IET BICET 2014 El-Gamal Et Al-Final

Proceeding of Brunei International Conference on Engineering and Technology, Institut Teknologi Brunei, Brunei Darussalam, November 1-3, 2014.

FLEXURAL STRENGTHENING OF RC BEAMS USING NEAR

SURFACE MOUNTED FIBRE REINFORCED POLYMERS

S.E. El-Gamal*, A. Al-Nuaimi*, A. Al-Saidy*, A. Al-Lawati*

* Department of Civil and Architectural Engineering, College of Engineering, Sultan Qaboos University, P.O. Box 33, Al-

Khodh 123, Oman, E mails: [email protected]; [email protected]; [email protected]; [email protected]

Keywords: Strengthening, Beams, Near Surface Mounted,

Fibre Reinforced Polymers.

Abstract

This paper presents an experimental study to investigate the

behaviour of Reinforced Concrete (RC) beams strengthened

in flexure with Near Surface Mounted (NSM) technique using

Glass and Carbon Fibre Reinforced Polymers (GFRP &

CFRP). An experimental program consisting of seven full

scale reinforced concrete beams were constructed and

strengthened in flexure with FRPs. The investigation included

three parameters; technique used (NSM or Hybrid), type of

FRP used (Carbon or Glass), and amount of FRP used. All

beams were tested under four point bending set-up. The test

results included behaviour, ultimate capacity, deflection,

cracking, and mode of failure. All strengthened beams

showed an increase in the capacity ranging between 55 and

133% compared with the reference beam. Most of the

strengthened beams had debonded in a sudden failure except

the NSM-GFRP strengthened beams. The NSM-CFRP

strengthened beams carried larger loads in terms of ultimate

capacity than the NSM-GFRP beams but they showed less

ductile behaviour. The NSM-GFRP strengthened beam,

however, showed good ductile behaviour with high deflection

values at ultimate load. This gave amble warning before

failure and can be considered as an advantage of

strengthening RC beams with NSM-GFRP system.

1 Introduction

Concrete structures in the Arabian Gulf countries and

other several places in the world are exposed to harsh

environmental conditions. This includes high temperatures,

humidity, and exposure to salt water as in the coastal

provinces of the Sultanate of Oman. These severe

environmental conductions result in significant deteriorations

of concrete structures at those regions. Consequently, the

rehabilitation and strengthening of these concrete structures

require materials that can withstand these severe conditions

for long time.

The advantages of fibre reinforce polymer (FRP)

materials such as high strength, light weight, and durability

make them a good choice in different strengthening

techniques. The near surface mounted (NSM) technique using

FRP bars (NSM-FRP) is one of the recent and promising

techniques that is currently used for the strengthening and the

rehabilitation of several RC concrete and masonry structures

[1-2]. In this technique, a groove is cut in the tension side of

the flexural element, and then the groove is partially filled

with a suitable adhesive. The GFRP bar is then inserted in the

groove and covered by a second layer of the adhesive.

The NSM technique has several advantages compared to

the externally bonded (EB) technique. The NSM technique

has less debonding problem and it does not need a lot of site

work as in the EB technique. In addition, the FRP bars in the

NSM technique are protected by concrete cover and adhesive

which make them less exposed to vandalism, temperature,

and damage. Furthermore, the surface of the strengthened

structure is almost unchanged. Due to these advantages, the

NSM-FRP technique is in many cases better than the EB-FRP

technique [3].

There are several research studies on the flexural

strengthening of RC structures using FRPs. Researches,

which focus on strengthening of RC structures using the NSM

technique and/or the EB technique and compare between

them are also available [4-8]. However, there is a lack of

researches that implement the hybrid system (NSM+EB) and

compare it with the NSM technique. Also, there is a lack of

research studies that investigate the utilization of GFRP in the

NSM technique and compare its behaviour with CFRP bars.

The objective of this research is to investigate the

flexural behaviour of RC beams strengthened in flexural with

FRPs. The effect of the following parameters are investigated

in this study; (a) technique used (NSM or Hybrid), (b) type of

FRP used (Carbon or Glass) and (c) the amount of FRP used

(one or two bars).

2 Experimental Work

2.1 Materials Used

2.1.1 Concrete

Ready mix concrete with a target concrete compressive

strength of 45 MPa was used. Compressive strength test and

tensile strength test were conducted using 6 cubes and

cylinders that were cast and cured according to [11]. Test

results show that the measured concrete compressive and

tensile strengths were 49.62 and 2.99 MPa, respectively.


2.1.2 Steel Bars

For all beams, deformed steel bars of 12 and 8 mm

diameter were used as tension and compression steel

reinforcement, respectively. The yield strength of the steel

bars was obtained to be 480 MPa by testing specimens in the

laboratory. Steel bars of 8 mm diameters were used for

stirrups.

2.1.3 FRP bars and sheets

Sand coated Carbon and Glass FRP bars of 2350 mm

length were used as a strengthening material. Table 1 presents

the characteristics of the CFRP and GFRP bars as provided by

the manufacturer. Carbon FRP sheets were also used as a

strengthening material. The ultimate load of the FRP sheets

was given by the producer to be 350 kN/m width. Figure 2

shows a photo of the carbon and glass FRP bars used in this

study.

Table 1: Characteristics of FRP bars used. Characteristic CFRP GFRP

Minimum guaranteed tensile strength (MPa) 1431 1100

Nominal tensile modulus (GPa) 120 52.5 2.5

Tensile strain (%) 1.33 2.15

Nominal bond strength (MPa) 16.5 14.0

Nominal cross-sectional area (mm2) 71.26 71.3

Figure 2: FRP bars and sheets used in this study.

2.1.3 Reinforcement Binder

Two types of resins where used in this work; MBRACE

Saturant epoxy resin for FRP sheets and MBRACE Laminate

Adhesive for FRP bars. They were used as a binder between

the fibres and concrete. The technical information for both

products can be found in [9, 10].

2.2 Test specimens

This experimental study included the construction of

seven simply supported beams of 2003002760 mm (width

depth length). Each beam was reinforced with 12 mm

diameter deformed steel bars in the tension side and 8 mm

diameter deformed steel bars in the compression side. In

addition, all beams had shear reinforcement to avoid shear

failure. The dimensions and reinforcement details of all

strengthened beams are shown in Figure 1.

Out of the seven beams, one beam was unstrengthened

(REF). The remaining six beams were strengthened with

FRPs. Out of which, two were strengthened with one or two

Figure 1: Dimensions of test specimens and setup.

bars of CFRP (CNSMB1 and CNSMB2); two were

strengthened with one or two bars of GFRP (CNSMB1 and

CNSMB2). The last two beams were strengthened with a

hybrid technique of NSM and EB. The first was strengthened

with one CFRP bars and one CFRP sheet (CHYP). The

second was strengthened with one GFRP bars and one CFRP

sheet (GHYP). Table 2 and Figure 3 show the test matrix and

the cross section of the seven beams, respectively.

Table 2: Control and strengthened test specimens.

Beam Strengthening

reinforcement

Strengthening

reinforcement

Equivalent Steel

ratio (%)

REF - - 0.427

CNSMB1 1 Bar Carbon 0.561

CNSMB2 2 Bars Carbon 0.695

GNSMB1 1 Bar Glass 0.561

GNSMB2 2 Bars Glass 0.695

CHYB 1 Bar + 1 Sheet Carbon+Carbon 0.611

GHYB 1 Bar + 1 Sheet Glass+Carbon 0.611

(where C: Carbon, G: Glass, NSM: Near Surface Mounted, HY: Hybrid, B:

Beam, 1: One FRP Bar, 2: Two FRP Bars)

(a) (b)

(c) (d)

Figure 3: Cross sections of the tested beams: (a) REF; (b)

CNSMB1 and GNSMB1; (c) CNSMB2 or GNSMB2; (d)

CHYP and GHYP.

Load

T8@100 mm 2-T8 mm

2-T12 mm

2760 mm

2360 mm

930 mm 500 930 mm

CFRP bars

GFRP bars

260

20

25

300

200

2T12

2T8

260

20

25

300

200

2T12

2T8

Epoxy CFRP or GFRP

260

20

25

300

200

2T12

2T8

85

260

20

25

300

200

2T12

2T8

CFRP sheet


2.3 Beams construction

After constructing all cages and before placing them in

the moulds, wood pieces (13 mm wide 20 mm depth) were

fixed at the bottom of the moulds to form the groove of the

NSM strengthened beams. After the placement of the steel

cages, concrete casting was conducted using the guidelines of

British Standard Code [11].

Concrete was poured in the moulds in three layers with

the usage of the vibrator to eliminate the presence of voids.

All specimens were cured in identical conditions for 28 days

before testing. Figure 4 shows the beams just after casting.

Figure 4: Fresh concrete after casting

For all strengthened beams, the grooves were cleaned

from dust and loose materials before inserting the FRP bars.

The two components of the epoxy resin were pre-mixed and

the mixture was added to the beam's grooves before/after

installation of the FRP bars. Figure 5 shows some of the NSM

beam after the installation of the bars.

Figure 5: NSM beam after the installation of the bars.

For the hybrid beams with NSM bars and external layer

of FRP sheets, after installing the NSM bars as described

above and keeping for a week, the bottom surface of the

beams was roughened sufficiently using a grinder. Then, all

dust and loose materials on the surface were removed and

cleaned. The two components of the epoxy resin were pre-

mixed and the mixture was added to the beam's surface before

and after the installation of the FRP sheet.

2.4 Test set-up and instrumentation

All beams were tested under four point bending test set-

up to failure as shown in Figure 6. The loading was applied

under a displacement control at a speed of (1 mm/min) using

an actuator. The measurement of all test data was recorded

using a static data logger and a computer at intervals of 1

second.

Two types of strain gauges were used for strain and

measurements. The first type of strain gauges was attached at

top of beams (mid-span) to measure concrete strain. The

second type of strain gauges was attached to the middle of

each bottom steel bars (before casting) to measure the strains

in the reinforcement. In addition, high accuracy LVDT was

attached on the first crack to measure the crack width.

Furthermore, three LVDTs were placed at bottom of the

beams to measure deflection.

Figure 6: Test set-up

3 Test Results and Discussion

Table 3 summarizes the results of the beams. The first

crack of the reference beam (REF) occurs at a load of about

20 kN. The beam failed by steel yielding at a load of about 56

kN followed by concrete crushing at a maximum load of 67

kN. The maximum measured deflection at yield was 11 mm,

while at concrete crushing it was about 58 mm. This big

difference demonstrates the good ductility of the reference

beam which is expected with steel reinforced concrete beams.

For all strengthened beams, the cracking loads range between

20 and 24 kN which were comparable with the reference

beam. This was expected as the cracking load depends mainly

on the gross moment of inertia of the beam which is not

significantly affected by adding the strengthening material.

Strengthening the beams increased the recorded yield loads

which ranged between 66 kN in GNSMB1 and 106 kN

CNSMB2 with an increase percent of about 18 to 89%,

respectively. The yield deflection, however, was slightly

affected as given in Table 3. The ultimate capacity of the

strengthened beams was significantly increased. This increase

ranged between a minimum value of 55% in beam GNSMB1

and 133% in beam CNSMB2. The number cracks at failure

was also increased in the strengthened beams. It ranged

between 19 to 35 cracks compared to only 15 in the reference

beam. In the coming sections, the effect of different

parameters will be discussed.


Table 3: Summary of test results. Beam

Name

Pcr

kN

Pyield

(kN)

yield (mm)

Pmax (kN)

Pmax (mm)

Capacity

increase

No. of

cracks

Failure

Mode

REF 20 56 11 67 58 _ 15 SYCC

CNSMB1 22 91 11 116 22 73% 19 SYDEB

CNSMB2 21 106 11 156 26 133% 26

GNSMB1 20 66 9 104 61 55% 22 SYCC

GNSMB2 20 83 11 136 54 103% 23 SYRUPb+

DEB

CHYB 24 112 12 145 24 116% 35 SY RUPs

DEB

GHYB 23 98 11 125 22 87% 30 SY RUPs

CC

Pcr: cracking load; Pyield: yield load; yield: yield deflection; Pmax: maximum load; Pmax: deflection at maximum load; SY: steel yielding; CC: concrete crushing; DEB: debonding of FRP bar; RUPb: rupture of FRP bar; RUPs:

rupture of FRP sheet.

3.1 Effect of the amount of FRP used

Figure 6 shows load versus mid-span deflections for

beam CNSMB1, CNSMB2 and REF. It can be noticed that

increasing the amount of FRP reinforcement increased the

capacity of the strengthened beams. Adding one or two CFRP

bars increased the capacity by 73 and 133%, respectively. The

mode of failure of CNSMB1 and CNSMB2 was similar. Both

beams failed by steel yielding followed by sudden debonding

of the CFRP bar; while the REF beam failed by steel yielding

followed by concrete crushing at collapse. The CFRP bars in

CNSMB1 and CNSMB2, however, did not totally split from

the beam. After checking the CFRP bars, it was observed that

the debonding happened between the external sand-coating

layer and the core of the CFRP bar. Figure 6 also shows that,

after cracking, the strengthened beams developed a slightly

larger initial stiffness than the REF beam. Increasing the

reinforcement increased the stiffness as expected. For

example; when comparing the deflection at service load

(assume 40 kN) with that measured in the reference beam it

was found that the deflections in CNSMB1 and CNSMB2

decreased by about 25 and 38%, respectively. After yielding

of the bottom steel reinforcement, the stiffness of the

strengthened beams was larger than that of the control beam

and it increased as the amount of reinforcement increased.

CNSMB1 and CNSMB2 behaved similarly to the reference

beam (REF) after the debonding failure but at a higher load

level because of the friction between bars and concrete.

Figure 6: Comparison between CNSMB1, CNSMB2 and REF

Figure 7 shows load versus mid-span deflections for

beams GNSMB1, GNSMB2 and REF. Again, it can be

noticed that increasing the amount of strengthening

reinforcement increased the capacity, stiffness after cracking,

and stiffness after steel yielding. The increase in the ultimate

capacity was about 55 and 103%, respectively, when one or

two GFRP bars were used. When comparing the deflection at

service load level (assume 40 kN) with that measured in the

reference beam it was found that the deflections in GNSMB1

and GNSMB2 decreased by about 8 and 15%, respectively.

The mode of failure of GNSMB1 and GNSMB2,

however, was not similar. GNSMB1 failed by steel yielding

followed by concrete crushing which was similar to that of

the reference beam. GNSMB2 failed by steel yielding

followed by rupture of the one of the GFRP bars and

debonding of the second bar. The two GFRP strengthened

beams showed high mid-span deflection at ultimate capacity.

For GNSMB1 and GNSMB2, the maximum measure

deflections at failure were 61 and 54 mm, respectively. These

high measured deflection values clearly show a good ductile

behaviour of both GNSMB1&2 which gives an amble

warning before failure. This can be considered an important

advantage of the RC strengthening beams with NSM-GFRP

bars.

Figure 7: Comparison between GNSMB1, GNSMB2 and REF

3.2 Effect of FRP type

The effect of the type of NSM-FRP bars used can be

clearly seen in Figure 8. It can be noticed that the beams

strengthened with CFRP bars gave higher capacity (about

30% increase) compared to the beams strengthened with

GFRP bars due to the higher strength of the CFRP bars. The

beams strengthened with CFRP bars also showed higher

stiffness after cracking and after yielding because of the

higher modulus of elasticity of the CFRP bars. The beams

strengthened with GFRP bars, however, showed much higher

mid-span deflection at failure (about 2 to 3 times) compared

to the beams strengthened with CFRP bars. These high

deflection values of the NSM-GFRP strengthened beam is a

big advantage as it shows much more ductile behaviour of

both GNSMB1&2. This gives an amble warning before

failure which is similar to the steel reinforced beam.


They type of FRP bars used also affected the mode of

failure of the strengthened beams. The CFRP strengthened

beams failed by steel yielding followed by sudden debonding

of the CFRP bars at low deflection values (22 &26 mm). The

GFRP strengthened beams, however, failed by steel yielding

followed by concrete crushing (GNSMB1) at a very high

mid-span deflection of 61 mm or by steel yielding followed

by rupture of the one of the GFRP bars and debonding of the

second bar (GNSMB2) at a high mid span deflection value of

54 mm. In general, it can be concluded that the NSM-CFRP

strengthened beams carried higher loads in terms of ultimate

capacity than the NSM-GFRP strengthened beams but they

showed much more brittle behaviour.

Figure 8: Effect of the type of FRP bars (NSM beams)

Similar observations were recorded with beams GHYP

and CHYP reinforced with a hybrid system (NSM and EB).

Figure 9 shows that CHYP, reinforced with NSM-CFRP bars

and EB-CFRP sheet, had higher capacity and stiffness

compared to GHYP, reinforced with NSM-GFRP bar and EB-

CFRP sheet. The capacity increase was about 33%. The

mode of failure, however, was different. The CHYB beam

failed by steel yielding followed by rupture of the CFRP sheet

followed directly by debonding of the CFRP bar at a low

deflection value of about 29 mm. In the GHYB beam, it failed

by steel yielding followed by rupture of FRP sheet followed

by concrete crushing at collapse at a high deflection value of

about 75 mm. After the rupture of the FRP sheet, it can be

noticed that the CHYP and GHYP beams showed a similar

behaviour as beams CNSMB1 and GNSMB1 reinforced with

one NSM carbon or glass bar, respectively.

Figure 9: Effect of the type of FRP bars (Hybrid beams)

3.3 Effect of technique used

The hybrid beams gave higher slightly lower capacities

when compared with NSM beam with two bars. In addition,

the mode of failure of the hybrid beams was different from all

NSM strengthened beams. Figure 10 shows a comparison

between all CFRP strengthened beams based on the

strengthening technique used. It can be noticed that both

CNSMB2 and CHYB showed almost similar behaviour

before failure. CNSMB2 failed by debonding of the CFRP

bars at a maximum load of about 156 kN at a corresponding

deflection of 26 mm. The CHYB beam failed by rupture of

CFRP externally bonded sheet at a maximum load of 145 kN

at a corresponding deflection of 24 mm. It can be noticed that,

after the rupture of CFRP externally bonded sheet, the load-

deflection of the CHYB beam was similar to that recorded in

the CNSMB1 beam as expected. With respect to REF, the

increase in ultimate load capacity of CHYB and CNSMB2

was 116 and 133%, respectively. This indicates that adding an

extra NSM-CFRP bar (CNSMB2) was more effective than

adding an extra externally bonded CFRP sheet (CHYB).

Figure 10: Effect of technique used (CFRP beams)

Figure 11 shows a comparison between all GFRP

strengthened beams. The three beams (GNSMB1, GNSMB2

and GHYB) failed at ultimate loads of 104, 136 and 125 kN,

respectively, with corresponding deflections of 61, 54 and 22

mm. It can be noticed that the GHYB beam had much more

brittle than the other two beams due to the rupture of the

externally CFRP sheet. It can be also noticed that, after


rupture of the CFRP sheet, the GHYB beam followed a

similar load-deflection curve as that of the GNSMB1 beam.

Figure 11: Effect of technique used (GFRP beams)

In general, it can be concluded that the hybrid system did

not show an extra advantage compared to the NSM technique.

It also needs more effort to roughness the surface of the beam

before bonding the FRP sheet. The NSM strengthening

technique gave higher capacities (with two bars) and better

ductility (in case of using GFRP bars).

4 Summary and Conclusions

To investigate the flexural behaviour of RC beams

strengthened by FRPs, seven full scale reinforced concrete

beams of 2003002760 mm (width depth length) were

constructed and strengthened with different types of FRPs.

The test parameters included the strengthening technique

(NSM and Hybrid), type of FRP used (carbon or glass) and

amount of FRP. Based on tests results of this research, the

following conclusions can be drawn:

All strengthened beams showed an increase in the ultimate capacity compared to the reference beam. This increase

ranged between 55 and 133%.

The NSM-CFRP strengthened beams gave larger capacities than the NSM-GFRP strengthened ones; however, they

showed much more brittle behaviour.

The NSM-GFRP strengthened beam showed a very good ductile behaviour with high deflection values at ultimate

load which were almost similar or even higher than that

recorded in the reference beam. This gives amble warning

before failure and can be considered as an important

advantage of using GFRP bars in the NSM strengthening

technique.

When compared with REF, the increase in ultimate capacity of the GNSMB1 and GNSMB2 was 55 and 103%,

respectively. Likewise, the increase in ultimate capacity of

the CNSMB1 and CNSMB2 was 73 and 133%,

respectively. This indicates that doubling the amount of the

FRP strengthening material increased the ultimate capacity

by about 85%.

The hybrid technique did not show an advantage compared to the NSM technique. In addition, it needs more effort to

roughness the surface of the beam before bonding the FRP

sheet. The NSM strengthening technique gave higher

capacities (with two bars) and better ductility (in case of

using GFRP bars).

Debonding was the major problem associated with NSM-FRP technique. In this study most of the strengthened

beams had deboneded in a sudden failure except the NSM-

GFRP beams which showed good ductile behaviour. It is

recommended to use strain control limits to prevent the

sudden collapse. In addition, it is recommended to conduct

more research to find ways to eliminate the deponding

problem in the NSM-FRP technique.

It is also recommended to conduct further research studied on the use of GFRP bars as NSM reinforcement since it

significantly increased the capacity and showed good

ductile behaviour.

References

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[2] Y. Al-Salloum, S. E. El-Gamal, T. Almusallam, S. Alsayed, and M. Aqel. Effect of harsh environmental conditions on the tensile properties of GFRP bars, Composites: Part B, 45(1), pp. 835844, (2013).

[3] L. De Lorenzis, J. G. Teng. Near-surface mounted FRP reinforcement: An emerging technique for strengthening

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[7] E. K. Castro, G. S. Melo, Y. Nagato. Flexural strengthening of RC T beams with near surface mounted (NSM) FRP reinforcements, FRPRCS-8, University of Patras, Patras, Greece, July 16-18, (2007).

[8] N. Bolghourjian, K. Soudki. Localized NSM GFRP rods for strengthening RC beams, FRPRCS-9, Sydney, Australia, July 13-15, (2009).

[9] http://www.basf-cc.com.sg/en/products/ConcreteRepairandProtectionSyst

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px

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Documents

IET BICET 2014 El-Gamal Et Al-Final