matter final

Bond durability of FRP bars Embedded in Fiber- Reinforced Concrete

CHAPTER – 1

1. INTRODUCTION

Composite materials offer many advantages when compared with

conventional steel reinforcement, such as high tensile strength, light weight, and

corrosion free. Their application in civil engineering has become more technically

attractive and economically viable. The last few decades have been marked by

degradation of numerous concrete structures due to the corrosion of steel

reinforcements that required costly repairs or replacements. To mitigate the corrosion

problem, several methods, such as epoxy coated rebars, synthetic membranes, or

cathodic protection, have been developed. To date, all of them have showed limited

success . In recent years, research has been carried out on fibre reinforced plastic

(FRP) bars as an alternative to steel reinforcement. These FRP rebars have already

shown a promising future to overcome the corrosion problem in many projects,

especially in bridge deck and parking garage design.

Short polypropylene fibres provide resistance to plastic and drying shrinkage,

and improve resistance to crack growth, impact loading, fatigue loading and freeze-

thaw durability. It was proven to have notable benefits to structures, especially under

service conditions. The combination of FRP reinforcement and short polypropylene

fibres may eliminate problems related to corrosion of steel reinforcement while

providing requisite strength, stiffness and desired ductility, which are shortcomings of

the plain concrete and FRP reinforcement system.

Considerable research efforts have been conducted on the bond behavior of

glass fiber reinforced plastic (GFRP) rebar in plain concrete. Different types of

FRP rebars have quite different bond characteristics, which are strongly dependent on

mechanical and physical properties of external layer of FRP rods.

Meanwhile, research has indicated that FRP materials are not immune to

Department Of Civil Engineering 1


long-term environmental degradation. Long-term performance must be investigated

and clearly understood before it can be widely applied to the field. This paper presents

a report on the long-term bond performance for this FRP/FRC hybrid system. For

comparison purposes, the corresponding bond performance of the FRP/plain concrete

system is also reported. The objective of this study was to qualitatively investigate the

benefits gained from fibres in the FRP reinforcing system. Thus, the different volume

fraction of fibre’s effect was not investigated in this research.

On the basis of the previous researches, it is still not clear whether the bond

degrades, or to what extent, after being subjected to various environmental agents.

Furthermore, in many regions of the world, it is not uncommon to experience freeze-

thaw cycles in the winter season and high temperature with high humidity in the

summer season. This paper provides insight to how the bond behaves after the long-

term environmental conditioning.

1.1 WHAT IS FIBER REINFORCED POLYMER?

Fiber reinforced polymer (FRP) is a composite that is a combination of two or

more materials to form a new and useful material with enhanced properties in

comparison to the individual constituents.

FRP’s consists of fibers and matrix. The matrix acts as the binder for FRP. The

various functions of matrix are

i) It binds the fibers together

ii) Protects the fibers from environmental degradation

iii) Transfers force between the individual fibers

iv) Provides shape to the FRP component.

Regardless of the fiber used in the FRP reinforcing bar, it is the resin matrix that plays

the major role in transferring forces from the surrounding concrete into the reinforcing

bar, and this resin matix is potentially susceptible to degradation at the concrete

interface . Once this degradation occurs , bond capacity can be reduced or lost

completely and in addition the reinforcing fibers themselves can be exposed to attack

leading to loss of the longitudinal strength and stiffness of the reinforcement.



1.2 WHAT IS FIBER REINFORCED CONCRETE?

Fiber Reinforced concrete can be defined as a composite material consisting of

mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly

dispersed suitable fibers. Some of the fibers used are steel fibers, polypropylene,

nylons, asbestos, coir, glass and carbon.

1.2.1 Effect of Fibers in ConcreteFibers are usually used in concrete to control plastic shrinkage cracking and

drying shrinkage cracking. They also lower the permeability of concrete and thus

reduce bleeding of water. Some types of fibers produce greater impact, abrasion and

shatter resistance in concrete. Generally fibers do not increase the flexural strength of

concrete, so it cannot replace moment resisting or structural steel reinforcement.

Some fibers reduce the strength of concrete. The amount of fibers added to a concrete

mix is measured as a percentage of the total volume of the composite (concrete and

fibers) termed volume fraction (Vf). If the modulus of elasticity of the fiber is higher

than the matrix (concrete or mortar binder), they help to carry the load by increasing

the tensile strength of the material. Increase in the aspect ratio of the fiber usually

segments the flexural strength and toughness of the matrix. However, fibers which are

too long tend to "ball" in the mix and create workability problems. Some recent

research indicated that using fibers in concrete has limited effect on the impact

resistance of concrete materials. This finding is very important since traditionally

people think the ductility increases when concrete reinforced with fibers. The results

also pointed out that the micro fibers is better in impact resistance compared with the

longer fibers.

1.2.2 Benefits of FRC

Controlled Plastic Shrinkage

Minimized Crack Growth

Reduced Permeability

Improved Surface Durability

Uniform Reinforcement In All Directions



CHAPTER – 2

2. EXPERIMENTAL SET UP AND PROCEDURES

2.1. EXPERIMENTAL PROGRAM

In this study, a total of 36 bond specimens were fabricated to study the effect

of various environmental agents on the durability of the FRP/FRC system. To

simulate the seasonal weather changes, specimens were subjected to combined

environmental cycles, consisting of freeze-thaw cycles and high temperature cycles,

while submerged in the salt solution. Then, bond behaviours were compared with

unweathered specimens to investigate the durability of this new hybrid system.

2.2 MATERIALS

2.2.1 Fiber-Reinforced Polymer Rods

Three types of commonly used FRP rods were adopted in this experimental study:

namely #8 (25-mm) GFRP, #4 (13-mm) GFRP, and #4 (13-mm) CFRP, as shown in

figure 2.1. The surface of the GFRP rods is tightly wrapped with a helical fibre strand

to create indentations along the bar, and sand particles are added to the surface to

enhance its bonding strength. The surface of the CFRP is very smooth, as shown in

figure 2.1. The resin used was epoxy-modified vinyl ester. The mechanical properties

of FRP rods as are shown in table 2.1.

2.2.2 Polypropylene Fiber

Currently, many fiber types are commercially available, including steel, glass,

synthetic, and natural fibers. To fulfill the completely nonferrous concept,

polypropylene fibre was used in this study. Fibres are fibrillated and commercially

available in 57-mm length. Some properties of interest as reported by the

manufacturer are listed as follows: specific gravity = 0:91;Young’s modulus = 3:5

GPa. Polypropylene fibers can

Improve mix cohesion, improving pumpability over long distances



Improve freeze thaw resistance

Improve resistance to explosive spalling in case of a severe fire

Improve impact resistance

Increase resistance to plastic

# 8 GFRP

# 4 GRRP

# 4 CFRP

Fig 2.1: Fiber-reinforced polymer rods used in this study

2.2.3 Concrete

The concrete mix used is shown in table 2.2. For practical application and quantitative

investigation of the benefits from the fibers, the volume fraction of fiber (Vf ) of 0.5%

was selected to make the FRC take the benefits from the fibers yet still ensure good

workability of the concrete. The air content of concrete and FRC and the 28-day

compression strengths are shown in table 2.3.

2.3 DISCUSSION OF VARIOUS ENVIRONMENTAL AGENTS

The environmental agents that have potential effects on the long term structural

behaviors of this FRP/FRC hybrid system are discussed as follows:

Table 2.1: Mechanical properties of Fiber-Reinforced Polymer Rods



Items #4 Carbonfiber-reinforced

polymer

#4 Glassfiber-reinforced

polymer

#8 Glassfiber-reinforced

polymerTensile strength

(MPa)2,069 690 551

Elastic modulus(GPa)

124 41 41

Fiber type Polyacrylonitrile(PAN)-basedcarbon fiber

E-glass fiber E-glass fiber

Resin type Epoxy-modifiedvinyl ester

Epoxy-modifiedvinyl ester

Epoxy-modifiedvinyl ester

Table 2.2: Concrete mix design

Concrete type Portland cementtype I (kg∕m3)

Water(kg∕m3)

Fly ash (Class C)(kg∕m3)

Sand(kg∕m3)

Coarse aggregate(limestone) (kg∕m3)

Volume fraction ofpolypropylene fiber

Air-entraining agent(mL∕m3)

Plain concrete 371 119 119 771 1,020 0% 296

Fiber-reinforced concrete

368 124 65 676 1,080 0.5% 266

Table 2.3: Concrete Properties

Concrete type

28-day compressive

strength (MPa) Air Content

Plain concrete 48 4.9%

Fiber reinforced

concrete

37 6%



2.3.1 Freeze-Thaw Effect

The freeze-thaw cycle is a serious environmental threat to bridge structures with a

poor-quality concrete. Researches showed that concrete can be damaged by freeze-

thaw cycles. With the addition of deicing salts, the damage can be greatly accelerated.

Owing to concrete’s permeable nature, water or deicing salt water can reach the

interface between the bars and concrete. Therefore, accumulated damage can occur to

concrete, FRP bars, and their interface by repeated freeze-thaw cycles. The selection

of the freeze-thaw cycle is subsequently discussed.

The internal temperature of the concrete was not the same as the air

temperature in the environmental chamber. It is important that the interface at the

FRP/concrete can reach the targeted internal temperature (i.e., less than 0°C during

the freezing period and greater than 0°C during the thawing period). Thermocouples

were embedded at the center of 100×200-mm concrete cylinders. Two such cylinders

were used (one placed in air, the other in water) to monitor the core temperature

inside the concrete. Several preliminary freeze-thaw cycles were conducted to

determine the appropriate air temperature and cycle time. A computer data-acquisition

system that records data at 3-min intervals was used to monitor the temperature

change. The temperature variation during the freeze-thaw cycle is shown in figure 2.2.

Testing revealed that the temperature in the concrete specimens changed at a much

slower rate than the ambient temperature. It was also found that a 9-h cycle,

consisting of a 6-h freezing regimen and 3-h thawing regimen, was adequate to obtain

the freezing-and-thawing conditions in the core of the specimens.

2.3.2 High Temperature

The coefficients of thermal expansion (CTEs) of steel reinforcement and concrete are

similar, as shown in table 2.4. Thus, little or no stresses will be induced between the

steel reinforcement and concrete owing to the temperature change for the RC

structure. However, as shown in table 2.4, the CTEs for fibers and concrete are

different. Furthermore, the resin materials used to bind the fibers have much larger

CTEs in comparison with concrete. Significant stresses at the interface of the two



Fig 2.2 : Cycle of freezing and thawing

materials could induce with the temperature variation, which may affect the bond

performances. When deciding the high temperature magnitude, the glass transition

temperature Tg was considered. The FRP materials should not be used at

temperatures greater than their glass transition temperatures. The glass transition

temperature for vinyl ester is approximately 93°C. Also, to accelerate the testing

process, a higher temperature than normal should be used. It was decided that the high

temperature cycle ranged from 35–60°C.

2.3.3 Ultraviolet Radiation

Polymeric materials can absorb ultraviolet (UV) radiation and, therefore, are

susceptible to reactions initiated by the absorption of UV energy. Generally, the

effects of UV exposure are confined to the top few microns of the surface. Thus,

the degradation from UV exposure may be a concern for the external application of

FRP materials. However, previous test results indicated that the mechanical properties

of the FRP bars were not significantly affected even by direct exposure to UV

radiation.



Table 2.4: Coefficient of thermal expansion of various materials

Type of

material

Longitudinal direction

(1061/K)

Transverse direction

(1061/K)

Aramid fiber -6.0 to -2.0 55 to 60

Carbon fiber -0.9 to 0.7 8 to 18

Glass fiber 5 to 15 5 to 15

Resin 60 to 140 60 to 140

Steel 12 12

Concrete 6 to 13 6 to 13

for the application of FRP materials in this project, FRP bars were protected by a

concrete cover. The degradation caused by UV radiation was expected to be

negligible and was not investigated in this study.

2.3.4 De-icing Salt Solution

Chloride penetration caused by the de-icing salt is a major cause of corrosion in steel-

reinforced highway structures. It may also affect the strength of the FRP materials. As

discussed previously, damage caused by the freeze-thaw cycles will be aggravated by

the use of salt solution. In this study, a solution of sodium chloride, 5% by weight,

was selected to simulate the de-icing salt solution. Specimens were submerged into

the salt solution in two large tanks.

2.3.5 Humidity Effect

The FRP rods are not waterproof. Moisture can diffuse into resin, leading to changes

in mechanical characteristics and in physical appearance (increase of volume). As a

consequence, the overall performance of the FRP/FRC hybrid system may be altered.

Because specimens in this study were submerged in salt water, the humidity effect on

the FRP/FRC system was covered automatically.



2.3.6 Alkaline Effect

When in contact with alkaline media, FRP material will degrade owing to the

chemical reaction with an alkaline solution. For this hybrid FRP/FRC system, FRP

rods were embedded in concrete, which is known to have a pH level as high as 13.5.

This alkaline environment can damage glass fibers through the loss of toughness and

strength. Several studies have been conducted on the effect of alkaline on the FRP

material and significant degradation for GFRP bars was reported. Direct immersion

into an alkaline solution was thought to be much more severe than real conditions.

Most likely, it is the mobility of the alkaline ions that greatly affects the test results.

To accelerate but not exaggerate the possible degradation effect from alkaline, FRP

rods were embedded in concrete, and the specimens were kept submerged in the de-

icing slat solution in this study.

2.4 SUMMARY OF ENVIRONMENTAL CONDITIONING IN THIS STUDY

A combined environmental cycle consisted of 20 freeze-thaw cycles and 20 high

temperature cycles as shown in figure 2.3. The same cycles were repeated 10 times.

Thus, a total of 200 freeze-thaw cycles and 200 high temperature cycles were

conducted to simulate the seasonal weather changes. It took approximately 100 days

to complete the 10 combined environmental cycles. After the environmental

conditioning, specimens were removed from the environmental chamber and allowed

to dry for 1 week before testing. Then, specimens were subjected to a pullout bond

test at room temperature (22°C). Compared with the typical weather in most regions

of the world, the determined environmental cycles were far more severe than the real

scenario, so the accelerated test was possible. Figure 2.4 shows the detailed testing

procedures.

2.5 TEST SPECIMENS

Test specimens were designed according to RILEM recommendations (7–II-128,

RILEM 1994) with a 5db embedment length, which was generally assumed to be able

to represent local bond behaviour. The FRP rods were embedded in concrete to a

predetermined length le in the concrete block. A PVC pipe was used as a bond



breaker at the first 5db length to minimize the bottom plate’s restraint effect on the

FRP bar and to eliminate any undesirable confinement that may affect the bond

characteristics. Details are shown in figure 2.5.

Chemical agents will attack the reinforcing materials and the bond between the

concrete and the reinforcing materials. In typical RC structures, cracks exist under

service conditions. The degradation effect is expected to be more pronounced at

locations at which cracks exist. Because the portion away from the cracks is not

directly in contact with solution, to better simulate the real situations, the portion at

the loaded end of the bond specimens was directly exposed to salt water, where as the

portion at the free end was coated with waterproof epoxy to protect it from direct

attack from salt water. Because the epoxy could induce unwanted mechanical

anchorages and change the bond behaviour when the bar was pulled out, all the epoxy

that stuck to the bar was removed after environmental conditioning. The notation

for specimens is as follows: the first character, V or D, indicates the unweathered

specimen or weathered specimen; the second character, P or F, indicates the plain

concrete or FRC; the third character (#4 versus #8) is the bar size in U.S.designation;

and the last character, C or G, indicates the reinforcement type, CFRP or GFRP. The

test matrix of the bond specimen is shown in table 2.5.

2.6 TEST SETUP AND PROCEDURES

The pullout tests were conducted in an MTS 880 machine. The test is run through

closed-loop displacement control using an external LVDT2 as a feedback device

(shown in figure 2.5).Because the FRP rods were weak in the transverse direction,

special anchorages were used to protect the reinforcement from crushing. The free end

of the FRP bar was embedded in a steel pipe using an expansive grout as an interface.

The pullout was then performed by pulling the steel pipe at one end, with the concrete

block encased in the steel reaction frame. The bar’s slip relative to the concrete was

computed from measurements of two LVDTs placed at both ends of the bar, as shown

in figure 2.5. To minimize the eccentricity effect, lead sheets were placed between the

concrete block and the reaction frame. The pullout tests were monotonic by increasing



Fig 2.3: Environmental cycles: a) One combined environmental cycle; b)

freezing and thawing cycle; c) high temperature cycle



Fig 2.4: Flow chart showing test procedures

the slip at a 0.76-mm∕ min rate. All measurements, including pullout load and

displacements (slips), were recorded by a computer-controlled data-acquisition

system.



Fig 2.5: Pull out test specimen and test setup

Table 2.5: Test Matrix

Specimen Number of specimens f’c (MPa) Vf (%)

VP4C 3 48 0

VP4G 3 48 0

VP8G 3 48 0

VF4C 3 37 0.5

VF4G 3 37 0.5

VF8G 3 37 0.5

DP4C 3 48 0

DP4G 3 48 0

DP8G 3 48 0

DF4C 3 37 0.5

DF4G 3 37 0.5

DF8G 3 37 0.5



CHAPTER – 3

3. TEST RESULTS AND DISCUSSIONS

3.1 TEST RESULTS

In the following discussion, the environmental conditioning’s effect on the specimen

appearances and the bond behaviors for both plain concrete specimens and FRC

specimens are presented.

3.1.1 Appearance of Specimen after Environmental Conditioning

i) Plain Concrete Specimens

In addition to concrete scaling on the surface, most specimens also showed some

damage on the concrete, especially at the corner areas. One DP4C specimen and one

DP4G specimen were severely damaged, and large portions of concrete were broken

apart, as shown in figure 3.1.

ii) Fiber-Reinforced Concrete Specimens

Damages were limited to the surfaces of the specimens. With the scaling of concrete

at the surfaces, fibers could clearly be observed. However, all of the FRC specimens

remained integrated. In comparison with the plain concrete specimens, the FRC

specimens were more immune to the environmental attack.

3.1.2 Environmental Conditioning Effect on Bond Behaviors

The test results are shown in table 3.1. A typical bond slip is shown in figure 3.2 .The

typical bond-slip responses at the loaded end and the free end are shown in figures 3.3

and 3.4, respectively. In this study, the average bond strength was calculated as

the pullout force over the embedded area of the bar. The slip on the side of loading

was calculated as the value of LVDT2 minus the elastic deformation of the FRP

bar between the bond zone and the location of LVDT2.

i) Plain Concrete Specimens

• Bond –slip response : Unweathered specimens showed fairly consistent test results



(a)

(b)

Fig 3.1 : Difference in appearance of plain and fiber-reinforced concrete specimens

after environmental conditioning: (a)#4 glass fiber-reinforced polymer;(b)#4 carbon

fiber-reinforced polymer

with the same testing parameters .However , test results for specimens subjected

to environmental conditioning were volatile. Different levels of damage on the

specimens were observed visually. In general, specimens with concrete more severely



Fig 3.2: Typical Bond Slip

damaged showed lower bond strength. In other words, the bond strength was strongly

dependent on the condition of the concrete. Figure 3.1 shows the most severely

damaged specimens (DP4C and DP4G). These specimens had large amounts of

concrete broken apart and thus showed very low bond strengths. As shown in figures

3.3 and 3.4, the bond-slip curves of all the tested specimens were softened after being

subjected to the environmental conditions.

• Failure modes: Most specimens had the same failure modes as the unweathered

specimens. However, the failure modes were changed for the DP4C specimens. All

three unweathered specimens, VP4C, failed in the reinforcement pullout. However,

two of the three DP4C specimens failed in concrete splitting; the other one failed

in reinforcement pullout. This was caused by the damage of concrete. Some portions

of the concrete were broken apart; thus, a smaller amount of concrete could resist the

splitting force caused by the reinforcement pullout.

• Ultimate bond strength: Ultimate bond strengths of all specimens were reduced, and

this effect was more pronounced in the smaller specimens (#4 bar specimens).

As shown in figure 3.5 , 23%, 56%, and 4% reductions were observed in ultimate

bond strength for DP4C, DP4G, and DP8G specimens, respectively.

• Design bond strength: Employment of the ultimate bond strength as a design value

is not appropriate because of the excessive associated slip, which will result in



untolerated crack width. Bond stress corresponding to 0.2-mm slippage of loaded end

or 0.05-mm slippage of free end for steel-reinforced structures can be defined as the

critical bond stress. The criterion of 0.2-mm slippage at the loaded end was decided

on the basis of half of the crack width limitation. As mentioned previously, after being

subjected to environmental conditioning, surface scaling occurred on most of the

specimens. During the pullout bond tests, the loose concrete at the loaded end would

be compacted under compression. Thus, the measured loaded-end slip could be

exaggerated. However, slips measured at the free end would not be affected.

Therefore to keep it comparable with limits imposed on the steel reinforcement, bond

strength corresponding to 0.05-mm slippage at the free end was adopted as the

designing bond strength. As shown in figure 3.6, 25%, 43%, and 11% reductions were

observed for DP4C, DP4G, and DP8G specimens, respectively.

ii) Fiber-Reinforced Specimens

• Bond-slip response: In general, test results of the FRC specimens showed good

consistency. The behavior of the specimens in the same testing group was similar.

Similar to the plain concrete specimens, all bond-slip curves were softened after being

subjected to the environmental cycles.

• Failure modes: Similar to the plain concrete specimens, most FRC specimens had

the same failure modes as the unweathered specimens. However, the failure mode of

one of the three DF8G specimens was changed from reinforcement pullout to concrete

splitting.

• Ultimate bond strength: Reductions of the bond strength were observed for most of

the FRC specimens, as they were for the plain concrete specimens. As shown in figure

3.4, 5% and 15% reductions were observed in the ultimate bond strength for DF4C

and DF4G specimens, respectively. However, little change was observed for DF8G

specimens.

• Design bond strength: Similar to the plain concrete specimens, reductions of the

design bond strength defined previously were observed for most of the FRC

specimens. As shown in figure 3.5, 12% and 16% reductions were observed in the



Table 3.1 Test Results of Durability Bond Tests

aP= Pullout failure; S= Splitting failurebSignificant bond reductions were observed for specimens whose concrete were badly

damaged after the environmental conditioning.



Fig 3.3: Typical loaded- end bond- slip curve (#4 GFRP): (a) Plain concrete

specimen; (b) Fiber-reinforced concrete specimen

Fig 3.4: Typical free- end bond- slip curve (#4 GFRP): (a) Plain concrete specimen;

(b) Fiber-reinforced concrete specimen



design bond strength for DF4C and DF4G specimens, respectively. However, little

changes were observed for DF8G specimens.

Fig 3.5: Reductions in ultimate bond strength

Fig 3.6: Reductions in design bond strength

3.2 DISCUSSIONS

In the following discussion, mechanisms responsible for the bond degradation are

presented. Bond durability affected by different testing parameters, including

specimen size, plain concrete or FRC, GFRP specimens or CFRP specimens, is also

discussed.



3.2.1 Bond Degradation Mechanisms

Bond is determined by the properties of its constituents (concrete and reinforcement)

and the interaction between them. Three possible reasons explain the bond

degradation:

i) Micro voids between reinforcement and concrete exist at the time of the specimen

fabrication (i.e., reinforcement is not in full contact with the concrete) .When

specimens are submerged in the solution, the solution will permeate into their

interface. The micro voids will gradually be filled up with solutions. The volume of

water will expand approximately 10% when frozen. Micro cracks will thus be created

if the stresses are larger than the tensile strength of the concrete. With the subsequent

freeze-thaw cycles, damage will build up, and more and bigger microcracks can be

created.

ii) FRP bar has a higher CTE than concrete. When the temperature increases, the

expansion rate of the FRP bar is larger than that of the concrete. Radial bursting force

will be imposed on the concrete surface at the interface. When the concrete stress is

larger than the tensile strength, cracks will develop. When the temperature reduces,

the contraction rate of FRP bar is bigger than that of concrete, and microgaps will

form along the interface.

iii) The FRP rods are not waterproof. Moisture can diffuse into the polymer resin to a

certain degree .Studies also show that some deterioration of the polymer resins may

occur because water molecules can act as resin plasticizers, thereby disrupting van der

Waals bonds in polymer chains. Furthermore, during the freeze-thaw cycles, water

will expand and lead to the cracking of the resin. The surface area is the most

vulnerable, and consequently, bar and concrete will not contact as tightly as before.

Bond is thus degraded. The first two mechanisms function together and degrade the

bond primarily by creating cracks along the interface. Bond degradation may also

come from degradation of reinforcement itself. All three mechanisms play a certain

role in the bond degradation, and the combined effects are likely to be even more

detrimental to the bond.



3.2.2 Specimen Dimension Effect on Bond Degradation

Compared with the large (#8) specimens, the small (#4) specimens showed greater

degradation effect. This phenomenon occurred to both plain concrete specimens and

FRC specimens. As shown in figure 3.5, the ultimate bond strengths reduced by 56%

for DP4G specimens, whereas only a 4% reduction was observed in DP8G specimens.

Similarly, the ultimate bond strengths were reduced by 15% for DF4G specimens,

whereas no noticeable changes occurred to DF8G specimens. As for the design bond

strength, the smaller specimens also showed a much more serious reduction, as shown

in figure3.6. Specimen dimensions’ effect on the bond durability can be explained in

the way that the salt solution attacks the bond. Salt solution can reach the interface

between bar and concrete in two ways, as shown in figure 3.7. One is through the

loaded end of the specimens (the free end was coated with waterproof epoxy).

Because the depth of the specimen that was immediately accessible to the solution

was independent of the size of the specimen, the absolute depths affected were the

same. On the other hand, the bigger specimens had a bigger embedment length; thus,

the ratio of affected area to the whole bonded area was smaller in the case of the large

specimens. The other way that salt solution can reach the interface is by permeating

through the concrete cover, as shown in figure 3.7. The cover of the #8 specimens was

twice as thick as the #4 specimens, thus the interface between reinforcement and

concrete was better protected.

3.2.3 Fibre Effect on Bond Degradation

With the addition of fibres, the durability of bond was significantly improved. As

shown in figure 3.5, an average reduction of 28% of bond strength was observed in

the plain concrete specimens, whereas only a 6% reduction was observed in the FRC

specimens. For the design bond strength, an average reduction of 26% was observed

in the plain concrete specimens, whereas only a 10% reduction was observed in the

FRC specimens, as shown in figure 3.6.It can be concluded that fibres can effectively

alleviate the bond deteriorations caused by environmental conditioning. As discussed,

cracks or voids were created during the environmental conditioning. Although the



Fig 3.7: Two ways of solution ingress

addition of fibers cannot increase the first cracking load, the fibers would restrict

further development of the cracks owing to the expansion of water or reinforcement.

Hence, the deteriorations would not be accumulated, or this deterioration effect would

happen at a much more moderate rate. Less bond degradation for the FRC specimens

could also be partly attributed to the fact that damage of the concrete was less after

environmental conditioning. The air content of the plain concrete used in this study

happened to be lower than that of the FRC, which may also be responsible for the

more severe damage of the plain concrete specimens.

3.2.4 Difference of Glass Fiber-Reinforced Polymer versus Carbon

Fiber-Reinforced Polymer

The bond degradation rate of the GFRP specimens was more severe than that of the

CFRP specimens. As shown in figure 3.5, the bond reduced by 23% in the DP4C

specimens and 56% in the DP4G specimens. Similarly, the bond reduced by 5% in the

DF4C specimens and 15% in the DF4G specimens. For the design bond strength, the

reductions were also larger in the GFRP specimens, as shown in figure3.6.As

discussed previously, the degradation of the reinforcement may partly contribute to

the bond strength degradation. Because of the attack by the salt water, the



reinforcement, especially the outer surface, was damaged. Thus, the contact area

would be reduced. Research has shown that the CFRP bar has superior durability

characteristics compared with the GFRP bar. Thus, less damage was expected in the

case of the CFRP bar, and hence, the CFRP specimens showed better durability of

bond. This phenomenon further supported the finding that bond strength degradation

is tightly related to the material degradation.



CHAPTER – 4

4. CONCLUSIONS

Long-term bond performance between FRP bars and FRC was investigated

and compared with that of FRP bars in plain concrete. The accelerated aging test was

accomplished by submerging specimens in salt solutions and subjecting them to 10

combined environmental cycles, each of which consisted of 20 freeze-thaw cycles and

20 high temperature cycles. The following conclusions can be drawn from this study:

• Three mechanisms cause bond degradation. The first is the expansion of solutions in

the microvoids at the interface. The second is the different CTE between

reinforcement and concrete. The first two mechanisms function together and degrade

the bond primarily by creating microcracks in concrete at the interface. The third

reason is the damage of the reinforcement, especially at its surface.

• With the addition of polypropylene fibers, the bond durability significantly

improved owing to the restriction of the crack development at the interface. The loss

of ultimate bond strength of FRP bars in plain concrete owing to aging effects was

found to be 28% on average, whereas only a 6% reduction was observed in the FRC

specimens. Similarly, design bond strength exhibited a 26% average reduction in the

plain concrete specimens, whereas only a 10% reduction was observed in the FRC

specimens.

• The larger specimens with thicker concrete cover and relatively smaller direct

exposed area to the solution of sodium chloride showed better bond durability.

• Degradation of bond is tightly correlated with the degradation of FRP bar. As

observed in this study, CFRP specimens had superior long-term bond durability

compared with GFRP specimens. This is attributed to the more durable characteristics

of the CFRP bar.



REFERENCES

Journals

Abdeldjelil Belarbi and Huanzi wang. (2012)” Bond durability of

FRP bars Embedded in Fibre-reinforced concrete” Journal of

Composites for Construction, Vol. 16, No. 4, August 1, 2012. ©ASCE,

16:371–380.

Robert, M., and Benmokrane, B. (2010). “Effect of aging on bond of

GFRP bars embedded in concrete.” Cem. Concr. Compos., 32(6), 461–

467.

Marta Baena, Lluís Torres, Albert Turon and Cristina Barris

(2009) “Experimental study of bond behaviour between concrete and

FRP bars using a pull-out test”. Composites: Part B 40 , 784–797

Bank, L. C., Puterman, M., and Katz, A. (1998). “The effect of

material degradation on bond properties of fiber reinforced plastic

reinforcing bars in concrete.” ACI Mater. J., 95(3), 232–243.

Bank, L. C., Gentry, T. R., and Barkatt, A. (1995). “Accelerated test

methods to determine the long-term behavior of FRP composite

structures: Environmental effects.” J. Reinf. Plast. Compos., 14(6),

559–587.


matter final

Documents

frp materials

frp reinforcement system

fiber reinforced polymer

bond durability of frp

longterm bond performance

different types of frp

corresponding bond performance

different bond characteristics