Structural performance of Eccentrically loaded GFRP ... · Structural performance of Eccentrically...

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING

Volume 2, No 1, 2011

© Copyright 2010 All rights reserved Integrated Publishing services

Research article ISSN 0976 – 4399

Received on September, 2011 Published on November 2011 395

Structural performance of Eccentrically loaded GFRP Reinforced concrete

columns Issa, M. S, Metwally I. M, Elzeiny S. M

Associate Professor, Reinforced Concrete Institute,

Housing and Building National Research Center, Giza, Egypt

[email protected]

doi:10.6088/ijcser.00202010119

ABSTRACT

This paper explores the behavior of GFRP and steel reinforced concrete columns when

subjected to eccentrically axial loads. Six columns of 150*150 mm cross section were tested.

Four of them had GFRP reinforcement and two had steel reinforcement. The concrete

strength of the GFRP reinforced columns was either 24.73 MPa or 38.35 MPa while for the

steel reinforced columns it was 24.73 MPa. The eccentricity was either 50 mm or 25 mm and

the tie spacing was either 80 mm or 130 mm. Large longitudinal deformations were recorded

for columns with GFRP reinforcement and for columns with large tie spacings. However, tie

spacing had no notable effect on the maximum lateral deflection and ductility of GFRP

columns of this research. The average maximum stress was about 60% of the concrete

compressive strength for columns with initial eccentricity of 50 mm. GFRP bars recorded

higher strains than steel bars and these strains were larger when the tie spacing was large. The

increase in the strength of the concrete was associated with reduction in the GFRP bar strain.

Two interaction diagrams were plotted for the columns and they present lower bound to the

obtained experimental results.

Keywords: GFRP bars, steel bars, eccentric load, column, ductility.

1. Introduction

Although the use of GFRP bars as reinforcement for concrete beams is becoming more

common, their use in the concrete columns is currently limited to the research areas. The use

of GFRP bars is a viable option particularly when corrosion resistance or electromagnetic

transparency is sought. The current design recommendations for concrete members

reinforced with GFRP bars such as those of the ACI 440.1R-06, 2006 do not cover GFRP

reinforced columns. Alsayed et al., 1999 tested concrete columns reinforced with GFRP rods

under axial concentric loads. Their results indicated that replacing the longitudinal steel bars

by GFRP bars reduced the axial capacity of the column by 13%. Also, replacing the steel ties

by GFRP ties reduced the axial capacity of the column by 10% regardless the type of the

longitudinal bars. Replacing the steel ties by GFRP ties had no influence on the load-axial

shortening curve up to about 80% of the ultimate load. They reported that using the ACI 318,

2008 formula to estimate the axial capacity of the GFRP reinforced columns overestimated

the actual capacity of the columns. Luca et al., 2009 tested five concrete columns one of them

was reinforced with steel bars and the other four were reinforced with GFRP bars. GFRP ties

were used with two different spacing. The columns were tested under pure axial load. They

reported that all the GFRP reinforced columns provided similar strength to that of the steel

reinforced columns. The failure mode was strongly influenced by the spacing of the GFRP

ties. Mirmiran et al., 2001 developed a rational method for the analysis of slender FRP

reinforced columns. They carried out a detailed parametric study. Their study recommends

Structural performance of Eccentrically loaded GFRP Reinforced concrete columns

Issa, M. S, Metwally I. M, Elzeiny S. M

International Journal of Civil and Structural Engineering

Volume 2 Issue 1 2011

396

reducing the current slenderness limit of 22 to 17 for FRP reinforced columns bent in single

curvature. It was also shown that the moment magnification method can be extended to FRP

reinforced columns. There is a considerable research about eccentrically loaded steel

reinforced columns as shown by Foster and Attard, 1997, Ibrahim and MacGregor, 1996, Lee

and Son, 2000 and Xie et al., 1996.

2. Experimental Program

2.1 Test Specimens

The test program included six columns of 150*150 mm cross section. Four columns had four

sand coated GFRP bars of diameter 12 mm and of mechanical properties as shown in Table

1.0. Two columns had four deformed high tensile steel bars of diameter 12 mm. The

mechanical properties of the steel bars are shown in Table 2.0. Two of the GFRP reinforced

columns had cylinder compressive strength of 24.73 MPa and are referred to as GN8 and

GN13. The other two had cylinder compressive strength of 38.35 MPa and are referred to as

GM8 and GM13. The remaining two steel reinforced columns had cylinder compressive

strength of 24.73 MPa. They are referred to by SN8 and SN13. All the ties were of 8 mm

diameter plain mild steel and were placed at spacing of 80 mm for columns GN8, GM8, and

SN8 and at spacing of 130 mm for columns GN13, GM13, and SN13. Each column had two

heads at its ends to facilitate the application of eccentric load and to reduce the concentration

of stress at the column ends, see Figure 1.0 and Table 3.0.

The specimens were cast horizontally. They were covered with wet burlap. Stripping was

done one day after casting. The water treatment continued for one week.

Table 1: Mechanical Properties of GFRP Bars

Ultimate Tensile Strength ffu (MPa) 347.50

Modulus of Elasticity Ef (GPa) 32.67

Rupture Strain εfu 0.05

Table 2: Mechanical Properties of Steel Reinforcement

Diameter

(mm) Fy (kN/mm

2) Fu (kN/mm

2) Elongation %

8

0.30 0.46 19.4

0.30 0.46 27.9

0.31 0.46 28.4

12

0.43 0.69 13.6

0.45 0.69 14.2

0.45 0.69 19.3





397

Table 3: Details of Specimens

Spec.

Cross

Section

(mm×mm)

Reinforcement Stirrup

Spacing

(mm)

f'c

(Mpa)

Initial

Eccentricity

(mm) Type Number Diameter

(mm)

GN8 150×150 GFRP 4 12 80 24.73 50

GN13 150×150 GFRP 4 12 130 24.73 25

GM8 150×150 GFRP 4 12 80 38.35 50

GM13 150×150 GFRP 4 12 130 38.35 50

SN8 150×150 Steel 4 12 80 24.73 50

SN13 150×150 Steel 4 12 130 24.73 50

Figure 1: Test Specimens

2.2 Material Properties

Two different concrete mixes were used as shown in Table 4.0. The first on is referred to by

'N' and gives a strength of 24.73 MPa while the second one is referred to by 'M' and gives a

strength of 38.35 MPa. A superplasticizer and silica fume were used with the second mix.

Table 4: Concrete Mix for One m3

Mix Type Cement

(kg)

Water

(kg)

Sand

(kg)

Dolomite-

Nominal size=10mm

(kg)

Silica

Fume

(kg)

Additive

(kg)

N 350 186 555 845 - -

M 533 140.6 799 1064 80.5 12.9





398

2.3 Instrumentation

The deformations of the specimens were measured using two LVDTs as shown in Figures 1.0

and 2. One of the LVDTs was located on the compression side of the specimen. It had a gage

length of 600 mm. Also a horizontal LVDT was attached at half the height of the specimen to

trace its lateral deflection during loading. Electrical resistance strain gage was mounted on

the middle of one of the longitudinal bars on the compression side to measure the stain. A

calibrated load cell was used to measure the applied load.

Figure 2: Test Setup

3. Test Results and Discussion

Six specimens were tested to failure. Maximum loads, steel strain at maximum load, concrete

compressive strain at maximum load, lateral displacement at maximum load, average

maximum stress and ratio of the average maximum stress to concrete strength are listed in

Table 5.0 and 6.0. The maximum moment is also listed in Table 6.0 and it is calculated as the

maximum load times the summation of initial eccentricity and maximum lateral deflection.

Table 5: Test Results- Part 1

Spec. f'c

(MPa)

Max.

Load, Pu

(kN)

Max. Concrete

Strain on the

Compression

Side

Max. Steel

Strain on the

Compression

Side

Max. Lateral

Deflection

(mm)

GN8 24.73 227.1 0.0056 >0.0016 5.21

GN13 24.73 425.8 0.0044 0.0025 2.8

GM8 38.35 535.8 0.0020 0.0018 *

GM13 38.35 490.9 0.0042 0.0024 *

SN8 24.73 331.5 0.0053 0.0026 *

SN13 24.73 343.4 0.0046 0.0031 6.52

*Not recorded due to problems with the measuring devices.





399

Table 6: Test Results- Part 2

Spec. f'c

(MPa)

Average Max.

Stress, fc (MPa) fc/f'c

Max. Moment

(kN.m)

GN8 24.73 10.09 0.408 12.54

GN13 24.73 18.92 0.765 11.84

GM8 38.35 23.81 0.621 -

GM13 38.35 21.82 0.569 -

SN8 24.73 14.73 0.596 -

SN13 24.73 15.26 0.617 19.41

*Not recorded due to problems with the measuring devices.

3.1 Failure Modes

In general, all the specimens failed by sudden crushing of the most compressed concrete

fibers on the compression face with the exception of specimen SN8 which showed spalling of

the cover before failure. The failure patterns are shown in Figure 3.0. From Table 5.0, the

maximum compressive face strains ranged between 0.0044 and 0.0056 for the normal

strength specimens and between 0.0020 and 0.0042 for the medium strength specimens.

GN8 GN13

GM8 GM13





400

SN8 SN13

Figure 3: Failure Modes

3.2 Deflection on the Compressive Face of Concrete

Figures 4 to 7 show the measured surface deformation for the compressive face of the tested

reinforced concrete columns. From Figure 4.0, it is clear that the higher strength columns

deform lesser than the lower strength ones at the same loading level. Also, steel reinforced

columns deform lesser than GFRP reinforced columns as shown in Figure 5.0. The columns

with greater tie spacing deform more than those with smaller tie spacing. However, this

increase was higher for the case of GFRP reinforced columns compared to steel reinforced

columns as presented in Figures 6.0 and 7.0.

0

100

200

300

400

500

600

0 2 4 6 8 10

deflection (mm)

loa

d (

kN

)

GN8

GM8

0

50

100

150

200

250

300

350

0 2 4 6 8 10

deflection (mm)

loa

d (

kN

)

GN8

SN8

Figure 4: Load-Compression

Face Deflection for GN8 & GM8


Face Deflection for GN8 & SN8





401

0

100

200

300

400

500

600

0 0.5 1 1.5 2 2.5 3

deflection (mm)

loa

d (

kN

)

GM8

GM13

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12

deflection (mm)

loa

d (

kN

)

SN8

SN13

3.3 Lateral Displacement

Figures 8.0 to 10.0 show that during the ascending part of the loading history, the lateral

displacement for all the specimens increased on a gradual rate until the maximum load where

they dropped suddenly. Comparing Figures 8.0 and 10.0, it can be noted that the confinement

had no effect on the maximum lateral deflection.

0

50

100

150

200

250

0 10 20 30 40 50deflection (mm)

load

(kN

)

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50deflection (mm)

loa

d (

kN

)

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35deflection (mm)

loa

d (

kN

)

Figure 10: Load-Lateral Displacement for SN13

3.4 Average Maximum Stress

Figure 6: Load-Compression Face

Deflection for GM8 & GM13


Face Deflection for SN8 & SN13

Figure 8: Load-Lateral

Displacement for GN8

Figure 9: Load-Lateral

Displacement for GN13





402

For all the specimens, the average maximum stress was calculated as the ratio between the

maximum load and the gross sectional area. The ratio between the average maximum stress

and the concrete compressive strength is also shown in Table 6.0. In general, this ratio

reached 60% for the tested columns with initial eccentricity equal to 50 mm.

3.5 Compression-Side Bar Strain

The compression-side bar strains are shown in Figures 11.0 to 14.0. The bar strains for the

medium strength specimen were smaller than those for the normal strength specimen as clear

from Figure 11.0. For corresponding specimens, the steel bar strains were smaller than the

GFRP bar strains, see Figure 12.0. From Figures 13.0 and 14.0, the strains for either GFRP

bars or steel bars at high loads are larger for the tie spacing of 130 mm relative to the tie

spacing of 80 mm.

0

100

200

300

400

500

600

0 0.0005 0.001 0.0015 0.002strain

loa

d (

kN

)

GN8

GM8

0

50

100

150

200

250

300

350

0 0.001 0.002 0.003 0.004 0.005 0.006strain

loa

d (

kN

)

GN8

SN8

0

100

200

300

400

500

600

0 0.0005 0.001 0.0015 0.002 0.0025

strain

loa

d (

kN

)

GM8

GM13

0

50

100

150

200

250

300

350

400

0 0.001 0.002 0.003 0.004 0.005 0.006

strain

loa

d (

kN

)

SN8

SN13

3.6 Ultimate Strength

The peak load carrying capacity of the column section can be defined in terms of its axial

load-bending moment interaction diagram. This diagram is drawn based on principles of

equilibrium and strain compatibility. The different points used to plot the interaction diagram

for steel reinforced columns shown in Figure 15.0 were found using the standard equations

and assumptions of ACI 318, 2008. However, for the interaction diagram of GFRP reinforced

columns, Figure 16.0, the following points were used.

Figure 11: Load-Compression Side

Bar Strain for GN8 & GM8GM13


Bar Strain for GN8 & SN8


Bar Strain for GM8 & GM13


Bar Strain for SN8 & SN13





403

Point 1: uniform axial compression (zero bending moment) which was calculated using the

following equation suggested by Alsayed et al., 1999,

GFRPfuGFRPgcu AfAAfP 6.0)(85.0 '

where f'c =28-day concrete cylinder compressive strength

Ag=gross area of the concrete cross section

AGFRP=total area of the GFRP bars

ffu=tensile strength of the GFRP bars

Point 2: pure bending moment and zero axial force.

Points 3 and 4: strain distribution corresponding to the maximum compressive strain (ɛcu) and

a tensile strain at the layer of GFRP bars less than their rupture strain.

The experimental results are compared to the theoretical interaction diagrams. These

interaction diagrams for both cases present lower bounds to the experimental data.

Figure 15: Interaction Diagram for Steel Reinforced Columns





404

Figure 16: Interaction Diagram for GFRP Reinforced Columns

3.7 Ductility

One measure of ductility can be given by

y

cD

85.0

where εy is obtained by drawing a tangent to the ascending part of the load-strain curve at

0.75Pu which intersects the horizontal line at Pu. εy is the strain corresponding to this point of

intersection. ε0.85 is the strain on the descending part of the load-strain curve at 0.85Pu. The

calculated ductilities are presented in Table 7.0 where the missing values are due to the

absence of the descending part of the load-strain curves.

Table 7: Ductility

Specimen Dc

GN8 2.04

GN13 2.11

GM8 -

GM13 -

SN8 2.40

SN13 1.70

From Table 7.0, it is clear that the spacing of the ties had small effect on the ductility for the

case of GFRP reinforced columns. However, for steel reinforced columns the increase in the

spacing of the ties resulted in reduction of the ductility.





405

4. Conclusions

1. Steel reinforced columns deform lesser than GFRP reinforced columns.

2. Columns with greater tie spacing deform more than those with smaller tie spacing.

The difference in deformations is clear for GFRP reinforced columns and is small for

steel reinforced columns.

3. In this research, tie spacing had no notable effect on the maximum lateral deflection.

4. For the tested columns with initial eccentricity equal to 50 mm, the average maximum

stress was about 60% of the concrete compressive strength.

5. The GFRP bar strain is smaller for the medium strength specimens compared to

normal strength specimens.

6. Steel bar strains were generally smaller than GFRP bar strains.

7. At high loads, the GFRP and steel bar strains are larger when the tie spacing is larger.

8. The plotted interaction diagrams present a lower bound to the obtained experimental

results.

9. Tie spacing had small effect on the ductility of the GFRP reinforced columns of this

research. Smaller ductility is obtained for steel reinforced columns with larger tie

spacings.

5. References

1. ACI Committee 318. (2008), "Building Code Requirements for Reinforced Concrete

(ACI 318-08)", American Concrete Institute.

2. ACI Committee 440.1R-06. (2006), "Guide for the Design and Construction of

Concrete Reinforced with FRP Bars", American Concrete Institute, Farmington Hills,

Michigan, 44 pp.

3. Alsayed, S. H., Al-Salloum, Y. A., Almusallam, T. H., Amjad, M. A. (1999),

"Concrete Columns Reinforced by Glass Fiber Reinforced Polymer Rods", Fourth

International Symposium on Fiber-Reinforced Polymer Reinforcement for Reinforced

Concrete Structures, SP-188, C. W. Dolan, S. H. Rizkalla, and A. Nanni, eds.,

American Concrete Institute, Farmington Hills, Mich., USA, pp 103-112.

4. De Luca, A., Matta, F., Nanni, A. (2009), "Behavior of Full-Scale Concrete Columns

Internally Reinforced with Glass FRP Bars under Pure Axial Load", Proceedings of

Composites and Polycon 2009, American Composites Manufacturers Association,

January 15-17, Tampa, USA, pp 1-10.

5. Foster, S. J., Attard, M. M. (1997), "Experimental Tests on Eccentrically Loaded

High-Strength Concrete Columns", ACI Structural Journal, 94(3), May-June, pp 295-

303.

6. Ibrahim, H. H. H., MacGregor, J. G. (1996), "Tests of Eccentrically Loaded High-

Strength Concrete Columns", ACI Structural Journal, 93(5), September-October, pp

1-10.





406

7. Lee, J., Son, H. (2000), "Failure and Strength of High-Strength Concrete Columns

Subjected to Eccentric Loads", ACI Structural Journal, 97(1), January-February, pp

75-85.

8. Mirmiran, A., Yuan, W., Chen, X. (2001), "Design for Slenderness in Concrete

Columns Internally Reinforced with Fiber-Reinforced Ploymer Bars", ACI Structural

Journal, 98(1), January-February, pp 116-125.

9. Xie, J., MacGregor, J. G., Elwi, A. E. (1996), "Numerical Investigation of

Eccentrically Loaded High-Strength Concrete Tied Columns", ACI Structural Journal,

93(4), July-August, pp 449-461.

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