Fiber reinforced polymers are successful alternative that have numerous advantages over traditional reinforcement methods giving structures a longer service life. The glass fiber reinforced polymer has significantly enhances the strength and ductility of concrete by forming perfect adhesive bond between concrete and the wrapping material. The research work on FRP confined circular column under the axial and eccentric compression loading has been done in past few years. In this project, behaviour of concrete columns confined by the GFRP (Glass Fiber Reinforced Polymers) jacket was investigated through analytical method. For this purpose, total 12 rectangular columns of different cross-section were taken for analysis. In this dissertation, 12 columns divided in to three groups of two, two, and eight columns. There are three groups of column first and second groups were reference columns. The third group consist of four subgroups depending on aspect ratio of rectangular shape of column viz. 1:1.25, 1:1.5, 1:1.75, and 1:2. Columns were modified in shape by using segmental cover. Each subgroup consist of two columns. The height of all columns were 700 mm. Comparative study using finite element based software was carried out with previous available literature.
Keyword: Ductility; Eccentricity; GFRP Confinement; High strength; Rectangular columns.
Most of the previous studies reported so far investigation the behaviour of concentric load as well as eccentric load. This paper investigates further in this direction with addition of shape modification.
Mr. P. B. KodagDepartment of Civil Engg.,Sinhgad College of Engineering,
Experimental investigation carried out by Rahul Raval and Urmil Dave 1 on effectiveness of GFRP wrapped square, rectangular and circular column. In results they were concluded GFRP shows increment in load carrying capacity. Square and rectangular section columns were found to experience lesser increment in strength as compared to that of circular columns. Riad benzaid et.al 2 done experimental research on behaviour of square column confined with GFRP flexible wrap. The parameters which were considered in this experiment were number of composite layers and the corner radius for square shape. They concluded that columns with GFRP wrap increase load carrying capacity, more axial load carrying capacity & increase column ductility.
M.N.S. Hadi et.al 3 explores new technique to maximize GFRP efficiency. They use GFRP confinement for square concrete columns, circularisation using segmental circular concrete covers. After experimental study researchers concluded that GFRP confinement reduce ultimate load coming on column. Steel stripe provide alternative for concrete confining. M.N.S. Hadi 4 has done another experimental work on FRP wrapped columns under eccentric loading. Researcher used three groups one with reference and another two represent CFRP and GFRP. Author found that FRP composite can significantly increase the strength of concrete column. Both concentrically and eccentrically, the CFRP wrapped columns resulted in higher loads and ductility compared to GFRP wrapped and steel reinforced columns.
M. N. S. Hadi 5 did experimental work using concrete segmental cover on square columns. He used two reference columns group one normal square column and all four corner round column. Other two groups consist CFRP wrapped and steel strip columns. At end, author concluded that all confined specimen showed high load carrying capacity compared with unconfined specimen. Among all three group CF achieved highest load carrying capacity followed by group SF.
Problem with CFRP is high cost, CFRP cost almost 6 times greater than GFRP sheets. The studies under taken in the past concentrated on studying FRP confined columns subjected to concentrated loads and also most of the studies have been limited to circular cross-sections and square columns. In view of this scarcity our goal is to study the behaviour of GFRP wrapped rectangular columns having different cross-section subjected to eccentric loading.
2 GLASS FIBER REINFORCED POLYMER
GFRP (Glass Fiber Reinforced Polymers) material is a type of composite material that is increasingly used in the construction industry in recent years. Due to their light weight, high tensile strength, and corrosion resistance and easy to implementation makes these material preferred solutions for strengthening method of reinforced concrete structural elements, typical form shown in Fig. 1
Fig 1 Typical form of glass fiber2.1 Advantages of GFRP
FRP composites are different from steel in that they possess properties that can vary in different directions (anisotropic) whereas steel has similar properties in all directions (isotropic).
GFRP have many other superior properties over conventional materials like: ultra-high strength, corrosion resistance, light weight, high fatigue resistance, nonmagnetic, high impact resistance, and durability.
2.2 Disadvantages of FRP
Due to the moisture effect matrix softening occur which leads to hydrolysis cause reduction dominant properties of composite, such as shear strength, stiffness, and mechanical properties.
Due to change in temperature mechanical properties of composite decreases, hence at low temperature premature brittle failure observed in structural members.
2.3 Applications of FRP
GFRP systems provide a very practical tool for lateral strengthening and retrofit of concrete structures, and are appropriate for:
Column confinement and ductility improvement
2.4 Type of GFRP
E-glass – Alkali free, highly electrically resistive glass made with alumina-calcium borosilicates. E-glass is known in the industry as a general-purpose fiber for its strength and electrical resistance.
S-glass – High strength glass made with magnesium aluminosilicates. Used where high strength, high stiffness, extreme temperature resistance, and corrosive resistance is needed.
C-glass – Corrosive resistant glass made with calcium borosilicates. Used in acid corrosive environments.E glass fiber is the most commonly used fiber in the fiber reinforced polymer composite industry. In this work E-glass fiber properties are used. Table 1 shows properties of E glass fiber.
E-GFRP sheets properties
1 Thickness(mm) 0.43
2 Tensile strength(Mpa) 2060
3 Modulus of elasticity(Gpa) 70-90
4 Density(gms/m3) 900
5 Young’s Modulus of Elasticity(N/mm2) 75,900
6 Specific gravity 2.56
Table 1 Properties of E-GFRP sheets
3 Numerical Modelling
The computation of loads acting on body and deformation caused due to that body gives us the collapse analysis of that body. In current project finite element model is developed to know the behaviour of innovative rectangular column under compressive loads as well as eccentric loads. Eccentric lode applies at 40 mm from the center of column. The results obtained from finite element model are compared with experimental tests. The FE model was generated using modelling software and finite element software.
For these modeling 12 short columns divided in to three groups of two, two, and eight columns is carried out. There are three groups of column first and second groups consist reference columns. The third group consists of four subgroups depending on aspect ratio of rectangular shape of column viz. 1:1.25, 1:1.5, 1:1.75, and 1:2.
3.1 Model Making
For this work various parts of columns created separately and assemble together in creo software. There are four model parts generated includes bar, stirrups, concrete box and GFRP outer cover.
1 Reinforcing bar has 10 mm diameter and 700 mm long. Material used for bar is steel fe415. Here 415 is minimum yield strength in N/mm2. Fig 2 shows reinforcing bar part
Fig 2 Reinforcing part
2 Stirrups used having diameter 6 mm but length of stirrups vary with aspect ratio and similar steel properties as bar. Stirrups paced in longitudinal direction of columns having 200 mm c/c distance. Fig 3 show stirrups
Fig 3 Stirrups
3 Concrete box this is made up with properties of concrete. M20 grade concrete properties are used. During modeling inside concrete material remove to create space for insert bar and stirrups when assemble together.
Fig 4 Concrete box
4 Outer GFRP cover is small thickness part having 0.43 mm thickness. Length varies with aspect ratio and this part use as outside cover to strengthening columns.
Fig 4 GFRP cover
3.2 Columns Assembly
1 Reference columns first group has two rectangular columns sizes 120 x 150 x 700 mm are generated in modeling software. There is no additional segmental concrete of GFRP cover are used. Fig 2 shown rectangular column.
Fig 5 a) Top view b) Front view of rectangular column
2 In second group two rectangular columns all four edges are rounded. Columns size 120 x 150 x 700 group columns. No additional cover provided in these columns.
Fig 6 a) Top view b) Front view round edge rectangular column
3 In third group having four subgroups size and segmental cover varies with columns size.
First subgroup consist two columns having aspect ratio 1:1.25. Columns size 120 x 150 x 700 mm with segmental concrete cover 12 mm at shorter side and 20 mm at larger side are provided. One layer GFRP wrap having thickness 0.43 mm wrapped around columns periphery.
Fig 7 a) Top view b) Front view 120 x 150 rectangular column with cover and GFRP
Second subgroup consist two columns having aspect ratio 1:1.5. Columns size 120 x 180 x 700 mm with segmental concrete cover 12 mm at shorter side and 20 mm at larger side are provided. One layer GFRP wrap having thickness 0.43 mm wrapped around columns periphery.
Fig 8 a) Top view b) Front view 120 x 180 rectangular column with cover and GFRP
Third subgroup consist two columns having aspect ratio 1:1.75. Columns size 120 x 220 x 700 mm with segmental concrete cover 12 mm at shorter side and 25 mm at larger side are provided. One layer GFRP wrap having thickness 0.43 mm wrapped around columns periphery.
Fig 9 a) Top view b) Front view 120 x 210 rectangular column with cover and GFRP
Fourth subgroup consist two columns having aspect ratio 1:2. Columns size 120 x 240 x 700 mm with segmental concrete cover 12 mm at shorter side and 25 mm at larger side are provided. One layer GFRP wrap having thickness 0.43 mm wrapped around columns periphery.
Fig 10 a) Top view b) Front view 120 x 240 rectangular column with cover and GFRP
4 ExperimentationThere are two type loading taken in to consideration, first one axial loading and another one eccentric loading. For this work all modeling software file save in to IGS and stp format then this file import in to finite element software. After importing geometry each part has need to specific material properties. Concrete and steel properties available but there are no GFRP properties so create new material and all GFRP properties include in outer cover. Pressure applies at top surface area of column while bottom surface consider as fix. Eccentric loading applies 40 mm apart from the center of column. Use split command and draws line and applies load on that line to get eccentric loading.
1 Rahul Raval and Urmil Dave, “Behavior of GFRP wrapped RC Columns of different shapes”, Elsevier Procedia Engineering, 51 (2013), pp-240-249. 2 Riad Benzaid, Nasr?Eddine Chikh ; Habib Mesbah, “Behavior of square concrete column confined with GFRP composite warp”, Elsevier Procedia Engineering, 54 (2013), pp-365-376. 3 Lei, X., Pham, T. M. ; Hadi, M. N. S., “Behaviour of CFRP wrapped square RCcolumns under eccentric loading”, Structural Engineering Conference, pp-1-8, Australia.
4 Muhammad N.S. Hadi “Comparative study of eccentrically loaded FRP wrapped columns”, School of Civil, Mining and Environmental Engineering, University of Wollongong, NSW 2522, Australia
5 Muhammad N. S. Hadi, M.ASCE1; Thong M. Pham2; and Xu Lei3″New Method of Strengthening Reinforced Concrete Square Columns by Circularizing and Wrapping with Fiber-Reinforced Polymer or Steel Straps” journal of composites for construction © asce / march/april 2013 / 229