Composites Part B Manuscript Draft Manuscript Number: JCOMB-D-11-00173 Title: An experimental investigation on flexural behavior of RC beams strenthened with prestressed CFRP strips using a durable anchorage system Article Type: Full Length Article Keywords: A. Carbon fiber; B. Debonding; B. Strength; D. Mechanical testing Abstract: This paper investigates the effectiveness and feasibility of a prestressed carbon fiber-reinforced polymer (CFRP) system for strengthening reinforced concrete (RC) beams. The proposed prestressing system with a novel anchorage allows the utilization of full capacity of the CFRP strips. Eight small-scale and two large-scale concrete beams strengthened different configuration of prestressed CFRP strips are tested under static loading conditions up to failure. The main parameters considered include the level of prestressing applied, ranging from 20 to 70% of the tensile strength of the CFRP strips, and the use of mechanical anchorages at both ends of the CFRP strips. Thanks to the durable anchorage, the full range of flexural behavior was investigated including post-debonding. The results indicate that the beams strengthened using prestressed CFRP strips exhibited a higher first-cracking, steel-yielding, and experimental nominal moments as the level of prestressing force increased up to a certain point. After analyzing prestress effects in small scale tests, an optimum prestress level for strengthening concrete beams using CFRP strips is proposed and verified in large scale tests.

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An experimental investigation on flexural behavior of RC beams

strengthened with prestressed CFRP strips using a durable anchorage

system

Young-Chan Youa, Ki-Sun Choi

a, and JunHee Kim

a,b

a Building Structure & Resources Research Division in Korea Institute of Construction

Technology, 1190, Simindae-Ro, Ilsanseo-Gu, Goyang-Si, Gyeonggi-Do, 411-712, Republic

of Korea

b Corresponding author, Tel:82319100230, Fax:82319100392, Email: junheekim@kict.re.kr

ABSTRACT

This paper investigates the effectiveness and feasibility of a prestressed carbon fiber-

reinforced polymer (CFRP) system for strengthening reinforced concrete (RC) beams. The

proposed prestressing system with a novel anchorage allows the utilization of full capacity of

the CFRP strips. Eight small-scale and two large-scale concrete beams strengthened different

configuration of prestressed CFRP strips are tested under static loading conditions up to

failure. The main parameters considered include the level of prestressing applied, ranging

from 20 to 70% of the tensile strength of the CFRP strips, and the use of mechanical

anchorages at both ends of the CFRP strips. Thanks to the durable anchorage, the full range

of flexural behavior was investigated including post-debonding. The results indicate that the

beams strengthened using prestressed CFRP strips exhibited a higher first-cracking, steel-

yielding, and experimental nominal moments as the level of prestressing force increased up to

a certain point. After analyzing prestress effects in small scale tests, an optimum prestress

*Manuscript

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level for strengthening concrete beams using CFRP strips is proposed and verified in large

scale tests.

Keywords: A. Carbon fiber; B. Debonding; B. Strength; D. Mechanical testing

1. INTRODUCTION

Strengthening using fiber reinforced polymer (FRP) materials has been widely used as an

alternative to steel plates in retrofitting reinforced concrete structures throughout most parts

of the world. Although past studies for non-prestressed FRP strengthening systems have

shown significant increases in the ultimate strength, no significant increase in serviceability

has been observed due to the possibility of premature debonding failure [1,2,3,4,5,6]. The

non-prestressed FRP strengthening systems hardly utilize the full capacity of externally

bonded FRP.

FRP can be utilized as either a form of sheets or strips. FRP strips are beneficially applied to

retrofit a flexural concrete member because FRP strips produced by closely controlled

pultrusion processes typically have enhanced mechanical properties and higher fiber volume

compared to hand laid-up FRP sheets. However, since strips have a smaller width and a

larger thickness than sheets, bonding stress at the interface between the FRP strips and

concrete surface tend to be higher for pultruded strengthening systems. In past research

premature debonding failures were reported more frequently for beams strengthened using

CFRP strips [2,3]. In order to maximize the utilization of the CFRP materials, these brittle

failure modes caused by debonding or delamination should be overcome and strengthening

with prestressing has been alternatively introduced [7,8,9].

The use of externally bonded prestressed CFRPs for strengthening concrete members has

only been studied within the last two decades [1,11,12,13]. Although many of the past studies

mainly focused on the use of CFRP sheets, it is worth reviewing them in order to characterize

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prestressing systems for flexural strengthening with CFRP strips. These can be divided into

three categories. In the category I, a camber is applied to the beam prior to installing the

CFRP [14]. The CFRP is bonded to the beam and prestressing is applied by releasing the

camber. In the category II, the prestressing force is applied to the CFRP using an independent

external reaction frame [1,9,13,13,15]. Since the initial prototype prestressing systems in

category I and II did not use a mechanical anchorage, failure occurred due to debonding of

the FRP sheets within the end zone at a relatively low level of prestress [1,9,13,14]. In the

category III, the prestressing force is applied to the CFRP by reacting against the

strengthened beam itself [8,12,16,17]. Adding mechanical anchorage within the end zone of

the beam can ensure more ductile behavior while also increasing the allowable level of

prestress which can be applied [11,15]. Although past research reported that the use of a

mechanical anchorage can result in a significant improvement in serviceability and strength

[5, 9,18], the most of them focused on CFRP sheets. For the application of CFRP strips, an

anchorage system should have higher-performance to utilize the full capacity of CFRP strips.

2. RESEARCH SIGNIFICANCE

Strengthening using externally bonded prestressed CFRP strips can maximize the utilization

of CFRP materials if taking advantage of excellent durability and structural improvement in a

prestressing system with an anchorage device. In this paper, to achieve practical use of a

prestressed strengthening system using CFRP strips, a novel high-performance mechanical

anchorage and jacking systems for CFRP strips are developed. Through a series of small-

scale experimental tests, the performance of the proposed prestressed strengthening system is

evaluated in terms of strength and deformability of strengthened concrete beams, and then

maximized allowable prestressing level in the new anchorage system is validated with a

large-scale test set. Test results indicate that the RC beams strengthened using the proposed

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prestressed CFRP system exhibit a significant increase in strength and serviceability over

non-prestressed FRP strengthened concrete beams as well as ordinary concrete beams.

Consequently, this study gives a better understanding of the flexural capacity from cracking,

via steel-yielding, FRP-debonding, concrete crushing, to rupture of FRP for bonded

prestressed-CFRP strips and an anchorage system.

3. EXPERIMENTAL PROCEDURE

The present experimental study investigates the effectiveness of prestressed CFRP strips to

strengthen a deteriorated concrete beams as well as to enhance the serviceability of pre-

cracked and damaged RC beams. A total of ten small and large-scale concrete beams were

strengthened using prestressed CFRP strips and tested to failure. The details of the

experimental programs are presented in the following section.

3.1 Test beams

The experimental program included a total of eight small-scale beams and two large-sale

beams. The details of the experimental program are given in Table 1. The small-scale beams

had nominal cross-sectional dimensions of 200 x 300 mm (7.87 x 11.8 in.) with a total length

of 2,700 mm (106.3 in.). The overall nominal dimensions of large-scale beams were 400 x

600 x 6,800 mm (15.75 x 23.62 x 267.71 in.). The small-scale beams included one

unstrengthened beam (Control-1), two strengthened beams using non-prestressed CFRP strips

without end anchorages (NFCB1:one FRP strip, NFCBW2:two FRP strips), one strengthened

beam using non-prestressed CFRP strips with end anchorages (PFCB1-0R), and four

strengthened beams using prestressed CFRP strips with end anchorage (PFCB1-2R, PFCB1-

4R, PFCB1-6R, and PFCB1-7R). The control beam and two strengthened beams using non-

prestressed CFRP strips were manufactured to compare the structural behavior of the

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prestressed strengthened beams using CFRP strips. Different levels of prestressing force were

used for each of the four small-scale prestressed beams. The target level of prestress was

selected relative to the ultimate tensile strength of the CFRP strips, which is 20, 40, 60, and

70%. The large-scale beams included one unstrengthened beam (Control-2) and one

strengthened beam using prestressed CFRP strips with end anchorages (PFCB2-5R). The

level of prestress for the large-scale beam (PFCB2-5R) was determined after analyzing the

behavior of the small-scale prestressed strengthened beams.

Fig. 1 shows the reinforcement details of the typical test beams. The beams are reinforced

with Grade 420 MPa (60 ksi) reinforcing bars. The small-scale beams were reinforced with

three D10 bars in tension and three D13 bars in compression. The two large-scale beams were

reinforced with three D19 bars in compression and five D22 bars in tension. The shear

reinforcement consists of D10 steel stirrups with a constant spacing of 100 mm (3.94 in.)

center-to-center along the length of the beam.

3.2 Prestressing system and operations

The prestressing system presented in this paper consists of mechanical anchorages and

jacking assemblage with hydraulic jacks as shown in Fig. 2. The anchorages consist of three

individual anchors; two fixed grip anchors which permanently attach the end of CFRP strips

to the beam and one jacking anchor which is used to apply the tension force to the CFRP

strips.

The anchorage system and jacking assemblage developed in this study were used to directly

tension the CFRP strips by jacking and reacting against permanent anchors mounted on the

strengthened concrete beam itself. Fig. 3 shows the jacking assemblage of a small-scale

strengthened beam. The prestressing force for the small-scale beam was applied with the

beams placed on the floor with the bottom face-up. The base plate of the grip anchor was

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installed at the dead end of the beam and fastened into position using several bolts anchored

directly into the concrete beam. After applying adhesive on the CFRP strips, the CFRP was

gripped by tightening the bearing plate of the grip anchor with a torque wrench. The jacking

force was applied to the CFRP strips using the jacking anchor and a hydraulic jacking

assemblage at the other end of the beam. When the desired level of prestress in the strips was

achieved, the CFRP strips was fixed to the beam using the other grip anchors located in front

of jacking assembly. The jacking system was removed and the beam was isolated to cure the

adhesive for one week.

Fig. 4 shows the loss of a prestressing strain in CFRP strips after the mechanical anchorage

was set. As shown in Fig 4, it was clear that the prestressing strain in CFRP strips rapidly

decreased with a removal of a jacking force, but the decreasing rate was reduced and

converged to a certain value. In this prestressing system, the total loss of the prestressing

strain in CFRP strips which was measured over 300 hours was 6% of the initial prestressing

strain.

The basic concept and prestressing process for the large-scale beams was similar to that of

the small-scale beams except the prestressing force was applied while the beams were placed

on a support with the bottom face-down. A specially designed jacking assemblage with

hydraulic jacks was used as shown in Fig. 5. The bottom surface of the large-scale concrete

beams was strengthened using two prestressed CFRP strips tensioned individually to

approximately 50% of their ultimate tensile strength.

3.3 Material properties

The mechanical properties of the concrete, steel reinforcing bars, and CFRP strips were

obtained from material tests. The measured compressive strength of the concrete after 28

days was between 16.4 to 20.7 MPa (2.34 to 2.96 ksi) for all of the tested beams. The CFRP

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materials used in this study were composed of pitch-based carbon fibers and epoxy resin

pultruded into 1.4 mm (0.055 in.) thick, 50 mm (1.97 in.) wide plates. The fiber volume

fraction of the strips was 68%. The mechanical properties of the CFRP strips were

determined from tension tests in accordance with ASTM D 3039. Test results showed that the

average ultimate strength of CFRP strips was 2,161 MPa (308.7 ksi) with a standard

deviation of 121 MPa (17.3 ksi). The average measured modulus of elasticity was 165 GPa

(23,571 ksi). A two-component epoxy adhesive was used to bond the CFRP strips to the

surface of the concrete beam. The average tensile shear strength of the epoxy adhesive was

measured according to ISO 4587 as 4.3 MPa (0.61 ksi).

3.4 Test set-up

The clear span of the small-scale and large-scale beams was 2,400 mm (7 ft 10.5 in.) and

6,400 mm (20 ft 11.9 in.) respectively. The beams were simply supported and loaded in

three-point quasi static loading as shown in Fig. 6. The load was applied gradually to the

beam with an MTS controller-testing machine. After yielding of the longitudinal

reinforcement, the load was applied using displacement control at a constant displacement

rate of 5 mm/min (0.2 in./min) until failure.

3.5 Instrumentation

All beams were fully instrumented to measure the applied load, deflections, and strains of

steel reinforcing bars, concrete, and CFRP strips. The distribution of cracks along the beams,

from first cracking to final failure, was also monitored. Deflections at mid-span were

measured using two linear variable differential transducers (LVDTs). Electrical resistance

strain gauges were installed at mid-span of the beam on the top and bottom reinforcing bars,

on the top face of the concrete, and on the CFRP strips along the beam with constant spacing

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of 150 mm (5.91 in.) as shown in Fig. 7. Pi gauges were placed at mid-span along the front

face of the beam to measure the strain distribution in concrete over the depth of the beam. All

the signals from LVDTs, strain gauges, and load cells were automatically recorded by a data

acquisition system, data logger TDS-303.

4. EXPERIMENTAL RESULTS

4.1 Crack patterns and failure modes

In the experimental test, a typical pattern of crack formation was observed. The first flexural

crack occurred in the mid-span of the beam, and was followed by the formation and

propagation of many smaller cracks which were symmetrically distributed about the mid-

span of the beam after yielding of longitudinal reinforcements. The crack formation pattern

was similar for all of the tested beams. Both deflection and cracking were reduced in

proportion to the level of prestress because the prestressed CFRP strips induce compressive

stresses. Furthermore, the presence of the bonded prestressed CFRP strips helped to distribute

the flexural cracks more evenly along the length of the beams resulting in smaller crack width.

This is similar to the trend reported previously by others [12,16]. These flexural cracks

continued to propagate with shear cracks at higher load levels, but shear failure was

effectively controlled by the stirrups spaced at 100 mm (3.94 in.).

The control beam (Control-1) failed in a conventional flexural manner with the concrete

crushing in compression in the mid-span of the beam. The beams strengthened with non-

prestressed CFRP strips without end anchorage (NFCB1 and NFCBW2) failed by the abrupt

debonding of CFRP strips starting from the center to the one end of the beam as shown in Fig

8.

In the non-prestressed strengthened beam with end anchorage, the debonding of CFRP strips

initiated on one shear span followed by a second debonding on the other shear span. The

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debonding was accompanied by a sudden drop of the applied load. The mechanical

anchorages at both ends of the CFRP strips delayed the abrupt propagation of debonding in

the CFRP strips. Consequently, the CFRP strips were only supported by the end anchorages

and the strengthening system behaved similarly to an unbounded system. The final failure of

this beam was governed by the rupture of the CFRP strips at the mid-span of the beam as

shown in Fig. 9.

From the experimental results, it was observed that the failure modes of the prestressed

strengthened beam were influenced by the level of prestress in the CFRP strips. Although the

flexural behavior and failure mode of the strengthened beams prestressed to not more than

60% of the CFRP tensile strength was similar to that of the non-prestressed strengthened

beams with end anchorage as shown in Fig. 10(a), the first-cracking and steel-yielding loads

are delayed due to the presence of the prestressing and this effects reduced the deflection

within the service load range due to the increased member stiffness. For the strengthened

beam prestressed to 70% of the CFRP tensile strength, failure occurred due to a rupture of the

CFRP strips without any occurrence of CFRP debonding as shown in Fig. 10(b). Following

the rupture of CFRP strips, the strengthened beams retained a reserve capacity comparable to

the strength of the unstrengthened control beams.

4.2 Flexural behavior and strengthening effects

The load-deflection response of the small-scale beams including the unstrengthened control

beam, non-prestressed strengthened beams, and the prestressed strengthened beams is given

in Fig. 11 to 13 respectively. The CFRP strips were of the same dimensions in all tested

beams. This approach allowed direct comparison of flexural behavior of the unstrengthened

beam, non-prestressed strengthened beams, and the prestressed strengthened beams with

different levels of prestress. The load-deflection response of the large-scale beams including

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the unstrengthened control beam and the prestressed strengthened beam was given in Fig. 14.

A summary of significant values characterizing the behavior of the tested beams is given in

Table 2.

Unstrengthened control beamThe unstrengthened small-scale control beam (Control-1)

showed a typical flexural behavior of a concrete beam tested under static loading as shown in

Fig. 11. The first-cracking and steel-yielding load of beam Control-1 were 18.2 kN (4.10

kips) and 40.4 kN (9.09 kips), respectively. The stiffness of the unstrengthened beams

decreased significantly after yielding of the longitudinal reinforcements. The experimental

nominal strength is defined in this study as the smallest of "the load when a FRP failed by

rupture" or "the load when a compressive strain in concrete reach 0.003. Since the control

beams do not have FRP reinforcements, the experimental nominal strength was determined

by the latter, as 47.0 kN (10.58 kips), and the maximum load achieved was 50.5 kN (11.36

kips). The unstrengthened large-scale control beam (Control-2) also displayed a typical

flexural load-displacement behavior similar to that of the small-scale control beam except the

difference of load level caused by the differences of scale.

Non-prestressed strengthened beams without end anchorageA considerable increase of the

measured maximum load was observed due to the addition of the non-prestressed CFRP

strips in the non-prestressed strengthened beams NFCB1 and NFCBW2 as shown in Fig. 11.

The maximum load of the beam NFCB1 achieved was 77.0 kN (17.33 kips) and that of the

beam NFCBW2 was 96.4 kN (21.69 kips). As shown in the figure, the one and two CFRP

strips approximately increase the load capacity of the strengthened beam compared to the

unstrengthened control beam by 60% and 100%, respectively. However, both strengthened

beams failed due to debonding of the CFRP strips from the beams The strain in the CFRP

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strips measured immediately prior to debonding failure was from 5,191 to 6,852 which

corresponded to nearly 40 to 53% of their ultimate tensile strain of the CFRP. As referred in

the literature review, the premature debonding resulted in hardly utilizing the full material

capacity of FRP reinforcement. If the ductility is defined the fraction of the strain at the

maximum load to the steel-yielding strain, the ductility of NFCBW2 is similar with that of

NFCB1unlike the strength enhancement.

Non-prestressed strengthened beams with end anchorageThe load-deflection behavior of

beam PFCB1-0R, which was strengthened with non-prestressed CFRP but with mechanical

anchorage installed, is given in Fig. 12 and compared to the behavior of beams Control-1 and

NFCB1. The presence of the mechanical anchorages on the non-prestressed strengthened

beams did not significantly affect the flexural behavior of the beam prior to debonding of the

CFRP strips. The first-cracking and steel-yielding load of beam PFCB1-0R were 24.5 kN

(5.51 kips) and 55.4 kN (12.47 kips) respectively, which represent almost same performance

in terms of strength and serviceability, compared to the beam without end anchorages. The

debonding load was 80.5 kN (18.11 kips), slightly larger than that of beam NFCB1, 77.0 kN

(17.33 kips).

However, the load-deflection behavior of beam PFCB1-0R after debonding is obviously

different from that of beam NFCB1. The first debonding of the CFRP strips occurred at one

shear span as the shear stress between strips and substrate exceeded the concrete shear and

tensile strength. At the same load level at which first debonding occurred, the second

debonding of the CFRP strips occurred at the other shear span, accompanied by a sharp drop

in load as can be clearly observed in Fig. 12. The behavior of the beam was similar to that of

an unbonded system due to the loss of full composite action after two debondings. The

further increase of the applied load was resisted by the unbonded CFRP strips fixed at both

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ends of the beam by the mechanical anchorage. The loss of full composite action caused

further loss of member stiffness and resulted in further increase in deflection as the load

increased as shown in Fig. 12. The final failure of beam PFCB1-0R was governed by the

rupture of the CFRP strips with a drastic drop in load.

The experimental nominal strength was 81.8 kN, (18.41 kips) which was similar to the

debonding load of the CFRP strips. The maximum load attained was 121.5 kN (27.34 kips),

50% higher than the experimental nominal strength. This indicates that the use of anchorages

at both ends of CFRP strips without prestress does not contribute to an increase of the

experimental nominal strength but significantly increase the maximum capacity of the

strengthened beam as shown in Fig. 12. Even if there is twice strength fluctuation due to

debonding, the mechanical anchorage also significantly increased the ductility of the FRP

strengthened beam.

Prestressed strengthened beams The addition of non-prestressed CFRP strips slightly

enhanced the serviceability of the beam with a little increase in first-cracking and steel-

yielding load. But a significant increase of serviceability was achieved when the applied

strips were prestressed with an end anchorage system. The delay in first-cracking load also

led to a delay in steel-yielding load, resulting in a significant increase in member stiffness.

The enhancements in serviceability were in proportion to the level of prestress in each beam.

The initial cracking load of beam PFCB1-4R, for example, prestressed using 40% of the

ultimate tensile strength of the CFRP strips was 24.0 kN (5.40 kips) greater than the first-

cracking load of the control beam. This represented an increase in first-cracking load over the

control beam of 133%. The maximum increase in first-cracking load was obtained in beam

PFCB1-7R prestressed using 70% of its ultimate tensile strength. The first-cracking load was

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42.7 kN (9.61 kips) greater than that of the control beam, corresponding to an increase over

the control beam of 235% as shown in Fig. 13 and Table 2.

For beams PFCB1-2R, PFCB1-4R, PFCB1-6R, and PFCB1-7R which were strengthened

with prestressed CFRP strips with different level of prestressing, 20, 40, 60 and 70% of their

tensile strength respectively, the steel-yielding occurred at a load 77 to 186% higher than that

of the control beam in proportion to the level of prestress. This was due to the compression

forces induced in the concrete and the steel reinforcement due to the prestressed CFRP strips.

Also, the experimental nominal strength of the beams was increased by 123 to 155%

compared to the control. This indicated that the prestressed CFRP strips with the proposed

anchorage system can greatly contribute to an increase of member strength as well as member

serviceability. Nevertheless, the maximum loads of each beam attained from the test were

almost same regardless of the level of prestress. This was because the failure was governed

by the rupture of the CFRP strips in all cases. The CFRP strain at failure was within the range

of 12,218 to 13,508 regardless of their initial prestressing strain as can be seen in Table 2.

Large-scale beams testThe feasibility of strengthening concrete beams using prestressed

CFRP strips with the proposed anchorage system was also verified by a large-scale beam test.

The unstrengthened control beam Control-2 and one strengthened beam PFCB2-5R

prestressed to 50% of their ultimate tensile strength using two CFRP strips were tested. As

shown in Fig. 14, a significantly increase in first-cracking and steel-yielding load as well as

an experimental nominal strength was observed in the large-scale beam PFCB2-5R. The

prestressed CFRP strips increased the first-cracking and steel-yielding load by 105 and 49%

respectively. The experimental nominal strength of the prestressed strengthened beam was

59% higher than that of the unstrengthened control beam. Also, the overall flexural behavior

of the large-scale beams was similar to that of the small-scale beams; two instant load drops

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after subsequent debondings of the CFRP strips, loss of further stiffness after a bonded

prestressing system switched into an unbonded system, and the final failure governed by the

rupture of the CFRP strips. In addition, the load-deflection curve exhibited large ductility

because the prestressing system with the end anchorage enables the strengthened beam to

have large deformability in spite of twice debonding.

Strain distribution in the CFRP strips

Fig. 15 shows the distribution of strains along the length of the prestressed CFRP strips at

different loading levels along one shear span of the beam. The linear strain distribution in the

CFRP strips was observed along the beam before steel-yielding, which was directly

proportional to the magnitude of the applied moment. After the steel reinforcing bars yielded,

the slope of the strain distribution increased dramatically within the region of steel-yielding,

representing the increased load carried by the CFRP strips. Finally, the slope of the strain

distribution along the shear span was drastically reduced after CFRP debonding, representing

the separation of the CFRP strips from the concrete substrate. This also indicated that the

bonded strengthening system behaved as an unbonded system due to loss of the full

composite action after debonding. However, a little strain gradient in the unbonded CFRP

strips remained due to a local friction force between the CFRP strips and concrete substrate.

The trend of the measured strains was similar for the small-scale and the large-scale beams as

shown in Fig. 15.

Optimum prestressing force

A new parameter, the experimental nominal strength of a concrete flexural member

strengthened using FRP materials was defined in this study as the smaller of "the load when a

FRP material failed by fracture" and "the load when a compressive strain in concrete reach

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0.003. This parameter will be compared to analytical nominal strength later in this paper.

Based on this analysis, the first-cracking load, the steel-yielding load, the experimental

nominal strength, and the maximum load attained for each tested beam are given in Table 2.

As shown in Table 2, the first-cracking and the steel yielding load were significantly

increased for the strengthened beams prestressed to not less than 40% of the ultimate tensile

strength of the CFRP strip compared to both the unstrengthened beam and the non-

prestressed strengthened beam. The delay in first-cracking and steel-yielding load increased

the overall member stiffness and contributed to the increase of serviceability. Also the

experimental nominal strength defined above increased in proportion to the level of prestress.

The strengthened beam prestressed using 70% of their ultimate tensile strength failed due to

rupture of the CFRP strips without any occurrence of CFRP debonding. This beam showed

an extremely brittle failure mode with a drastic decrease of load after failure. A ductile failure

mode, on the other hand, was observed in the beam prestressed to not more than 60% of the

ultimate tensile strength of the CFRP strips with a significant increase of serviceability and

flexural maximum strength. The two subsequent debondings of the CFRP strips prior to

CFRP rupture transformed the bonded prestressing system into an equivalent unbonded

system. The stress redistribution in the unbonded CFRP strips reduced the large strain around

mid-span of the beam, and delayed the rupture of the CFRP strip while increasing the

deflection until the unbonded CFRP strips reached their ultimate rupture strain. This failure

mechanism led to a ductile failure mode for the RC beams strengthened using prestressed

CFRP strips prestressed to not more than 60% of their ultimate tensile strength.

The average debonding strain on the CFRP strips measured was about 6,852, which

corresponds to nearly 50% of the ultimate tensile strain of the CFRP strips. Prestressing the

CFRP strips to not more than 60% of their ultimate tensile strength ensured a ductile failure

mode for the CFRP strengthened beams with a significant increase in both serviceability and

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

16

strength. This mechanism was verified by the large-scale beam test in this paper. An optimum

level of prestressing, 50% of the ultimate tensile strength of the CFRP strips is proposed

when strengthening a concrete beam using the prestressed CFRP system.

5. ANALYTICAL PREDICTION

In this section, the flexural behavior of a section strengthened by prestressed CFRP with the

proposed anchorage system is predicted on the basis of simple mechanics with recent code

equations. The cracking moment, yielding moment, and nominal moment at the first

debonding state are calculated by using strain compatibility and internal force equilibrium.

The three moments are compared with the test results.

The prestressing force (Pi) of CFRP bonded on the bottom induces eccentric moment as well

as compressive axial force. Fig. 16 shows the stress distribution of a prestressed FRP-

strengthened section. The compressive strain in the bottom-end of the section (fb) and the

corresponding moment (Mpi) are calculated in equations (1) and (2).

(1)

(2)

where Ag, Ig, and rg are the gross sectional area, the moment of inertia of a gross concrete

section neglecting reinforcement, and radius of gyration of a gross section, respectively; y2

and ep are the distance of neutral axis from extreme tension fiber and the eccentric distance

from the bottom of a beam; Z2 is the section modulus of the FRP-strengthened section. The

prestressing force increase the moment capacities at cracking, yielding, and ultimate states by

the prestressing-induced moment (Mpi) since it resists the sagging deformation characteristic

of a beam experiencing bending. Therefore, cracking moment(Mcr), yielding moment (My),

and nominal moment (Mn) are computed as following equations (3) and (4) and (5).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

17

(4)

(5)

where d, h, k, k2, and a are the distance from extreme compression fiber to centroid of tension

reinforcement, the height of a beam, ratio of depth of neutral axis to reinforcement depth

measured from extreme compression fiber, ratio of the distance between the extreme

compression fiber and the resultant of the compressive force to the depth of the neutral axis,

and the depth of equivalent rectangular stress block, respectively; f and fd are the strain level

in the FRP reinforcement and the debonding strain of externally bonded FRP reinforcement.

fr, f`c, and Ef are the flexural tensile strength, specified compressive strength of concrete, and

tensile modulus of elasticity of FRP.

An ultimate state controlled by concrete crushing is assumed to occur if the compressive

strain reaches 0.003. In the test specimens strengthened with prestressed FRP strips lower

than 60% of the CFRP tensile strength, the compressive strain in concrete reached 0.003

immediately when the FRP strips debonded. Therefore, the nominal moment is determined

(3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

18

when the FRP debonding occur. This study adopts the following debonding strain equation

(ACI440.2R-08) modified from the original form proposed by Teng et al.[19,20]

(6)

where n and tf are the number of FRP strips and the thickness of a FRP strip.

Table 3 lists both analytical and experimental moments at cracking, yielding, and ultimate

states. The analytical approach predicts the similar increase rate of moment capacities at each

state as prestressed force increases. While the analytical cracking and yielding moments are

acceptably close to the measured moments in the experimental tests, the analytical nominal

moment is much lower than the experimental nominal moment by 38~57% because using the

ACI debonding stain equation have the nominal moments predicted conservatively. After the

first debonding points, the prestressed-FRP and the anchorage system indeed allow the

strengthened beams to reach the maximum moments until the FRP strips rupture in the

experimental tests. The 11th

column shows the maximum strength when the FRP strips

rupture, and they are all similar because the failure modes are controlled by rupture of the

FRP strips. The 12th

column presents the fraction of the maximum moments measured in the

experimental tests to the analytical nominal moments.

6. CONCLUSIONS

Eight small-scale and two large-scale tests were conducted to investigate the flexural

behavior of RC beams strengthened by bonded prestressed-CFRP strips and an anchorage

system. Most of past studies focused on FRP sheets or relatively low level of prestress on

FRP plates and flexural behavior up to debonding of FRP reinforcements. However, this

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

19

study include effects on relatively high level of prestress on CFRP strips as well as the

flexural behavior after debonding. The research results and findings are presented below.

1. The beam strengthened using non-prestressed CFRP strips without end anchorage

failed due to debonding of the CFRP strips at about 50% of the ultimate tensile

strength of the CFRP with a slight increase in serviceability.

2. Prestressed CFRP strips can significantly improve serviceability of reinforced

concrete beams by increasing member stiffness because of increase and delay of first-

cracking and steel-yielding load, respectively in proportion to the level of prestress

compared to an unstrengthened concrete beam. The enhancement in serviceability

was also verified in a large-scale beam test.

3. Significant increases in the flexural strength of the beams were also observed. The

first-cracking and steel-yielding strength increased by 133 to 235% and 77 to 186%

and the experimental nominal strength increased by 123 to 155% over the

unstrengthened control beams. The enhancement in strength was also verified in a

large-scale beam test.

4. The proposed prestressing system with end anchorages allowed the concrete beams

prestressed by not more than 60% of the ultimate tensile strength of the CFRP strips

to resist additional loads(up to maximum strength) in a ductile manner. On the other

hand, the beam prestressed to 70% of the ultimate tensile strength of the CFRP strips

showed an extremely brittle failure mode caused by rupture of the CFRP strips

without any occurrence of CFRP debonding.

5. Debondings of the CFRP strips was observed at two stages of loadings which

transformed the bonded prestressing system into an unbonded system. In this

debonding mechanism, the proposed durable anchorage system helps strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

20

distribution along the FRP strips and induces a large deformability to the CFRP

rupture.

6. In strengthening a concrete beam using prestressed CFRP strips, a maximum

allowable prestressing force, 50% of their ultimate tensile strength is, in authors

opinion, proposed with some safety margin.

7. The computed nominal moment of a prestressed-FRP strengthened beam are

underestimated in comparison with the experimental nominal moment because of

using the ACI debonding strain equation.

ACKNOWLEDGMENTS

This work is a part of research project supported by the Korea Institute of Construction and

Transportation Fund under the E01-01. The authors wish to express their gratitude and

sincere appreciation to the authority of KICTTEP for financing this research work

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4. ACI Committee 440. Guide for the Design and Construction of Externally Bonded

FRP Systems for Strengthening Concrete Structures (ACI440.2R-08). American

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prestressed fiber reinforced plastic sheets. ACI Struct J 1992;89(3): 235-244.

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FRP sheets. In: Proceedings of the Second International Symposium on Non-Metallic

(FRP) Reinforcement for Concrete Structures (FRPRCS-2). Ghent, Belgium, Aug,

1995. p. 568-575.

12. El-Hacha R, Grren M, Wight G. Innovative system for prestressing fiber reinforced

polymer sheets. ACI Struct J 2003;100(3):305313.

13. Triantafillou TC, Deskovic N. Innovative prestressing with FRP sheets: Mechanics of

Short-Term Behavior. J Eng Mech 1991;117(7):1652-1672.

14. Saadatmanesh H, Ehsani M. RC beams strengthened with GFRP plates: Part I:

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Experimental study. J Struct Eng 1991;117(11):3417-3433.

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23

TABLES AND FIGURES

List of Tables:

Table 1 Beam list and strengthening details

Table 2 Summary of test results

Table 3 Experimental and analytical moment capacity

List of Figures:

Fig. 1 Details of beams (Note: 1 mm = 0.039 in.) (a) Small-scale beam (b) Large-scale

beam

Fig. 2 Schematic drawing of prestressing system

Fig. 3 Jacking assemblage for the small-scale beams

Fig. 4 Loss of prestressing strain in CFRP strips

Fig. 5 Jacking assemblage for the large-scale beam

Fig. 6 Test set-up (Note: 1 mm = 0.039 in.)

Fig. 7 Position of strain and PI gages (Note: 1 mm = 0.039 in.)

Fig. 8 Typical debonding failure of beam strengthened using non-prestressed CFRP strips

without end anchorage

Fig. 9 Typical failure for beam strengthened using non-prestressed CFRP strips with end

anchorage

Fig. 10 Typical failure of beam strengthened using prestressed CFRP strips (a) Not more

than 60% of ultimate tensile strength of CFRP strips (b) 70% of ultimate tensile strength of

CFRP strips

Fig. 11 Load-deflection response of non-prestressed strengthened small-scale beams

without end anchorage (Note: 1 mm = 0.039 in.)

Fig. 12 Load-deflection response of non-prestressed strengthened small-scale beams

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

24

with/without end anchorage (Note: 1 mm = 0.039 in.)

Fig. 13 Load-deflection response of prestressed strengthened small-scale beams (Note: 1

mm = 0.039 in.)

Fig. 14 Load-deflection response of prestressed strengthened large-scale beams (Note: 1

mm = 0.039 in.)

Fig. 15 Strain distribution in CFRP strips at different loading levels (Note: 1 mm = 0.039

in.) (a) Small-scale beam (PFCB1-6R) (b) Large-scale beam (PFCB2-5R)

Fig. 16 - Stress distribution of a prestressed FRP-strengthened section

1

Table 1 Beam list and strengthening details

Beam External

prestressing Anchorage

Number of

CFRP strips

Level of

prestressing

force*

Scale

Control-1

NFCB1

NFCBW2

PFCB1-0R

PFCB1-2R

PFCB1-4R

PFCB1-6R

PFCB1-7R

Control-2

PFCB2-5R

Not strengthened

Non-prestressed

Non-prestressed

Non-prestressed

Prestressed

Prestressed

Prestressed

Prestressed

Not strengthened

Prestressed

None

None

None

End anchorages

End anchorages

End anchorages

End anchorages

End anchorages

None

End anchorages

None

1

2

1

1

1

1

1

None

2

None

None

None

None

20%

40%

60%

70%

None

50%

Small-scale

Small-scale

Small-scale

Small-scale

Small-scale

Small-scale

Small-scale

Small-scale

Large-scale

Large-scale

* relative to ultimate strength of CFRP strips

Table

2

Table 2Summary of test results

Name

Jacking

Strain of

CFRP

strip

Cracking

load

Yield

load

Debonding

Load of CFRP

strip

Experimental

Nominal load

(c=0.003) a)

Maximum load

Final

strain of

CFRP

strip b)

jak_f ()

Pcr

(kN)

Py

(kN)

Pde

(kN)

de ()

Pn

(kN)

n_f ()

Pmax

(kN)

max_f ()

f_f ()

Control-1

NFCB1

NFCBW2

PFCB1-0R

PFCB1-2R

PFCB1-4R

PFCB1-6R

PFCB1-7R

Control-2

PFCB2-5R

-

-

-

-

2,367

5,011

7,410

8,069

-

6,648

18.2

13.7

60.7

24.5

26.4

42.4

51.8

61.0

58.3

119.8

40.4

56.3

69.9

55.4

71.6

85.2

100.5

115.5

262.3

390.6

-

77.0

98.4

80.5

105.0

120.1

119.6

-

-

461.1

-

6,852

5,192

7,002

8,309

6,882

6,023

-

-

7,217

47.0

74.6

- c)

81.8

105.0

120.2

119.6

-

290.5

461.1

-

6,477

- c)

7,109

8,309

6,787

6,023

-

-

6,287

50.5

76.4

96.4

121.5

123.0

125.2

122.8

126.5

328.5

502.1

-

6,852

5,191

12,218

10,317

7,239

6,098

4,987

-

8,645

6,852

5,191

12,218

12,684

12,250

13,508

13,056

-

15,293

Note : a) load when the strain at concrete compressive fiber reach 0.003

b) f_f= jak_f +max_f c) fail to measure

1 kN = 0.225 kip

3

Table 3 - Experimental and analytical moment capacity

Name

Mcr(kN-m) Mya)(kN-m) Mn(kN-m)

cal exp exp

/cal cal exp

exp

/cal cal expn

b) expn

/cal expm

c) expm

/cal

PFCB1-0R 10.5 14.7 1.31 28.4 33.2 1.17 34.6 49.1 1.42 72.9 2.11

PFCB1-2R 15.9 15.8 0.95 33.9 43.0 1.27 40.0 63.0 1.57 73.8 1.84

PFCB1-4R 22.4 25.4 1.10 40.3 51.1 1.27 46.5 72.1 1.55 75.1 1.61

PFCB1-6R 28.9 31.1 1.05 46.8 60.3 1.29 53.0 71.8 1.35 73.7 1.39

PFCB2-5R 160.5 179.7 1.12 472.5 585.9 1.24 502.9 691.6 1.38 753.2 1.50

a) Moment at steel-yielding b) Experimental nominal moment at c=0.003

c) Experimental maximum moment at CFRP rupture

1

2,700

D10@1003-D10

150 2,400 150

3-D13

CFRP strips

1,900145

As : 3-D10

A's : 3-D13

Stirrup : D10@100

300

200

Concrete cover: 30mm

CFRP Strips

(a) Small-scale beam

6,800

D10@1005-D22

3-D19

CFRP Strips

6,0006,000400 400

Anchorage

As : 5-D22

As : 3 -D19

600

400

Concrete cover : 30mm

2-CFRP strips

(b) Large-scale beam

Fig. 1Details of beams (Note: 1 mm = 0.039 in.)

Figure

2

Base Plate

Bearing Plate

Anchor Bolt

Concrete Beam

CFRP StripsBearing Plate

Base Plate

Concrete

Prestressing Force

Grip Anchor at Dead End

Grip Anchor at Live EndJacking

Anchor

Fastening Bolt

Fig. 2Schematic drawing of the prestressing system

3

Fig. 3Jacking assemblage for small-scale beams

4

Fig. 4Loss of prestressing strain in CFRP strips

5

Fig. 5Jacking assemblage for large-scale beam

6

Concrete beam

Rubber pad

2,400 (small-scale beam)

6,400 (large-scale beam)

Hydraulic actuator

CFRP strips

Grip

anchor

Fig. 6Test set-up (Note: 1 mm = 0.039 in.)

7

Concrete gaue

L1, L

2PI Gage

Strain Gage CFRP strip

150 150 150 150 150 150

Grip

anchor

10

404040

40404040

10

Fig. 7Position of strain and PI gages (Note: 1 mm = 0.039 in.)

8

Fig. 8Typical debonding failure of beam strengthened using non-prestressed CFRP

strips without end anchorage

9

Fig. 9Typical failure for beam strengthened using non-prestressed CFRP strips with

end anchorage

10

(a) Not more than 60% of ultimate tensile strength of CFRP strips

(b) 70% of ultimate tensile strength of CFRP strips

Fig. 10Typical failure of beam strengthened using prestressed CFRP strips

11

Fig. 11 Load-deflection response of non-prestressed strengthened small-scale beams

without end anchorage (Note: 1 mm = 0.039 in.)

12

Fig. 12 Load-deflection response of non-prestressed strengthened small-scale beams

with/without end anchorage (Note: 1 mm = 0.039 in.)

13

Fig. 13 Load-deflection response of prestressed strengthened small-scale beams (Note:

1 mm = 0.039 in.)

14

Fig. 14 Load-deflection response of prestressed strengthened large-scale beams (Note:

1 mm = 0.039 in.)

15

(a) Small-scale beam (PFCB1-6R)

(b) Large-scale beam (PFCB2-5R)

Fig. 15Strain distribution in CFRP strips at different loading levels (Note: 1 mm =

0.039 in.)

after CFRP

debonding

before

steel

yielding

after steel

yielding

16

-Pi/Ag +Pi*ep*y1/Ig

+ =

A's

As

Af

y1(kd)

-Pi*ep*y2/Ig

y2(ep)

-Pi/Ag

-Pi/Ag(1-ep*y1/rg2)

-Pi/Ag(1+ep*y2/rg2)

Fig. 16 - Stress distribution of a prestressed FRP-strengthened section