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USE OF FIBRE COCKTAILS TO INCREASE THE SEISMIC PERFORMENCE OF BEAM-COLUMN JOINTS

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USE OF FIBRE COCKTAILS TO

INCREASE THE SEISMIC

PERFORMENCE OF BEAM-COLUMN

JOINTS

Dr.P.PERUMAL1 *

, B.THANUKUMARI2

1Professor and Head ,Department of Civil Engineering, Government College of Engineering, Salem,TamilNadu,

India.636 011.

2Assistant Professor and Head, Department of Civil Engineering, Cape Institute of Technology, Levengipuram,

Tirunelveli District, Tamil Nadu, India.627 114. Abstract :

The seismic performance of seven one fourth scale exterior beam-column joints was examined using M20 concrete.

The first and second specimens were designed and detailed without and with seismic load and was cast without fibers. The remaining five specimens were similar to the first one but various combinations of cocktail fibre concrete in the joint region. Cocktail fibre consists of constant % (1.5) of steel fibre and 0 to 0.6% polypropylene fibre

.

The properties of ultimate strength, ductility, energy dissipation capacity and joint stiffness were compared. It was determined that the cocktail fibre combinations of 1.5% of steel fibre and 0.2% of polypropylene fibre have shown the best performance considering the energy dissipation capacity and ductility factor.

Keywords: reverse cyclic load; cocktail fibre; energy absorption; hysteresis loop; beam-column joint; stiffness and

ductility.

1. INTRODUCTION

The recent earthquakes revealed the importance of the design of reinforced concrete (RC) structures with ductile behaviour. Ductility can be described as the ability of reinforced concrete cross sections, elements and structures to absorb the large energy released during earthquakes without losing their strength under large amplitude and reversible deformations. Generally, the beam-column joints of a RC frame structure subjected to cyclic loads such as earthquakes experience large internal forces. Consequently, the ductile behaviour of RC structures dominantly depends on the reinforcement detailing of the beam-column joints. Numerous investigations [1 and 2], have been

reported about the behaviour and reinforcement detailing of beam-column joints under reverse cyclic loading. In these researches, factors affecting the behaviour of RC beam-column joints were studied. In brief, the results of these investigations showed that the shear strength and ductility of RC beam-column joints increased as the compressive strength of concrete and the amount of transverse reinforcement increased. Moreover, for adequate ductility of beam-column joints, the use of closely spaced hoops as transverse reinforcement was recommended in various earthquake codes for RC structures. Previous experimental investigations [3, 4, 5, 6 and 7] have identified the steel

fiber reinforced concrete as having a potential solution. By introducing the steel fibers as secondary reinforcement in the beam column joints, ductility can be achieved at a lower cost and the fibrous concept can offer saving in material and labour costs [1]. The present study deals with the conventional reinforcement detailing and provides cocktail

fiber [8 and 9] (combinations of steel and polypropylene fibers) reinforced concrete in the beam-column joint region.

This cocktail fiber reinforced concrete in the joint region has more energy absorption capacity, more stiffness and more ductility.

2.1.Research Significance

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Dr. P.Perumal et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 3997-4006 dissipating capacity, displacement capacity, stiffness and outstanding damage tolerance, which make the joints attractive for reducing the need for costly post-earthquake repairs.

2. MATERIALS AND METHODS

Six specimens designed as per IS 456:2000 [10] and one designed as per IS 1893 (Part 1): 2002 [11] and detailed as

per IS 13920-1993[12]

. The remaining five specimens were similar to the first one but various combinations of cocktail fibre concrete in the joint region. Out of five fibre specimens four specimens were cast by using (constant 1.5% of steel fibre and 0 to 0.6% polypropylene fibres). The fifth fibre specimen was cast by using 1.5 % of polypropylene fibre only.Table 1 shows thedetails of specimens cast and their identification

.

Test specimen was reduced to one- fourth scale [13] to suit the loading and testing facilities. Original joint design had contained beam

and column sections of 380 X 450 mm and 380 X 380 mm respectively. One fourth joint design includes beam and column with 90 X 110 mm and 90 X 90 mm respectively. Original joint beam reinforcement at top was 1.52 % and at bottom was 1% of gross area. Original joint column reinforcement is 2.4 %. These percentages of steel

reinforcement ratio were also used for the model. Percentage of reinforcement of hoops at the joint was 2.13 % and other portion was 1.07 %. Fig. 1 and 2 show the one fourth scale of seismic joint and ordinary joint respectively. For seismic joint this hoop spacing is 20 mm for a distance of 180 mm from the face of the column in the beam and at a distance of 90 mm from the bottom and top of the beam in the column. After casting the specimens were left to dry for 24 hours at laboratory temperature and cured for about 28 days.

Table 1- Details of Test specimens

Sl.No Identification Specimen Joint seismic Reinforcement requirement %of fibre

Steel Polypropylene

1 I O2 Without - -

2 I S2 With - -

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Dr. P.Perumal et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 3997-4006

Fig. 2 Fibre Joint

2.1.Material Properties

Plain concrete mix and fibrous concrete mix in casting the test specimens by using M20 concrete. The first mix

1:1.76:2.69 (cement: sand: C.A) was used to cast specimens I O2 and I S2 and by mixing 1.5% of polypropylene fiber only was used for casting I F52 specimen. The second mix (fibrous mix) 1:1.64:2.5 (cement: sand: C.A) was used to cast remaining four specimens by adding various proportions of polypropylene fiber 0 to 0.6% and constant 1.5% of steel fibers. The concrete mix was designed according to Indian Standard Recommended Guidelines. . OPC

53 (Ordinary Portland cement) grade cement, river sand passing through 4.75mm IS sieve and coarse aggregates less than 10 mm size were used in this investigation concrete mixes. The macro crimped steel fibers and polypropylene fibers (synthetic fibers) were used in this investigation.

2.2.Experimental Setup and Procedure

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Fig. 3 Schematic Diagram of Test set-up

Fig. 4 Cyclic displacement loading history applied on the specimens

3. RESULTS AND DISCUSSIONS

3.1. Load-displacement hysteresis results

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Dr. P.Perumal et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 3997-4006 by I F12 specimen is 17.8. It is 68% greater than the specimen cast by using ordinary concrete (I O2) and 11% greater than the specimen cast by using 1.5% steel fibre and 0.2% of polypropylene fibre (I F22).The increase in polypropylene fibre will decrease the ultimate load carrying capacity.

Fig. 5 Load Displacement Plot for I F 22

The amount of accumulated hysteresis energy of a beam column connection subjected to reverse cyclic loading was calculated as the area under the peak value of the beam tip force-displacement hysteresis loop up to the related displacement level. Table 2 and Fig.6 shows the energy dissipation capacity of all the specimens cast by using M20

concrete subjected to reverse cyclic loading. From the figure it is observed that I F22 had maximum energy dissipating capacity compared to all the specimens. The energy absorption power was increased by 87 % by adding only steel fibre and 205 % by adding cocktail with the combination of 1.5% steel fibre and 0.2 % polypropylene fibre.

Table 2 Energy Dissipating capacity and Ultimate Load

Sl.No Specimen Id Ultimate load (PKn u) dissipation (EEnergy cu)

in kNmm Positive Negative

1 I O2 10.2 -10 247.6

2 I S2 15.4 -13.4 282.6

3 I F12 17.2 -16.8 464.4

4 I F 22 15.6 -15.4 755.2

5 I F32 11.6 -13.4 588.4

6 I F 42 12.4 -12.2 560

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Fig. 6 Energy Dissipation Capacity of All the Specimens

3.2. Displacement Ductility Factor

P

U2

0.5P

u2

dy

2

P

U1

du

2

0.8P

u1

0.5P

u1

Load

d

u2

Actual

Curve

Idealisded

elasto

Plastic curve

0.8P

u2

d

y1

d

y

=d

y1

+ d

y2

d

u

=d

u1

+ d

u2

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Dr. P.Perumal et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 3997-4006 Table 3 Displacement Ductility Factor and Ultimate Stiffness

Specimen Yield

Displacement dy (mm)

Ultimate displacement

du (mm)

Displacement Ductility Factor

(du/dy)

Ultimate Stiffness Id

I O2 30 45 1.50 0.8

I S2 31 60 1.94 0.84

I F12 31 75 2.41 1.16

I F 22 34 90 2.68 1.04

I F32 26 75 2.90 0.9

I F 42 20 75 3.75 0.88

I F 52 40 60 1.50 0.6

The displacement ductility factor is defined as Ψ= du / dy (1)

Where du is the deflection corresponding to ultimate load; and, dy the deflection corresponding to yielding of steel.

In order to calculate load at yield point, following steps were used. For each specimen, the load-displacement envelope was used to define the yield and maximum displacements following the method used by Shannag et al., (2008) [14]. In this method, the yield displacement is defined from the line drawn between the origin and 50% load

capacity point of the curve shown in Fig 7, this line is extended to intersect the 80% load capacity horizontal line. This point is assumed as the yield displacement. The other point intersect, the curve, is the 80% load displacement (du).The displacement ductility was calculated from the ratio between du / dy. Displacement ductility values for all

the specimens are presented in Table 3.

From the table it is observed that the specimen cast by using normal concrete exhibited low displacement ductility values and poor seismic performance. The specimen I F42 (1.5% of steel fibre +0.6% of polypropylene fibre) had the maximum ductility factor. The excess polypropylene fibre increases the ductility.

3.3. Stiffness Behaviour

In the case of reinforced concrete beam-column joints, stiffness of the joint gets reduced when the joint is subjected to cyclic/repeated/dynamic loading. This reduction in stiffness is due to the following reasons.

During cyclic loading, the materials, viz. concrete and steel, are subjected to loading, unloading, and reloading processes. This will cause initiation of micro-cracks inside the joint and will sometimes lead to the fatigue limit of the materials. This, in turn, increases the deformations inside the joints, thus resulting in reduction in the stiffness. Hence, it is necessary to evaluate degradation of stiffness in the beam-column joints subjected to cyclic or repeated loading. In order to determine the degradation of stiffness, the following procedure was adopted.

The recorded loads and corresponding displacements at the end of each half cycle were used to calculate a secant stiffness of each specimen during each cycle of test [3]. The secant stiffness in each cycle was calculated using

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Fig. 8 Stiffness Behaviour of M20 Concrete Specimens

The values of the secant stiffness obtained for each cycle are plotted for all the specimens. The degradation of the secant stiffness is plotted, the ultimate stiffness versus corresponding cycle number for each specimen tested. Fig.8 shows the stiffness plots for all specimens and Table 3 shows the ultimate stiffness of all the specimens. It may be noted from this figure that as the number of cycles increases, stiffness decreases. It may also be noted from these figures that the addition of steel fibres does not have any effect on the first cycle. However, as the number of cycles increases, the rate of degradation of stiffness decreases in the case of specimens additionally reinforced with fibres. The above behaviour may be attributed to the fact that at the first cycle, micro cracks would not have initiated and hence the fibres were not effective in the absence of formation of cracks. As the number of cycles increases, micro-cracks develop and fibres, which are distributed at random, intercept these micro-cracks and bridge across these micro-cracks. This action will control further propagation of cracks and will result in higher energy demand for bonding and pull-out of fibres in the vicinity of cracks. During this process, stiffness of the joint with fibres will not undergo much reduction when compared to that without fibres.

4. CONCLUSIONS

The results of reversed cyclic loading tests performed on one fourth scale exterior beam column joint models indicate that the fibre reinforced concrete is an appealing alternative to conventional confining reinforcement for providing adequate ductility. Steel fibre bridging across cracks in the concrete mix increase the joint shear strength. The synthetic fibres increase the ductility (large strain capacity) and energy dissipating capacity, the most important properties required for earth quake resistant structures. The specimen I F22 which is formed by using M20 with fibre

reinforcement in the joint region, consisting of 1.5% of steel fibre and 0.2% of polypropylene fibre have best performance considering the energy dissipation capacity and ductility factor but the ultimate load carrying capacity is reduced by adding polypropylene fibre. The addition of fibre cocktail to concrete prevents the brittle failure of the joint. The addition of polypropylene fibre increases the energy dissipation capacity when the dosage of polypropylene fibre is 0.2%. Further increase in polypropylene fibre was found to reduce the strength of the joint and also energy dissipating capacity. The rate of degradation of stiffness decreases in the case of specimens additionally reinforced with fibres.

Hence the cocktail combination of 1.5% of steel fibre and 0.2% of polypropylene fibre is highly recommended in beam-column joints subjected to reverse cyclic loading for M20.

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Dr. P.Perumal et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 3997-4006 [2] C.V.R. Murty, Durgesh C. Rai, K.K Bajpai and K. Jain, Effectiveness of Reinforcement Details in Exterior Reinforced Concrete

Beam-Column Joints for Earthquake Resistance, ACI Structural Journal,.Vol.100, No. 2, pp. 149-156, 2003.

[3] Andre Filiatrault, Sylvain Pineau, and Jules Houde, Seismic Behaviour of Steel Fibre Reinforced Concrete Interior Beam-Column Joints, ACI Structural Journal, Vol.92, No. 5, September-October, pp. 543-551, 1995.

[4] N. Ganesan, P.V Indira and Ruby Abraham, Fibre Reinforced High Performance Concrete Beam-Column Joints Subjected to Cyclic Loading, ISTE Journal of Earthquake Technology, Vol. 44, No. 3-4, pp.445-456, 2007.

[5] M. Gebman, Application of Steel Fiber Reinforced Concrete in Seismic Beam- Column Joints, in Department of Civil Engineering. Spring 2001, San Diego State University.

[6] Gustavo J. Parra- Montesinos, “High-Performance Fibre Reinforced Cement Composites An Alternative for Seismic Design of Structures”, ACI Structural Journal, Vol.102, No. 5, pp. 668-673, 2005.

[7] Paul, R.Gefken and Melvin, R.Ramey, Increased Joint Hoop Spacing in Type 2 Seismic Joints Using Fibre Reinforced Concrete, ACI Structural Journal, Vol.86, No. 2, March-April, pp. 168-172,1989.

[8] Lars Kutzing, Use of Fibre Cocktail to Incerase the ductility of High Performance Concrete (HPC), Institute for Massivbau and Baustoffechnologie i Gr.Universitat Leipzig,LACER No. 2, pp.125-134, 1997.

[9] Lars Kutzing, Some aspects of designing High Performance Concrete with Fibre Cocktails, Dipl-Ing, Institute of concrete and building materials,University of Leipzig, LACER No. 3, pp.143-151, 1998.

[10] Indian Standard Plain and Reinforced Concrete Code of Practice IS 456:2000 Bureau of Indian Standards, New Delhi.

[11] Indian Standard Criteria for Earthquake resistant Design of Structures, Part I Genaral Provisions and Buildings, IS 1893 (Part I) 2002 Bureau of Indian Standards, New Delhi.

[12] Indian Standard Ductile Detailing of reinforced Concrete Structures subjected to Seismic Forces. Code of Practice: IS 13920-1993 (Part 1):2002. Bureau of Indian Standards, New Delhi.

[13] Ziad Bayasi and Michael Gebman., Reduction of lateral reinforcement in seismic Beam column connection via application of steel fibres, V.99, No. 6, ACI Structural Journal, November-December 2002, pp. 772-780.

[14] Jamal Shannag M, Nabeela Abu- Dyya and Ghazi Abu- Farsakh, Lateral load Response of HighPerformence Fibre Reinforced Concrete Beam-Column Joints, ELSEVIER Journal of Construction and Building Materials 19, pp. 500-508, 2005.

[15] Mustafa Gencoglu and Ilhan Eren. An Experimental Study on the Effect of Steel Reinforced Concrete on the Behaviour of the Exterior Beam-Column Joints Subjected to Reverse Cyclic Loading, Turkish J. Eng.Sci. 26 pp. 493-502, 2002.

[16] B. Thanukumari and P. Perumal, An Experimental Study on the Behaviour of M20 Concrete with Cocktail Fibre in Exterior Beam-Column Joints Subjected to Reverse Cyclic Loading, IETECH Journal of Civil and Structures, Vol. 2, No. 2, pp. 065-070, 2009. [17] B. Thanukumari and P. Perumal, Behaviour of M60 Concrete Using Fibre Cocktail in Exterior Beam-Column Joint Under Reverse

Figure

Table 1- Details of Test specimens
Fig. 1 Seismic Joint
Fig. 2   Fibre Joint
Fig. 3 Schematic Diagram of Test set-up
+5

References

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