BEHAVIOUR OF GFRP R.C. SLABS IN FLEXURE IN LOCAL ENVIRONMENT - AN EXPERIMENTAL STUDY

M V Venkateshwara Rao*, JNT University, Hyderabad, India

P Jagannadha Rao, Vasavi College of Engineering, India M V Sheshagiri Rao, JNT University, Hyderabad, India

K Jagannadha Rao, CBIT Hyderabad, India

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35th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 – 27 August 2010, Singapore

BEHAVIOUR OF GFRP R.C. SLABS IN FLEXURE IN LOCAL ENVIRONMENT - AN EXPERIMENTAL STUDY

M V Venkateshwara Rao*, JNT University, Hyderabad, India P Jagannadha Rao, Vasavi College of Engineering, India M V Sheshagiri Rao, JNT University, Hyderabad, India

K Jagannadha Rao, CBIT Hyderabad, India

Abstract Most of the failures in concrete structures are due to corrosion of reinforcement, particularly in aggressive environment. This has prompted the researchers to develop an alternative non corrosive, non metallic material as reinforcement. Some preliminary investigations were carried out using different fiber resin ratios for preparing GFRP bars and an optimum fiber resin ratio of 7:3 was arrived. The tensile strength of GFRP bars is comparable to that of the mild steel as per the tests carried out, but the modulus of elasticity is about 25-30 percentage of that of steel bars. This paper deals with the preliminary investigations carried out on small slab panels supported on all four edges with effective spans of 0.9m x 0.6m which is a part of large research problem undertaken with different ratios of long span to short span with different support conditions. The test results are compared with similar slab panels reinforced with conventional mild steel bars. Key Words: GFRP bars, steel bars, corrosion, slab panels 1. Introduction

Research on concrete is a continuous process world over, to improve the performance of concrete to meet the functional, strength, economy and durability requirements. As concrete is weak in tension hither’ to steel is being used in tension zone to strengthen the concrete. Depending on the type of members, different types of fibers are also used now as crack arresters in concrete. Because of corrosion problems associated with steel, necessity of new non corrosive materials has arisen. Fiber reinforced polymer (FRP) is a composite material made of fibers and resin. Glass Fibre Reinforced Polymer (GFRP) is one such composite material. Aramid, Carbon and Basalt fibers are some other fibers in use.

Extensive work was carried out in Rapid City, USA on basalt fibres and basalt reinforced concrete beams (Ramakrishnan & Panchalan 2004)(4). Braided or weaved FRP bars made of glass fibers are more economical, since these bars are made of Calcium Alumina Boro Silicates, which is much cheaper than both Carbon and Aramid fibers. The weight to volume ratio is a major advantage in all the fiber reinforced polymer bars. Based on the observations of the present investigations and available literature, the weight of the glass fiber bars is approximately one third of that of steel bars of

same diameter and length, while the tensile strength is comparable to that of steel (Braun & Eskridge, 2004)(5).

GFRP plain rods were also tried by P.J.Rao, et al (1) in their investigation on “Behavior of GFRP reinforced beams in Flexure”. All the beams failed much below the expected load because of failure in bond. In the same investigation they used silica coated bars in the beams which gave same flexural strength as that of conventionally reinforced beams using HYSD bars. Hence silica coated GFRP bars are directly used in the present investigations on slabs.

Luciano et al (6) used deformed GFRP bars to study the behavior of one way slabs. The

variables studied included the reinforcement ratio, rebar diameter and rebar spacing. A good comparision of experimental and analytical results was made. Due to the reduction in the stiffness of these bars they will undergo large deflections(7). Laterally restrained FRP slabs such as those in bridge deck slabs, exhibit arching action and show better service behavior compared to the equivalent laterally restrained steel reinforced slabs (8). The results of El-Ragaby et al (9) showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with GFRP composite bars compared to the steel reinforced ones.

Glass fibers are economical and lighter in weight compared to other fibers. Therefore, the

objective of the present study is to find out the suitability of GFRP bars as flexural reinforcement in slabs. Silica coated GFRP bars with fiber resin ratio of 7:3 with a tensile strength of 360MPa were used in the present investigation. The modulus of elasticity of these bars was found to vary between 55 GPa to 60GPa.

2. Experimental Program

The proposed experimental program was divided into number of phases using different support conditions and different ratios of long span to short span. In the present paper, the experimental work related to second phase with the ratio of long span to short span of 1.5 was reported. All the slabs are supported on all four edges.

A total number of 15 slabs were cast dividing into 3 groups. The thickness of the slab in all the

groups was 75mm. In the 1st group 5 slabs of size 1.05m x 0.75m were cast with an effective span of 0.9m x 0.6m. The reinforcement in both the directions (main and distribution) consists of 6mm silica coated GFRP bars @ 100mm c/c at bottom and 125 mm c/c above the bottom layer 3 numbers, and 2 control specimen using 6mm mild steel bars with 85mm c/c at bottom and 115mm c/c above the bottom layer were cast. The slab dimensions in the second group of 5 numbers are also of same size with an effective span of 0.9mx0.6m. The reinforcement in both directions consists of 6mm silica coated GFRP bars @ 150mm c/c at bottom and 190mm c/c above the bottom layer 3 numbers, and two control specimen using 6mm mild steel bars with 125mm c/c at bottom and 170mm c/c above the bottom layer were cast. The slab dimensions in the third group of 5 numbers are also of same size with an effective span of 0.9mx0.6m. The reinforcement in both directions consists of 6mm silica coated GFRP bars @ 200mm c/c at bottom and 250mm c/c above the bottom layer 3 numbers, and two control specimen using 6mm mild steel bars with 165mm c/c at bottom and 220mm c/c above the bottom layer were cast. In the control specimen using 6mm mild steel bars, the cover and lever arm have been so adjusted in all the 3 groups to give same theoretical moment as that of GFRC reinforced slabs.

All the slabs were under reinforced. The moment of resistance was calculated as per Bureau

of Indian Standards IS456:2000 and the guidelines given by ACI 440 for GFRP bars. The stress block of IS456 was used with little modifications.

3. Test Procedure

Uniformly distributed load was applied on the entire slab by using a steel box filled with sand covered with a thick plate at top and the load applied through a hydraulic jack. The proving ring used was of capacity 250kN with a least count of 0.25 kN. The load was applied at an increment of 2.5 kN and the central deflection of the slab was measured for each increment of the load. Least count of the

dial gauge used was 0.01mm. Dial gauge was removed immediately after the formation of the first crack.

4. Test Results and Discussions

The load carrying capacity of slabs reinforced with GFRP bars are almost equal to that of the corresponding control specimens. The experimental values in all the slabs are higher by 1 to 20 percent than the theoretical values (Table 1) of corresponding slabs. Similar trends were observed for moments also (Table.2).

The load vs deflection graphs depicts that the failure of slabs with GFRP bars is ductile

though the failure of GFRP bars in direct tension was observed to be sudden and brittle with a splintering type of failure. The margin between first crack load and the ultimate load observed also indicates the ductile behavior.

5. Conclusions

The load carrying capacity of slabs using silica coated GFRP bars as reinforcement is almost the same as compared to conventionally reinforced slabs with mild steel reinforcement. Failure loads and moments in slabs with silica coated GFRP reinforcement are higher than the theoretical values both for GFRP reinforced slabs and the conventionally reinforced slabs. Both GFRP reinforced slabs and conventionally reinforced control slabs showed almost equal deflections throughout. More definite conclusions about the usage of GFRP reinforcement in slabs, in the Indian context, can be made at the end of all the phases of present investigation.

Table 1. Test Results

Material (Reinforcement)

Effective Span (m) 0.9 x 0.6

Theoretical Ultimate load(kN)

Experimental Pu Pcr First Crack

Load Pcr (kN)

Ultimate Load Pu (kN)

GFRP 1 97.62 65.77 98.73 1.5 2 66.45 52.45 78.73 1.52

3 50.71 33.60 59.56 1.75

M.S 1 97.62 78.45 91.23 1.163 2 66.45 64.32 76.23 1.185 3 50.71 41.87 56.23 1.32

Table 2. Comparison of Theoretical and Experimental Moments

Material (Reinforcement)

Effective Span (m) 0.9 x 0.6

Bending Moments % Increase Theoretical

(kNm) Experimental

(kNm)

GFRP 1 3.655 3.712 1.5

2 2.488 2.947 18.48 3 1.898 2.23 17.45

M.S 1 3.655 3.685 1 2 2.488 2.854 12.84 3 1.898 2.105 9.88

References 1) P.J.Rao, K.J.Rao, N.V.R.C.Bala Bhaskar and M.V.Seshagiri Rao; “.Flexural Behaviour of GFRP

Reinforced Beams in Local Environment -An Experimental Study” Proceedings of the International Conference “ISEC-2008”, held at Melbourne, Australia during -----

2) IS 456-2000 Code of practice for plain and reinforced concrete, Beaureau of Indian Standards, New Delhi.

3) ACI 440 – Guide for The Design and Construction of Concrete Reinforced with FRP bars.

4) Ramakrishnan, V. & Ramesh, K. Panchalan 2004. A New Construction Material - Non Corrosive Basalt Rebar Reinforced Concrete, Proceedings of the International Conference ICACC-2004, 16-18 December 2004. Hyderabad: India.

5) J. Braun & M. Eskridge 2004. Concrete Reinforced with Glass Fiber Bars: Innovative and Cost-

Effective Solutions, Proceedings of ICFRC International Conference on Fiber Composites, High Performance Concretes and Smart Materials, January 8-10, 2004: 337-351. Chennai: India.

6) Luciano Ombres, Tarek Alkhrdaji, Antonio Nanni; “Flexural Analysis of One Way Concrete Slabs

Reinforced with GFRP Rebars”, International Meeting on Composite Materials, PLAST 2000, Proceedings, Advancing with Composites 2000, Ed.I. Crivelli-Visconta, Milan, Italy, May 9-11, 2000, pp 243-250

7) Theriault, M. and Benmokrane, B., “Effects of FRP Reinforcement Ratio and Concrete Strength on Flexural Behaviour of Concrete Beams”. Journal of Composites for Constructions, ASCE, 1998, Vol.2, No.1, pp. 7-16

8) Susan E. Taylor. and Barry Mullin., “Arching action in FRP reinforced concrete slabs”. Construction and Building Materials, 20 (2006), 71-80

9) Amr El-Ragaby, Ehab El-Salakawy. and Brahim Benmokrane. “Fatigue analysis of bridge deck slabs reinforced with E-glass/Vinyl ester FRP reinforcement bars”. Composites: Part B 38 (2007), 703-711

Fig. 1 Load Deflection Curvve ( Group 1)

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16 18 20

Deflection (mm)

Load(kN)

GFRP of 6mm @ 100mm (B), 125mm(T) (1)

GFRP of 6mm @ 100mm (B),125(t) (2)

GFRP of 6mm @ 100mm (B) ,125mm (T) (3)

control specimen of 6mm @ 85mm(B), 100mm (T)

Fig.2 Load Deflection Curve ( Group 2)

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18

Deflection(mm)

Load(Kn)

GFRP OF 6mm @ 150B,190T (1)

GFRP OF 6mm @ 150B,190T (2)

GFRP OF 6mm @ 150B,190T (3)

CONTROL SPECIMEN OF125B,170T

Fig. 3 Load Deflection Curve ( Group 3)

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14

Deflection(mm)

Load(Kn)

GFRP OF 6mm @ 200B,250T (1)

GFRP OF 6mm @ 200B,250T (2)

GFRP OF 6mm @ 200B,250T (3)

CONTROL SPECIMEN OF 165B.220T

Plate 1. Test Setup (Slab subjected to U.D.L)

Plate2. Failure pattern of GFRP slab with an effective span of 0.9m x0.6m with spacing of bars

150mm centre to centre on bottom and 190mm centre to centre on top.

Plate 3. Failure pattern of GFRP slab with an effective span of 0.9m x0.6m with spacing of bars

100mm centre to centre on bottom and 125mm centre to centre on top.

Plate 4. Failure pattern of control specimen with an effective span of 0.9m x0.6m with spacing of

bars 85mm centre to centre on bottom and 115mm centre to centre on top.

Plate 5 Failure pattern of GFRP slab with an effective span of 0.9m x0.6m with spacing of bars 200mm centre to centre on bottom and 250mm centre to centre on top.