strengthening of steel structures with fiber reinforced polymers

1. MANUFACTURING AND
STRENGTHING OF STEEL
STRUCTURES WITH FIBER
REINFORCED POLYMER
COMPOSITES

2. Definitionofcomposite
• Two or more chemically distinct materials which when
combined have improved properties over the individual
materials. composites could be natural or synthetic.
• Composites are combinations of two materials in which one of
the material is called the reinforcing phase, is in the form of
fibers, sheets, or particles, and is embedded in the other material
called the matrix phase.
• The reinforcing phase consist of fibers which provide strength
and stiffness.
• The common type of fibers are carbon, glass, aramid, basalt
etc..
• The matrix protects and transfer the load between fibers. The
polymer matrix composite consist of Resin. The common type
of resins are thermosetting materials such as polyester, epoxy's
and thermo plastic materials such as polyvinyl, polyethylene.

5. • Carbon type FRP: Glass type FRP:
• Aramid type FRP: Basalt type FRP:

6. • FRP composite materials have experienced a continuous
increase of use in structural strengthening and repair
applications around the world in the last decade. Because,
• The main advantages of FRP:
 Light weight- easy to handle and transport.
 high strength to weight ratio.
 Corrosion resistant-will not corrode.
 Non magnetic.
 impervious to pests and woodpecker attack.
 material properties in different directions can be tailored for a
particular application.
 Environmentally safe- no leaching of toxic chemicals in to soil.

7. • MANUFACTURING PROCESS:
Forming processes for Thermosetting matrix composites:
 wet/Hand lay up and spray up techniques.
 Pultrusion.
 filament winding.
 Autoclave moulding.
 Resin transfer moulding.
Forming processes for Thermoplastic matrix composites:
 Injection moulding.
 Film stacking.
 Diaphragm forming.
 Thermo plastic tape laying.

9. • SPRAY UP PROCESSES:
in spray up process liquid resin matrix and chopped
reinforcing fibers are sprayed on to the mold surface. The fibers
are chopped in to fibers of 25-50mm length and then sprayed by
an air jet at a predetermined ratio between the reinforcing and
matrix phase. The spray up method permits rapid formation of
uniform composite coating, however mechanical properties of
the material are moderate since the method is unable to use
continuous reinforcing fibers.

10. • FILAMENT WINDING:
• Filament winding method involves a continuous filament of
reinforcing material wound on to a rotating mandrill in layers at
different layers. If a liquid thermosetting resin is applied on the
filament prior to winding, the process is called the Wet
Filament winding. If the resin is sprayed on to the mandrel with
wound filament ,the process is called Dry Filament Winding.

11. • PULTRUSION:
Pultrusion is a process where composite parts are manufactured by
pulling layers of fibers impregnated with resin, through a heated
die, thus forming the desired cross sectional shape with no part
length limitation.

13. INJECTION MOULDING:
• It is a closed mold process in which molten polymer mixed with
very short reinforcing fibers under high pressure in to a mold
cavity through an opening.
• Polymer fiber mixture in the form of pellets is fed in to an
injection molding machine through a hopper. The material is
then conveyed forward by a feeding screw and forced in to a
split mold.
• Screw of injection molding machine is called reciprocating
screw since it not only rotates but also moves forward and
backward according to the steps of the molding cycle.
• The polymer is held in the mold until solidification and then the
mold opens and the part is removed from the mold by ejector
pins.

15. STRENGTHENING OF STEEL STRUCTURES WITH FIBER
REINFORCED POLYMER COMPOSITES
• More recently, the use of FRP composites in combination with
steel, particularly in the strengthening of steel structures, has
received much attention.
This shows typical stress-strain responses of FRP composites in contrast
with that of mild steel, where it is clearly seen that FRP composites
exhibit a linear elastic stress-strain behavior before brittle failure by
rupture.

16. Appropriate use of FRP in the strengthening of steel structures
• Since steel is also a material of high elastic modulus and
strength. The main advantage of FRP over steel in the
strengthening of steel structures is its high strength to weight
ratio. Another advantage of FRP, which applies only to FRP
laminates to follow curved and irregular surfaces of a structure.
This is difficult to achieve using steel plates. Material properties
in different directions can be tailored for a particular application.
As a result of these advantages fibers oriented in circumferential
direction can be used to confine steel tubes/shells or concrete
filled steel tubes to delay or eliminate local buckling problems
in steel tubes there by enhancing the strength and seismic
resistant of such structures.
• Steel plates can also be attached by welding to strengthen
existing steel structures, but the bonding of FRP laminates is
superior to the welding of steel plates in the following
situations.

17.  bonding of FRP laminates for enhanced fatigue resistance has
the advantage that the strengthening process does not
introduce new residual stresses.
 in certain applications(oil storage tanks, chemical plants) where
the risk must be minimized, welding needs to be avoided when
strengthening a structure; bonding of FRP laminates is then a
very attractive alternative;
 high strength steels suffer significant local strength reductions
in heat affected zones of welds, so bonded FRP laminates offer
an local strength compensation method.
BOND BEHAVIOUR BETWEEN FRP AND STEEL:
In all bond critical applications , the interfacial behavior between
FRP and steel is of critical importance in determining when failure
occurs and how effectively the FRP is utilized.

18. ADHESION FAILURE:
• In an FRP-to-steel bonded joint, adhesion failure may occur at the
steel/adhesive interface or at the FRP/adhesive interface. Failure occur
between FRP/adhesive interface only when the FRP is manufactured
through wet lay up process. When a pultruded FRP strip is used such
failure can be avoided.
• The adhesion strength of steel/adhesive interface result from both
chemical bonding and mechanical bonding between the two adhereds.
• When the two adherends are in intimate contact, the strength of
chemical bonding depends mainly on the chemical composition of the
steel surface and that of the adhesive and whether they are chemically
compatible.
• The strength of mechanical bonding depends mainly on the roughness
and topography of steel surface.
• Existing approaches of steel surface treatment generally aim to enhance
the two bonding mechanisms by: (1) cleaning the surface; (2) changing
the properties of the surface. The most popular approaches include
solvent cleaning and mechanical abrasion through grit blasting.

19. BOND STRENGTH:
The bond strength is the ultimate tensile force that can be resisted by the
FRP plate in a bonded joint test before the FRP plate debonds from the
substrate.
Existing studies have shown that the bond strength of an FRP-to-steel
bonded joint initially increases with the bond length, but when the bond
length reaches a threshold value, any further increase in the bond length
does not lead to a further increase in the bond strength.
FLEXURAL STRENGTHENING OF STEEL BEAMS:
• Similar to an RC beam, a steel beam can be strengthened by bonding an
FRP plate to its tension face. The bonded FRP plate can enhance not
only the ultimate load but also the stiffness of the beam. A number of
failure modes are possible for such FRP plated steel beams, include
(1)plate end debonding; (2)intermediate debonding; (3)lateral buckling;
(4) local buckling of the compression flange and web.

20. FATIGUE STRENGTHING:
• One of the most important aspects of FRP strengthening of steel
structures is its capability to improve their fatigue life.
• Debonding along the FRP-to-steel plates interface is also a key issue of
concern in the fatigue strengthening of steel beams. Where both plate
end debonding and intermediate debonding are possible.
• Debonding near the crack tip can lead to significant increase in the stress
intensity factors, which is detrimental to the fatigue life of the
strengthened structure.
• The fatigue strengthening of steel structures generally aims to reduce the
stress intensity factors at a crack tip and thus increase their post-crack
fatigue life.
STRENGTHENING OF STEEL STRUCTURES AGAINST LOCAL BUCKLING:
• Under local compressive stress, due to concentrated loads local
buckling failure is likely to occur. Such failure may be prevented by
bonding FRP patches.

21. • Hollow steel tubes are used in many structures. Local buckling can
occur in these tubular members when they are subjected to axial
compression alone or in combination with cyclic lateral loading.
• A typical local buckling mode of circular hollow steel tubes involves the
appearance of an outward bulge near the base and is often referred to as
ELEPHANT’S FOOT buckling. It appears after yielding and the
appearance of this inelastic local buckling mode normally signifies the
exhaustion of the load carrying capacity and the end of ductile response.

22. CONCLUSION:
• External bonding of FRP reinforcement has been clearly established as a
promising alternative strengthening technique for steel structures by
existing research.
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