THE EFFECT OF TRANSVERSE STEEL REINFORCEMENT ON THE BEHAVIOR OF CONCRETE BEAMS REINFORCED WITH GLASS POLYMER REBARS

THE EFFECT OF TRANSVERSE STEEL REINFORCEMENT ON THE

BEHAVIOR OF CONCRETE BEAMS REINFORCED WITH GLASS

POLYMER REBARS

Seyed Mostafa Tabatabaei

Department of Civil,College of Engineering,Zahedan Branch,Islamic Azad University, Zaheden , Iran Mahmoud-Reza Hosseini-Tabatabaei

Department of Civil,College of Engineering,Zahedan Branch,Islamic Azad University, Zaheden , Iran

ABSTRACT

In this study, through preparing an experimental program, flexural behavior of glass fiber reinforced polymer (GFRP) concrete beams is investigated using finite element method. For this purpose, 4 concrete beams with square section were modeled in Abaqus Software. In all beams, 4 GFRP No.12 rebars and 10 steel rebars (A3) No.8 were used with space of 250mm vertically to resists against shear cracks in beam.

Beam no.1 is without transverse reinforcement; beam no.2 has a row of transverse reinforcement in beneath and is placed in plate of vertical reinforcements. Beam no.3 has a row of transverse reinforcement above the beam and is placed in plate of vertical reinforcements; beam no.4 has 2 rows of transverse reinforcement in upper and lower part of beam and is placed in plate of vertical reinforcements. Beams are exposed to force of 6tons gradually. In each beam, values of displacement and strain in mid part of beam are compared to each other. Obtained results show that force-displacement diagram of GFRP beams has been almost linear to the final step and equal in all beams. Adding transverserebar has led to fewer drops in mid part of beam compared to beams without transverserebar. Moreover, all beams were deformed under certain load; although in beams with transverse reinforcement, more load resistance and deformations were observed.

Keywords: Glass Fiber Reinforced polymer, finite element, transverserebars

INTRODUCTION

Experimental investigations show thatsteel fibers can be used as stirrups in beams, frames and slaps and also as strengthening shear reinforcement in precast beams with fragile body. Reinforcement fibers can be added to concrete mixture in place or on critical areas of members made of Prestressed concrete to remove secondary reinforcements. Fiber reinforced concrete can be used forstrengthening plasticity and seismic resistance of structures.

LITERATURE REVIEW

Non-metallic resistant fibers, such as carbon fiber, glass and aramid surrounded in a polymer matrix have shown high potential to reinforce concrete.Fiber-reinforced polymer area available in different forms includingRebar, networks, cables, cords, tendons, sheets and a variety of Construction profiles and are usually known as FRP. With the recent advancements in this field, large number of studies has been reported till now from different perspectives of structural use of FRP in various studies. FRPs have been used to reinforce structures [1].

Ramana et al [3]have studied behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening.This paper summarizes the results of experimental and analytical studies on the

flexural strengthening of reinforced concrete beams by the external bonding of high-strength, light-weight carbon fiber reinforced polymer composite (CFRPC) laminates to the tension face of the beam. Four sets of beams, three with different amounts of CFRPC reinforcement by changing the width of CFRPC laminate, and one without CFRPC were tested in four-point bending over a span of 900 mm. The results indicate that the most increase in first crack and ultimate anchors is about 150-230%. In this study, increase in stiffness of reinforced beams has been considerable and about 110%.

Mansur et al [4] have investigatedShear strengthening of RC deep beams using externally bonded FRP systems.This study explores the prospect of strengthening structurally deficient deep beams by using an externally bonded fibre reinforced polymer (FRP) system. Six identical beams were fabricated and tested to failure for this purpose. One of these beams was tested in its virgin condition to serve as reference, while the remaining five beams were tested after being strengthened using carbon fibre wrap, strip or grids. The results of these tests are presented and discussed in this paper. Test results have shown that the use of a bonded FRP system leads to a much slower growth of the critical diagonal cracks and enhances the load-carrying capacity of the beam to a level quite sufficient to meet most of the practical upgrading requirements.

Obaidat et al [5] have studied retrofitting of Reinforced Concrete Beams using Composite Laminates.This paper presents the results of an experimental study to investigate the behaviour of structurally damaged full-scale reinforced concrete beams retrofitted with CFRP laminates in shear or in flexure. The main variables considered were the internal reinforcement ratio, position of retrofitting and the length of CFRP.

Increase in maximum load of reinforced samples was about 23% for shear reinforcement and to 7-33%

for flexural reinforcement. Moreover, the reinforcement has led to change in failure mode to fragile failure. On the other hand, strengthening beams has led to reduction of width of crack compared to control beams. The experimental results, generally, indicate that beams retrofitted in shear and flexure by using CFRP laminates are structurally efficient and are restored to stiffness and strength values nearly equal to or greater than those of the control beams. It was found that the efficiency of the strengthening technique by CFRP in flexure varied depending on the length. The main failure mode in the experimental work was plate debonding in retrofitted beams.

Pannirselvam et al [6] have conducted a study under the title of Strength Modeling of Reinforced Concrete Beam with Externally Bonded Fiber Reinforcement Polymer Reinforcement.This research study presents the evaluation of the structural behavior of reinforced concrete beams with externally bonded Fibre Reinforced Polymer (FRP) reinforcements. Three different steel ratios with two different Glass Fibre Reinforced Polymer (GFRP) types and two different thicknesses in each type of GFRP were used.

Totally fifteen rectangular beams of 3 m length were cast. Three rectangular beams were used as reference beam (Control Beams) and the remaining were fixed with GFRP laminates on the soffit of the rectangular beam. The variables considered for the study includes longitudinal steel ratio, type of GFRP laminates, thickness of GFRP laminates and composite ratios. Flexural test, using simple beam with two- point loading was adopted to study the performance of FRP plated beams interms flexural strength, deflection, ductility and was compared with the unplated beams. The test results show that the beams strengthened with GFRP laminates exhibit better performance. The flexural strength and ductility increase with increase in thickness of GFRP plate. The increase in first crack loads was up to 88.89% for 3 mm thick Woven Rovings GFRP plates and 100.00% for 5 mm WRGFRP plated beams and increase in ductility interms of energy and deflection was found to be 56.01 and 64.69% respectively with 5 mm thick GFRP plated beam. Strength models were developed for predicting the flexural strength (ultimate load, service load) and ductility of FRP beams. The strength model developed give prediction matching the measurements. The deflections at which first cracks appeared at the tension zone of the beams were higher for GFRP plated beams. The maximum reductions in first crack load were up to 50.59% for 3 mm thick plating and up to 58.59% for 5 mm thick plating.

Yield loads increased substantially due to the bonding of GFRP plates. The increase level of achieved by WRGFRP plates was higher than those achieved by CSMGFRP plates. The increase in yield load was up to 40.00% for 3 mm thick CSMGFRP and 128.57% for 5 mm CSMGFRP, 103.33% for 3 mm WRGFRP and 200.00% for 5 mm WRGFRP plating. Yield deflection values were marginally lower for GFRP plated beams compared to the unplated beams. The reduction in yield deflection ranged from 7.99- 28.03% for 3 mm GFRP plated beams and from 5.19% to 28.54% for 5 mm GFRP plated beams.WRGFRP plating resulted in substantially higher ultimate load levels compared to CSMGFRP plating. Increase of ultimate strength ranged from 42.86-103.33% for 3 mm WRGFRP plating and from 60.00-200.00% for 5 mm WRGFRP plating. The increase in deflection ductility ranged from 30.30- 56.01% with 5 mm CSMGFRP plating and from 35.16-64.69% with thick 5 mm for WRGFRP plating.

Jahangiri and Khaloohave investigatedthe behavior of reinforced concrete deep beams with pop-up body using finite element analysis. In this study, they have investigated use of finite element method to analyze continuous concrete reinforced beams with pop-up structure. Obtained results from this study showed that with the increase in pop-up size, shear strength of beams is decreased. The decrease is equal to 15% for square pop-ups and 20% for rounded pop-ups. Through changing the pop-ups into rounded form, ultimate strength of beams is increased to 2-13% and the maximum increased strength for beams with large pop- ups is in span of internal shear. Rounded pop-ups have higher ductility than square pop-up to about 2- 16%.

Park et al [7] have studiedStrut-and-Tie Method (STM) for CFRP Strengthened Deep RC Members. For analysis of CFRP strengthened deep reinforced concrete (RC) members, the strut-and-tie method (STM) is also a powerful analysis tool since a bonded CFRP element acts as an additional tension tie. In this paper, a practical analysis and design process for CFRP strengthened deep RC members using the STM is presented. In addition, seven effective factor models accounting for reduction of strength in cracked concrete were also investigated.A total of 17 experimental deep beam test results were compared with the proposed STM approach results. It has been shown that the proposed STM approach with an effective factor model depending on the strut angle provides the best agreement with the test results.

El Maaddawy and Sherif [8] have conducted a study under the title of FRP composites for shear strengthening of reinforced concrete deep beams with openings.This paper presents the results of a research work aimed at examining the potential use of externally bonded carbon fiber reinforced polymer (CFRP) composite sheets as a strengthening solution to upgrade reinforced concrete (RC) deep beams with openings. A total of 13 deep beams with openings were constructed and tested under four-point bending. Test specimen had a cross section of 80 × 500 mm and a total length of 1200 mm. Two square openings, one in each shear span, were placed symmetrically about the mid-point of the beam. Test parameters included the opening size, location, and the presence of the CFRP sheets. The structural response of RC deep beams with openings was primarily dependent on the degree of the interruption of the natural load path. Externally bonded CFRP shear strengthening around the openings was found very effective in upgrading the shear strength of RC deep beams. The strength gain caused by the CFRP sheets was in the range of 35–73%. A method of analysis for shear strength prediction of RC deep beams containing openings strengthened with CFRP sheets was studied and examined against test results.

O.Chaallal , B.Benmokrane[10] have found that GRP rebars are very light and show elastic behavior till the time of fracture and have very high ultimate flexural strength and low ultimate strain and low elasticity entry. Moreover,its Coefficient of thermal expansion is similar to concrete. GFRP rebars act properly under pressure similar to similar reinforced rebars and metal rebars; although they would be failed more than these rebars. GFRP rebars used in this study show elastic and linear behavior under pressure and tension till the tile of fracture. The rebars have high strength to weight ratio. Final strain and elasticity modulation of these rebars is low and about 1.8% and 42GPa. Obtained results showed that it is possible to use them in construction industry. Moreover, mechanical and physical properties of the

materials allows using them to design concrete structures such as beams, slabs and columns reinforced by the composite rebars. One cases of using the rebars is areas with coastal zones or regions with ice cycles and concretes including salt, nonmagnetic structures or electricity insulators and non-military structures non-detectable by radars and underground structures. Initial cost of building such GFRP concrete structures seems to be higher than conventional concrete structures; although initial cost can't be good criterion of real estimation of construction cost of the structures during their lifecycle. For example, costs that should be considered in these calculations include maintenance, repair and replacement costs. The problem of corrosion in some structures appears just 5 years after the construction. The cost of repairing and maintenance reaches to considerable amount after a few years, if it is not more than initial cost. This issue can enhance the opportunity to use the materials for real time structures. A perspective of possible and different fibers usable in construction industry is presented in study of V.S Parameswaran [11].

In study of C Cuchiara et al [12] under the title of effectiveness of stirrups and steel fibers as shear reinforcement (2003), results of experiments on rectangular beams with simple bases and made of fiber reinforced concrete with and without stirrups under effect of 2-point symmetric lateral load are presented.

A.F Ashour has conducted a study under the title of bending and shear capacity of GFRP concrete beams by 2005 and has presented the results of experiments on 12 beams with GFRP rebars and under effect of 4-point loading system. All experimental samples had no compressive and shear rebar and were divided to two groups based on compressive strength of concrete. Two failure modes of shear and bending modes were observed. Bending failure behavior was mainly as a result of rupture of GFRP rebars and in middle part of span or in beneath of loading points.

Equations of SCI Guide for the design and construction of concrete reinforced with FRP rebars [13]

Tensile strength of design of polymer reinforcements

( )

ffu

would be obtained as follows:

* fu E

fu C f

f = (1)

Where;

C

^E is reduction coefficient related to environmental conditions that can be obtained using table 1 depending on type of fibers and environmental conditions.

*

ffu Refers to tensile strength presented by producing company of FRP reinforcement determined by tensile test.

Table 1: environmental reduction coefficient for fibers under different environmental conditions [13]

Environmental conditions Type of fibers Reduction coefficient related to environmental conditions (CE) Concrete is not in direct contact

with soil and air Carbon 1

Glass 0.8

Aramid 0.9

Concrete is in direct contact

with soil and air Carbon 0.9

Glass 0.7

Aramid 0.8

TENSILE STRENGTH

As yield is impossible in FRP, tensile strength is the final criterion. FRP rebars under compression are weaker than tension. Compressive strength is depended on smooth or ribbed surface of rebar.

Compressive strength of GFRP is about 317MPa to 470; although its tensile strength is about 552 to 896MPa. Approximate correlation between compressive strength and cement ratio of concrete containing aggregates and concrete containing sand is presented in table 2 based on ACI213 −R 79 standard [14].

Table 2: approximate correlation between average compressive strength and cement ratio [14]

Compressive strength MPa

Cement ratio ₃

m kg

All aggregates Composition of sand and aggregate

17.24 240-305 240-305

20.68 260-335 250-335

27.58 320-395 290-395

34.47 375-450 360-450

41.37 440-500 420-500

An adequate empirical equation is presented to estimate final strength by ACI544 Committee (for fiber reinforced concrete):

(2)

Where; S refers to final strain of matrix and l/d is length to diameter ratio; V refers to volume of fibers considered to estimate random effects. A and B are constants that can be obtained through tracing curve against composite strength.

MECHANICAL PROPERTIES OF STIRRUPS

Yield stress of stirrups is equal to 340MPa and final stress is equal to 500MPa (rebar A3). Poisson coefficient and elasticity modulation for reinforcement materials is considered respectively to 0.3 and 200GPa. Strain of yield and fracture of reinforcements are respectively equal to 0.02 and 0.05. Special weight of steel is equal to 77kn/m^3.

MODELING IN ABAQUS SOFTWARE [15]:

ANALYSIS OF BEHAVIORAL MODEL OF CONCRETE PLASTIC DAMAGE (CDP)

Fracture criterion in plastic limit of material is expressed under compositional stresses. The criterion is divided to two main groups based on response of the material to hydrostatic pressure. In most cases, ductile behavior is recognized under title of depended on hydrostatic pressure and non-metal materials like soil, rocks and concrete are among these materials and are depended on pressure.

( ) ^{l d}

V

(

^V

)

^BV

( )

^l_d

AS

S_c = 1− _c + _r

Plasticity state of damage of concrete is one hybrid model developed by Kachanov and is completed by Rabotnov et al. the combined equation of materials gives following damage using Isotropic scalar quantity [16]:

(2)

Where; d refers to measurement; Cauchy refers to Cauchy stress factor ( ). In this equation,refers to variable stiffness drop. Hence, initial elasticity stiffness (not damaged) of strain tensor is considered as tensor of . However, elastic stiffness is declined. Effective strain tensor is defined as follows:

(3)

Where;refers to plastic strain. In regard with formulation, it is required to consider drop variations.

(4)

A series of strain tensors and stiffness (smoothness) of variables is dominant in CDP model. Stiffness drop has been firstly isotropic and as drop variable, dc is defined in compression area and dt is define din tension area. Therefore, Cauchy tension associated with effective stress tensor based on drop scalar parameter (1-d) is:

(5)

The damage state in tensile and compressive modes is divided independently to two types of stiffness variables of and which refer respectively to equivalent of plastic strain under tension and compression. Completion of stiffness variables as presented as follows:

(6)

Cracking (tension) and crunch (compression) illustrated in concrete varies with the increase in amount of stiffeners (emollients). The variablescontrol the growth resulted from the drops in level of elastic stiffness. Fluidity level can determine distance of a surface in stress mode with failure or damage status:

(7)

Plastic current would be controlled by potential function of based on following equation:

(8)

Plastic potential function of G is also defined in distance of effective stress.

4 concrete beams with square section are modeled in Abaqus software. In all beams, 4 GFRP rebars No.12 and 13 steel rebars No.6 are applied with spaces of 250mm laterally to resist against shear cracks in beam. Beam No.1 is applied without transverse reinforcement. Beam no.2 has one row of transverse reinforcement in lower part and is placed in plate of vertical reinforcements. Beam no.3 has one row of transverse reinforcement in upper part of beam and is placed on plate of vertical reinforcements. Beam 4

has two rows of transverse reinforcement in upper part and lower part of beam and is placed in plate of vertical reinforcements. Beams are exposed to 8ton load gradually. In each beam, values of displacement and strain in middle of beams are compared with each other (figure 1-12). Elasticity modulation of concrete and polymer fibers is respectively equal to 18.8 and 42GPa [9]. Moreover, final strain of polymer fibers is equal to 0.18 [9]. Also, to model the concrete in Abaqus software, damaged concrete model is used [15].

Figure 1: stress counter of beam no.1 Figure 2: displacement counter of beam no.1

Figure 3: stress counter of beam no.2 Figure 4: displacement counter of beam no.2

Figure 5: stress counter of beam no.3 Figure 6: displacement counter of beam no.3

Figure 7: stress counter of beam no.4 Figure 8: displacement counter of beam no.4

Figure 9: stress counter of reinforcements of beam no.2 Figure 10: stress counter of reinforcements of beam no.1

Figure 11: stress counter of reinforcements of beam no.3 Figure 12: stress counter of reinforcements of beam no.4

Figure 13: diagram of displacement per load in modeled beams 1, 2, 3 and 4 CONCLUSION

Mercury displacement in millimeters Number 1

Mercury displacement in millimeters Number 3

Mercury displacement in

millimeters Number 4 Mercury displacement in millimeters Number 2

In all beams and under constant load, beams with more transverse reinforcement led to more tolerance of load than beams with fewer transverse reinforcements. Under equal load, strain in beam with lower transverse reinforcement percent has been more than beam with higher percent of transverse reinforcement. Load-displacement curve is continued relatively in linear form to the final loads after beginning point related to non-cracked section for all GFRP rebars. This indicates lack of yield of GFRP to the moment of failure. GFRP beams have high ductility and energy precipitation ability. With the increase in amount of transverse reinforcement in member, failure of beams would happen in higher final load and increase in transverse reinforcement in members can result in increase in lower level of load- displacement curve.

REFERENCES

1. PanahiDorche M and Arabzadeh, A, (2009), parameters affecting shear reinforcement of deep FRP concrete beams, 8th International Congress of Civil Engineering, Shiraz University, Shiraz

2. V.P.V. Ramana, T. Kant, S.E. Morton, P.K. Dutta, A. Mukherjee, Y.M. Desai, " Behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening", Elsevier Journal, Composites: Part B 31 (2000) 461- 470.

3. M.R.Islam, M.A.Mansur, M.Maalej, "Shear strengthening of RC deep beams using externally bonded FRP systems", Department of Civil Engineering, Chittagong University of Engineering and Technology, Bangladesh, Department of Civil Engineering, National University of Singapore, Singapore, 2005.

4. Y.T. Obaidat, S. Heyden, O. Dahlblom, G. Abu-Farsakh and Y. Abdel-Jawad, "Retrofitting of Reinforced Concrete Beams using Composite Laminates", Master Thesis, Jordan University of Science and Technology. 2007.

5. N. Pannirselvam, P.N. Raghunath and K. Suguna, "Strength Modeling of Reinforced Concrete Beam with Externally Bonded Fiber Reinforcement Polymer Reinforcement", American J. of Engineering and Applied Sciences 1 (3): 192-199, 2008.

6. Sangdon Park, Riyad S. Aboutaha, "Strut-and-Tie Method (STM) for CFRP Strengthened Deep RC Members", Journal of Structural Engineering, ASCE, 2009 .

7. Tamer El Maaddawy and SayedSherif, "FRP composites for shear strengthening of reinforced concrete deep beams with openings", Structural Engineering Dept., Al-Wasl Al-Gadeed Consultants, Dubai, United Arab Emirates,2009 .

8. N. Dash, "Strengthening of Reinforced Concrete Beams Using Glass Fiber Reinforced Polymer Composite", Department of Civil Engineering NIT Rourkela - 769008, 2009.

9. O., Chaallal, B., Benmokrane, "Glass fibre reinforced plastic (GFRP) rerebars for concrete structures", construction and building materials, vol. 9, no. 6, pp. 353-364, 1995

10. V., Parameswaran, "Fiber reinforced concrete: a versatile construction material", building and environment, vol. 26, no. 3, pp. 301-305, 1991

11. C., Cucchiara, I., La.Mendola, M., Papia, "Effectiveness of stirrups and steel fibers as shear reinforcement", cement & concrete composites, no. 26, pp. 777–786, 2004

12. ACI committee 440, "Guide for the design and construction of concrete reinforced with FRP rebars", vol. 2003

13. M.M., Rafi, A., Nadjai, F., Ali, D., Talamona, "Aspects of behavior of CFRP reinforced concrete beams in bending", construction and building materials, vol. 22, pp. 277–285, 2008

14. Ramezanianpoor, AA, (2004), microstructure, properties and elements of concrete, Tehran, Amir Kabir University

15. Abaqus Analysis User's Manual(6.14)