Carbon fibre reinforcement in concrete

Bachelor of Engineering Thesis Carbon fibre reinforcement in concrete By Blessing Chabvuta 2016 Supervisors Rob Wolff School of Engineering and Inf...
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Bachelor of Engineering Thesis

Carbon fibre reinforcement in concrete By

Blessing Chabvuta 2016

Supervisors Rob Wolff School of Engineering and Information Technology Charles Darwin University

Sabaratnam Prathapan School of Engineering and Information Technology Charles Darwin University

School of Engineering and Information Technology

Abstract In this report carbon fibre in the form of braided rope was considered for the possibility of use in reinforcing concrete. Braided carbon fibre ropes were considered for applications such as tendons for prestressed concrete or grids for slabs and beams. Experimental investigations that were carried out by Cortis, Kaczmarcyzyk, & Pearce, (2013) reported a low bonding strength between concrete and braided carbon fibre rope. This means braided carbon fibre despite having exceptional mechanical properties may not be used for prestressed concrete. However, braided carbon fibre may be used for post-tensioning of concrete. Grid reinforcement may be the only way to effectively to use carbon fibre for concrete reinforcement. Application of grid reinforcement in concrete was reported to increase the pull out load (Seo & Djamaluddin, 2006). This could be due to large surface area of the grid that is exposed to concrete bonding in comparison with adhesive bonding on ropes. Therefore, grid carbon fibre reinforcement may be used to reinforce concrete slabs, wall panels and beams.

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Carbon Fibre reinforcement in concrete

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Acknowledgement I am grateful to my primary supervisor Rob Wolff for his support, guidance and mentoring over the course of the study. I would also like to thank my second supervisor A/Prof Sabaratnam Prathapan for their support and expert advice. I would also like to thank the school of Engineering and Information technology department.

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Table of Contents List of figures ............................................................................................................................ 5 List of tables .............................................................................................................................. 6 1

Introduction ........................................................................................................................ 7

2

Scope .................................................................................................................................. 8

3

Alternative reinforcement options ...................................................................................... 9

4

5

6

3.1.

Stainless steel ............................................................................................................... 9

3.2.

Carbon fibre ............................................................................................................... 10

3.3.

Glass fibre .................................................................................................................. 11

3.4.

Aramid fibre ............................................................................................................... 12

3.5.

Comparison of reinforcement options ....................................................................... 13

Carbon fibre ...................................................................................................................... 15 4.1.

History of carbon fibre ............................................................................................... 15

4.2.

Production process of carbon fibre ............................................................................ 16

4.3.

Carbon fibre reinforcement concept .......................................................................... 18

Existing carbon fibre reinforcement ................................................................................. 19 5.1.

Carbon fibre reinforced polymer bars ........................................................................ 20

5.2.

Carbon fibre reinforced polymer cable ...................................................................... 21

5.3.

Carbon fibre reinforced polymer grid ........................................................................ 22

5.4.

Carbon fibre textile reinforcement ............................................................................. 23

5.5.

Concrete reinforced with CFRP ................................................................................. 24

5.6.

Benefits of using CFRP reinforcement ...................................................................... 26

5.7.

Problems of using CFRP reinforcement .................................................................... 27

Reinforcing concrete using carbon fibre without resin .................................................... 28 6.1.

Carbon fibre rope ....................................................................................................... 29

6.2.

Arrangement of carbon fibre ropes in concrete ......................................................... 31

6.3.

Application of braided carbon fibre rope in concrete ................................................ 32

6.3.1.

Pre-stressed concrete .......................................................................................... 32

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6.3.2.

Pre-stressing bed ................................................................................................. 32

6.3.3.

Anchorage system .............................................................................................. 33

6.3.4.

Bond strength ...................................................................................................... 34

6.4.

Braided carbon fibre grid ........................................................................................... 36

6.4.1.

Fabrication of carbon fibre grid .......................................................................... 37

6.4.2.

Adhesive options for grid reinforcement ............................................................ 37

6.4.3.

Concrete for braided carbon fibre grid ............................................................... 39

6.5.

Properties of concrete reinforced with braided carbon fibre grid .............................. 41

6.5.1.

Bond strength ...................................................................................................... 41

6.5.2.

Flexural behaviour .............................................................................................. 43

6.5.3.

Failure mode ....................................................................................................... 43

7

Discussion......................................................................................................................... 44

8

Recommendations ............................................................................................................ 46

9

Conclusion ........................................................................................................................ 46

10 References ........................................................................................................................ 47

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List of figures Figure 1: Strain in a rectangular beam subjected to bending ................................................. 18 Figure 2:Ribbed carbon fibre reinforced polymer bars (Zongao Carbon, 2016)...................... 20 Figure 3: Carbon Fibre Reinforced Cables for prestressed concrete ........................................ 21 Figure 4:Carbon fibre reinforced polymer grid ........................................................................ 22 Figure 5 Comparison of steel and textile reinforcement .......................................................... 23 Figure 6:Insulated architectural wall panels for external building wall ................................... 24 Figure 7: Carbon fibre grid was used to reinforce double tee beam......................................... 25 Figure 8:Shade grove parking garage in Maryland used double Tee beams reinforced with carbon fibre grid (Altusgroup, 2016) ........................................................................................ 25 Figure 9 : Symphony House in Philadelphia (Dave, 2009) ...................................................... 26 Figure 10:High strength braided carbon fibre .......................................................................... 30 Figure 11:Typical arrangement of steel reinforcement in concrete .......................................... 31 Figure 12: Possible way of arranging small diameter carbon fibre rope in concrete ............... 31 Figure 13:Typical pre-stressing bed schematics ....................................................................... 32 Figure 14: Steel coupling device for anchoring braided carbon fibre (Tokyo rope mfy.co.ltd) .................................................................................................................................................. 33 Figure 15: Components of the steel coupling device (Tokyo rope mfy.co.ltd) ........................ 33 Figure 16: Pre-stressing bed with steel coupling and braided carbon fibre (Tokyo rope mfy.co.ltd) ................................................................................................................................ 34 Figure 17: Standard pull out test for braided carbon fibre rope ............................................... 34 Figure 18;Rapture of braided carbon fibre and cracking of concrete (Cortis, Kaczmarcyzyk, & Pearce, 2013) ............................................................................................................................ 35 Figure 19: Braided carbon fibre grid reinforcement ................................................................. 36 Figure 20: Self consolidating concrete being applied over a grid reinforcement ..................... 39 Figure 21: Typical volume percentage of constituents in self-consolidating concrete and traditional concrete (Andrawes, Shin, & Pozolo, 2009). .......................................................... 40 Figure 22: Average bond capacity (information used to draw the graph was obtained from Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000)) ............................................................. 42

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List of tables Table 1: Material properties of PX35 carbon fibre produced by Zoltek .................................. 10 Table 2: Mechanical properties of different types glass fibre .................................................. 11 Table 3: Material properties of continuous aramid fibre .......................................................... 12 Table 4: Mechanical properties of different alternative reinforcement options ....................... 13 Table 5: Chemical resistance of Fibres and stainless steel (Newhook & Svecova, 2007) ....... 14 Table 6 Properties of matrix material ....................................................................................... 19

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1 Introduction The corrosion of steel reinforcement is a major concern with regards to durability of structures that are exposed to aggressive chemical environment. Common examples are bridge decks, concrete columns and parking garages that are in direct contact with sea water. The corrosion of concrete due to chloride ions is recognised to be the main cause for structural deterioration. Ingress of chloride ions into concrete removes the passive protective oxide layer, thereby exposing the steel reinforcement to corrosion. In order to eliminate or reduce corrosion, research into chloride ion corrosion has recommended increasing the concrete cover, coating the steel reinforcement with epoxy, using stainless steel and cathodic protection of steel reinforcement as ways to increase the service life of structures but none of these methods is effective (Neville, 1995). Replacing steel reinforcement with carbon fibre may be a solution to improve on the durability of concrete. Over the years carbon fibre has been used to reinforce concrete in the form of a composite material known as Carbon Fibre Reinforced Polymer (CFRP). CFRP have exceptional mechanical strength and does not corrode in chloride environment such as sea water. However, the epoxy used in fabrication of CFRP is not stable at high temperatures above 120°C (Huang, 2009). This report considers the possibility of using carbon fibre without thermosetting resin to reinforce concrete in the form of a rope. Continuous carbon fibre rope was reported to be ineffective in transferring force with 50% reported to work as flexural reinforcement in concrete (Djamaluddin, Yuichi, Toshiaki, & Takayaki, 2000). Braiding carbon strands was considered as a way on improving interlocking between carbon filaments and also produces a surface that can bond well with concrete. Alternatively, braided carbon fibre rope may be used to make grid reinforcement for slabs and beams. In order to successfully use braided carbon fibre rope and grid, to reinforce concrete, the bond strength between concrete and reinforcement had to be considered. Research done by Cortis, Kaczmarcyzyk, & Pearce, (2013), reported a low bond strenth which was about 36% that of steel reinforecemnt and concrete. However, a grid reinforcement may result in improved bonding to concrete. Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000) report an increase in pull out load when grid reinforcement with more than three crossing members were used in concrete. The low bond strength that was reported means braided carbon fibre may not be used to prestressed concrete. The only effective way of reinforcing concrete with carbon fibre may be to use a grid reinforcement instead of rope.

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2 Scope High modulus carbon fibre which is produced from Polyacrylonitrile (PAN) and petroleum pitch material has exceptional mechanical and corrosion resistance properties. This material may be able to be used as a replacement for steel reinforcement in concrete that is exposed to sea water. The carbon fibre filaments can be combined to form a yarn which is then braided together to form a rope. Braided carbon fibre rope is a flexible material which requires tensioning before casting concrete. Therefore, carbon fibre rope may be used to reinforce prestressed concrete that is fabricated in a factory. Alternatively, braided carbon fibre may be used to form grid reinforcement similar to steel mesh. Structural adhesive may be used to hold the crossing members of the grid. In order to achieve the scope, this study was divided into four main areas which are: 

Investigate the current use of carbon fibre in concrete



Explore ways of reinforcing concrete with carbon fibre rope



Examine the bond strength between carbon fibre rope and concrete



Research into failure mechanism of beams reinforced with carbon fibre rope

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3 Alternative reinforcement options 3.1.

Stainless steel

The word steel means iron makes the bulky part of stainless steel compound, while stainless implies the absence of staining, rusting or corroding in environments where carbon steel is susceptible to corrosion (Baddoo, 2008). In order to achieve the excellent properties that stainless steel possess, different elements are added in controlled amounts each for specific purposes. For instance, about 11% chromium is added to carbon steel to prevent it from rusting. Chromium forms a passive protective oxide layer that protects the underlying steel from corrosion. In some more hostile environment, such as in sea water, carbon and a higher percentage of chromium (up to 27%) are added to prevent pitting corrosion (Lo, Shek, & Lai, 2009). Other elements such as nickel and molybdenum can also be added to improve the properties of stainless steel. Molybdenum has a powerful effect in improving the resistance to pitting in chloride environments (Cunat, 2004). There are four different types of stainless steel that are suitable for reinforcing concrete. These are 2205 stainless steel, Type 316LN, 18Cr-3Ni-12Mn and Type 304LN (Maggie & Schnell, 2002). With regard to the hostile marine environment and the reinforcing properties of stainless steel, this report considered 2205 stainless steel and Type 316LN as more suitable material that may be used as alternative to carbon steel. Stainless steel 2205 is a duplex with a microstructure that is made up of austenite and ferrite phase. The duplex structure, along with its chemical composition, gives the alloy an excellent combination of strength and corrosion resistance (Magee & Schnell, 2002). It has a higher percentage of chromium, molybdenum and nitrogen which provide exceptional superior chloride pitting and crevice corrosion resistance. The duplex microstructure allows it to resist chloride stress-induced corrosion and cracking. According to Bourgin, Chauveau, & Demelin, (2006), the pitting resistance of stainless steel 2205 is approximately 50%, which is higher than the other stainless steel grades. Stainless steel Type 316LN has high yield and tensile strength. It has a higher nitrogen content which enhances its resistance to pitting and crevice corrosion. The low percentage of carbon makes it resistant to intergranular corrosion and it should, therefore, be considered for concrete reinforcement exposed to chloride ions (Magee & Schnell, 2002).

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3.2.

Carbon fibre

Carbon fibre is a fibrous material with an approximately 95% carbon content. It is produced by heating organic material at high temperatures above 200°C. The major starting materials that are used to produce carbon fibre are Polyacrylonitrile (PAN) and petroleum pitch. According to the information provided on Zoltek website, a company that produce carbon fibre, 90 percent of commercial carbon fibre is produced from PAN with the remaining 10 percent from petroleum pitch material. Carbon fibre has excellent mechanical properties when compared to steel. It has high modulus of elasticity ranging from 200 to 800GPa and an ultimate elongation of 0.3 to 2.5% where low elongation refers to high stiffness and vice versa (Chaea, et al., 2015). It does not absorb water and is further resistant to chemical solutions that corrode steel, such as sea water. Carbon fibre does not stress corrode and neither shows any sign of creep relaxation. It is however, a quite conductive, such that when it is joined to steel, it undergoes galvanic corrosion (Chaea, et al., 2015). Table 1 illustrates the mechanical properties of a carbon fibre brand (PX35) that is produced by Zoltek. Table 1: Material properties of PX35 carbon fibre produced by Zoltek Material properties of a PX35 continuous carbon fibre towel produced by Zoltek 50k carbon fibre towel Tensile

4.137

strength

MPa

Tensile

242 GPa

modulus Elongation

1.5%

Density

1.81g/cm3

Fibre

7.2µm

diameter Carbon

95%

content Spool length

1500m

Spool mass

5.50 Kg

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3.3.

Glass fibre

Glass fibre is a more commonly used form of fibre reinforcement when compared to carbon fibre or aramid fibre. It is produced by drawing melted oxide (a mixture of silica oxide, limestone, fluorspar, boric acid and clay) into filaments which range in sizes from 3 to 24nm.The glass fibre that is produced, has a higher tensile strength, electrical resistance, acid resistance and costs less when compared to carbon and aramid fibres. There are two types of glass fibre that are mostly used to reinforce concrete which are, the S-glass fibre and the Eglass fibre. The S-glass fibre has higher tensile strength, stiffness and ultimate strain than the E-glass which is also more expensive and degrades in alkaline environments. Other types of glass fibre are the C and Alkaline resistant (AR) glass fibre. The C glass fibre has chemical stability in an acidic environment and the (AR) glass fibre is used to minimise weight loss in alkaline environment (Benmoktane, Chaallalt, & Masmoudi, 1995). Mechanical properties of different types of glass fibre are summarised in table 2. Table 2: Mechanical properties of different types glass fibre Parameter

Tensile

E-

S-

C-

AR

Glass

Glass

Glass

3.45

4.30

3.03

2.50

72.40

86.90

69.00

70.00

4.80

5.00

4.80

3.60

2.54

2.49

2.49

2.78

AR glass fire

strength [MPa] Tensile modulus [GPa] Ultimate strain Density g/cm3

Information in table 2 was obtained from Benmoktane, Chaallalt, & Masmoudi (1995)

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3.4.

Aramid fibre

Aramid fibre is a short name for aromatic polyamide, a group of synthetic products with exceptional strength and heat resistance. It has a breaking strength of 3045MPa, which is about five times that of steel and it is able to retain strength and high modulus of elasticity at temperatures as high as 300°C. When an aramid fibre is loaded under tension, it can undergo elastic deformation. A good example of a common aramid fibre is Kevlar which is used in areas where high strength, high modulus, high toughness, thermal dimensionality stability, low creep and light weight is needed. Kevlar is commonly used in reinforced tyres, circuit board reinforcement, helmets and as a replacement for asbestos. Although aramid fibre has good mechanical properties for a wider range of applications, it absorbs moisture. This attribute makes aramid fibre more sensitive to the wet or humid environment in comparison with glass fibre which is more durable. Aramid fibre may also undergo an oxidative reaction when exposed to UV light resulting in change in colour and loss in strength (Fibremax Composite, 2014) . Table 3 summarises mechanical properties of continuous aramid fibre spool. Table 3: Material properties of continuous aramid fibre Material

properties

of

continuous Aramid fibre spool

aramid fibre Tensile strength

2950 MPa

Tensile modulus

99 GPa

Elongation

2.9 %

density

1.44 g/cm3

Fibre diameter

14µ

Information contained in table 3 was obtained from Fibre Technologies International, (2016)

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3.5.

Comparison of reinforcement options

Mechanical properties for different materials that may be used to reinforce concrete are summarised in Table 4 Table 4: Mechanical properties of different alternative reinforcement options Fibre type

Tensile strength [MPa]

Modulus of elasticity [GPa]

Percentage Coefficient Density elongation of thermal g/cm3 expansion [× 𝟏𝟎−𝟔

Carbon fibre High strength

3500

200-240

1.3-1.8

-1.2 to –0.1

1.76

High modulus

2500-4000

350-650

0.4-0.8

7 to 12

1.80

ordinary

780-1000

38-40

2.1-2.5

-1.6 to -0.9

High modulus

3000-3500

400-800

0.4-1.5

-1.6 to -0.9

Kevlar 29

3620

82.7

4.4

NA

Kevlar 49

2800

130

2.3

Kevlar 129

4210

110

Kevlar 149

3450

172-179

1.9

Twaron

2800

130

2.3

Technora

3500

74

4.6

NA

1.39

E-Glass

3500-3600

74-75

4.8

5.0

2.54

S-Glass

4900

87

5.6

2.9

2.60

1800-3500

70-76

2.0-3.0

NA

NA

2205 Duplex steel

620-900

200

25

13.7

7.81

316LN stainless steel

515-620

190-210

35-40

19.5

8.00

PAN

Pitch

Aramid fibre 1.44 1.44 NA

1.44

NA

1.47 1.44

Glass fibre

AR Glass Stainless steel

Table 4 is based on data obtained from Newhook & Svecova,(2007) It can be seen from Table 4 carbon fibre, glass fibre and aramid fibre have high tensile strength when compared with stainless steel. However, this property alone can not be used to select the best material as they have almost the same tensile strength. Carbon fibre has a higher modulus of elsticity when it is compared with all the other material listed in Table 4. From the information obtained Table 4, it can be concluded that carbon fibre has exceptional mechanical properties, therefore, it may potential to reinforce concrete. Blessing Chabvuta

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Materials required to reinforce concrete that is in direct contact with sea water must be able to resist corrosion. An experimental investigation was conducted by Newhook & Svecova, (2007), to determine behaviour of different reinforcing material in damaging environment. Materials such as carbon fibre, aramid fibre and stainless steel were placed in hydrochloric acid, sulphuric acid, sodium hydroxide and brine to determine their stability. It was observed that carbon fibre did not corrode in all the chemicals. The results of this experiment are summarised in table 5. Table 5: Chemical resistance of Fibres and stainless steel (Newhook & Svecova, 2007) Material

Acid resistance

Alkaline resistance

High strength

Good

Excellent

High modulus

Excellent

Excellent

Ordinary

Excellent

Excellent

High modulus

Excellent

Excellent

Kevlar 49

Poor

Good

Tenchora

Good

Good

E-Glass

Poor

fair

S-Glass

Good

Poor

Alkaline Resistance glass

Good

Good

2205 Duplex steel

Good

Good

316LN stainless steel

Good

Good

Carbon fibre PAN

pitch

Aramid fibre

Glass fibre

Stainless steel

Information in table 5 was obtained from Newhook & Svecova, (2007)

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4 Carbon fibre 4.1.

History of carbon fibre

Carbon fibre is a fibrous material with a high carbon content of up to 95 percent. It is produced by heat treatment of organic material at temperatures ranging from 1000ºC to 1500ºC.The earliest commercial carbon fibre is attributed Thomas Edison’s carbonisation of cotton and bamboo fibres for an incandescent lamp. The filaments produced were able to withstand high temperature which was required within the incandescent light bulb. However, the filaments had weak mechanical properties and were not considered as reinforcing material. Practical commercial use of carbon fibre started in the late 50s in pursuit of improving ablative material for rockets (Walsh, 2001). During World War II, an American company called Union Carbide Corporation explored the production of high strength carbon fibre from Rayon and polyacrylonitrile precursor material. The research performed by Union Carbide lead to the production of carbon fibre from Rayon and PAN in 1959 and 1961 respectively. Previous research in the 1950 by DuPont has reviewed that acrylic material could be thermally stabilised, while work conducted by Shindo in Japan and Watts demonstrated that using tension through carbonisation process increases the mechanical properties of carbon fibre. A considerable amount of research was performed during 1960 to 1970 to improve on performance to price ratio of carbon fibres. Different organic materials such as cotton, linen, sisal and other man-made polymers like polyester, polyamides, and polyvinyl chloride was tried as precursor material for carbon fibre. However, only Rayon, PAN and pitch received recognition because of the excellent mechanical properties of the carbon fibre produced. The demand for carbon fibre increased in 1980 due to advances in aerospace industry. PAN based carbon fibre was extensively used compared to carbon fibre from other precursor material such as pitch and rayon. This was mainly due to the low cost of producing PAN compared with spinnable pitch. In the mid-90s, a new cost effective PAN based carbon fibre made from modified textile precursor was aggressively promoted by companies such as Zoltek and Fortar for commercial application. According to Walsh (2001), a goal was announced to reach a carbon fibre price level of $11/Kg by the year 2000. This received wide attention and increased application development. An overall trend in improved performance/price ratio for both pitch and PANbased manufacturer has sustained growth. Blessing Chabvuta

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4.2.

Production process of carbon fibre

About 90% of today’s commercial carbon fibre is produced from Polyacrylonitrile (PAN) precursor (McConnell, 2008). Production of carbon fibre from a PAN-based precursor consists of five main steps. These are Polymerization, spinning, oxidation, carbonisation, surface treatment and sizing; and these steps are described below: Polymerisation Polymerization is the first step in the production of carbon fibre. In this step, polymeric feedstock known as a precursor is produced. Acrylonitrile monomer is combined with plasticized acrylic monomer and a catalyst such as titanic acid, sulphuric acid, and sulphur dioxide or methyl acrylic acid. The mixture is continuously stirred to blend the ingredients and ensure consistency and purity and initial formation of free radical. This leads to the polymerization reaction to produce acrylic fibres. After acrylic fibres have been washed and dried, the acrylonitrile now in powder form is dissolved in either organic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAC), or dimethylformamide (DMF) or aqueous solvent such as zinc chloride. Organic solvents prevent contamination by trace metals that can upset thermal oxidation. At this point, the solvent-powder slurry is the consistency of maple syrup. This stage is important as it determines the success of the next phase of fibre formation (McConnell, 2008). Spinning Wet spinning is performed where the dope is immersed in a liquid coagulation bath and extruded through holes in a spinneret made from precious metals. The number of spinneret holes determines the filament count of the PAN fibre. Wet spanned fibres which are gelatinous and fragile are drawn by rolling through a wash which removes excess coagulant, then dried and stretched to continue orientation of PAN polymer. Filament external shape and internal cross section are determined by the degree to which the selected solvent and coagulant have penetrated the precursor fibre, the amount of applied tension and percentage of filament elongation. The last step of this stage is the application of finishing oil to prevent tacky filaments from clumping white PAN fibres, then dried and wound onto bobbins. The process combines oxygen molecules from the air with PAN fibre in the warp and causes polymeric chains to start cross-linking. This increases the density of the fibres (McConnell, 2008).

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Carbonization Carbonisation occurs in an inert atmosphere inside a series of specially designed furnaces that progressively increases the processing temperature. During the carbonisation process, noncarbon elements are removed. Carbonisation starts at low temperature around 700°C to 800°C and ends in a high-temperature furnace at 1200°C to 1500°C. Fibre tensioning is continued throughout the production process. The number of furnaces determines the modulus desired in the carbon fibre (McConnell, 2008). Surface treatment The surface treatment is the last step in the production PAN based carbon fibre. The surface treatment is performed to enhance adhesion and one of the common methods used involves pulling fibres through an electrochemical or electrolytic bath that contains solutions such as sodium hypochlorite or nitric acid. These chemicals etch or roughen the surface the surface of each filament (McConnell, 2008). A special type of coating is applied at a ration that varies from 05 to 5 percent the weight of carbon fibre. This coating process protects the fibre during handling, processing and also holds individual filaments together in tows.

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4.3.

Carbon fibre reinforcement concept

High modulus carbon fibre has a potential to reinforce concrete based on the mechanical and chemical properties it possesses. Carbon fibre has high tensile strength and requires a large force to undergo small deformations. When a concrete beam reinforced with carbon fibre is subjected to bending deformation, the neutral axis moves closer to the reinforcing material reducing the compressive stress developed in the concrete (Hayashiida, Takeda, Yamasaki, & Nakamura, 1997). This is attributed to carbon fibre undergoing small deformation as compared to steel reinforcement which as low tensile strength. The higher the modulus of reinforcing material, the smaller is the deformation of the material. Figure 1 illustrates a strain diagram for a simply supported carbon fibre reinforced beam under bending. Compression Effective depth d

Carbon fibre

Intermediate modulus

Neutral

Reinforcement cross sectional area

Reinforcement position

Figure 1: Strain in a rectangular beam subjected to bending

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5 Existing carbon fibre reinforcement Carbon fibre is strong in one direction (along with the fibre) and brittle when it is loaded perpendicular to the direction of the fibre. In order to improve the mechanical properties, and handling of material at the construction site, carbon fibre can be converted to a composite material known commercially as Carbon Fibre reinforced polymer (CFRP). Carbon fibre reinforced polymer consists of parallel carbon fibre filaments impregnated in a matrix material which is usually epoxy. The matrix material acts as a binder, helps to transfer forces between fibre filaments and also protects the fibre against physical and chemical attack. There are two types of matrix material commercially used in civil engineering work which are thermosetting and thermoplastic resins, and of these two, the thermosetting resin is widely used to make the composite material. Epoxy and vinyl ester are the two mainly used thermosetting resins to make carbon fibre composite reinforcement. Epoxy is expensive when compared to a vinyl ester but it has good mechanical properties. These exceptional mechanical properties make it a preferable matrix choice. Table 6 illustrates the difference in mechanical properties of epoxy and vinyl ester. Table 6 Properties of matrix material Material

Density

Tensile strength

Tensile modulus Failure strain

Kg/m3

[MPa]

[GPa]

[%]

Vinyl ester

1000-1450

20-100

2.1-4.1

1.0-6.5

epoxy

1100-1300

55-130

2.5-4.1

1.5-9.0

Data in Table 6 was obtained from Caroline ( 2003). There are different methods of making CFRPb reinforcements which are hand lay-up, pultrusion, filament winding and moulding. The mechanical properties of resulting CFRP depends on matrix that is used, direction and volume of carbon fibre. In some cases, there is a limit on the volume of carbon fibre that can be used to make the polymer. For instance, at least 60% fibre volume in required to make carbon fibre reinforced polymer bars and rods. These bars can be used in the same manner as steel bars. CFRP can also be made in the form of cables for prestressed concrete and grids for concrete slabs and walls.

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5.1.

Carbon fibre reinforced polymer bars

Carbon fibre reinforced composite rods have been used to reinforce both precast and cast in place concrete. They are produced through an automated process known as pultrusion. In this process, carbon fibre filaments are drawn through a bath of epoxy resin or any other thermosetting resin followed by a dye which gives the characteristics cross section. After completing the baking process, the final product produced has a smooth surface which prevents good bonding to concrete. Bonding between the carbon fibre reinforcement and concrete is critical as it determines the ultimate serviceability and load carrying capacity the reinforced structure. To improve the bonding properties of the carbon fibre rods, the following methods have been employed: 

Deforming the surface of the bars by creating ribs (Figure 5) or indents. Ribbed bars can be manufactured from a combination of protrusion and compression moulding technique.



Surface treatment using sandblasting or epoxy coated sand.



Spirally winding the protrusion rod carbon fibre row and sand coat.

Figure 2:Ribbed carbon fibre reinforced polymer bars (Zongao Carbon, 2016)

Carbon fibre reinforced polymer bars are used to reinforce concrete in the same way as steel. Their main advantage over steel reinforcement is that they can be tailor-made to resist specific loads.

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5.2.

Carbon fibre reinforced polymer cable

Carbon fibre reinforced polymer cable shown in figure 3 is a composite material used for prestressing and post-tensioning of concrete. CFRP cables are produced through a pultrusion which is a process for the continuous extrusion of polymer profile. Carbon fibre rovings are pulled through an impregnated bath of epoxy resin, then a forming die and finally curing area. The cables produced have good mechanical and physical properties that can be used in concrete. These properties include high specific strength and stiffness, corrosion resistance and outstanding fatigue behaviour. Carbon fibre filaments that are used in making the cables are aligned in a continuous and parallel profile. The carbon fibres have a good parallel alignment and are continuous (Saiidi, 2016). In prestressed concrete the carbon fibre cables can be held by anchorage device, and are then tensioned before the concrete is cast around them. When the concrete has reached the required strength, the cables can be cut from the anchorage device, therefore, stress is transferred to the concrete.

Figure 3: Carbon Fibre Reinforced Cables for prestressed concrete

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5.3.

Carbon fibre reinforced polymer grid

The carbon fibre reinforced polymer grid, also known as carbon grid is made using a poltrusion process. It was reported by Rizkalla, et al.,( 2013) that, increasing the diameter of the carbon fibre reinforced polymer rods reduces their overall effectiveness due to shear lag mechanism required to activate all the filaments within the cross section. A solution to this issue was to reduce the diameter to make small CFRP rods that can be cross-linked to form a reinforcement mesh as highlighted in figure 4. CFRP grid can be used as reinforcement to replace steel mesh in slabs and wall panels.

Figure 4:Carbon fibre reinforced polymer grid

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5.4.

Carbon fibre textile reinforcement

Carbon fibre textile reinforcement is a polymer material made of carbon fibre and epoxy resin. Continuous carbon fibre yarns or roving are processed in a planar structure by a textile technique to produce an optimum alignment and arrangement of the fibre. Epoxy resin is then applied and the material is allowed to cure. Figure 5 demonstrates the difference between steel reinforcement and textile reinforcement used in concrete.

Steel reinforced concrete Figure 5 Comparison of steel and textile reinforcement

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5.5.

Concrete reinforced with CFRP

Carbon fibre reinforced polymers have been used to reinforced concrete mainly in the form of a grid. The carbon fibre grids come in rows of different sizes; it is a light and flexible material which makes it easy to handle at a construction site. The carbon fibre reinforced polymer grid is used to make insulated architectural panels for external building walls (see Figure 6).

Figure 6:Insulated architectural wall panels for external building wall The picture shown in figure 6 was taken from Altusgroup, (2016). Figure 6 demonstrates how carbon fibre was used in the face panel to replace steel mesh reinforcement and as a mechanical link between the inner and outer sections of the concrete wall. Less concrete cover is needed to protect the reinforcing material as carbon fibre grid does not corrode. This saves material and produces thin, strong and durable concrete walls.

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Carbon fibre grids have been used to replace steel mesh in double Tee beams (Figure 7). A practical application example of this application is found in a project at the University of Maryland, which successfully incorporated double tee beams within the construction of parking garage (see figure 8)

Figure 7: Carbon fibre grid was used to reinforce double tee beam

Figure 8:Shade grove parking garage in Maryland used double Tee beams reinforced with carbon fibre grid (Altusgroup, 2016)

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5.6.

Benefits of using CFRP reinforcement

Carbon fibre reinforced composite material has a number of benefits when used to reinforce concrete structures. CFRP does not corrode in harsh environments such as sea water; therefore concrete structures that are reinforced with CFRP have a greater life span compared to those with steel reinforcement. Non-corrosion nature of CFRP means less concrete cover is required to protect the reinforcement. It was reported that reinforcement of double tee beams using carbon fibre grids uses 8mm concrete cover instead of 20mm required to protect steel reinforcement. This is a reduction of approximately 12mm in concrete cover and therefore saves construction material (Altusgroup, 2016). A reduction in concrete cover for structures reinforced using CFRP also decreases the total weight of the structure built. The symphony house in Philadelphia (see figure 9) for example used wall panels reinforced with carbon fibre grid. The weight of the wall panels was reported to weigh 40% less compared to a conventional six-inch steel reinforced precast concrete (Dave, 2009)

Figure 9 : Symphony House in Philadelphia (Dave, 2009)

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5.7.

Problems of using CFRP reinforcement

Although carbon fibre reinforced polymer composite material has excellent mechanical properties for reinforcing concrete, there is a downside that has reduced their application in the construction industry. The main problem is the instability of epoxy thermosetting resin at high temperature. When concrete reinforced with CFRP is exposed to elevated temperature (above 200°C), differential thermal expansion between the concrete and CFRP takes place (Galati, Nanni, Dharani, Focucci, & Aiello, 2006). In addition, degradation of epoxy resin also occurs. In the event of elevated temperature, the glass transition temperature which is relatively low is often reached resulting in weakening of the bond between individual fibres and a loss in adhesion bonding between concrete and CFRP. A loss in adhesion bond of between 80-90% was reported in a study conducted by Katz, Berman & Bank (1999). According to the Authors, a loss in adhesion bonding resulted in a decrease in strength of the concrete structure; therefore, structures reinforced with CFRP are less likely to meet the fire endurance standard of 90 minutes. CFRP are expensive reinforcing material when it is compared with the price of steel reinforcement. The price of CFRP is about ten times that of steel tendons (Burgoyne, 2009) making construction process uneconomical. Therefore, CFRP may not be used a direct replacement of steel reinforcement despite having excellent mechanical properties.

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6 Reinforcing concrete using carbon fibre without resin The main problem that has prevented the use of carbon fibre reinforced polymer is the cost and instability of epoxy resin at temperatures above 120°C. One way of overcoming the problem, may be to use carbon fibre without resin, in the form of a rope. Carbon fibre rope is a flexible material which requires tensioning to keep in position before concrete is cast. It may be used as a good alternative for steel tendons used in reinforced pre-stressed concrete. Alternatively, it may be used to make a reinforcing grid where longitudinal ropes are linked to crossing members by a structural adhesive. Carbon fibre rope is a flexible material which requires application on tensional force to keep in position when pouring concrete. Therefore, reinforcing concrete using carbon fibre rope is only applicable for precast concrete produced in a factory. Although it is possible to make long beams and wide slabs using carbon fibre rope, transportability problems limit the size of precast concrete structures that can be produced. For example, by considering the width of roads and turning circles of long vehicles, precast concrete beams reinforced with carbon fibre rope must be approximately 12m which is the length of most transport vehicles. In order to determine the feasibility of using carbon fibre rope to reinforce concrete, the bonding between concrete and the rope must be investigated. Pre-stressed concrete relies on bonding between concrete and reinforcement to transfer force, therefore a strong bonding is critical. For structures that may be reinforced with grid made from carbon fibre rope, flexural bonding and failure mode needs to be investigated.

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6.1.

Carbon fibre rope

Carbon fibre has a smooth surface which results in poor bonding to concrete when it is used as reinforcement. In order to make a rope, carbon fibre filaments can be combined to form a thick continuous rope. In a continuous rope, the filaments are not linked together resulting in poor transfer of force between filaments. In an experimental study, for the application of continuous carbon fibre as flexural reinforcement in concrete Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000) observed that approximately 50 percent of carbon fibres were involved in transferring force. A braided carbon fibre rope may be a solution to improve concrete bonding and effective transfer of forces between strands. Carbon filaments are combined together to form strands which is then braided into a rope. The braiding process produces a load bearing core that is made up of strands intertwined together to form an interlocking structure. At least three carbon fibre strands are required to form a braided rope and the properties of the rope increase with an increase in the number of stands. For example, it was it was reported by Michael, Kern, & Heinze, (2016) that increasing the number of strands would decrease the polygonal effect and with it the pressure between strands thus creating a smooth and denser surface. Although a single strand of carbon fibre rope may have the same strength as braided carbon fibre rope of the same cross-sectional area, there are several advantages which make braided carbon fibre rope a more preferred choice. These are: 

If one carbon fibre strand were to break, the rope will remain intact whereas a single

stranded large continuous carbon fibre rope would fail with potentially catastrophic consequences. 

Thin braided carbon fibre rope has a higher strength in contrast to thick continuous carbon fibre rope. Therefore, using thin braided carbon fibre rope saves material; reduces weight and the cost of producing the rope.



Multi-stranded braided carbon fibre rope is more flexible than a single stranded carbon fibre rope of the same diameter.

Commercially, there exists a high strength carbon fibre rope (figure 10) that is being produced by a Chinese company Hi-tech Carbon Co Limited. This type of braided carbon fibre rope is produced from Polyacrylonitrile (PAN) through a classified woven technique and has the potential to be used as concrete reinforcement. The rope is characterised by its exceptional properties which include high strength, light weight, good corrosion resistance, non-melting Blessing Chabvuta

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under high temperature and excellent thermal stability. It is produced in different diameters ranging from 1mm to 5mm and has the high tensile strength of between 450-500MPa (AliExpress, 2016)

Figure 10:High strength braided carbon fibre The picture in figure 10 was obtained from AliExpress, (2016)

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6.2.

Arrangement of carbon fibre ropes in concrete

When a simply supported beam is subjected to bending deformation, the top section of the beam will be in compression while the bottom section will be tension. Reinforcement is placed on the bottom section of the beam to help concrete to resist tension force. In concrete that is reinforced using steel, 8mm or 12mm bars are arranged as shown in figure 11. The same arrangement may be adopted for carbon fibre rope. However, large diameter carbon fibre rope must be avoided as they are not efficient in transferring force. A reduction in overall effectiveness due to shear lag mechanism required to activate all fibres in a cross section was reported by Rizkalla, et al, (2013). A similar scenario was reported by Djamaluddin, Yuichi, Toshiaki, & Takayaki,( 2000), where approximately 50% of carbon fibre filaments in a continous rope were reported to work as flexural reinforcement. In order to effectively use carbon fibre, thin carbon fibre ropes about 2mm in diamter may be arranged in a close spacing as shown in figure 12. The arrangement in figure 12 increases the surface area for bonding to concrete and also increases the effeciency in carrying the tensile force.

Figure 11:Typical arrangement Figure 12: Possible way of arranging small diameter of steel reinforcement in carbon fibre rope in concrete concrete

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6.3.

Application of braided carbon fibre rope in concrete

Thin braided carbon fibre ropes may be used for pre-stressed concrete where tension force is applied on the cables to keep them in position before casting concrete around them. They may not be used for cast in-situ concrete because of handling problems at the construction site. 6.3.1. Pre-stressed concrete Braided carbon fibre ropes may be used to reinforce prestressed concrete. Prestressed concrete is a structural material that allows for predetermined engineering stresses to be applied to members to counteract stresses that will occur when service loads are applied. Prestressing concrete can be done in two ways which are pre-tensioning and post-tensioning. In pre-tensioning, three or more 2mm braided carbon fibre ropes can be combined to make a pre-tensioning tendon of approximately 8mm in diameter. Stress is applied to the pretensioning cable and concrete is cast around it. Afterwards, when the concrete has reached the desired strength, the cables are cut at the anchorage. Through the bond between the concrete and the pre-tensioned braided carbon fibre rope, a compressive force is applied onto the concrete. This technique relies on the bond strength between the cables and the concrete to be effective in transferring compressive forces. In post-tensioning, cables consisting of three or more 2mm braided carbon fibre rope encased in a duct or sleeve that is positioned in the form before concrete is cast. Afterwards, when the concrete has gained the desired strength before service loads are applied, the cables are pulled tight and anchored against the outer edge of the concrete. 6.3.2. Pre-stressing bed Pre-stressing bed (see figure 14) contains equipment that is used either for the pre-tensioning or post-tensioning technique. The equipment consists of a chuck which is commonly used on the non-stressing end of the bed, also known as Bayonet grip. The bayonet grip is made up of a wedge and a barrel. On the stressing end of the bed, where tension is applied through a hydraulic jack, is an open grip. In an open grip, the wedge is held together by an O-ring

Figure 13:Typical pre-stressing bed schematics

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6.3.3. Anchorage system The anchorage system for connecting braided carbon fibre strands to the hydraulic jack is very important for transfer tension force to the tendons. Braided carbon fibre rope is a brittle material when loaded at a right angle to the fibre direction. It is also susceptible to abrasion when a conventional anchorage system that is used on steel tendons is applied. Steel coupling equipment (see figure 14) produced by Tokyo ropes may be used as an anchorage device. The steel coupling device is made up of a stainless steel sleeve and an attached joint coupler in which to anchor with steel strands. A steel mesh sheet and braided steel grip to provide friction between the steel sleeve and the braided carbon fibre rope and they also avoid direct contact of the wedge with braided carbon fibre. To anchor the steel stands to the coupler a chunk is used inside the coupler. Figure 15 and 16 show the components for a steel coupling device and the final set up of a pre-stressing bed that uses the steel coupling device.

Figure 14: Steel coupling device for anchoring braided carbon fibre (Tokyo rope mfy.co.ltd)

Sleeve for braided carbon fibre

Wedge for braided carbon fibre rope

Joint coupler

Mesh sheet

Braid grip

Wedge for steel strand

Figure 15: Components of the steel coupling device (Tokyo rope mfy.co.ltd) Blessing Chabvuta

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Figure 16: Pre-stressing bed with steel coupling and braided carbon fibre (Tokyo rope mfy.co.ltd)

6.3.4. Bond strength Determining the bond strength between braided carbon fibre rope and concrete is critical in order to identify the practicality of using braided carbon fibre as reinforcement. Bond strength determines the efficiency in transferring stresses from pre-tensioned ropes to concrete. A high bond strength is required to prevent pretension members from premature failure, in regards to split failing or pull out failure. In order to determine the bond strength between braided carbon fibre rope and concrete, a pullout test can be performed. The setup for the pull out test is illustrated in Figure 17 where a pull out force is applied on the braided carbon fibre rope until failure.

Figure 17: Standard pull out test for braided carbon fibre rope

In this thesis report, experimental investigation to determine the bond strength between braided carbon fibre reinforcement and concrete was not carried out. However, findings from past experiments on similar studies were used to estimate the strength of braided carbon fibre to concrete. Cortis, Kaczmarcyzyk, & Pearce, (2013) performed an experimental investigation to determine the bonding strength between concrete and braided fibre ropes. They performed a standard pull-out test on 7mm diameter Siltex prestressed carbon fibre ropes, that were embedded in six samples of 150mm by 20mm concrete cylinders. The test reported an average bond strength of 4.17 MPa which was 36 percent of the bond strength between concrete and similar diameter steel bars (11.44MPa). The load response was reported as linear Blessing Chabvuta Carbon Fibre reinforcement in concrete Page | 34

load displacement behaviour, followed by rope de-bonding and failure was due to anchorage slippage during pulling load which influenced elongation behaviour (Cortis, Kaczmarcyzyk, & Pearce, 2013). Due to the low bond strength reported on braided carbon fibre rope, a similar pullout test was conducted to determine the bond strength using 10mm diameter technora fibre rope ribbed with glass beads at the core. A 5.5 times increase in bonding strength was reported and the failure mode was due to crushing of glass beads and cracking of concrete as shown in figure 18. The ripped braided carbon fibre rope was reported to show an enhancement in bond strength but in cost comparison with steel reinforcement, the ripped rope was four times more expensive than a steel reinforcement (Cortis, Kaczmarcyzyk, & Pearce, 2013).

Figure 18;Rapture of braided carbon fibre and cracking of concrete (Cortis, Kaczmarcyzyk, & Pearce, 2013)

The low bond strength reported on braided carbon fibre ropes makes braided carbon fibre rope not suitable for prestressing concrete. Prestressing relies on the bond strength between the braided carbon fibres and surrounding concrete to transfer stress from prestressed strands to concrete. It can be concluded that braided carbon fibre ropes will have a long development length and prestressed members would fail prematurely due to pull out failure. A solution to increase the bond strength between braided carbon fibre rope and concrete will be to increase the surface area for bonding by making a reinforcement mesh. Blessing Chabvuta

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6.4.

Braided carbon fibre grid

Low bond strength (4.17 MPa) between braided carbon fibre and concrete was reported by Cortis, Kaczmarcyzyk, & Pearce, (2013). In order to improve the bond strength between concrete and braided carbon fibre reinforcement, a solution is to increase the surface area for bonding. One way of increasing the surface area will be to form a grid reinforcement system (see figure 19) using braided carbon fibre ropes and high-temperature structural adhesives such as Mastersil 800. Longitudinal members

Crossing members Figure 19: Braided carbon fibre grid reinforcement The grid reinforcement system is made up of 2mm braided carbon fibre rope in parallel arrangement with 10mm spacing between longitudinal reinforcement. Crossing members will be spaced out at 20mm. Structural adhesive may be used as the joint between longitudinal members and crossing members which helps in transferring force. The advantages of using grid reinforcement are:  

Increases the surface area of braided carbon fibre rope that can be bonded to concrete. The grid reinforcement system is easy to layout at a construction site and maybe used for precast as well as cast in-situ concrete.

A braided carbon fibre grid reinforcement system may be used to replace steel mesh. During the construction process, the grid is placed under tension force to keep it in position when concrete is poured.

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6.4.1. Fabrication of carbon fibre grid Fabrication of carbon fibre grid reinforcement for concrete can be achieved through an automated system. A robot may be used to lay out longitudinal and crossing carbon fibre rope, structural adhesive is then applied to the joint. A structural adhesive is defined as a substance capable of holding materials together by surface attachment (Hartshan, 2012).The forces of attraction hold the materials together. Structural adhesive bond is characterised by the following properties: 

High strength adherents are involved.



Adhesive is capable of transferring stress between adherents without loss in structural integrity.



Bond structure maintains stable over a long period of time.

6.4.2. Adhesive options for grid reinforcement Mastersil 800 This is one component, high-performance silicon elastomer compound for bonding. It is mainly used in electronic and aerospace industry. The compound can bond to a variety of substances that includes glass, metal and plastics. It can be cured at room temperature but the curing time depends on the thickness of the layer and humidity. It has ultra-high temperature resistance with a serviceable temperature range of up to 300°C and has high corrosion resistance. Unlike other silicones, mastersil 800 does not depolymerise when exposed to temperatures of up to 300°C and high pressure. (Masterbond, 2016) EP42HT-2 adhesive This is room temperature curable, two component epoxy adhesive with high temperature and chemical resistance. It has a service temperature of 230°C and easy to use by just applying contact pressure while curing. The material has outstanding strength properties (Masterbond, 2016) EP17HT-LO A one component heat cured epoxy system for bonding. It has excellent physical properties and chemical resistance. The glass transition temperature of 225°C is higher than that of commercial epoxy and has a high service temperature of 345°C.The adhesive is easy to use but requires a minimum curing temperature of 150°C (Masterbond, 2016). Blessing Chabvuta

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EP46HT This is a two-part epoxy system for high-performance structural bonding. It is used in applications which require a temperature resistance of up to 315°C. The material readily cures at elevated temperature and seals with high mechanical strength. It is easy to use by just applying contact pressure and has outstanding heat resistance (Masterbond, 2016).

UV 25 This has a glass transition temperature of 180°C and can readily cure by exposing the material to UV radiation. Comparison of adhesive options By comparing the five structural adhesives described in sections above based on service temperature. Mastersil 800 has the better option because it has a high service life of 300°C. Although the temperature for Mastersil 800 is less than that of EP 17HT AND EP46HT of 345 and 315°C, Mastersil can cure rapidly at room temperature whereas the EP17HT require minimum curing temperature of 150°C

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6.4.3. Concrete for braided carbon fibre grid Self-consolidating concrete Carbon fibre grid has a rectangular opening measuring 10mm x 20mm.Therefore; concrete mixture must not contain aggregates which are more than 10mm in size. Self-consolidating concrete may be used for carbon fibre grid reinforcement. This type of concrete is highly flowable and does not segregate. It can spread into form, fills the form work and can encapsulate even most congested reinforcement. Plasticisers are used to create flowing concrete that meets high performance requirements. In order for concrete to be considered as self-consolidating concrete, it must be able to meet three minimum requirements which are: 

Ability to flow



Passing ability



Resistance to segregation

Self-consolidating concrete may be used for precast slabs, wall panels and beams that may be strengthened with carbon fibre grid. Figure 20 illustrates the practical application of selfconsolidating concrete to make precast concrete slab.

Figure 20: Self consolidating concrete being applied over a grid reinforcement

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Self-consolidating concrete composition Self-consolidating concrete has a high paste volume which allows it to flow. In addition, a typical mix has less or small aggregates and high sand to coarse aggregate ratio than conventional concrete. Figure 21 illustrates the percentage composition of aggregates in selfconsolidating concrete and conventional concrete.

Figure 21: Typical volume percentage of constituents in self-consolidating concrete and traditional concrete (Andrawes, Shin, & Pozolo, 2009).

Mechanical properties of self-consolidating concrete Hardened self-consolidating concrete is considered to share similar mechanical properties with conventional concrete in terms of strength and modulus of elasticity. The main difference between the two types of concrete is the creep and shrinkage properties. Selfconsolidating concrete is designed to have a higher paste content or fine aggregate compared to typical concrete mixtures, which would likely cause an increase in shrinkage (Andrawes, Shin, & Pozolo, 2009).

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6.5.

Properties of concrete reinforced with braided carbon fibre grid

In order to successfully apply braided carbon fibre grid as reinforcement in concrete, there are a number of properties that have to be investigated. These properties include the bond strength between braided carbon fibre and concrete, flexural and failure modes of beams reinforced with braided carbon fibre grids. 6.5.1. Bond strength Identifying the bonding strength of the grid system to the concrete is pivotal, in order to determine the feasibility of using braided carbon fibre as reinforcement in concrete. A standard pull-out test can be performed to determine the strength of the bond between braided carbon fibre grid and concrete. In this thesis report, no standard tests were conducted; however, there exist similar tests that were performed in previous research in the field. These test results were used to estimate the bond strength between carbon fibre grid and concrete. Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000), performed a number of pull out tests which were aimed to determine the bond capacity of the carbon fibre grids. Samples with just longitudinal reinforcement (zero grid), longitudinal reinforcement with variable number of crossing members were tested. The results from the pull out test were compared with data obtained from a grid reinforcement model. It was reported that the load increased to a maximum value and then it decreased. The load decreased to about 4kN and was followed by slip propagation without significant change in load. The bonding strength was attributed to adhesion or friction between the rope and the concrete. It was also reported that increasing the number of crossing members in a carbon fibre grid increases the pullout load. The pull out load was reported to double when the number of crossing members was increased from 1 to 3 as shown in Figure 22. However, there was no significant change in pull out load when crossing members of the grid were increased from 3 to 5 (Figure 22). This information reinforces the idea that, the bond strength between braided carbon fibre rope and concrete may be increased by introducing a grid reinforcement system up to a point. The strain distribution was reported to increase from 0 on the free end to a maximum on the loaded end. The difference in strain between segments along the longitudinal cable separated by a crossing member indicated that some of the tensile force is carried by the joints in the grid.

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35 30

P max (kN)

25 20 15 10 5 0 1

2

3

4

number of grids

Figure 22: Average bond capacity (information used to draw the graph was obtained from Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000))

It was reported that the bond capacity of the cable alone was about 10% of the specimen with five grids, therefore, it was concluded that the existence of more than three grids increases the pullout load approximately 10 times. Although the experiment was conducted using continuous carbon fibre rope, the bonding strength to concrete may be increased by using braided carbon fibre. The bonding strength between braided carbon fibre ropes to concrete was reported to be equivalent to 36% of that of deformed steel bars to concrete (Cortis, Kaczmarcyzyk, & Pearce, 2013). This is higher when it is compared with that of continous carbon fibre rope. In an experiment to determine the bonding strength of continuous carbon fibre rope to concrete, Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000) reported a bonding strength equivalent to 10% of deformed steel bars to concrete.

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6.5.2. Flexural behaviour The flexural behaviour of beams reinforced with carbon fibre grid must be checked in order to determine the failure mode. A flexural beam test can be conducted by applying a point load to a simply supported beam. The applied load can be increased until the beam fails where the failure mode and maximum load sustained can then be determined. Experiments to determine the behaviour of concrete beams using UCAS method were performed by Ohta et al (2013). A series of experiments were conducted to determine the influence of a grid system on the flexural behaviour of relatively short concrete beams. It was reported that initially, all beams were uncracked and stiff but upon further loading, a crack appeared in the mid-span where the applied moment exceeded the cracking moment causing a reduction in stiffness. This was greater in beams reinforced with continuous carbon fibre without resin than steel. It was attributed to the fact that the cracks were wider and crack spacing narrower for beam reinforced with carbon fibre but no shear reinforcement. 6.5.3. Failure mode When a beam is loaded under simply supported conditions and the applied point load is increased continuously until it breaks, the beam will fail by the crushing of concrete instead of shear failure. When a carbon fibre grid reinforced beam is loaded for the flexural test, a single crack will appear at the midpoint and on further loading; the crack widens and propagates fast upwards near the top of the compression side. The single crack will continue to widen until concrete starts to crush (Djamaluddin, Yuichi, Toshiaki, & Takayaki, 2000). A maximum load of 92.7 kN was reported Djamaluddin, Yuichi, Toshiaki, & Takayaki, (2000)and this was followed by crushing of concrete until the load reached 124.4 kN, then the load started to decrease followed by an increase in deflection. The propagation of a single crack was attributed to the fact that, the carbon fibre cables could not be bonded perfectly with surrounding concrete. In comparison with a steel reinforced concrete beam, a crack will appear at the midpoint and widen and then propagate upwards. With further loading, an inclined crack will start from the support and propagate to the load point. This is due to the fact that, the beam had no shear reinforcement, therefore, shear capacity must be sustained by concrete.

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7 Discussion Braided carbon fibre rope has the potential to reinforce pre-stressed concrete. The rope is flexible, therefore requires the application of tensile force to keep in position before concrete is poured. One good application for braided carbon fibre rope will be, as a tendon for pretensioned concrete. Three 2mm diameter braided carbon fibre ropes may be combined to make reinforcing tendon similar to steel cables. Pre-tensioned precast concrete relies on the bond between concrete and reinforcing tendons in transferring stress. In an experimental investigation to determine the bond strength between concrete and braided carbon fibre rope, Cortis, Kaczmarcyzyk, & Pearce, (2013) reported a low bond strength which was about 36% the bond strength between concrete and steel reinforcement. Due to the low bond strength braided carbon fibre rope may not be an ideal material for pretension precast concrete. However, braided carbon fibre rope may be used as post-tensioning reinforcement in concrete. Post-tensioning is dependent on the tensile strength of the reinforcing material and is not bonded to concrete. In addition, braided carbon fibre may not corrode and is stable at high temperatures (above 1000ºC). Areas of application include bridges where it can be used a steel tendon replacement. A grid reinforcement may be the only way to reinforce concrete using carbon fibre. This is due to the grid having a large surface area exposed for bonding with concrete as compared to tension rope reinforcement. Seo & Djamaluddin, (2006) reported an increase in bonding of approximately 10 times when a grid reinforcement was used compared to the adhesive bond capacity of cables alone. Although the experiment conducted by Seo & Djamaluddin, (2006), used CFRP cables instead of braided carbon fibre rope, the same phenomenon may be experienced when a grid using braided carbon fibre rope is used. However, the bonding values may be slightly less as CFRP cables have better adhesion bond compared to braided carbon fibre. Although braided carbon fibre has exceptional mechanical properties which make it a possible replacement for steel, there are limitations in the application of braided carbon fibre rope for concrete reinforcement. Braided carbon fibre rope is a flexible material which would deflect due to its own weight. Due to the nature of this reinforcing material, it is hard to keep it in position when concrete is poured. One way to overcome this deflection problem is to apply tensile force and cast concrete around it. This makes reinforcing with braided carbon fibre only limited to precast concrete that is fabricated in a factory. Application for cast in place concrete may not be possible. Blessing Chabvuta

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Transport related problems play a huge role in limiting the application of braided carbon fibre in concrete. Precast concrete reinforced with braided carbon fibre rope may be limited to approximately 12m in length. This is mainly due to transport vehicle length capacity in terms of turning circles. Dimensions of concrete structures such as width of slab may also limit the application of the material in construction. It may not be possible to transport wide concrete slabs to the construction site and the fabrication is also limited to the size of the factory.

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8 Recommendations Tests to determine the bond strength between concrete and braided carbon fibre were not carried out. It is therefore required to determine the actual bonding strength as it depends of the other factors such as type of concrete and strength of concrete. In order to determine whether braided carbon fibre cannot be applied for prestressed concrete, practical investigation must be conducted. The strength of the adhesive used in fabricating carbon fibre grid need to be investigated and its behaviour at high temperature. In order for the grid to be used as a reinforcing material, the reinforced material must be able to meet the fire endurance standards of 90 minutes. MasterSil 800 was chosen because it has a higher glass transition temperature compared to that of epoxy. Although MasterSil 800 was reported to be stable at high temperature, there is a need to investigate its behaviour and durability in different chemical environments.

9 Conclusion Braided carbon fibre rope has exceptional mechanical and non-corrosion properties which makes it a good alternative concrete reinforcement option to steel. 2mm braided carbon fibre rope may be combined to form a prestressing tendon for precast concrete. A low bonding strength between braided carbon fibre and concrete was reported by (Cortis, Kaczmarcyzyk, & Pearce, 2013). This means braided carbon fibre rope may not be used as pre-stressing tendons in concrete as the technique relies on bonding in transferring stress from tendon to concrete. Braided carbon fibre grid was reported to have enhanced bonding strength due to large surface are of the reinforcement exposed to bonding with concrete. In conclusion braided carbon fibre rope cannot be used as a replacement of steel reinforcement.

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10

References 1. AliExpress. (2016). 2mm carbon fibre braided cord for vacuum furnace/ carbon cord/ carbon twisted rope. Retrieved from http://www.aliexpress.com/store/product/2mmCarbon-Fiber-Braided-Cord-for-Construction-carbon-fiber-woven-Carbon-FiberTwist-Rope/1305179_32266145782.html 2. Altusgroup.

(2016).

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double

tee.

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http://altusprecast.com/products/double-tees/ 3. Andrawes, B., Shin, M., & Pozolo, A. (2009). Transfer ad development length of prestressing tendons in full scale AASHTO prestressed concrete girders using self consolidating

concrete.

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