Degradation of Glass Fiber Reinforced Concrete Due to Environmental effects

7/30/2019 Degradation of Glass Fiber Reinforced Concrete Due to Environmental effects

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Seminar report

On

Degradation of Glass Fiber Reinforced Concrete Due toEnvironmental Effects

SUBMITTED

TO

VIVESWARAIAH TECHNOLOGICAL UNIVERSITY

BELGAUM

FOR THE PARTIAL FULFILLMENT OF B.E.(CIVIL ENGINEERING)

BYDIVAKAR.M.K

Reg. No: -4AI06CV010

8st Semester

Under The Guidance of:

Dr. M. NAGESH

Department of Civil Engineering

ADICHUNCHANAGIRI INSTITUTE OF TECHNOLOGY(Affiliated To Visveswaraiah Technological University)

CHIKMAGULAR


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ADICHUNCHANAGIRI INSTITUTE OF TECHNOLOGYCHIKMAGULAR

CERTIFICATE

This is to certify thatMR. DIVAKAR.M.K. bearing university USN4AI06CV010

has submitted the seminar report on Degradation of Glass Fiber ReinforcedConcrete Due to Environmental Effects in partial fulfillment of the 8st semesterBE course as prescribed by the Visveswaraiah Technological University during

the academic year 2009-2010, under the guidance of Dr. M. NAGESH.

Dr. M.NAGESHDepartment of Civil Engineering


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ACKNOWLEDGEMENT

I express my deep sense of gratitude to Dr M.NAGESH, Department of CivilEngineering, AIT, for his guidance and help through out this seminar work.

I will remain thankful to all the faculty members of Department of Civil

Engineering, AIT for their support during the course of this work.

Finally I express gratitude to my parents, fellow students and friends.

DIVAKAR.M.K.

ADICHUNCHANAGIRI INSTITUTE OF TECHNOLOGY

CHIKMAGALUR


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Degradation of Glass Fiber Reinforced Concrete Due toEnvironmental Effects

Abstract:

In the past several years, fiber reinforced polymer (FRP) reinforcing bars have

been introduced as an alternative to steel reinforcing bars. Glass fiber reinforced

polymer bars are one of the widely used polymer bars. The properties of Glass

fiber reinforced polymer (GFRP) detoriate significantly with time when subjected

to environmental conditions. Water absorption often causes polymer swelling,

which can lead to stresses that result in debonding of the fibers from the matrix.The fibers themselves can suffer chemical attack. In the case of unidirectional

fiber-reinforced composites, though, as is the case here, the tensile strength in

the fiber direction is dependent predominantly on the fiber strength. Therefore,

there is an overall degrading in the properties of GFRP with time. The objective

of this paper is to study the degradation of characteristics of Glass FRP

reinforcing bars due to environmental effects with regard to the areas described.

- degradation of tensile strength

- direct shear capacity

- predicted deflections due to creep

- bond behavior and development length

- effects of thermal expansion on cracking of FRP reinforced concrete


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CONTENTS

Introduction

Fiber Reinforced Polymer Composite

Glass Fiber Reinforcement

GFRP Composite vs. Steel Reinforced Concrete

Deleterious effects of several environments on

fibers and matrices

Environmental effects on properties of GFRP bars

- Degradation of tensile strength

- Direct shear capacity

- Predicted deflections due to creep

- Bond behavior and development length

- Effects of thermal expansion on cracking of FRP

reinforced concrete


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INTRODUCTION

Concrete and steel were always considered the most important, and the most

commonly used structural materials. The development of new high-performance

composite materials that are stronger and more durable than conventional

materials (e.g., Portland cement concrete, steel, wood, and masonry) is

important to the construction industry.

FRP reinforcing bars and tendons are composite materials made up of

unidirectional fibers embedded in a polymeric matrix. The most popular fiber

materials used to make the bars are glass, aramid, and carbon. The most

commonly used polymeric resins are polyester, epoxy, vinyl ester, and polyimide.

Of particular interest are glass fiber reinforced polymers (GFRP) because they

have the lowest initial costs. The use of glass fiber reinforced polymer (GFRP)

bars, under research since the 1970s,to reinforce concrete is a promising

alternative to the use of steel reinforcement. GFRP bars exhibit lighter weight,

are non-conductive, exhibit high tensile strength, and have been reported to be

non-corrosive when compared with conventional steel reinforcement.

The use of glass fiber-reinforced polymer (GFRP) composites is becoming

increasingly common in construction, both in new construction, as in the use of

reinforcing bars employed to reinforce concrete bridge decks, and in the repair of

deteriorated structures, as in reinforcing concrete columns by wrapping with

GFRP layers. In each case, the benefits of GFRPs are well-recognized: high

strength-weight ratio; corrosion and fatigue resistance; ease of handling; and

ease of fabrication. Widespread utilization of these materials, though, is hindered

by the lack of long-term durability and performance data on which to base designcalculations. In addition, there is concern that the mechanical properties of a

hybrid material system may deteriorate much faster than that suggested by the

property degradation rates of the individual components making up the hybrid

system. This is an especially relevant concern since GFRP composites used in

infrastructure applications are intended to have a service life in excess of 50


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years. In view of these considerations, there is a need to make an extensive time

dependent analysis on the mechanical properties of GFRP.

Fiber Reinforced Polymer Composite:


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A composite is a mixture of two or more phases (materials).A better or unique

combination of properties is realized when different materials (or phases) are

combined. FRP is a two phase composite constituting of matrix andreinforcement.

Matrix :

It is the continuous phase and surrounds the reinforcements. It is made from

polymer. Its function is to

Bind the reinforcements (fibers/particulates) together

Transfer load to the reinforcements

Protect the reinforcements from surface damage due to abrasion or

chemical attacks.

High bonding strength between fiber and matrix is important.

Reinforcement :

The term reinforcement implies some property enhancement.

Itis the dispersed phase, which normally bears the majority of stress. Different

types ofFibres or Filaments are continuous fibres, discontinuous fibres and

whiskers. They are all easy to break (low ductility).


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Types of commonly used Fiber Reinforced Polymer Composite are:

Glass fiber-reinforced polymer composites (GFRPs)

o Most common fiber usedo High strength

o Good water resistance

o Good electric insulating properties

o Low stiffness

.

Carbon fiber-reinforced polymer composites (CFRPs)

o Good modulus at high temperatures

o Excellent Siffness

o More Expensive than glass

o Brittle


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o Low electric insulating properties

Aramid fiber-reinforced polymer composites (AFRPs)

o Superior resistance to damage (energy absorber)

o Good in tension applications (cables, tendons)

o Moderate Stiffness

o More Expensive than glass

Typical Properties Of Continuous and Aligned GFRP,

CFRP, AFRP

1 psi = 6.895kPa


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GLASS Fiber Reinforcements

Glass fiber reinforcements are classified according to their properties. At present

there are five major types of glass used to make fibers. The letter designation is

taken from a characteristic property:

A-glass is a high-alkali glass containing 25% soda and lime, which offers

very good resistance to chemicals, but lower electrical properties.

C-glass is chemical glass, a special mixture with extremely high chemical

resistance.

E-glass is electrical grade with low alkali content. It manifests better

electrical insulation and strongly resists attack by water. More than 50% of

the glass fibers used for reinforcement is E-glass.

S-glass is a high-strength glass with a 33% higher tensile strength than E-

glass.

D-glass has a low dielectric constant with superior electrical properties.

However, its mechanical properties are not so good as E-or S-glass. It is

available in limited quantities.


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GFRP vs. Steel Reinforced Concrete

Modulus of Elasticity

Glass Fiber reinforced polymer (GFRP) bars have lower modulus of elasticity

than steel bars(fig 1). For this reason when GFRP bars are used as flexural

nonprestressed reinforcement in concrete sections, the stress in the GFRP is

limited to a relatively small fraction of its tensile strength. This limit is necessary

to control width of cracks in tension zone at service. In GFRP reinforced sections

crack patterns at lower load levels are similar to those of steel-reinforced

sections, but as loads increase beyond the service level, crack spacing

decreases and crack width increases relative to steel reinforcement.

Fig- 1

Creep and Shrinkage


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Creep and shrinkage behavior in GFRP-reinforced members is similar to that in

steel-reinforced members.

Shear Strength

The concrete contribution to shear strength is reduced in beams with GFRP

longitudinal reinforcement because of smaller concrete compression zones,

wider cracks, and smaller dowel forces. A reduction factor proportional to the

modular ratio, EFRP /Esteel, is typically applied to concrete shear contribution

equations for conventional beams, although such an approach underestimates

the shear strength in flexural members with larger amounts of GFRP longitudinal

reinforcement.

Chemical Attack

GFRP bars have been reported to be non-corrosive when compared with

conventional steel reinforcement. As a result of this, GFRP bars have been used

as the main reinforcement for prestressed concrete bridge beams, bridge decks,

tunnel linings, waterfront structures and buildings near waterfronts. The mostobviousadvantage for bridge deck applications is that they are non-reactive to

chlorides.Although GFRP bars do not corrode, they do experience a loss of

strength with time, particularly in an alkaline environment, such as concrete.

Tensile Strength

GFRP bars have higher strength ffu, than the specified yield strength fy of steel

reinforcing bars. However, because the modulus of elasticity of GFRP bars, Ef, is

lower than that for steel, Es, the higher strength cannot be effectively exploited.

Stress-Strain Behavior


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The stress-strain behavior ofGFRP bars is linear elastic to failure, with no yield

plateau. This requires a different design approach than for concrete reinforced with

mild steel bars, in which proper design ensures that the steel yields before the

concrete crushes. With GFRP reinforcement, designs in which the concrete crushes

before the GFRP ruptures are encouraged.

Thermal Conductivity

GFRP materials have relatively lowerthermal conductivity than steel. Hence the

temperature gradient between the surfaces of GFRP composite structures is

lower than the conventional steel reinforced structures.


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Deleterious effects of several environments on fibers andmatrices

Since our study is on the time dependent deformation on the properties of GFRP

bars when exposed to environmental conditions, it is very much necessary to

study effects of environment on them.

Water: Polymeric fibers and matrices absorb moisture. Moisture absorption

softens the polymers. There is not sufficient data for the rate of deterioration of

carbon and glass fibers.

Weak acids: Bridges in industrialized areas may be exposed to weak acids from

acid rain and carbonization, with pH values between 4 and 7. Weak acids can

attack glass fibersand polyester matrices.

Strong acids: Accidental spillage may cause strong acids to come in contactwith bridge components. Strong acids can attack glass fibers, aramid fibers and

polyester and epoxy matrices.

Weak alkalis: Concrete containing pozzolans can have pH values between 7

and 10. Weak alkalis can attack glass fibers and polyester matrices.

Strong alkalis: Typical portland cement concretes have pH values greater than

10 and can cause degradation of glass fibers. Strong alkalis can attack glass

fibers, aramid fibers, and polyester matrices.

High temperatures: Carbon and glass fibers are resistant to high temperatures.

However, high temperatures adversely affect aramid fibers and polymeric

matrices.


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Ultraviolet radiation: Carbon and glass fibers are resistant to ultraviolet

radiation. However, ultraviolet radiation adversely affects aramid fibers and

polymeric matrices.

Environmental effects on properties of GFRP bars

In this presentation the characteristics of Glass FRP reinforcing bars is studied

with regard to the areas described as follows.

- Degradation of tensile strength

- Direct shear capacity

- Predicted deflections due to creep

- Bond behavior and development length

- Effects of thermal expansion on cracking of FRP reinforced concrete

Specifications of GFRP Reinforcing Bars used:

GFRP bars with diameters of 0.5 in., 0.625 in., and 0.75 in. provided by three

different manufacturers were used in the experiments. The bars containedapproximately

70 percent of unidirectional glass fibers by volume and 30 percent resin. The

bars are identified as bar P, V1, and V2.

Bartype P is made with polyethylene terephthalate (PET) polyester matrix, and

E-glass fibers. Bar type P has a noncircular cross section due to the impression

of surface lugs. The surface of bar type P was finished with lugs and had no sand

coating. Figures below show the surface and cross section of each bar type .

Bar type V1 contains E-glass fibers embedded in a vinyl ester resin. Bar type V1

is made with external helical fiber wrapping with an average spacing of 1.04 in.

The surface of the bar is coated with fine sand.


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Bar type V2 is composed of E-glass fibers embedded in a vinyl ester resin. Bar

type V2 has a circular cross section and is coated with coarse sand. Figures 2

and 3 show the surface and cross section of this bar type.

Fig. 2Cross-section of GFRP bar types P, V1, and V2


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Fig. 2Surface of GFRP bar types P1, V1, and V2

Degradation of tensile strength

It is well known that polymers absorb moisture and the polymer matrix softens as

a result of this moisture absorption. Because the pore humidity in concrete

seldom drops below 72 percent, GFRP bars are continually exposed to a moist

environment. Beginning with the onset of hydration, concrete exhibits a high pH,

usually between 12 and 12.5. In addition, glass fibers have been reported to

deteriorate in alkaline environments.As a result, the mechanical characteristics

of GFRP bars embedded in concrete would be expected to change over time,

since concrete is a moist environment with a high pH solution. Although the

fundamental reason for implementing the use of FRP bars has been to eliminate

conventional steel reinforcement deterioration due to corrosion, FRP bars,

especially GFRP bars, could exhibit significant loss of tensile strength.


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The tensile strength of GFRP bars degrades with time while in contact with

simulated concrete pore solution. The overall average tensile strength

reductions were 1 percent at 26 weeks and 7 percent at 50 weeks. The Youngs

modulus of the GFRP bars tested had a tendency to increase with time. The

overall average increase of the Youngs modulus was 1 percent at 26 weeks and

9 percent at 50 weeks. The tests results can be used to make predictions for

exposure periods similar to those studied.

After the test 18 circular markers corresponding to 3 bar types with 6 exposure

conditions each were drawn for the times of 26 and 50 weeks (fig 4). An overall

average value of the relative tensile strength was obtained using the averagevalues from the relative tensile strength of each of the exposure conditions

mentioned. The middle line shown in the Figure 4 shows the location of the

overall average relative tensile strength. The overall average tensile strength

values are 0.99 at 26 weeks and 0.93 at 50 weeks. The top line in Figure 4

connects the maximum relative tensile strengths observed from all of the

specimens tested for each test time. The maximum relative tensile strength

values observed were 1.09, 1.13, and 1.09 at 0, 26, and 50 weeks of exposure,

respectively. The lower line in the Figure 4 connects the minimum relative tensile

strengths observed from all of the specimens tested for each test time. The

minimum relative tensile strength values observed were 0.86, 0.86, and 0.76, for

0, 26, and 50 weeks of exposure, respectively. The overall standard deviations

for the relative strength at 0, 26, and 50 weeks were 0.054, 0.063, 0.070,

respectively. The overall coefficients of variation at 0, 26, and 50 weeks were

0.054, 0.063, and 0.075, respectively.


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Fig. 4 Summary of tensile strength results.

DIRECT SHEAR STRENGTH OF GFRP

Structural elements without shear reinforcement such as bridge decks or slabs

are other applications where GFRP bars may fail in direct shear or a combination

of tension and direct shear.

GFRP bars were tested at a constant load rate in direct shear. The degradation

over time of the direct shear strength of GFRP bars was determined by testing

specimens previously exposed to different solutions in uncapped ends

conditions. Bar types P, V1, and V2 were exposed for 51, 71, and 71 weeksrespectively.

Figure 5 shows a comparison of the direct shear strength results. The ordinate

indicates the shear strength results after exposure relative to the unexposed


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shear strength. It can be observed that the most severe condition is the high pH

exposure. In this case the direct shear strength reductions amount to 2, 4, and 9

percent of the original shear strength values for bar types P, V1, and V2,

respectively. All silicate glasses become especially susceptible to decomposition

when in contact with a solution with pH values higher than approximately 9 or 10.

Thus, larger shear strength degradations are expected to occur in GFRP bars

exposed to high pH solutions. Nevertheless, a shear strength increase of

approximately 3 percent relative to the original value was recorded for bar types

P after 48 weeks of exposure to distilled water, and a relative increase of

approximately 2 percent was measured in bar type V1 after 68 weeks of

exposure in a high pH solution with chlorides. The apparent shear strength

increase may result from the high variability of the mechanical properties of

GFRP bars.

In the case of a unidirectional composite subjected to shear forces, failure may

occur by matrix shear failure, matrix shear failure with fiber debonding, fiber

debonding or shear rupture of fibers. Figures below illustrate the failure modes of

bar types P, V1, and V2, respectively.


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Failure of bars type P.

combination of constituent debonding and shear rupture of the matrix and the

glass fibers failed primarily by direct shear, that is to say, mostly by matrix

shear failure and fiber rupture

Failure of bars type V1.


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Failure of bars type V2.

It can be observed in the Figures that bar types P and V2 tend to fail by a

combination of constituent debonding and shear rupture of the matrix and the

glass fibers. Bar types V1 failed primarily by direct shear, that is to say, mostly by

matrix shear failure and fiber rupture as illustrated in the Figure.

Predicted deflections due to creep

Creep can be defined as the increase in length of a bar loaded with a constant

force over time, beyond the initial (elastic) deformation.

Six GFRP bars were placed in creep frames located inside a controlled-

environmental room with an average temperature of 88 F and a relative humidity

of 67 percent for a period of 6 months. The specimens were loaded to simulated

service load conditions and a load equivalent to approximately 23 percent of the

ultimate tensile load of the GFRP bars was applied to the specimens. The load

applied to the bars was 6,900 lb. The bars used in the test had a diameter of

0.625 in. Thus, the stresses present in bars were 20,110 psi, 20,710 psi, and

20,170 psi, for specimens with bar types P, V1, and V2, respectively. These

stresses correspond to 24, 23, and 27 percent of the ultimate strength of the bar

types P, V1, and V2, respectively.


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From the data shown in Table, it can be determined that specimens

manufactured without fibers wrapped round the bar (P and V2) showed an

increase in strain beyond the initial elastic strain on the order of 2 percent. This

value was computed by dividing the creep strain by the elastic strain. Specimens

V1, which were manufactured with fibers wound around the bar, exhibited an

increase in strain beyond the initial elastic strain on the order of 6 percent over

the 6-month period.

Hence GFRP bars can creep between 2 and 6 percent over six months, when

stressed at about 23 percent of the ultimate strength of the bar.

Creep strain of GFRP bars at six months


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Bond behavior and development length

A set of specimens was exposed outdoors and another set was exposed indoors

under high temperature and high humidity conditions. During planning of the test

it was believed that a continuously moist and hot environment would accelerate

bond degradation to a measurable degree, if any occurred, between the FRP

bars and concrete. Thus, some specimens were left outdoors and some were

exposed in the controlled conditions, both for a period of 16 months.

Tables below contain the results of specimens that failed by bar

rupture as well as by pullout.


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Average bond stresses at failure for the 0.5 in. diameter bars

Average slip at loaded end of 0.5 in. diameter bars at failure


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Results of the test program indicate

that a continuously wet concrete

environment may degrade the bond

properties of GFRP bars more than

an outdoor exposure, with slippage at

failure load increasing by as much as

30 percent after 16 months of

exposure. This is important because

any bond strength degradation

increases the required development length of a reinforcing bar. Since the

specimens exposed to a high moisture environment showed more bond strength

degradation, it is expected that reinforced concrete elements exposed to high

moisture environments, such as beams and decks of a bridge spanning a body of

water or bridge piers founded under water, could exhibit higher bond strength

degradation.


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Effects of thermal expansion on cracking of GFRPreinforced concrete

The high transverse coefficient of thermal expansion of FRP bars causes

concern for cracking in slabs. As such, it is important to know the depth of a safe

concrete cover to be used with FRP bars in concrete for the typical bar sizes and

temperatures expected to develop in a concrete structure. FRP bars can undergo

expansions or contractions without stressing the concrete before the concrete

has set. However, once the concrete sets, tensile stresses develop in the

concrete as a result of the differential thermal expansion between the FRP bars

and the concrete, at temperatures above the temperature present in the concrete

at setting

The setting temperature of the specimens was assumed to occur at 95 F. After

the heat lamps were turned on, the specimen was visually monitored

continuously for surface cracking.

For P bar types, cracks were observed for bar cover depths of 1 in.,

2 in., and 3 in. These cracks were observed on the surface of the concrete

specimen over the bars with cover depths of 1 in. when the temperature on the

surface of the center bar was 239 F. This indicates that an increase of 144 F

from the setting temperature is required to cause cracking. The temperature on


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the surface of the slab when the specimen cracked for the 1 in. cover depth

samples was 292 F.

Cracks on the specimen with the 2 in. cover depths occurred

when the temperature at the surface of the center bar was 206 F. Thus, a

temperature increase of 111 F at the level of the reinforcement was required to

crack the concrete over the FRP bar. The temperature at the surface of the slab

when the specimen cracked on the 2 in. cover depth sample was 292 F.

Some small cracks were observed on the surface of the slab

over the 3 in. cover. The temperature recorded at the depth of the bars was 168

F. This represents a temperature rise of 73 F from the setting temperature. The

temperature on the surface of the slab when the specimen cracked for the 3 in.

cover depth was 292 F.

The experimental results indicated that a typical 8 in. thick concrete bridge deck

reinforced with GFRP bars would not experience cracking on the surface due to

thermal expansion for concrete covers of 1, 2, and 3 in. and GFRP reinforcement

with a diameter 0.75 in. or smaller. This assertion would be valid for conditions

where a temperature rise of less than 54 F from the concrete setting

temperature takes place and the concrete compressive strength is 5880 psi or

higher.

Specimen with bars type P after testing


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Conclusion

FRP composites have many excellent structural qualities and some examplesare high strength to weight ratio, material toughness, and fatigue endurance.

Other highly desirable qualities are high resistance to elevated temperature,

abrasion, corrosion, and chemical attack.

Although FRP reinforcing bars may provide potential benefits for the performance

of reinforced concrete structures, some key issues are needed to be considered

while designing of GFRP reinforced concrete elements. These issues are the

evaluation of the tensile strength degradation of GFRP bars with time after

exposure to simulated concrete pore solutions, the evaluation of the deterioration

of the direct shear strength of GFRP bars exposed to simulated concrete pore

solutions, estimation of the creep induced deflections of GFRP reinforced

concrete elements, study of the degradation of the bond strength between GFRP

barsand concrete, and evaluation of the cracking of GFRP reinforced concrete

elements dueto thermal expansion.


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FUTURE WORK

An investigation that exposes GFRP bars over longer periods of time, preferably

under different stress levels, is required in order to make reliable, long-term

residual tensile strength predictions and to obtain adequate environmental

strength reduction factors.

Additional long-term creep tests on FRP bars reinforced with glass, aramid, andcarbon fibers with different bar diameters and under different stress levels are

necessary.

The degradation of the bond strength between concrete and GFRP bars

needs to be investigated further. Experimental research is required in this area,

especially long term bond strength tests in order to determine whether the bond

strength of GFRP bars degrades faster than their tensile strength.


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REFERENCES

1. Francisco Aguiniga Goana, Characterization Of Design Parameters For Fiber

Reinforced Polymer Composite Reinforced Concrete Systems, Research Report, Dec2003.

2. Nawy, E. G., and Neuwerth, G. E., Behavior of Fiber Glass Reinforced Concrete

Beams,Journal of the Structural Division, ASCE, V. 97, No. ST9, September1971, pp. 2203-2215.

3. Michaluk, C. R., Rizcalla, S. H., Tadros, G., and Benmokrane, B., FlexuralBehavior of One-Way Concrete Slabs Reinforced by Fiber Reinforced Plastic

Reinforcements,ACI Structural Journal, V. 95, No. 3, May-June 1998, pp. 353-365.

4. Theoretical and Experimental Analysis of GFRP Bridge Deck under Temperature

Gradient, ASCE Journal, 24 February 2006.

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Degradation of Glass Fiber Reinforced Concrete Due to Environmental effects