FLEXURAL STRENGTH OF RC BEAMS WITH … · and textile or fabric sheet. Using of FRP bars in concrete structure has been widely accepted ... The approach of RC beam flexural strengthening

http://www.iaeme.com/IJCIET/index.asp 134 [email protected]

International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 12, December 2017, pp. 134–143, Article ID: IJCIET_08_12_016

Available online at http://http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=12

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

FLEXURAL STRENGTH OF RC BEAMS WITH

MULTIPLE LAYERS OF CFRP SHEET

Nadzirah Musa, Bashar S Mohammed, and Mohd Shahir Liew

Civil and Environmental Engineering Department, Universiti Teknologi PETRONAS, 32610,

Bandar Seri Iskandar, Perak, Malaysia

Parnam Singh

RNC Technology (M) Sdn Bhd, 47650, Subang Jaya, Selangor, Malaysia

ABSTRACT

Carbon Fiber Reinforced Polymer (CFRP) composite sheets has been universally

used in the construction industry for the last twenty years as a common technique for

strengthening, rehabilitation and retrofitting purpose of the RC structures. Five RC

beams with different layers of CFRP sheets were prepared and put under four-point

load. The aims of this research are to investigate the efficiency of multiple layers of

CFRP and how different number of layers of CFRP will influence the RC beam

behavior. Besides, to identify the optimum number of CFRP sheets should be used to

strengthen the RC beams. It was proven that the load carrying capacity of the

strengthened RC beams were increased compared to the control beams (B1).

However, the outcomes from the load-deflection curve shows that the optimum number

of CFRP sheets layer was three layers with load increment by 14.63%. This is because

when the beam was strengthened by four layers of CFRP sheets, it shows the lowest

load increment by 2.23%. The failure mode experienced by the strengthened beams

was concrete cover separation, which is categorized as shear failure.

Keywords: RC Beam, Strengthening, CFRP, External bonding, Flexural behavior

Cite this Article: Nadzirah Musa, Bashar S Mohammed, Mohd Shahir Liew and

Parnam Singh, Flexural Strength of Rc Beams with Multiple Layers of Cfrp Sheet,

International Journal of Civil Engineering and Technology, 8(12), 2017, pp. 134–143

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=12

1. INTRODUCTION

Strengthening and repairing of Reinforced Concrete (RC) beams in shear and flexural with

externally bonded CFRP composite sheets has been universally used in the construction

industry for the last twenty years. CFRP materials can be considered as a common system

used both for post-planned and pre-planned building.

The main reasons of using CFRP in this industry are because of its high strength to weight

ratio, high stiffness, light weight, flexible and easy to install, non-magnetic properties, high

Structural concepts’ instruction model for architecture students regarding motivation and creativity

promotion


tensile strength and non-corrosive as compared to other materials [7,8,12,13]. Researchers

have shown that the use of CFRP contribute to higher stiffness, ductility and strength of the

RC structures. Alferjani et al. [1] have reported that FRP in civil engineering applications can

be classified into three categories as follows: applications for pre-planned construction

(design), repair and rehabilitation applications, and architectural applications.

There are three different types of CFRP composites, which are: solid bar, inflexible plate,

and textile or fabric sheet. Using of FRP bars in concrete structure has been widely accepted

to substitute the use of steel reinforcement in the design stage of concrete structure [15].

While FRP plate and fabric types normally are being used in structural strengthening and

retrofit purposes. FRP plate is used to strengthen the concrete structure by placing the plate on

straight surface. While, FRP fabrics is flexible and can be used to wrap the structural RC

member and it is available in continuous sheets that can be easily cut to fit any geometry [16].

Many studies have been carried out and proven repeatedly to strengthen existing RC

structure using CFRP either by externally bonded reinforcement (EBR) or near surface

mounted (NSM) technique. The approach of RC beam flexural strengthening by placing

CFRP plates and sheets at the underneath of the beam via epoxy adhesives had denoted an

improvement in the load carrying capacity and beam’s stiffness [12]. For NSM technique,

CFRP material is placed inside the concrete cover, which to have high protection against

outside contact, wear and vandalism actions, as well as from the effects of warmth and fire

[10]. From previous research by Fathelbab et al. [9] have reported that the use of CFRP sheets

for strengthening could enhance the load capacity of RC structure as well as the RC structure

toughness. In his study, it was proven that the load capacity increment is in the range of

79.8% and 107.7%.

The efficiency of CFRP in the purpose of RC beam’s strengthening relies on few factors,

including the stiffness of CFRP plies, quantity of the thermosetting resin, compressive

strength, number of layer of CFRP sheet, wrapping scheme, and fiber orientation angle of the

FRP. Bsisu et al. [3] have suggested the using of multiple layers of CFRP is better than one

layer with the desired thickness to achieve the desired strength required for strengthening the

concrete. In his eleven beams experimented, with different width and number of CFRP sheets

plies, they give variant contribution for each parameter. It has been shown that by using

multiple layers of wide CFRP sheets give result in a high development for the beam strength

and capacity, but will reduce the ductility of concrete beams. However, very limited and

scarce studies have been done to determine the effectiveness of using multi CFRP layers in

strengthening of RC beams.

Therefore, the aims of this research are to investigate the efficiency of multiple layers of

CFRP and to identify the optimum number of CFRP sheets layers in order to strengthen and

restore the loss of the RC beams’ load capacity.

2. METHODOLOGY

2.1. Geometry of the beams and Test Setup

A total of five RC beams were tested. All specimens with 1.4 m effective span, 230 mm in

height and 100 mm in width. The main reinforcement comprised of two 10 mm deformed bars

and two 8 mm plane hanger bars. Shear reinforcement comprised of 8 mm plain bars stirrup

with 50 mm spacing in shear region as shown in Figure 1. The distance between two point

loads was maintained to be 400 mm, while the span between point load and beams support

was set to be 500 mm for all beams as shown in Figure 1.

Javad Tabatabaei, Mehdi Mahmoudi Kamelabadi and Mahyar Javidruzi


Figure 1 Configuration of the beams design (dimension in mm)

One beam, B1 was designed without any additional CFRP sheet attached and considered

as control beam. While the remaining of the beams B2, B3, B4, and B5 were strengthened

with one, two, three and four layers of CFRP sheets, respectively with different anchorage

length at the center of the beam’s soffit for flexural strengthening as summarized in Table 1.

Table 1 CFRP Configuration

Beam

CFRP Sheets

No of Layers Distance of CFRP

between support (mm)

B1 - -

B2 1 413.1

B3 2 377.1

B4 3 349.5

B5 4 326.2

The load was applied to the simply supported beams using hydraulic jack and the load

values were controlled using the load cell placed beneath the hydraulic cylinder with a

capacity of 500 kN. The concentrated load was transmitted to the RC beam through a spreader

beam and acted as two point load support. The load increment was set to be 0.1 kN/s until

failure. All beams were instrumented with strain gauge of different length of 84 mm, 5 mm

and 60 mm to measure the strain values on the concrete, steel reinforcement bar, and CFRP

sheets, respectively. In order to measure the deflection, one transducer (LVDTs) was also

used and put at the center of the beam’s span as shown in Figure 2.

Figure 2 Configuration of experiment set-up

RighLef


promotion


2.2. Materials

All RC beams were casted and designed with average compressive strength of 25.88 MPa

after 28 days. While the measured yield strength of the 10 mm deformed steel reinforcement

bar was 437.38 MPa.

The CFRP sheets, comprised of 100 mm width and thickness of 0.129 mm for each layer

were externally bonded to the concrete beam’s soffit by using two part of epoxy adhesive

(Sikadur 330) with the mix proportion of 4:1 ratio and cured at room temperature. The

mechanical properties of materials are given in Table 2.

Table 2 Mechanical properties of material

Materials fy

(MPA) εy (%)

fu

(MPA) εu (%) E (GPa)

Steel

(10 mm) 437.38 0.219 503.10 - 200

Concrete - - 25.88 - 23.91

CFRP a

(tf = 0.129mm) - - 4000 1.7 230

Source: Product Data Sheet SikaWrap – 230 C

3.0 RESULTS AND DISCUSSIONS

3.1 Load-deflection curve and failure mode

Load-deflection behavior for all beams and experimental results are drawn in Figure 3 and

tabulated in Table 3. The load carrying capacity of the strengthened beams have been

increased when externally bonded of CFRP sheets have been added. The increment load in the

range of 2.23% and 14.63%

Figure 3 Load-deflection behavior

The tested beams showed two different failure modes. For the control beam (B1), the

failure was by concrete crushing at the mid-span of the beam after the steel has been yielded

as shown in Figure 4. While the failure for the strengthened RC beams B2, B3, B4, and B5

were by concrete cover separation, due to crack formation at the end of CFRP sheets from the

tension face of the concrete as shown in Figure 5-8. The initial crack pattern for strengthened

beams seems similar with control beam which at the moment constant zone. However, when



the steel yielded, all loads were distributed to the CFRP sheets at the underneath of the beams,

which leading to the concrete cover separation failure mode as shown in Figure 9. This failure

mode is categorized as shear failure [11].

Based on the results shown in Table 3, the ultimate load of the strengthened beams B2,

B3, and B4 increased consistently as the number of layers of CFRP sheets was increased by

6.27%, 10.15% and 14.63%, respectively as compared to beam B1 due to restraining effect of

CFRP sheet to the concrete. However, for beam B5, the load carrying capacity shows the

lowest load increment only by 2.23% as compared to beam B1 and decreased by 10.81% as

compared to beam B4. This result shows that the optimum number of CFRP sheets to increase

the strength capacity of the RC beam is only up to three layers, otherwise the effect of CFRP

sheet in flexural strengthening the RC beams will not be affected. However, the use of

externally bonded of CFRP sheets decreased the beam’s ductility as the number of CFRP

layers increased as shown in Table 3.

Table 3 Summary of load-deflection result

Beam

Experimental result Analytical

results Ductility

Index

Mode

Failure Py (kN)

Δy

(mm) Pu (kN)

Δu

(mm)

Δf

(mm)

Ultimate

Load (kN)

B1 42.08 4.26 66.97 21.50 65.42 42.23 15.37 Flexural at

mid span

B2 58.67 5.22 71.17 7.15 59.68 74.07 11.42

Concrete

cover

separation

B3 58.98 5.10 73.77 9.75 55.40 98.77 10.86

Concrete

cover

separation

B4 64.17 4.88 76.77 11.78 31.22 123.46 6.40

Concrete

cover

separation

B5 67.37 5.28 68.47 10.93 25.02 148.15 4.74

Concrete

cover

separation

Note: Py = Load at yield;

Pu = Ultimate Load;

Δy = Deflection at yield;

Δu = Deflection at ultimate failure;

Δf = Deflection at final failure

Figure 4 Concrete crushing after tension steel yielded failure mode (B1)


promotion


Figure 5 Crack pattern at final load (B2)






Figure 9 Concrete cover separation failure mode (B2, B3, B4, and B5)

3.2. Relationship between load and strain

The strains were measured for tension steel, compression of the reinforced concrete and

CFRP sheets to determine the ability of the material over load applied. Figure 10 shows the

load–strain curves of mid span of CFRP sheet, top concrete and steel for all beams. The steel

strains, top concrete strains and CFRP strains are originally almost similar at loads before the

concrete’s first crack. After the beams reach yield point which is 2187μm/m, the strains of

steel were 3315μm/m, 2781μm/m, 2708μm/m, 2601μm/m and 5061μm/m for B1, B2, B3, B4,

and B5 respectively. As plotted in the graph below, the strain in the steel seems exceeded both

strain for CFRP and to concrete, except for beam B1 without CFRP because there is no

strengthening material to prevent the beam from being crushed at the centre of the beam’s

span. At failure load, the strain of CFRP for beam B5 shows the lowest value compared to

beam B2 with one layer of CFRP shows the highest value of strain, which are 579μm/m and

3936μm/m respectively.


promotion


Figure 10 Load-strain responses of CFRP, top concrete and steel for B1, B2, B3, B4, and B5

Meanwhile, the strain values at the left and right end of beam’s soffit were plotted to

compare the strain between each beams with different layers of CFRP as shown in Figure 11

and Figure 12 respectively. It shows that B1, without CFRP sheets attached underneath the

beam has maximum value of strain at the left end and minimum value of strain at the right

end of the beam. The strain values at the left end of the beam seem increased consistently as

the load applied increased. However, the strain values at the right end of the beam seem

decreased when the beam reached yield point as shown in Figure 12 except for the control

beam B1.

Figure 11 Load-strain responses at left end of beam’s soffit

Figure 12 Load-strain responses at right end of beam’s soffit



4. THEORETICAL MODEL

In order to develop a design approach for the flexural members strengthened with CFRP by

externally bonded system, the model of the moment capacity specified in the ACI 440.2R-08

was adopted. The moment capacity of the flexural strengthening member was expressed in the

following form:

�� = �� − �� + ѱ��[ℎ − ��

] (1)

where �� is the nominal bending moment, �� and �� are the cross-sectional areas of the

longitudinal steel reinforcement and FRP respectively, fs is the ultimate steel tensile stress, ��

is the effective ultimate tensile stress of the FRP, h is the total depth of the beam, �� is the

effective depth of the steel reinforcement, c is the position of the neutral axis, and ѱ�=0.85 is

a safety factor, and �� is an parameter given in Section 10.2.7.3 of ACI Building Code [2].

The values of the load carrying capacity of the beams estimated by ACI Building Code [2]

was correlated to the experimental values as tabulated in Table 3. It shows that the values of

experimental result were lower than the analytical one, except for control beam (B1). This

result due to maximum strength of CFRP, which is 4000 MPa was calculated in the analytical

approach. Besides, one other possible reasons for the difference was due to the anchorage

length of the CFRP was neglected in the Equation (1), which will affect the total strength of

the externally bonded of CFRP sheets.

5. CONCLUSIONS

The following conclusions are summarized based on the results obtained:

1. The load carrying capacity of strengthened RC beams was increased up to 14.63%

from the control beam. However, if more than three layers of CFRP sheets were

added, the load carrying capacity of the beams will reduced. It shows that the optimum

number of CFRP layer used for strengthening purpose limit to 3 layers.

2. The use of externally bonded CFRP sheets will decreased the beam’s ductility as the

number of CFRP sheets increased.

3. At failure load, the strain of CFRP for beam B5 with four layers of CFRP shows the

lowest value as compared to beam B2 with one layer of CFRP shows the highest value

of strain, which are 579μm/m and 3936μm/m respectively.

7. ACKNOWLEDGEMENTS

The author would like to thanks to all Technologist of Concrete Laboratory, Universiti

Teknologi PETRONAS for the contributions during the course of this work to conduct all

experiments.

REFERENCES

[1] Alferjani, M.B.S., et al., Use of carbon fiber reinforced polymer laminate for

strengthening reinforced concrete beams in shear: a review. Int. Refereed J. Eng.

Sci.(IRJES), 2013. 2(2): p. 45-53.

[2] American Concrete, I., Building code requirements for structural concrete (ACI 318-05)

and commentary (ACI 318R-05). 2004: American Concrete Inst.

[3] Bsisu, K.A.-D., S. Srgand, and R. Ball, The Effect of Width, Multiple Layers and Strength

of FRP Sheets on Strength and Ductility of Strengthened Reinforced Concrete Beams in

Flexure. Jordan Journal of Civil Engineering 2015. 9(No 1).


promotion


[4] Cheng, H.T., B.S. Mohammed, and K.N. Mustapha, Experimental and analytical analysis

of pretensioned inverted T-beam with circular web openings. International Journal of

Mechanics and Materials in Design, 2009. 5(2): p. 203-215.

[5] Cheng, H.T., B.S. Mohammed, and K.N. Mustapha, Interaction diagram and response

surface plot of pretensioned inverted T-beam with circular web openings. International

Journal of Modelling, Identification and Control, 2009. 7(2): p. 209-218.

[6] Cheng, H.T., B.S. Mohammed, and K.N. Mustapha, Ultimate load analysis of

pretensioned inverted T-beams with circular web openings. Frontiers of Architecture and

Civil Engineering in China, 2009. 3(3): p. 262-271.

[7] El-Enein, H.A., et al., Flexural strengthening of reinforced concrete slab–column

connection using CFRP sheets. Construction and Building Materials, 2014. 57: p. 126-

137.

[8] El-Maaddawy, T. and B. El-Ariss, Behavior of concrete beams with short shear span and

web opening strengthened in shear with CFRP composites. Journal of Composites for

Construction, 2012. 16(1): p. 47-59.

[9] Fathelbab, F.A., M.S. Ramadan, and A. Al-Tantawy, Strengthening of RC bridge slabs

using CFRP sheets. Alexandria Engineering Journal, 2014. 53(4): p. 843-854.

[10] Firmo, J.P., et al., Flexural behaviour of partially bonded carbon fibre reinforced polymers

strengthened concrete beams: Application to fire protection systems design. Materials &

Design (1980-2015), 2015. 65: p. 1064-1074.

[11] Gao, B., C.K.Y. Leung, and J.-K. Kim, Failure diagrams of FRP strengthened RC beams.

Composite Structures, 2007. 77(4): p. 493-508.

[12] Hawileh, R.A., et al., Effect of flexural CFRP sheets on shear resistance of reinforced

concrete beams. Composite Structures, 2015. 122: p. 468-476.

[13] Masuelli, M.A., Introduction of Fibre-Reinforced Polymers-Polymers and Composites:

Concepts, Properties and Processes. 2013: INTECH Open Access Publisher.

[14] Mohammed, B.S., L. Ean, and M. Malek, One way RC wall panels with openings

strengthened with CFRP. Construction and Building Materials, 2013. 40: p. 575-583.

[15] Nor, N.M., et al., Carbon Fiber Reinforced Polymer (CFRP) as Reinforcement for

Concrete Beam. 2013.

[16] Soudki, K. and T. Alkhrdaji. Guide for the design and construction of externally bonded

FRP systems for strengthening concrete structures (ACI 440.2 R-02). 2005.

[17] Tian, C.H., B.S. Mohammed, and K.N. Mustapha. Application of response surface

methodology in finite element analysis of deflection of pretensioned inverted T-beam with

web openings strengthened with GFRP laminates. In Information and Computing Science,

2009. ICIC'09. Second International Conference on. 2009. IEEE.S. K. Pathak, V. Sood, Y.

Singh, and S. Channiwala, Real world vehicle emissions: Their correlation with driving

parameters, Transportation Research Part D: Transport and Environment, vol. 44, pp. 157-

176, 2016.

[18] Devendiran.S, Manivannan K, Venkatesank, Arun Tom Mathew, Abhijeet Thakur, and

Ashish Chauhan V, Free Vibration of Damaged and Undamaged Hybrid cfrp/Gfrp

Composite Laminates, International Journal of Mechanical Engineering and Technology

(IJMET) Volume 8, Issue 8, August 2017, pp. 349-360.

[19] Javaid Ahmad, Dr. Javed Ahmad Bhat, Umer Salam, Behavior of Timber Beams Provided

with Flexural as well as Shear Reinforcement in the form of Cfrp Strips, International

Journal of Advanced Research in Engineering and Technology (IJARET), Volume 4,

Issue 6, September – October 2013, pp. 153-165.

Documents

FLEXURAL STRENGTH OF RC BEAMS WITH … · and textile or fabric sheet. Using of FRP bars in concrete structure has been widely accepted ... The approach of RC beam flexural strengthening