zz applications of frp composites in civil engineering

Skeletal frames constructed from reinforced concrete RC or steel columns and beams were in-filled with non-load-bearing or semi-load-bearing GFRP panels manufactured by the wet lay-up pr

During the introduction of FRP composites into the

building and construction industry in the 1970s glass

fibres were used in a polyester matrix as a construction

material Skeletal frames constructed from reinforced

concrete (RC) or steel columns and beams were

in-filled with non-load-bearing or semi-load-bearing

GFRP panels manufactured by the wet lay-up

pro-cess or by the spray-up technique to form structural

buildings Several problems developed owing to a

lack of understanding of the FRP material, mainly

arising from insufficient knowledge of its in-service

properties relating to durability and the enthusiasm

of architects and fabricators for developing geometric

shapes and finding new outlets for their products

without undertaking a thorough analysis of them

Consequently, to improve certain physical properties

of the FRP some additives were incorporated into

the polymers by the fabricators without a full

under-standing of their effect on the durability of the FRP

material, or indeed were omitted in cases where

additives should have been added

Advanced polymer composites did not enter the

civil engineering construction industry until the middle

to late 1980s; polyester and epoxy polymers were used

initially and vinylester was introduced in the 1990s

From the 1970s, universities, research institutes and

industrial firms have been involved in researching the

in-service, mechanical properties of FRPs and in the

design and testing of structural units manufactured

from fibre/polymer composites This was followed

by the involvement of interested civil engineering

consultants undertaking industrial research and the

utilisation of the structural material in practice The

application of advanced polymer composites, over

the past 35 years for the building industry and the

past 25 years for the civil engineering industry, can

be conveniently divided into some specific areas, which

will be discussed briefly in this chapter:

• Building industry:

• infill panels and new building structures

• Civil engineering industry:

• civil engineering structures, fabricated en­ tirely from advanced polymer composite

ma-terial, known as all-polymer/fibre composite structures

• bridge enclosures and fairings

• bridge decks

• external reinforcement rehabilitation and retrofitting to RC structures (including FRP confining of concrete columns)

• external reinforcement rehabilitation and retrofitting to steel structures

• internal reinforcement to concrete members

• FRP/concrete duplex beam construction

• polymer bridge bearings and vibration absorbers

All these, other than the first, involve a combination

of advanced polymer composites and conventional construction materials and are therefore often termed

composite construction.

FRP composites are durable and lightweight and consequently they can fulfil many of the requirements

of structural materials for many forms of construction Ideally when new civil engineering structures are manufactured from polymer composite systems the component parts should be modular to provide rapid and simple assembly An example of the importance

of this is in the installation of highway infrastructure, where any construction or long maintenance period

of the infrastructure will cause disruption to traffic flow and will be expensive

The examples of the applications of polymer fibre composites in those areas that we will discuss in this chapter have been chosen to illustrate all the areas of use listed above

43.1 The building industry

During the 1970s two sophisticated and prestigious GFRP buildings were developed and erected in the

Applications of FRP

composites in civil

engineering

UK, Mondial House, the GPO Headquarters in

London (Berry, 1974) and the classroom of the

primary school in Thornton Clevelys, Lancashire;

these are discussed below Other FRP buildings that

were erected during this period were Covent Garden

Flower Market (Roach, 1974; Berry, 1974), the

American Express Building in Brighton (Southam,

1978), and Morpeth School, London (Leggatt, 1974,

1978) These structures played a major role in the

development of polymer composite materials for

construction Because of the relatively low modulus

of elasticity of the material, all except one of these

buildings were designed as folded plate systems

and erected as a composite modular system, with

either steel or reinforced concrete units as the main

structural elements and the GFRP composite as the

load-bearing infill panels The exception to this is

the classroom of the primary school, Thornton

Clevelys, Lancashire, UK (Stephenson, 1974), which

is entirely manufactured from GFRP material

43.1.1 MondiAl House, eReCted on tHe

noRtH bAnk oF tHe tHAMes in

london 1974

This building was clad above the upper ground floor

level and the panels were manufactured from glass

fibre polyester resin The outer skin of the panel

included a gel coat that used isophthalic resin,

pig-mented white, with an ultraviolet stabiliser backed

up with a glass fibre reinforced polymer laminate; the

latter used a 3 oz per square foot chopped strand mat

and a self-extinguishing laminating resin reinforced

with 9 oz per square foot glass fibre chopped strand

mat reinforcement Some degree of rigidity was

obtained from a core material of rigid polyurethane

foam bonded to the outer skin and covered on the

back with a further glass-reinforced laminate; this

construction also provided thermal insulation Further

strength and rigidity were obtained by the use of

lightweight top-hat section beams, manufactured as

thin formers and incorporated and over-laminated

into the moulding as manufacture proceeded The

effect of the beams was transferred to the front of

the panel by means of glass-fibre reinforced ties or

bridges formed between the polyurethane foam at

the base of each beam The face of the beam was

reeded on the vertical surfaces in order to mask any

minor undulations and to provide channels off

which the water ran and thereby cleaned the surface

The reeding also gave the effect of a matt panel

without reducing the high surface white finish

The structure was visually inspected in 1994 by

Scott Bader and the University of Surrey and the

degradation was found to be minimal It was

demolished in 2007 to allow for redevelopment of that area A part of the composite material from the demolished structure was analysed at the University

of Surrey for any variations in the mechanical properties due to the degradation of the composite material during its life (Sriramula and Chryssantho-poulos, 2009)

43.1.2 An ‘All-PolyMeR CoMPosite’

ClAssRooM oF PRiMARy sCHool, tHoRnton Clevelys, lAnCAsHiRe, uk, 1974

The classroom, shown in Fig 43.1, is an ‘all-

composite’ FRP building in the form of a geometrically modified icosahedron, and is manufactured from 35 independent self-supported tetrahedral panels of chopped strand glass-fibre reinforced polyester com-posite Twenty eight panels have a solid single skin GFRP composite and in five of these panels circular apertures were constructed to contain ventilation fans

In the remaining seven panels non-opening triangular windows were inserted The wet lay-up method was utilised to manufacture the E-glass fibre/polyester composite skins The inside of the panels has a 50

mm thick integral skin phenolic foam core acting

as a non-load bearing fire protection lining to the GFRP composite skins The icosahedron structure

is separated from the concrete base by a timber hardwood ring The FRP panels were fabricated onto a mould lining of Perspex with an appropriate profile to give a fluted finish to the flat surfaces of the panels The edges of the panels were specially shaped to provide a flanged joint, which formed the connection with adjacent panels Sandwiched between two adjacent flanges is a shaped hardwood batten, which provides the correct geometric angle between

Fig 43.1 The ‘all-polymer composite’ classroom of the

primary school, Thornton Clevelys, Lancashire, UK.

the panels; the whole is bolted together using

galvanised steel bolts placed at 450 mm intervals

The external joint surfaces between the adjacent

panels were sealed with polysulphide mastic The

glass windows were fixed in position on site by

means of neoprene gaskets The classroom was

designed by Stephenson (1974)

When the classroom structure was under

con-struction in 1974 a fire test at the BRE Fire Research

Station was undertaken on four connected GFRP

panels, with the integral skin phenolic foam in place

At the same time, tests were also undertaken on an

identical geometrically shaped school system used

at that time The results demonstrated that the GFRP

classroom had over 30 minutes fire rating whereas

the existing school system had only 20 minutes

These two descriptions of the Mondial House

and the school classroom at Thornton Clevelys have

been based on Hollaway (2009)

43.2 The civil engineering industry

The ‘all-polymer composite’ structure systems – like

those of the building industry produced to date –

have tended to be single prestigious structures,

manufactured from ‘building blocks’, Hollaway and

Head (2001) The advantages of this are:

• the controlled mechanised or manual factory

manufacture and fabrication of identical structural

units

• the transportation to site of the lightweight units,

which can be readily stacked; it is more

econom-ical to transport lightweight stacked FRP units

than the heavier steel and concrete units

McNaughton (2006) said: ‘The majority of the

Network Rail’s bridges in the UK are 100 years old

and are constructed in a variety of materials, for

example cast iron, wrought iron, steel, reinforced

concrete, brick, masonry and timber Future

con-struction is likely to use more complex forms of

composite construction, in particular fibre reinforced

polymers, which are already being used to strengthen

bridges’

Examples of some of these ‘more complex

struc-tures’ are the Aberfeldy Footbridge, Scotland (1993),

the Bonds Mill Single Bascule Lift Road Bridge,

Oxfordshire (1994) (Head, 1994), Halgavor Bridge

(2001) (Cooper, 2001), the road bridge over the

River Cole at West Mill, Oxfordshire (2002) (Canning

et al., 2004), the Willcott Bridge (2003) (Faber

Maunsell, 2003), the New Chamberlain Bridge,

Bridgetown, Barbados (2006) and the Network Rail

footbridge which crosses the Paddington–Penzance railway at St Austell, UK (2007) An innovative

£2 million Highways Agency super-strength FRP composite bridge (The Mount Pleasant Bridge) was installed in 2006 over the M6, between Junctions

32 and 33; the structure won the National Institu-tion of Highways and TransportaInstitu-tion Award for Innovation in June 2007

All these structures were of modular construction, manufactured utilising advanced composite materials; for the construction to be successful the material had to be durable, and assembly of the units had to

be rapid and simple with reliable connections As

we have already seen advanced polymer composite materials are durable and lightweight and conse-quently they fulfil these requirements, provided that the initial design of the basic building modular system is properly undertaken and the material properly installed

A number of bridges have used the concept of

the Maunsell structural plank, shown in Fig 43.2.

43.3 Bridge enclosures and

fairings

It is a requirement that all bridge structures have regular inspection and maintenance, which will often cause disruption to travellers, particularly if closure of roads and interruption to railway services are required Furthermore, increasingly stringent standards are causing the cost of closures to be high, particularly if maintenance work is over or beside busy roads and railways Most bridges that have been designed and built over the last 30 years do not have good access for inspection, and in Northern Europe and North America deterioration caused by de-icing salts is creating an increasing maintenance workload

The function of ‘bridge enclosures’ is to erect a

‘floor’ underneath the girder of a steel composite bridge to provide access for inspection and main-tenance The concept was developed jointly by the Transport Research Laboratory (TRL, formerly TRRL) and Maunsell (now AECOM) in 1982 to provide a solution to the problems Most bridge enclosures that have been erected in the UK have utilised polymer composites These materials are ideal because they add little weight to the bridge, are highly durable, and as they are positioned on the soffit of the bridge they are protected from direct sunlight

The floor is sealed on to the underside of the edge girders to enclose the steelwork and to protect it

from further corrosion Once the enclosures have been

erected the rate of corrosion of uncoated steel in the

protected environment within the enclosure is 2–10%

of that of painted steel in the open (McKenzie, 1991;

1993) The enclosure space has a high humidity;

chloride and sulphur pollutants are excluded by

seals and when condensation does occur (as in steel

girders) the water drops onto the enclosure floor,

which is set below the level of the steel girders from

where it escapes through small drainage holes

Figure 43.3 shows an example of the enclosure

on the approach span of the Dartford River Bridge

(QE2) where it passes over the Channel Tunnel rail

link (CTRL) (before the train rails were laid)

43.4 Bridge decks

The development of FRP deck structures has been

based generally on the pultruded systems, but

occasion-ally on moulded structures Recently FRP deck

80

3

Plank cross section Connector

cross section

Box beam cross section 2310

80 × 80 voided connector

603 × 80 voided plank

Notes (i) All dimensions are in millimetres (ii) All voids are 80 × 76 mm Key

760

Fig 43.2 The Maunsell structural plank (Hollaway and Head, 2001, by permission, Elsevier).

Fig 43.3 Photograph of the enclosure on the approach

span of the Dartford River Bridge (QE2) where it passes over the CTRL (before the train rails were laid) (Courtesy of AECON).

replacements in conjunction with FRP superstructure

replacement for road bridges have been carried out

This type of construction is becoming popular for

replacement decks of bridges up to 20 m span

Figure 43.4 illustrates a typical cross-section of a

bridge deck The reasons for FRP material being

used in particular circumstances are:

• the bridge deck is the most vulnerable element

in the bridge system because it is exposed to the

direct actions of wheel loads, chemical attack,

and temperature/moisture effects including freeze–

thaw shrinkage and humidity; FRP material

characteristics satisfy these requirements

• reduced future maintenance (FRP composites are

durable materials)

• quick installation owing to pre­fabrication and

easy handling

In the USA over 100 concrete bridge decks have

been replaced by FRP deck installations, most of

which have been built using proprietary experimental

systems and details The lack of standardisation is

a challenge to bridge engineers, who traditionally

have been accustomed to standard shapes, sizes and

material properties The first FRP European bridge

deck and superstructure replacement was conceived

and developed under the innovative European ASSET

Project led by Mouchel Consulting It culminated

in 2002 in the construction of the West Mill Bridge

over the River Cole in Oxfordshire; the beam and

deck structures were manufactured by the pultrusion

technique

The first vehicle-carrying FRP bridge deck in the

UK to span over a railway replaced the existing

over-line bridge at Standen Hey, near Clitheroe,

Lancashire; it has a span of 10 metres, weighs

20 tonnes and was completed in March 2008 This

is the first of Network Rail’s six trial sites in the country The consultants Tony Gee and Partners were respon sible for the design of the deck, which comprises three layers of ASSET panel deck units made from E-glass fibres in the form of biaxial mats within a UV-resistant resin matrix

Composite Advantage (CA) has recently built (April 2008) a new ‘drop-in-place’ GFRP composite prefabricated integral beams and deck bridge super-structure, 6.77 m long by 19.0 m wide (22 feet by

62 feet) in Hamilton County, Ohio, USA No heavy lifting equipment was required and it took one day

to install (Composite Advantage, 2008)

A new single carriageway road bridge over the M6 motorway (UK) has recently been completed

by the UK Highways Agency The superstructure comprises a novel pre-fabricated FRP deck spanning transversely over, and adhesively bonded to, two longitudinal steel plate girders The Mouchel Group designed the FRP bridge deck, which provides general vehicular access to an equestrian centre

(Fig 43.5); this was designed for unrestricted traffic

loading (Canning, 2008)

43.5 External reinforcement to

reinforced concrete (RC) structural members

The repair, upgrading and strengthening/stiffening

of deteriorated, damaged and substandard infra-structure has become one of the fastest growing and

There are three types of FRP deck:

1 Honeycomb: core construction provides considerable flexibility in tailored depth,

however the wet lay-up method now employed requires painstaking attention to quality control in the bonding of the top and bottom face material to the core.

2 Solid core sandwich: solid core decks have foam or other fillers in the core.

3 Hollow core sandwich: consists of pultruded shapes fabricated together to form

deck sections FRP decks typically have continuous hollow core patterns, as shown above.

Wearing surface

Hollow core

Sandwich beam

Fig 43.4 A typical cross-section of an FRP bridge deck.

most important challenges confronting the bridge

engineer worldwide It is generally much less

expensive and less time consuming to repair a bridge

or building structure than to replace it

Civil infrastructure routinely has a serviceable life

in excess of 100 years It is inevitable that some

structures will eventually be required to fulfil a role

not envisaged in the original specification It is often

unable to meet these new requirements, and

con-sequently needs strengthening Changes in use of a

structure include:

• Increased live load For example, increased traffic

load on a bridge; change in use of a building

resulting in greater imposed loads

• Increased dead load For example, additional

load on underground structures owing to new

construction above ground

• Increased dead and live load For example,

widening a bridge to add an extra lane of traffic

• Change in load path For example, by making

an opening in a floor slab to accept a lift shaft,

staircase or service duct

• Modern design practice An existing structure

may not satisfy modern design requirements; for

example, owing to the development of modern

design methods or to changes in design codes

• Design or construction errors Poor construction

workmanship and management, the use of inferior

materials, or inadequate design, can result in

deficient structures that are unable to carry the

intended loads

• New loading requirements For example, a

struc-ture may not have originally been designed to carry blast or seismic loads

• Material deterioration For example, concrete

degradation by the alkali–silica reaction or corrosion of steel reinforcement in marine or industrial environments or from the de-icing salts used on highways, all of which were discussed

in Chapter 24

• Structural deterioration The condition of a

structure will deteriorate with time owing to the service conditions to which it is subjected In some cases this deterioration might be slowed or rectified by maintenance, but if unchecked the structure will become unable to perform the purpose for which it was originally designed

• Fatigue This is a secondary cause of structural

degradation, and it can govern the remaining life

of a structure, as discussed in Chapter 2

Structural degradation can also result from hazard events, such as impact (for example, ‘bridge bashing’

by over-height vehicles), vandalism, fire, blast load-ing or inappropriate structural alterations durload-ing maintenance A single event may not be structurally significant, but multiple events could cause significant cumulative degradation to a structure

The following discussions and examples illustrate the strengthening of members by external bonding

of FRP plates or members These will be considered

as un-stressed at the time of bonding onto the structural beam It is however possible to pre-stress the plate before bonding it onto the beam; this is known as active flexural strengthening This topic will not be discussed here but further reading on it may be

found in Teng et al (2002) and De Lorenzis et al

(2008), and a practical example is cited in Hollaway (2008)

Many experimental and analytical research inves-tigations have been undertaken on reinforced con-crete beams strengthened by FRP composites; some

of these are discussed in Triantafillou and Plevris

(1991), Hollaway and Leeming (1999), Teng et al

(2002), Concrete Society Technical Reports (2000, 2003), Oehlers and Seracino (2004) and Hollaway

and Teng (2008) Both flexural and shear upgrading

can be undertaken using FRP composites

43.5.1 ReHAbilitAtion oF degRAded FlexuRAl RC stRuCtuRAl beAMs using FRP PlAtes

Within the scope of ‘strengthening’ concrete, it is essential to differentiate between the terms repair, rehabilitation, strengthening and retrofitting; these

Fig 43.5 Craning in the 100-tonne FRP deck onto

the supports of the bridge over the M6 (Courtesy of

Mouchel).

terms are often erroneously interchanged but they

do refer to four different structural upgrading

procedures

• Repair to an RC structural member implies the

filling of cracks by the injection of a polymer

into the crack

• Rehabilitation of a structural member (of any

type) refers to the improvement of a functional

deficiency of that member, such as caused by severe

degradation, by providing it with additional

strength and stiffness to return it to its original

structural form

• Strengthening of a structural member is specific

to the enhancement of the existing designed

performance level

• Retrofit is used to relate to the upgrading of

a structural member damaged during a seismic

event

Bonding of FRP plates to the adherend

As with all bonding operations the adherends must

be free of all dust, dirt and surface grease

Conse-quently, the concrete or steel surface onto which

the composite is to be bonded must be grit blasted

to roughen and clean the surface It will then be air

blasted to remove any loose particles and wiped with

acetone or equivalent to remove any grease before

the bonding operation The surface preparation of

component materials of FRP composite plate

bond-ing to concrete surfaces is described in Hutchinson

(2008)

The thickness of the adhesive and FRP composite

plate would generally be about 1.0–1.5 mm and

about 1.2 mm, respectively; the total length of the

FRP plate as delivered to site would be of the order

of 18 metres It is possible to roll the material into

a cylinder of about 1.5 metre diameter for

trans-portation and for bonding the plate onto the beam

in one operation

Power actuated (PA) fastening ‘pins’ for

fastening FRP composites

This method, which has been recently developed, is

known as the Mechanically-fastened unbonded FRP

(MF-UFRP) method and is a viable alternative to

the adhesive bonding of a preformed pultruded or

a prepreg rigid plate It mechanically fastens the

FRP plate to the RC beam by using many closely

spaced steel power-actuated (PA) fastening ‘pins’

and a limited number of steel expansion anchors

The process is rapid and uses conventional hand

tools, lightweight materials and unskilled labour In

addition, the MF-UFRP method requires minimal

surface preparation of the concrete and permits immediate use of the strengthened structure The advantage of using multiple small fasteners as opposed

to large diameter bolts, which are generally used for anchorages, is that the load is distributed uniformly over the FRP strip and this reduces the stress con-centrations that can lead to premature failure The method was developed by researchers at the University

of Wisconsin, Madison, USA (Bank, 2004) Bank

et al (2003a, 2003b) have discussed the

streng-thening of a 1930 RC flat-slab bridge of span 7.3 m

by mechanically fastening the rigid FRP plates using the MF-UFRP method

Unstressed FRP plates

Figure 43.6 shows an FRP composite flexural plate

bonded in position The plate material used for the bonding or the MF-UFRP operations is generally the high-modulus (European Definition) CFRP, AFRP (Kevlar 49) or GFRP composite These will be fabricated by one of three methods:

• the pultrusion technique, in which the factory made rigid pre-cast FRP plate is bonded onto the degraded member with cold-cure adhesive polymer

• the factory made rigid fully cured FRP prepreg plate, which is bonded to the degraded member with cold-cure adhesive polymer

• the low­temperature mould prepreg FRP prepreg/ adhesive film placed onto the structural member and both components are cured simultaneously

on site under pressure and elevated temperature (see Chapter 41, section 41.1.2)

The third method for the bonding operation is superior to the precast plate and cold-cure adhesive systems (first and second methods) as the site com-paction and cure procedure of the prepreg and film adhesive ensure a low void ratio in the composite and an excellent join to the concrete The current drawback to this method is the cost; it is about twice

as expensive as the other two, and the currently preferred manufacturing system for upgrading is either the first or the second method With these systems the plate material cannot be reformed to cope with any irregular geometry of the structural member In addition, a two-part cold-cure epoxy adhesive is used to bond the plate onto the substrate This is the Achilles’ heel of the system, particularly

if it is cured at a low ambient temperature since without post cure the polymerisation of the polymer will continue over a long period of time; this incom-plete polymerisation might affect the durability of the material

Near-surface mounted (NSM) FRP composite

reinforcement technique

This is another method for the rehabilitation of RC

structural members CFRP, AFRP and GFRP

com-posites can be utilised and generally the cross-section

of the member is either circular or rectangular Grooves

are cut into the surface of the member, generally

into the soffit of the concrete beam, but if the cover

to the steel rebars is insufficient for this the grooves

may be cut into the vertical side of the beam as

near to the bottom of the section as is practical The

NSM FRP reinforcement is embedded and bonded

into this groove with an appropriate binder (usually

high-viscosity epoxy or cement paste) Figure 43.7

shows the position of NSM bars in an RC structural

member

The NSM reinforcement can significantly increase

the flexural capacity of RC elements Bond may be

the limiting factor to the efficiency of this technique

as it is with externally bonded laminates A review

of the technique has been given by De Lorenzis and

Teng (2007)

NSM FRP reinforcement has also been used to

enhance the shear capacity of RC beams In this

case, the bars are embedded in grooves cut into the

sides of the member at the desired angle to the axis

Utilising NSM round bars, De Lorenzis and Nanni

(2001) have shown experimentally that an increase

in capacity as high as 106% can be achieved, thus

when stirrups are used a significant increase can be

obtained

Flexural strengthening of pre-stressed

concrete members

Limited research has been undertaken on

strength-ening pre-stressed concrete (PC) members; the fib

have reported that less than 10% of FRP-strengthened bridges as of 2001 are pre-stressed (fib Task Group 9.3, 2001) Strengthening usually takes place when all long-term phenomena (creep, shrinkage, relaxation) have fully developed, which may com-plicate the preliminary assessment of the existing condition As in RC strengthening, the required amount of FRP will generally be governed by the ultimate limit state design in PC members Addi-tional failure modes controlled by rupture of the pre-stressing tendons must also be considered, and consideration should be given to limitations on cracking

Soffit Plate Adhesive layer U-strip composite anchor

Uniformly distributed load

Plated RC beam with FRP U-strip end anchorage

Section of beam

Fig 43.6 An FRP flexural plate bonded in position with cold-cure adhesive.

NSM FRP composite rod [either GFRP

or CFRP]

Steel rebar Steel stirrups

High viscosity epoxy or cenemt paste adhesive surrounding the NSM bar Main tensile steel rebars

Fig 43.7 Near-surface mounted (NSM) FRP composite

reinforcement technique.

Seismic retrofit of RC columns

The properties of FRP composites (their light weight

and tailorability characteristics) provide immense

advantages for the development of structural

com-ponents for bridges and buildings in seismic regions

The retrofit of RC structures improves the strength

of those members that are vulnerable to seismic

attack

The seismic retrofit of RC columns tends to change

the column failure mode from shear to flexural

failure, or to transfer the failure criteria from column

to joint and/or from joint to beam failure, depending

upon the strengthening parameters This technique

is used in existing reinforced concrete columns

where insufficient transverse reinforcement and/or

seismic detailing are provided; three different types

of failure mode can be observed under seismic input

These are:

• Column shear failure mode: This mode of failure

is the most critical one The modern seismic

co-lumn designs contain detailed transverse or shear

reinforcement, but the shear strength of existing

substandard columns can be enhanced by providing

external shear reinforcement or by strengthening

the column through composite fibres in the hoop

direction

• Confinement failure at the flexural plastic hinge:

Subsequent to flexural cracking, the cover-concrete

will crush and spall; this is followed by buckling

of the longitudinal steel reinforcement, or a

com-pression failure of the concrete, which in turn

initiates plastic hinge deterioration

• Confinement of lower ends of columns: Some

bridge columns have lap splices in the column

reinforcement; these are starter bars used for

ease of construction and are located at the lower

column end to form the connection between the

footings and the columns This is a potential

plastic hinge region and it is advantageous to

provide confinement by external jacketing or

continuous fibre winding in this area

None of these failure modes and associated column

retrofits can be viewed separately since retrofitting

for one deficiency may shift the seismic problem

to another location and a different failure mode

without necessarily improving the overall deformation

capacity

The confinement of RC columns can be undertaken

by fabricating FRP composites using techniques such

as the wet lay-up, the semi-automated cold-melt

factory-made pre-impregnated fibre or the automated

filament-winding processes The fib have discussed

the use of prefabricated (pre-cured) elements in the

form of shells or jackets that are bonded to the concrete and to each other to provide confinement (fib Task Group 9.3, 2001) The wet lay-up and the prefabricated systems are generally placed with the principal fibre direction perpendicular to the axis of the member The concrete column takes essentially axial load therefore the ratio of the areas

of the circumferential to axial fibres of the composite

is large thus providing confinement to the concrete This allows the tensile strength in the circumferen-tial direction to be virtually independent of the axial stress value A review of the effectiveness of FRP composites for confining RC columns has been given

in De Lorenzis and Tepfers (2003)

43.5.2 sHeAR stRengtHening oF degRAded RC beAMs

Shear strengthening of RC beams and columns may

be undertaken by bonding FRP laminates to the sides

of the member The principal fibre direction is parallel

to that of the maximum principal tensile stresses, which in most cases is at approximately 45° to the member axis However, for practical reasons it

is usually preferable to attach the external FRP reinforcement with the principal fibre direction per-pendicular to the member axis Various researchers – El­Hacha and Rizkalla (2004), Triantafillou (1998) – and current design recommendations – El-Refaie

et al (2003) and Ibell and Silva (2004) – have shown

that an FRP-shear-strengthened member can be modelled in accordance with Mörsch’s truss analogy Further information on this topic can be found in

Lu et al (2009).

43.6 Upgrading of metallic

structural members

Advanced polymer composite materials have not been utilised to upgrade metallic structures to the same extent as they have been for reinforced con-crete structures However, as a result of research into this subject, which commenced at the latter part of the 20th century (Mertz and Gillespie, 1996; Mosallam and Chakrabarti, 1997; Luke, 2001; Moy, 2001; Tavakkolizadeh and Saadatmanesh,

2003; Cadei et al., 2004; Moy, 2004; Luke and Canning, 2004, 2005; Photiou et al., 2006; Holla-way et al., 2006; Zhang et al., 2006), there have

been a number of applications of CFRP to metallic structures that have shown that the technique can have significant benefits over alternative methods

of strengthening

The number of applications to date in the UK

has led to the publication of two comprehensive

guidance documents:

1 ICE Design & Practice Guide FRP Composites

– Life Extension and Strengthening of Metallic

Structures (Moy, 2001).

2 CIRIA Report C595 Strengthening Metallic

Structures using Externally-Bonded FRP (Cadei

et al., 2004).

Design guidance has also been published recently

by the Italian National Research Council (CNR,

2006), Schnerch et al (2006) and ISIS (Canada),

2007

FRP strengthening can be used to address any of

the structural deficiencies described in the concrete

section The reasons for using FRP to rehabilitate

a metallic or concrete structure may be similar;

however, the way in which the FRP works with an

existing metallic structure can often be very different

to that in a concrete structure

The FRP composite plate material used for the

bonding operation is either the ultra-high-modulus

(European definition) or the high-modulus (European

definition) CFRP, AFRP (Kevlar 49) or possibly

GFRP composites and these will be fabricated by

one of four methods:

1 The pultrusion technique, in which the factory

made rigid pre-cast FRP plate is bonded onto

the degraded member with cold-cure adhesive

2 The factory made rigid fully cured FRP prepreg

plate bonded to the degraded member with

cold-cure adhesive

3 The low-temperature mould prepreg FRP prepreg/

adhesive film placed onto the structural member

and both components compacted and cured under

vacuum at an elevated temperature

4 Vacuum infusion (The Resin Infusion under Flexible

Tooling (RIFT) process)

Figure 43.8 shows the upgrading of a curved steel

structural beam by a carbon fibre/epoxy composite

prepreg

It should be mentioned that the ultra high-modulus

carbon fibre composite has a low strain to failure,

of the order of 0.4% strain, and a modulus of

elasticity of the composite of about 40 GPa, so the

system will fail with a small inelastic characteristic

The high-modulus CFRP composites have a value

of ultimate strain of the order of 1.6% strain for

modulus of elasticity of 28 GPa This implies that

the material is ductile and is unlikely to fail in a

rehabilitation situation by ultimate strain but by

some other method (Photiou, 2006)

43.7 Internal reinforcement to

concrete members

FRP rebars for reinforcing concrete members are generally fabricated by the pultrusion method (Nanni, 1993; ACI, 1996; Pilakoutas, 2000; Bank, 2006) The rebars can be manufactured from carbon, aramid and glass fibres using epoxy or vinylester polymers The surfaces of pultruded composites are smooth and therefore it is necessary to post-treat them to develop a satisfactory bond characteristic between the concrete and the rebar Several techniques are used for this, including:

• applying a peel­ply to the surface of the pultruded bar during the manufacturing process; the peel ply is removed before encasing the bar with concrete, thus leaving a rough surface on the pultruded rebar

• over­winding the pultruded rebar with additional fibres

• bonding a layer of sand with epoxy adhesive to the surface of the pultruded rod; this is a secondary operation at the end of the pultrusion line The features and benefits of using FRP rebars are:

• they are non­corrosive – they will not corrode when exposed to a wide variety of corrosive elements, including chloride ions, and are not susceptible to carbonation-initiated corrosion in

a concrete environment

Fig 43.8 The upgrading of a curved steel structural

beam by the carbon fibre/epoxy composite low-temperature mould prepreg (Courtesy of Taylor Woodrow, UK, and ACG Derbyshire, UK).