EXPERIMENTAL INVESTIGATION ON REHABILITATION OF REINFORCED

CEMENT CONCRETE INTERIOR BEAM-COLUMN JOINTS USING CFRP

AND GFRP SHEETS

R.V.S.RAMAKRISHNA Research Scholar, Department of Civil Engineering JNT University Kakinada,

Kakinada - 533 003, Andhra Pradesh, India. E mail : sivaramar@yahoo.com

V.RAVINDRA

Professor of Civil Engineering Department and Registrar, JNT University Kakinada, Kakinada - 533 003, Andhra Pradesh, India

E mail : ravindra.vipparthy@gmail.com Abstract

This paper proposes a rapid rehabilitation scheme for moderately damaged reinforced concrete interior beam - column joints under the static loading. Eight interior beam - column joints were casted and designated as virgin specimens and tested up to failure . Out of eight specimens, four of the specimens were externally wrapped with glass fibre reinforced polymer sheets and other four specimens with carbon fibre reinforced polymer sheets. These rehabilitated specimens were tested up to failure. The performances of the rehabilitated beam-column joint specimens were compared with the virgin beam-column joint specimens. Experimental results illustrated that the rehabilitated specimens of glass fibre reinforced polymer and carbon fibre reinforced polymer beam-column joint specimens exhibited an improved load carrying capacity and a higher rate of stiffness than the virgin specimens. Key Words: Rehabilitation; Carbon Fibre; Glass Fibre; Matrix

1. Introduction

Rehabilitation of structures to higher seismic zones of several cities and towns in the country has also necessitated in evolving new strategies. Recent earthquakes have demonstrated that most of the reinforced concrete structures were severely damaged during earthquakes and they need major repair works. One of the techniques of strengthening the RC structural members is through external confinement by high strength fibre composites which can significantly enhance the strength, ductility and will result in large energy absorption capacity of structural members. Fibre materials are used to strengthen a variety of reinforced concrete elements to enhance the flexural, shear and axial load carrying capacity of elements. Beam-column joints, being the lateral and vertical load resisting members in reinforced concrete structures are particularly vulnerable to failures during earthquakes and hence their rehabilitation is often the key to successful seismic rehabilitation strategy. FRPC based strengthening strategy could be an attractive option in order to restore joints. In addition to being lighter, thinner and easier to implement FRPC reinforced joints have the virtue of making the joints more ductile. This property is extremely desirable for seismic rehabilitation of structures. However, a direct extension of the strategies adopted for beams and columns are difficult as such the behavior of beam-column connections are complex and still not completely understood. Survey of existing constructions reveals that rehabilitation of structures is necessary in three conditions. The structure is inadequately designed for the present load conditions. The inadequately detailed for the present loading. This also includes those structures that are found deficient under seismic conditions. The structure is

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damaged and requires rehabilitation. The motivation behind this program is to examine the performance of fibre reinforced polymer composites (FRPC) in rehabilitation of damaged joints. 2. Materials Properties GFRP / CFRP Table 1. Materials Properties GFRP / CFRP

Name Glass fibre Carbon fibre

Colour White Black

Technical data of fibre E-Glass,900 gsm 230 gsm

Table 1 (Continued) 

Modulus of elasticity 73 kN/mm² 240 kN/mm²

Tensile strength 3400 N/mm² 3800 N/mm²

Total weight of sheet in main direction

900 g/m² 230 g/m²

Density 2.6 g/cm³ 1.7 g/cm³

Ultimate Strain % 4.5 1.55

Thickness for static design wt/density 0.342 mm 0.117 mm

Safety factor for static design 1.5 (recommended) 1.2 (recommended)

3. Experimental Work

3.1. Specimen details: The experimental programme consist of the testing eight reinforced concrete interior beam-column joint specimens. The columns had a cross section of 150 mm x 150 mm with an overall length of 750 mm and the beams had a cross section of 150 mm x 150 mm with an overall length of 750 mm. The cantilever length of 300 mm on either side of the column. The column was reinforced with 4 numbers of 10mm diameter tor steel bars and the beam was reinforced with 2 numbers of 10 mm diameter tor steel bars each as tension and compression reinforcement. The lateral ties in the columns of the specimens were 6 mm diameter bars with the spacing of 90 mm c/c. Beams had double legged stirrups of 6 mm diameter mild steel bar at 90 mm c/c. There was no transverse reinforcement in the joint region apart from the main reinforcements along the columns and beams. They were designed such that failure would be due to flexural in the joint during the test, so as to evaluate the contribution of GFRP and CFRP to the flexural capacity of joint.

Fig 3.1: Reinforcement details for the test specimens Fig 3.2 Schematic diagram

UTM

UTM

0.3 m 

 

0.3 m 

Specimen

150 mm  

 

300m

m  

300m 300m

150m

300mm 

4#10mm ф bars

6 mm ф  two 

legged 

stirrups @ 90

150mm  

150mm  

4#10mm

6 mm ф  lateral ties @ 90 mm 

/

150mm  150mm  

Loading Jack

Pro ring

Dial gauge

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All the eight reinforced concrete beam-column joint (virgin) specimens casted and cured for one month. The experimental programme consist of rehabilitation using glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP). Out of these eight virgin specimens four glass fibre specimens named as GSP1, GSP2, GSP3 & GSP4 and other four Carbon Fibre Specimen named as CSP1, CSP2, CSP3 & CSP4 before conducting test. 3.2. Application Procedure of GFRP & CFRP Wrapping:

Grinding the surface from joint up to 150 mm and to get an even surface. All projectins are grounded off.

Apply MBrace Primer to be prepared concrete surface area.Work site must be thoroughly ventilated during the application of chemicals.

Mix the two packed MBrace Saturant two pack and apply to the primed concrete specimen using brush. The fibre sheet must be cut before application of MBrace Saturant into prescribed sizes using scissors or

cutters. On the saturant fix the sized glass fibre/ carbon fibre sheets and roll in the beam longitudinal direction.

Fig – 3.3 Wrapping of Carbon Fibre Reinforced Polymer Sheet Fig – 3.4 Application of Glass Fibre Reinforced Polymer Sheet

3.3. Test Setup: The specimens were fixed on universal testing machine such that the both ends of column were fixed by UTM. The projections of beam length 300 mm on either side of the column were fixed by proving ring attached with hydraulic jacks. Only one end beam was loaded by means of hydraulic jack and readings are taken from proving ring. Other end of the beam also have same arrangement but only for supporting purpose. Packing plates were placed on either side of the column. The hydraulic jack and proving ring was seated vertically. A dial gauge was placed on top of the application of load on the beam for measuring deflections. The least count of dial gauge is 0.01mm. The whole arrangement of test setup was shown in Fig – 3.5.

Fig – 3.5 Test arrangement for Virgin Specimen Fig – 3.6 Test arrangement for Rehabilitated specimen

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4. Test Results and Graphs

The loads and the corresponding deflections on virgin and rehabilitated specimens were plotted on graphs. These results were obtained by conducting load test on virgin specimens and rehabilitated specimens. The graphs are plotted based on loads and deflections of both the specimens

Fig 4.1 Load Deflection Curve for GSP 1 Fig 4.2 Load Deflection Curve for GSP 1

Fig 4.3 Load Deflection Curve for GSP 3 Fig 4.4 Load Deflection Curve for GSP 4 Observations

1. It is clearly understood from above graphs fig – 4.1,4.2,4.3,4.4 that glass fibre specimen have exhibited more ultimate load carrying capacity than virgin specimen.

2. For each load increment the corresponding deflections on glass fibre specimens were less than that of virgin specimens.

3. From graph it is observed that the relation between load vs. deflection curve is almost linear up to first crack.

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Fig 4.5 Load Deflection Curve for CSP 2 Fig 4.6 Load Deflection Curve for CSP 1

Fig 4.7 Load Deflection Curve for CSP 3 Fig 4.8 Load Deflection Curve for CSP 4

Observations

1. It is clearly understood from graphs in Fig -4.5,4.6,4.7,4.8 that carbon fibre specimen have more ultimate load carrying capacity than virgin specimen.

2. For each load the corresponding deflections on carbon fibre specimens were less than that of virgin specimens.

3. From graph it is observed that the relation between load vs. deflection curve is almost linear up to first crack. 5. Discussion on test results

5.1.1. Load Study: With reference to the test results, the loads on virgin specimens at first crack stage are compared to the loads on glass fibre specimens at first crack stage. It is observed that the load carrying capacity of glass fibre specimens are increased when compared to the virgin specimens. From these values the percentage of increase in load carrying capacity of glass fibre specimens over virgin specimens are tabulated in table. 2.

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Table 2 Yield points of the glass fibre specimens

5.1.2. Deflection Study: The deflections at first crack load on virgin specimens and glass fibre specimens are tabulated in table. 3. It is observed that the deflections were decreased. in glass fibre specimen when compared to the virgin specimen and percentage of reduction in deflections were also mentioned. Table.3 Deflection points of glass fibre specimens

5.1.3. Stiffness Study: With reference to the above test results, the stiffness values were calculated for both virgin and glass fibre specimens and the values shows that the rehabilitated with glass fibre specimens have more stiffer than virgin specimens. The percentage of increase in stiffness in case of glass fibre specimens over virgin specimens are mentioned in table.4. Table.4 Stiffness points of glass fibre specimens

Sample No

Load at First crack on

Virgin Specimen

(kN)

Corresponding Deflections(mm) Stiffness

of Virgin

specimen (kN/mm)

Stiffness of Glass

fibre specimen (kN/mm)

Percentage of Increase in stiffness Virgin

Specimen

Glass fibre

specimen

GSP1 5.5 2.97 1.15 1.85 4.78 158.38

GSP2 7.5 12.65 2.73 0.59 2.75 366.10

GSP3 8.0 5.76 2.93 1.39 2.73 96.40

GSP4 6.0 8.85 1.92 0.68 3.13 360.29

5.2. Comparison between Virgin Specimen and Carbon Fibre Specimen.

5.2.1. Load Study: With reference to the test results, the load on virgin specimens at first crack stage are compared to the load on carbon fibre specimens at first crack stage. It is observed that the load carrying capacity of carbon fibre specimens are increased when compared to the virgin specimens. From these values the percentage of increase in load carrying capacity of carbon fibre specimens over virgin specimens are tabulated table. 5..

Sample No

Load at First Crack (kN)

Percentage of Increase in Strength Virgin Specimen Glass Fibre Specimen

GSP1 5.5 9.0 63.63

GSP2 7.5 9.5 26.67

GSP3 8.0 8.5 6.25

GSP4 6.0 9.0 50.00

Sample No

Deflections at First Crack(mm) Percentage of Decrease in Deflection Virgin Specimen Glass Fibre Specimen

GSP1 2.97 1.15 61.28

GSP2 12.65 2.73 78.42

GSP3 5.76 2.93 49.13

GSP4 8.85 1.92 78.31

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Table.5 Yield points of the carbon fibre specimens

Sample No

Load at First Crack (kN)

Perscentage of Increase in Strength Virgin Specimen

Carbon Fibre Specimen

CSP1 7.0 10.0 42.86

CSP2 4.5 8.5 88.89

CSP3 6.5 10.0 53.85

CSP4 5.5 7.5 36.36

5.2.2. Deflection Study: The deflections at first crack load on virgin specimens and carbon fibre specimens are tabulated table. 6. It is observed that the deflections were decreased in carbon fibre specimen when compared to the virgin specimen and percentage of reduction in deflections were also mentioned. Table.6 Deflection points of carbon fibre specimens

Sample No

Deflections at First Crack(mm) Perscentage of Decrease in

Deflection Virgin Specimen Carbon Fibre Specimen

CSP1 4.76 3.52 26.05 CSP2 1.28 0.84 34.38 CSP3 5.40 2.95 45.37 CSP4 7.58 4.85 36.02

5.2.3. Stiffness Study: With reference to the above test results, the stiffness values were calculated for both virgin and carbon fibre specimens and the values shows that the rehabilitated with carbon fibre specimens have more stiffer than virgin specimens.The percentage of increase in stiffness in case of carbon fibre specimens over virgin specimens are mentioned in table.6. Table.7 Stiffness points of carbon fibre specimens

Sample No

Load at First crack on Virgin

specimen (kN)

Corresponding Deflections(mm) Stiffness

of Virgin specimen (kN/mm)

Stiffness of Carbon

fibre specimen (kN/mm)

Percentage of

Increase in stiffness Virgin Specimen

Carbon fibre

specimen CSP1 7.0 4.76 3.52 1.47 1.99 35.37 CSP2 4.5 1.28 0.84 3.52 5.36 52.27 CSP3 6.5 5.40 2.95 1.20 2.20 83.33 CSP4 5.5 7.58 4.85 0.73 1.13 54.79

6. Conclusions

Based on the experimental investigations carried out on the virgin and rehabilitated beam-column joint specimens using GFRP and CFRP wrapping, the following conclusions were drawn. The rehabilitation technique using wrapping system for the damaged R.C.C interior beam – column joints have proved to be effective. The rigidity and ultimate load carrying capacity of the restored beam was improved with decrease in deflections. Both glass and carbon composite materials can be efficiently used for rehabilitation of reinforced concrete joints. Joints can exhibit enhanced performance for different reinforcement detailing and damage states. Considerable increase in yield load can be achieved by use of glass and carbon reinforced polymer materials. Considerable increase in first crack load can be achieved by using glass and carbon reinforced polymers. Tests on rehabilitated specimens suggest that FRP not only restores its original strength but also

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considerable enhancement in its yield load and initial stiffness. The rehabilitated specimens are stiffer than the virgin specimen and the crack widths in the rehabilitated specimens are relatively less. References

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