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The body shells of Formula 1 racing cars are constructed with FRP
composites
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Why the excitement?
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The UK highway bridge assessment programme, carried out over the
last few years revealed various degrees of structural inadequacy in
these national assets and thus created a need for technically
competent but economical strengthening solutions. The fledgling
external steel plate bonding market expanded, but for larger
structures the technique was rather cumbersome and involved a
substantial labour input with consequential cost implications.
In recent times, there has also been a realisation that the
disruption costs and environmental concerns associated with
demolishing and constructing the nations buildings every twenty
years or so cannot be sustained indefinitely. Thus, the building
industry has been forced to examine ways of repairing, strengthening
and modifying its buildings to cater for changes in use and general
deterioration.
By searching for improved modern techniques, the construction
industry suddenly discovered the riches of another world in which
strength/weight ratios were king and corrosion where applicable
resulted from stress, not the movement of ions. The people working
in this New World wore white overalls, worked in warm buildings and
had calendars of racing cars, aircraft and ships on their office
walls.
Behind the faade of this strange environment, new materials
composed of glass, aramid and carbon fibres were discovered. When
bathed in a bath of polymer resin, there resulted a fibre reinforced
polymer composite. These materials possessed tensile strengths up to
ten times greater than, and a density of only one fifth, that of
steel.
It is not difficult to see why the construction industry has been
stimulated and excited by the prospect of technology transfer from
the world of advanced composite materials to the hitherto
conservative world of construction. External Strengthening of
Concrete Structures Using FRP Composites.
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Flexural Strengthening |
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It is increasingly rare for buildings to remain structurally
unaltered during their life. The need to introduce holes for
vertical service risers, form openings for new stairs or elevators
and cater for increased applied loads, has created many
opportunities for FRP Composites to be utilised in the modification
of building structures. In the civil engineering sector it is
generally the desire to improve load capacity that leads to the
consideration of an external strengthening solution. |
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Applying thixotropic adhesive
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A recent project in Poole, Dorset illustrates the usefulness of FRP
as a flexural strengthening reinforcement. A building was undergoing
a major refurbishment resulting from a change of use when it was
realised that the floor slab was in need of strengthening to
withstand not only new loads, but also to cater for significant
loads for existing masonry walls. The ribbed slab imposed
geometrical limits on the design as the ribs were only 150mm wide.
Carbon fibre laminates were chosen as the reinforcement, in
preference to other types of fibre-reinforced composites primary on
the basis of cost and quality control of the FRP product. However
further advantage was the greater stiffness of carbon fibres over
those of glass and aramid, as in certain areas the presence of
concentrated loads was causing deflections that were unacceptable.
By removing some of the loads before stiffening the floor slab, then
reinstating them afterwards it was possible to reduce the overall
deflections.
In dealing with modification of structures it is important that
serviceability and fire implications are not overlooked. Since it is
rarely possible to remove self-weight from a structure, the build-up
of stresses in the strengthened section must be considered. It is
generally unacceptable for existing reinforcement to yield under the
effects working loads and therefore it is occasionally necessary to
apply more reinforcement for the purposes of improving the elastic
section properties of a section than would be necessary for
an-increase in strength alone. On the Poole project care was taken
to ensure that floor slab ribs supporting relatively stiff walls
above would not be the subject of significant deformation.
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Multiple carbon fibre laminates as external reinforcement
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It is usually not necessary to fire protect external reinforcement
providing that the fire limit state requirements of BS8110 Part 2
are met. If the structure does not satisfy the British Standard
requirements any protection to the bonded reinforcement must ensure
that the adhesive and resin binder temperature of is limited to a
level lower than their glass transition temperatures.
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Strengthening of Concrete Structures Using FRP Composites Shear
Strengthening
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In the UK, there have been relatively few applications of FRP
Composites for shear strengthening, but in appropriate circumstances
the materials can be successfully employed.
During assessment of an open spandrel arch bridge in Leeds, it
became apparent that one of the arch ribs was deficient in shear
capacity. This situation was rectified by bonding layers of carbon
fibre fabric to the prepared surface of the concrete. In these
circumstances the fabric acts in a similar manner to the leg of a
shear link and assists in arresting diagonal cracks. Unfortunately,
the development of bond strength either side of the crack severely
limits the extent of any strengthening. Tests have shown that
fabrics can become debonded at strains as low as 0.004, whereas many
fibres reach their ultimate tensile capacity at strains in excess of
0.015. Most testing for shear has therefore been carried but with
carbon fibres; which have elastic moduli between 230-640KN/mm.
Conversely glass fibres may only have an elastic modulus of
60KN/mm. |
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FRP composite shear strengthening by in-situ lay-up
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Fabrics are particularly useful for the strengthening columns and
other unusual shapes. Fibres can be orientated to suit the
directions of tensile stress and if necessary multi-axial fabrics
can be specified. In general though, inmost situations encountered
with civil engineering and building structures, an insitu lay-up of
uni-directional fibres will suffice. |
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Fabrics can be wound around difficult shapes
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Design Guidelines |
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The joint interests of the FRP Composites and construction
industries has led to several collaborative projects whose aims are
to produce guidance for designers. Of particular relevance to
concrete structures is the work being undertaken by the Concrete
Society. Their aim is to formulate design guidelines for publication
later this year. Other work is in progress by the Highways Agency,
who are drafting an Interim Advice Note on Strengthening Concrete
Bridge Supports using Fibre Reinforced Plastics.
Given the wide range of composite types and materials available it
is unlikely that advice on all potential types of strengthening can
be dealt with in a single publication. Specialist designers should
always be sought before using the technique for the first time or in
all but standard flexural strengthening structures.
In Japan and the USA, a major market for fabric based systems is in
the area of concrete confinement. By wrapping circular columns it is
possible to produce dramatic increases in axial strength and strains
to failure. However, designers should be aware that confinement can
maintain stability in the case of seismic loading, the strains
involved can be unacceptable under normal in-service loading.
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Research and Development |
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Whilst the behaviour of strengthened structure in flexure has become
relatively well understood there are still areas requiring
assessment. These include the effects of composite thickness on
interfacial and interlaminar shear capacity and methods of effective
fire protection.
Shear strengthening with FRP Composites has not been as extensively
researched as flexural strengthening and is more difficult to
quantify due to the greater number of geometrical variables.
Researchers at Oxford Brookes University in the UK are currently
attempting to correlate their own results with those obtained from
other projects around the world. If successful they will increase
confidence in this particular application of composites.
There are many other areas of research in progress of which many
will add to our understanding and promote confidence in materials
that will undoubtedly play a major part in the future of new and
existing concrete structures.
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Conclusions |
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The way in which FRP Composites have been embraced by the
construction industry is an expression of the desire to provide new
solutions to old questions. Specialist design and construction
advice is readily available and reinforced through comprehensive
research and testing programmes. There are many potential users for
these materials which provide they are currently designed and
installed will prove an invaluable tool in the strengthening of
concrete structures.
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