Design of footbridge with GFRP reinforced concrete

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DET TEKNISK-NATURVITENSKAPELIGE FAKULTET

BACHELOROPPGAVE

Studieprogram/spesialisering:

Bachelor Bygg, Konstruksjonsteknikk

Vår semesteret, 2016

Åpen

Studenter:

Bengt Olav Skogland Eirik Hartveit Hansen Ørjan André Kristiansen

………

signaturer

Faglig ansvarlig: Luis Manuel Rocha Evangelista, Universitetet i Stavanger

Veileder: Kathrin Sandstad, Statens Vegvesen

Tittel på oppgaven: Design of footbridge with GFRP reinforced concrete Norsk tittel: Dimensjonering av gangbru med GFRP armert betong

Studiepoeng: 20 poeng

Emneord:

Footbridge, GFRP RC, costs, design, CSA S806.

Sidetall: 83

+ vedlegg/annet: 57

Stavanger, 18.05.16

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Preface

This bachelor thesis was carried out in collaboration with Bruseksjonen Region Vest from Statens Vegvesen and the Department of Structural Engineering and Materials Science at the University of Stavanger. The work was completed during a period of 18 weeks throughout spring 2016 lasting from early January until the middle of May and is the final part of our Bachelor of Science degree.

The theme of this thesis is bridge design using Glass Fibre Reinforced Polymers (GFRP) as reinforcement in concrete replacing conventional steel rebars. Material properties, design guidelines and initial costs were evaluated considering GFRP and differences highlighted. Design considerations were made using the standard CSA S806-12 from Canadian Standards Association and complimentary equations from researchers in the field.

The main objective has been to design the bridge deck from the already existing footbridge Sandvedhagen in Sandnes municipality using GFRP rebars as reinforcement. It was commissioned by Statens Vegvesen that the needs of other material alternatives are increasing with time, and a feasibility study considering GFRP reinforcement would be an interesting topic to discuss further.

We would like to thank our supervisor Luis Evangelista, Associate Professor at the University of Stavanger, for exceptional guidance and feedback through the entire working period. Also, we would like to thank Kathrin Sandstad, João Devesa and Magne Langeteig at Statens Vegvesen for providing necessary information, documents and drawings when needed.

Bengt Olav Skogland 18.05.16

Eirik Hartveit Hansen University of Stavanger

Ørjan André Kristiansen

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Abstract

Due to corrosion of steel reinforcement in concrete structures, the search of other material compositions and design methods has been carried out throughout the world. Glass Fibre Reinforced Polymer (GFRP) is one type of material that has been tested and evaluated as a reliable alternative. This thesis has introduced this relatively new material emphasizing historical development through a «state of the art» presentation. Also the different aspects of why, when and where to use this material as well as different design approaches have been presented.

Designing a bridge deck from an already existing footbridge was then carried out using design approaches given in the Canadian standard CSA S806.

Calculating loads acting on the bridge deck and the combination of these was done according to Eurocodes and custom guidelines for Statens Vegvesen. Both hand calculations and a numerical computer program were used to retrieve the loads. The analysis of all the load combinations on the bridge deck was done using CSI Bridge, which is an analysis and design software for the engineering of bridge systems. The bridge deck was divided into a beam and a side slab because of its cross sectional geometry and designed for both ultimate limit state and serviceability limit state.

Furthermore, the costs related to the amount of materials needed to achieve required resistance were evaluated. Initial costs were calculated for both steel and GFRP and the predicted deviance was verified by the results. Also life cycle costs were discussed, but due to lack of exact data it was here only made a rough estimate regarding the annual costs considering maintenance work for repairing corrosion damages.

It was concluded based on retrieved results that the use of GFRP as reinforcement instead of steel can be implemented as a liable and sustainable alternative to steel reinforcement. On the other hand, more attention should be given to the design methods and application of these. Also, the difference in initial costs may be a reason for many contractors not to choose GFRP as their solution.

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Sammendrag

Korrosjon av stålarmering i betongkonstruksjoner har ført til økt leting etter andre materialer og dimensjoneringsmetoder over hele verden. Glassfiber armerte polymerer er ett av disse materialene som har blitt testet og vurdert som et pålitelig alternativ. Denne oppgaven har introdusert dette relativt nye materialet ved å gjøre rede for den historiske utviklingen og dens posisjon i vitenskapen. Ulike forhold som diskuterer og forklarer hvorfor, når og hvor dette materialet kan brukes samt flere ulike dimensjoneringsmetoder har blitt presentert.

Etter en innføring ble det foretatt en dimensjonering av et brudekke med bruk av GFRP. Dimensjoneringen ble gjennomført på grunnlag av regler og retningslinjer gitt i den kanadiske standarden CSA S806. Beregning av laster som virket på brudekket ble gjort både for hånd og ved hjelp av et regneprogram på data i samsvar med de retningslinjer som er gitt i Eurokodene og Statens Vegvesens egne håndbøker. Selve analysen av lastkombinasjonene på brudekket ble gjort ved hjelp av et analyseprogram som heter CSI Bridge. Brudekket ble delt opp i en bjelke og en utkraget plate på grunn av dets tverrsnitts geometri. Det ble dimensjonert for både bruddgrense og bruksgrense.

Videre ble kostnadene i forhold til mengder materialer som var nødvendig for å tilfredsstille kravene. Kostnader for både stål og GFRP ble beregnet og den antatte forskjellen ble bekreftet av resultatene. I tillegg ble kostnadene som påløper en bro gjennom dens brukstid prøvd evaluert, men eksakte data viste seg å være vanskelig å oppdrive. Det ble derfor gjort et grovt estimat på hvor stor andel av årlige tilvirkede midler som går til reparasjon av korrosjonsskader.

Oppgaven konkluderer med at bruken av GFRP som armering i stedet for stål kan innføres som et pålitelig og bærekraftig alternativ. Det må allikevel tas hensyn til at dette er et nytt materiale, og beregningsmetoder og retningslinjer bør evalueres og diskuteres ytterligere. I tillegg anslås det at den store forskjellen på materialkostnader er en viktig grunn til at byggherrer og bestillere ikke velger GFRP.

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List of symbols

Greek

𝛼𝛼 - Bar location

𝛼𝛼𝑏𝑏 - Bond dependent coefficient

𝛼𝛼𝜇𝜇 - Angle between the bond resistance direction and the bar axis 𝛽𝛽 - Coefficient of effect of the load duration or cyclic loadings 𝛽𝛽_𝑑𝑑 - Bond reduction factor

∆𝑐𝑐 - Additional concrete cover for cast-in-place structure (10 mm)

∆𝑇𝑇𝑀𝑀,ℎ𝑒𝑒𝑒𝑒𝑒𝑒/𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐- Temperature differences in bridge

∆𝑇𝑇𝑁𝑁,𝑐𝑐𝑐𝑐𝑐𝑐/𝑒𝑒𝑒𝑒𝑒𝑒 - Maximum contraction/expansion temperature interval

𝛿𝛿 - Total deflection

𝛿𝛿₁ - Deflection considering uncracked section 𝛿𝛿₂ - Deflection considering cracked section

𝜀𝜀_{𝑐𝑐,𝑚𝑚} - Compressive strain in concrete

𝜀𝜀𝑐𝑐𝑐𝑐 - Total shrinkage strain

𝜀𝜀_{𝑐𝑐𝑐𝑐} - Ultimate compressive strain of concrete 𝜀𝜀_{𝑓𝑓𝑑𝑑} - Design tensile strain of GFRP

𝜀𝜀_𝑐𝑐 - Longitudinal strain at mid-depth of the section 𝜀𝜀_{𝑠𝑠𝑚𝑚} - Mean strain in reinforcement

𝜂𝜂 - Factor of effective height of compression zone 𝜂𝜂₁ - Cracking inertia factor

𝜃𝜃 - Angle of the diagonal compressive stress 𝜆𝜆 - Factor of effective strength

𝜆𝜆_𝑐𝑐 - Factor to account for concrete density

𝜇𝜇 - Non-dimensional moment capacity

𝜇𝜇_{𝑓𝑓𝑓𝑓𝑒𝑒} - Bond strength

𝜉𝜉 - Non-dimensional neutral axis depth

𝜉𝜉𝑐𝑐𝑓𝑓 - Ratio between cracking moment and design moment at SLS 𝜉𝜉ℎ - Effects of the 𝑑𝑑𝑏𝑏, tensile strength and casting position 𝜌𝜌 - GFRP reinforcement ratio

𝜌𝜌_𝑒𝑒 - Density of air

𝜌𝜌_{𝑓𝑓𝑓𝑓𝑐𝑐} - Density of GFRP RC

𝜌𝜌_{𝑓𝑓𝑏𝑏} - Balanced GFRP reinforcement ratio 𝜙𝜙 - Reduction factor

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𝜙𝜙_𝑐𝑐 - Resistance factor for concrete

𝜙𝜙_𝑓𝑓 - Resistance factor for GFRP reinforcement 𝜙𝜙(𝑡𝑡, 𝑡𝑡₀) - Creep number

𝜔𝜔_{𝑁𝑁/𝑀𝑀} - Factors for unfavourable temperature combinations

Latin

𝐴𝐴 - Constant, relative to the material and degradation process 𝐴𝐴_𝑏𝑏 - Area of one GFRP rebar

𝐴𝐴_𝑓𝑓 - Area of GFRP reinforcement

𝐴𝐴_{𝐹𝐹𝐹𝐹} - Area of GFRP shear reinforcement perpendicular to the axis of a member within the distance 𝑠𝑠𝐹𝐹

𝐴𝐴𝑓𝑓𝑒𝑒𝑓𝑓,𝑒𝑒 - Wind force reference area

𝐴𝐴_𝑒𝑒 - Area of transverse GFRP reinforcement

𝐴𝐴_{𝐹𝐹𝐹𝐹} - Minimum area of transverse shear reinforcement

𝑎𝑎 - Shear span

𝑎𝑎_𝑑𝑑 - Depth of equivalent rectangular stress block 𝑏𝑏 - Width of cross section

𝑏𝑏𝑤𝑤 - Minimum effective web width

𝐶𝐶𝑒𝑒 𝑐𝑐𝑓𝑓 𝑧𝑧 - Wind load factor for x- or z-direction

𝑐𝑐 - Minimum concrete cover

𝑐𝑐𝑒𝑒𝑐𝑐𝑒𝑒 - Factor for altitude effects on wind

𝑐𝑐𝑑𝑑𝑑𝑑𝑓𝑓 - Factor for wind direction

𝑐𝑐𝑒𝑒𝑓𝑓𝑐𝑐𝑏𝑏 - Factor for wind return period

𝑐𝑐𝑓𝑓(𝑧𝑧) - Factor for terrain roughness affecting wind at height z

𝑐𝑐𝑠𝑠𝑒𝑒𝑒𝑒𝑠𝑠𝑐𝑐𝑐𝑐 - Seasonal wind factor

𝑐𝑐0(𝑧𝑧) - Factor for orographic effects on wind at height z 𝑐𝑐𝑒𝑒 - Minimum side concrete cover

𝑑𝑑 - Effective depth 𝑑𝑑_𝑏𝑏 - Bar diameter

𝑑𝑑_𝑐𝑐 - Distance from tension side to center of closest bar

𝑑𝑑_{𝑐𝑐𝑠𝑠} - Min of the distance from the closest concrete surface to the center of rebar OR ²₃ of c-c distance of the bars being developed ≤ 2.5db

𝑑𝑑_{𝑒𝑒𝑐𝑐𝑒𝑒} - Total depth of bridge deck including railings

𝑑𝑑_𝐹𝐹 - Effective shear depth

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𝐸𝐸_𝑐𝑐 - Modulus of elasticity of concrete 𝐸𝐸_𝑓𝑓 - Modulus of elasticity of GFRP rebars 𝐸𝐸_𝑠𝑠 - Modulus of elasticity of steel

𝐸𝐸𝑎𝑎 - Activation energy

𝐹𝐹_𝑒𝑒 - Factor taking into account the yield strength hooked GFRP rebar 𝐹𝐹_𝑤𝑤 - Wind force

𝑓𝑓_𝑐𝑐′ - Specified compressive strength of concrete 𝑓𝑓_{𝑐𝑐𝑑𝑑} - Design compressive strength of concrete 𝑓𝑓_{𝑐𝑐𝑒𝑒} - Splitting tensile stress of concrete 𝑓𝑓𝑓𝑓 - Tensile stress in GFRP rebars 𝑓𝑓𝑓𝑓𝑏𝑏 - Bond strength of GFRP rebars

𝑓𝑓_{𝑓𝑓𝑑𝑑} - Design tensile strength of GFRP rebars

𝑓𝑓_{𝑓𝑓,𝑠𝑠} - Creep rupture stress limit

𝑓𝑓_ℎ - Tensile stress developed by the hook 𝑓𝑓_𝑅𝑅 - Bar surface properties

𝑓𝑓_𝑓𝑓 - Mechanical of GFRP rebars and concrete 𝑓𝑓𝑠𝑠ℎ - Nominal tensile stress of hooked GFRP rebar 𝑓𝑓𝑠𝑠𝑒𝑒 - Stress in steel reinforcement

𝑓𝑓𝑐𝑐 - Ultimate tensile strength of GFRP reinforcement 𝑓𝑓𝑦𝑦𝑓𝑓 - Effective yield strength of GFRP rebar

𝑓𝑓𝑦𝑦𝑒𝑒 - Yield stress of GFRP rebar 𝐺𝐺 - Dead load

ℎ1 - Distance from the centroid of reinforcement to neutral axis ℎ₁ - Distance from tension face to the neutral axis

𝐼𝐼𝑐𝑐𝑓𝑓 - Cracked moment of inertia 𝐼𝐼𝑒𝑒 - Effective moment of inertia 𝐼𝐼_𝑔𝑔 - Gross moment of inertia

𝐼𝐼_𝐹𝐹(𝑧𝑧) - Turbulence intensity at height z 𝑗𝑗_𝑑𝑑 - Resistant moment arm

𝐾𝐾 - Strength and serviceability factor

𝑘𝑘 - Ratio of effective depth and depth of the elastic neutral axis 𝑘𝑘_𝑏𝑏 - Bond behaviour factor

𝑘𝑘_𝑑𝑑 - Degradation rate �_{𝑒𝑒𝑑𝑑𝑚𝑚𝑒𝑒}¹ � XIV

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𝑘𝑘_𝑚𝑚 - Coefficient taking into account the effect of moment on section 𝑘𝑘_𝑒𝑒 - Top factor considering wind calculations

𝑘𝑘_𝑓𝑓 - Coefficient taking the effect of reinforcement rigidity into account

𝑘𝑘_𝑠𝑠 - Factor considering size effect on shear strength

𝑘𝑘𝑠𝑠𝑐𝑐𝑓𝑓1/𝑠𝑠𝑐𝑐𝑓𝑓2 - Temperature change distribution factor

𝑘𝑘₁ - Bar location factor 𝑘𝑘₂ - Concrete density factor 𝑘𝑘₃ - Bar size factor

𝑘𝑘₄ - Bar fiber factor 𝑘𝑘₅ - Bar surface factor

𝐿𝐿 - Span length

𝐿𝐿_𝑏𝑏 - Splice length

𝐿𝐿_𝑔𝑔 - Distance from the support to the point where design MEd,SLS =Mcr

𝐿𝐿_ℎ𝑏𝑏 - Basic development length 𝑙𝑙_𝑑𝑑 - Development length

𝑙𝑙_{𝑑𝑑𝑏𝑏} - Development length of straight GFRP rebar 𝑙𝑙_𝑒𝑒 - Embedded length of GFRP rebar

𝑙𝑙_𝑒𝑒 - Tail length of bent GFRP rebar

𝑀𝑀 - Moment capacity

𝑀𝑀𝑐𝑐𝑓𝑓 - Cracking moment

𝑀𝑀𝐸𝐸𝑑𝑑 - Design moment for ultimate load case

𝑀𝑀𝐸𝐸𝑑𝑑,𝑆𝑆𝑆𝑆𝑆𝑆 - Design moment for service load case

𝑀𝑀_𝑓𝑓 - Design moment at ULS 𝑀𝑀𝑅𝑅𝑑𝑑 - Moment capacity

𝑀𝑀𝑠𝑠 - Moment due to G + sustained portion of variable load

𝑛𝑛𝑓𝑓 - Ratio between E-modulus of FRP reinforcement and concrete 𝑃𝑃 - Maximum point load at failure

𝑃𝑃𝐸𝐸𝑑𝑑𝑔𝑔𝑒𝑒 - Self weight of edge beam as a point load

𝑃𝑃𝑓𝑓𝑒𝑒𝑑𝑑𝑐𝑐𝑑𝑑𝑐𝑐𝑔𝑔 - Self weight of railings as a point load

𝑄𝑄𝑓𝑓𝑐𝑐𝑓𝑓 - Horizontal traffic load

𝑄𝑄_{𝑠𝑠𝑒𝑒𝑓𝑓𝐹𝐹} - Service vehicle load

𝑞𝑞_{𝑓𝑓𝑓𝑓} - Uniformly distributed traffic load

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𝑞𝑞_𝑚𝑚 - Mean wind velocity pressure 𝑞𝑞_𝑒𝑒(𝑧𝑧) - Peak velocity pressure at height z 𝑅𝑅 - Universal gas constant

𝑟𝑟_𝑏𝑏 - Bend radius of bent GFRP rebar

𝑠𝑠 - Spacing of longitudinal GFRP reinforcement

𝑠𝑠_{𝑓𝑓,𝑚𝑚𝑒𝑒𝑒𝑒} - Largest crack distance

𝑠𝑠_{𝑒𝑒𝑓𝑓𝑒𝑒𝑐𝑐𝑠𝑠} - Spacing of transverse GFRP reinforcement

𝑠𝑠_𝐹𝐹 - Spacing of shear reinforcement

𝑇𝑇 - Temperature

𝑇𝑇_𝑒𝑒 - Applied tensile load

𝑇𝑇𝑒𝑒,𝑚𝑚𝑑𝑑𝑐𝑐/𝑚𝑚𝑒𝑒𝑒𝑒 - Lowest/highest bridge temperature ratio

𝑇𝑇𝑚𝑚𝑑𝑑𝑐𝑐/𝑚𝑚𝑒𝑒𝑒𝑒 - Lowest/highest temperature expected to occur in the bridge

𝑇𝑇0 - Initial bridge temperature where the bridge is constrained 𝑢𝑢 - Bond stress for straight rebar

𝑢𝑢_{𝑒𝑒𝑒𝑒𝑠𝑠𝑒𝑒} - Average bond stress

𝑢𝑢_{𝑒𝑒𝑓𝑓} - Bond stress for transverse reinforcement 𝑉𝑉_𝑐𝑐 - Shear resistance provided by concrete 𝑉𝑉_{𝐸𝐸𝑑𝑑} - Design shear

𝑉𝑉𝑓𝑓 - Design shear at ULS

𝑉𝑉_𝑓𝑓 - Shear resistance of GFRP RC member 𝑉𝑉_{𝑠𝑠𝐹𝐹} - Shear resistance provided by GFRP stirrups 𝑣𝑣_𝑏𝑏 - Basic wind velocity

𝑣𝑣_𝑏𝑏,0 - Reference wind velocity 𝑣𝑣_𝑚𝑚(𝑧𝑧) - Mean wind velocity

𝑤𝑤 - Crack width

𝑥𝑥 - Neutral axis depth

𝑧𝑧 - Height of bridge deck above ground

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List of figures

Figure 1: Weightman Bridge in Niagara Falls, Ontario, Canada (Kamal & Boulfiza, 2011) ... 2

Figure 2: Lleida footbridge in Lleida, Spain ("Lleida Footbridge," 2006) ... 2

Figure 3: Elkhorn North bridge in Washington, Nebraska, USA ("Elkhorn North Bridge," 2016) 3 Figure 4: Aberfeldy footbridge in Aberfeldy, Scotland ("Aberfeldy Footbridge," 2015) ... 3

Figure 5: Stress and strain profile of GFRP RC beam (fib Bulletin 40, 2007) ... 12

Figure 6: Relation between PR and predicted service life (Robert & Benmokrane, 2012) ... 16

Figure 7: a) 90 degr. hook, b) 180 degr. hook ... 23

Figure 8: a) Contact lap splice (Preferred), b) Non-contact lap splice ("CRSI," 2016) ... 24

Figure 9: Figure 9: Transfer of force through bond ... 29

Figure 10: Parameters for deflection calculation (AutoCAD) ... 32

Figure 11: Cross section of Sandvedhagen footbridge (AutoCAD) ... 36

Figure 12: Definition of wind forces (NS, clause 8.1(3)) ... 39

Figure 13: Load situation for service vehicle (NS-EN 1991-2:2003) ... 41

Figure 14: Temperature differences (AutoCad) ... 44

Figure 15: Temperature loads ... 47

Figure 16: Bridge spans and axes (AutoCAD) ... 48

Figure 17: Considered sections for design (AutoCAD) ... 49

Figure 18: Load situations for side slab with Qserv(left) and q(right) (AutoCAD)... 49

Figure 19: Flexure reinforcement at mid-span (AutoCAD) ... 50

Figure 20: Flexure reinforcement near support (AutoCAD) ... 51

Figure 21: Flexure reinforcement at side-span (AutoCAD) ... 52

Figure 22: Distribution of flexure reinforcement (AutoCAD) ... 52

Figure 23: Distribution of shear reinforcement (AutoCAD) ... 53

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List of tables

Table 1: Usual tensile properties of reinforcing bars (ACI Committee 440, 2006, tb.3,3) ... 7

Table 2: Different bar diameter vary with the critical temperature (ComBAR®, 2015) ... 19

Table 3: Fire rating and minimum concrete cover (ComBAR®, 2015) ... 19

Table 4: Unbraced length vs Ultimate compressive strength of the GFRP (Deitz et al., 2000) .. 20

Table 5: Results self-weight ... 37

Table 6: Results wind forces ... 40

Table 7: Results traffic loads ... 42

Table 8: Recommended ksur values (NS-EN 1991-1-5:2003+NA:2008, 2010) ... 44

Table 9: Temperature combinations ... 45

Table 10: Load combinations (NS EN 1990:2002) ... 46

Table 11: Calculated values for shear forces and design moments for SLS and ULS ... 48

Table 12: Cost analysis for steel vs GFRP... 55

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1 State of the Art

1.1 History

The involving of steel as reinforcement in structural concrete design during the final decades of the 19^th century was revolutionary (Sozen, Ichinose, & Pujol, 2014). The compressive strength of concrete is very high, but the resistance to tensile forces on the other hand is low (NTNU Institutt for konstruksjonsteknikk, 2011). As a result of these mechanical properties, the application of concrete alone in structural elements was limited mainly to carry compressive loads.

The idea of using steel with concrete in structural elements came from the fact that steel has a very high tensile strength. Combining the compressive strength of concrete and tensile strength of steel led to a technical revolution in the construction industry (Kurrer, 2009). Although steel-reinforced concrete has proved to be a success regarding both structural performance and durability, the deterioration of concrete due to corrosion is a worldwide problem. High maintenance and repair costs, reduced service-life and failure due to cracking or delamination are some of the problems that occur (Bertolini, Elsener, Pedeferri, Redaelli, & Polder, 2013).

Many solutions to solve this have been developed, and among them, is the glass fibre reinforced polymer (GFRP) rebar, which was considered in this thesis.

The GFRP rebars were produced as an experiment in the late 1960s when steel reinforcement could not be applied into polymer-impregnated concrete. The experiment was a success and the GFRP bar became a reliable alternative. But GFRP as a replacement to steel in conventional concrete was not recognized as a viable solution nor commercially available in the market until the late 1970s (ACI Committee 440, 1996). Germany was a leading nation on the subject at the time and have since 1978 conducted several large scale tests using GFRP reinforcement in bridge engineering. The world’s first highway bridge was finished in the city of Dusseldorf in 1986 (Meier, 1992). Since then, the need of a non-metallic reinforcement alternative considering the corrosion problem has increased and the applications of GFRP are continuously expanding throughout the world. Today, there are several bridges being built using GFRP rebars as reinforcement throughout the world (ACI Committee 440, 2006).

Examples of bridges built or rehabilitated with GFRP throughout the world:

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Figure 1: Weightman Bridge in Niagara Falls, Ontario, Canada (Kamal & Boulfiza, 2011)

Figure 2: Lleida footbridge in Lleida, Spain ("Lleida Footbridge," 2006)

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Figure 3: Elkhorn North bridge in Washington, Nebraska, USA ("Elkhorn North Bridge,"

2016)

Figure 4: Aberfeldy footbridge in Aberfeldy, Scotland ("Aberfeldy Footbridge," 2015)

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1.2 Field of application

1.2.1 Internal and external

Fibre-reinforced polymers can be placed internally in structural elements as reinforcement or externally as a strengthening solution (Bakis et al., 2002).

Internal FRP reinforcement, either prestressed or non-prestressed, have been developed in Europe since 1970s (Taerwe & Matthys, 1999). The appearance of FRP reinforcement in structures can be several: rebars, grids, sheets, fibres and strips are some of the forms it comes in today (ACI Committee 440, 1996; Bakis et al., 2002; Portnov, Bakis, Lackey, & Kulakov, 2013). Also under each category there are different types, i.e. solid rebars, hollow rebars, rebars with non-circular cross section, among others.

FRP used externally on elements or members can be used as a repair method on strengthening of existing structures, either for a temporary case or as a permanent solution (Yang et al., 2015). For the purpose of this thesis it was considered the reinforcement as non-prestressed, internal GFRP rebars.

1.2.2 Durability

The significant resistance to corrosion is one of the main reasons why the GFRP bar is used in highly corrosive environments (Portnov et al., 2013). The fib technical report bulletin 40 (fib Bulletin 40, 2007) states that:

The alkaline environment of concrete normally provides the necessary protection to conventional steel reinforcement from the environments.

Nonetheless, when exposed or when the alkaline environment is neutralised, conventional steel corrodes and leads to spalling of the concrete cover.

When considering structures in or nearby marine climates and other structures which are exposed to either marine salts or de-icing road salts, the GFRP bar can be applied. Examples on structures can be bridge decks, sea walls and noise barriers near highways. Because of GFRP bar’s high corrosion resistance the service-life can be extended (ACI Committee 440, 2006). However, there are researches saying that the alkaline environment in concrete can damage the brittle glass fibres (Brahim Benmokrane, Wang, Ton-That, Robert, &

Rahman, 2002), both in strength and through embrittlement i.e. loss of ductility.

The resin acts as protective surface for the glass fibres. Both these allegations

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considered, the decision of using GFRP as choice of reinforcement should be thoroughly studied and evaluated.

1.2.3 Electromagnetic neutrality

Steel can, due to its high electromagnetic conductivity, interfere with magnetic fields (fib Bulletin 40, 2007). Equipment and systems such as magnetic resonance imaging (MRI) at hospitals and the magnetic levitation railway system in Japan, MAGLEV (Nakashima, 1994), are applications where electromagnetic neutrality should be achieved. The GFRP reinforcement can satisfy this requirement and can be used either as a structural element on its own or as reinforcement in concrete (fib Bulletin 40, 2007).

1.2.4 Costs

Because of the appearance of steel as reinforcement in concrete, road administrations across the world today struggle with a large portion of ageing bridges (Mara, Haghani, & Harryson, 2014). Each year they assign large amount of money to perform maintenance work. In Norway it has been estimated that it would take 11-19 billion NOK to repair all the bridges in need of any type of maintenance work (NTB, 2015). Although steel provides low initial costs with high structural performance, the use of GFRP may contribute to overall lower costs.

Considering the change that the Earth’s population are experiences these days it is also important to consider the sustainability of available materials and solutions. Mara et al. (2014) states that both energy consumption and emissions to air, water and soil are higher for steel than that of GFRP. One should therefore widen the aspect of which materials and design to use in the future due to limited resources and environmental considerations.

1.3 Material Properties

1.3.1 Introduction

If one looks at the term GFRP, or Glass Fibre Reinforced Polymers, two elements can vary. The first one is fibre reinforcement. In addition to glass fibres, which were considered in this thesis, there are also carbon, aramid and basalt 5

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fibres being used in the same way, each with different properties. The second is polymers,and refers to the polymeric matrix that covers the fibres, often referred to as resin. Since GFRP is a composite material, there are many variations within these two elements. Almost every manufacturer has their own, trademarked GFRP rebar product with different material properties. Based upon the literature review, it was not possible to identify a classification system for GFRP materials.

In spite of that, it is important to mention that such classification system is paramount if GFRP are to succeed in future applications in current structures.

1.3.2 Resin

The resin has two main functions; bond the fibres to transfer and distribute the loads and to provide cover from environmental attacks. There are two main types of resin, thermoplastic and thermosetting. Thermoplastic can be heated up, reformed and cooled down again several number of times, whereas the thermosetting resins are irreversibly formed. Thermosetting resins are most common type in reinforcement and are divided in three types; epoxy resin, polyester resin and vinyl ester resin. (FIB Task Group 9.3, 2007)

Epoxy resin have many advantages, such as high mechanical performance, easy processing and high corrosion resistance. This comes at a relatively high cost and long curing time however. Polyester resins can be formulated to have strong UV resistance or fire resistance. Its main disadvantage is a high volumetric shrinkage as well as reduced corrosion resistance when used as a fibre-matrix composite. Vinyl ester resin is the most common due to it being cheaper than epoxy while offering a good combination of properties of the two previously mentioned resins. (FIB Task Group 9.3, 2007)

1.3.3 Glass Fibres

Among the variety of glass fibres available, E-glass fibres are the general- purpose type of glass fibres and most common reinforcement for composites.

There is also a variant of E-glass called ECR-glass, which has an improved resistance to corrosion against most acids. E-glass accounts for the majority of glass fibre production and therefore the most economical alternative, while the

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more expensive S-glass (or S2-glass, HS2/HS4, T-glass)¹ has a higher tensile strength and greater modulus of elasticity.(Gibson, 1994)

1.3.4 Properties of GFRP

There are some things to keep in mind regarding the properties. As mention earlier, the components of different GFRP composites can vary, giving different performances. Volume, type of fibre and resin, orientation of fibres and quality control during manufacture are some of the factors influencing the mechanical properties of GFRPs. In service, the properties will also be affected by loading history/duration, temperature and moisture(B. Benmokrane, Chaallal, &

Masmoudi, 1995). The main properties will be described in the subsections below.

1.3.4.1 Tensile Properties

The most important property when used as internal reinforcement is the tensile strength of the GFRP bar. The main purpose of the bars (whether steel of FRP) is to withstand the tensile stresses in the concrete member. The main difference from traditional steel is that, while steel has a plastic behaviour after reaching its yield stress, GFRP does not and will rupture once reaching yield stress. On the other hand, GFRP has a higher tensile strength that steel (Table 1).

Steel GFRP

Nominal yield stress 276 to 517 MPa -

Tensile Strength 483 to 690 MPa 483 to 1600 MPa

Elastic Modulus 200 GPa 35.0 to 51.0 GPa

Yield strain % 0.14 to 0.25 -

Rupture strain % 6.0 to 12.0 1.2 to 3.1

Table 1: Usual tensile properties of reinforcing bars (ACI Committee 440, 2006, tb.3,3)

The diameter of a GFRP bar influences its tensile strength. It’s been reported that an increase of the diameter from 9 mm to 22 mm lead to a reduction of ultimate tensile strength of 40%. This is because of matrix cracking becoming more pronounced (Faza & GangaRao, 1993). It is recommended that

1 These are trademarked names for equivalent glass fibre products. HS2/HS4® by Sinoma in China, T-glass® by Nittobo in Japan and S-2-glass® by AGY in the U.S.

(Gardiner, 2009). S-2-glass is often referred to as S-glass, but S-glass is out of production.

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manufactures provide tensile strength for all diameters they produce (FIB Task Group 9.3, 2007).

1.3.4.2 Compression properties

The compressive strength of GFRP are generally much lower than the tensile strength (about 40%-60% of tensile). As internal reinforcement in concrete (and most other applications), the compressive strength is not a primary concern. (B.

Benmokrane et al., 1995) 1.3.4.3 Shear properties

Due to GFRP bars being Anisotropic, any resisting capabilities perpendicular to the longitudinal axis is quite low. A bar can be cut in half with a handsaw (ACI Committee 440, 1996). If necessary, the shear strength can be improved by braiding or winding fibres transverse to the main fibres during manufacture (ACI Committee 440, 2006).

1.3.4.4 Chemical properties

GFRP was considered as an alternative to steel reinforcement mainly due to chemical properties, especially for bridge decks and near marine environments.

This is because GFRP has a high resistance to acids (road salt, seawater) compared to steel. Nevertheless, early fibre-resin compositions showed corrosion in alkaline environment such as fresh concrete (FIB Task Group 9.3, 2007). This is not an ideal scenario for GFRP used as internal reinforcement.

Tests of the effect of alkaline attack on FRP rods have been performed both in alkaline solutions and in concrete. Vinyl ester resin and Epoxy resin are the least vulnerable to chemical attacks, but Vinyl ester resin is the cheapest of the two and most common in GFRP rebar manufacturing. (Sawpan, Holdsworth, &

Mamun, 2014). Most GFRP manufacturers also use E-CR glass fibre, which has increased chemical resistance. Sawpan et al. (2014) concluded in their tests of durability of GFRP rebars in high alkaline concrete; that E-CR fibre coated in vinyl ester polymer did not degrade notably.

UV-rays also damage some resin, but this is less of a concern when used as internal reinforcement. It should be kept in mind regarding storage of the GFRP rebars, if the resin does not have the additives to prevent UV-deterioration (FIB Task Group 9.3, 2007).

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1.3.4.5 Thermal properties

The thermal expansion coefficient of FRP materials naturally depend on the type of fibre, resin and volume fraction. It is important that this expansion is similar to concrete in order to prevent internal stresses. GFRP have a similar longitudinal expansion to hardened concrete, but the transverse expansion can be up to 8 times greater. This could lead to cracking. (Masmoudi, Zaidi, & Gérard, 2005). The decomposition temperature 𝑇𝑇_𝑔𝑔 is governed by the resin and is reported to be in the range of 60 to 170°C. Even though 𝑇𝑇_𝑔𝑔 of the resin is reached, the longitudinal strength of FRP bars is not significantly affected until around 300°C (Katz, Berman, & Bank, 1999).

1.4 Design Guidelines

The design approach regarding GFRP rebar concrete members naturally follows some of the same principles as the traditional steel reinforced members since the method in which the materials are utilized is the similar (ex.

equilibrium on the cross section, strain compability, assume plane sections remain plane (ACI Committee 440, 1996)). With that in mind, there are differences in material properties and composition that need to be considered for design using GFRP rebars. Simply replacing the steel bars with GFRP bars is a very poor (and possibly dangerous) way of utilizing the full potential of this composite material (Portnov et al., 2013).

1.4.1 Concrete cover

The concrete cover’s primary intention is protecting the reinforcement from corroding. As previous mentioned, corrosion of steel reinforcement is the main reason why GFRP is considered a reliable alternative (Portnov et al., 2013). This because of its non-corrosive material properties. Considering all environments possible, the concrete cover for steel reinforcement in Eurocode 2 (2004) is governed by predefined exposure- and durability classes representing different environmental conditions. With an increase of external alkaline environment follows a larger concrete cover.

For GFRP reinforcement, the procedure of determine the minimum concrete cover differ from that used for steel. The Canadian Standard Association (2012) clause 8.2.3 recommends:

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𝑐𝑐 = 𝑚𝑚𝑎𝑎𝑥𝑥 � 2𝑑𝑑30 𝑚𝑚𝑚𝑚�^𝑏𝑏 (1) Where:

𝑐𝑐 - Minimum concrete cover 𝑑𝑑_𝑏𝑏 - Diameter bar

Schöck provides another approach, more similar to EC2 (ComBAR®, 2015):

𝑐𝑐 = 𝑑𝑑_𝑏𝑏+ ∆𝑐𝑐 (2)

Where:

∆𝑐𝑐 - 10 mm for cast-in-place concrete

These approaches can differ from each other with large bar diameters, but the approach by CSA was chosen for further calculations for conservative reasons.

1.4.2 Flexural design

In traditional reinforced concrete design there are two failure modes, yielding of tensile steel (under-reinforced) or crushing of concrete in compression (over- reinforced). Of these two; the preferred failure mode has been the yielding of steel (Mosley, 2012). This is achieved by considering a balanced section, and through safety factors, you end up generally with an under-reinforced section.

This changes with the use of FRP however, due to the lack of plasticity in GFRP bars, which results in a sudden failure mode for the reinforcement as well.

Although both failure mode are sudden, crushing of concrete is marginally more desirable, because it show some signs of plastic behaviour (Nanni, 1993). To control which failure mode that is achieved, a balanced reinforcement ratio (as with steel) ρfb is considered. Assuming perfect bond, compatibility of strains, equilibrium of internal forces and linear behaviour of FRP up to failure, the following equation is given for ρfb (Torres, Neocleous, & Pilakoutas, 2012):

𝜌𝜌𝑓𝑓𝑏𝑏 = 𝜂𝜂𝜆𝜆𝑓𝑓_{𝑐𝑐𝑑𝑑}

𝑓𝑓𝑓𝑓𝑑𝑑 � 𝐸𝐸_𝑓𝑓𝜀𝜀_{𝑐𝑐𝑐𝑐} 𝐸𝐸𝑓𝑓𝜀𝜀𝑐𝑐𝑐𝑐+ 𝑓𝑓𝑓𝑓𝑑𝑑�

(3)

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Where:

𝜌𝜌_{𝑓𝑓𝑏𝑏} - Balanced GFRP reinforcement ratio

𝜂𝜂 - Factor of effective height of compression zone 𝜆𝜆 - Factor of effective strength

𝑓𝑓𝑐𝑐𝑑𝑑 - Design compressive strength of concrete 𝑓𝑓𝑓𝑓𝑑𝑑 - Design tensile strength of FRP rebars 𝐸𝐸_𝑓𝑓 - Modulus of elasticity of FRP rebars 𝜀𝜀_{𝑐𝑐𝑐𝑐} - Ultimate compressive strain of concrete

The American guideline ACI 440.1R-06 uses practically the same model.

Torres et al. (2012) state that the values of η and λ proposed in Eurocode 2 (CEN, 2004, clause 3.1.6(3))can be used directly when designing for failure due to concrete crushing (𝜌𝜌 > 𝜌𝜌𝑓𝑓𝑏𝑏, 𝜀𝜀𝑐𝑐𝑚𝑚= 𝜀𝜀𝑐𝑐= 0.0035and 𝜀𝜀𝑓𝑓 < 𝜀𝜀𝑓𝑓𝑑𝑑). Considering strain compatibility, the non-dimensional neutral axis depth ξ is given in equation (4), and from equilibrium of internal forces a normalized reinforcement ratio (5) is obtained:

𝜉𝜉 = 𝑥𝑥 𝑑𝑑 =

𝜀𝜀_{𝑐𝑐,𝑚𝑚}

𝜀𝜀𝑐𝑐,𝑚𝑚+ 𝜀𝜀𝑓𝑓𝑑𝑑 (4)

Where:

𝜉𝜉 - Non-dimensional neutral axis depth 𝑥𝑥 - Neutral axis depth

𝑑𝑑 - Effective depth

𝜀𝜀_{𝑐𝑐,𝑚𝑚} - Strain in concrete (= 𝜀𝜀_{𝑐𝑐𝑐𝑐} when considering concrete crushing)

𝑇𝑇 = 𝐶𝐶 ⇒ 𝜌𝜌𝐸𝐸_𝑓𝑓𝜀𝜀_{𝑓𝑓𝑑𝑑}

𝑓𝑓_{𝑐𝑐𝑑𝑑} = 𝜂𝜂𝜆𝜆𝜉𝜉 (5)

Where:

𝜌𝜌 - GFRP reinforcement ratio

𝑇𝑇 = 𝐶𝐶 - Represent the equilibrium of forces in the cross section (tensile and compressive)

𝜀𝜀𝑓𝑓𝑑𝑑 - Design tensile strain of GFRP

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The non-dimensional moment capacity is expressed as:

𝜇𝜇 = 𝑀𝑀_{𝑅𝑅𝑑𝑑}

𝑓𝑓_{𝑐𝑐𝑑𝑑}𝑏𝑏𝑑𝑑²= 𝜌𝜌𝐸𝐸_𝑓𝑓𝜀𝜀_{𝑓𝑓𝑑𝑑}

𝑓𝑓_{𝑐𝑐𝑑𝑑} (1 − 0.5𝜆𝜆𝜉𝜉) (6)

Where:

𝜇𝜇 - Non-dimensional moment capacity 𝑀𝑀_{𝑅𝑅𝑑𝑑}- Moment capacity

𝑏𝑏 - Width of cross section

Figure 5: Stress and strain profile of GFRP RC beam (fib Bulletin 40, 2007)

ACI 440.R1-06 suggest a similar equation and, in addition, the following model:

𝑀𝑀_{𝑅𝑅𝑑𝑑} = 𝜙𝜙𝐴𝐴_𝑓𝑓𝑓𝑓_𝑓𝑓�𝑑𝑑 −𝑎𝑎

2� (7)

𝑎𝑎 = 𝐴𝐴_𝑓𝑓𝑓𝑓_𝑓𝑓

0.85𝑓𝑓_𝑐𝑐′𝑏𝑏 (8)

Where:

𝐴𝐴_𝑓𝑓 - Area of reinforcement

𝑓𝑓_𝑐𝑐′ - Specified compressive strength of concrete 𝑓𝑓_𝑓𝑓 - Tensile stress in GFRP rebars

𝜙𝜙 - Reduction factor (concrete crushing: =0.65, GFRP rupture: =0.55)

For moment capacity by failure due to GFRP rupture (under reinforced) the same equations can be used, but through a trial and error process by assuming values for εc,m and checking force equilibrum (ACI Committee 440, 2006; Torres et al., 2012). ACI440.1R does not take into account the possible variation in the

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lever arm, which lead Torres et al. (2012) to propose this equation as an improvement to the simplification of the interativ process:

µ = 𝑀𝑀𝑅𝑅𝑑𝑑

𝑓𝑓_{𝑐𝑐𝑑𝑑}𝑏𝑏𝑑𝑑²= 𝜌𝜌𝐸𝐸_𝑓𝑓𝜀𝜀_{𝑓𝑓𝑑𝑑}

𝑓𝑓_{𝑐𝑐𝑑𝑑} (1 − 0.514𝜆𝜆𝜉𝜉𝜂𝜂) (9)

1.4.3 Shear and punching shear design

1.4.3.1 Shear

Shear design with GFRP is also based on the steel design recommendations, with modifications to account for the properties of GFRP. Researchers have argued that since steel shear design rely on plasticity theory and stress- redistribution, assumptions are not compatible with the behaviour of GFRP, therefore the models for GFRP design are not safe (Stratford & Burgoyne, 2003).

However, experimental evidence shows that, provided that shear cracks and shear resistance from both shear reinforcement and concrete are controlled, the assumptions provide good predictions of the shear capacity (fib Bulletin 40, 2007).

Bentz, Massam and Collins (2010) concluded in a research paper where they tested shear strength for large concrete members, that GFRP RC members shows the same size-effect and strain effect as steel RC members. They also found that the Canadian Standard Association (CSA) shear provisions for steel gave good predictions for shear strength (Bentz, Collins, & Massam, 2010). The CSA S806-12 gives the following shear capacity equation:

𝑉𝑉_𝑓𝑓 = 𝑉𝑉_𝑐𝑐+ 𝑉𝑉_{𝑠𝑠𝐹𝐹} (10)

Where:

𝑉𝑉_𝑓𝑓 - Shear resistance of GFRP RC member 𝑉𝑉_𝑐𝑐 - Shear resistance provided by concrete 𝑉𝑉𝑠𝑠𝐹𝐹 - Shear resistance provided by FRP stirrups

For members with an effective depth not exceeding 300 mm and for members with an effective depth greater than 300 mm with equal or greater transverse shear reinforcement according to equation (11); shear capacity of concrete (𝑉𝑉_𝑐𝑐) is given as:

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𝑉𝑉_𝑐𝑐 = 0.05𝜆𝜆_𝑐𝑐𝜙𝜙_𝑐𝑐𝑘𝑘_𝑚𝑚𝑘𝑘_𝑓𝑓(𝑓𝑓_{𝑐𝑐𝑑𝑑})¹³𝑏𝑏_𝑤𝑤𝑑𝑑_𝐹𝐹 (11)

𝑘𝑘𝑚𝑚 = �𝑉𝑉_{𝐸𝐸𝑑𝑑}𝑑𝑑

𝑀𝑀_𝑓𝑓 ≤ 1.0 (12)

𝑘𝑘_𝑓𝑓 = 1 + (𝐸𝐸_𝑓𝑓𝜌𝜌)¹³ (13)

𝐴𝐴𝐹𝐹𝐹𝐹 = 0.07�𝑓𝑓𝑐𝑐𝑑𝑑 𝑏𝑏_𝑤𝑤𝑠𝑠_𝐹𝐹 0.4𝑓𝑓𝑓𝑓𝑐𝑐

(14)

Where:

𝜆𝜆𝑐𝑐 - Factor to account for concrete density 𝜙𝜙_𝑐𝑐 - Resistance factor for concrete

𝐴𝐴_{𝐹𝐹𝐹𝐹} - Minimum area of transverse shear reinforcement 𝑏𝑏_𝑤𝑤 - Minimum effective web width

𝑑𝑑_𝐹𝐹 - Effective shear depth, taken as the greater of 0.9d or 0.72h 𝑓𝑓_{𝑓𝑓𝑐𝑐} - Ultimate tensile strength of GFRP reinforcement

𝑘𝑘𝑚𝑚 - Coefficient taking into account the effect of moment at section 𝑘𝑘𝑓𝑓 - Coefficient taking into account the effect of reinforcement rigidity 𝑀𝑀_𝑓𝑓 - Design moment at ULS

𝑠𝑠_𝐹𝐹 - Spacing of shear reinforcement 𝑉𝑉_{𝐸𝐸𝑑𝑑} - Design shear at ULS

If member has an effective depth greater than 300 mm and with less transverse shear reinforcement than according equation (14), the shear capacity in equation (11) shall be multiplied with a size effect factor:

𝑘𝑘_𝑠𝑠 = 750

450 + 𝑑𝑑 ≤ 1.0 (15)

The shear capacity of FRP stirrups (𝑉𝑉_{𝑠𝑠𝐹𝐹}) shall be computed as:

𝑉𝑉_{𝑠𝑠𝐹𝐹} =0.4𝜙𝜙_𝑓𝑓𝐴𝐴_{𝐹𝐹𝐹𝐹}𝑓𝑓_{𝐹𝐹𝑐𝑐}𝑑𝑑_𝐹𝐹

𝑠𝑠_𝐹𝐹 𝑐𝑐𝑐𝑐𝑡𝑡𝜃𝜃; 𝑓𝑓_{𝐹𝐹𝑐𝑐} ≤ 0.005𝐸𝐸_𝑓𝑓 (16)

𝜃𝜃 = 30° + 7000𝜀𝜀_𝑐𝑐 (17)

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Where:

𝜙𝜙_𝑓𝑓 - Resistance factor for GFRP reinforcement

𝐴𝐴_{𝐹𝐹𝐹𝐹} - Area of GFRP shear reinforcement perpendicular to the axis of a member within the distance sv

𝜃𝜃 - Angle of the diagonal compressive stress 𝜀𝜀𝑐𝑐 - Longitudinal strain at mid-depth of the section

Regarding the failure mode problem in flexure design, Bentz et al. (2010) mention that stirrups failing in shear provide a more gradual failure than the sudden flexural failure.

Due to the lower shear stiffness of GFRP bars, the dowel capacity (shear strength provided by longitudinal reinforcement) has been reported to be 70%

lower than that of steel. Therefore, this contribution can be neglected for usual bar diameters (Oller, Marí, Bairán, & Cladera, 2015).

1.4.4 Durability of GFRP rebars

In the last decade, GFRP in concrete constructions (bridges, columns, members, slabs) has been tested and used in some few countries (USA, Canada, Japan, England, and some few European countries) (fib Bulletin 40, 2007).

Different assumption and environment can have a major influence from different countries and continents. The weather has a certain effect on the durability of a structure over a long period of time. As durability is one of the keywords for designing a structure, GFRP reinforced concrete is no exception. Although, the technology these days has good guidelines and applications, there are still barriers to accomplish. Of particular concerns, we are looking at the effects of aging, effects of saline and moist, and various temperatures.

1.4.4.1 Long term prediction

Chen, Davalos and Ray (2006) tested the long-term behavior of GFRP rebars in concrete structures, using short-term data of accelerating aging test and based on Arrhenius relation (Nelson 1990):

𝑘𝑘 = 𝐴𝐴 ∙ 𝑒𝑒𝑥𝑥𝑒𝑒 ∙ �−𝐸𝐸𝑎𝑎 𝑅𝑅𝑇𝑇 � 𝑡𝑡𝑟𝑟𝑎𝑎𝑛𝑛𝑠𝑠𝑓𝑓𝑐𝑐𝑟𝑟𝑚𝑚𝑒𝑒𝑠𝑠

𝑖𝑖𝑛𝑛𝑡𝑡𝑐𝑐 →1 𝑘𝑘 =

1 𝐴𝐴 ∙ exp

𝐸𝐸𝑎𝑎 𝑅𝑅𝑇𝑇

(18)

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𝑙𝑙𝑛𝑛1 𝑘𝑘 =

𝐸𝐸𝑎𝑎 𝑅𝑅 ∙

1

𝑇𝑇 − ln(𝐴𝐴) (19)

Where:

𝑘𝑘_𝑑𝑑 - Degradation rate �_time¹ �

𝐴𝐴 - Constant, relative to the material and degradation process 𝐸𝐸𝑎𝑎 - Activation energy

𝑅𝑅 - Universal gas constant

𝑇𝑇 - Temperature (0 K = -273,15°C)

Arrhenius plot can estimate the service life by reaching the established tensile strength retention levels (PR) for any temperature. In the experiment from (Robert & Benmokrane, 2012), which they could predict the tensile strength retention as a function of time for temperatures of 10°C (Nordic condition) and 50°C (Middle East), immersed. This gave the authors a figure (figure that showed the relation between predicted service life and the PR).

Their conclusion for long-term prediction for retention of tensile strength of GFRP rebar embedded in moist and saline solution would decrease by 25% after 200 years and 35 years when immersed in isotherm temperature of 10°C and 50°C. While the service life needed for reaching a tensile strength retention of less than 70% could be concluded as infinite.

Figure 6: Relation between PR and predicted service life (Robert &

Benmokrane, 2012)

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1.4.4.2 Effects of saline and moist

Corrosion is a big problem when it comes to steel rebars. Using GFRP rebar could be an alternative solution to deterioration of structures such as building’s and bridges for long- term durability (Robert & Benmokrane, 2012).

To test the GFRP bars (embedded in concrete), a saline solution (3% NaCl) where made to simulate a marine environment or the use of deicing salt in a temperature of 50 °C (and 70°C). The concrete element was immersed for 365 days and showed no significant microstructural changes. Furthermore, no effect or changes by moisture absorption and high temperatures between bars and concrete, resin and fibers, in the glass transition where observed during the test (Brahim Benmokrane et al., 2002).

1.4.4.3 Sustained loading and freeze- thaw condition

According to Alves, El-Ragaby and El-Salakawy (2011) should freeze- thaw cycle along with the sustained load condition give increased bond strength.

Increased bond strength could also be achieved in specimens with different concrete covers added. In addition, a decrease in bond stiffness and some specimens with certain cover showed increase in slip (displacement between concrete and GFRP bar).

From their pullout test, specimens subjected to sustained load and freeze condition with inside concrete temperature of -18°C to +4°C, a short embedment length of 5dd was tested in an environment of -20°C to +15°C in a period of 250 freeze-thaw cycles (four months) with a relative humidity of 50%. The result showed a 10% degradation in both compressive and tensile concrete strength after 28 days, while specimens not subjected to freeze- thaw actually increased in both compressive and tensile concrete cover strength with values of 10% and 15% compared to those at 28 days.

Bars with a smaller diameter of 16 mm showed to be 35% higher in bond strength than the larger bars (19 mm).

Bond slip specimens subjected to conditioning scheme with concrete cover of 2.5db, showed increase in bond strength up to 40% for diameters of 16 mm and 19 mm. With hardly any change in the bond slip properties which indicated that sustained loads and freeze-thaw actually enhanced the bond properties. In fact, alleges (Alves et al., 2011) that the GFRP rebar absorbs moisture, absorption of 0.42% (16 mm) and 0.21% (19 mm). This especially in condition during thawing and enlarged cross- sectional areas. Furthermore, enhancing the friction mechanism, which improves the bond resistance. The thermal stresses seem to

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minimize the concrete surrounding the GFRP rebar, but a deterioration like this is not enough to reduce the resistance at the interface of the shear forces.

Schöck ComBAR (ComBAR®, 2015) is the manufacture GFRP rebar chosen for this thesis. The durability properties is described as the newest generation of GFRP rebar. The long-term strength of GFRP has different factors depending on maximum temperature, frequency, amplitude of the temperature changes where the rebar’s is exposed. Shöck ComBAR claims that the long-term strength of GFRP will decrease with time. However, a new safety concept is developed and is to guarantee the same level of safety in any design of GFRP reinforced concrete members. In addition, allowing an efficient and economic structure at the same time.

1.4.5 Fire design of GFRP rebar

This chapter shows the latest research regarding the fire design and behavior of GFRP structures. When designing ALS (Accidental Limit State) for bridge constructions, fire should be one of the topics to consider even though the possibility is very small. Because GFRP is more combustible and “softer” material than steel, could a high temperature in concentrated area change the properties in durability and bond strength. In this thesis, the fire design calculations were not considered.

The main conclusion from the research by Carvelli, Pisani and Poggi (2013) concerns a concentrated heating area that would give damage in the concrete element and partial bonding of the GFRP rebar without making the construction element to collapse. It’s most vulnerable aspect is overlapping areas and significant reduction of the load carrying capacity because of this.

The bond and mechanical properties will according to (Pagani, Bocciarelli, Carvelli, & Pisani, 2014) deteriorate as the temperature reaches close to the GFRP value transition temperature, depending on the resin. In addition, solution for possible fire underneath a bridge deck could be achieved by increasing the concrete cover.

Håndbok N400 (2014) clause 5.6.5 states that the characteristic values as an accidental load caused by a fire or explosion shall be determined for each project individually. For bearing construction parts concerning bridge related elements, which is considered as fire hazard, must be dimensioned for fire. Furthermore, construction details should be formulated in such way that consequences of fire are minimized.

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The Canadian Standard S806-12 (2012) contains fire performance, fire resistance, noncombustibility, flame spread and smoke development for GFRP.

Choosing GFRP rebar’s from Schöck ComBAR® (ComBAR®, 2015) and based on their information a GFRP rebar could catch fire when exposed directly to open flame since it does not contain any fire retardant, but would stop burning after a short period. As the temperature increases the bond between the surrounding concrete and bar, which is provided by the matrix, will become softer/melt (about 600 C^°). This will finally lead to loss of bond strength. Because of the critical temperatures, the factor of safety is set as 1.0.

Bond strength (N/mm²)

3.0 2.5 2.0 1.5 1.0 0.5

Critical temperature

(Cᵒ)

192 202 211 225 238 336

Table 2: Different bar diameter vary with the critical temperature (ComBAR®, 2015)

Table 2 shows the surface temperature of the bar. This temperature of concrete can be taken as the centerline of the rebar.

By using Schöck ComBAR® products, a concrete cover can be used as fire rating to make it more fire resistant:

Fire rating R30 R60 R90 R120

c 30 50 65 85

Table 3: Fire rating and minimum concrete cover (ComBAR®, 2015)

1.4.5.1 Bridge accidental events

As it is known, accidents can happen at any time and anywhere, even when the chances are small. Regarding bridges, fire scenarios should be enlightened and considered because of its possibilities. Cases such as:

- A vehicle/object burning on the bridge deck:

In this particular event, it will cause no harm to the embedded GFRP rebar, due to the protection of asphalt concrete and concrete cover. This is because the heat source is over the deck.

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- When a fire initiates below the bridge:

A more severe scenario, with a higher risk of damaging the bridge

structure. This can impair the strength and durability, but also debonding of the GFRP rebars can occur, mainly in areas where the rebars are overlapped.

- High temperatures:

When temperature reaches as high as 600°C, the strength of the structure decreases rapidly. Temperatures higher than 600°C happens only in extreme conditions (e.g. truck carrying inflammable substance).

1.4.6 Compression Design

The purpose of the compressive test is to determine ultimate compressive strength and compressive modulus of elasticity for the GFRP rebar’s (Deitz, Harik, & Gesund, 2000).

There have been conducted few research programs for determining the compressive properties of GFRP rebar’s. Kobayashi & Fujisaki (1995) preformed a test to find properties of FRP in compression. They used several different types of FRP rebars, one of them was a GFRP rebar. The conclusion of their test was that the GFRP rods had a compression strength equal to 30% of their tensile strength and the GFRP rods were affected by cyclic loading. In addition, the test showed about 20% to 50% reduction in the compression capacity of the rods reinforcement under repeated loading, all under observation.

An experiment of compressive properties of GFRP were made. Test results from this experiment experienced failing due to buckling and crushing with different unbraced GFRP lengths and diameters. The results of ultimate compressive strength versus unbraced length of GFRP rebars are listed below (Deitz et al., 2000):

Table 4: Unbraced length vs Ultimate compressive strength of the GFRP (Deitz et al., 2000)

Comp.

strength (MPa)

Crushing Crushing and

Buckling Buckling Unbraced length (mm)

500-400 X 60-115

450-250 X 110-215

250-100 X 215-380

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According to Dietz et al.’s conclusion (2000), GFRP reinforced concrete members should be considered fully braced for compression. The design was based on crushing compressive properties.

Esfahani et al. (2013) states that compressive strength for concrete does not influence the bond strength of GFRP rebar’s for spliced beams.

1.4.7 Bond length

Bond length is defined as the minimum embedment length in which the bar can reach its ultimate strength and is given as 𝑙𝑙_𝑑𝑑 (ACI Committee 440, 2006):

𝑙𝑙_𝑑𝑑=

𝛼𝛼 𝑓𝑓_{𝑓𝑓𝑓𝑓}

0.083√𝑓𝑓′𝑐𝑐 − 340 13.6 + 𝑐𝑐𝑑𝑑𝑏𝑏

𝑑𝑑_𝑏𝑏 (20)

Where:

𝛼𝛼 - Bar location 𝑑𝑑_𝑏𝑏 - Bar diameter

𝑓𝑓_{𝑓𝑓𝑓𝑓} - Mechanical properties of GFRP rebars and concrete

𝑐𝑐

𝑑𝑑𝑏𝑏 - Max 3.5 mm

𝑐𝑐 - Minimum concrete cover

𝑓𝑓′_𝑐𝑐 - Specified compressive strength of concrete

The Canadian Standard Association presents a similar equation for development length (CSA Standards, 2012):

𝑙𝑙𝑑𝑑= 1.15 ∙ 𝑘𝑘₁𝑘𝑘₂𝑘𝑘₃𝑘𝑘₄𝑘𝑘₅ 𝑑𝑑_{𝑐𝑐𝑠𝑠} ∙ 𝑓𝑓𝑓𝑓

�𝑓𝑓′𝑐𝑐∙ 𝐴𝐴𝑏𝑏 (21)

Where:

𝑘𝑘₁ - Bar location factor 𝑘𝑘₂ - Concrete density factor 𝑘𝑘₃ - Bar size factor

𝑘𝑘₄ - Bar fiber factor 𝑘𝑘5 - Bar surface factor

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𝑑𝑑_{𝑐𝑐𝑠𝑠} - Minimum of the distance from the closest concrete surface to the center of rebar OR ²₃ of center-center distance of the bars being developed ≤ 2.5db

𝑓𝑓_𝑓𝑓 - Design tensile stress in GFRP reinforcement at ULS 𝐴𝐴_𝑏𝑏 - Area of one rebar

1.4.8 GFRP of hooked rebars

The main objective was to investigate the bond behavior for anchorage of hooked GFRP rebars in concrete. The hooked rebar’s values are based on a test from Ehsani, Saadatmanesh, and Tao (1996) who developed a guideline for hooked rebar’s. In their experiment, 24 specimens were made with a hooked radius diameter of three bar. All the specimens were tested in a steel reaction frame. At each monotonic test, all load values were measured. The corresponding nominal tensile stress is calculated:

𝑓𝑓_𝑠𝑠ℎ =

⎝

⎛ 4𝑇𝑇^𝑒𝑒 𝜋𝜋 �𝑑𝑑2 �^𝑏𝑏

2

⎠

⎞ (22)

Where:

𝑇𝑇_𝑒𝑒 - Applied tensile load

Tensile stress developed by the hook:

𝑓𝑓ℎ = 𝜉𝜉ℎ�𝑓𝑓′𝑐𝑐 (23)

Where:

𝜉𝜉ℎ - Effects of the rebar diameter, tensile strength and casting position

For considering strength and serviceability for GFRP rebars, a factor K is computed as:

𝐾𝐾 = ^�

𝐿𝐿𝑑𝑑ℎ 𝑑𝑑𝑏𝑏�

�^{𝑓𝑓𝑠𝑠𝑠𝑠}

�𝑓𝑓′𝑠𝑠� → 𝑔𝑔𝑖𝑖𝑣𝑣𝑒𝑒𝑠𝑠 𝐿𝐿𝑒𝑒𝑒𝑒. = 0.205 ∙ �^𝑑𝑑_{�𝑓𝑓′}^𝑏𝑏^∙𝑓𝑓^{𝑦𝑦𝑓𝑓}

𝑠𝑠 � (24)

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Where:

𝑓𝑓_{𝑦𝑦𝑓𝑓} - Effective yield strength of GFRP rebar (approx. 80% of ultimate strength (Ehsani et al., 1996))

Figure 7: a) 90 degr. hook, b) 180 degr. hook

According to Ehsani et al. (1996), K= 0.205 is an adequate value for GFRP hooked rebars. It is recommended to use ^𝑓𝑓₅₁₇^{𝑦𝑦𝑓𝑓} and 0.7, taking into account the yield strength for GFRP hooked rebars, and other than 517 MPa and covers that is larger.

The modification factor:

𝐹𝐹_𝑒𝑒 = 𝑓𝑓_{𝑦𝑦𝑓𝑓}

517 ⋅ 0.7 (25)

From the 𝐿𝐿_{𝑒𝑒𝑒𝑒.} we get calculation of basic development length 𝐿𝐿_ℎ𝑏𝑏 (ACI Committee 440, 2006):

𝐿𝐿ℎ𝑏𝑏= 152 ∙ � 𝑑𝑑_𝑏𝑏

�𝑓𝑓′𝑐𝑐� ∙ 𝐹𝐹𝑒𝑒 (26)

For the hooked rebar near the critical section (consider the equation above) should not be less than 8 times the bar diameter of 152 mm to prevent direct pullout failure (Ehsani et al., 1996).

According to (CSA Standards, 2012) for GFRP rebar’s should the provided calculated development length not be less than 12𝑑𝑑_𝑏𝑏or 230 mm. This also applies to tail length of a bent bar (𝑙𝑙𝑒𝑒). Furthermore, the bend radius (𝑟𝑟𝑏𝑏) should not be less than 3𝑑𝑑𝑏𝑏^.These interfaces is to be used with development length of bent/

hooked bar and can be taken either as:

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Design of footbridge with GFRP reinforced concrete

BACHELOROPPGAVE

Preface

Abstract

Sammendrag

Table of contents

List of symbols

List of figures

List of tables

1 State of the Art

1.1 History

1.2 Field of application

1.2.1 Internal and external

1.2.2 Durability

1.2.3 Electromagnetic neutrality

1.2.4 Costs

1.3 Material Properties

1.3.1 Introduction

1.3.2 Resin

1.3.3 Glass Fibres

1.3.4 Properties of GFRP

1.4 Design Guidelines

1.4.1 Concrete cover

1.4.2 Flexural design

1.4.3 Shear and punching shear design

1.4.4 Durability of GFRP rebars

1.4.5 Fire design of GFRP rebar

1.4.6 Compression Design

1.4.7 Bond length

1.4.8 GFRP of hooked rebars