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Flexural Strengthening of RC Beams

with NSM Steel Bars

Md. Akter Hosen, Mohd Zamin Jumaat, Kh. Mahfuz Ud Darain, M. Obaydullah,

and A. B. M. Saiful Islam

AbstractIncreased service loads, imposed cyclic loading anderror in design and construction encourage the re-construction of

existing structural elements for strengthening. Externally bondedreinforcement (EBR) and near surface mounted (NSM) techniques,

are the main strengthening approaches traditionally used till date.This paper presents an experimental investigation on the flexuralstrengthening of reinforced concrete (RC) beams with NSM

technique using different ratios of steel bars. Four-point bending testswere carried out up to failure on four rectangular RC beams (125 mmwidth by 250 mm depth by 2300 mm length) strengthened withdifferent ratios of steel reinforcement (different yield and ultimate

strengths). Yield and ultimate strengths, flexural failure modes,cracking behavior and ductility are reported and discussed based onmeasured load and deflection. The test results show that flexuralstrength increased up to 53.02% and energy and deflection ductilityimproved.

KeywordsBending stiffness, Ductility, Flexural strengthening,NSM.

I.INTRODUCTION

EINFORCEDconcrete structures all over the world require

rehabilitation or strengthening at some in their life span

due to various reasons such as mechanical damage,

environmental effects, increased service loads, and errors in

design and construction. There are a number of methods forstrengthening existing reinforced concrete structures.

Externally bonded reinforcement (EBR) and the near surface

mounted (NSM) technique are among the most popular

strengthening methods [1-5]. The EBR technique involves the

external bonding of strengthening materials such as steel

plates or fiber reinforced polymer (FRP) laminates. However,

this technique suffers from the high possibility of premature

failure such as debonding of longitudinal laminates,

delamination and other types of premature failure.

Md. Akter Hosen is with the Civil Engineering Department, Faculty of

Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.

(Corresponding authors Cell: +60183704547;

Mohd Zamin Jumaat is with the Civil Engineering Department, Faculty of


Kh. Mahfuz Ud Darain is with the Civil Engineering Department, Faculty

of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.

M. Obaydullah is with the Civil Engineering Department, Faculty of


A. B. M. Saiful Islam is with the Civil Engineering Department, Faculty of


This prevents the strengthened members from achieving

ultimate flexural capacity [6-8]. Moreover, the externally

bonded steel or FRP plates are vulnerable to thermal,

environmental and mechanical damage. Therefore, the NSM

strengthening technique offers an effective substitute to the

EBR technique. In the NSM method the surrounding concrete

protects the NSM bars or strips from thermal, environmental

and mechanical damage, as well as delaying or preventing

premature failure. The NSM technique was first used in the

1940s in Finland where steel bars were put into grooves to

strengthen a bridge deck slab [9]. The first experimentalresearch on the NSM technique using CFRP strips in grooves

cut into the concrete specimens was conducted by [10]. A

number of experimental studies have investigated the flexural

behavior of RC beams strengthened with NSM bars or strips

using FRP materials [11-16]. FRP reinforcement has various

advantages such as high strength, light weight, resistance to

corrosion and potentially high durability but is highly

expensive and not readily available. In addition, FRP

reinforcements have little ductility. On the other hand, steel

bars are readily available, less expensive, show adequate

ductility, long-term durability and bond performance [17].

NSM strengthening using steel bars has been used on masonrybuildings and arch bridges [18]. Almusallam et al. [19]

investigated the experimental and numerical behavior of RC

beams strengthened in flexure with NSM steel or GFRP bars

and found that NSM bars promoted the flexural capacity of

RC beams.

In this paper the structural behavior of RC beams

strengthened with NSM steel bars and exposed to flexural

loading is investigated. Due to its economic feasibility, steel is

used as the strengthening material. It provides a cost-effective

solution which is also structurally efficient. The test variables

are strengthening reinforcement ratio and yield strength. Load,

deflection and strain data are analyzed to understand cracking

behavior, failure modes, ductility and bending stiffness of thetested beams.

II.EXPERIMENTAL PROGRAM

A. Test Matrix

A total of four RC beam specimens were tested. The first

beam specimen was the control beam with no strengthening

and the remaining beam specimens were strengthened with

steel bars of different diameters.

R

International Conference on Food, Agriculture and Biology (FAB-2014) June 11-12, 2014 Kuala Lumpur (Malaysia)

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TABLE I

TEST MATRIX

Notation DescriptionStrengthened barstrength (MPa)

Yield Ultimate

CB Control beam

(unstrengthened)-

NSM1 Beam strengthened with

1-6 mm steel bar580 650


1-8 mm steel bar

379 536


1-10 mm steel bar520 570

B. Specimen Configuration

The specimen dimensions and reinforcement details are

shown in Fig. 1. The beams were designed as under reinforced

beams to initiate failure in flexure, in accordance with the ACI

code [20]. The cross-sectional dimensions of the beams were

125 mm x 250 mm and the length of the beams was 2300 mm

with 2000 mm as the effective span and a shear span of 650

mm. Three types of steel bars, 12 mm, 10 mm and 6 mm in

diameter, were employed in constructing the beam specimens.

The internal tension reinforcement of all beams consisted of

two deformed steel bars, 12 mm in diameter, which were bent

ninety degrees at both ends to fulfill the anchorage criteria.Two deformed steel bars, 10 mm in diameter, were used as

hanger bars in the shear span zone. The shear reinforcement

consisted of smooth steel bars, 6 mm in diameter, distributed

along the length of the beams as shown in Fig. 1.

(a)

Control beam

(b)

Strengthened beam

(c)

Cross-section of beam and groove

Fig. 1 Specimen design details

C. Material Properties

All beam specimens were cast used normal concrete.

Crushed granite was used as coarse aggregate and the

maximum size of the coarse aggregate was 20 mm. Natural

river sand was used as fine aggregate. Fresh tap water was

used to hydrate the concrete mix during the casting and curing

of the beams, cubes, prisms and cylinders. Concrete consisted

of 224 kg/m3 of water, 888 kg/m3 of fine aggregate, 892

kg/m3of coarse aggregate and 420 kg/m3of OPC and a 0.50

water/cement ratio. The 28 days average compressive strength

of the concrete was 40 MPa based on tests of three 100 mm x

100 mm x 100 mm concrete cubes. The yield and ultimate

strength of 12 mm steel bar was 550 MPa and 640 MPa

respectively. The modulus of elasticity for all bars was 200

GPa. Sikadur 30, an epoxy adhaesive, was used to bond the

strengthening materials to the concrete substrate. Sikadur 30

has two components namely component A and component B.

Component A is white in color while component B is black.

800 mm 800 mm700 mm

2000 mm

6 mm @ 50 mm

2 No.10M

2 No.12M

2000 mm

1900 mm

250 mm

125 mm

NSM Bar

Epoxy1.5 db

1.5 db



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The two components were mixed in a ratio of 3:1 until a

uniform grey color was achieved. The density was 1.65

kg/liter at 23C after mixing. The bond strength with steel is

21 MPa and with concrete is 4 MPa. The compressive, tensile

and shear strengths of this adhesive vary with curing time as

shown in Table II.

TABLE II

PROPERTIES OF SIKADUR30

Strength Curing time Amount (MPa)

Compressive strength

7days

95.00

Tensile strength 31.00Shear strength 19.00

Fig. 2 Instrumentation and loading set-up.

D. Strengthening Procedure

In the NSM technique strengthening bars are placed into

grooves cut into the concrete cover of the RC beams and

bonded using an epoxy adhesive groove filler. In this

investigation the installation of the strengthening steel bars

began with the cutting of grooves with the dimensions

specified in Table 1 into the concrete cover of the beam

specimens at the tension face in the longitudinal direction.

The grooves were made with a special concrete saw with a

diamond blade. A hammer and a hand chisel were used to

remove any remaining concrete lugs and to roughen the lower

surface of the groove. The grooves were cleaned with a wire

brush and a high pressure air jet. The details of the grooves

are shown in Fig 1. The grooves were half filled with epoxy

and then the steel rod was placed inside the groove and lightly

pressed. This forced the epoxy to flow around the inserted

steel bar. More epoxy was used to fill the groove and the

surface was leveled. To ensure the epoxy achieved full

strength, the beam was left for one week.

E. Test Set-up

The instrumentation of the beams is shown in Fig. 2. To

measure the deflection at beam mid span, one vertical linear

variable differential transducer (LVDT) was used.

To measure strains a number of gauges were used. Two 5

mm strain gauges were attached to the middle of the internal

tension bars. A 30 mm strain gauge was placed on the top

surface of the beam at mid span. Demec gauges were attached

along the depth of the beam at mid span.

All beams were tested in four-point bending using an

Instron Universal Testing Machine. Tests were carried out

using two controls. The first was load control, which was

close to the yield capacity of the beam and the second was

displacement control until failure of the beam. All data were

recorded at 10 second intervals. The rate of the actuator was

set to 5 kN/min during load control and 1.5 mm/min duringdisplacement control. A Dino-Lite digital microscope was

used to measure crack widths on the beams during testing.

III. EXPERIMENTAL RESULTS

The leading aspects considered in this study are cracking

load, crack width, yield load, ultimate load and modes of

failure. The results of the tested beams are given in Table III.

TABLE IIISUMMARY OF BEAM TEST RESULTS

A. Load-Deflection Curve

Fig. 3 shows the load versus midspan deflection for all

beam specimens. As can be seen from the figure, the curves

exhibit a tri-linear response defined by elastic, concrete

cracking to steel yielding and steel yielding to failure stages.

In the first stage, the behavior of all the beams is linear and

elastic. Before the first crack, the bond between the steel bar,

Beam

ID

Pcr

(kN)%Pcr

Py

(kN)% Py

Pu(kN)

%Pumax

(mm)

Failure

mode

CB 15.75 - 65.00 - 74.37 - 33.61 FLNSM1 17.00 7.94 75.00 15.38 80.00 7.57 41.68 FL

NSM2 20.00 26.98 90.00 38.46 101.0 35.81 40.23 FL

NSM3 22.00 39.68 100.00 53.85 113.8 53.02 39.13 FL



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adhesive and concrete is perfect. The first cracking load

increased by 7.94%, 26.98% and 39.68% for NSM1, NSM2

and NSM3 respectively, compared the control beam. In the

second stage, cracking initiates in the concrete section of the

maximum moment zone of the beam. At the beginning of this

stage, the cracks do not pass through the integrant materials

because of their higher tensile strength and low elastic

modulus. As the loading increases, the cracks become more

widespread and new flexural cracks arise. The use of NSMsteel bars increased the yield load of the strengthened beams

by 15.38%, 38.46% and 53.85% for NSM1, NSM2 and

NSM3 respectively over the control beam. In the third stage,

the internal tension steel reinforcement has yielded and the

NSM steel bar controls the cracks and the width of the cracks

up to failure of the beam. The ultimate load increased by

7.57%, 35.81% and 53.02% for NSM1, NSM2 and NSM3

respectively over the control beam.

Fig. 3 Load-midspan deflection of beams

B. Mode of Failure

The failure modes of all the beams are shown in Fig. 4. The

beam specimens exhibited two modes of failure. The firstfailure mode was by concrete crushing at the top fiber of the

RC beam cross-section and the second failure mode was by

yielding of the tension steel reinforcement. The cracking

pattern was similar for all the beams. At first, a fine flexural

crack developed at the midspan of the beam. As the external

load increased, additional cracks developed at the neutral axis

or beyond the neutral axis, with a notable increase in the

deflection of the beam. However, all strengthened beam

specimens showed narrower and finer cracks compared to the

control beam. This is due to the greater stiffness of the

strengthened beam specimens.

(a)

CB

(b)

NSM1

(c)

NSM2

(d)

NSM3

Fig. 4 Failure modes of beams

C. Crack Behavior

Fig. 5: Load-crack width

The loads versus crack widths of the beam specimens are

shown in Fig. 5. The first crack load of CB, NSM1, NSM2

and NSM3 were 15.75 kN, 17 kN, 20 kN and 22 kN

respectively. All strengthened beams showed higher first

crack loads compared to the control beam. Thus, the use of

near surface mounted steel bars increased the first crack load.

The total number of cracks of CB, NSM1, NSM2, and NSM3

were 11, 16, 17 and 19 respectively and the average crack

spacing of each beam was 240 mm, 115 mm, 125 mm and

100 mm respectively. NSM2 had wider crack spacing than

NSM1 and NSM3. This was because NSM2 was strengthened

with a low strength steel bar. However, all strengthened

beams showed smaller crack spacing than the control beam.

Thus, it can be said that beams strengthened with NSM steel

0

20

40

60

80

100

120

0 20 40 60

Load(kN)

Deflection (mm)

CB

NSM1

NSM2

NSM3

0

10

2030

40

50

60

70

80

90

0 0.2 0.4 0.6 0.8

L

oad(kN)

Crack width (mm)

CB

NSM1NSM2

NSM3



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bars exhibit more and finer cracks with closer spacing than

nonstrengthened beams.

D. Ductility

TABLE IVDUCTILITY OF TESTED BEAM

Beam

ID

Deflection ductility Energy ductility

y( mm)

u(mm)

d Ey

(kNmm)

Eu

(kNmm)E

CB 9.12 26.56 2.91 320.64 1661.25 5.18NSM1 11.74 12.74 1.08 488.25 560.00 1.15NSM2 10.68 20.18 1.88 557.57 1474.59 2.64

NSM3 12.14 23.06 1.90 679.20 1858.96 2.73

In this study two ductility indexes, deflection ductility and

energy ductility, were examined and are presented in Table 4.

The deflection ductility index is expressed as the ratio

between the deflection at ultimate load (u is the mid-span

deflection at ultimate load) and the yield load (y is the mid-

span deflection at yield load). The energy ductility index is

expressed as the ratio between the energy at ultimate state (Eu

is the area under the load-mid span deflection curve up to

ultimate displacement) and the yield state (Ey is the area

under the load-mid span deflection curve up to yield

displacement). The strength index is the ratio between the

maximum bending moment and the yield bending moment.

The deflection ductility index was reduced by 62.88%,

35.39%, and 34.71% for NSM1, NSM2 and NSM3

respectively over the control beam. The energy ductility index

was reduced by 77.79%, 49.03%, and 47.30% for NSM1,

NSM2 and NSM3 respectively over the control beam. The

strength index decreased by 6.14% and 1.75% for NSM1 and

NSM2 respectively over the control beam. However, the

strength index for NSM3 remained the same as the control

beam. The overall reduction in ductility of the strengthened

beams is most likely due to the increased tensionreinforcement ratio (tension steel bars and strengthened bars).

E. Bending Stiffness

The bending stiffness (EI)exp was computed experimentally

by applying the following equation:

( )

MEI =

expWith

aL

PM

2=

Andd

sc

+

= (1)

Where,M = bending moment (kN-m), P= applied load (kN),

La= distance between the first loading point and support, =

curvature (m-1),c

= top fiber concrete compression strain

ands

= internal tension steel strain. The bending stiffness

versus applied load are shown in Fig.6. Decrease in bending

stiffness is a result of crack formation. After the first crack

occurred, the bending stiffness of the beams decreased very

rapidly over a small increase in load. However, after the

yielding of the tension steel reinforcement, the bending

stiffness stabilizes. Thus, strengthening with NSM steel bars

not only enhances the strength of RC beams but also increases

the bending stiffness and in this way improves the mechanical

behavior of the strengthened members.

Fig. 6 Bending stiffness-load

IV.CONCLUSION

The investigation carried out in this paper on RC beams

strengthened with NSM steel reinforcement has shown that

this is a very effective strengthening technique. The following

conclusions can be derived from the experimental and

proposed theoretical model results:

-

The NSM steel reinforcement enhances the load-

deflection response of the RC beams. At any load level,

the deflections of the strengthened beams were

meaningfully less than that of the control beam except in

the case of NSM1 which was strengthened with a plain

steel rebar.

- Beams strengthened with NSM steel reinforcement have

significantly increased yield and ultimate capacities with

improvement in ductility (deflection and energy).- Increasing the amount of NSM steel reinforcement from

28.27 mm2 to 78.54 mm2 increased the ultimate load

capacity from 7.57% to 53.02%.

- The mode of failure observed in all strengthened beams

was flexural failure.

ACKNOWLEDGMENT

The authors gratefully acknowledge the financial support

from the University of Malaya High Impact Research Grant

under Account No-UM.C/HIR/MOHE/ENG/36.

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