Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

205

FLEXURAL STRENGTHENING OF RC BEAMS USING EXTERNALLY BONDED

FRP SHEETS THROUGH FLEXIBLE ADHESIVE BONDING

J.G. Dai1, T. Ueda1, Y. Sato1 and T. Ito 2

1Division of Built Environment, School of Faculty of Engineering Hokkaido University, Japan, Email: daijg@eng.hokudai.ac.jp

2P.S. Mitsubishi Construction Co., Ltd., Japan ABSTRACT This paper reports summarily a flexible FRP/concrete bonding system for flexural strengthening of RC members. The bonding adhesive used in this system has very low elastic modulus but large elongation up to tensile fracture. Therefore, bonding line between FRP and concrete can bear large shear deformation to delay or prevent the occurrence of local interface debonding. Results from pullout tests of FRP sheet/concrete interfaces, laboratory tests on static and fatigue performance of RC members strengthened with FRP sheets, and a full-scale test on a T-beam of 15m span are reported in this paper. Failure mechanisms of strengthened members in cases of normal bonding and flexible bonding systems are clarified comparatively. Compared to the normal adhesive bonding system, the flexible one has proven its promising validity in strengthening RC beams in terms of their ultimate strength and deformability. However, this technique is favored for the ultimate limit state strengthening purpose only since it contributes less stiffness enhancement under the serviceability limit state. KEYWORDS FRP, RC beams, flexural strengthening, interface, flexible bonding INTRODUCTION Externally bonded FRP reinforcement (FRP EBR) has proven its efficiency in strengthening RC members in flexure through extensive researches and engineering practices in the past decade. A primary concern for this technique is local debonding of FRP/concrete interfaces, which affects negatively the structural integrity and long-term durability of strengthened members. Macro-propagation of the local interface debonding usually leads to a sudden drop in loads and loss of ductility of the whole FRP/RC composite system. In general, stresses in FRP materials are limited in design guidelines to prevent the failure triggered in the bond line [JSCE 2001, fib 2001, ACI 2002]. However, the stress limitation in the meantime indicates that the advantage of FRP in high strength cannot be fully utilized. Strain limitation in FRP also means reduced deformability and the moment redistribution ability of the whole strengthened systems. To improve the flexural performance of RC members strengthened with FRP, traditional prestressing system, mechanical anchorage systems (U-shape wrapping, mechanical bolting and so on), and some other novel bonding technologies like near surface mounting (NSM) and mechanical fastening for the bond interface as a substitute of adhesive bonding have been developed [Nanni 1999, Rizkalla and Haasan 2001, Lamanna 2002, Bank 2004]. Efforts are also being put into developing new bonding adhesives. Two types of soft adhesives are available for optimizing the flexural performance of FRP strengthened RC members. One is moderately soft adhesive. Its elastic modulus can be as low as 0.39GPa, which is less than one quarter of those of most commonly used commercial adhesives. The bond interfaces using it can improve the interfacial fracture energy and the pullout capacity [Sugiyama et al. 2000, Dai et al. 2001 and 2003]. Another new type is a very soft one called flexible bonding adhesive, the elastic modulus of which is less than 10MPa and its elongation is about 100%. It is expected that the low elastic modulus is able to release the local stress concentration of FRP sheet/concrete interfaces and the large deformability can prevent local debonding even if large interface slip occurs. Experimentations for verifying the efficiency of this new bonding system were performed jointly by Shimizu Construction Co., Nisseki Mitsubishi Co., and Hokkaido University. Some results were reported previously [Maeda et al. 2001, Sato et al. 2002, Ueda and Sato 2002]. This paper provides a summary report on this flexible bonding system. The contents include the bond characteristics of FRP/concrete joints, strength and ductility of FRP strengthened RC beams, fatigue durability of this bonding system, and test results on a 15m span bridge T-beam.

206

DESCRIPTION OF FLEXIBLE BONDING SYSTEM Fig.1 shows a sketch of FRP-concrete bonded interface using the flexible bonding system, in which wet lay-up bonding method is applied. Resins used in the bonding layer (see layer 2 in Fig.1) and impregnating matrix of FRP sheets (see layer 1 in Fig. 1) usually are same in conventional wet lay-up processes. However, they are different in the flexible bonding system, in which soft resin can not be misused for the impregnating matrix of FRP sheets. The bonding layer likes a sandwich filling composed of (1) a layer of 250 to 400g/m2 conventional primer depending on conditions of concrete substrate; (2) a layer of 600g/m2 flexible bonding adhesive. The thickness is about 0.5mm but also is adjustable; and (3) a layer of 500g/m2 conventional resin for the first layer of FRP sheets. Brand of the bonding adhesive used in this system is EE-50, the mechanical properties of which are shown in Table 1 and also compared with those of normal (FR-E3P) and moderately soft adhesives (SX-325 and CN-100). Fig.2 shows tensile stress-strain curve of EE-50 compared with those of FR-E3P, SX-325 and CN-100. It is indicated that the flexible one, EE-50 behaves linearly up to the tensile rupture as normal adhesives do. Its rupture strength is rather low however its rupture strain is very large. The moderately soft ones have yielding points and before that show some nonlinearity. Their deformability after the yielding point is large but is difficult to utilize since concrete usually fractures before the adhesives yield. However, experimental results still showed that the moderately soft adhesives, SX-325 and CN-100 can delay the interface debonding and improve the interface fracture energy [Dai and Ueda 2003].

FRP sheets impregnated with

normal resin (layer 3)

Flexible bonding

adhesive (layer 2)

Normal primer (layer 1)

Concrete substrate 0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

Flexib le bonding adhesive

M oderate ly soft adhesive

FR -E3P SX-325 CN -100 EE-50 FP-NS

σ(M

Pa)

ε

N orm al adhesive

Fig.1 A sketch of flexible bonding system Fig.2 Tensile stress-strain relationships of different adhesives

Table 1 Material Property of Different Adhesives

Types of adhesives Normal Moderately soft Flexible Primer

Brand FR-E3P SX-325 CN-100 EE-50 FP-NS

Elastic modulus (GPa) 2.41 1.0 0.39 0.003 2.45

Poisson’s ratio 0.38 0.38 0.45 N/A 0.38

Tensile strength (MPa) 44.7 15.9 11.8 2.5 48.1

Elongation (%) 2.0 45 50 100 1.9

Material provider Nittetsu

Composites

Toho

Earth-Tech.

Tohto

Kasei

Nisseki

Mitsubishi

Nittetsu

Composites

*Tests for adhesives were carried out by Sho-bond Co. Ltd. and Toho Co. Ltd.; N/A: Not available. PULLOUT TESTS OF FRP/CONCRETE BONDED JOINTS USING FLEXIBLE BONDING SYSTEM Pullout tests for CFRP/concrete joints using the flexible bonding system were performed (see Fig.3). The bonded length was 300mm. Two layers of high strength type of CFRP sheets with tension stiffness of 50.6GPa/mm were used. Concrete substrate had the compressive strength of 35.0MPa. For CFRP/concrete joints using the flexible bonding system, the typical interface failure occurred in the flexible bonding layer (see Fig.3), which is different from the mode of concrete fracture in cases of normal and moderately soft adhesives [Dai et al. 2003]. The pullout force-slip relationship of the CFRP/concrete joint using the flexible bonding layer is shown in Fig.4. Others using normal (FR-E3P) and moderately soft (CN-100) adhesives, which were reported by the authors previously [Dai et al. 2003], are also included in Fig.4 for the purpose of comparison. It is indicated in Fig.4 that the stiffer the adhesive, the greater the initial overall bonding stiffness. The interfaces using FR-E3P and CN-100 showed significant decrease in the overall bonding stiffness after the initial peeling. However, the overall bonding stiffness in the case of EE-50 was almost kept constant up to the maximum pullout capacity. The peak pullout loads were 39.3kN, 51.0kN, and 41.1kN in the case of FR-E3P, CN-100 and EE-50,

207

respectively. The moderately soft adhesive CN-100 achieved the highest pullout load. The interfaces using EE-50 and FR-E3P had almost the same pullout capacity. However, the conclusion on the comparison depends on the applied bonded length. As shown in Figs.5.a and 5.b, relationships between the pullout capacity and the bond length are different when different adhesives are used. In these two figures the predicted lines in the cases of FR-E3P and CN-100 are drawn based on the model proposed by the authors [Ueda and Dai, 2004]. The relationship in the case of EE-50 is plotted based on experimental results [Shimizu and Nisseki Co., 2001]. In general, FRP/concrete interfaces using FR-E3P and CN-100 had reached their maximum pullout capacity when the bond length increased up to 300mm. However, the pullout force still showed increasing tendency even if the bond length increases up to 400mm in the case of EE-50. It is possible for the flexible bonding system to achieve higher pullout force capacity if a much longer bond length is available.

0.0 0.3 0.6 0.9 1.2 1.5 1.80

10

20

30

40

50

60Initial debonding

Types of adhesives: Normal FR-E3P Moderately soft CN-100 Flexible EE-50

Pullo

ut fo

rce

(kN

)

Slip(mm)

Initial debonding

Fig.3 Pullout test of CFRP/concrete joints Fig.4 Pullout force-slip relationships

0 100 200 300 4000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Eftf=38.4kN/mm flexible adhesive, ta=0.5mm flexible adhesive, ta=1.0mm normal adhesive moderately soft adhesivePu

ll ou

t for

ce p

er b

ond

wid

th (k

N/m

m)

Bond length(mm)0 100 200 300 400

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Eftf=76.8kN/mm Flexible adhesive, ta=0.5mm Flexible adhesive, ta=1.0mm Nomal adhesive Moderately soft adhesive

Pullo

ut fo

rce/

bond

wid

th (k

N/m

m)

Bond length(mm)

Fig.5 Pullout force-bond length relationships: (a) Eftf=38.4kN/mm; (b) Eftf=76.8kN/mm

Pullout capacity is an important reference for selecting optimum bonding adhesives. It should be noticed that concrete blocks in pullout tests are not cracked. However, tensile forces in FRP are transferred along many cracked sections in actual flexural strengthening cases. These local cracks of concrete make the debonding mechanisms in flexural strengthening cases more complex than those in pullout cases. To understand well how the flexible bonding system works in actual member strengthening cases, detailed investigations on the local interfacial behaviors are necessary. Fig.6 compares the strain distributions in FRP in pullout tests of CFRP/concrete joints using normal, moderately soft and flexible adhesives, respectively. It can be seen that the softer the bonding adhesive is, the more widely the strains in FRP distribute over. In other words, the effective bond length increases if the adhesive becomes soft. In the case of EE-50, strains in FRP distribute actively in the whole bonded area. This smooth strain distribution nature in FRP sheets releases local bond stress concentration and avoids local brittle failure in concrete. In the meantime, the ability of FRP/concrete interfaces in accommodating local slips is largely improved as is shown in Fig.6. When EE-50 is used the local bond stress does not show softening even at a large slip of 0.9mm. For FR-E3P and CN-100, however, macro debonding of the interfaces has occurred under that slip. As is verified in the following flexural tests, this τ-s nature of FRP sheet/concrete interfaces using EE-50 prevents the interfaces from local debonding near flexural cracks.

P

300mm

Concrete

FRP

208

0 5 10 15 20 25 300

1000

2000

3000

4000

5000

6000

Different adhesives Normal FR-E3P Moderately soft CN-100 Flexible EE-50

Stra

in(1

0-6)

Distance from loaded point(cm)

Load transfer area

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

1

2

3

4

5

6

7 Normal adhesive FR-E3P(Dai et al.2005) Moderately adhesive CN-100(Dai et al.2005) Flexible bonding adhesive EE-50 exp.

τ (M

Pa)

s(mm)

Fig.6 Strain distribution in FRP for different adhesives Fig.7 τ-s relationships for different adhesives

STREGNTH AND DUCTLITY OF RC BEAMS STRENGTHENED WITH FRP SHEETS IN FLEXURE Laboratory tests for FRP strengthened RC beams were carried out to compare the flexible bonding system and normal adhesive bonding system. Dimensions of all tested beams are shown in Fig.8. All specimens were designed to fail in flexure and reinforced longitudinally with deformed rebars of 13mm diameter (D13), the yielding strength of which was 360MPa. Compressive strength of concrete was 35.0MPa. One directional CFRP sheets were used in the tests and had the tensile strength of 4120MPa and elastic modulus of 236GPa. The design thickness of each FRP layer was 0.167mm. The number of CFRP layers ranged from 1 to 3 in the experiments. The above-mentioned normal bonding adhesive FR-E3P and flexible adhesive EE-50 were used for the purpose of comparison. The thickness of flexible bonding layer was 0.5mm. All tested specimens are listed in Table 2, in which specimen SP-0 was a control beam, specimens SP-C1(s), SP-C2(s), SP-C3(s) were strengthened with 1, 2 and 3 layers of CFRP sheets, respectively. Specimen codes with and without the postfix of “s” represent flexible adhesive and normal adhesive, respectively.

Fig. 8 Dimension of tested beams Fig. 9 Failure modes

Table 2 Flexural test results of RC beams strengthened with FRP

Specimen Py (kN)

Pu (kN)

δy (mm)

δu (mm)

εfrp,max (µs)

Pu/Py

δu/δy Mode of failure

SP-01 40.2 44.1 4.08 25.7 - 1.1 6.3 Yielding of steel and concrete crushingSP-C11 44.4 78.3 5.22 21.7 7,790 1.76 4.2 Peeling of FRP from concrete substrateSP-C1s1 42.0 101.0 6.81 33.4 14,800 2.4 4.9 Fracture of FRP SP-C21 53.5 109.0 5.79 16.4 7,170 2.04 2.8 Peeling of FRP from concrete substrateSP-C2s1 46.8 148.0 7.62 40.7 13,100 3.16 5.3 Delamination of covercrete SP-C32 68.6 108.0 6.62 14.8 6,708 1.57 2.2 Delamination of covercrete SP-C3s2 58.9 116.6 9.68 21.2 8,664 1.98 2.2 Delamination of covercrete

Note: Py: load at the yielding of steel reinforcement; Pu: the maximum load capacity; δy: deflection at the yielding of steel reinforcement; δu: the ultimate deflection; and εfrp,max: the maximum strain observed in FRP. 1: Experimentation was performed in Shimizu Construction Co. [Maeda et al. 2001]; 2: Experimentation was performed in Hokkaido Univ. [Sato et al. 2002].

(a) Peeling of FRP

(b) Delamination of covercrete

200

200 150

200

165

5@100

900 6@50

8@100

Strain gauge on steel Strain gauge on Sheet

209

0 10 20 30 400

20

40

60

80

100

120

140

160

SP-C2s

SP-C3sSP-C3 SP-C2

SP-C1

Load

(kN

)

Deflection(mm)

Normal adhesive Flexible adhesive

SP-0

SP-C1s

Fig. 10 Load－deflection curves Fig.11 Strain distributions in steel and FRP when steel yields

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Ave

rage

bon

d st

ress

(MPa

)

Load(kN)

SP-C1 SP-C1s SP-C2s SP-C2s SP-C3s SP-C3s

Fig. 12 Strain distributions in steel Fig.13 Average bond stress-load relationships and FRP at the ultimate state

The mode of failure, load-carrying capacity and deflection at the yielding of reinforcement and the ultimate states, and the maximum strain achieved in CFRP sheets are presented in Table 2. Failure mode of the control beam SP-0 was the yielding of rebar followed by concrete crushing. Specimens SP-C1 and SP-C2 using normal adhesive bonding failed due to the peeling of FRP from concrete substrate in the shear span and the spalling of covercrete within the constant moment zone (see Fig.9.a). Specimen SP-C1s using the flexible bonding system failed due to the fracture of CFRP sheets. Specimens SP-C2s, SP-C3 and SP-C3s failed due to the delamination of covercrete from the mid-span till the end of FRP sheets (see Fig.9.b). It can be seen that ripping of covercrete usually tended to occur in the zone with large moment (see Fig.9.a). The covercrete likes teeth between CFRP sheets and reinforcement. The yielding of steel reinforcement and the corresponding bond deterioration under mix-mode stress condition are possible factors making the covercrete to split at the height of steel/concrete interface. The strengthened RC beams failed in the covercrete rather than in the flexible bonding layer, which was observed in pullout tests [Shimizu and Nisseki Co., 2001]. The reason is that the bond stress-slip behaviors of FRP/concrete interfaces can influence greatly the size of the yielding zone of steel. Using flexible bonding layer increased the length of the yielding zone, shifting the failure from the FRP/concrete interface to covercrete. Naturally, amount of FRP and steel amount also affects the stress balance in the concrete teeth. To simulate the interactions among them accurately with consideration of change of adhesives, steel, and FRP’s properties is still challengeable at this moment. Special discrete modeling technique may help to handle with this difficulty [Dai et al. 2005]. Compared to Specimens SP-C1 and SP-C2, Specimens SP-C1s and SP-C2s increased their ultimate capacity Pu by 29% and 36%, respectively. The observed maximum strains in CFRP sheets increased from 7,790 to 14,800µ and 7,170 to 13,100µ in the cases of one and two layers of CFRP sheets, respectively. The effect of using flexible bonding layer on improving the ultimate member capacity is rather obvious. However, Specimen SP-C3s increased its ultimate capacity by 8% only compared to Specimen SP-C3. The maximum strain in CFRP sheets increased from 6,708 to 8,664. Effect of the flexible bonding layer was not as significant as those in the one and two layers cases. The ultimate capacity of SP-C3s was even lower than that of SP-C2s. It is considered

0

500

1000

1500

2000

2500

3000

0 200 400 600 800 1000Location from mid-span(mm)

(µ)

sp-c2 steel sp-c2 FRP sp-c2s steel sp-c2s FRP

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000Location from mid-span (mm)

Stra

in (

µ)

sp-c2 steelsp-c2 FRPsp-c2s steelsp-c2s FRP

Yielding zone

Anchorage zone

Stra

in

210

that the anchorage length was not sufficient for the case of three layers of CFRP sheets under the testing shear span. Fig.10 shows the load-deflection curves of all specimens. It can be seen that the flexible bonding system not only improved the ultimate capacity but also increased the deformability, which had both positive and negative effects. Before the steel yielded, specimens using flexible adhesive showed comparatively low bending stiffness. And also, compared to the normal bonding adhesive cases, the specimens using flexible bonding layer had decreased load Py when the steel yielded, however, had a much higher ratio of the ultimate capacity to the yielding one Pu/Py (see Table 2). After steel yielded, the increase of moment capacity was mainly contributed by the increase of the tensile force in FRP sheets. The rapid increase of the tensile force in FRP sheets is the factor leading to local stress concentration. On the other hand, the decrease of the bending stiffness after steel yields makes the width of flexural cracks develop more rapidly. These two factors trigger local interface debonding easily at the tip of flexural cracks (mid-span debonding) in the normal adhesive bonding system. However, flexible bonding layer can release the local stress concentration and also can keep deforming compatibility with the increased crack width by providing sufficient interface shear deformation. So that the integrity of FRP/concrete interface can be kept and in the meantime the steel can yield further toward the supports, providing more ductility (see δu/δy in Table 2). Therefore, FRP strengthened RC members can achieve enough ductility through appropriate interface design even though FRP is a linear material. Figs.11 and 12 show the strain distributions in FRP sheets and steel at the yielding of steel and the ultimate state, respectively. When the normal adhesive was used, the strains in FRP sheets were higher than those in steel in both flexural and shear zones. This is in accordance with the plane section assumption. However, when the flexible adhesive was used, FRP had higher strain than steel in shear zone but both FRP and steel had approximately same strain in the flexural zone. Consequently, FRP was less efficiently used at the yielding point of steel. However, when macro-debonding occurred at the ultimate state, strains in FRP sheets behaved two different manners in cases of normal and flexible adhesives (see Fig.12). In the yielding zone where steel had yielded, FRP strain showed very small gradient in both the normal and flexible bonding systems, meaning that the covercrete failure had occurred there. In the anchorage zone, the strain distribution in FRP was linear and active in a wide range in the flexible bonding case, however, was bilinear and active in a comparatively narrow range in the normal bonding case. The situation was rather similar to that observed in pullout cases [see Fig.6]. So higher average bond stress could be achieved in the anchorage zone in the flexible bonding cases as is shown in Fig.13, in which the length of the anchorage zone was assumed a constant 570mm from the end of CFRP for the purpose of comparison. One exception in Fig.13 is that specimen SP-C3s did not achieve higher average bond strength than SP-C3. That was possibly due to the shortage of anchorage length, which may not become a concern in cases of beams with larger spans. FATIGUE DURABILITY OF RC BEAMS FLEXURALLY STRENGTHENED WITH FRP Fatigue tests on 5 RC beams strengthened with FRP using normal and flexible bonding systems were performed (see Table 3). Dimensions of all specimens were the same as is shown in Fig.8. The maximum and minimum fatigue loads were 40kN and 10kN, respectively. The frequency of fatigue load was 4Hz. The maximum stress levels of steel were about 90%, 75% and 60% of its yielding strength in cases of one, two and three layers of FRP sheets, respectively. Steel in specimen FSP-C1s fractured after 1,295,703 fatigue cycles without debonding in the FRP/concrete interface. Other specimens all kept their integrity after 2,000,000 fatigue cycles and then were given static tests. The ultimate capacity and deflections obtained from the static tests conducted after the fatigue cycles are listed in Table 3. The testing load-deflection curves are shown in Fig.14. In general, the more layers of CFRP sheets were used, the smaller the plastic deflections were observed after 2000,000 fatigue cycles. Moreover, specimens FSP-C2s and FSP-C3s using flexible bonding system had more plastic deflections than specimens FSP-C2 and FSP-C3 (see Fig.14). This is because that use of flexible bonding system caused lower initial bending stiffness as had shown in the static tests. The bending stiffness of all specimens degraded rapidly with the fatigue cycles in the beginning, and then degraded slowly (see Fig.15). However, the specimens using flexible bonding system contributed to more smooth degradation of the bending stiffness (see Fig.15). The reason is that flexible bonding system could avoid the local bond deterioration at the vicinities of concrete’s cracks triggered by local stress concentration. This nature could make the flexible bonding system favourable for elongating the fatigue life of RC slabs experienced with damages [Maeda et al. 2003]. After 2,000,000 fatigue cycles, the ultimate capacity Pu,f obtained from the following static tests did not show decrease in both normal and flexible bonding adhesive cases compared to the initial static capacity Pu,s (see Table 3). Only specimen FSP-C2s was an exception. The ultimate capacity of specimen FSP-C2s decreased from 148.0kN to 109.8kN after 2,000,000 cycles. Since specimens FSP-C2s for static test and fatigue tests were prepared and tested in Shimizu Co. and Hokkaido Univ., respectively, there might have some deviation on the thickness of flexible layer or some environmental effects on the adhesive properties. It is a future task to study how sensitive the member performance is to the thickness of flexible layer for different FRP stiffness and also the environment

211

effects. On the whole, it can be concluded that the strengthened RC members using the flexible bonding system has equal or better fatigue performance than those using the normal adhesive bonding.

Table 3 Fatigue tests of RC beams strengthened with FRP Specimen Pu,s

(kN) Pu,f (kN)

δu,s (mm)

δu,f (mm)

Experienced fatigue cycle Mode of failure

FSP-C1s 58.8 101.0 33.4 32.0 1,295,703 Fracture of steel reinforcement FSP-C2 101.9 109.0 16.4 16.7 2,000,000 Peeling of FRP from concrete substrate FSP-C2s 109.8 148.0 40.7 21.4 2,000,000 Delamination of covercrete FSP-C3 115.6 108.0 14.8 14.8 2,000,000 Delamination of covercrete FSP-C3s 117.6 116.6 21.2 23.1 2,000,000 Delamination of covercrete

Note: All the fatigue tests were performed in Hokkaido University. Pu,s: static ultimate capacity; Pu,f: ultimate capacity from static tests after fatigue cycles; δu,s: ultimate deformation in the static test, and δu,f: ultimate deformation from static tests after fatigue cycles.

Fig.14 Load-deflections curves Fig.15 Degradation of the stiffness of all specimens

A FULL-SCALE TEST ON A 15-M SPAN T-BEAM FLEXURALLY STRENGTHENED WITH FRP Prior to its demolition, a reinforced concrete bridge T-beam with a span of 15.0m was strengthened using the flexible bonding system and tested to failure. It was observed that the longitudinal bars in the T-beam were corroded before the strengthening (see Fig.16). The T-beam had a tension reinforcement ratio of 1.8%. Compressive strength of concrete was 24.0MPa. Longitudinal bars had the yielding strength of 295.0MPa. The deteriorated covercrete was replaced by new mortar layer and then two layers of CFRP sheets with tension stiffness of 76.8kN/mm were attached to the bottom of the beam. The width of FRP sheets was the same as that of the T-beam’s web part. The T-beam had a testing span of 14,660mm and a 500mm constant moment zone (see Fig.17) . The mode of failure of the T-beam strengthened with CFRP sheets was crushing of top flange concrete after steel yields. The peeling of CFRP sheets from concrete did not occur although the maximum strain in the CFRP sheets had achieved 12,000µ, which is 80% of the fracture strain of CFRP. The whole T-beams kept its integrity up to the crushing of top flange concrete, after which the loading was stopped for the safety concern. The capacity of the T-beam strengthened with CFRP was 102.0kN. The capacity of the un-strengthened T-beam was 89.0kN, which was estimated by FEM analysis. Since the corrosion of main bars had occurred before the strengthening, the un-strengthened T-beam might have an ultimate capacity smaller than 89.0kN. So the ultimate capacity of T-beam was fully recovered and increased at least by 14.6% by the flexible bonding system. The obtained load-deflection curve is shown in Fig.18. It can be seen that the bending stiffness before steel yielded almost had no change before and after the strengthening. It was verified again that the flexible bonding system is not suitable for serviceability strengthening purpose. Fig.19 shows the measured strain distributions in FRP sheets. It can be seen that the strain in CFRP sheets distributed in linear manner in the shear span before the steel yields, whereas a bilinear manner at the ultimate state. Different from that observed in the laboratory tests for RC beams with small spans, the bond stresses were mainly transferred between the FRP sheets and concrete within the yielding zone which was about 4 meters (see Fig.19). The large strain gradient within the yielding zone indicates that the increased tensile force in FRP sheets and the increased crack width after the steel yielded did not cause any debonding of FRP from concrete, verifying the effects of the flexible bonding system. Since the span of the T-Beam was very long, the slip at the end of FRP was not observed. It is expectable that tensile

0 20 300

50

100

FSP-C2FSP-C2sFSP-C3FSP-C3s

P (kN) Degradation of stiffness

δ (mm)

FSP-C1s FSP-C2 FSP-C2s FSP-C3 FSP-C3s

FSP-C1

212

force in CFRP could increase further and the active bond stress zones would shift from yielding zone toward the anchorage zone as that having occurred in Fig. 12 if the crushing of concrete did not occur. The long shear span is very favourable for the flexible bonding system to develop its full interface bond capacity without having to concern about the anchorage failure.

Fig.16 Photo of tested T-beam Fig. 17 Dimension of the T-Beam

0

2000

4000

6000

8000

10000

12000

14000

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000Distance from mid-span (mm)

Stra

in o

f FRP

(µ) P=1040kN(max)

P=950kN

P=800kN(yielding)

P=400kN

Fig. 18 Load-deflection curve of tested T-beam Fig.19 Strain distribution in FRP sheets

CONCLUSIONS The following conclusions can be drawn up for the flexible bonding system reported in this paper: 1. FRP/concrete interfaces using the flexible bonding system need much longer anchorage length to develop

fully its interfacial bond force-transferring capacity. With a sufficient long anchorage length, flexible bonding adhesive can achieve higher pullout capacity. FRP sheet/concrete interfaces with normal and flexible bonding system show big differences in their local bond stress-slip behaviours. The flexible bonding system has lower interfacial bonding stiffness but can ease the local stress concentration and can bear large interfacial slip while avoiding local debonding.

2. Compared to the normal adhesive bonding, flexible bonding system can improve the ultimate capacity of FRP strengthened RC beams. So the high strength, an advantage of FRP, can be utilized more efficiently. Different from those in pullout tests, the failure modes of strengthened RC members using the flexible bonding system may shift from the debonding of FRP/concrete interface to the ripping of concrete covercrete. The latter mode is triggered by the yielding of steel reinforcement rather than the stress concentration at FRP/concrete interfaces. Therefore, the flexible bonding system contributes a ductile failure. But the improving effects of the flexible bonding system may be eliminated in cases of larger FRP amount and shorter shear span.

3. Contribution of the flexible bonding system to the ultimate capacity of FRP strengthened RC beams is mainly after the yielding of steel reinforcement. Hence the flexible bonding system is appreciated for the ultimate state strengthening rather than the serviceability strengthening purpose.

4. The strengthened RC members using flexible bonding system have an equal or better fatigue performance than those using normal adhesive bonding. Compared to the normal bonding system, the flexible bonding system contributes a smooth degradation of member stiffness although it leads to a comparatively low initial stiffness.

Corroded rebar

900

500

1550

170

830

250

7330

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120 140

Control beam (FEM ana.)Strengthened beam (exp.)

P(kN)

δ (mm)

213

5. The flexible bonding system seems particularly favourable for the RC members with larger span. The strengthened FRP/RC composite system can keep its integrity up to a ductile failure and gain high strength efficiency in FRP.

ACKNOWLEDGMENTS The authors gratefully acknowledge Shimizu Construction Co. and Nisseki Mitsubishi Co. for their providing adhesive materials and related data sources. REFERENCES ACI committee 440. 2R., Guide for the Design and Construction of Externally Bonded FRP Systems for

Srengthening Concrete Structures. American Concrete Institute, Farmington Hills, MI. 2002. Bank, L. C.(2004), Mechanically-fastened FRP(MF-FRP)-a viable alternative for strengthening RC members,

Proceedings of the Second International Conference on FRP Composites in Civil Engineering-CICE 2004, Seracino eds., pp.3-15.

Dai, J.G.; Sato, Y.; and Ueda, T. (2001), Experimental Study on Effects of Resins on Bond Force Transfer of Carbon Fiber Sheet-Concrete Interface. Proceedings of the 7th Japan International SAMPE Symposium & Exhibition, pp. 379-38.

Dai, J.G. and Ueda, T. (2003). “Local Bond Stress Slip Relations for FRP Composites-concrete Interfaces.” Proceedings of FRPRCS-6, Edited by Tan, K. H., Vol. l, 143-152.

Dai, J.G.; Ueda, T.; Sato, T., and Nagai, K.(2005), Prediction of Tension Stiffening Behaviors of FRP Sheet Strengthened RC tensile members by Discrete Modeling, Proceedings of ConMat'05, Vancouver, Canada

fib bulletin 14, Externally Bonded FRP Reinforcement for RC Structures, 2001. JSCE, Recommendations for Upgrading of Concrete Structures with Use of Continuous Fiber Sheets, 2001. Lamanna A. J.: Flexural Strengthening of Reinforced Concrete Beams with Mechanically Fastened Fiber

Reinforced Polymer Strips, Doctoral dissertation, University of Wisconsin-Madison, 2002. Maeda, T.; Komaki, H.; Tsubonai, K. and Murauei, K.(2001), Strengthening Effects of CFRP Sheet Bonding

using Soft Resins, Proceedings of the Japan Concrete Institute, Vol.23, No.1, pp.817-822(in Japanese). Maeda, T.; Komaki, H.; Uehigashi, Y. ; and Matsui, S.(2003), Fatigue Performance of RC Slabs Strengthened

with CFRP Sheets using Flexible Bonding Adhesives, Proceedings of the Japan Concrete Institute, Vol.25, No.2,pp.1885-1890(in Japanese).

Nanni, A.; Alkhrdaji, T.; Barker, M; Chen, G; Mayo, R; and Yang, X.(1999), Overview of Testing to Failure Program of a Highway Bridge Strengthened with FRP Composites, Proceedings of the 4th International Symposium on Non-Metallic(FRP) Reinforcement for Concrete Structures (FRPRCS-4), SP-188, C. W. Dolan, S. Rizkalla, and A. Nanni, eds., American Concrete Institute, Farmington Hills, Mich., pp.69-75.

Rizkalla S.; and Haasan T. (2001), Various FRP Strengthening Techniques for Retrofitting Concrete Structures, FRP Composites in Civil Engineering, Vol. II, pp.1033-1040.

Sato, Y.; Ito, T.; Komaki, H. and Maeda, T.(2002), Flexural Behaviors of Reinforced Concrete Beams Strengthened by CFS with Soft Layer, Proceedings of the Japan Concrete Institute, Vol.24, No.2, pp.1375-1380(in Japanese).

Shimizu Co. and Nisseki Co., (2001), Bonding Tests for CFRP Sheets Bonded to Concrete, Technical Report (in Japanese).

Sugiyama, Y.; Morikawa, H.; Kobayashi, H.; and Nakajima, J.(2000), Bond and Flexural Strength of RC Beams Bonded with Continuous Fiber Sheet in Consideration of Resin Material, Proceedings of the Japan Concrete Institute, Vol.22, No.3, pp.289-294(in Japanese).

Ueda, T. and Sato, Y. (2002), New Approach for Usage of Continuous Fiber as Non-Metallic Reinforcement of Concrete, Structural Engineering International, No.2, pp.111-116.

Ueda, T.; and Dai, J.G. (2004), New Shear Bond Model for FRP-Concrete Interface-from modeling to application, Proceedings of the Second International Conference on FRP Composites in Civil Engineering-CICE 2004, Seracino eds., Adelaide, Australia, pp.69-81.

214