while it is considered that the case of NSMR hashigh bond capacity so that the internal force is supported enoughuntil the concrete within the embedded depth of retrofit materialsare generally dropped out. It is assumed that the not-clear contri-bution of shear key as shown in Fig. 7 is attributable to the fact thatbond strength between NSM FRP and concrete is high so that the

overall internal force is decided by drop-out failure of concrete. Be-cause of this, there was not a difference of strength between spec-imens with different number of shear key.

2.3.3. Strain of FRPTypical strain distribution of FRP was illustrated in Fig. 8 for

EBR, NSMR with one-layer and three-layer. In each figure, the hor-izontal axis is the location of strain gauge from concrete block sur-face to load direction while (–) distance shows non-bonded parts.

Overall, the high strain was observed in the location close toload direction and the longer distance resulted in smaller valuesof strain, meaning that the closer to the stress direction results inhigher stress taking ratio so that the anchorage in this part isimportant for effective anchorage of FRP.

0

40

80

120

160

0 1 2 3 4

Loa

d (k

N)

Displacement (mm)

E300

N-300-3

N300-1

Pδ

Fig. 5. Load–displacement curve of specimen with bond length of 300 mm.

0

40

80

120

160

0 1 2 3 4

Loa

d (k

N)

0

40

80

120

160

Loa

d (k

N)

Displacement (mm)

0 1 2 3 4

Displacement (mm)

N150-1

N150-3

N300-1

N300-3

N200-1

N200-3

P

P

δ

δ

(a) One layered NSMR specimen

(b) Three layered NSMR specimen

Fig. 6. Comparison of load–displacement curve between specimens with differentbond length.

0

40

80

120

160

0 1 2 3 4

Loa

d (k

N)

Displacement (mm)

N200-1

N200-1-2S

N200-1-1S

Pδ

Fig. 7. Comparison of load–displacement curve between specimens with differentshear key condition.

S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727 723

Compared to EBR specimen, NSMR specimen illustrates high le-vel of strain, interpreted as the capability excellence of NSMRagainst EBR in terms of effectiveness in using FRP plate. What

extraordinary is that as for NSMR specimens at 300 mm in bondlength, the strain of FRP plate at 10 mm in bonding location recordshigher strain level compared to that of non-bonded parts and thisphenomenon is observed from low load level. It means that stressis concentrated to the connecting part of embedded part and inparticular, where embedding starts while well-distributed strainof the total length in non-bonded section is observed.

From the comparison of strain distribution between specimenswith one-layer and three-layer, it can be seen that the formershows rapid decrease from concrete surface at around ultimatestate while the latter shows linear decrease. This means the in-crease of number of layer may effect on the stress concentration.In order to find this effect, the effect of space between FRPs mustbe included.

2.3.4. Comparison of strengthTable 4 shows the initial and ultimate loads of each specimen

with failure pattern. All of NSMR specimen failed showing concretebreak out failure while EBRs de-bonding failure. Therefore, thepeak strengths of EBRs are less than those of NSMRs. In Fig. 9, ulti-mate loads of all specimens were compared and the values for Nseries specimens mean average value of each three specimens.From the figure, even if same amount of FRP is used, the strengthcan be varied according to retrofit method. The bond strength ofNSMR is at least 1.7 times in one-layer and at least 2.3 times inthree-layer than that of EBR.

3. Evaluation of bond strength of NSM FRP

In order to evaluate bond strength of NSM FRP, severalresearchers presented a few formulae based on experimental data.The strengths of test specimens are calculated by using those for-mulae and compared with test results to check usefulness.

Eq. (1) represents formula by Ali [8] to calculate the bondstrength of NSM FRP. In the equation, only concrete strength andsectional shape of FRP are considered as main factors to affectthe bond strength of NSM FRP without considering the effect ofbond length.

sf ¼ 0:54ffiffiffiffiffifck

pb0:4

f t0:3f 6 fyf ½N=mm2� ð1Þ

where fck is concrete compressive strength, bf and tf are width andthickness of FRP, respectively, fyf is yield strength of FRP.

Seracino et al. [9] found that strip width had more influence onfailure load than strip thickness from a nonlinear regression anal-ysis for his test data and suggested Eq. (2) as a modified one. What

Fig. 8. Strain distribution.

724 S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727

have changed are the values of exponents of width and thicknessand the unit of calculated result. In the manner of parameter

consisting the formula, there are not any changes between Eqs.(1) and (2).

Table 4Test results.

Specimen Initial crack Pcr (kN) Ultimate state Failure patterna

Load (kN) du (mm)

E150 41.18 44.13 0.79 DE200 49.03 53.94 1.015 DE300 48.05 64.72 1.1 DN150-1 86.29 88.26 0.92 C (Type-1)N200-1 89.24 90.21 1.28 C (Type-1)N300-1 98.06 125.52 1.57 C (Type-2), YN150-1-1S 86.78 90.22 1.5 C (Type-1)N200-1-1S 96.1 100.02 1.715 C (Type-2)N300-1-1S 95.12 100.03 2.62 YN150-1-2S 74.53 90.22 1.29 C (Type-1)N200-1-2S 86.29 101.99 1.74 C (Type-2)N300-1-2S 89.24 115.72 2.2 C (Type-2)N150-3 65.7 130.43 1.59 C (Type-3)N200-3 129.44 133.39 1.82 C (Type-3)N300-3 117.67 158.87 2.01 C (Type-3)N150-3-1S 105.91 134.35 1.92 C (Type-3)N200-3-1S 103.95 152.00 1.89 C (Type-3)N300-3-1S 119.64 155.93 3.06 C (Type-3), YN150-3-2S 106.89 136.31 2.05 C (Type-3)N200-3-2S 101.98 146.12 2.28 C (Type-3)N300-3-2S 119.64 136.31 2.22 C (Type-3), Y

a Failure pattern: C = concrete break out; D = debonding of FRP; and Y = yield/fracture of FRP; type in blank means the failure pattern shown in Photo 2.

Fig. 9. Comparison of ultimate load.

S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727 725

PIC ¼ abffiffiffiffiffifck

pb1:36

f t0:21f 6 bf tf f yf ½kN� ð2Þ

where a is 0.19 as mean, b is 1.0 when L is not less than 200 mm andL/200 when L is less than 200 mm.

he

bfbe1

Fig. 10. Failure surface of concre

In addition, Seracino et al. [10] suggested another equation Eq.(3) which contains the factors of width-to-thickness and stiffnessof FRP to predict the de-bonding resistance of EB and NSM plate-to-concrete joints.

PIC ¼ ap0:85/0:25f f 0:33

ck

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiLperðEAÞf

q6 bf tf f yf ½kN� ð3Þ

where Lper = 2bf + tf, /f = bf/tf, ap is 1.0 as mean and 0.85 as lower 95%confidence limit.

Based on the failure mechanism, such as tension yield of FRP,bond failure of epoxy between concrete and FRP, and concretebreak out failure, Seo [11] suggested below equations for each fail-ure pattern.

Tf 1 ¼ /bf tf f yf ðtension failure of FRPÞ; ½kN� ð4Þ

Tf 2 ¼ ksf ð2bf ldÞ ðshear failure of epoxy bondÞ; ½kN� ð5Þ

Tf 3 ¼ 0:57bffiffiffiffiffifck

pAcf ðsplit failure of concreteÞ; ½kN� ð6Þ

Tf ¼ minfTf 1; Tf 2; Tf 3g; ½kN� ð7Þ

where k is effective reduction factor of bond, sf is shear strength ofepoxy, ld is bond length, b is experimental coefficient Acf is surfacearea of split failure of concrete.

When concrete split failure governs, its strength can be calcu-lated by using the relationship between split failure strength ofconcrete and the area of failure surface for both single and groupedFRP. From the observation of failure shape of bond specimens, itwas found that concrete failure shape was similar to con-failureshown in Fig. 10. Especially, if several FRPs are embedded andthere is not enough space between those, group failure occurs. Inthe evaluation of split strength of concrete, therefore, the surfaceof group failure should be used in the calculation.

Seo suggested Eq. (8) to calculate the surface area includingreduction factor of bond length, k. It reflects the strength variationdue to bond length.

Afr ¼ 2bf

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðkheÞ2 þ b2

e1

qþ ðkheÞðtf þ be1Þ ð8Þ

Based on the observation of test results, 0.3 he was suggested asan effective width of projected surface, be1.

For the comparison, the bond strengths of test specimens werecalculated by using above formulae and presented in Table 5 andFigs. 11–14. Most close mean to the test results was found in thecalculated results by Seo. Also the results by Seo showed lest stan-dard deviation among all formulae.

Overlapped area

he

te of NSMR with FRP plate.

Table 5Evaluation of bond strength of NSM FRP.

Name Test result Pexp (kN) Seracino-1 Seracino-2 Ali Seo

Pcal (kN) Pexp/Pcal Pcal (kN) Pexp/Pcal Pcal (kN) Pexp/Pcal Pcal (kN) Pexp/Pcal

N150-1 44.1 37.10 1.19 61.05 0.72 62.71 0.70 44.99 0.98N200-1 45.1 49.46 0.91 61.05 0.74 62.71 0.72 49.74 0.91N300-1 62.8 49.46 1.27 61.05 1.03 62.71 1.00 59.91 1.05N150-1-1S 45.1 37.10 1.22 61.05 0.74 62.71 0.72 44.99 1.00N200-1-1S 50.0 49.46 1.01 61.05 0.82 62.71 0.80 49.74 1.01N300-1-1S 50.0 49.46 1.01 61.05 0.82 62.71 0.80 59.91 0.83N150-1-2S 45.1 37.10 1.22 61.05 0.74 62.71 0.72 44.99 1.00N200-1-2S 51.0 49.46 1.03 61.05 0.84 62.71 0.81 49.74 1.03N300-1-2S 56.9 49.46 1.17 61.05 0.95 62.71 0.92 59.91 0.97N150-3 65.2 88.36 0.74 134.40 0.49 94.12 0.69 72.41 0.90N200-3 66.7 117.82 0.57 134.40 0.50 94.12 0.71 77.24 0.86N300-3 79.4 117.82 0.67 134.40 0.59 94.12 0.84 86.89 0.91N150-3-1S 67.2 88.36 0.76 134.40 0.50 94.12 0.71 72.41 0.93N200-3-1S 75.5 117.82 0.65 134.40 0.57 94.12 0.81 77.24 0.98N300-3-1S 79.4 117.82 0.66 134.40 0.58 94.12 0.83 86.89 0.90N150-3-2S 68.2 88.36 0.77 134.40 0.51 94.12 0.72 72.41 0.94N200-3-2S 73.1 117.82 0.62 134.40 0.54 94.12 0.78 77.24 0.95N300-3-2S 79.4 117.82 0.58 134.40 0.51 94.12 0.72 86.89 0.78Average 0.89 0.68 0.78 0.94Standard deviation 0.25 0.17 0.08 0.07

0.0

0.5

1.0

1.5

2.0

100 150 200 250 300 350

Test

/ C

al.

Bond length (mm)

1 layer

3 layerEq.(1)

Fig. 11. Comparison of strength by Eq. (1) and test data.

0.0

0.5

1.0

1.5

2.0

100 150 200 250 300 350

Test

/ C

al.

Bond length (mm)

1 layer

3 layerEq.(2)

Fig. 12. Comparison of strength by Eq. (2) and test data.

0.0

0.5

1.0

1.5

2.0

100 150 200 250 300 350

Test

/ C

al.

Bond length (mm)

1 layer

3 layerEq.(3)

Fig. 13. Comparison of strength by Eq. (3) and test data.

0.0

0.5

1.0

1.5

2.0

100 150 200 250 300 350

Test

/ C

al.

Bond length (mm)

1 layer

3 layerEq.(7)

Fig. 14. Comparison of strength by Eq. (7) and test data.

726 S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727

S.-Y. Seo et al. / Composite Structures 95 (2013) 719–727 727

4. Conclusion

(1) From the comparison NSMR and EBR with same amount ofFRP plate, NSMR is capable to increase the bond strengthby over 1.5 times than EBR. Even if strain distribution ofFRP plate of NSMR decreases in line with its location fromload direction, which is similar to EBR, however, the magni-tude of strain in NSMR is much higher than that of EBR. Fromthis, the NSMR is more effective than EBR in the manner ofstructural resistance.

(2) The de-bonding resistance depends on the bond length. Onthe other hand, the effects of shear key in NSMR with FRPplate are tedious, largely attributable to peel off failure ofconcrete in general. Therefore, additional studies are consid-ered as being required to appreciate the share key effect forobjects with short bond length.

(3) When same amount of FRP is divided into three layers withkeeping certain space, the bond resistance increases about1.5 times due to the increase of bonded area. More studyfor the suitable space is necessary since the increase of bondresistance depends on the space of FRPs.

(4) From the review of previous equations to predict the bondstrength of NSMR with FRP plate and the comparison withtest result, most reliable prediction of bond strength is pos-sible by using Seo’s in which the effect of bonded length isconsidered.

Acknowledgements

This research was supported by 2011 Basic Science ResearchProgram through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science and Technology(2011-0011350) and a grant from the Academic Research Programof Korea National University of Transportation in 2012.

References

[1] De Lorenzis L, Rizzo A, La Tegola A. A modified pull-out test for bond of nearsurface mounted FRP rods in concrete. Composites Part B: Eng2002;33(8):589–603.

[2] Sharma SK, Mohamed Ali MS, Goldar D, Sikdar PK. Plate-concrete interfacialbond strength of FRP and metallic plated concrete specimens. Composites PartB: Eng 2006;37(1):54–63.

[3] Yao J, Tang JG, Chen JF. Experimental study on FRP-to-concrete bonded joints.Composites Part B: Eng 2005;36(2):99–113.

[4] Nami A. FRP reinforcement for bridge structures, proceedings of structuralengineering conference beams. J Compos Constr (ASCE) 2000;5(1):12–7.

[5] De Lorenzis L, Nanni A, La Tegola A. Flexural and shear strengthening ofreinforced concrete structures with near surface-mounted FRP rods. In:Proceeding of the 3rd international conference on advanced compositematerials in bridges and structures (ACMBS III); 2000. p. 521–8.

[6] EI-Hacha R, Rizkalla SH. Near surface-mounted fiber-reinforced polyumerreinforcements for flexural strengthening of concrete structures. ACI Struct J2004;101(5):717–26.

[7] Yost JR, Gross SP, Dinehart DW, Mildenberg JJ. Flexural behavior of concretebeams strengthened with near surface-mounted CFRP strips. ACI Struct J2007;104(4):430–7.

[8] Ali NSM, Oehlers DJ, Friffith MC, Seracino R. Interfacial stress transfer of nearsurface-mounted FRP-to-concrete joints. Eng Struct 2008;30:1861–8.

[9] Seracino R, Jones NM, Ali NSM, Page MW, Oehlers DJ. Bond strength of nearsurface-mounted FRP strip-to-concrete joints. J Compos Constr (ASCE)2007;11(4):401–9.

[10] Seracino R, Rashid R, Oehlers DJ. Generic debonding resistance of EB and NSMplate-to-concrete joints. J Compos Constr (ASCE) 2007;11(1):62–70.

[11] Seo S, Yoon S, Kwon Y, Choi K. Bond behavior between near surface-mountedfiber reinforced polymer plates and concrete in structural strengthening. JKorea Concr Ins 2011;23(5):675–82 (written in Korean).