Shear Strengthening of RC Beams Using Hybridized FRP Composite

Sang-Su Ha

Assistant Professor, Department of Architectural Engineering, Kangnam University, Yongin 449-702, Korea

Dong-Uk Choi

Professor, Department of Architectural Engineering, Hankyong National University, Anseong 456-749, Korea

Thomas H.-K. Kang

Assistant Professor, Department of Architecture and Architectural Engineering, Seoul National University, Seoul 151-744, Korea.

Chin Yong Lee

President, Carecon, Daekyeong Building 2F, 563, Seongnae-dong, Gangdong-Gu, Seoul, Korea

ABSTRACT

This study investigated the effect of shear strengthening of reinforced concrete (RC) beams using hybridized Fiber Reinforced Polymer (FRP) composite. The hybridized FRP composite (HF) consisted of glass fiber (GF) and carbon fiber (CF). The HF can be more economical than CF and demonstrates pseudo-ductility when subjected to tension. A total of seven RC beam specimens were manufactured and tested in shear. Test variables were the type of fibers (CF, GF, and HF), method of shear strengthening (U-wrap, I-type, and U-wrap+I-type), and amount of fibers. All specimens were subjected to four-point loading. A control beam without any shear reinforcement failed by shear in the shear span. All specimens shear-strengthened with CF, GF, or HF sheets failed by debonding of the FRP from the concrete substrate. The failure experimentally determined loads matched closely with those theoretically predicted using ACI 440.2R-08 equations. Based on the results of the tests, the quantitative effects with shear strengthening were from 56% to 137% higher than BS-N without any shear strengthening, and the specimens with the mixed U-wrap and I-type showed the most shear strengthening effects in terms of ultimate strength. However, the U-wrap method was more effective when the strengthening effect was evaluated based on the amount of fibers used.

KEYWORD

Shear strengthening, hybridized FRP composite (HF), glass fiber (GF), carbon fiber (CF), pseudo-ductility

S1A03

h=240

d=205

1. INTRODUCTION Externally bonded FRP (fiber-reinforced polymer) systems have been used to strengthen existing concrete structures. After the initial development of externally bonded FRP systems for the retrofit of concrete structures began in the 1980s in both Europe and Japan, the number of strengthening projects using FRP systems worldwide has increased dramatically [1]. Although FRP materials have many advantages such as corrosion, durability, and lightness, the disadvantages include linear elastic stress-strain relationship and brittle rupture. Engineered hybrid carbon-glass fiber-reinforced polymer sheets consisting of carbon fibers (CF) and glass fibers (GF) can exhibit some limited ductility known as pseudo-ductility [2]. This is attributed to the fact that CF has higher elastic modulus (ECF) than that of GF and less ultimate strain (εu-CF) than GF. The objective of this paper is to understand shear strengthening effects of reinforced concrete members with externally bonded FRP systems of different types. 2. TEST PLANNING 2.1 Test Specimen w/o Shear Strengthening A specimen without any shear strengthening is shown in Fig. 1. The specimen dimensions are 160(w) x 240(h) x 2700(L) mm. Three tension and two compression reinforcing bars, respectively, with 19-mm diameters were provided. Transverse reinforcing bars were not used in the shear span to induce shear failure.

Fig. 1 Test specimen details and test set up

2.2 Area of Strengthening Materials The CF sheets, GF sheets, and specially designed HF (Hybrid carbon-glass fiber-reinforced polymer) sheets were used as shear strengthening materials of the test specimens. As shown in Table 1, the areas of CF and GF rovings were 0.446 mm2 and 0.870 mm2, respectively. The cross-sectional areas of the carbon and glass rovings extracted from an HF sheet were 1.607 mm2 and 0.850 mm2, respectively.

Table 1 Physical characteristics of FRPs

Type CF GH HF Carbon Glass

Weight (gram) 0.401 1.106 1.446 1.081 Specific gravity 1.80 2.54 1.80 2.54 Length (mm) 500 500 500 500 Area (mm2) 0.446*1 0.870 1.607 0.850 Mix proportion - - CF:9GF=1:4.76*2 Note *1) A=W/(ρL)=0.401g/(0.0018g/mm3 x 500mm)=0.446 *2) Carbon: one roving (48K), Glass: 9 rovings (2200tex) 2.3 Material Properties The coupon tests of FRP sheets were carried out in the laboratory to evaluate the mechanical properties, which were obtained using ISO/FDIS 10406-2[3]. A test coupon had a length of 600 mm including two 150 mm grips at end as shown in Fig. 2.

Fig. 2 Dimensions of a tensile test coupon

The coupon tests were carried out using a 1,200-kN UTM (universal testing machine) while maintaining a displacement-controlled loading rate of 1 mm/min. Fig. 3 shows a tensile test under progress. Fig. 4 shows the comparisons of stress-strain relationships for three different types of FRP sheets. The tensile behavior of CF and GF sheets was characterized by a linear elastic stress-strain relationship until failure. In contrast, HF sheets using the mixed CF and GF show pseudo-ductility as shown in Fig. 4. The values of material properties for three different types of FRP sheets are summarized in Table 2.

850 850 850

2700

75 75

D10@85850

b=16000

clear cover = 15 b=16000

2-D19 (compression reinforcing bar) D10

3-D19 (tension reinforcing bar)

P/2 P/2

LVDT

Strain gauges All units : mm

FRP clip

150 mm 300 mm 150 mm

FRP clip FRP sheet

Fig. 3 A tensile test in progress

Fig. 4 Comparison of tensile

Table 2 Results of coupon tests

Type 1st peak Final r

EHF (GPa)

εu_C_HF (%)

σu_C_HF (MPa)

εu_

(%)HF 76.0 2.082 1582 3.255

ECF (GPa)

- - εu_

(%)CF 174.0 - - 0.780

EGF (GPa)

- - εu_

(%)GF 61.1 - - 3.108

The yield strengths of D19 and D10 462.3 MPa and 489.5 MPa, respectively, and the compressive strength of concrete determined fromcylinders was 34.8 MPa at the time of testing 2.4 Method of Shear StrengtheningAs described in ACI 440.2R-08 [1], there are three types of FRP wrapping schemes used to shear strength of prismatic, rectangular beamscolumns as illustrated in Fig. 5.

test in progress

test results

sults of coupon tests Final rupture

u_G_HF (%)

σu_G_HF (MPa)

3.255 1903 u_CF (%)

σu_CF (MPa)

0.780 1357 u_GF (%)

σu_GF (MPa)

3.108 1899

D19 and D10 rebars were respectively, and the

determined from at the time of testing.

Method of Shear Strengthening 1], there are three

types of FRP wrapping schemes used to increase prismatic, rectangular beams, or

Fig. 5 Typical attachmentstrengthening using FRP sheets

The complete-wrapping efficient. However, this method is not suitable in beam applications. In this paper, U-wrap and I-type, were thereforeTest specimens BS-CF-U and BSshown in Fig. 6. Table 3 variables: strengthening methodamount of strengthening. For tspecimen, both I-type and UI-type on top of U-wrap.strengthening was the full range of Table 3 Summary of shear strengthening tests

Beam index

FRP type

Method of streng-thening

BS-N - - BS-CF-U CF U-wrap

BS-CF-UI CF U-wrap I-type

BS-GF-U GF U-wrap

BS-HF-U HF U-wrap

BS-HF-I HF I-type

BS-HF-UI HF U-wrap I-type

Notes. *1) 8.5 x 24 rovings x 0.446*2) 2.3 x 24 rovings x 0.446 mm*3) 8.5 x 40 rovings x 0.870 mm*4) 33 rovings x 1.607 mm2/ roving *5) 33 x 9 rovings x 0.850 mm*6) 9 rovings s x 1.607 mm2/ roving *7) 9 x 9 rovings x 0.850 mm2/

Fig. 6 Beam specimens after

(a) Complete wrapping

(b) U- wrap

attachment schemes for shear

strengthening using FRP sheets

wrapping system is the most efficient. However, this method is not suitable in

In this paper, only two methods, therefore employed. U and BS-HF-UI are 3 summarizes the test

strengthening method, fiber types, and For the U-wrap + I-type

type and U-wrap were applied The region of shear

range of the shear span.

Summary of shear strengthening tests

Amount of strengthening

- *1ACF = 90.98mm2

ACF = 90.98mm2 *2ACF = 24.62 mm2

*3AGF = 295.8mm2

*4ACF =53.03mm2

*5AGF =252.45mm2

*6ACF =14.46 mm2

*7AGF = 68.85 mm2

ACF = 65.49 mm2

AGF = 321.3 mm2

x 0.446 mm2/ roving mm2/ roving mm2/ roving roving

2/ roving roving / roving

Beam specimens after shear strengthening

wrap (c) I-type

One control specimen had no shear strengthening (BS-N), whereas the rest of the specimens were shear strengthened using CF, GF, and HF sheetstwo-part epoxy was used as adhesive.epoxy-to-FRP ratio of 1.5:1 by volume was used for all specimens. 2.5 Test Set Up A 500-kN load cell was used to Two loading points were located in the center of the beam with 850-mm center-to-Three LVDTs were used to measure deflections the constant moment region under the specimenshown in Fig. 7.

Fig. 7 Beam test set-up 3. Results of Tests 3.1 BS-N specimen without shear strengtheningFig. 8 shows the load (P)-displacement (relationship for the BS-N specimen shear strengthening. The final failure shear at the shear span between the and reaction point as shown in Fig.

Fig. 8 Load-displacement curve of BS

no shear strengthening N), whereas the rest of the specimens were

shear strengthened using CF, GF, and HF sheets. A part epoxy was used as adhesive. An

1 by volume was used

used to apply the load. re located in the center of

-center distance. were used to measure deflections in

under the specimen as

up

N specimen without shear strengthening displacement (d)

N specimen without any inal failure occurred in

the loading point shown in Fig. 8.

curve of BS-N specimen

The values of maximum loadingwere 81.8 kN and 8.51 mmconcrete nominal shear strengthusing Eq. (11-3) of ACI 318As no stirrups were used nominal shear strength, Vequals to Vc. The test value experiment was 40.9 kN. The conservative side when compared to theoretical value as expected 3.2 Specimens with shear All specimens strengthening sheets failed by shear in the shear spanin Fig. 9, the sheets in the shear off away from concrete surfaceThe maximum loads varied variables such as the types of FRP sheets and attachment schemes, although patterns were similar. The susing U-wrap and U-wrap+I type strengths than that of the BS

(a) BS-CF-UI (b) BS

(c) BS-HF-I (d) BS

Fig. 9 Final failure of specimens with shear strengthening

3.3 Comparison of Test Results Table 4 summarizes the test shear force, deflection, and strain at failure also analytical values calculated 440.2R-08 equations [1]. As shown in Table 4, the measured of BS-N specimen without strengthening was 40.9 kN. The shear strengthsBS-CF-U, BS-GF-U, and BSU-wrap method increased 133%, respectively.

of maximum loading and displacement mm, respectively. The

shear strength, Vc, calculated 3) of ACI 318-05 [4] was 32.89 kN.

were used in the shear span, the Vn of BS-N specimen

. The test value obtained from this he test value was on the

conservative side when compared to the as expected.

hear strengthening All specimens strengthening using CF, GF, and HF

in the shear span. As shown in the shear span were peeled

concrete surface close to ultimate. varied depending on the test

such as the types of FRP sheets and although all the final failure The specimens strengthened wrap+I type revealed higher the BS-HF-I.

UI (b) BS-CF-UI

I (d) BS-HF-UI

Final failure of specimens with shear strengthening

Test Results Table 4 summarizes the test results in terms of shear force, deflection, and strain at failure and

calculated using ACI measured shear strength without any shear

kN. The shear strengths of U, and BS-HF-U utilizing the

increased by 113%, 115%, and

Table 4 Measured test data versus analytical

Beam index Test Vmax,

kN

BS-N 40.90 (1) 8.51 (1)BS-CF-U 87.0 (2.13) 18.58 (2.18)BS-CF-UI 92.0 (2.25) 23.32 (2.74)BS-GF-U 87.8 (2.15) 20.46 (2.40)BS-HF-U 95.4 (2.33) 28.92 (3.40)BS-HF-I 63.6 (1.56) 12.98 (1.53)BS-HF-UI 96.8 (2.37) 33.16 (3.90)Notes. *: Vn calculated based on ACI 440.2Rultimate strain of concrete at failure; εcs

Fig. 10 Comparison of P-d

Fig. 11 Strains in tensile reinforcement at failure Test values of the specimens with Uwere greater than those of the specimens with U-wrap only both in terms of strength and deflection. The strength of BS-HFincreased by only 56 % over BS-N.Fig. 10, the shear strengthening effects of the specimens (BS-HF-U, BS-HF-UI) sheets were greater than those of the specimens using CF or GF sheets both in terms of strength

analytical results

Dmax mm

Strain at failure (x 10-6)

εcc εcs εts

8.51 (1) -657 -743 1064 32.918.58 (2.18) -1924 -1,098 5284 58.823.32 (2.74) -2125 -1,506 6397 83.0320.46 (2.40) -1722 -1,113 5314 62.5328.92 (3.40) -3056 -1527 7639 70.9212.98 (1.53) -1064 -945 2008 53.1133.16 (3.90) -3260 -2175 9339 95.92

alculated based on ACI 440.2R-08; Δmax: maximum displacement at center of cs: strain of compression reinforcement; εts: strain of tension

curves

tensile reinforcement at failure

Test values of the specimens with U-wrap+I-type than those of the specimens with

both in terms of strength and HF-I with I-type

N. As shown in shear strengthening effects of the

UI) using HF than those of the specimens

both in terms of strength

and deflection. Fig. 11 depicts the relation of in tension reinforcementspecimen did not exhibit while the rest of the specimens significantly larger than the yield strainof the U-wrap, shear strengthening effects significantly greater thanspecimens utilizing the U-same time showed the effects. However, considering the shear strengthening, the more effective than the Uin Table 5. Table 5 Shear strengthening effects amount of shear strengthening

Beam index

Vmax kN

BS-N 40.90 BS-CF-U 87.0 BS-CF-UI 92.0 BS-GF-U 87.8 BS-HF-U 95.4 BS-HF-I 63.6 BS-HF-UI 96.8 Notes. *1) 90.98mm2 x (240+240+160)mm=*2) 90.98mm2x640mm + 24.62mm*3) 295.8mm2 x 640mm=189312*4) (53.03+252.45)mm2 x 640mm=*5) (14.46+68.85) x (850+850)mm=*6) 195507 + 141627[*4)+*5)]=337134m

3.4 Nominal shear strength based on the ACI 440.2R-08 The nominal shear strengthusing Eq. (11-1) through Eq. (11440.2R-08. As described in the nominal shear strength

Theoretical. Vn,

kN *

Test-to-theory Ratio

32.90 (1) 1.24 58.80 (1.79) 1.48 83.03 (2.52) 1.11 62.53 (1.90) 1.40 70.92 (2.16) 1.20 53.11 (1.61) 1.47 95.92 (2.91) 1.01

maximum displacement at center of the specimen; εcc: the tension reinforcement.

the relation of load-versus-strain tension reinforcement. Only BS-HF-I

exhibit yielding at failure, pecimens resulted in strains

significantly larger than the yield strain. In case wrap, shear strengthening effects were

than the I-type. Also, the -wrap and I-type at the

the greatest strengthening owever, considering the amount of

the U-wrap method was the U-wrap + I-type method

hear strengthening effects versus amount of shear strengthening

Amount of Strengthening

mm3 -

58,227 *1 100,081 *2 189,312 *3 195,507 *4 141,627 *5 337,134 *6

x (240+240+160)mm= 58227mm3 24.62mm2x700mm=100081mm3 189312mm3

mm=195507mm3

(850+850)mm= 141627 mm3 =337134mm3

Nominal shear strength based on the ACI

he nominal shear strength (Vn) was calculated 1) through Eq. (11-11) of ACI

As described in the ACI 440.2R-08, the nominal shear strength of an

FRP-strengthened concrete member can be determined by adding the contribution of the FRP external shear reinforcement to the contribution from the reinforcing steel and the concrete (Eq. 1).

Vn = (Vc + Vs + ψVf) (1) where, Vc : nominal shear strength provided by concrete Vs : nominal shear strength provided by stirrups Vf : nominal shear strength provided by FRP ψ: FRP strength reduction factor (= 0.85) The values determined using Eq. (11-1) through Eq. (11-11) of ACI 440.2R-08 are also shown in Table 4. The test value for the BS-N without any shear strengthening was 24% higher than the predicted value. For all specimens with shear strengthening, the test values recorded were 1% to 48% higher than the predicted values. Given that all specimens failed in shear, these theoretical values are considered to correspond well to the test values. The effects of shear strengthening were significant (56% to 137% higher when compared with BS-N with no shear strengthening), as shown in numbers in the parenthesis in the second column of Table 4. 4. Conclusions (1) The final failure mode of the BS-N

specimen without any shear strengthening was the shear failure in the shear span, and all specimens shear strengthened with CF, GF, and HF sheets failed by debonding of the FRP from the concrete substrate in the shear span.

(2) The ACI 440-2-08 equations for the nominal shear strength of FRP-strengthened concrete members corresponded to the test values relatively well.

(3) It was found that the U-wrap method was more effective than the I-type method by comparing the test results between BS-HF-I and BS-HF-U.

(4) The effects of shear strengthening were 56% to 137% greater than the BS-N without any shear strengthening.

(5) The specimens utilizing U-wrap and I-type methods at the same time exhibited the greatest shear strengthening effect in terms of strength and deflection. However, considering the amount of FRP used for the shear strengthening, the U-wrap method was more effective than the U-wrap + I-type

method. (6) The engineered HF sheets in this study were

effectively utilized for the purpose of shear strengthening. Using the HF, effective shear strengthening can be achieved while using relatively small amount of CF.

ACKNOWLEDGEMENT The work presented in this paper was supported by Business for International Cooperative R&D between Industry, Academy, and Research Institute funded by Korean Small and Medium Business Administration in 2010 (Grants No. 00041602-1).

REFERENCES

[1] ACI Committee 440, “Guide for the Design

and Construction of Externally Bonded FRP Systems for Strengthening concrete Structures (ACI 440.2R-08),” American Concrete Institute, Farmington Hills, MI, 2008.

[2] Choi, D.-U.; Kang, T. H.-K.; Ha, S.-S.; Kim, K.-H.; and Kim W., “Flexural and Bond Behavior of Concrete Beams Strengthened with Hybrid Carbon-Class Fiber-Reinforced Polymer Sheets,” ACI Structural Journal, V. 108, No. 1, 2011, pp. 90-98.

[3] ISO/FDIS 10406-2, “Fibre-Reinforced Polymer (FRP) Reinforcement of Concrete-Test Methods–Part 2: FRP Sheets,” International Organization for Standardization, 2008.

[4] ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-08),” American Concrete Institute, Farmington Hills, MI, 2008, 456 pp.