Flexural Strengthening of RC Beams with NSM Steel Bars · 2019-04-18 · strengthening of reinforced concrete (RC) beams with NSM technique using different ratios of steel bars. Four-point

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

Abstract—Increased service loads, imposed cyclic loading and

error in design and construction encourage the re-construction of existing structural elements for strengthening. Externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques, are the main strengthening approaches traditionally used till date. This paper presents an experimental investigation on the flexural strengthening of reinforced concrete (RC) beams with NSM technique using different ratios of steel bars. Four-point bending tests were carried out up to failure on four rectangular RC beams (125 mm width by 250 mm depth by 2300 mm length) strengthened with different 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 on measured load and deflection. The test results show that flexural strength increased up to 53.02% and energy and deflection ductility improved.

Keywords—Bending stiffness, Ductility, Flexural strengthening, NSM.

I. INTRODUCTION EINFORCED concrete 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 for strengthening 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 author’s Cell: +60183704547;

Mohd Zamin Jumaat is with the Civil Engineering Department, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.

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

A. B. M. Saiful Islam is with the Civil Engineering Department, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.

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 experimental research 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 masonry buildings 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 the tested 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)

http://dx.doi.org/10.15242/IICBE.C614512 8

TABLE I TEST MATRIX

Notation Description Strengthened bar strength (MPa)

Yield Ultimate CB Control beam

(unstrengthened) -

NSM1 Beam strengthened with 1-6 mm steel bar 580 650



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/m3 of coarse aggregate and 420 kg/m3 of 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 φ12 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 mm 700 mm

2000 mm

φ6 mm @ 50 mm 2 No.10M

2 No.12M

2000 mm

1900 mm

250 mm

125 mm

NSM Bar

Epoxy 1.5 db

1.5 db



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 23°C 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 SIKADUR® 30

Strength Curing time Amount (MPa) Compressive strength

7days 95.00

Tensile strength 31.00 Shear 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 during displacement 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 III

SUMMARY 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) %Pu ∆max

(mm) Failure mode

CB 15.75 - 65.00 - 74.37 - 33.61 FL NSM1 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



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 NSM steel 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 first failure 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)

CBNSM1NSM2NSM3

0102030405060708090

0 0.2 0.4 0.6 0.8

Load

(kN

)

Crack width (mm)

CBNSM1NSM2NSM3



bars exhibit more and finer cracks with closer spacing than nonstrengthened beams.

D. Ductility

TABLE IV DUCTILITY OF TESTED BEAM

Beam ID

Deflection ductility Energy ductility ∆y

( mm) ∆u

(mm) μd

Ey (kNmm)

Eu (kNmm) μE

CB 9.12 26.56 2.91 320.64 1661.25 5.18 NSM1 11.74 12.74 1.08 488.25 560.00 1.15 NSM2 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 tension reinforcement ratio (tension steel bars and strengthened bars).

E. Bending Stiffness The bending stiffness (EI)exp was computed experimentally

by applying the following equation:

( )φMEI =exp With aLPM

2=

And d

sc εεφ

+= (1)

Where, M = bending moment (kN-m), P = applied load (kN), La = distance between the first loading point and support, φ =

curvature (m-1), cε = top fiber concrete compression strain

and sε = 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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Flexural Strengthening of RC Beams with NSM Steel Bars · 2019-04-18 · strengthening of reinforced concrete (RC) beams with NSM technique using different ratios of steel bars. Four-point