Alternative reinforcement for concrete

Alternative reinforcement for concrete John L. Clarke

When correctly specified and constructed, reinforced concrete is a cost-effective and durable construction material. However, the specification and/or use of inappropriate materials can lead to poor durability, particularly in severe exposure conditions and with poor levels of workmanship. The major problem is the corrosion of the embedded steel reinforcement. The principal approach for achieving durability is the selection of appropriate concrete materials and mixes, which are covered extensively elsewhere in these volumes. Another approach, and one that is becoming more widespread, is the use of alternative reinforcement materials that will be more durable. These include: • coated reinforcement (either galvanized or fusion-bonded epoxy) • stainless steel • fibre composites This chapter briefly describes the first two materials, considering their properties and giving examples of their use in practice. The chapter concentrates on fibre composite materials, generally known as FRPs (fibre reinforced polymers), which consist of glass,

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Alternative reinforcement for concrete

carbon or aramid fibres combined with an appropriate resin to form a solid. The topics covered include: • • • • • • •

properties of fibres and resins manufacturing processes and properties of composites research and development advantages and disadvantages amended design rules for concrete reinforced with fibre composites Health and safety considerations applications and demonstration projects

The use of fibre composite reinforcement is still very much in its infancy but it is predicted that its use in specialist applications will grow rapidly. This chapter provides the necessary introduction to the materials.

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26.2.1 Galvanized steel reinforcement Galvanized steel reinforcing bars have been successfully used in several countries over the past 50 years (Australia, Bermuda, Netherlands, Italy, the UK, and the USA) and consumption is increasing. The main advantages of galvanized steel are: • • • • •

it it it it it

delays the initiation of corrosion and cracking has very good performance in carbonated concrete tolerates higher chloride migration levels than uncoated steel provides protection to the steel during storage has longer life in cracked carbonated concrete than uncoated bar.

Hot-dipped galvanized steel is produced by dipping clean and fluxed steel into a bath of molten zinc. The layer formed on the surface of the steel usually consists of a thin outer coating of pure zinc on a series of layers of zinc/iron alloys with increasing iron content. The performance of galvanized steel in concrete as reported in the literature (Andrade et al., 1995) is contradictory. Although it has been used successfully in practice, laboratory studies suggest that its performance would not be cost-effective. The factors behind this divergence of views, currently the object of discussion, are: • • • • •

the pH of the cement paste the bond between the reinforcing bars and the concrete chromate passivation of the galvanized steel the structure and thickness of the zinc coating the resistance of the zinc coating to corrosion induced by chloride ions

Zinc is passive in most cement pastes as the pH of uncarbonated cement pastes is 1213.5. A passive layer would be formed when pH < 13.3, the upper limit for passivation, due to the formation of a layer of calcium hydroxyzincate, inhibiting further corrosion. The passivating process results in an homogenous zinc depletion of about 10 ~tm. A more protective film is produced from pure zinc than from an iron-zinc alloy. It is recommended

Alternative reinforcement for concrete

that an external pure zinc layer of at least 10 ktm and a total galvanized layer of at least 80-85 l.tm are needed to provide suitable protection when embedded in concrete. In concrete made from a cement with exceptionally high soluble alkali, film formation could be inhibited during the setting period and corrosion of the zinc in the hardened concrete will depend on the environment (humidity, chloride penetration). During the formation of the passive layer, hydrogen is evolved. Although the evolution of hydrogen raises the spectre of embrittlement, the reinforcing bars normally used in construction are not susceptible to hydrogen embrittlement. Similarly, the hydrogen evolved during the pickling process (pickling is part of the preparation of the surface prior to the application of the zinc, using a weak acid) before galvanizing does not cause a problem. Galvanizing is not generally recommended for steels with a tensile strength above 700800 N/mm 2 , i.e. not for prestressing steels here the risk of hydrogen embrittlement is more severe than for unstressed reinforcement. Several reports (Andrade et al., 1995) compare the reduction in bond strength of galvanized and uncoated steels, both plain and deformed. Reduction in bond is attributed to the formation of hydrogen bubbles at the interface between the bar and the concrete. It has been suggested (Andrade et al., 1995) that this can be overcome by adding chromate to the concrete mix or giving the bars a chromate passivation treatment. On the other hand, the zincates p r o d u c e d - which are less expansive and more soluble than iron corrosion products in the cement environment - could diffuse into the pores of the concrete and make the concrete more dense locally, increasing the bond strength above what would be expected for uncoated bar. In practical terms, most construction is carded out with deformed bar and it is probable that the evolution of hydrogen will not affect the bond strength of galvanized deformed steel reinforcement. However, the use of a passivation agent is still debated. The most effective is a chromate but its use as a concrete admixture raises a number of serious environmental and health questions on-site and would certainly be rejected by cement manufacturers and contractors. It would be more appropriate to use chromated bars as, in the first instance, it would restrict the amount of chromate used and ensure it was where it was needed. It would furthermore provide additional corrosion protection before use and ensure that poor storage would not lead to white rust on the reinforcing bar. Zinc coatings remain passive in carbonated concrete and the rate of corrosion is much lower than for uncoated steel. This makes galvanized steel reinforcement ideal for use in concrete which is at risk from carbonation. As regards corrosion resistance in chloride-contaminated concrete, the distinction has to be made between cast-in chloride and that which penetrates from the outside. Cast-in chloride may attack the zinc coating before and during the formation of the passive calcium hydroxyzincate whereas chlorides penetrating from the outside will find the passive layer already formed and so may be less dangerous. Though zinc can be depassivated and attacked in the presence of chloride ions, the tolerance of galvanized steel to chloride is higher than that of uncoated steel. Galvanizing protects the steel against chloride ingress because it is more tolerant to chloride, requiting a higher concentration for depassivation and it corrodes more slowly in chloride-contaminated conditions.

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Alternative reinforcement for concrete

26.2.2 Fusion-bonded epoxy-coated steel reinforcement Fusion-bonded epoxy-coated reinforcement (FBECR) has been developed over the past 25 years to combat the huge economic cost of deterioration of reinforced concrete structures caused by reinforcement corrosion. The reputation of the first FBECR was doubtful, with many failures reported in the USA. The critical processes in the manufacture of FBECR are now better understood. There are ASTM (ASTM A775M-01 Standard specification for epoxy-coated reinforcing steel bars) specifications for FBECR. It has been recognized that, with good manufacturing techniques, the quality and performance of the FBECR can be improved significantly. It is also recommended that no cracking should be allowed on any part of the coating when a bar is subjected to the specified bend test (ASTM A755). The chemical resistance of epoxy coating to alkalis is very good, with powder coatings being slightly better than liquid-applied coatings (Andrade et al., 1995). Suitably formulated epoxy coatings exhibit the necessary mechanical properties such as good adhesion, formability, impact resistance and abrasion resistance. As most bars are coated in straight lengths, they must be capable of being bent without rupture of the epoxy coating. To do this, the epoxy coating relies on its flexibility and its adhesion to the steel. The flexibility of the coating depends upon its formulation and its thickness: the thinner the layer, the more flexible the coating but if it is too thin its ability to protect against corrosion falls. For good corrosion protection and adequate flexibility, a coating thickness of 180-300 ~tm is recommended (ASTM A775M-01). The abrasion resistance of epoxy coatings is usually good and wear resistance is slightly better with powder coatings than with liquid-applied coatings (Andrade et al., 1995). The adhesion of epoxy powder coatings to steel is in most cases good but pre-treatment of the steel is important. The best adhesion is obtained from steel that has been blast cleaned. Some patterns are more difficult to clean than others. The adhesion of an epoxy coating to an uncleaned or inadequately cleaned bar is poorer than a clean bar as the epoxy coating adheres to the contaminants rather than the steel and produces poor protection. One important structural requirement that may be affected by the presence of the epoxy coating is the bond strength between the steel and the concrete, normally determined by pull-out tests. Epoxy coatings can cause a certain degree of slippage between the coated bar and the embedding concrete. An acceptable level of bond strength is generally considered to be 80 per cent (Andrade et al., 1995) of that of an equivalent uncoated bar. Reduction of bond strength is slightly less important for deformed bar as the deformations transmit the load from the bar to the concrete. However, the geometry of the deformation can be very important and affect the pull-out strength of epoxy-coated bar (Swamy, 1988). The epoxy coatings used today to protect reinforcing steel contain no corrosioninhibiting pigments but act solely as a barrier against the environment. No epoxy coating is completely impermeable to oxygen and moisture, but diffusion can be reduced if the coating is as dense (free from pores) as possible. If aggressive substances can reach the exposed steel because of damage to the coating, corrosion will be concentrated at these points and the cross-section will be reduced at these points. To control the spread of corrosion beneath the film, some manufacturers of epoxy-coated bars now pre-treat the bars before the epoxy coating is applied.

Alternative reinforcementfor concrete 26.2.3 Stainless steel r e i n f o r c e m e n t ~.~:~:: :~.:~::.~ :~:.~.~:~

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General Stainless steels are iron-based alloys containing at least 10.5 per cent chromium whose corrosion resistance increases with alloying metal content. The alloys are commonly identified by their micro structure: martensitic, ferritic, austenitic and duplex (with a microstructure of ferrite and austenite). Although these terms relate to the microstructure of the alloys, they also define the ranges of chemical composition as the microstructures of these alloys tend only to be stable in specific composition ranges. Interest in the use of stainless steels as concrete reinforcement is attributable to their increased resistance to corrosion particularly in chloride-containing media. Of the wide range of possible alloys, only a few have been investigated (Concrete Society, 1998) for their suitability as reinforcement for concrete.

Ferritic stainless steels Tests have shown (Concrete Society, 1998) that the threshold chloride content of concrete for corrosion of ferritic stainless steels is higher than for plain carbon steel. However, the susceptibility of ferritic stainless steels to corrosion with increasing chloride content suggests that, unless the level of chloride contamination can be contained below the threshold level (which has yet to be defined but is likely to be below 1.9 per cent chloride ion content with respect to the cement content), the use of ferritic stainless steels as reinforcement should be limited. At present there are no standards for ferritic stainless steel reinforcing steel.

Austenitic stainless steels Long-term studies of austenitic stainless steels have shown (Concrete Society, 1998) that they are resistant to corrosion in chloride-contaminated concrete. The critical chloride content has not been determined but is likely to be above 3.2 per cent chloride ion with respect to the cement content. Work carried out on welded stainless steels has shown (Concrete Society, 1998) that it is essential to remove all oxide and scale produced during welding. If oxide remains on the surface, the steel is at risk from corrosion in chloride-contaminated concrete. Any blueing created by grinding austenitic stainless steel would create a similar corrosion hazard. To ensure the best possible performance, the surface condition of austenitic stainless steel is therefore important: certainly it should be oxide-free. At present there are only Standards for types 304 and 316 stainless steel reinforcing bars. Table 26.1 shows the chemical compositions, which are specified in BS 6744: Austenitic stainless steel bars for the reinforcement of concrete. The use of the 316 grade austenitic steel is recommended in chloride-contaminated environments, as the molybdenum provides better corrosion resistance. (Since the publication of BS 6744 in 1986 there have been many developments which now mean that updating is required to cover the latest chemical and physical properties of the stainless steels available. A BSI committee is currently reviewing this standard and it is proposed that the BS 6744 classification system should no longer be used. It will be replaced by the system used in BS EN 10088-1: Stainless steels. Part 1. List of stainless steels. Thus the commonly used names of 304 and 316 will be replaced.)

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Alternative reinforcement for concrete Table 26.1 Chemical composition of austenitic steels complying with BS 6744 Grade

C

Si

Mn

P

S

Cr

Mo

Ni

304S31

0.07

1.0

2.0

0.045

0.030

17.0-19.0

-

8.0-11.0

316S33

0.07 max

1.0 max

2.0 max

0.045 max

0.030 max

16.5-18.5

2.5-3.0

11.0-14.0

Duplex stainless steels Laboratory studies (Concrete Society, 1998) have shown that duplex stainless steels can be more corrosion-resistant than austenitic type stainless steels. There are currently no Standards for duplex stainless steel reinforcing bars.

Reinforcing bar clad in stainless steel Provided the cut ends are well protected, reinforcing bar clad in austenitic stainless steel performs as well as solid austenitic stainless steel. When the cost of manufacture of such steel became too high, production ceased. However, new manufacturing techniques have been developed and clad bars are now available again. There are currently no Standards for reinforcing bars clad with stainless steel.

26.3.1 Background .....................................

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FRP materials have been used for many years in the aerospace and automotive industries, where their high strength and low weight have shown distinct advantages over traditional materials such as aluminium and steel. They are slowly being adopted by the construction industry either as construction materials in their own right or for use in conjunction with traditional materials such as concrete. This section aims to give a brief introduction to FRP materials and to give indicative values for the properties appropriate to those materials likely to be used in connection with concrete bridges.

26.3.2 Materials Currently, the most suitable fibres are glass, carbon or aramid. Each is a family of fibre types and not a particular one. Typical values for the properties of the materials are given in Table 26.2. The fibres all have a linear elastic response up to ultimate load, with no significant yielding. These values should only be taken as indicative; actual values should be obtained from the particular manufacturer. The fibres are used in various forms, either in the form of ropes or fabric materials or, more generally, combined with a suitable resin to form a composite as described later. In each case the strength and stiffness will be lower than the anticipated value from Table 26.2, for a composite they will be in the region of 65 per cent. The common forms in which fibres are used may be summarized as follows: • for embedded reinforcement: composite rods or grids

Alternative reinforcement for concrete Table 26.2 Typical fibre properties

Fibre

Tensile strength (N/mm 2)

Modulus of elasticity (kN/mm 2)

Elongation (%)

Specific density

Carbon - high strength Carbon - high modulus

3430-4900

230-240

1.5-2.1

1.8

2940--4600

390--640

0.45-1.2

1.8-2.1

Aramid - high strength and high modulus

3200-3600

124-130

2.4

1.44

Glass

3500

75

4.7

2.6

• for prestressing: fibre ropes or composite rods • for external repair and strengthening: sheet material consisting of pure fibres or fibres pre-impregnated with part-cured resins (known as prepreg materials), composite plate material or preformed composite shells • for permanent formwork: preformed composite shells The main advantages of fibre composites are that they are lighter and stronger than steel and, with the correct resin and fibre combination, should prove to be more durable. However, there are possible disadvantages which include the lack of any yield at the ultimate load. This will be considered further in the section on design. There is a wide choice of resins available, many of which, though not all, are suitable for forming composites. (The Draft Canadian Highways Bridge Design Code (Bakht, et al., 1996) specifically prohibits the use of polyester resins for embedded FRP material.) The choice will depend on the required durability, the manufacturing process and the cost. Thermosetting resins are generally used but they have the major disadvantage that, once they have fully cured, the composites cannot be bent to form hooks, bends and similar shapes. One possible alternative is the use of suitable thermoplastic resins, which are now being developed. With these resins the composite components can be warmed and bent into the required shapes. On cooling the full properties of the resins are restored. However, there is likely to be distortion of the fibres in the region of the bend, which will lead to a reduction of the strength locally.

26.3.3 Manufacturing processes The most widely used manufacturing process for forming composite rods is pultrusion. The fibres, which are supplied in the form of continuous rovings, are drawn off in a carefully controlled pattern through a resin bath which impregnates the fibre bundle. They are then pulled through a die which consolidates the fibre-resin combination and forms the required shape. The die is heated which sets and cures the resin allowing the completed composite to be drawn off by suitable reciprocating clamps or a tension device. The process enables a high proportion of fibres to be incorporated into the crosssection and hence relatively high strength and stiffness are achieved. However, the sections have a smooth surface, which provides insufficient bond if they are to be used as

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reinforcement in concrete. Hence, a secondary process, such as overwinding with additional fibres, is required to improve the bond. There are variations to the process, such as the use of a braided rope, which is then impregnated with resin. The resulting profile provides a good bond with the concrete. As indicated above, thermosetting resins are generally used at present. Once formed these cannot be bent into the range of shapes currently used by the concrete industry, and hence different manufacturing processes are required to form specials. Filament winding, in which resin-impregnated fibres are wound round a mandrel of the required shape, has been used to manufacture shear links. Other manufacturing processes, such as filament arranging, are being developed for more complex shapes. Fibre composite two-dimensional reinforcement grids, and even three-dimensional grids, are made by a number of different patented processes.

26.3.4 Short-term properties The physical properties of a composite will depend on the type and percentage of fibres used. Typically a pultruded composite would have about 65 per cent of fibre by volume. Thus with glass the ultimate strength might be 1200 N/mm 2 rising to 2000 N/mm 2 for carbon. The elastic modulus will be about 40 kN/mm 2 for glass fibre composites and may be 150 kN/mm 2 for carbon fibre composites. As composites are not currently manufactured to a common standard, their properties will vary from one manufacturer to another. Thus all design must be on the basis of the actual properties, as supplied by the appropriate manufacturer.

26.3.5 Long-term properties Creep rupture, or stress rupture as it is often known, is the process by which a material with a permanent high load applied to it will creep to failure. This will be particularly important for prestressed structures and for reinforced structures with a high permanent load. There is a reasonable amount of data to confirm the performance for a few years under stress but there is still a degree of uncertainty about the exact form of the long-term response. Thus for design purposes, relatively large factors of safety are currently proposed (The Institution of Structural Engineers, 1999) to allow for the uncertainty, which may be revised as more test data become available. The phenomenon of creep rupture does not appear to have any effect on the short-term strength. Thus an FRP element that has been loaded to a significant level for a period of time will retain its initial short-term strength. Similarly the stiffness of FRP is largely unaffected by permanent load (Institution of Structural Engineers, 1999). The effect of corrosive actions on the fibre is complicated and varies from fibre to fibre. Some corrosive elements attack the internal bonds, breaking the long-chain molecules into short lengths, reducing the strength but not the stiffness. Other factors attack the fibre from the outside, physically removing some of the fibre, which will change both the strength and the effective stiffness. The effect of the resin is to shield the fibres from chemical attack. The durability of the resin itself, and its permeability to aggressive substances, will affect the durability of the FRP bar. Because of the lack of research data,

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relatively large safety factors have to be applied to the measured short-term properties. Future testing will allow these factors to be modified and will indicate the most suitable combinations of resins and fibres to resist attack.

26.3.6 Health and safety Loose fibres on the surface of the FRP bars may cause irritation and a few people may suffer an allergic reaction to the resin matrix. Hence, during assembly of the reinforcement cage sensible precautions should be taken, such as wearing gloves. At all times, current Health and Safety Regulations, as indicated by the manufacturer of the FRP material, should be followed. When cutting the material, suitable dust extraction should be used. Once the FRP material is embedded into the concrete there will be no problems as far as health and safety are concerned. Where adhesives are used, it should always be used in accordance with the manufacturer's recommendations.

26.4.1 Review of materials and manufacturing processes A number of manufacturers make FRP reinforcement for concrete, some of which are given in Table 26.3. Carbon and glass are the most common fibres, though aramid is also used. Rods are made by pultrusion, as described in section 26.3.3, with a secondary process to form a surface with adequate bond properties. This may consist of removing the smooth outer layer of resin, overwinding with additional fibres or adding a sand layer to form a mechanical key. Table 26.3 Details of some available types of FRP reinforcement Manufacturer

Country

Trade name

Fibre type

Comments

Fibreforce

UK

Eurocrete bar

Glass or carbon

Hughes Brothers

USA

Aslan

Glass

International Grating

USA

Kodiak

Glass

Mitsubishi

Japan

Leadline

Carbon

Prestressing material

Nefcom

Japan

Nefmac

Carbon, glass or hybrid

Grid material

Tokyo Rope

Japan

CFCC

Carbon

Prestressing material

The Japanese NEFMAC is a grid material made by a special process which forms successive layers of composite material in the two directions, which are then compressed together (Sugita, 1993). The above are all with thermosetting resins, and hence cannot be bent once the resin has fully cured. Some attempts have been made to bend partly cured elements, though this is not generally satisfactory as it leads to displacement of the fibres around the bend. Thus various techniques are being considered for forming shapes such as shear links and

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Alternative reinforcement for concrete

hooks. These include filament winding, to form a cylinder or box. These can be cut to form appropriate closed shapes. An alternative approach is filament placing, in which the resin-impregnated fibres are wound round pins to give the required shape.

26.4.2 Advantages and disadvantages Advantages The main advantage of FRP reinforcement should be improved durability, provided an appropriate combination of fibre and resin has been used. Thus FRP reinforcement would be most beneficial in highly corrosive environments, such as bridges in marine environments or those subjected to de-icing salts. The improved durability should lead to the possibility of a lower specification for the concrete and lower covers. The rods have high strength, probably twice that of normal high-yield steel, leading in some situations to lower reinforcement percentages. FRP would be particularly beneficial for precast members in which the reinforcement is required mainly for handling, transport and erection. Here the long-term behaviour of the material would not be an issue; unlike steel, it would not cause any damage to the concrete if it were to degrade.

Disadvantages One of the disadvantages of FRP reinforcement is its relatively low stiffness compared to steel; for glass it may be 25 per cent, for carbon 75 per cent or more. This will result in increased deflections and crack widths. In addition, the bond stress may be lower, again increasing crack widths. However, the latter will only be important from the point of view of aesthetics and not durability. A major practical disadvantage at present is the difficulty of forming shear links, hooks etc. All such items will have to be factory made as the current bars cannot be bent on site. This may change with the introduction of thermoset resins, which can be bent once warmed, though the properties at the bend may be seriously affected.

26.4.3 Summary of research ....................

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Brief overview of research Throughout the world there are programmes of work developing the use of FRP in concrete structures. In Japan, North America and in Europe, considerable effort is being put into the development of embedded FRP reinforcement for concrete. The main areas being investigated around the world are: • selection of suitable resins and fibres • development of appropriate manufacturing techniques • investigations to determine the durability of FRP rods exposed to aggressive environments either directly or embedded in concrete • determination of the structural behaviour through testing and analytical techniques • economic and feasibility studies • development of case studies of trial structures and components • development of suitable design guidance

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Tests on reinforced concrete members Much of the early work was concerned with the bond between FRP reinforcement and concrete. Over the last 10 years or so there have been many programmes of tests on simple beams, considering both the flexural and the shear behaviour. Limited work has been carried out on frames, columns and slabs. The work has been reported in a number of major international conferences (Nanni and Dolan, 1993; Taerwe, 1995; E1-Badry, 1996; Japan Concrete Institute, 1997; Burgoyne, 2001).

Durability One very important aspect of the use of FRP rods embedded in concrete is their durability. Surprisingly, this was not considered to any great extent in any of the early research programmes worldwide. It is important to note that carbons and aramids are inherently more durable in an alkaline environment than the standard E-glass. AR-glass (Alkali Resistant glass) has been used for some time for GRC (glass reinforced cement) and is used in some composite rods. However, many resins degrade in the highly alkaline concrete environment. The Draft Canadian Highways Bridge Design Code specifically prohibits the use of polyester resins for embedded FRP material (Canadian Standards Association, 1996). Thus the manufacturers' claims of long life need to be demonstrated. Currently, trials are being carried out on the resins and fibres in isolation and in the form of the composite, both in a range of artificial aggressive environments and embedded in concrete, with a view to developing the necessary confidence in the long-term properties of the materials. The accelerated laboratory testing is being backed up by data from specimens on exposure sites. An important aspect in the development of new materials is the construction of demonstration structures, as outlined below. While they may not be economic, because the materials themselves are not yet fully understood and the design approaches are not fully developed, they give valuable experience of practical construction aspects and an indication of the long-term performance. They thus develop the necessary confidence in the new materials.

26.4.4 Design guidance Modified design rules for use with FRP reinforcement have been developed for BS 8110: Structural use of concrete, Part 1, Code of practice for design and construction (1997): Part 2, Code of practice for special circumstances (1985) for buildings and BS 5400: Steel, concrete and composite bridges, Part 4, Code of practice for design of concrete bridges (1990) for bridges (Institution of Structural Engineers, 1999). The proposals are broadly in line with the codes being developed in Japan (Japanese Ministry of Construction, 1995; Japan Society of Civil Engineers, 1997) and in North America (Canadian Standards Association, 1996; Bakht et al., 1996; ACI, 2001). For detailed design guidance, reference should be made to the Institution of Structural Engineers document. The following significant differences when designing with FRP reinforcement are summarized as follows: Because FRP materials have a straight-line response to ultimate, with no yielding, it is appropriate to use only elastic methods of analysis. For design purposes it should be assumed that no redistribution of the elastic bending moments and shear forces will take place.

• Analysis

26/14 Alternative reinforcement for concrete •

•

•

•

•

•

•

• •

•

7 f a c t o r s The effective strength and the effective stiffness of embedded reinforcement may change with time, due to alkali attack, depending on the types of resin and fibre used in the composite. Appropriate factors of safety applied to the short-term values will be required to take account of the changes. Durability The quality of the concrete will be governed mainly by strength considerations and the cover by the aggregate size and the size of the reinforcing bar. Design crack widths will be controlled by aesthetic considerations and, possibly, watertightness of the structure. F l e x u r e The basic principles are unchanged. The design equations and design charts given in the Codes are not appropriate as they assume yielding of the reinforcement at ultimate. Because of the relatively low stiffness of FRP, it is likely that failure will occur by compression of the concrete and not by reaching the ultimate capacity of the tensile reinforcement. Shear The shear capacity of the concrete cross-section should be calculated on the basis of an equivalent area of steel, transformed on the basis of the modular ratio. The strain in FRP shear reinforcement should be limited. Serviceability Because of the lower stiffness, deflections and crack widths may become dominant design criteria. However, as indicated above, with no limitation on crack width required from the point of view of durability, aesthetics will be the only criterion. Thus the current rules could be relaxed considerably. Columns The strength of bars in compression should be ignored and the column designed on the basis of the concrete area alone. (There would appear to be considerable scope for using hoop reinforcement in columns, providing containment to the concrete and hence increasing its load-carrying capacity.) Bond Because there are no agreed Standards, it will be necessary to determine the ultimate bond stress for the particular material and then use this in design with an appropriate partial safety factor. Fire FRP reinforcement is generally not recommended for structures for which fire is a significant design consideration. D e t a i l i n g Reinforcement cages should be assembled with non-metallic ties. Lapping of reinforcement should be satisfactory, but the relatively low bond strength may lead to uneconomic overlaps. Once formed, FRP reinforcement cannot be bent to form shear links etc. Thus the designer will be required to work not only to standard shape codes but also to fixed dimensions. Construction There should be no significant problems during the assembly of the reinforcement cage. Any cut ends of bars, or damaged areas, should be sealed with a suitable resin. During the casting of the concrete the reinforcement will have a tendency to float, because of its low density, and allowance must be made for this during fixing.

Care must be taken with the storage of FRP material prior to use, to avoid damage to the resin matrix. In particular, reinforcement should be protected from the effects of prolonged exposure to UV light, which can lead to the degradation of some resins.

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26.4.5 Applications Introduction This section concentrates on the use of FRP reinforcement materials in bridges but also considers a number of other applications, chiefly marine and coastal, that are relevant because chloride ingress is the prime cause of corrosion of the embedded steel.

Bridges The first footbridge in Britain, and probably in Europe, using glass FRP reinforcement was built at Chalgrove in Oxfordshire in 1995 (Clarke et al., 1998). The bridge is a simple slab, 1.5 m wide and 5 m long reinforced on both faces with glass FRP bars. The structure was load tested and then monitored for about a year. Shortly after, a second footbridge was built in Oxfordshire using the concept of Supercover (Arya, 1996). This is a conventionally reinforced structure with additional cover to protect the bottom steel. To control cracking, the additional cover is reinforced with a layer of glass FRP bars. In Norway, the Oppegard footbridge was built in 1997 on a golf course near Oslo (Grostad et al., 1997; Haugerud and Mathisen, 1997). It has a span of about 10 m and consists of twin-arched beams, reinforced with glass FRP bars and stirrups made from a glass FRP with a thermoplastic matrix, which could be bent once warmed. The bridge had horizontal prestressed ties containing aramid tendons. In Denmark, carbon FRP was used for the unstressed reinforcement and shear links for the 90 m long concrete footbridge at Herning in Denmark (Christoffersen et al., 1999). The same FRP material was used for the longitudinal and transverse prestressing cables and for the cable stays. In the USA, glass FRP bars were used in a bridge in Arkansas in 1994, though the actual application is unclear. The McKinleyville Bridge in West Virginia is a 54 m long, three-span continuous structure which was completed in 1996 (Thippeswamy et a/.,1998). It carries two lanes of traffic. The 230 mm thick concrete deck spans between the main steel girders which are 1.5 m apart. Two types of glass FRP reinforcing bars were used. The structure was load tested on completion in 1996. In Canada the five-span Taylor Bridge in Headingly, Manitoba, has a total length of 165 m. A number of the 1830 mm deep precast beams were reinforced with carbon FRP both for the shear links and also for longitudinal reinforcement (Rizkalla et al., 1998). The links projected from the tops of the beams to provide longitudinal shear reinforcement. Two different types of carbon FRP material were used. The beams were prestressed using the same materials. Carbon FRP was also used for the reinforcement of part of the deck slab and glass FRP rods were used to reinforce the safety barrier. A large number of sensors were built into the structure and are being monitored remotely. The bridge was opened in October 1997. In North America, and particularly Canada, there is a growing interest in the use of steel-free bridge decks, though this would appear to be generally for steel-concrete composite bridges. The concrete deck spanning between the main girders is designed using the principle of compressive membrane action (Cole, 1998). Hence, tension reinforcement is not required but some nominal anti-crack reinforcement is provided. This may be short, chopped fibres or else FRP bars, as were used for the Chatham Bridge in Ontario (Hearn, 1998) or FRP grid material, as was used in the Joffre Bridge in Sherbrooke, Canada (Benmokrane, 1997).

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FRP grid material has been proposed for bridge parapets in Ontario (Maheu, 1994).

Other applications Apart from bridges, fibre composite reinforcement has been used in a number of different marine and coastal applications. Descriptions are given in the proceedings of various conferences (Nanni and Dolan, 1993; Taerwe, 1995; E1-Badry, 1996; Japan Concrete Institute, 1997; Burgoyne, 2001). A significant marine application was a replacement fender support beam for a jetty in the Middle East (Grostad et al., 1997) using glass FRP bars and thermoplastic links. Glass FRP bars were recently used in the sprayed concrete repair of a sea wall in West Palm Beach, Florida, as nominal anti-crack reinforcement. In the USA, Japan and France glass fibre composite bars have been used in a number of applications in which stray electrical currents in steel reinforcement would be a problem. To date these have been mainly under sensitive electronic equipment, such as in hospitals or military installations. As indicated above, FRP reinforcement and prestressing tendons have been used for the support beams for experimental maglev systems in Japan. A number of other applications are being actively considered.

26.5.1 Review of materials and manufacturing processes A number of different tendon systems have been developed, using carbon, glass or aramid fibres, as shown in Table 26.4. This is not an exhaustive list as there are many other Japanese systems which have not yet been used elsewhere. Table 26.4 Details of some available types of prestressing systems Trade name (manufacturer)

Country of origin/use

Fibre type

Arapree (Nippon Aramid plus Italian Company)

Netherlands, Italy, Japan, Portugal

Aramid

CFCC (Tokyo Rope)

Japan, USA, Canada, Germany

Carbon

Fibra (Mitsui/Shinko Wire)

Japan

Braided aramid

Leadline (Mitsubishi)

Japan, USA

Carbon

Parafil (Linear Composites)

UK, Norway

Aramid rope

Technora (Teijin)

Japan

Aramid

It should be noted that one system, Parafil, consists of parallel Kevlar fibres without any resin. Thus it is not strictly speaking a composite but for the purposes of this report it has still been classed as an FRP. The remaining materials are all produced by variations of the pultrusion process, as described earlier. Their properties will depend on the particular amount and fibre used, but will be in line with those given in section 26.3.4. Standard barrel and wedge type anchors as used for steel strands are not appropriate for fibre composites as they cut into the surface layer, damaging the material and leading

Alternative reinforcement for concrete 26/17

to premature failure. Hence manufacturers have had to develop special anchorage systems for composite tendons. These are generally stainless steel barrels into which the tendons are bonded using specially formulated adhesive grouts. It is likely that the anchorages will have to be installed on the ends of the tendons under factory conditions, to ensure proper workmanship and a fully cured resin. The anchors can then be proof tested, to, say, 10 per cent above design ultimate load, before the tendon is installed in the structure. The design of the end-block, and of any parts of the structure through which the tendon has to pass, must be such as to allow the passage of the anchorage and not just of the tendon. In addition, some manufacturers have developed wedge systems which grip the individual strands without cutting into them. These can obviously be fitted to the ends of the strands once installed in the structure, as with steel strands. As indicated above, Parafil is the only system in which the fibres are used in isolation, rather than in the form of a composite. Anchorage is by means of a wedging system, using a tapered barrel and an internal spike drawn into the middle of the rope (Burgoyne, 1993).

26.5.2 Advantages and disadvantages

Advantages The major advantage of FRP prestressing tendons is their improved durability. They do not require the traditional protection provided by grout to bonded steel tendons; in fact, bonding is difficult for some of the systems because of the smooth surface texture. Thus they are ideally suited for use as unbonded tendons, either internal or external. Their light weight will make installation easier than with equivalent steel tendons.

Disadvantages From the point of view of construction, one disadvantage will be that the end anchorages of FRP tendons will generally have to be installed in the factory, making installation more difficult than with simple steel strand. Appropriate openings will have to be formed throughout to allow the complete anchor to pass through. In addition, their low elastic modulus will require large extensions during the stressing process. As with all external tendons, appropriate protection against accidental damage must be provided. In addition, the possibility of vandalism must be considered with FRP tendons as some can be easily cut. In some situations, fire might be an additional hazard. Where necessary, protection against ultraviolet light should be provided; advice should be sought from the manufacturer.

26.5.3 Summary of research A large amount of development work has been carried out around the world on FRP prestressing tendons (Burgoyne, 1993; Wolff and Meisseler, 1993; Gerritse, 1993), looking at the development of appropriate anchorage systems as well as the behaviour of prestressed concrete beams. A number of studies have looked at the behaviour of beams in flexure (e.g. Yonekura et al., 1996). Because the tendons are generally unbonded and because the elastic modulus

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of FRP will be relatively low, the ultimate strength is dictated by the concrete failing in compression. Because of the lack of any yield, it has been suggested that local high strains in a bonded tendon at a crack might lead to sudden, brittle failure. Hence work has been carried out at Cambridge University (Burgoyne, 1997) on partially bonded tendons. This leads to a more ductile behaviour. Little experimental work has been carried out on the shear behaviour of beams with FRP tendons, though Yonekura et al., (1993) tested a number of I-beams prestressed with either carbon or aramid tendons. Limited studies have looked at the thermal behaviour of pretensioned elements. The coefficient of thermal expansion along an FRP bar is controlled by the fibres and will be significantly lower than that of concrete. Transversely, it will be due largely to the resin and will be significantly higher. Cracking of some elements prestressed with an aramid FRP has been attributed to this high transverse expansion (Gerritse, 1993).

26.5.4 Design guidance In principle there should be few changes required when designing with fibre composite tendons rather than steel ones. One significant factor that has to be taken into account, however, is the phenomenon of stress rupture, that is, the fact that a tendon stressed to a certain level will creep to failure. This will limit the applied stress in FRP tendons. The draft Canadian Bridge Design Code (Bakht et al., 1996) gives the following maximum permissible jacking stresses as percentages of their ultimate strengths: Carbon fibre tendons Glass fibre tendons Aramid fibre tendons

65 55 40

Depending on the type of tendon being used (see Table 26.3), the member may be pretensioned or post-tensioned. Many of the tendons are smooth, making grouting difficult if not impossible. If grouting is used it should be resin based rather than cement based. The member may be designed in accordance with the current rules for bonded tendons (using BS 5400 Part 4) or unbonded tendons (using BD 58/94: Highways Agency, 1994) as appropriate. When designing with bonded tendons, and clauses that assume that the tendon has yielded should be ignored as they are not applicable to FRP which is elastic to failure. In determining losses, creep will be controlled by the fibres in the tendons and not the resins. The relaxation in the tendons will be offset by the lower elastic modulus leading to losses that are similar to those for steel. In addition, with grouted anchorages as described above, there is likely to be some loss due to creep in the resin grout. Appropriate information will have to be obtained from the manufacturer. The clauses in BS 5400 for the design of end blocks are intended for bonded prestressing tendons. When using unbonded FRP tendons it will be appropriate to use the slightly more detailed clauses in BS 8110, which cover both bonded and unbonded tendons.

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26.5.5 Applications Prestressing tendons have probably been the widest use of fibre composites in concrete structures to date, though this is likely to change rapidly. A number of bridges have been built worldwide, generally with conventional steel for the unstressed reinforcement. In Germany and Austria a total of five road bridges and footbridges have been built prestressed with glass-fibre composite tendons, using the Polystal system. The first highway bridge, the Ulenbergstrasse Bridge in Dusseldorf, which has two spans each of about 20 m, was opened to traffic in 1986 (Wolff and Meisseler, 1993). The bridge has been monitored and load tested periodically. However, it is not clear whether the system is still commercially available. Also in Germany, carbon fibre tendons have been used for one bridge (Zoch et al., 1991). In Spain, aramid FRP tendons were used for a cantilevered roadway in Spain (Casas and Aparicio, 1992). As mentioned earlier, carbon FRP tendons were used for the 90 m long cable-stayed concrete footbridge at Heming in Denmark (Christoffersen et al., 1999). Carbon FRP was also to be used for the unstressed reinforcement and for the stays. Carbon fibre tendons are proposed for the three-span Dintelhaven Bridge near Rotterdam (Hordijk, 1998) which will have a total length of about 370 m. Short lengths of the proposed cables will undergo long-term trials before the bridge is constructed. In Japan the emphasis of development has been on carbon or aramid composites. At least 10 bridges have been built to date (Noritake et al., 1993; Tsuji et al., 1993). These take a variety of forms. There are a number of road bridges, ranging from 7 m single-span up to four-span with a total length of about 80 m. A number of foot and cycle bridges have been built, the longest having a clear span of about 75 m. In addition, there have been a number of unconventional footbridges built; there is a 55 m long floating structure and a stressed ribbon bridge with a clear span of about 45 m. A further application has been for maglev structures. In North America part of one bridge in South Dakota has been stressed with glass and carbon tendons (Iyer, 1993) and a bridge in Calgary contains carbon fibre composite strands (Anon, 1993). A number of other bridges are currently being planned. In Canada the five-span Taylor Bridge in Headingly, Manitoba, has a total length of 165 m. A number of the 1830 mm deep precast beams were prestressed with carbon FRP tendons (Rizkalla et al., 1998). Two different types of carbon FRP material were used. Carbon FRP was also used for the reinforcement of the beams and for part of the deck, see section 26.3.5. A large number of sensors were built into the structure and are being monitored remotely. The bridge was opened in October 1997. To date no concrete bridge has been built in the UK with FRP tendons, though they have been used for a prototype stressed masonry footbridge.

This chapter has described alternative forms of reinforcing bars and prestressing tendons for concrete, concentrating on fibre composite materials. It has demonstrated that FRP materials are at an advanced stage of development and have been used in a variety of applications around the world.

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ACI (2001) AC1440. IR Guide to the design and construction of concrete reinforced with FRP bars. American Concrete Institute, Farmington Hills, USA. Andrade, C. et al. (1995) Comit6 Euro-International du Beton, State of the Art Report, Coating Protection for Reinforcement, Thomas Telford, London. Anon (1993) Carbon-fibre strands prestress Calgary span. Engineering News Record, 18 October, 21. Arya, C. (1996) Supercover concrete. FRP International, IV, Issue 3, 4. Bakht, B. et al. (1996) Design provisions for fibre reinforced structures in the Canadian highway bridge design code. In E1-Badry, M.M. (ed.), Advanced Composite Materials in Bridges and Structures, Canadian Society for Civil Engineering, Montreal, pp. 391-406. Benmokrane, B. (1997) NEFMAC for a highway bridge in Canada. FRP International, V, Issue 4, 5. Burgoyne, C.J. (1993) Parafil ropes for prestressing tendons. In Clarke, J.L. (ed.), Alternative Materials for the Reinforcement and Prestressing of Concrete, Blackie Academic & Professional, Glasgow, pp. 102-126. Burgoyne, C.J. (1997) Rational use of advanced composites in concrete. Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Japan Concrete Institute, Vol. 1, pp. 75-88. Burgoyne, C.J. (2001) Fibre-reinforced Plastics for Reinforced Concrete Structures, Thomas Telford, London, (two volumes). Canadian Standards Association, (1996) Canadian Highways Bridge Design Code, Section 16, Fibre Reinforced Structures. Casas, J.R. and Aparicio, A.C. (1992) A full scale experiment on a prestressed concrete structure with high strength fibres; the North ring-road in Barcelona. In Proceedings of FIP-XIInternational Congress, Hamburg, T15. Christoffersen, J., Hauge, L. and Bjerrum, J. (1999) Footbridge with carbon-fibre-reinforced polymers, Denmark. Structural Engineering International, 9, No. 4, November, 254-256. Clarke, J.L., Dill, M.J. and O. Regan, P. (1998) Site testing and monitoring of Fidgett Footbridge. In Virdi, K.S., Garas, F.K., Clarke, J.L. and Armer, G.S.T. (eds), Structural Assessment - The Role of Large and Full-scale Testing, E&FN Spon, London, pp. 29-35. Cole, M. (1998) Arching action. New Civil Engineer, 28 May, 32-33. Concrete Society, (1998) Guidance on the use of stainless steel, Technical Report 51, The Concrete Society, Crowthorne. Gerritse, A. (1993) Aramid-based prestressing tendons. In Clarke, J.L. (ed.) Alternative Materials for the Reinforcement and Prestressing of Concrete, Blackie Academic & Professional, Glasgow, pp. 172-201. Grostad, T., Haugerud, S.A., Mathisen, L.L. and Clarke, J.L. (1997) Case studies within Eurocrete - Fender in Qatar and bridge in Norway. In Non-Metallic (FRP) Reinforcement for Concrete Structures, Japan Concrete Institute, Sapporo, Japan, Vol. 1, pp. 657-664. Haugerud, S.A. and Mathisen, L.L. (1997) The design and development of a novel FRP reinforced bridge. In Proceedings of IABSE Conference on Composite Construction- Conventional and Innovative, Innsbruck, Austria, pp. 765-770. Hearn, N. (1998) Strength without steel, Innovator; the Newsletter of ISIS Canada, ISIS Canada, University of Manitoba, Winnipeg. Highways Agency (1994) Design Manual for Roads and Bridges, Volume 1, Highway Structures: Approval Procedures & General Design: Section 3, General Design; Part 9, Design of bridges and concrete structures with external and unbonded prestressing. Hordijk, D.A. (1998) A concrete balanced cantilever box girder bridge in the Netherlands with carbon fibre prestressing cables. In Stoelhurst, D. and den Boer, G.P.L. (eds), Challenges for Concrete in the Next Millennium. A.A. Balkema, Rotterdam & Brookfield, pp. 29-33.

Alternative reinforcement for concrete 26/21 Institution of Structural Engineers (1999) Interim Guidance on the Design of Reinforced Concrete Structures using Fibre Composite Reinforcement. The Institution of Structural Engineers, London, p. 116. Iyer, S.L. (1993) Advanced composite demonstration bridge deck. In Nanni, A. and Dolan, C.W. (eds), Fibre reinforced plastic reinforcement for concrete structures, SP 138, American Concrete Institute, Detroit, 831. Japan Society of Civil Engineers (1997) Recommendations for Design and Construction of Concrete Structures using Continuous Fiber Reinforcing Materials, Japan Society of Civil Engineers, Tokyo. Concrete Engineering Series 23. Japanese Ministry of Construction, (1995) Guidelines for structural design of FRP reinforced concrete building structures, Japanese Ministry of Construction. Maheu, J. (1994) NEFMAC barrier walls. FRP International, II, Issue 2, 7. Noritake, K. et al. (1993) Practical applications of aramid FRP rods to prestressed concrete structures. In Nanni, A. and Dolan, C.W. (eds), Fibre Reinforced Plastic Reinforcement for Concrete Structures, SP 138, American Concrete Institute, Detroit, pp. 853-873. Rizkalla, S. et al. (1998) The new generation. Concrete International, June, 35-38. Sugita, M. (1993) NEFMAC grid type reinforcement. In Clarke, J.L. (ed.), Alternative Materials for the Reinforcement and Prestressing of Concrete, Blackie Academic & Professional, Glasgow, 55-82. Thippeswamy, H.K., Franco, J.M. and GangaRao, H.V.S. (1998) FRP reinforcement in bridge deck. Concrete International, June, 47-50. Tsuji, Y., Kanda, M. and Tamura, T. (1993) Applications of FRP materials to prestressed concrete bridges and other structures in Japan. PCI Journal, July-August, 50. Wolff, R. and Meisseler, H.J. (1993) Glass fibre prestressing system. In Clarke, J.L. (ed.),Alternative Materials for the Reinforcement and Prestressing of Concrete, Blackie Academic & Professional, Glasgow, pp. 127-152. Yonekura, A. et al. (1993) Flexural and shear behaviour of prestressed concrete beams using FRP rods as prestressing tendons. In Nanni, A. and Dolan, C.W. (eds), Fibre Reinforced Plastic Reinforcement for Concrete Structures, SP 138, American Concrete Institute, Detroit, pp. 525548. Zoch, P. et al. (1991) Carbon fibre composite cables: a new class of prestressing members. In Proceedings of 70th Annual Convention of the Transportation Research Board, Washington, DC.

Clarke, J.L. (ed.) (1993) Alternative Materials for the Reinforcement and Prestressing of Concrete, Blackie Academic & Professional, Glasgow. There have been a number of international conferences in recent years which have reported on the wide range of research and development being carried out around the world and on current applications for FRP reinforcement and prestressing. Some are listed below. Benmokrane, B. and Rahman, H. (eds) (1998) Durability of Fibre Reinforced Polymer (FRP) Compositesfor Construction, Department of Civil Engineering, University of Sherbrooke, Quebec, Canada, 706. E1-Badry, M.M. (ed.) (1996) Advanced Composite Materials in Bridges and Structures, Canadian Society for Civil Engineering, Montreal. Japan Concrete Institute (1997) Non-Metallic (FRP) Reinforcement for Concrete Structures, Tokyo, Japan, pp. 728 and 813 (2 volumes). Nanni, A. and Dolan, C.W. (eds) (1993) Fibre-Reinforced- Plastic Reinforcement for Concrete Structures, American Concrete Institute, Detroit, SP-138, 977. Taerwe, L. (ed.) (1995) Non-Metallic (FRP) Reinforcement for Concrete Structures, E&FN Spon, London, p. 714.