Concrete reinforced with FRP rebarsEvaluation of durability and behaviour in the Service Limit State (SLS)

David Ottosson

Civil Engineering, master's level

2021

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering

II

Preface The work presented in this thesis is a part of my master’s degree in Civil Engineering at the

department of Civil, Environmental and Natural Resources Engineering at Luleå University of

Technology.

Without the support and encouragement from Linda this master thesis would never had been

completed, so I would like to extend my great gratitude to her.

I would also want to thank my supervisors throughout this work, Professor Björn Täljsten and

Tekn. Dr Cosmin Popescu.

Åtvidaberg, Juni 2021

David Ottosson

III

Summary

One of the most common building materials is concrete and it has been for a long time. To

overcome its low tensile capacity concrete structures are normally reinforced with steel rebars.

The use of FRP (Fibers Reinforced Polymers) bars in concrete structures has emerged as an

alternative to conventional steel reinforcement, due to the corrosion of steel in aggressive

environments. FRP has been used as internal reinforcement for more than 30 years, bridges and

parking garages are examples of structures in harsh environments where FRP is a good

replacement for steel reinforcement. This due to the higher strength of FRP compared to steel

and non-corrosive properties, however FRP as internal reinforcement is not commonly used in

Scandinavia.

This work has been divided into four parts, a Literature survey, a Literature study on durability,

structural behaviour in the serviceability limit state and a FE analysis of previously carried out

laboratory tests. In the literature survey the material FPR is described with its components,

manufacturing process, history and various applications. A literature study was done to

determent the long-term durability of GFRP by accelerated laboratory tests for durability, then

compared to field tests on durability of GFRP rebars. The accuracy of FRP design international

standards has been evaluated in terms of serviceability limit stat, such as ACI 440.1R-15, ISIS

and a variant of Eurocode 2 (EC2). The design models for deflection available for these

standards were compared to a database of experimental studies collected by the author. The

stiffness of structures reinforced with FRP is such an important parameter so a non-linear

calculation using ATENA software was conducted. Results were compared to laboratory tests

performed at Denmark Technical University (DTU).

In several accelerated laboratory experiments where bare FRP bars were exposed to different

harsh environments the degradation of strength was significant, where an alkaline solution at

elevated temperature was the harshest environment for the GFRP bars. When GFRP rebars are

embedded in concrete the degradation was significantly lower (around 40 percentage points),

the concrete protects the GFRP rebars considerably. The largest rate of degradation on GFRP

rebars is in the initial state, in comparison to steel which starts to corrode when carbonation

and/or chloride penetration critical levels reaches the reinforcement. In field studies there were

small signs of degradation of the GFRP rebars, mainly in tropical climates. De-icing salts have

a limited effect on the degradation. Laboratory experiments are very conservative with

unrealistic harsh environments compared to the natural harsh environments. Therefore, after 20

years of service in harsh environment there were no or small signs of degradation on the GFRP

rebars which indicates the validity of GFRP.

All three standards evaluated had a large spread on the predicted deflection compared to the

experiments, with ACI 440.1R-15 as the most conservative standard with a mean value of the

deflection ratio at 0.81. The mean value of the deflection ratio when using ISIS was 0.87,

slightly less conservative but with the same spread as ACI 440.1R-15. The calculation using a

variant of EC2 had the most spread of results, but with a mean value of the deflection ratio at

0.93, this excluding 11 beams that had an unrealistic prediction due to the wrong prediction of

the crack moment. The FEM model created had a similar stiffness as compared to the

experiment from DTU, which indicates that the use of Atena was accurate for calculating the

IV

deflection of the beams. Although the ultimate load was not well predicted, probably due to the

failure mode crushing of concrete in the compressive zone.

Despite this, there are many structural parts where FRP could be beneficial, for example in

splash zones, in edge beams and slabs etc. This could bring down the costs for maintenance and

also prolong the life span of the structure.

V

Sammanfattning

Ett av de vanligaste byggnadsmaterialen är betong och har varit det under en längre tid. För att

kompensera för betongens låga draghållfasthet förstärks konstruktionerna med armering,

vanligtvis av stål. Användningen av FRP (Fibers Reinforced Polymers) armering i betongen

har blivet ett alternativ till den traditionella användningen av stålarmering. FRP har använts

som armering i mer är 30 år, byggnader och anläggningar som broar och parkeringshus är några

exempel på konstruktioner i med tuffa miljöer där FRP armering är ett bra alternativ till

stålarmering. Detta för att FRP har en högre brottgräns jämfört med stål och inte rostar i tuffa

miljöer.

Detta arbete har delats in i fyra delar, en litteraturundersökning, en litteraturstudie om hållfast-

heten, en undersökning om pålitligheten hos standarder angående bruksgränstillståndet och en

FE-analys av tidigare utfört laboratorie-experiment. I litteraturundersökningen beskrives

materialet FPR med dess komponenter, tillverkningsprocess, historik och några tillämpningar.

En litteraturstudie gjordes för att undersöka den långsiktiga hållfastheten av GFRP armering

genom accelererade laboratorietester som jämfördes med fältförsök på hållfastheten hos GFRP

armering. Noggrannheten i FRP standarder för internationella standarder har utvärderats i ter-

mer av bruksgränstillståndet, som ACI 440.1R-15, ISIS och en variant av Eurokode (EC2). De

designmodeller för nedböjning som finns i dessa standarder jämfördes mot en databas med ex-

perimentella studier som samlats av författaren. Styvheten hos balkar armerade med FRP är en

viktig parameter så en icke-linjär beräkning med ATENA-programvara utfördes. Resultaten

kalibrerades mot laboratorietester utförda vid Danmarks tekniska universitet (DTU).

I accelererade laboratorieexperiment utsattes av GFRP armering för olika tuffa miljöer, den

alkaliska miljön visade sig ha störst påverkan på hållfastheten. När GFRP stängerna är inbäd-

dade i betong är nedbrytningen betydligt lägre, med cirka 40 procentenheter, betongen ger

GFRP armering ett betydande skydd. Nedbrytningshastigheten på GFRP stänger är störst i den

initiala fasen, jämfört med stål som börjar korrodera när karbonering och/ eller kloridgenom-

trängning når kritisk nivå till armeringen. I fältstudier fanns det små indikationer på nedbrytning

av GFRP armeringen, främst i tropiska klimat. Tö salt visade endast en begränsad effekt på

nedbrytningen. De experimentella laboratorier försöken var väldigt konservativa med orealist-

iskt tuffa miljöer jämfört med naturligt tuffa miljöer. Det fanns enbart små eller inga tecken på

nedbrytning hos GFRP armering efter 20 års exponering i till tuffa miljöer, vilket tyder på att

GFRP är ett bra alternativ till stål.

Alla tre utvärderade standarder hade stor spridning på resultat av beräknad nedböjning jämfört

med de experimentella, ACI 440.1R-15 som den mest konservativa standarden med ett medel-

värde för nedböjningskvoten på 0.81. Medelvärdet för nedböjningskvoten vid användning av

ISIS var 0.87, något mindre konservativ men med samma spridning som ACI 440.1R-15. Be-

räkningen med en variant av EC2 hade mest spridning av resultaten men medelvärdet för av-

böjningsförhållandet vid 0.93, detta exklusive 11 balkar som hade en orealistisk beräknad ned-

böjning på grund av fel förutsägelse av sprickmomentet. FEM-modellen hade en liknande styv-

VI

het jämfört med experimentet från DTU vilket indikerar att användningen av Atena var tillför-

litlig för att beräkna nedböjning av balkarna. Detta trots att inte brottlast överensstämde, tro-

ligtvis på grund av felkrossning av betong i kompressionszonen.

Trots detta finns det många byggnader och anläggningar delar där FRP skulle vara fördelaktigt,

några exempel är vid skvätt zoner, i kantbalkar eller i plattor mm. Detta skulle kunna sänka

underhållskostnader och förlänga livslängden på byggnader och anläggningar.

VII

VIII

Notations and abbreviations

Roman letters

a Distance from the support to one of the two-point loads applied

Af Area of FRP

b Width of a beam

d Effective height in a cross section

Ec Modulus of elasticity for concrete

EII Stiffness of an uncracked section

EIII Stiffness of a cracked section

fc Compressive strength of concrete

fctm Tensile strength of concrete

h Height of a beam

Icr Moment of inertia of a cracked section

Ie Effective moment of inertia

Ig Moment of inertia for a gross section

Itr Moment of inertia for a transformed section.

k Factor used in the calculation of moment of inertia for a cracked section

L Length of the beam

Ma Applied moment

Mcr Crack moment

nf Ratio between the modules if elasticity for concrete and FRP

P Point load

yt Neutral axis from the top

IX

Greek letters

β A term depending on the load is short or long term

γ Factor depending on the cracked moment and the applied moment

Δ Deflection

ΔI Deflection of an uncracked section

ΔII Deflection of a cracked section

ζ Coefficient that termers where in between the states the section is

ζf Factor depending on the modulus of elasticity of concrete and FRP and the

reinforcement ratio

λ Factor that takes the density of concrete in consideration.

ρf Reinforcement ratio

ξ Factor depending on the cracked moment and the applied moment

X

Table of content

1 Introduction ............................................................................................................................. 1

1.1 Background .................................................................................................................. 2

1.2 Aim and Objectives ..................................................................................................... 2

1.3 Method ......................................................................................................................... 3

1.4 Limitations ................................................................................................................... 3

1.5 Thesis structure ............................................................................................................ 4

2 Literature survey ..................................................................................................................... 5

2.1 Fiber reinforced polymers ........................................................................................... 5

2.2 Polymer matrix ............................................................................................................ 6

2.3 Fibers ........................................................................................................................... 6

2.4 Manufacture process .................................................................................................... 8

2.5 History of FRP ............................................................................................................. 8

2.6 FRP as internal reinforcement ................................................................................... 10

2.7 Applications of GFRP ............................................................................................... 12

3 Durability .............................................................................................................................. 17

3.1 Laboratory tests ......................................................................................................... 19

3.1.1 Test on rebars ..................................................................................................... 19

3.1.2 Tests with rebars embedded in concrete ............................................................ 26

3.2 Field tests ................................................................................................................... 32

3.3 Summary of durability ............................................................................................... 35

4 Study in the Service Limit State (SLS) ................................................................................. 36

4.1 ACI 440.1R-15 .......................................................................................................... 37

4.2 ISIS ............................................................................................................................ 39

4.3 Eurocode 2 ................................................................................................................. 40

4.4 Summary .................................................................................................................... 43

5 FEM analysis ......................................................................................................................... 44

5.1 Description of the experiment ................................................................................... 44

5.2 Description of the FE model ...................................................................................... 48

5.3 Results from the FEM analysis. ................................................................................. 50

5.3.1 BF3O16 .............................................................................................................. 50

5.3.2 BF3O32 .............................................................................................................. 52

6 Analysis ................................................................................................................................. 54

XI

6.1 Durability ................................................................................................................... 54

6.2 Service limit state analysis ........................................................................................ 56

6.2.1 ACI 440.1R-15 ................................................................................................... 57

6.2.2 ISIS ..................................................................................................................... 59

6.2.3 Eurocode 2 .......................................................................................................... 61

6.3 Atena models ............................................................................................................. 64

7 Conclusions ........................................................................................................................... 66

8 Future work ........................................................................................................................... 67

9 Reference ............................................................................................................................... 68

Appendix AA ........................................................................................................................... 72

1

1 Introduction

To overcome its low tensile capacity concrete structures are normally reinforced with steel

reinforcement. The steel rebar is protected from corrosion by the alkaline environment in the

concrete. This protection is normally satisfied when the structure is not subjected to an

aggressive environment, has enough concrete cover, a limited number of cracking and good

concrete quality. However, over time carbonation, in particular if the structure is affected by

chlorides from de-icing salts or seawater the reinforcement will start to corrode due to the

reduction of alkalinity in the concrete. The annual cost of repairing concrete bridges in the USA

alone is roughly $8.3 billion including maintenance, repair, replacement and the cost of capital,

With the traffic delays and lost productivity the cost indirect could be up to ten times higher

(ECi, n.d.).

In Scandinavia, primarily in Norway and Sweden, with both cold winters and long coastlines

where chlorides affect the concrete structures by seawater and the use of de-icing salts which

makes the steel reinforcement corrode. The structures are also becoming older which has led to

extensive carbonation. Stainless steel, epoxy-coated reinforcement and cathodic protection of

reinforcement are some of the noncorrosive reinforcements in the market. With stainless steel

10-15 times more expensive than conventional steel and cathodic protection reinforcement

needing regular maintenance to function satisfied, these alternatives are not optimal (Almgren,

et al., 2018). To solve this corrosion problem a new material is needed, fibre reinforced

polymers (FRP) could be the answer. FRP is a non-corrosive material that can withstand harsh

and chloride environments and has high tensile strength compared to steel. In Scandinavia FRP

is primarily used as a strengthening and retrofitting material in existing buildings. FRP is not

commonly used as internal reinforcement, due to the lack of standards and knowledge about

the material in the construction industry. In USA, Japan and Canada there are existing standards

for FRP as internal reinforcement and an increasing interest in the material proves that FRP is

a viable reinforcement for concrete in harsh environments, especially glass fiber reinforced

polymer (GFRP) with its lower cost compared to the other FRP (ACI 440.1R-15, 2015). Some

advantages of GFRP are:

• Higher strength, regularly twice the strength or more compared with steel.

• High corrosion and chemical resistance, which lowers maintenance costs.

• Thermal and electrical nonconductive.

• Low weight, GFRP weighs only ¼ of the weight steels does.

There are certain disadvantages as well, for example durability problems such as degradation

due to alkaline and moist environments, lower elasticity modulus than steel, unfamiliarity in

the building industry and larger initial cost for a structure (Täljsten & Blanksvärd, 2018).

Generally being 3-4 times higher per kg compared to steel (Gardiner, 2020) the cost of the

construction increases by approximately 1-2%. More research is needed on the subject to design

effective structures with GFRP and to develop effective standards.

2

Despite this, the negative aspects are often smaller than the positive properties when used in

aggressive environments. A life cycle cost analysis done by Dhinakaran, et al., (2016) on GFRP

reinforced beams showed a cost-saving of 40% after 50 years in comparison to steel reinforced

beams in harsh environments.

1.1 Background

There are certain disadvantages as well with GFRP as internal reinforcement, for example

durability problems such as degradation due to alkaline and moist environments, lower

elasticity modulus than steel, unfamiliarity in the building industry and larger initial cost for a

structure (Täljsten & Blanksvärd, 2018). To determine the long-term durability of GFRP

accelerated tests are conducted with corrosion solutions, that both bare bars and bars embedded

in concrete are tested. This is to simulate the reality, but from actual structures reinforced with

GFRP there are samples gathered and tested to determine the durability after a time in service.

Additionally, with new material that has different properties, in this case steel compared to

GFRP, the standards tend to be more conservative. This leads to uneconomical designs which

reduce the material attractiveness.

1.2 Aim and Objectives

The aim of this thesis is to investigate if the use of GFRP reinforcement is beneficial in harsh

environments such as marine environments and structures where de-icing salt is used in

comparison to steel. In the thesis, the following will be presented and researched.

• A literature study regarding GFRP rebars exposed to a harsh environment, both bare

rebars and bars embedded in concrete.

• Accuracy of existing FRP design guidelines in the calculation of the deflection.

• The accuracy of FEM analysis compared to an experimental test of a GFRP reinforced

concrete beam.

3

1.3 Method

This work is a qualitative study where data has gathered from the literature. The work has been

divided into four parts, a Literature survey, a Literature study on durability, structural behaviour

in the serviceability limit state and a FE analysis of previously carried out laboratory tests. In

the literature survey the material FPR is described with its components, manufacturing process,

history and various applications. A literature study was done to determent the long-term

durability of GFRP by accelerated laboratory tests for durability, then compared to field tests

on durability of GFRP rebars. The accuracy of FRP design internationally standards has been

evaluated in terms of serviceability limit stat, such as ACI 440.1R-15 (2015), ISIS (2007) and

a variant of Eurocode 2 (EC2) (Täljsten & Blanksvärd, 2018). The design models for deflection

available for these standards were compared to a database of experimental studies collected by

the author. The stiffness of structures reinforced with FRP is such an important parameter so a

non-linear calculation using ATENA software was conducted. Results were compared to

laboratory tests performed at Denmark Technical University (DTU) by Jensen (2006).

1.4 Limitations

The Literature study on durability was limited to the latest research, only studies from the last

five years were included to get the latest research with the newest types of GFRP. No

experiments were conducted on durability or structural behaviour in this thesis due to

economic reasons, all data were retrieved from the literature. In the evaluation of the

serviceability limit state, a limited number of beams were included in the database, due to

difficulties to find articles with the deflection graph or with all the required data. For example,

not all articles did not have the strength of the FRP described. Two different FE models were

created based on the DTU tests.

4

1.5 Thesis structure

The thesis has the following structure:

Chapter 1 – Introduction

The introduction includes a background and a description of the purpose, method and limitation

of the thesis.

Chapter 2 – Literature survey

The material FRP is presented in more detail, with material properties of each component, his-

tory, material properties compared to steel and some projects are presented.

Chapter 3 – Durability

The degradation process of GFRP rebars is described and experiments on durability from the

literature are described with the results presented. Three different subchapters are presented,

laboratory tests on bare bars, laboratory tests on rebars embedded in concrete and field studies.

Chapter 4 – Serviceability Limit State (SLS)

The deflection was calculated using three standards, ACI, ISIS and a variant of Eurocode. The

results were compared to the experiments retrieved from the literature.

Chapter 5 – FE analysis

A comparison of FE-analysis and experiments from Denmark Technical university was carried

out.

Chapter 6 – Analyses

All the results from chapter 3-5 were analysed.

Chapter 7 – Conclusion

General conclusion was presented that summarize the work.

Chapter 8 – Future research

suggestions for future research are presented.

5

2 Literature survey

In this chapter the material FRP is presented, with two constituents: the polymer matrix and the

fibres. The manufacturing process is described briefly. Furthermore, FRP as an internal

reinforcement is presented with material properties of some commercially available rebars. In

addition, some reference projects are presented.

2.1 Fiber reinforced polymers

Compbell (2010) describes a composite as a material structure that consisting of at least two

macroscopically identifiable materials that work together to achieve a better result in strength

and durability for example. Meaning composite materials have been used for as long as

humans have constructed houses, for example straws mixed with clay used to make stronger

walls and flooring.

The most common composite is made of a polymer, which together with a reinforcing material

forms a strong material. The reinforcing material gives the composite its strength and stiffness.

The property of the composite is governed by the matrix and the properties of the reinforcing

material in combination, the amount of each component as well as the orientation of the

reinforcing material. The most used reinforcing material is fibres, which have high tensile

strength and stiffness. Figure 1 shows two different types of fibres, short continuous and long

discontinuous fibres. There are different orientations of the fibre as well, which determine the

properties of the material, shown in Figure 1 (Compbell, 2010).

Figure 1 Typical reinforcement orientations and length (Compbell, 2010).

For example, internal reinforcement has continuous fibres in one direction which gives the

material high tensile strength in said direction. Woven reinforcement (weaves) has the same

6

continuous fibres, but the cloth pattern gives the material tensile strength in two directions.

During the curing of the matrix there is crimping of the woven fabrics causing the volume

decrease of the matrix. Weaves are very good at strengthening existing structures in sheer force,

for example a beam wrapped in an FRP weave (Compbell, 2010).

2.2 Polymer matrix

Two types of the matrix are commonly used in FRP, thermosetting and thermoplastic matrices.

Thermosetting matrixes are polymers that after hardening have inversed formation. The bond

between molecules and the molecules themselves is strong. Thermoplastics are polymers that

do not have cross-links between the molecules. They can repeatedly be reshaped by subjecting

them to temperature cycles that reach above their forming temperature (Fib, 2007).

The main function of the matrix is to distribute force to the fibres, bind the fibres together and

protect them. The composite is directly affected by the chemistry and physical properties of the

matrix. In composite generally, the matrix constitutes 30-60% of the total volume for FRP. For

internal reinforcement rebars thermosetting resin is usually used, out of these are epoxy,

polyesters and vinyl ester the most common. Some properties of this matrix are shown in Fel!

Ogiltig självreferens i bokmärke. (Fib, 2007).

Table 1 Properties of common thermosetting resin (Fib, 2007).

2.3 Fibers

Commonly used fibres are carbon, glass and aramid fibres, shown in Fel! Hittar inte

referenskälla.. The fibres absorb the forces in the composite due to their strength and stiffness.

The reason for this is their preferential orientation of molecules along the direction of the fibre

as well as lower numbers of defects. All the different types of fibres have a linear elastic

behaviour with an abrupt failure. Glass fibres are isotropic, with the same mechanical and

thermal properties in all directions, whereas carbon and aramid fibres are anisotropic. Figure 2

shows the FRP material properties in general compared to steel.

Matrix Tensile

strength

[MPa]

Modulus of

elasticity

[GPa]

Ultimate

tensile strain

[%]

Thermal

expansion

coefficient [10-

6/C]

Poisson´s

coefficient

Polyester 34,5-104 2.1-3.45 1.0-6.5 55-100 0.35-0.39

Epoxy 55-130 2.75-4.1 1.5-9.0 45-65 0.48-0.4

Vinyl ester 73-81 3.0-3.5 4.0-5.0 50-75 0.46-0.49

7

Table 2 Properties of common fibers

Fibre type Tensile

strength

[MPa]

Modulus of

Elasticity [GPa]

Elongation

[%]

Coefficient of thermal

expansion [10-6/°C]

Poisson’s

coefficient

Carbon (HM) 2500-4000 350-800 0.2-0.9 -1.2-(-0.1) 0.20

Carbon (HS) 6000 240 -2.0 -0.6-(-0.2) 0.20

Aramid 3500-4100 70-130 2.5-5.0 2.0 0.35

E-glass 2000-3700 72-77 3.0-4.5 5.0 0.22

S-glass 3500-4900 80-90 4.2-5.4 2.9 0.22

AR glass 3000-3300 71-74 3.0-4.3 - -

Figure 2 General properties for GFRP, CFRP, AFRP compared to steel (Fib, 2007).

The thickness of the fibres is in between 3-20 μm. To improve the wetting properties and to

create better adhesion between the matrix and the fibres the surface of the fibres is coated with

sizing. The fibres are also provided with a coating out of a coupling agent that provides a

flexible layer at the interface and improves the strength of the bond as well as reducing the

number of voids in the material. The most commonly used fibre is glass, due to its low cost. In

high temperatures the tensile strength of glass fibres is reduced, but that is not a problem for

FRP in normal construction. A reduction of strength can also occur with chemical corrosion or

sustained loads. The most inexpensive glass fibre is E-glass, which has a wide application in

the FRP industry. S-glass is more expensive but comes with higher tensile strength and modulus

of elasticity, but due to its higher cost S-glass is not as frequently used as E-glass. Alkali-

resistant (AR) glass fibres have added zirconium to prevent corrosion by alkali attack. Carbon

fibres possess the most strength, have higher modulus and better durability, but are 10-30 times

more times than glass fibres (Fib, 2007).

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6 7

Stre

ss [

MP

a]

Strain [%]

GFRP

CFRP

AFRP

Steel

8

2.4 Manufacture process

FRP materials are most commonly manufactured by pultrusion, braiding and filament winding.

In braiding two or more yarns interlock to form an integrated structure. Filament winding is a

process where continuous fibres are impregnated with matrix resin and then wrapped around a

mandrel. The pultrusion process, seen in Figure 3, is the most common technique for

manufacturing continuous length of FRP bars that have a constant or nearly constant profile

(straight and bend bars). From the creel continuous strands of reinforcing material are drawn,

then pulled through a resin tank where the reinforcing material is saturated with resin. After

that, the material is shaped to the correct profile and lastly heated and cooled. With internal

rebars the surface of the bars is usually braided or sand-coated to ensure a good bond with the

concrete (ISIS, 2007).

Figure 3 A simplified description of the Pultrusion process. (ISIS, 2007)

2.5 History of FRP

The first application of FRP was a boat fabricated in the mid-1930s, but due to the high

brittleness the material did not perform well enough. Later, during the Second World War the

U.S navy, U.S force and British air force began to look into FRP due to its low weight,

unaffected by magnetism, nonconductive and resistance to corrosion. The aluminium alloy,

which was used had corrosion and fatigue problems, so the military searched for replacement

material. Fiberglass was the first FRP used, followed by carbon and aramid. The uses of FRP

spread in the military from ships and aircraft to space applications.

The start for civil uses of FRP was after the Second World War in oil pipes. Due to non-

corrosive behaviour and chemical resistance GFRP fitted well as pipes in both the oil and

chemical industry. Since then, the uses have far exceeded the military use in different

applications, shown in Figure 4 (Sentler, 1992). When FRP was introduced in the 1960’s into

the marine industry they quickly become the largest consumers of FRP and nowadays most of

the leisure boats are made from FRP. Some years later FRP was introduced into sports

applications to provide equipment with high strength and low weight and fishing rods, racket

9

sport and bikes were made of FRP. A decade later the car industry became the biggest consumer

of FRP, which they continue to be. In the 1970’s and 1980’s the number of industries that used

FRP grew even bigger, which led to civil airplanes, the electricity industry and the

manufacturing of windmills rotors and towers being mainly constructed using composites

(Compbell, 2010). In Figure 4 the use of composite in different disciplines can be seen. The

construction industry stands for a big part of the consumption.

Figure 4 Distribution of FRP (Pathan, et al., 2017).

History of FRP in the construction industry.

Research for FRP in the building construction industry began in the 1950’s, mainly in the UK,

USA and the former Soviet Union. Although it wasn´t until the 1970’s that the quality was good

enough to satisfy the needs, thanks to a new manufacturing technique, pultrusion. Composite

got larger attention due to corrosion of existing buildings in the 1980’s. The first FRP system

was a prestressed pultruded GFRP rebar in a pedestrian bridge, 6.7 meter long. The first road

bridge was made in Europe, Düsseldorf Germany, it was a 47 m long bridge reinforced with

prestressed GFRP rebars built-in 1987, but because of high stresses in the rebars the bridge had

fatigue problems. At the same time in the Nederland’s constructions with AFRP were made

prestressed, due to the low commercially viable of GFRP. In the mid-1990’s Japan was the

leading country in FRP, with more than 100 demonstration or commercial projects. During this

time, they developed the first standard for constructing with FRP. A decade later China raised

to be the largest user of FRP for new structures, in all from bridge decks to works underground.

In the 2000’s Canada as immersed as the leading country using FRP as internal reinforcement,

Canadian Highway Bridge Design Code (CAN/CSA-S6-06) is today one of the most

established standards. The USA standard ACI 440.1R-15 has also immersed as one of the most

used standards worldwide (Täljsten & Blanksvärd, 2018).

Application with CFRP begun somewhat later, in the early 1990’s in Switzerland and was used

as external reinforcement. Compared to the early applications of GFRP, this was a commercial

success that led to strengthening and retrofitting in existing structures to be the largest

application for FRP in the construction industry (ACI 440.1R-15, 2015).

10

2.6 FRP as internal reinforcement

The use of externally bonded FPR for retrofitting concrete structures is today commonly used

and accepted worldwide, Scandinavia included. The use of internal reinforcement has not had

the same rate of success, even though it is frequently used in both North America, Asia and the

middle east. In Sweden there is a lack of knowledge for the construction industry, the

constructor, the entrepreneur and the civil engineers all lack the proper knowledge to design

structures with FRP. The owner of structures does not consider the long-term cost and rather

just looks at the immediate cost. With calculations and standards on how to use FRP as internal

reinforcement would result in more frequently used, this was the case when calculation method

and guidelines for strengthening with FRP was introduced in Sweden (Täljsten & Blanksvärd,

2018).

In conventional reinforced structures (RC) using steel, the method is to design a balanced

section meaning that concrete will crush at the same load as the steel rebars yields. However,

with the safety factors used, the section is in general under-reinforced, which means that the

strength of the rebars is fully utilized. When the steels yield the structure suffers from large

deflection and the failure mode has high ductility, which alerts people in the structure that for

example a beam is failing. With steel the stiffness ratio is similar to that of normal concrete,

therefore, the neutral axis depth is around the middle of the overall effective depth for a

balanced rectangular section. The strength to stiffness ratio for FRP reinforcement is magnitude

greater compared to concrete, therefore the neutral axis depth is very close to the compressive

end for a balanced section, shown in Figure 5.

Figure 5 Strain distribution for a steel and GFRP RC balanced section (Fib, 2007).

Balanced FRP RC element which fully utilizes the strength of the FRP, there the majority of

the section is subjected to tensile strains, see Figure 5. This leads to larger flexural deflection

and a greater strain gradient in the compressive zone. Prestressing or post-tensioning the FRP

reinforcement will eliminate most of the problems above but makes the construction process

much more difficult and expensive. However, by its nature, RC cracks in tension with the FRP

rebars to prevent or control the opening cracks. Due to the large difference in stiffness for a

cracked and un-cracked section the stress in the rebars also differs between the two states in the

11

section. This causes high surface shear stress on the rebars which can cause slip of the rebars

(Fib, 2007).

As previously mentioned, the fibres used in FRP have a linear behaviour, this means that FRP

also has a linear behaviour, there is no yielding for FRP as for steel. Therefore, the FRP RC is

usually designed as an over-reinforced section, concrete cursing has some ductility proven in

experiments compared to rupture failure of FRP. With an over-reinforced section, the

reinforcement is not fully utilized, which increases the cost of the structure. Also, the partial

safety factors for FRP material become somewhat irrelevant when the strength is not fully used.

The tensile strength is much higher for FRP in comparison to steel, the modulus of elasticity is

generally lower for FRP leading to the serviceability limit stats (SLS) often governs the design.

The deflection should be within acceptable limits to prevent loadbearing structures from

damaging non-structural elements. Because FRP does not corrode like steel, the crack width

criteria are higher, although crack control is still important for aesthetic reasons, creep rupture

and shear effects. The strength and stiffness of FRP are dictated by several factors: different

properties in fibres and matrix, fibre-volume fraction, curing and the manufacturing process.

For example, a change in the diameter of GFRP from 9.5mm to 22.2 mm can reduce the tensile

strength by 40 percent. This is due to shear lag, where the outer fibres are more affected

compared to the inner core fibres It is crucial that the properties of each FRP product is retrieved

directly from the manufacture (Fib, 2007).

There is no standard for manufacturing GFRP which leads to properties given by the

manufacturers. Three different products of GFRP are presented in Fel! Ogiltig självreferens i

bokmärke., Combar from Schöck, fiberglass rebar from Owens corning and V-ROD from

Pultrall. For reference, steel is included in the table. GFRP has lower weight and stiffness

compared to steel but higher strength in comparison to steel, in Figure 6 a comparison between

GFRP and steel can be seen in a stress/strain graph. All properties are gathered directly from

the manufacturer's website, if the ultimate strain was not given it was calculated from the other

properties. There is a lot more manufacture on the market.

Table 3 GFRP rebars from different manufacture, retrieved from the manufacture’s website.

Product Density

[kg/m]

Tensile

strength

[MPa]

Modulus

of

elasticity

[MPa]

Ultimate

tensile

strain

[%]

Combar Ø8 0.111 1 500 60 000 2.5

Combar Ø16 0.442 >1 200 60 000 2.5

Owens corning Ø 16 0.476 1000 60 300 1.7

V-ROD 46 Ø15 0.404 1000 46 000 2.17

V-ROD 60 Ø 15 0.442 1100 60 000 1.8

Steel 15 Ø16 1.58 500-700 200 000 6-12

12

Figure 6 Stress/strain graph with different GFRP rebars and common steel, graph done on the rebars from

Balanced FRP RC element which fully utilizes the strength of the FRP, there the majority of

the section is subjected to tensile strains, see Figure 5. This leads to larger flexural deflection

and a greater strain gradient in the compressive zone. Prestressing or post-tensioning the FRP

reinforcement will eliminate most of the problems above but makes the construction process

much more difficult and expensive. However, by its nature, RC cracks in tension with the FRP

rebars to prevent or control the opening cracks. Due to the large difference in stiffness for a

cracked and un-cracked section the stress in the rebars also differs between the two states in the

section. This causes high surface shear stress on the rebars which can cause slip of the rebars.

As previously mentioned, the fibres used in FRP have a linear behaviour, this means that FRP

also has a linear behaviour, there is no yielding for FRP as for steel. Therefore, the FRP RC is

usually designed as an over-reinforced section, concrete cursing has some ductility proven in

experiments compared to rupture failure of FRP. With an over-reinforced section, the

reinforcement is not fully utilized, which increases the cost of the structure. Also, the partial

safety factors for FRP material become somewhat irrelevant when the strength is not fully used.

The tensile strength is much higher for FRP in comparison to steel, the modulus of elasticity is

generally lower for FRP leading to the serviceability limit stats (SLS) often governs the design.

The deflection should be within acceptable limits to prevent loadbearing structures from

damaging non-structural elements. Because FRP does not corrode like steel, the crack width

criteria are higher, although crack control is still important for aesthetic reasons, creep rupture

and shear effects. The strength and stiffness of FRP are dictated by several factors: different

properties in fibres and matrix, fibre-volume fraction, curing and the manufacturing process.

For example, a change in the diameter of GFRP from 9.5mm to 22.2 mm can reduce the tensile

strength by 40 percent. This is due to shear lag, where the outer fibres are more affected

compared to the inner core fibres It is crucial that the properties of each FRP product is retrieved

directly from the manufacture.

There is no standard for manufacturing GFRP which leads to properties given by the

manufacturers. Three different products of GFRP are presented in Fel! Ogiltig självreferens i

bokmärke., Combar from Schöck, fiberglass rebar from Owens corning and V-ROD from

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8

Stre

ss [

MP

a]

Strain [%]

Combar Ø8

Combar Ø16

Owens corning Ø16

V-rod 46; Ø15

V-ROD 60; Ø16

Steel

13

Pultrall. For reference, steel is included in the table. GFRP has lower weight and stiffness

compared to steel but higher strength in comparison to steel, in Figure 6 a comparison between

GFRP and steel can be seen in a stress/strain graph. All properties are gathered directly from

the manufacturer's website, if the ultimate strain was not given it was calculated from the other

properties. There is a lot more manufacture on the market.

.

The bond strength of GFRP is presented in documents from the different manufactures. Schöck

reports that the Combar has an equivalent bond property to steel from the tensile test. The

Combar is ribbed to achieve a bond to the concrete and recommended anchor length equal to 5

and 10 times the diameter of the rebar (Schöck, 2018). Pultrall reported higher bond strength

for their products than steel reinforcement. (Pultrall, N.D). A straight bar cannot be bent on a

construction site the same way as steel rebars can. Therefore, the rebars need to be

manufactured into the correct shape before installation.

2.7 Applications of GFRP

There are few applications of GFRP as internal reinforcement located in Scandinavia, but one

of them is a small bridge, build in 1994, located in Ottarp, Sweden. The bridge can be seen in

Figure 7. The cantilever was reinforced with 18 ϕ12 GFRP rebars in the bottom longitudinal

and 22 ϕ12 as transverse reinforcement in the bottom. The cantilever was also reinforced with

33 GFRP ϕ12 stirrups. The middle element was reinforced with GFRP rebars, 18 ϕ 12

longitudinal at the bottom and 33 ϕ12 transverses at the bottom and top. The material quality

of the GFRP was 950 MPa for tensile strength and 50 GPa for modulus of elasticity. The slab

was reinforced with conventional steel. After 10 years of use samples from the bridge were

taken to evaluate the durability of GFRP and no signs of degradation were found (Täljsten &

Blanksvärd, 2018).

Figure 7 Shows a small bridge in Sweden reinforced with GFRP (Täljsten & Blanksvärd, 2018).

In a new water treatment plant (WTP) located in Thetford mines in Quebec, Canada GFRP

reinforcement was used in the slab on the ground, walls and cover slab for two tanks, see Figure

8. The whole WTP covers an area of 1 812 m2 and produces 22 000 m3 drinkable water daily.

In the purification process chlorine is added to the water, this chlorine damages the surrounding

concrete if it is reinforced with steel due to corrosion. To extend service life, reduce

14

maintenance costs, avoid the corrosion problem and improve the life cycle cost efficient GFRP

reinforcement was used in the two chlorine tanks. These two tanks had a volume capacity of

approximately 2 500 m3, with a high of 4.65 m, a width of 24 m and a length of 23 m. The

thickness of the exterior and the middle walls, cover slab and foundation wall was 350 mm and

the thickness of interior walls was 300 mm. The GFRP used was manufactured by Pultrall and

was sand coated. Three different types were used which can be seen in Table 4 (Mohamed &

Benmokrane, 2014).

Figure 8 GFRP reinforcing in the two water tanks (Mohamed & Benmokrane, 2014).

Table 4 The different properties of the GFRP rebars used in the tank project at the WTP in Thetford

mines (Mohamed & Benmokrane, 2014).

Bar diameter

(mm)

Grade Guaranteed

tensile strength

(MPa)

Modulus of

elasticity (GPa)

Guaranteed

tensile strain (%)

15 II 934 55.4 1.69

15 III 1 105 64.7 1.71

20 III 1 059 62.6 1.69

A bridge located in New Brunswick, Canada was rehabilitated in 2015, the bridge was a six-

span slab-on girder supported on five piers, with a total length of 130 m, see Figure 9. The new

slab was a 197 mm retrofitted slab with overhang cantilevers reinforced with grade III (60GPa)

TUF-BAR, #5 (15M), with a tensile strength of 1150 MPa. The type of fiber was E-CR glass

and the matrix was vinyl ester. A total of 21065 m GFRP rebars were used in this project

(TUFBAR, u.d.).

15

Figure 9 Retrofired slab on girder bridge reinforced with GFRP in New Brunswick, Canada

(TUFBAR, u.d.).

In the Saint-Roch neighbourhood of Québec, Canada there is a 40-year-old, two-story (A & B)

parking garage under a multi-storey residential building. The total area of the two levels was 3

160 m2, the slab was a two-way flat slab supported by columns and retaining walls. The slab

had a thickness of 230 mm, increased to a thickness of 367 mm by a concrete cover of about

25mm over the columns. De-icing salt caused spalling of the concrete and clear corrosion of

the steel which led to a costly rehabilitation of the slabs on level A. This deterioration led to the

slabs needed a costly rehabilitation of slab on level A. The main supporting elements were

decided to be repaired at a later time. To prevent the potential for corrosion and provide a

maintenance-free structure GFRP reinforcing bars were used in the new structure flat slabs. The

GFRP used was sand-coated bars grade III with a young’s modulus of 60 GPa, manufactured

by Pultrall (V-rods) and the matrix used was vinyl ester. Four different diameters were used

with different tensile strength, #15 (1 323 MPa), #22 (1 405 MPa), #25 (1 113 MPa) and #32

(1 140 MPa). A total of 40 000 longitudinal meters of GFRP were used, see Figure 10 for the

reinforcement in the slab. (Ahmed, et al., 2016).

16

Figure 10 Level A reinforcement in the parking garage in Quebec, Canada (Ahmed, et al., 2016).

The world’s largest project with GFRP was completed in December 2020 in Jizan, Saudi Arabia

and it is a mitigation channel. With a total length of 23 km and a width of up to 80 meters, it is

a massive structure and a total of 11 000 kilometres of GFRP rebars were used to reinforce the

concrete. In Saudi Arabia, the combination of the surrounding sand consisting a lot of salt and

considerably changes in temperature between day and night leading to cracks in the concrete.

In the region, a lot of the construction consists of repairing existing concrete structures due to

corrosion of steel. The reason for using GFRP was the demand from the city of Jizan for a

service life of more than 100 years. The reinforcement can be seen in Figure 11 (Gardiner,

2020).

Figure 11 picture of the

GFRP reinforcement (Gardiner, 2020).

With the reinforcement weight of only 25% of steel, the installation was very fast because of

the reinforcement being easier to move and position, as well as demanding fewer workers.

When the rebars were placed they were tied together with stainless steel wires. The cost of

GFRP rebar is 3-4 times higher than steel when calculated in dollars per pound, but when

17

calculating cost by volume GFRP is a viable option for steel, especially when the life span is

factored in harsh environments (Gardiner, 2020).

18

3 Durability

This chapter introduces common challenges related to FRP composites. How different

environments affect the GRP rebars and what kind of tests have been carried out. As well as

accelerated laboratory tests examine the durability of GFRP rebars are examined. There are two

different methods presented, when rebars are lowered directly in a harsh environment, e.g.,

heated water saturated with different ions and when rebars are embedded directly in concrete

and then lowered in different corrosive solutions. The last subchapter is about field studies on

GFRP rebars where samples have been taken out from actual structures and examined.

Durability in a new material is essential for its uses. Even though research begun in the 1960s

for the use of GFRP in construction the durability aspects are still not fully understood, one

reason can be the development of new types of GFRP rebars. There are severe harsh

environments that degrade the strength of GFRP. The most interesting environment that

degrades GFRP is the alkaline environment found in concrete. Moist and seawater are also

environments that degrade the GFRP rebars. Aggressive environments can affect the matrix,

the fibers and the interface between fiber and matrix. These three factors need to maintain their

functions for the rebars to maintain its function over time.

Concrete has an internal pH value of around 13 due to the presence of calcium, sodium and

potassium hydroxide. This creates a passivating oxide layer around the reinforcement, usually

steel. This layer prevents any direct contact with water and oxygen that prevents corrosion of

the steel reinforcement. Carbonation of concrete, penetration of chloride ions and sulfuric acids

is the most ordinary reason why the steel corrodes. Factors that affect the protection are concrete

W/C ratio, cement type, curing process humidity and CO2/ chloride/ acid concentration.

Carbonation occurs everywhere but in moist climates and at elevated temperatures, the

protecting layers are neutralized and the pH value goes down leading the carbonation to occur

even faster. In the marine environment, swimming pools and concrete bridges in cold regions

the protection is neutralized by penetration of chlorides. These chlorides can come with the

wind or direct contact with salts or de-icing salts on roads. FRP degradation is affected by

different factors then steel reinforcement, carbonation and chloride does not have a significant

effect on FRP (Fib, 2007).

In GFRP rebars water molecules disrupt Van-der-Waals bonds in the polymer’s chains, which

damage the matrix. This leads to changes in modulus, strength, failure strain and toughness. As

well as swelling which causes cracking and debonding between fibers and matrix. The fibers

can also dissolve due to water. The pressure is accelerated by temperature, with an expansile

increase with temperature over 60 °C. Therefore, in laboratory tests moisture uptake is of

importance to monitor (Fib, 2007).

In concrete the pH value can vary between 11.5 and 14.0 and will degrade the tensile strength

and stiffness in GFRP bars. Alkali produces ions in water, Leading the moisture in concrete to

damage the FRP further. High temperature, higher stress levels in the rebars and extended time

will the degradation process. Most of the literature included indicates that vinyl ester resin has

superior resistance compared to other resins. The degradation process is lower when comparing

19

embedded FRP bars against external ones, due to ions having more mobility in a solution (Fib,

2007).

Ultraviolet light will not affect GFRP rebars embedded in concrete, but if handled incorrectly,

ultraviolet light with direct sunshine can cause degradation of the rebars. This due to the

degradation of the polymer constituents. This means that the rebars need to be protected from

direct sunlight before embedded in concrete (Fib, 2007).

To understand degradation in GFRP rebars, researchers perform accelerated laboratory

experiments to find a model for how much degradation may occur in 50-100 years. Two

different accelerated tests are generally used to determine the long-term durability of GFRP.

The first method is when bars are immersed in a corrosion solution and the temperature is

elevated 40-80 °C to accelerate the degradation of the GFRP rebars, see Table 5 for corrosion

solutions. The other simulation is to embed GFRP rebars in concrete, then immerse the whole

element in corrosion solution with a higher temperature, as in the first method. After a

predetermined time, the rebars are tested in tensile and shear tests. To simulate the service load

a sustained stress is often induced in the experiments on the GFRP rebars (Fib, 2007).

Table 5 Different standard corrosion solution, seawater solution (ASTM D665-03, 2003) and concrete

pore solution (alkaline solution) (ACI 440.1R-15, 2015)

Solution Type Quantities (Gram per litre) pH

Seawater NaCl

24.53

MgCl2

5.2

Na2SO4

4.1

CaCl2

1.16

KCl

0.71

8.1

Concrete pore

solution

NaOH

2.11

KOH

19.63

Ca(OH)2

2.1

- - 13.4

There are standards such as ASTM D7705, that say how to perform accelerate tests which

gives the possibility to compared studies. With Arrhenius law, it is possible to estimate what

different exposed time and temperature does and make long-term prediction durability of

GFRP based on the accelerated test. Together with the alkaline solutions, the law is used to

predict long-term exposure to concrete environments. The primary assumption of this model

is that the single dominant degradation mechanism of the material will not change with time

and temperature during the exposure, but the degradation rate will accelerate with the increase

in temperature (Tu, et al., 2019).

In practical service it is harder to evaluate the durability of RC FRP structures, comparing to

steel rebars there are some indications of corrosion on the bars due to the volume increase,

which can lead to cracks and spalling in the concrete. There are not the same indications for RC

FRP structures due to the degradation process. A sample can be taken from an actual structure

after a certain time and be evaluated with the following tests, but the degradation is lower and

therefore harder to detect. The pH value can be controlled, which indicates what condition the

concrete has. Scanning electron microscopy (SEM-analysis) is a test process that scans a sample

with an electron beam to produce a magnified, high-resolution image for analysis. This to

evaluate if there are voids or debonding between fiber and resin/matrix. To see which

substances there are in a material energy-dispersive X-ray spectroscopy (EDX) can be used.

With the analysis, the rebar substance can be compared to rebar of the same kind before

embedded in concrete, if for example there are any Na+, K+ or OH- in the surrounding concrete

20

it is a sign of degradation. The horizontal shear strength can be tested on small GFRP samples

to directly evaluate degradation. Fibre content and glass transition temperature can be evaluated

to see degradations in samples among other tests. Although it is possible to evaluate an existing

structure reinforced with FRP condition a sample is needed and tested, with no external

indications. When GFFP rebars are loaded with more than moderate stress limit progressive

rupture of the fiber filaments with consequent failure of the rebars may occur, known as creep.

To prevent failure of this kind the standards ACI 440.1R-15 (2015) and ISIS (2007) have set a

limited stress level of 0,25fu for ISIS and 0,2fu in ACI 440.1R-15 (2015). This limited stress is

relatively restricted, but due to the low modulus of elasticity the stress in GFRP rebars should

be low in-service load to minimize the deflection. In practice that means in service limit state

for long term loads the stress in the rebars cannot exceed 20% of the tensile strength according

to ACI 440.1R-15 (2015) and 25% according to ISIS (2007).

3.1 Laboratory tests

All the tests are accelerated using high temperatures and different harsh solutions. The studies

use the same standards to have simulant prerequisites, ASTM Standards.

3.1.1 Test on rebars

Tu, et al (2020) investigated a new generation of GFRP bars to evaluate their performance with

sustained load and harsh environments exposing, such as seawater and concrete environments.

A total of 162 specimens were tested with the parameters: temperature, sustained load, exposed

time and environment. The different temperatures used were 23 °C, 40 °C and 60 °C and the

sustained loads were set to 0%, 20% and 40% of the ultimate tensile strength for the rebars. The

time of exposure was 30, 60 and 90 days, after that tensile strength test, residual modulus of

elasticity was conducted and an SEM observation was made. The results were compared to ACI

440.1R-15 (2015) and a prediction of degradation was plotted using Arrhenius theory (Tu, et

al., 2020).

The results from the tensile tests can be seen in Figure 12, 13 and 14. Degradation of the GFRP

rebars when immersed in concrete, was much higher compared to the samples immersed in

seawater. The degradation rate becomes more obvious with higher stress and temperature. With

a stress level of 40% it was clear that damages in the form of micro-cracks in the matrix

enhanced the degradation, which the authors draw as a conclusion. Therefore, GFRP rebars

should not have a service load above 40% of ffu. When rebars are directly exposed to the harsh

environment the stress level of 20% ffu also affects the degradation, but when the temperature

was 23 °C there was no degradation at all on rebars immersed in seawater for 90 days with 0%,

20% and 40% level of stress. The same result was not found for the concrete solution, where

an increase in degradation was found for 23 °C. All except the rebars immersed in a concrete

solution with the temperature 60 °C and stress level 40% met the requirements in the residual

tensile properties of ACI 440.1R-15 (2015) (Tu, et al., 2020).

21

Figure 12 GFRP result with no stress. S stands for seawater, C stands for concrete environment and the

following number is the temperature (Tu, et al., 2020).

Figure 13 GFRP result with 20% of ffu stress. S stands for sea water, C stands for concrete environment and

the following number is the temperature (Tu, et al., 2020).

50

55

60

65

70

75

80

85

90

95

100

0 30 60 90

Ten

sile

str

engt

h r

eten

tio

n (

%)

Exposure time (days)

G/S/23/0

G/S/40/0

G/S/60/0

G/C/23/0

G/C/40/0

G/C/60/0

50

55

60

65

70

75

80

85

90

95

100

0 30 60 90

Ten

sile

str

engt

h r

eten

tio

n (

%)

Exposure time (days)

G/S/23/20

G/S/40/20

G/S/60/20

G/C/23/20

G/C/40/20

G/C/60/20

22

Figure 14 GFRP result with 20% of ffu stress. S stands for seawater, C stands for concrete environment and

the following number is the temperature (Tu, et al., 2020).

SEM analyses were only made for samples at 60 °C, after 90 days in both the environments.

Results from the SEM observations showed no damages on the fibres themselves, but there

were voids between fibre/matrix. Therefore, the interface between fibre/matrix is the damaged

areas, not the fibres themselves. With higher stress the number of voids increased, this is in line

with the tensile strength retention. Indicating debonding and that the rebars were significantly

affected by the harsh environment and sustained load. The degradation of tensile strength and

tensile strains tends to remain at the same level regardless of temperature and stress, which

means no significant effect on the modulus of elasticity in either of the samples exposed to

seawater or concrete environment. Neither stress nor temperature had significant effects on

effect on the modulus of elasticity. Figure 15 shows changes over time for strain under different

sustained loads. The maximum strain increases at 20% and at 40% load, where 3% respective

7% of the initial strain. This result indicates that the guideline for creep failure stress is

conservative when compared to the design strain in ACI 440.1R-15 (2015), where the creep

stress is 0,2ffu (Tu, et al., 2020).

Figure 15 Change in axial strain under different stress levels. Where a is immersed in seawater and b is

immersed in a concrete environment (Tu, et al., 2020).

50

55

60

65

70

75

80

85

90

95

100

0 30 60 90

Ten

sile

str

engt

h r

eten

tio

n (

%)

Exposure time (days)

G/S/23/40

G/S/40/40

G/S/60/40

G/C/23/40

G/C/40/40

G/C/60/40

23

In Figure 16 a prediction of degradation in GFRP bars with the Arrheneius Theory and test

results are seen. The prediction indicates that a 50% reduction in strength may be reached in

1,5; 1,2 and 0,65 years for 0%, 20% and 40% stress level. The tests were implemented in the

laboratory environment, were the samples where in direct contact with the corrosion solutions

and accelerated process (Tu, et al., 2020).

Figure 16 Prediction of degradation in GFRP bars exposed to concrete pore solution at 20 °C

(Tu, et al., 2020).

Another study that tested GFRP was conducted by Zhang & Deng (2019) they used the same

type of harsh environments and stress levels as Tu, et al., (2020) but in compressive load and

temperature of 40 °C, 60 °C and 80 °C. A total of 165 specimens were tested after 16, 60 and

90 days of exposure and observed with an SEM analysis. The results of the compressive test

can be seen below in Figure 17 and 18. After being immersed in salt solution the remaining

strength in the samples was 17-45% and in the alkaline solution 30-76 % for the different

temperatures. The degradation was larger in an alkaline solution than in the salt solution, this

was in line with the results from Tu, et al., (2020). The failure mode of the bars immersed in an

alkaline solution had a brush-like appearance due to the fracture of the fibers and the debonding

of the fiber/matrix interface, this indicated degradation of that interface. Compared to the failure

of the unconventional bars and salt solution immersed bars which had a longitudinal splitting

of the resin matrix (Zhang & Deng, 2019).

24

Figure 17 Compressive strength retention immersed in salt solution (Zhang & Deng, 2019).

Figure 18 Compressive strength retention immersed in alkaline solution (Zhang & Deng, 2019).

Increasing the stress level from 0% to 20% had less impact than increasing the stress level from

20% to 40%. For instance, the salt solution at 80 °C, after 90 days of strength retention with the

stress of 20% and 40% were 7.5% respective and 17.3% lower compared with no stress level.

For alkaline solution at the same temperature and time, with a stress of 20% and 40% were

18.2%-, respective 45.4% lower strength retention than with no stress (Zhang & Deng, 2019).

The temperature has a great impact as well as time. When comparing the impact of temperature

for sustained loads, higher temperatures lead to higher impact for the sustained load. When the

sustained load was increased from 0% to 40%, the amplitude of degradation was 4.5% at 40

°C, 8% at 60°C and 12% at 80 °C for the salt solution and for the alkaline solution the amplitude

of degradation was 7% at 40°C, 12% at 60 °C and 20% at 80 °C. Compering the impact of

temperature when samples were immersed in 80 C to 40 °C at 0%, 20% and 40% sustained

25

load, the salt solution the reduction of compressive strength where 16%, 20.5% and 23.5% and

for the alkaline solution 26%, 32% and 39% In the study a prediction module were created and

plotted, which was very similar to the experimental result. The module can be used to predict

the long-term compressive strength retentions of GFRP bars (Zhang & Deng, 2019).

The specimens immersed in salt solution at 80 °C for 90 days with no stress had just minor

damages in the surface of the rebars, which was seen from the SEM analysis. The specimen

also had microcracks in the cross-section, which can lead to debonding between fibers/matrix.

When a sample was immersed in a salt solution with the same temperature, but with a sustained

load of 40%, the damages in the rebars were more significant. There was a distinct border

between the damaged and the undamaged areas. The SEM observation for the samples

immersed in an alkaline solution found similar results, higher stress level leads to more damages

in the samples, the same connection is seen for the increase in time. All test results had in

common that no damages on the fibers themselves could be seen, meaning the matrix had

sufficient protection (Zhang & Deng, 2019).

A third study investigated the degradation of alkaline solution (K). In this study tap water (W)

and moisture concrete solution (M CON) was used as a harsh environment. The temperatures

used were 20 °C, 40 °C and 60 °C, the sustained stress was inducing a tensile strain of 3000με

(S3), the same stress recommended by ISIS (2007) and higher stress by inducing a tensile strain

of 5000 με (S5), the strain was achieved via elastic bending. All 334 samples were measured

for moisture absorption, SEM, FTIR (Fourier Transform Infrared Spectroscopy), EDX, tensile

flexural and inter-laminar shear tests (Fergani, et al., 2018).

Figure 19, 15 and 16 shows the moisture absorption and strength retention from tensile tests.

Moisture uptake was lower when a stress was applied to the sample, due to the bending of the

sample causing the cracks to be closed in the compressive zone of the sample. With different

stress distributions the result will look different. When exposed to different temperatures the

moister absorption was slightly higher for the samples exposed to an alkaline solution than for

the samples exposed to tap water. This due to the higher content of free hydroxide ions in the

alkaline solution which acts as a solvent for the polymer (Fergani, et al., 2018).

The initial phase had larger degradation than the rate decreases. The attribution for this is high

diffusion of water molecules which damages the interface between fiber/matrix, this can be

seen in Figure 19, 20 and 21. The rate of water absorption is similar to the rate of retention. For

lower temperatures the exposure of K and M CON were similar, but with higher temperatures

the exposure to M CON had a larger impact. A Higher level of stress also led to higher

degradation. All samples subjected to the sustained stress equivalent to 3000με were within the

recommended limits seen in the Canadian code for high durability bars. The samples subjected

to the sustained stress equivalent to 5000με were the only ones having a lower average strength

retention (Fergani, et al., 2018).

26

Figure 19 Tensile strength (TNS) for samples immersed in alkaline solution and measurement of moisture

absorption (MAb) (Fergani, et al., 2018).

Figure 20 Tensile strength (TNS) for samples immersed in moisture concrete solution and measurement of

moisture absorption (MAb) (Fergani, et al., 2018).

27

Figure 21 Tensile strength (TNS) for samples immersed in moisture concrete solution with sustained load

and measurement of moisture absorption (MAb (Fergani, et al., 2018)).

SEM images gave more evidence of degradation on the rebars, after 8760 h in tap water or an

alkaline solution cracks, debonding and voids were seen for a temperature of 60 °C. With lower

temperatures 20 °C & 40 °C no significant evidence of degradation could be seen. There were

also damages on the fiber itself when immersed in an alkaline solution for 8760 h with a

temperature of 60 °C shown by the SEM images. Despite the damages seen in the analysis,

there were no significant changes in the elastic modulus, regardless of condition and time

(Fergani, et al., 2018).

3.1.2 Tests with rebars embedded in concrete

In the previous subchapter, GFRP bars were immersed directly into a harsh environment.

Manalo, et al., (2020) investigated bare GFRP rebars immersed in harsh environments and then

compared to GFRP rebars embedded in concrete then immersed in harsh environment. The

harsh environments used were tap water, salt solution and alkaline solution, together with three

different temperatures: 23 °C, 60 °C and 80 °C with the exposure time of 28, 65 and 112 days.

An interlaminar shear strength (ILSS) test was conducted to evaluate the mechanical properties

to see long-term durability. SEM and FTIR observations were done to evaluate microstructure

changes in the GFRP bars, as well as analysed moisture uptakes in each specimen directly

exposed to an aggressive environment.

Figure 22 and 23 illustrate the results from the ILSS tests, as expected both temperature and

higher exposure time increased the degradation. The retention in strength was higher in the bare

bars in comparison to the bars embedded in concrete when exposed to a similar environment.

When exposed to an alkaline solution with a temperature of 80 °C for 112 days, the ILSS test

showed a retained strength of 68% for GFRP bars in concrete compared to 23% retained

strength in bare bars. The most aggressive environment where the alkaline solution in both

cases, followed by tap water. The least harsh environment was salt solutions (Manalo, et al.,

2020).

28

Figure 22 Bare GFRP bars in different solutions. TW is tap water, SS is salt solution, AS is alkaline

solution (Manalo, et al., 2020).

Figure 23 GFRP bars embedded in concrete then immersed in different solutions. TW is tap water, SS is

salt solution, AS is alkaline solution (Manalo, et al., 2020).

The SEM analysis showed similar results as the ILSS, meaning that the interface between

fibres/matrix had fewer voids in the GFRP rebars embedded in concrete compared with the bare

bars. There was no evidence that the fibre itself had been damaged, but there was some

debonding between the matrix/fibre. The FTIR spectra did not show any significant changes in

the chemical structure of the GFRP bars, only an increase of OH- ions when bare GFRP bars

were immersed in an alkaline solution. This was only observed at the surface of the bars, which

indicates that water absorption only was concentrated in the thin-rich area of the bars (Manalo,

et al., 2020).

The durability of GFRP bars embedded in concrete in various environments was also

investigated by Jia, et al., (2020). A total of 150 specimens were created by casing concrete

around a bar of GFRP in a cylinder-shaped mold. Tested parameters for this study were

degradation in tap- and seawater, different W/C, effects of humidity for the rebars and the

effects of concrete covering (Jia, et al., 2020). In Table 6Fel! Ogiltig självreferens i

bokmärke. an overview of the changed parameters can be seen.

29

Table 6 Details of the specimens in various exposure conditions (Jia, et al., 2020).

Container

designation

Exposure

condition

Temperature

[°C]

Water to

cement ratio

(W/C)

Cover depth

[mm]

Number of

specimens

Tank 1 (T1) Tap water 40 0,4 47 12

Tank 2 (T2) Tap water 60 0,31; 0,4; 0,6 47 36

Tank 3 (T3) Seawater 40 0,4 47 12

Tank 4 (T4) Seawater 60 0,4 47 12

Humidity

chamber 1 (H1)

RH 98 % 60 0,4 47

62

12

14 (*)

Humidity

chamber 2 (H2)

RH 85 % 60 0,4 47

62

12

14 (*)

Humidity

chamber 3 (H3)

RH 75 % and

changed to RH

65 %

60 0,4 62 14 (*)

Humidity

chamber 4 (H4)

RH 75 % 60 0,4 47 12

Total 150 specimens.

*including 2 for conductivity test.

Results show that both in tap water and seawater there was a process of degradation in the

GFRP bars. In these experiments, the exposure to seawater had a bigger impact on the GFRP

bars compared to exposure to tap water, but the mechanics in the degradations are different

depending on the type of water. Tap water has a greater impact on the GFRP itself, the moisture

gain is greater and the concentration of ions in the bars is bigger than with seawater. On the

other hand, with seawater, the depth of penetration is higher when embedded in concrete. When

immersed in 60 °C tap water or seawater for 120 days the tensile strength was reduced to 61,6%

for tap water and 60,7% for seawater. When the samples were immersed in 40 °C respective 60

°C tap water for 120 days the result was clear; the temperature had affected the durability.

Tensile strength was reduced by 39,0 % for a water temperature of 60 °C and for 40 °C water

the strength was reduced by 19,8% (Jia, et al., 2020).

Unexpectedly the result showed that lower W/C led to a larger reduction in strength. With a

W/C ratio of 0,6, 0,4 and 0,31 the reduction of strength was 33,7%, 35,7% respective 38,5%

after 120 days with a temperature of 60 °C. Concrete with a lower W/C-ratio had lower porosity,

which leads to a lower amount of ion reaching the rebars, but with a lower W/C-ratio the

concentration of ions in the concrete itself is higher. This result indicates that the ions produced

by the concrete reaction are more harmful to the rebars than the ions from the surrounding

environment.

The result for the humidity and concrete cover tests is illustrated in Figure 24, there was a big

difference for retention in strength from 85% RH up to 95% RH. For moisture to efficiently

penetrate the concrete the RH moist be relatively high. The results also clearly indicate that

concrete cover has a negative effect on the retention strength. This is due to the moisture

gradient generated along the cover zone, which means more moisture lingers in the concrete

(Jia, et al., 2020).

30

Figure 24 Tensile strength retention of GFRP rebars exposed to different humidity and with different

concrete cover in 60 °C (Jia, et al., 2020).

Due to the corrosion of steel in the concrete, freshwater is used. In some regions, freshwater is

a shortage resource were using seawater could be a solution if steel reinforcement were

exchanged with GFRP rebars. Khatibmasjedi, et al. (2020) evaluated the durability of GFRP

reinforcement in concrete with seawater compared to conventional concrete. The samples were

immersed in seawater at 60 °C for 24 months. The researcher evaluated tensile and shear

properties of the rebars, as well as bond strength and an SEM analysis were made. The result

indicated that there was no significant difference in the degradation of the GFRP bars in the

different concrete mixes. Figure 25 shows the results from the transverse shear test after

exposure for 24 months. The average decrease of mechanical properties for both concrete mixes

was tensile strength 21%-26%, tensile modulus 6%-12%, horizontal shear strength 21%-26%

and transverse shear strength 25%-28% (Khatibmasjedi, et al., 2020).

31

Figure 25 Transverse shear test, mix A is conventional concrete and mix B is with seawater

(Khatibmasjedi, et al., 2020).

An article written by He, et al., (2017) investigated the consequences of concrete cracking and

how it affects durability. 84 beams, 1100 mm long, 80 mm wide and 110 mm high were

fabricated. One GFRP rebar was placed in the middle of the beam vertical and 30 mm from the

bottom horizontally. Tested parameters where the impact of cracks for the durability and

sustained loads immersed in tap water or an alkaline solution, seen in Table 7. The temperature

for the beams immersed in tap water was 23 °C and 60 °C for the beams immersed in an alkaline

solution (He, et al., 2017).

Table 7 Different test setup (He, et al., 2017).

Environment Process

method

Sustained

load level

Name 6-month

aging

numbers of

specimens

12-month

aging

18-month

aging

Unconditioned … RE 4 4 4

Tap water Without pre-

cracks

20 % TW20 4 4 4

Tap water With pre-

cracks

20 % TPW20

4 4 4

Tap water With pre-

cracks

40 % TPW40 4 4 4

Alkaline

solution

Without pre-

cracks

20 % AS20 4 4 4

Alkaline

solution

With pre-

cracks

20 % APS20 4 4 4

Alkaline

solution

With pre-

cracks

40 % APS40 4 4 4

32

The most degradation of GFRP bars was seen in the initial state, this was in line with previous

studies. For the beams immersed into tap water the difference between a pre-cracked beam and

a beam without any cracks was not significant. After 18 months with higher sustained load

TPW40 showed more degradation compared to TPW20, 87.3% compared to 84.2% of retention

of tensile strength. The beams immersed in an alkaline solution followed the same trend but

had a higher degradation. The degradation was faster in the initial phase, but due to changes in

temperature during the experiment, it is difficult to compare and make an assumption. There

were differences between pre-cracked and uncracked beams, which could indicate that cracks

have a larger impact in an alkaline solution (He, et al., 2017), this can be seen in Figure 26.

Figure 26 Retention of tensile strength for the rebars (He, et al., 2017).

An SEM observation was conducted on the rebars after 18 months of aging, no difference was

observed between the micrographs of the beams with or without cracks. After 18 months of

aging the microstructure in the GFRP rebars with 20% sustained load showed no significant

evidence of degradation. The rebars with a sustained load of 40% showed some damage in the

microstructure after 18 months for both solutions. There where debonding between fibre/matrix

in TPW40 and debonding between fibre/matrix and damages to the fibres itself in APS40 The

FTIR analysis showed no changes in the matrix’s chemical structure (He, et al., 2017).

33

3.2 Field tests

It is important that the laboratory experiments are compared to field studies in order to examine

the validity of the experiments. Because the experiments are accelerated to simulate a part of

the service age of the rebars, therefore the rebars needs to be tested to calibrate the equations

and method used in the predictions. A concrete core is taken from the structure to be analysed,

small samples of rebars are extracted from the concrete core and then tested. In Table 8 all the

tests and results from four different articles can be seen.

Table 8 Different test and the results from four studies, if there are a “-“ means that no data where

available.

Different

Data/Tests

(Gooranorimi &

Nanni, 2017)

(Benmokrane, et

al., 2018)

(Al-Khafaju, et al.,

2021)

(Ramanathan, et

al., 2021)

Description Sierrita da la Cruz

Creek bridge are

24.1 m long 13,8 m

wide with 7 spans.

Span 6 & 7 had

GFRP bars in the

top of the slab.

At abridge from

Val-Alain in

Quebec in Canada

there were a barrier

walls are fully

reinforced with

both bent and

straight GFRP

rebars.

GFRP rebars was

extracted from 11

different bridges in

the USA for

assessment of

durability.

Samples from a

dry-dock was

taken from Pearl

Harbor in Oahu,

Hawaii. With a lot

of drying/wetting

cycles.

Thermal range -6 °C to 35 °C -30 °C to 30 °C Estimated freeze

thaw cycle duration

90-200 days.

18 °C - 30 °C

Years in service

before the tests

15 years 11 years 15-20 years 18 years

Use of de-icing

salt

Yes Yes yes No

pH 11-12 (good) 12,3 (good) 12.1-10 -

Corrosion

possible if steel

were used

Yes, carbonations

depth was 32 mm.

- Four bridges had

carbonations or/and

chloride penetration

depth that could

reach the

reinforcement.

Yes, chloride

penetration had a

depth of 50 mm.

SEM analysis No signs of

degradations, very

small voids between

concrete and resin

(due to

preparations).

No signs of

degradation, very

small voids

between concrete

and resin (due to

preparations).

Bent bars had some

minor voids, but

most likely from

the manufacturing

Very small

damages on some

fibers (0.05%),

could be due to the

preparations. One

bridge had more

damages to the

fibers due to

abraded during

manufacturing.

Some small

microcracks due to

No significant

damages to the

fiber itself.

Although at the

surface of the

rebars there were

some voids,

debonding

between

fiber/matrix and

cracks in the

matrix.

34

process according

to the author.

preparations and

degradation.

EDS analysis No signs of

degradations but

there whereno

reference samples so

hard to make a

conclusion.

No signs of

degradations

No signs of

chemical attacks on

the fibers. In one

bridge Na+ from the

surrounding was

found that could, be

chemical attack of

the matrix

No comparison

possible.

Indications for

possible alkaline

attack where seen.

Present of Cl- and

K+ in the matrix.

Horizontal shear

strength

One small increase

and one decrease in

strength compered

to GFRP

manufactured in

2015,

Samples may have

been damaged

during extraction,

making a conclusion

impossible.

Two samples with a

loss of

approximately 15 %

of strength.

Three bridges had

control bars, the

retained strength

was 72%, 76% and

92%.

The samples with

72% and 76%

remained strength

was smaller than

the

recommendations,

this could explain

the low values.

Horizontal shear

tests indicated

some damage to

the GFRP in the

layers of surface,

but the core had

no damages.

Fiber content Very close to the

original. Meet the

recommendations.

No signs of

degradations, but

the

recommendations

of fiber content

were not met.

Only one bridge

had lower fiber

mass content than

today’s

recommendations

(70%)

No significant

degradations

X-ray

Fluorescence

spectrometry

- No signs of

degradations

- -

Water absorption

- Increase of

absorption in

straight bar, but still

under the

requirements.

Lower absorption

for bent bars.

Three bridges had

too high-water

absorption, but the

samples were

fabricated before

the standard, no

conclusion can be

made of

degradation.

Higher water

absorption than

the standard due

to degradation or

non-existing

standards when

fabricated

Solid-state NMR

spectroscopy

- No signs of

degradation.

- -

FTIR spectra of

the surface.

- No signs of

degradation.

- -

Authors comment

on GFRP

The study provides a

partial confirmation

for GFRP bars

maintaining their

microstructural

integrity after 15

years.

- The tests indicates

that GFRP can be

considered a

promising

replacement for

steel.

The results and

recent advance in

GFRP production

technology GFRP

is an attractive

replacement for

steel.

35

For Gooranorimi & Nanni (2017) the horizontal shear strength test was only accomplished for

two different samples with different diameters. The author also mentions that some damages

from when the samples were extracted could make the results invalid. The study by

Benmokrane, et al., (2018) only had two samples to examine the shear strength as well, the

result showed retention of 85% from the original strength. The fracture morphology tests done

on the samples had no indication of degradation in the fiber/matrix interface, which contradicts

the shear strength tests. More samples are needed for a conclusion on the shear strength tests to

be made (Benmokrane, et al., 2018).

In the study by Al-Khafaju, et al., (2021) the shear strength obtained better results on

degradation for one bridge, with a retention of the strength of 92%. Shear strength was tested

on samples from two other as well bridges, although the samples were not long enough to be

tested according to ASTM D4475, meaning conclusions are hard to make. Ramanathan, et al.,

(2021) found evidence of degradation, with a loss of 30% of the shear strength by continuous

drying/wetting cycles leading to cracking of the matrix, explaining the large loss in strength.

The manufacturing process has a big impact as well, if there are voids after the manufacturing

process degradation will start instantly (Ramanathan, et al., 2021).

In a study by Al-Khafaju (2021) a tensile test was conducted on samples from one bridge. The

samples were too short for the standard test in ASTM D7205, but if the samples were sliced to

a flat coupon therefore the samples were sufficient to be tested according to ASTM D3039.

With calculations on the estimated strength loss in tensile, the result was 2.5% loss of the

original tensile strength and the elasticity modulus was 20% higher than the original rebar.

When hypothesized that the degradation rate of the samples was linear, the strength would

decrease by 15% in 100 years of service. Based on the creep rupture, the strength of GFRP

varies with log-time and the degradation rate is larger in the initial stats the authors suggest

degradation of only 3.6% in 100 years. More research needs to be done to make good

predictions with this method (Al-Khafaju, et al., 2021).

36

3.3 Summary of durability

The result of laboratory tests was that an alkaline solution was the most harmful environment,

the temperature and sustained stress for bare GFPR rebars had also a big influence. He, et al.,

(2017), Khatibmasjedi, et al (2020) and Manalo, et al., (2020) examined the amount of

degradation of rebars embedded in concrete. Comparing the results to the experiments on bare

bars the rebars embedded in concrete were less affected by the corrosive solutions, 40

percentage point less degradation in strength. In Manalo, et al., (2020) the comparison between

bare bars and bars embedded in concrete immersed in a harsh environment was done in the

same study, where the concrete protected the rebars significantly. It is at the interface between

fibres and matrix where degradations most often occur. All studies presented evidence claiming

that the fibres get protected by the matrix and do not get damaged by harsh environments.

In the field studies one environment stood out from the rest. A wall located in Hawaii with a

tropical environment and drying/wetting cycles. The samples retrieved from the wall had signs

of degradations in the shear strength tests and water absorption in the surface layer, but not in

the core of the GFRP bars (Ramanathan, et al., 2021).

In one of the bridges from the study by Al-Khafaju et al., (2021) tensile strength was tested and

estimated/ predicted that the degradation of the GFRP rebars could be 3.6% in 100 years of

service. Small indications of micro-cracks in the SEM images were seen, also indicating

degradations. There was no evidence of degradation in the EDS analysis for ten of eleven

bridges, one had some Na in the matrix, but not in contact with the fibres (Al-Khafaju, et al.,

2021).

In the studies written by Gooranorimi & Nanni (2017) and Benmokrane, et al., (2018) there

were no significant signs of degradation, only small defects on the samples but according to the

authors the defects came from the preparations. The horizontal shear strength test was in both

cases insufficient to draw any conclusions. The harsh environment for the samples was exposed

to de-icing salt and repeated freeze/thaw cycles in both studies (Gooranorimi, et al., 2018)

(Benmokrane, et al., 2018).

37

4 Study in the Service Limit State (SLS)

One of the most important criteria to consider when designing GFRP reinforced structures is

the service limit state (SLS), this is because of the general low elasticity modulus of GFRP,

about ¼ of steel. The two main criteria’s in SLS is deflection and control of cracks, it is of

importance that deflection is within acceptable limits for the building to remain functional. The

size and extension of cracks are related to deflection, but the width is limited due to

environmental degradation and mainly due to the degradations of steel bars. FRP bars are not

affected by the external environment as the steel, therefore the environmental effect of cracking

is not studied in this thesis. Deflection after immediate, sustained static and dynamic loads need

to be considered as well. The calculating deflection caused by short-term loads can be done

with a fairly simple equation, by calculating the effective moment of inertia which gives the

stiffness of the section combined with modulus of elasticity for concrete. Calculating the

deflection of a long-term load has more parameters to consider, which makes it more

complicated. Parameters are, magnitude and duration of the load as well as material

characteristics, creep and shrinkage of concrete, formation of new cracks and widening of

existing cracks). When comparing the time-dependent deflection due to shrinkage and creep of

FRP-reinforced structure to steel-reinforced structure the time-versus-deflection curves are

similar. This means that the same fundamental approach can be used (ACI 440.1R-15, 2015).

Choosing a minimum thickness of the section and adapting the stress level in the bars at service,

an GFRP reinforced section can have the same ratio of span to deflection as in a steel reinforced

section (ISIS, 2007).

Steel-reinforced concrete is assumed to be approximately linear elastic at the service limit state,

the same assumption is made for FRP-reinforced concrete in this calculation. Further

approximations state that plane sections remain plane, meaning no bond slip occurs. In figure

27 a simplified version of the section can be seen, more rigorous methods are available but not

considered here. Moment of inertia for an uncracked section is equal to the gross moment of

inertia, Ig. A section cracks when the applied moment is larger than the cracking moment, Mcr.

This will reduce the stiffness of the member due to the tension force is only taken by the

reinforcement (ACI 440.1R-15, 2015) (ISIS, 2007).

Figure 27 A cracked section of a loaded beam (ACI 440.1R-15, 2015).

Different standards have evolved through the years which are evaluated in this thesis, ACI

440.1R-15 (2015), ISIS (2007) and a report from Täljsten & Blanksvärd (2018) based on

38

Eurocode 2 (EC2) were used to calculate the deflection in beams. The beams used were

experimental beams retrieved from included literature, for each standard the method used to

calculate the deflection is presented in the subchapters below. The studies retrieved from the

literature were collected in a database with the geometric properties and results included, see

appendix A. According to Eurocode 2 (2008) the deflection is recommended not to exceed

L/250, to preserve the function of the structure and aesthetic. The Deflection value

corresponding to the admissible limit in EC2, L/250, was marked on the load-displacement

graphs retrieved from the studies investigated. With said load the deflection was calculated with

the method described in the subchapters below and later compared to L/250. The deflection

ratio is the experimental (L/250) deflection divided with the calculated deflection for each

standard. With a deflection ratio below 1 the deflection is overestimated and therefore safe,

with a ratio over 1 the deflection is underestimated and deflection is predicted lower than the

experiment.

4.1 ACI 440.1R-15

ACI 440.1R-15 (2015) method is based on a rectangular section with a single layer

reinforcement due to most of the research focus herein. The concept presented can be applied

with members with different geometry and multiple layers with FRP reinforcement, but with

less evidence from the literature that this theory applies equally (ACI 440.1R-15, 2015).

For a single reinforced rectangular section, the moment of inertia of the cracked section is

calculated using an elasticity analysis that recalculates the stiffness of the rebars to concrete.

The ratio between modules of elasticity for concrete and FRP is called nf.

𝐼𝑐𝑟 =𝑏𝑑3

12𝑘3 + 𝑛𝑓𝐴𝑓𝑑2(1 − 𝑘)2 (1)

Where k is

𝑘 = √2𝜌𝑓𝑛𝑓 + (𝜌𝑓𝑛𝑓)2 − 𝜌𝑓𝑛𝑓 (2)

And nf is the ratio between modules of elasticity for concrete and FRP.

The cracking moment is estimated to be, where λ is a factor that takes the density of concrete

in consideration. For normal concrete λ=1.

𝑀𝑐𝑟 =

0,62𝜆√𝑓𝑐𝐼𝑔

𝑦𝑡

(3)

To determine the stiffness the moment of inertia is multiplayer with modulus of elasticity for

concrete. Since the stiffens varies between 𝐸𝑐𝐼𝑔 and 𝐸𝑐𝐼𝑐𝑟 an effective moment of inertia was

39

introduced. This so the transition from Ig to Icr should be gradual. The expression to calculate Ie

is shown below.

𝐼𝑒 =𝐼𝑐𝑟

1 − 𝛾 (𝑀𝑐𝑟

𝑀𝑎)

2

[1 −𝐼𝑐𝑟

𝐼𝑔]

≤ 𝐼𝑔𝑤ℎ𝑒𝑟𝑒 𝑀𝑎 ≥ 𝑀𝑐𝑟

(4)

The factor 𝛾 depends on the ratio between the cracking moment and the applied moment, for

simply supported beam with two equal point loads applied the factor is.

𝛾 =

3(𝑎 𝐿⁄ ) − 4𝜉(𝑎 𝐿⁄ )

3(𝑎 𝐿⁄ ) − 4(𝑎 𝐿⁄ )

(5)

Where ξ is

𝜉 = 4 (𝑀𝑐𝑟

𝑀𝑎) − 3 (6)

For a simply supported beam with only one point load in the middle γ is

𝛾 = 3 − 2 (𝑀𝑐𝑟

𝑀𝑎) (7)

The reinforcement ratio is.

𝜌𝑓 =𝐴𝑓

𝑏 ∙ 𝑑 (8)

If 𝑀𝑎 < 𝑀𝑐𝑟 but with a small different the deflection will be significantly underestimated with

the use of gross section properties. This due to cracking in the concrete from shrinkage and the

tensile strength of concrete can vary in the member.

The deflection is calculated with the flowing equation.

∆=𝑃 ∙ 𝑎(3𝐿2 − 4𝑎2)

48𝐸𝑐𝐼𝑒

(9)

The result from the calculation of the deflection according to ACI 440.1R-15 can be seen in

Figure 28, with the point on the left side the deflection is underestimated and the deflection is

overestimated with the point on the right side of the line. A deflection ratio was also calculated,

with experimental over calculated deflection, so if the deflection ratio is under one the

deflection is overestimated. The mean value for all the deflection ratio calculated using ACI

440.1R-15 was 0.81, see appendix A for more data on each beam.

40

Figure 28 The result from calculation of the deflection according to ACI 440.1R-15 (2015), see appendix A

for database.

4.2 ISIS

There are many equations that are the same in ISIS (2007) as they are in ACI 440.1R-15 (2015),

calculating Icr (2), k (3), ρf (7), nfrp and Δ (8). Although the cracking moment (11), where the

neutral axis is (10) transformed moment of inertia (13) and the effective moment of inertia (14)

are different. Calculation the cracking moment.

𝑀𝑐𝑟=

𝐼𝑡𝑟 ∙ 𝑓𝑐𝑡𝑚

ℎ2

(10)

The position of the neutral axis is calculated.

𝑦𝑡 =

𝑏 ∙ ℎ ∙ℎ2 + (𝑛𝑓 − 1) ∙ 𝐴𝑓 ∙ 𝑑

𝑏 ∙ ℎ

(11)

The transformed moment of inertia for the section.

0

4

8

12

16

20

24

28

0 4 8 12 16 20 24 28

Exp

erim

en

tal d

eflection

[m

m]

Calculated deflection [mm]

Deflection, ACI 440.1R-15

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

41

𝐼𝑡𝑟 =

𝑏 ∙ ℎ3

12+ 𝑏 ∙ ℎ ∙ (

ℎ

2− 𝑦𝑦𝑡)

2

+ (𝑛𝑓 − 1) ∙ 𝐴𝑓 ∙ (𝑑 − 𝑦𝑡)2

(12)

Using the effective moment of inertia approach the effective moment of inertia for the cross

section.

𝐼𝑒 =

𝐼𝑡𝑟 ∙ 𝐼𝑐𝑟

𝐼𝑐𝑟 + (1 − 0.5 ∙ (𝑀𝑐𝑟

𝑀𝑎)

2

) (𝐼𝑡𝑟 − 𝐼𝑐𝑟)

(13)

The result from the calculation of the deflection according to ISIS (2007) can be seen in Figure

29, The point on the left side shows that the deflection is underestimated and the point on the

right side shows that the deflection is overestimated. A deflection ratio was also calculated,

with experimental over calculated deflection, so if the deflection ratio is under 1 the deflection

is overestimated. The mean value for all the deflection ratio calculated using ISIS (2007) was

0.87, see appendix A for more data on each beam.

Figure 29 The result from the ISIS (2007) standard from the database, see appendix A.

4.3 Eurocode 2

In the SBUF rapport done by Täljsten & Blanksvärd (2018) a method extracted from Eurocode

2 with modifications regarding specifications of materials where used. The first step is to

0

4

8

12

16

20

24

28

0 4 8 12 16 20 24 28

Exp

erim

en

tal D

eflection

[m

m]

Calculated Deflection [mm]

Deflection ISIS

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandr 2020

Chen 2021

42

calculate the stiffness of an uncracked section, which means that the concrete is taking all the

load.

𝐸𝑐 = 22000 (𝑓𝑐 + 8

10)

0.3

(14)

𝐸𝐼𝐼 = 𝐸𝑐

𝑏 ∙ ℎ3

12

(15)

To calculate the stiffness in a cracked section a partition coefficient that takes the tension

stress in consideration. αef is the ratio between concrete and FRP modulus of elasticity.

𝜁𝑓 = 𝑛𝑓 ∙ 𝜌𝑓 ∙ (√1 +

2

𝑛𝑓 ∙ 𝜌𝑓− 1)

(16)

The stiffness of the section in a cracked stated.

𝐸𝐼𝐼𝐼 = 0.5 ∙ 𝑏 ∙ 𝑑3 ∙ 𝜁𝑓 ∙ 𝐸𝑐 ∙ (1 −

𝜁𝑓

3)

(17)

The deflection in both states can now be calculated.

∆𝐼=𝑃𝑎(3𝐿2 − 4 ∙ 𝑎2)

48𝐸𝐼𝐼 (18)

∆𝐼𝐼=

𝑃𝑎(3𝐿2 − 4 ∙ 𝑎2)

48𝐸𝐼𝐼𝐼

(19)

The location of the neutral axis from above.

𝑦𝑡 =

𝑏ℎℎ2 + (𝑛𝑓 − 1)𝐴𝑓𝑑

𝑏ℎ + (𝑛𝑓 − 1)𝐴𝑓

(20)

Cracked moment.

𝑀𝑐𝑟 =𝑓𝑐𝑡𝑚

𝑏ℎ3

12𝑦𝑡

(21)

43

Coefficient to determine where in between the stats the section is. In this cases β=1, for short

term loads.

𝜁 = 1 − 𝛽 (

𝑀𝑐𝑟

𝑀𝐸𝑑)

2

(22)

The final deflection of the section is appreciated to.

∆= 𝜁∆𝐼𝐼 + (1 − 𝜁)∆𝐼 (23)

The result from the calculation of deflection according to a variant of Eurocode 2 can be seen

in Figure 30Figure 29, with the point on the left side the deflection is underestimated and the

deflection is overestimated with the point on the left side of the line. A deflection ratio was also

calculated, with experimental over calculated deflection, so if the deflection ratio is under one

the deflection is overestimated. The mean value for all the deflection ratio was calculated using

EC2 was 0.93, see appendix A for more data on each beam. The point with a calculated

deflection that was only 0.5-2 mm on the beams was according to EC2 not cracked, but in

reality, the section of beams was cracked.

Figure 30 The result from the Eurocode from the database. Deflection ratio is experimental/calculated.

0

4

8

12

16

20

24

28

0 4 8 12 16 20 24 28

Exp

erim

en

tal d

eflection

[m

m]

Calculated Deflection [mm]

Deflection EC2

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

44

4.4 Summary

All three standards evaluated had a large spread on the predicted deflection, with ACI 440.1R-

15 (2015) as the most conservative standard with a mean value of the deflection ratio of 0.81.

The mean value of the deflection ratio for ISIS (2007) was 0.87, slightly less conservative but

with the same spread as ACI 440.1R-15 (2015). The calculation using EC2 according to

Täljsten & Blanksvärd (2018) had the most spread of results shown in Figure 30 but the mean

value for the deflection ratio at 0.93 which was the closest to 1, this excluding 11 beams that

are red marked in appendix A.

45

5 FEM analysis

An experiment done by Jensen (2006) performed at Denmark Technical University (DTU) was

evaluated with the FEM software Atena. A total of four T-beams were fabricated, two of them

reinforced with GFRP, while the other two with steel rebars. All beams was subjected to loading

in a four-point bending, see Figure 31 for the test set-up (Jensen, 2006).

Figure 31 Picture of the bending experiment in Denmark (Jensen, 2006).

5.1 Description of the experiment

Concrete structures reinforced with GFRP are facing other failure modes compared to steel

reinforced structures where the steel yields at the ultimate limit state. An FRP reinforced

structure has either a brittle tensile failure in the FRP bar or a quite ductile compressive failure

in the concrete. For both types of beams enough anchorage is always necessary. Before the test

specimens were manufactured, a proper design was carried out to find the most suitable cross-

section. The designer came up with a T-section, as in Figure 32, providing both compressive

failure for the larger FRP bars and aiming for tensile failure in the smaller FRP bars. The

relatively large section implies that problems caused by the magnitude of scales are minimized.

Figure 32 Geometry of the section (Jensen, 2006).

46

All beams where 4,2 m long and subjected to four-point bending, the test setup can be seen in

Figure 33.

Figure 33 Test setup with the four-point bending (Jensen, 2006).

ComBAR was used as reinforcement for the two GFRP beams, one beam had three 16 mm bars

(BF3O16) and the other had three 32 mm bars (BF3O32). Steel stirrups and compressive

reinforcement were used due to the aim of the study were to examine the flexure behaviour.

BF3O16 were designed to fail with GFRP rupture and BF3O32 was designed to fail due to

concrete crushing. Figure 34 shows the reinforcement configuration. Besides compressive

reinforcement at the flange, transverse reinforcement was used to prevent cracking and

extensive deformations of the flange.

Figure 34 cross section with the reinforcement configuration (Jensen, 2006).

To reinforce the beams so fail in flexural and not shear the stirrups had the configuration as

shown in Figure 35. All stirrups had a diameter of 8 mm and were made of steel with a yield

strength of 550 MPa.

47

Figure 35 Stirrup’s configuration (Jensen, 2006).

Jensen (2006) performed tensile strength tests on all the rebars the obtain the exact data from

the used reinforcement. Both transversal and longitudinal reinforcement made of steel had a

characteristic yield stress of 550 MPa, but the tensile test indicated an average yield stress of

653 MPa. The results of the tensile tests on GFRP reinforcement can be seen in Table 9.

Table 9 Result from the tensile test on the GFRP rebars (Jensen, 2006).

Diameter [mm] 16 32

Number of specimens modulus of elasticity [-] 3 2

Average experimental modulus of elasticity [MPa] 61427 49254

Standard deviation [MPa] 893 569

Number of specimens rupture stress [-] 2 9

Average experimental rupture stress [MPa] 1242 N/A

Standard deviation [MPa] 130 N/A

The results from the test on the 16 mm specimens were similar to the properties given by the

manufacturer, meaning the tensile material the test was valid. The tensile test of the 32 mm

rebar mechanisms contributed to the failure: failure of the ribs, failure of the socket and the

matrix. These problems are common and difficult to avoid, but due to the test on 16 mm rebars

the manufactures data can be considered enough for the 32 mm specimens. The stiffness of the

GFRP with a higher diameter was as expected lower than the specimens with a 16 mm diameter.

The characteristic compressive strength of the concrete was 40 MPa. Both compressive and

tensile tests were conducted on concrete cylinders, the test can be seen in Table 10.

48

Table 10 Results from the tensile and compressive test of the concrete (Jensen, 2006).

Beam BF3O16 BF3O32

Compressive number of specimens [-] 4 4

Average experimental compressive

strength [MPa]

46.8 48.8

Standard deviation [MPa] 2.5 4.0

Tensile modulus of elasticity number of specimens [-] 3 2

Average tensile modulus of

elasticity [MPa]

41568 47175

Standard deviation [MPa] 656 6244

Tensile number of specimens [-] 3 3

Average experimental tensile

strength [MPa]

3.5 3.4

Standard deviation [MPa] 0.4 0.1

The two beams with GFRP reinforcement had different reinforcement ratio and were designed

to fail in different modes, Table 11 shows the preliminary design.

Table 11 The two different GFRP reinforced beam with some data (Jensen, 2006).

Beam Balanced rein-

forcement ratio

Actual reinforce-

ment ratio

Expected failure

mode.

Moment capacity

[kNm]

BF3O16 0.139 0.052 Rupture of GFRP 213.9

BF3O32 0.139 0.225 Concrete crushing 557.5

Instruments that were used to measure the deflection, strain and crack for the test were Linear

Variable Displacement Transducers (LVDT) for displacement, Strain gauges for strains, a

manual measuring system for cracks and optical strain measuring equipment for strains, cracks

and displacements, Figure 36 shows where LVDT instruments were put and cuts are illustrated

as well.

49

Figure 36 Illustration where LVDT was measured and different cuts (Jensen, 2006).

5.2 Description of the FE model

Atena is a nonlinear finite element-based software designed to analysis reinforced concrete.

Atena simulates the actual behaviour of concrete structures, which can be seen in real time

during the analysis. Static, dynamic, creep, thermal, moisture, fatigue and seismic analyses can

be executed with Atena. FRP is available to the analyst in Atena as well as fiber reinforced

concrete, fiber wrapping and lamellas (Cervenka, 2015).

In this analysis Atena engineering 2D was used to simulate the experiment. Due to symmetry

only half of the beam has been modelled in Atena to save computational time, the half beam

was 2000 m long. All the values from the experiment were put into the program. An average

value of the thickness was used, due to 2D modelling, the geometry of the experiment had

slopes on the side of the web and flange, see Figure 32. The total thickness of the flange is

217.5 mm and the web 810 mm, under the applied load there is a steel plate simulation of the

HE180B beam used in the experiment. The beam is simply supported, there is a steel plate

under the beam at the end as in the experiment, the other end (middle of the beam) is restricted

in the x-axis, so the half beam acts like the whole beam. The load applied is predetermined

deformation at a rate of 1 mm/min. The concrete behaviour is based on the formulation by

Červenka (2020), which builds on the constitutive models for tensile and compressive

behaviour, see Figure 37. In Figure 38 the ft is the tensile strength of concrete, Gf is fracture

energy of concrete and wc is the crack opening at the complete release of stress. The mechanical

properties of the concrete were retrieved from the experimental tests on the beam described in

chapter 5.1.

50

Figure 37 Uniaxial stress-strain law for concrete (Cervenka, 2015).

Figure 38 Exponential crack opening law (Cervenka, 2015).

The material properties for the GFRP rebars as well as the steel rebars and stirrups were

retrieved from the experiment. Predetermined steel behaviour was used for steel and linear

behaviour for GFRP. Figure 39 shows the model for beam BF3O16 in pre-processing when no

analysis is done. To achieve the best results in Atena the mesh size should be around 50 mm,

in this case 50 mm was used. Quadrilateral’s elements were used as it is the standard setting in

the program and fitted well with the module. To compute the stiffness of the model a standard

incremental and iterative Newton-Raphson method was used (Cervenka, 2015).

51

Figure 39 Illustration of the half beam modelled in Atena 2D.

5.3 Results from the FEM analysis.

The two GFRP reinforced beams from Jensen’s (2006) experiment is compared to the two

different models that were created in Atena 2D.

5.3.1 BF3O16

In Figure 40 the results from the FE-analysis are compared to the result from the experiment on

beam BF3O16 done by Jensen (2006). Beam BF3O16 was under reinforced and design to fail

in rupture of the GFRP reinforcement, but in both experiment and in the FE model the failure

mode was crushing of concrete at the top. The stiffness of the specimen in FE analysis was in

good agreement with the experimental, but the crack moment was not a perfect match. The

moment when the experimental beam cracks are around 20 kNm and in the module around 28

kNm. The maximal load of the experimental beam was 295 kNm and the maximal load in the

module was 216 kNm, the module only predicted 71% of the capacity of the beam. In Figure

41Figure 44 the manually measured crack pattern is shown from the experiment and can be

compared to the crack pattern from the FE analysis in Figure 42.

52

Figure 40 Results and comparison to the experiment done by Jensen (2006) on beam BF3O16.

Figure 41 Manuel measured crack patterns from the experiment at ultimate load for beam BF3O16

(Jensen, 2006).

Figure 42 Crack patterns from the FE-analysis at ultimate load for beam BF3O16, the crack width is given

in meters.

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140 160

Mo

me

nt [k

Nm

]

Nedböjning [mm]

BF3016

Experiment Model

53

5.3.2 BF3O32

BF3O32 was designed to fail due to crushing of the concrete at the top, which happened in the

model but in the experimental, the failure mode was flange separation. As for BF3O16 the

stiffness of the beam was well predicted in the module, there was a decrease in stiffness at the

end of both the experiment and in the model see Figure 43. At the end of both the model and

the experimental, the stiffness decreased somewhat the crack moment was also well predicted

in the module in beam BFO32, with the experimental crack moment 32kNm and in the module

a crack moment of around 33kNm. The maximal load for the experimental was 457 kNm and

in the module 310kNm, the module only predicted 68% of the capacity of the beam. In Figure

44 the manually measured crack pattern is shown from the experiment and can be compared to

the crack pattern from the FE analysis in Figure 45.

Figure 43 Results and comparison to the experiment done by Jensen (2006) on beam BF3O32.

Figure 44 Manuel measured crack patterns from the experiment at ultimate load for beam BF3O16

(Jensen, 2006).

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70 80 90

Mo

me

nt

[kN

m]

Deflection [mm]

BF3032

Exprement cykel 1 Expriment cykel 3 Modell

54

Figure 45 Crack patterns from the FE-analysis at ultimate load for beam BF3O16, the crack width is given

in meters.

55

6 Analysis

6.1 Durability

When comparing the field studies to the accelerated laboratory test, it is clear that the harsh

environments used in the laboratory are too aggressive. For example, the retention strength of

rebars immersed in an 80°C alkaline solution for 112 days was only 23% (Manalo, et al., 2020),

none of the field tests indicates similar degradation. With a lower temperature of 23°C the

samples from Manalo, et al., (2020) also had a big degradation in strength, only 39% of

retention strength after 112 days which indicates that the alkaline solution is very strong. The

concrete is protecting the GFRP rebars, when samples were embedded in concrete the retention

after 112 days for the alkaline solution was 67% and 80% when the temperature was 80°C

respective 23 °C (Manalo, et al., 2020).

With the service state often govern the design, the allowed sustained stress on the rebars is an

important parameter. Experiments done by Tu, et al., (2020), Zhang & Deng, (2019) and

Fergani, et al., (2018) gave significant results on bare bars that the increase of sustained load

from 20% ffu to 40% ffu had a big impact. For example, in Tu, et al., (2020) study the retention

strength of the GFRP bars when immersed in the 60 °C alkaline solution for 90 days and with

a sustained stress 20% ffu and 40% ffu, the retentions strength difference was 13.01 percentage

points between the sustained loads. The study by He, et al., (2017) also applied sustained stress

of 20% ffu and 40% ffu but on GFRP rebars embedded in concrete, but the difference was only

3.1 percentage in retentions strength after 18 months of exposure to tap water. With the same

setup but immersed in the alkaline solution the difference was 2.5 percentage points in strength

retention. One obvious factor is the protection of the concrete, with the harsh environment is

not in direct contact with the GFRP rebar. Another explanation for the different rate of

degradation comparing bare bars and bars embedded in concrete can be the sustained load in

the laboratory experiments the was induced with direct tensile in the rebars. Compared with the

study done by He, et al., (2017) where rebars embedded in concrete were examined, the

sustained load was a bending moment on the beam. The amount of stress in the GFRP rebar

was the same but it was inducted in different methods, which could have an impact.

In service state the deflection is an important factor, the decrease of tensile strength does not

affect the deflection, but the modulus of elasticity does which in all the studies did not suffer

from any significant degradation. This due to the degradation of tensile strength and strain of

GFRP rebars remains at the same level. As the service stat often governs the design some

degradation of the tensile strength does not make the structure fail. It is of great importance that

the modulus of elasticity does not suffer any degradation for that reason.

56

SEM analysis is a valid method for evaluating durability for both laboratory experiments and

for existing structures, one negative aspect is that the samples could be damaged when prepared.

The results from the shear test and SEM analysis in laboratory experiments were not

contradicted. When the shear or tensile test could not be executed in the field due to lack of

data or minimal samples from the rebars meaning SEM analysis is a valid option. From Al-

Khafaji, et al., (2021) one bridge had a valid shear test that indicated degradations, the same

degradation was seen on the SEM and EDS analysis. There was only one bridge with clear signs

of degradations where de-icing salt was used, because there were only one of all the bridges

with degradation it indicates that de-icing salt has minimal effects on structure reinforced with

GFRP. This conclusion is strengthened by the laboratory experiments where salt solutions were

not as harsh as the concrete environment. The field study done by Ramanathan et al., (2021)

done on a dry-dock in Hawaii had the most degradation. The higher temperature is constant due

to ocean climate, which can affect the degradation of the GFRP rebars. From the laboratory,

test temperature has a big impact and the worst solution was a simulation of concrete, which is

in line with the dry-dock.

From all the laboratory tests the degradation rate is higher at the initial state, then the rate slows

down. This means in the field test if there are limited signs of degradation in the initial state the

rate of degradation could slow down with time. This indicates that if the initial state is over for

the structure in the field the risk could possibly be reduced. The question is how long is the

intimal state in fields? For Zhang and Deng, (2019) after 60 days the degradation rate was

reduced for all the different temperatures used (40 °C, 60 °C and 80 °C). But the degradation

had reduced the strength significant in compered to field study done by Gooranorimi & Nanni

(2017) and Benmokrane, et al., (2018) there no signs of degradation was observed, of the initial

stat has passed then the possibility of degradations on the GFRP reinforcement is reduced.

The higher degradation in the initial state when GFRP rebars are exposed to harsh environments

is explained by the high alkalinity of the interstitial solution due to the cement hydration

reaction. Through diffusion the OH- ions and H2O molecules penetrate the internal GFRP bar

which causes the surface layer of the matrix to swell up, which leads to an osmotic pressure

that promotes the emergence and eventually creates micro-cracks in the GFRP bar. With time

the concrete gradually hardens, the moisture in the concrete is consumed by the reaction to the

cement and evaporated. Carbonation reaction also occurs that neutralizes the alkaline

environment in the concrete, which stop the adverse effect on the tensile properties of the GFRP

bars and instead the concrete protects the GFRP bars from the erosion of outside aggressive

media (He, et al., 2017). This indicates that the intimal state is the most dangerous for

degradation of GFRP rebars embedded in concrete for real structures in the field as well

supports Al-Khafajum et al., (2021) prediction of only 3.6% reduced tensile strength after 100

years, with 2.5% reduced tensile strength after the initial 17 years of service. Carbonation and

chloride penetration are the two different processes that initial corrosion of steel embedded in

concrete. For all constructions outside, eventually one of these processes will occur, but there

are two ways to prevent the penetration of carbonation and chloride and that is a concrete cover

and the concrete W/C (Almgren, et al., 2018). Therefore, there is no degradation of steel in

concrete in the initial state and the degradation does not start until the carbonation or chloride

57

penetration reaches the steel reinforcement, which is the complete opposite of GFRP

reinforcement.

All field studies were done on over 11 years old structures, some even up to 20 years old. In

that time development of new types of GFRP rebars has developed. The durability test must be

executed on the new generation of GFRP all the time, which means that the GFRP rebars

evaluated in the field’s studies are out of data, but if the old GFRP rebars do meet up to the

recommendations of today that indicate a good future for GFRP. The results from the field

studies indicate the validity of GFRP as internal reinforcement.

6.2 Service limit state analysis

The deflection is governed by the stiffness of the section, which can be estimated with different

methods. The factors that affect the estimation of the stiffness are modules of elasticity for

concrete and the estimation of the effective moment of inertia for the section. Parameters

affecting Ie are modules of elasticity for the reinforcement, reinforcement ratio and crack

moment. All three standards have different methods for estimating the stiffness of a section.

This chapter will evaluate some of the parameters to investigate which parameters affect the

standards the most and why over- and underestimations occur. When calculating the stiffness

of a section the transverse reinforcement is not considered. A study done by Khorasani et al.,

(2019) examined the effect of transverse reinforcement in 20 simply supported beams that had

a low-reinforcement ratio and a high reinforcement ratio. For both cases the amount of

transverse reinforcement and the stiffness of the section increased considerably, which indicates

that transverse reinforcement influences the stiffness (Khorasani, et al., 2019).

The results from each standard had a significant disparity regarding overestimations and large

underestimations. The majority of the deflection ratios for all standards had a range between

0.8 and 1, which is a good prediction. The result with a conservative standard agrees with the

literature (Saleh, et al., 2019) (El-Nemr, et al., 2018) (Goldston, et al., 2017).

In Table 12 different methods for calculating the modulus of elasticity for concrete are

presented with an example for concrete with a compressive strength of 40 MPa. Ec is a very

important material property when calculating the deflection, see equation 10 and when

calculating nf. In an experimental study done by Vakhshouri & Nejdai (2018) they predicted Ec

and compared the result to the standards, the comparison indicated that the standards

underestimated the Ec for normal concrete and overestimated Ec for high strength concrete

(Vakhshouri & Nejadi, 2018). The results from the calculation done in this thesis did not

indicate an overestimate of Ec but some underestimation on the normal concrete for ACI

440.1R-15 and ISIS (2007).

58

Table 12 Different equation to calculate the modulus of elasticity for concrete.

Standard ACI 440.1R-15

(2015)

ISIS (2007) EC2 (Täljsten &

Blanksvärd, 2018)

Method of calculating

Ec

𝐸𝑐 = 4500√𝑓𝑐 𝐸𝑐 = 4500√𝑓𝑐 𝐸𝑐 = 22000 (

𝑓𝑐 + 8

10)

0.3

Example of Ec with

fc=40 MPa

𝐸𝑐 = 28460 𝑀𝑃𝑎 𝐸𝑐 = 28460 𝑀𝑃𝑎 𝐸𝑐 = 35220 𝑀𝑃𝑎

6.2.1 ACI 440.1R-15

As previously mentioned, the modulus of elasticity for concrete has a big influence on the

stiffness, with higher strength concrete leading to higher modulus of elasticity. In Figure 46 a

graph was created to show the impact of different concrete qualities for the deflection ratio.

Small indications can be seen towards an overestimation of deflection in ACI 440.1R-15 (2015)

for higher strength concrete, but more studies with high strength concrete are necessary before

any conclusions can be made.

Figure 46 The influence of concrete strength on the deflection in ACI 440.1R-15 (2015).

For the deflection ratio below 0.5 there are some indications that the rebars with smaller

diameters as which in this case is linked to small reinforcement area have an influence on the

overestimations in ACI 440.1R-15 (2015). This can be seen in Figure 47 and 48, the predictions

for the rebars with a larger diameter is a bit more accurate.

0.00

0.50

1.00

1.50

2.00

0 20 40 60 80 100 120 140

Deflection

ratio

Concrete strength [MPa]

Concrete strength ACIAlsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

59

Figure 47 The influence of bare diameter on the deflection in ACI 440.1R-15 (2015).

Figure 48 The influence of reinforcement area on the deflection in ACI 440.1R-15 (2015).

0.00

0.50

1.00

1.50

2.00

0 5 10 15 20 25 30

Deflection

ratio

Φ (mm)

Diameter rebar. ACI Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

0.00

0.50

1.00

1.50

2.00

0 200 400 600 800 1000 1200

Deflection

ratio

Reinforcment area [mm2]

Reinforcment area. ACIAlsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

60

Parameters for the strength of GFRP rebars, ratio between reinforcement ratio and balanced

reinforcement ratio, ratio between applied moment and crack moment was also investigated but

no patterns were found. Modulus of elasticity of the GFRP rebars were also investigated without

any results of patterns in the prediction of deflection compared to the experiments.

6.2.2 ISIS

For the deflection ratio below 0.5, there are some indications that rebars with small diameters,

in this case linked to small reinforcement area, have an influence on the overestimations in ISIS

(2007). This can be seen in Figure 49 and 50, the rebars with larger diameter are predicted more

precise.

Figure 49 The influence of bare diameter on the deflection in ISIS (2007).

0.00

0.50

1.00

1.50

2.00

0 5 10 15 20 25 30

Deflection

ratio

Φ [mm]

Diameter rebar. ISISAlsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

61

Figure 50 The influence of reinforcement area on the deflection in ISIS (2007).

When the ratio between the reinforcement ratio and the balanced reinforcement ratio is lower,

meaning balanced or under reinforced section there are larger underestimations of deflection.

All three beams with a deflection ratio over 1.5 have a reinforcement ratio between 0.8 and 1.5,

see Figure 51. Although, there are beams with the same reinforcement ratio that have a good

prediction and beams with overestimation deflection, which makes it hard to draw any

conclusions.

0.00

0.50

1.00

1.50

2.00

0 200 400 600 800 1000 1200

Deflection

ratio

Af [mm2]

Reinforcment area. ISISAlsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

62

Figure 51 The influence of reinforcement ratio divided with balanced reinforcement ratio on the deflection

in ISIS (2007).

Parameters such as the strength of concrete, the strength of GFRP rebars, ratio between applied

moment and the crack moment were also investigated, but no patterns were found. Modulus of

elasticity of the GFRP rebars was also investigated without any results of patterns in the

prediction of deflection compared to the experiments.

6.2.3 Eurocode 2

As mentioned in chapter 4.3 there were eleven beams with an unrealistic deflection ratio, in

Figure 52 and 49 these beams are not included but they are in Figure 54.

As previously mentioned modulus of elasticity is affected by the concrete strength. In Figure

52 a graph illustrates the influence of concrete quality on the EC2 method, they show a clearer

result than other standards have done. With lower strength of the concrete there can be some

underestimations of the deflection, compared with higher strength concrete. The trend points

towards an overestimation with higher strength concrete.

0.00

0.50

1.00

1.50

2.00

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Deflection

ratio

ρf / ρfb

Ratio between reinforcment ratio and balenced reinfocment ratio. ISIS

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-Sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandr 2020

Chen 2021

63

Figure 52 The influence of concrete strength on the deflection in EC2, with not all beams.

From Figure 53 there is a trend towards the largest underestimations of deflection being

influenced by the reinforcement ratio. All the largest underestimations are with low ratio

between the reinforcement ratio and the balanced reinforcement ratio, but at the same time there

are good predictions and overestimations with the same ratio, which makes it hard to draw any

conclusions.

0.00

0.50

1.00

1.50

2.00

0 20 40 60 80 100 120 140

Deflection

ratio

fc [MPa]

Concrete strength. EC2Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

64

Figure 53 The influence of reinforcement ratio divided with balanced reinforcement ratio on the deflection

in EC2 with not all beams.

The eleven beams that have an unrealistic deflection are cased because the applied moment is

lower than the calculated crack moment, which can be seen in Figure 54. When the crack

moment is lower than the applied moment the section does not crack. The deflection for an

uncracked section is calculated with equation 19. In the experiments the section is cracked

resulting in larger deflection than an uncracked section. Therefore, it is clear that the crack

moment for beams reinforced with GFRP rebars needs to be examined.

0.00

0.50

1.00

1.50

2.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00

Deflection

ratio

ρf / ρfb

Ratio between reinforcment ratio and balenced reinfocment ratio. EC2

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

65

Figure 54 Ratio between applied moment and crack moment, EC2 with all the beams.

6.3 Atena models

The purpose of the FEM analysis was not to calibrate the program, but only to examine the

accuracy of the program. The models corresponded well with the experiment in terms of

stiffness and for BF3O32 also the crack moment, as shown in Figure 40 and 43, although the

module and experiment did not correspond regarding ultimate load. The calculated capacity on

beam BF3O16 was 213.4 kNm which was close the result from the FE-analysis, but the

predicted failure mode was rupture of FRP and in the analysis the concrete was crushed. For

beam BF3O32 the calculated capacity was 447.5 kNm and in the FE-analysis the maximal

capacity was 457 kNm. The results from the experiments done by Jensen (2006) were gathered

from a graph, not the data itself, which means it is an estimation of the result and errors can

occur. This could be the reason why the crack load comparison of beam BF3O16 did not

correspond. However, it is not an explanation why the maximum load for each beam is

underestimated. With the concrete crushing in both models, it is possible that incorrect

calculations have been made, it is on the other hand hard to say if the errors were made by the

program or by the author.

When comparing the crack patterns on the experiment with the FE-analysis for both the beams

it is hard to make any conclusions, because of the different interfaces between them. However,

they both have flexural cracks in the middle and shear-flexural cracks closer to the support. But

for BF3O16 the experimental crack pattern had horizontal cracks in the middle of the beam at

the bottom, no similar crack was seen in the FE-analysis. Between beam BF3O16 and BF3O32

there are inequalities, the crushing of the concrete is more concentrated for beam BF3O16

0.00

10.00

20.00

30.00

40.00

0.0 1.0 2.0 3.0 4.0 5.0

Deflection

Ratio

MEd / Mcr

Ratio between applied moment and moment at the first crack with all the values. EC2

Alsayed 2000

Saikia 2007

Kassem 2011

Kalpana 2011

Al-sunna 2012

El-Nemr 2013

Barris 2013

Ju 2016

Godston 2016

Godston 2017

El-Nemr 2018

Saleh 2019

Abdelkarim 2019

Khorasani 2019

Ramachandra 2020

Chen 2021

66

compared to BF3O32. For BF3O32 the largest cracks are not at the bottom of the beam as they

are for beam BF3O16, which is strange. One reason for this could that in beam BF3O32 had

more reinforcement, preventing cracks at the bottom.

67

7 Conclusions

In several accelerated laboratory experiments where bare FRP rebars were exposed to differ-

ent harsh environments the degradation of strength was significant, where an alkaline solution

at elevated temperature was the harshest environment for the GFRP bars. When GFRP rebars

are embedded in concrete the degradation was significantly lower (around 40 percentage

points), the concrete protects the GFRP rebars considerably. The largest rate of degradation

on GFRP rebars is in the initial state, in comparison to steel which starts to corrode when car-

bonation and/or chloride penetration critical levels of reaches the reinforcement. In field stud-

ies, there were small signs of degradation of the GFRP rebars, mainly in tropical climates. De-

icing salts have a limited effect on the degradation. Laboratory experiments are very con-

servative with unrealistic harsh environments compared to the natural harsh environments.

Therefore, after 20 years of service in harsh environment there were no or small signs of deg-

radation on the GFRP rebars which indicates the validity of GFRP.

All three standards evaluated had a large spread on the predicted deflection compared to the

experiments, with ACI 440.1R-15 as the most conservative standard with a mean value of the

deflection ratio at 0.81. The mean value of the deflection ratio when using ISIS was 0.87,

slightly less conservative but with the same spread as ACI 440.1R-15. The calculation using a

variant of EC2 had the most spread of results, but with a mean value of the deflection ratio at

0.93, this excluding 11 beams that had an unrealistic prediction due to the wrong prediction of

the crack moment. The FEM model created had a similar stiffness as compared to the experi-

ment from DTU, which indicates that the use of Atena was accurate for calculating the deflec-

tion of the beams. Although the ultimate load was not well predicted, probably due to the fail-

ure mode crushing of concrete in the compressive zone.

The use of GFRP in Scandinavia has been limited. There are many possible reasons for this,

one being the lack of knowledge for clients, designers and contractors. If no one asks for it,

not one will learn how to design and consequently, no one will build with GFRP reinforce-

ment. Another reason could be the different behaviour, particularly in the service limit state,

compared to traditional steel reinforced concrete structures. To obtain the same stiffness be-

haviour after cracking considerably more GFRP is needed.

Despite this, there are many structural parts where FRP could be beneficial, for example in

splash zones, in edge beams and slabs etc. This could bring down the costs for maintenance

and also prolong the life span of the structure.

68

8 Future work

One possible reason for GFRP rebars not being used as internal reinforcement in Scandinavia

is the lack of national standards and Eurocode, a standard that would potentially solve the

knowledge deficit for the building construction industry. The situation was similar for external

strengthening of concrete structures with FRP, when guidelines where published the use

increased significantly in Scandinavia. Although the initial cost for constructing with GFRP as

internal reinforcement is higher it is of importance to consider the life cycle costs, mainly

because less maintenance is needed but also because the expected life span of the structures

will be longer.

In the accelerated laboratory tests the GFRP bars are exposed to corrosive solutions, both as

bare bars and as bars embedded in concrete. With the Arrheneius theory the long-term

performance of GFRP rebars is predicted, but the results for this thesis and other articles in the

reference list indicate that the prediction from accelerated laboratory tests is too aggressive

compared to field studies for bars cast into concrete. Further research is needed to establish the

relationship between the degradation of GFRP bars in laboratory simulated conditions to field

conditions, to provide practical and efficient guidelines for GFRP RC structures.

This thesis indicates that long-term exposure to concrete has a limited effect on degradation for

the GFRP rebars as internal reinforcement. More field studies are needed on GFRP rebars to

determine the long-term durability. Furthermore, the structures that are examined in the articles

included in chapter 5.3 are needed to be examined at a later date to see further exposer to harsh

environments. With such comparison, the prediction of long-term durability of GFRP rebars

would be even better because there are two values to compare the result with.

The recommended sustained stress on the GFRP rebars from the service load is only at 20% of

the ultimate tensile strength in ACI 440.1R-15. From the durability chapter there are indications

for when rebars is embedded in concrete the increased stress in rebars change from 20% to 40%

does not affect the degradation notedly. Articles in this thesis did point out that the ACI 440.1R-

15 is conservative on this topic as well. Future research needs to establish a more effective

recommendation on maximum service stress in the rebars to have more effective use of the

material.

The SLS predictions are the most critical when designing GFRP RC structures, this thesis has

examined one of the parameters in SLS the deflection. There was a large spread in the results

from the calculation of the deflection in all the evaluated standards, however with no new

equations developed or suggestions to any changes in the standards from this thesis more

reaches are necessary to obtain more accurate predictions. There are some indicators of the

parameters affecting the deflection calculation for each standard in this thesis that could be used

to develop or modify the existing equations.

The short-term stiffness properties of a beam reinforced with GFRP were examined in this

thesis, long-term properties as creep, fatigue, shrinkage and cyclic loading are not examined.

FE modelling could be used to examine these properties in a GFRP RC structure as well as

experiments. The sustained load for a GFRP RC structure is conservative in the existing

standards, future research is needed to modify the standards to make them more efficient.

69

9 Reference Abdelkarim, O., Ahmed, E., Mohamed, H. & Benmokrane, B., 2019. Flexural strength and

serviceability evaluation of concrete beams reinforced with deformed GFRP bars.

Engineering Structures, Volume 186, pp. 282-296.

ACI 440.1R-15, 2015. Quide for the design and construction of structural concrete reinforced

with fiber-reinforced polymer (FRP) bars, Farmington Hills: American concrete institute.

Ahmed, E., Benmakrane, B. & Sansfacon, M., 2016. Case Study: Design, Construction, and

Performance of the La Chancelière Parking Garage’s Concrete Flat Slabs Reinforced with

GFRP Bars. Journal of Compostites for Construction, 21(1).

Al-Khafaju, A. et al., 2021. Durability assessment of 15- to 20- year-old GFRP bars extracted

from bridges in the US. II: GFRP bar assessment. Joutnal of *Composites for Construction,

25(2).

Almgren, T. et al., 2018. Betong- och Armeringsteknik. Göteborg/Stockholm: Sveriges

Byggindustier.

Alsayedm, S., Al-Salloum, Y. & Almusallam, T., 2000. Performance of glass ®ber reinforced

plastic bars as a reinforcing material for concrete structures. Composites, Volume 31, pp. 555-

567.

Al-Sunna, et al., 2012. Deflection behaviour of FRP reinforced concrete beams and slabs: An

experimental investigation. Composites, Volume 43, pp. 2125-2134.

ASTM D665-03, 2003. Standard Test Method for Rust-Preventing Characteristics of

Inhibited mineral Oil in hte Presence of Water, s.l.: ASTM International.

Barris, C., Torres, L., Comas, J. & Miàs, C., 2013. Cracking and deflections in GFRP RC

beams: An experimental study. Composites, Volume 55, pp. 580-590.

Benmokrane, B., Nazair, C., Loranger, M.-A. & Manalo, A., 2018. Field Durability Study of

Vinyl-Ester-Based GFRP Rebars in Concrete Bridge Barriers. Journal of Bridge

Engiineering, 1 December.23(12).

Cervenka, J., 2015. Atena Program Documentation par 4.1, Prag: Cervenka Consulting.

Červenka, V., Jendele, L. & Červenka, J., 2020. Atena Program Documentation Part 1,

Prague: Červenka, Consulting.

Chen, X. et al., 2021. Experimental and Theoretical Study on the Flexural Behavior of

Recycled Concrete Beams Reinforced with GFRP Bars. Journal of Renewable Materials,

9(6), pp. 1169-1188.

Compbell, F., 2010. Structural Composite Materials. Materials Park ed. s.l.:ASM

international.

Dhinakaran, G., Gowrisankar, S. & Jeyasehar, A., 2016. Life cycle cost analysis of glass fiber

reinrorced polymer reinforced concrete beam. Asian Journal of Civil Engineering, Volume

17, pp. 315-323.

70

ECi, n.d. corrosioninstrument. [Online]

Available at: https://corrosioninstrument.com/gn/

[Accessed 14 05 2021].

El-Nemr, A., Ahmed, E. & Benmokrane, B., 2013. Flexural Behavior and Serviceability of

Normal- and High-Strength Concrete Beams Reinforced with Glass Fiber-Reinforced

Polymer Bars. ACI Structural Journal, 110(6), pp. 1077-1087.

El-Nemr, A., Ahmed, E., El-Safty, A. & Benmokrane, B., 2018. Evaluation of the flexural

strength and serviceability of concrete beams reinforced with different types of GFRP bars.

Engineering Structures, Volume 173, pp. 606-619.

Eurokode2, 2008. Eurokod 2: dimensionering av betongkonstruktioner. Del 1.1: Allmänna

regler och regler för byggnader, Stockholm: Swedish Standards Institute.

Fergani, H. et al., 2018. Durability and degradation mechanisms of GFRP reinforcement

subjected to severe environments and sustained stress. Construction and Building Materials,

Volume 170, pp. 637-648.

Fib, 2007. FRP reinforcment in RC structures, stuttgart: International Federation for

Structural Concrete.

Gardiner, G., 2020. Composite rebar for future infrastructure. CompositesWorld, 29

December.

Goldston, M., Remennikov, A. & Sheikh, M., 2016. Experimental investigation of the

behaviour of concrete beams reinforced with GFRP bars under static and impact loading.

Engineering Structures, Volume 113, pp. 220-232.

Goldston, M., Remennikov, A. & Sheikh, M., 2017. Flexural behaviour of GFRP reinforced

high strength and ultra high strength concrete beams. Construction and Building Materials,

Volume 131, pp. 606-617.

Gooranorimi, O., Claure, G., Suaris, W. & Nanni, A., 2018. Bond-slip effect in flexural

behavior of GFRP RC slabs. Composite Structures, Volume 193, pp. 80-86.

Gooranorimi, O. & Nanni, A., 2017. GFRP Reinforcement in Concrete after 15 Years of

Service. Journal of Composites for Construction , 1 Oktober.21(5).

He, X., Dai, L. & Yang, W., 2017. Durability and degradation mechanism of GFRP bars

embedded in concrete beams with cracks. Plastics, Rubber and Composites, 46(1), pp. 17-24.

ISIS, C. r. n., 2007. Reinforcing concrete strucutres with Fibre reinforced polymers, s.l.:

Canada corporation.

Jensen, J., 2006. FRP rebars to reinforce conrete structures, Lyngby: Technical University of

Denmark.

Jia, D. et al., 2020. Durability of glass fibre-reinforced polymer (GFRP) bars embedded in

concrete under various environments. I: Experiments and analysis. Composite Structure,

Volume 234.

Ju, et al., 2016. A Modified Model for Deflection Calculation of Reinforced Concrete Beam

with Deformed GFRP Rebar. International Journal of Polymer Science, Volume 1, pp. 1-10.

71

Kalpana, V. & Subramanian, 2011. Behavior of concrete beams reinforced with GFRP

BARS. Journal of Reinforced Plastics and Composites, 30(23), pp. 1915-1922.

Kassem, C., Farghaly, A. & Benmokrane, B., 2011. Evaluation of Flexural Behavior and

Serviceability Performance of Concrete Beams Reinforced with FRP Bars. Journal of

Composites for construction, 15(5), pp. 682-695.

Khatibmasjedi, M., Ramanathan, S., Suraneni, P. & Nanni, A., 2020. Durability of

commercially available GFRP reinforcement in seawater-mixed concrete under accelerated

agin conditions. Journal of Composite for Construction, 24(4).

Khorasani, A., Esfahani, M. & Sabzi, J., 2019. The effect of transverse and flexural

reinforcement on deflection and cracking of GFRP bar reinforced concrete beams.

Composites, Volume 161, pp. 530-546.

Manalo, A. et al., 2020. Comparative durability of GFRP composite reinforcing bars in

concrete and in simulated concrete enviroment. Cement and Concrete Composites, Volume

109.

Mohamed, H. & Benmokrane, B., 2014. Design and Performance of Reinforced Concrete

Water Chlorination Tank Totally Reinforced with GFRP Bars: Case Study. Journal of

Composites for Construction, 18(1).

Murthy, A. et al., 2020. Performance of concrete beams reinforced with GFRP bars under

monotonic loading. Structures, Volume 27, pp. 1274-1288.

Pathan, S., Nawaj, M. & Periyasamy, A., 2017. Composites, High performance synthetic;

manufacturing, recent developments and applications, Maharashra: Textile & engineering

institute.

Pultrall, N.D. Direct comparision between steel and V-ROD, Thetford mines: Pultrall.

Ramanathan, S., Benzecry, V., Suraneni, P. & Nanni, A., 2021. Condition assessment of

concrete and glass fiber reinforced polymer (GFRP) rebar after 18 years of service life. Case

studies in Construction Materials, Volume 14.

Saikia, B. et al., 2007. Strength and serviceability performance of beams reinforced with

GFRP bars in flexure. Construction and Building Materials, Volume 21, pp. 1709-1719.

Saleh, Z., Goldston, M., Remennikov, A. & Sheikh, M., 2019. Flexural design of GFRP bar

reinforced concrete beams: An appraisal of code recommendations. Journal of Building

Engineering, Volume 25.

Schöck, 2018. Reinforce safely without steel, Baden Baden: Schöck Bauteule GmbH.

Sentler, L., 1992. Fiberkompositer som armering, Lund: Byggforskningsrådet.

TUFBAR, n.d. TUFBAR.COM. [Online]

Available at: https://www.tuf-bar.com/case-studies/cs-pokemouche-river-bridge/

[Accessed 9 may 2021].

Tu, J., Xie, H. & Gao, K., 2020. Prediction of the Long-Term Performance and Durability of

GFRP Bars under the Combined Effect of a Sustained Load and Severe Environments.

Materials, 13(10).

72

Tu, J. et al., 2019. Durability prediction of GFRP rebar based on elastic modulus degradation.

Frontiers in Materials, Volume 6.

Täljsten, B. & Blanksvärd, T., 2018. Kompositarmering, Stockholm: SBUF.

Vakhshouri, B. & Nejadi, S., 2018. Empirical models and design codes in prediction of

modulus of elasticity of concrete. Frontiers of Structural and Civil Engineering, 13(2).

Zhang, X. & Deng, Z., 2019. Durability of GFRP bars in the simulatd marine environment

and concrete environmnet under sustained compressive stress. Construction and Building

Materials , Volume 223, pp. 299-309.

73

Appendix AA The database is presented in Table 13, the orange numbers are the mean value for the deflection ratio for each study.

Table 13 The results from the database, the orange numbers are the mean value for the deflection ratio for each study and the red is bad results excluded in the results.

Investigator Specimen b

(mm)

h

(mm)

d

(mm)

fc

(MPa)

ffu

(MPa)

Ef

(MPa)

bars Pexp

(kN)

Δexp

(mm)

ΔEC2

(mm)

Δexp/

ΔEC2

ΔISIS

(mm)

Δexp/

ΔISIS

ΔACI

(mm)

Δexp/

ΔACI (Alsayedm, et

al., 2000)

II 200 210 158 31 700 35630 4Φ19 20 10.8 10.49 1.03 11.79 0.92 11.63 0.93

III 200 260 211 31 886 43370 4Φ12,7 26 10.8 10.52 1.03 12.26 0.88 12.04 0.90

IV 200 300 248 41 700 35630 2Φ19 40 10.8 8.04 1.34 10.31 1.05 10.06 1.07

V 200 250 198 41 700 35630 4Φ19 33 10.8 9.77 1.11 10.85 1.00 10.69 1.01

1.13

0.96

0.98

(Saikia, et al.,

2007)

FG1SOC 180 250 225 68 972 49000 4Φ10 93 5.36 5.63 0.95 5.26 1.02 5.42 0.99

FG1GOC 180 250 225 64 972 49000 4Φ10 81 5.36 4.57 1.17 4.45 1.21 4.63 1.16

FG1SFPC 180 250 225 45 972 49000 4Φ10 78 5.36 4.88 1.10 4.56 1.18 4.70 1.14

FG1GFP

C

180 250 225 39 972 49000 4Φ10 81 5.36 5.27 1.02 4.83 1.11 4.96 1.08

FG2SOC 180 250 216 42 464 49620 7Φ10 139 5.36 6.67 0.80 5.76 0.93 5.87 0.91

FG2SFC 180 250 216 38 464 49620 7Φ10 124 5.36 5.92 0.90 5.16 1.04 5.27 1.02

FG2GFC 180 250 216 38 464 49620 7Φ10 124 5.36 5.92 0.90 5.16 1.04 5.28 1.02

0.98

1.07

1.05

(Kassem, et al.,

2011)

G1-6 200 300 222 39 617 40000 6Φ12,7 50 11 14.23 0.77 14.06 0.78 14.49 0.76

G1-8 200 300 222 39 617 40000 8Φ12,7 57 11 13.59 0.81 12.93 0.85 13.21 0.83

G2-6 200 300 223 39 747 36000 6Φ12 53 11 18.59 0.59 18.01 0.61 18.50 0.59

G2-8 200 300 223 39 747 36000 8Φ12 53 11 14.52 0.76 14.08 0.78 14.46 0.76

0.73

0.76

0.74

74

(Kalpana &

Subramanian,

2011)

M20-D16 200 250 196 20 900 55000 2Φ16 55.8 6.4 6.34 1.01 6.23 1.03 6.30 1.02

M20-D20 200 250 194 20 900 55000 2Φ20 65 6.4 5.35 1.20 5.22 1.23 5.26 1.22

M20-D24 200 250 192 20 900 55000 2Φ24 86.5 6.4 5.62 1.14 5.42 1.18 5.45 1.18

M40-D16 200 250 196 40 900 55000 2Φ16 76.6 6.4 8.46 0.76 8.11 0.79 8.20 0.78

M40-D20 200 250 194 40 900 55000 2Φ20 94.5 6.4 7.63 0.84 7.15 0.89 7.20 0.89

M40-D24 200 250 192 40 900 55000 2Φ24 123 6.4 7.76 0.82 7.18 0.89 7.21 0.89

M60-D16 200 250 196 60 900 55000 2Φ16 101 6.4 11.29 0.57 10.57 0.61 10.66 0.60

M60-D20 200 250 194 60 900 55000 2Φ20 130 6.4 10.56 0.61 9.66 0.66 9.70 0.66

M60-D24 200 250 192 60 900 55000 2Φ24 165 6.4 10.31 0.62 9.32 0.69 9.35 0.68

0.84

0.88

0.88

(Al-Sunna, et

al., 2012)

BG1 150 250 220 48 665 42800 2Φ9,53 18.5 8.4 0.22 29.16 7.19 1.17 8.45 0.994

BG2 150 250 219 48 620 41600 2Φ12,7 22.3 8.4 -0.23 24.19 6.59 1.27 7.14 1.177

BG3 150 250 193 47 670 42000 4Φ19,05 47 8.4 2.63 2.43 6.50 1.29 6.55 1.28

1.24

1.15

(El-Nemr, et al.,

2013)

N2#13G2 200 400 344 33 1639 67000 2Φ13 36.4 15 11.05 1.36 15.49 0.97 16.49 0.91

N3#13G1 200 400 344 33 817 48700 3Φ13 36.4 15 10.28 1.46 14.37 1.04 15.27 0.98

H2#13G2 200 400 344 66 1639 67000 2Φ13 36.4 15 0.93 16.06 9.64 1.56 12.34 1.22

H3#13G1 200 400 344 66 817 48700 3Φ13 36.4 15 0.93 16.06 8.98 1.67 11.42 1.31

N5#15G2 200 400 323 33 1362 69300 5Φ15 73 15 15.14 0.99 15.20 0.99 15.33 0.98

N6#15G1 200 400 320 33 762 50000 6Φ15 73 15 17.43 0.86 17.49 0.86 17.61 0.85

H5#15G2 200 400 323 66 1362 69300 5Φ15 85.6 15 16.13 0.93 16.31 0.92 16.52 0.91

H6#15G1 200 400 320 66 762 50000 6Φ15 85.6 15 18.60 0.81 18.83 0.80 19.04 0.79

N5#15G3 200 400 323 33 1245 59500 5Φ15 75.9 15 18.07 0.83 18.02 0.83 18.14 0.83

75

N2#25G3 200 400 338 33 906 60300 2Φ25 72.3 15 14.02 1.07 14.07 1.07 14.19 1.06

H5#15G3 200 400 323 66 1245 59500 5Φ15 90.6 15 19.88 0.75 19.87 0.75 20.07 0.75

H2#25G3 200 400 338 66 906 60300 2Φ25 86.3 15 15.32 0.98 15.45 0.97 15.63 0.96

1.00

1.04

0.96

(Barris, et al.,

2013)

N-212-D1 140 190 156 30 1321 63437 2Φ12 21.1 7.2 7.89 0.91 7.73 0.93 7.88 0.91

N-216-D1 140 190 154 30 1015 64634 2Φ16 30.5 7.2 7.89 0.91 7.38 0.98 7.44 0.97

N-316-D1 140 190 154 30 1015 64634 3Φ16 40.4 7.2 7.82 0.92 7.25 0.99 7.28 0.99

N-212-D2 160 190 136 30 1321 63437 2Φ12 20.1 7.2 8.89 0.81 9.25 0.78 9.53 0.76

H-316-D1 140 190 154 50 1015 64634 3Φ16 46.1 7.2 8.58 0.84 7.76 0.93 7.80 0.92

0.88

0.92

0.91

(Ju, et al., 2016) FB-1 180 230 186 27 841 42100 2Φ9,53 21 6.4 5.38 1.19 7.62 0.84 7.68 0.83

FB-2 180 230 186 27 841 42100 3Φ9,53 25 6.4 5.45 1.17 6.76 0.95 6.75 0.95

FB-3 180 230 174 27 841 42100 5Φ9,53 30 6.4 5.67 1.13 6.49 0.99 6.46 0.99

1.16

0.92

0.92

(Goldston, et al.,

2016)

40-#2-0.5-

S

100 150 128 40 732 37500 2Φ6,35 5.1 8 8.10 0.99 14.37 0.56 16.26 0.49

40-#3-1.0-

S

100 150 126 40 1764 55600 2Φ9,53 8.3 8 10.66 0.75 10.81 0.74 11.13 0.72

40-#4-2.0-

S

100 150 125 40 1642 48600 2Φ12,7 9.7 8 9.57 0.84 9.29 0.86 9.46 0.85

80-#2-0.5-

S

100 150 128 80 732 37500 2Φ6,35 5.1 8 0.72 11.18 6.80 1.18 11.42 0.70

80-#3-1.0-

S

100 150 126 80 1764 55600 2Φ9,53 8.7 8 8.03 1.00 9.60 0.83 10.24 0.78

80-#4-2.0-

S

100 150 125 80 1642 48600 2Φ12,7 10.6 8 8.74 0.92 9.00 0.89 11.62 0.69

0.90

0.84

0.70

76

(Goldston, et al.,

2017)

80-#2-0.5-

S

100 150 128 80 732 37500 2Φ6,35 4.5 9.6 17.43 0.55 27.72 0.35 25.77 0.37

80-#3-1.0-

S

100 150 126 80 1764 55600 2Φ9,53 7.6 9.6 17.84 0.54 19.89 0.48 19.21 0.50

80-#4-2.0-

S

100 150 125 80 1605 48600 2Φ12,7 9.9 9.6 17.76 0.54 18.88 0.51 18.48 0.52

120-#2-

0.5-S

100 150 128 120 732 37500 2Φ6,35 4.1 9.6 0.89 10.81 17.48 0.55 16.62 0.58

120-#3-

1.0-S

100 150 126 120 1764 55600 2Φ9,53 7.9 9.6 16.73 0.57 19.43 0.49 18.56 0.52

120-#4-

2.0-S

100 150 125 120 1605 48600 2Φ12,7 9.9 9.6 16.48 0.58 17.87 0.54 17.31 0.55

0.56

0.49

0.51

(El-Nemr, et al.,

2018)

3#13G1 200 400 344 34 817 48700 3Φ13 37 15 9.29 1.61 18.53 0.81 18.94 0.79

5#13G1 200 400 344 39 817 48700 5Φ13 47 15 10.00 1.50 15.76 0.95 15.96 0.94

2#13G2 200 400 344 34 1639 67000 2Φ13 37 15 9.87 1.52 19.91 0.75 20.37 0.74

3#15G1 200 400 343 39 751 48100 3Φ15 37 15 6.15 2.44 14.12 1.06 14.52 1.03

4#15G1 200 400 343 39 751 48100 4Φ15 58 15 13.92 1.08 19.25 0.78 19.38 0.77

2#15G2 200 400 343 29 1362 69300 2Φ15 41 15 11.25 1.33 17.88 0.84 18.12 0.83

2#15G3 200 400 343 34 1245 59500 2Φ15 44 15 13.31 1.13 21.45 0.70 21.74 0.69

6#15G1 200 400 320 34 751 48100 6Φ15 70 15 15.39 0.97 19.69 0.76 19.75 0.76

5#15G2 200 400 323 29 1362 69300 5Φ15 73 15 13.99 1.07 17.63 0.85 17.69 0.85

5#15G3 200 400 323 34 1245 59500 5Φ15 74 15 15.83 0.95 20.00 0.75 20.06 0.75

2#20G1 200 400 340 39 728 47600 2Φ20 28 15 0.3 19.9 8.41 1.78 8.98 1.67

3#20G1 200 400 340 42 728 47600 3Φ20 61 15 11.73 1.28 16.09 0.93 16.19 0.93

2#22G1 200 400 339 39 639 46400 2Φ22 49 15 10.25 1.46 15.68 0.96 15.86 0.95

3#20G2 200 400 340 48 1082 52500 3Φ20 73 15 13.63 1.10 17.80 0.84 17.89 0.84

2#25G1 200 400 338 48 666 53200 2Φ25 70 15 12.43 1.21 16.48 0.91 16.57 0.91

2#25G2 200 400 338 48 1132 66300 2Φ25 99 15 16.12 0.93 19.69 0.76 19.74 0.76

2#25G3 200 400 338 34 906 60300 2Φ25 72 15 12.57 1.19 15.96 0.94 16.02 0.94

77

1.22

0.91

0.89

(Saleh, et al.,

2019)

47-0.5-4 100 150 128 47 732 37500 2Φ6,35 4.8 8 2.03 3.95 11.21 0.71 20.14 0.40

47-1.0-4 100 150 126 47 1764 55600 2Φ9,53 8.4 8 8.71 0.92 10.60 0.75 13.71 0.58

47-2.0-4 100 150 125 47 1605 48600 2Φ12,7 9.7 8 7.81 1.02 9.02 0.89 11.32 0.71

66-0.5-4 100 150 128 66 732 37500 2Φ6,35 4.8 8 0.6 13.24 7.40 1.08 18.55 0.43

66-1.0-4 100 150 126 66 1764 55600 2Φ9,53 8.6 8 7.65 1.05 10.03 0.80 13.50 0.59

66-2.0-4 100 150 125 66 1605 48600 2Φ12,7 10.6 8 8.04 1.00 9.40 0.85 11.96 0.67

1.00

0.85

0.56

(Abdelkarim, et

al., 2019)

B1-35-12 200 300 246 34 1166 65000 2Φ12 34.5 10.8 12.78 0.85 15.79 0.68 17.57 0.61

B2-35-16 200 300 244 34 1122 63000 2Φ16 47.3 10.8 13.06 0.83 14.67 0.74 15.83 0.68

B3-35-20 200 300 242 34 1117 69000 2Φ20 57.3 10.8 10.71 1.01 11.76 0.92 12.56 0.86

B4-35-25 200 300 240 34 1340 65000 2Φ25 69.1 10.8 9.92 1.09 10.76 1.00 11.43 0.94

B5-65-12 200 300 246 68 1166 65000 2Φ12 40.0 10.8 11.66 0.93 16.24 0.67 18.77 0.58

B6-65-16 200 300 244 68 1122 63000 2Φ16 56.4 10.8 14.17 0.76 16.24 0.67 17.73 0.61

B7-65-20 200 300 242 68 1117 69000 2Φ20 72.7 10.8 12.91 0.84 13.98 0.77 15.00 0.72

B8-65-25 200 300 240 68 1340 65000 2Φ25 92.7 10.8 12.81 0.84 13.51 0.80 14.36 0.75

0.89

0.78

0.72

(Khorasani, et

al., 2019)

2D16-

8S70-N

250 250 214 30 775 46000 2Φ16 45 8 7.97 1.00 9.32 0.86 10.30 0.78

2D16-

10S110-N

250 250 212 30 775 46000 2Φ16 38 8 6.12 1.31 7.64 1.05 8.64 0.93

2D16-

8S35-N

250 250 214 30 775 46000 2Φ16 47 8 8.51 0.94 9.83 0.81 10.82 0.74

2D16-

10S55-N

250 250 212 30 775 46000 2Φ16 39.5 8 6.56 1.22 8.05 0.99 9.04 0.88

5D10-

8S70-N

250 250 217 30 789 44000 5Φ10 48 8 9.07 0.88 10.40 0.77 11.43 0.70

78

5D10-

10S110-N

250 250 215 30 789 44000 5Φ10 41 8 7.22 1.11 8.72 0.92 9.74 0.82

5D10-

8S35-N

250 250 217 30 789 44000 5Φ10 44 8 7.95 1.01 9.35 0.86 10.36 0.77

5D10-

10S55-N

250 250 215 30 789 44000 5Φ10 38.5 8 6.47 1.24 8.02 1.00 9.05 0.88

3D18-

8S70-N

250 250 213 30 800 42000 3Φ18 65 8 8.16 0.98 8.93 0.90 9.63 0.83

3D18-

10S110-N

250 250 211 30 800 42000 3Φ18 66 8 8.47 0.94 9.27 0.86 9.99 0.80

3D18-

8S35-N

250 250 213 30 800 42000 3Φ18 62 8 7.70 1.04 8.48 0.94 9.16 0.87

3D18-

10S55-N

250 250 211 30 800 42000 3Φ18 65 8 8.32 0.96 9.11 0.88 9.83 0.81

5D14-

8S70-N

250 250 214 30 825 45000 5Φ14 58.5 8 6.63 1.21 7.36 1.09 7.98 1.00

5D14-

10S110-N

250 250 212 30 825 45000 5Φ14 60 8 6.98 1.15 7.73 1.03 8.37 0.96

5D14-

8S35-N

250 250 214 30 825 45000 5Φ14 72.5 8 8.60 0.93 9.33 0.86 10.02 0.80

5D14-

10S55-N

250 250 214 30 825 45000 5Φ14 69.5 8 8.19 0.98 8.91 0.90 9.59 0.83

1.06

0.92

0.84

(Murthy, et al.,

2020)

GFRP-1S-

10

100 200 160 40 673 43000 2Φ10 29.9 5.4 8.04 0.67 8.85 0.61 10.84 0.50

GFRP-2S-

10

100 200 160 40 673 43000 2Φ10 29.9 5.4 8.04 0.67 8.85 0.61 10.84 0.50

GFRP-3S-

10

100 200 160 40 673 43000 2Φ10 24.3 5.4 6.02 0.90 6.93 0.78 8.70 0.62

GFRP-4S-

13

100 200 159 40 673 43000 2Φ13 34.6 5.4 6.22 0.87 6.74 0.80 8.16 0.66

GFRP-5S-

13

100 200 159 40 673 43000 2Φ13 41.5 5.4 7.69 0.70 8.21 0.66 9.83 0.55

0.76

0.69

0.57

79

(Chen, et al.,

2021)

G-1-0 150 200 161 42 1025 51200 2Φ8 19 7.2 9.09 0.79 13.18 0.55 18.42 0.39

G-2-0 150 200 160 42 1173 55600 2Φ10 23 7.2 8.83 0.82 11.00 0.65 14.40 0.50

G-3-0 150 200 158 42 1037 59500 2Φ14 34 7.2 8.81 0.82 9.74 0.74 11.94 0.60

G-1-50 150 200 161 40 1025 51200 2Φ8 19 7.2 9.21 0.78 13.17 0.55 18.30 0.39

G-2-50 150 200 160 40 1173 55600 2Φ10 23 7.2 9.45 0.76 11.52 0.63 14.89 0.48

G-3-50 150 200 158 40 1037 59500 2Φ14 31 7.2 7.66 0.94 8.63 0.83 10.67 0.67

G-1-100 150 200 161 39 1025 51200 2Φ8 20 7.2 10.62 0.68 14.35 0.50 19.48 0.37

G-2-100 150 200 160 39 1173 55600 2Φ10 20 7.2 7.32 0.98 9.58 0.75 12.80 0.56

G-3-100 150 200 158 39 1037 59500 2Φ14 28 7.2 6.79 1.06 7.79 0.92 9.73 0.74

0.85

0.68

0.52

Mean

value

0.93

0.87

0.81