Abstract: Steel is the

most common used material in reinforced Concrete in Civil Engineering

applications. But, because of the corrosive characteristics of steel, the

durability of the concrete cannot be ensured in Alkaline medium. In order to

prevent this drawback, Glass Fibre Reinforced Polymer can be proposed as an

alternative material to steel which has excellent properties such as high

strength, light weight, flexibility, and resistance to chemical harm. Because

of its flexibility, it can be preferred in concrete beams of curved structures

such as bridges, arches, roads, Water tanks, etc. But larger deflections can be

expected because of its low modulus of elasticity. As it is a new material to

the industry, the flexural performance should be tested especially for

curvature effects. This research addresses the finite element modelling of

curvature effects on flexural performance of GFRP concrete beams.

Keywords:

GFRP, FRP, FEM,

Flexure, Reinforcement Bars

1.

Introduction

Glass

Fibre Reinforced Polymer (GFRP) bars have been developed in recent times as an

alternative material to steel reinforcement in Civil Engineering applications.

This material is being successfully used as internal reinforcement bars in new

concrete structures which are exposed to corrosive environment.

Originally,

Fibre Reinforced polymer materials were introduced in aerospace and automotive

industries due to their superior strength and light weight. FRP is being used

as internal reinforcement in concrete structures in the forms of bars, pre

stressing tendons and rods.

When

a new material gets introduced to the industry, experimental testing on its

properties and failure modes should be carried out. But as it consumes more

time and cost effective simulation of a numerical model will be more effective

which can even give more results that cannot be obtained by experimental

studies.

The

purpose of this research is simulating a numerical model to obtain the

curvature effects on flexural performance of GFRP concrete beams. Due to the

low modulus of Elasticity of GFRP, it is possible to produce bars with large

radius curves. But this will induce bending stresses along the bar. And a

stress reduction can be also expected in the bent portion. So, when it will be

subjected to bending loads, failure modes will be more severe. Thus, the

flexural behaviour should be properly investigated.

This

research will utilize Finite element modelling using ABAQUS software by

implementing experimental results data for material properties.

2.

Manufacture of GFRP Reinforcing Bars

Many

manufacturing methods are being carried out to produce GFRP reinforcement bars

such as pultrusion, wet lay-up, pull-winding, filament winding and injection

modelling. But, Pultrusion manufacturing is the most typical method to produce

strong and light weight bars, rods and tendons. This is an automated

manufacturing method which doesn’t require much labours and is cost effective.

In

this method, glass fibres are pulled out from the spools through a device which

coats them polymer resin. Then, heat-treatment will be given for those fibres

and later it will be cut to the appropriate lengths. This is an ideal process

which can only produce constant cross of bars but in different shapes,

diameters, and tensile strengths. The bars which are produced in this method

will have smooth surface. Further, in order to improve the bond mechanisms,

various surface treatments will be applied between the concrete and bars.

3.

Material Behaviour of FRP bars

When FRP bars are used for reinforcement

bars in concrete structures, the fibres are oriented parallel to the bar’s

length and also continuous. Comparing to the transverse direction, this orientation

makes FRPs highly orthotropic with high strength and stiffness. The tensile

behaviour of FRP bars are linear-elastic up to failure. Thus, FRP bars rupture

don’t shows the plastic behaviour or yielding like steel reinforcement.

Therefore, it’s more brittle than steel bars.

As steel is considered as an isotropic

material, the compressive properties can be said equal to the tensile

properties. But, FRP is an anisotropic material, where the compressive strength

is significantly lower than the tensile strength. In the literature it is said

that the compressive strength of FRP bars are 35% of its tensile strength (Kobayashi &

Fujisaki, 1995). But some other

findings show that it is 80% of its tensile strength (Chaallal & Benmokrane, 1993). Other mechanical

properties such as ultimate tensile strength, ultimate strain and tensile

modulus of elasticity will vary according to their fibre types, manufacturing

process, resin matrix type and uses of additives. But, it has been proved that

the tensile modulus of elasticity of FRP is considerably lower than the steel. (Generally

less than 100GPa, steel- 200GPa). Comparatively it has also a lower axial

stiffness. As larger deflection can be expected, the design of concrete

structures with FRP reinforcement bars should be governed by Serviceability

limit states.

Other than

glass fibres, Carbon and Aramid fibres are also widely used for structural

applications. Figure 1 shows different types of FRP bars. It has been proved

that Carbon and Aramid FRPs has excellent modulus of elasticity, long term

behaviour, fatigue behaviour, alkaline resistance comparing to the GFRP. But

GFRP was found more economical comparing to the other FRPs. E-glass is the most

commonly used grade because of its low cost and modulus of elasticity ranging.

4.

Tests on GFRP reinforced

concrete beams

An experimental study of flexural

behaviour of GFRP concrete beams was carried out for 24 concrete beams with

type a and type b where type a is uncrack specimen and type b is specimen with pre-crack

in mid span (Barris C. ,

Torres, Turon, Baena , & Mias, 2008). A four-point

bending load test was conducted to evaluate the short-term serviceability

behaviour. The Strain behaviours were investigated and variation in neutral

axis depth along the cracking was also observed. It was also found that the

crack width is depended on the bond coefficient.

Another experimental study was conducted

on crack width of GFRP concrete beams and the cracking behaviour of 15 beams

were observed (Barris C. ,

Torres, Vilanova, Mias, & Liorens, 2016). The Digital Image Correlation

(DIC) technique was used to measure the crack widths, over the flexural zone.

It was found that the crack width in reinforced concrete flexural members

depend on the cover, bar spacing, bond stress between concrete and rebar, ø/?eff

ratio and strain level of the reinforcement.

An investigation was carried out on

Flexure-Shear Analysis of concrete beam reinforced with GFRP bars (Ramadass &

Job Thomas, 2010).

The influence of longitudinal reinforcement ratio, vertical reinforcement

ratio, and compressive strength of concrete, and shear strength of GFRP was

analysed. The nominal shear strength (Vn) of the concrete section can

be evaluated using equation (1), and the nominal shear strength resistance (Vn*)

corresponding to the flexural capacity of the beam subjected to concentrated

load can be computed by equation (2).

(1)

(2)

Where Vf is the shear resistance, Vc is

the shear strength of concrete section without stirrups Mn refers

the nominal moment of resistance of the section and a is the shear span. It was

found that the shear strength of the beam increases with the increase of

longitudinal and vertical reinforcement. The a/d ratio at failure mode change

also increase with the increment of

concrete length and amount of vertical reinforcement.

Another investigation

was done to strengthen the reinforced concrete beams with CFRP and GFRP. 3

control beams, 3 CFRP strengthened beams, and 3 GFRP strengthened beams were

totally casted and magnetized apparatus with Kinear Variable differential

transformer (LVDT) was used to measure the displacements.in this study, shear

edges of the beams were wrapped twice by CFRPs and GFRPs. Both materials have

shown a successful results in yielding. Even the corner rounding was

successful, double wrapping was not given a successful results (Onal, 2014).

Behaviour of reinforced concrete beams

reinforced with GFRP bars were also investigated to compare the strength,

reinforcement deformation, displacement, and some anchorage aspects between the

GFRP & steel-reinforced beam (Giongo, Paultre,

& Tavares, 2008). A four point bending test was

conducted and found longitudinal reinforcement stiffness is the m main

parameter which controls the behaviour of the reinforced concrete beams. It was

concluded that the design procedure carried out here was not able to ensure the

flexural capacity. But it was clearly found that GFRP reinforced beams to be

designed for serviceability limit state instead of ultimate limit state. Also

CFRP has shown a higher failure reduction than GFRP.

Another experimental test study was

conducted and three point bending test was carried out for simply supported

concrete beams reinforced with longitudinal GFRP bars and GFRP stirrups. A

finite element analysis was performed for this test. The objective was to investigate the

influence of longitudinal GFRP reinforcing bar’s arrangement on the effective

strength of GFRP stirrups. The strain behaviour of the shear stirrups was also

investigated. It was observed that when the shear reinforcement ratio

increases, the crack sizes at the peak load was also increased (Stoner, 2015).

5.

Finite Element Modelling of

Concrete Beams Reinforced with FRP

Ferreira et al (2001) has simulated a

model for the finite element analysis of concrete beams reinforced with GFRP longitudinal

reinforcement bars (Stoner, 2015). The formulated

model utilized two dimensional degenerated concrete shell elements based on a

first order shear deformation theory (Stoner, 2015). Several

experimental tests was conducted to the concrete beams and the experimental

results were analysed. A strong correlation between the experimental results

and the numerical model was found. The generated model was applicable to shells

and plates of arbitrary shapes.

Rafi et al (2007) performed a two

dimensional non-linear finite element analysis of simply supported concrete

beams reinforced with Carbon fibre reinforced polymer (CFRP) bars (Stoner, 2015). In this study, a smeared crack approach has

been modelled to analyse the tensile behaviour. The cracking criteria can be

explained by fracture energy (Gf) and Gf can be computed

using equation (3).

(3)

Where a = 80.6, n = 0.32 and ønmax

is the maximum size of aggregate. The non-linear analysis was performed by

incorporating the material models and element formation into the finite element

analysis software DIANA. An excellent numerical stability was observed and

strong correlation between the experimental results and numerical results was

found.

Demenico et al. (2014) proposed a finite

element-based limit analysis approach to predict the peak load and failure

mechanism of concrete members reinforced with FRP bars (Stoner, 2015). The proposed

analysis consists both Elastic Compensation Method (ECM) and the Linear

Matching method (LMM). The implementation of the ECM and LMM was conducted

using the Finite element software ADINA. Even the model was able to provide the

precise upper and lower boundary on the peak load, the model has not provided

accurate predictions for under reinforced beams. This was concluded as a

rupture of FRP bars which represented a brittle failure even the proposed

methodology has focused on the plastic behaviour. Because the brittle failure

of under reinforced concrete element is not considered in practical

applications, this limitation was acceptable.

6.

Conclusion

By now, many experimental studies and as

well as numerical studies has been carried out on GFRP concrete beams to

investigate their strength, flexural behaviour, shear behaviour, crack widths,

mechanical properties and comparative behaviour with other FRPs. The curvature

effects of GFRP beams have not been widely investigated till now. Even it has

been introduced to the structural applications because of its non-corrosive

behaviour, it has many limitations too. Especially, because of the low modulus

of Elasticity, excess deflections can be expected under bending. And as it is a

brittle material, it can be fail in rupture under bending.in most of the investigations,

the correlation between the experimental results and numerical model fit very

well. ABACUS, ANSYS, and ADINA were widely used Softwares for the finite

element modelling of GFRP beams in past studies.

Acknowledgements

It’s my privilege to express my

gratitude to my research supervisor Dr.J.C.P.H.Gamage for her support and

guidance for this research. I would like to extend my thanks to the instructors

who were very helpful and motivate. Also, I would like to thank my colleagues

who are doing the research with me.

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