Bond is necessary to transfer forces from the concrete in to the FRP, bond failure implies complete loss of composite action. Four different bonding failures are discussed below:
• debonding in the concrete cover near the surface along a weakened layer,
• debonding at the interface between concrete and adhesive,
• debonding between adhesive and FRP
Peeling-off failure is associated with the propagation of the localized debonding. Peeling-off failures can be distinguished according to the initiation of debonding. Debonding can result in peeling-off at: flexural cracks, shear cracks, unevenness of the concrete surface and in the anchorage zones
Fig. 5.7 Interface bond failure modes for EBR FRP strips 5.5.2 Failure Modes of Near Surface Mounted FRP Reinforcement
5.5.2.1 Interfacial Failure Modes
Interfacial failure modes can develop in two modes as a pure interfacial failure or as a cohesive shear failure in the adhesive. Pure interfacial failure can be identified by the absence of adhesive remained at the FRP surface after failure. Cohesive shear failure can be identified by the presence of adhesive on both FRPand concrete after failure.
(1) Failure at reinforcement adhesive interface
The pure interfacial mode can be critical for bars with smooth or lightly sand-blasted surfaces, when the bond relies on adhesion instead of mechanical interlock between bar and adhesive.
(2) Failure at the epoxy concrete interface
Interfacial failure was found critical only in case of precast grooves due to their even surface. When this type of failure develops the bond
stress is lower than usual, but failure is more ductile due to the residual friction at adhesive and concrete interface.
5.5.2.2 Cover Splitting
The mechanism of cover splitting in case of round bars is similar to the splitting bond failure of steel deformed bars, but due to the softer deformations of the FRP bars the splitting tendency is not as intense. Splitting is caused by the radial component of the bond stress. Multiple types of cover splitting were observed, incase of epoxy adhesive concrete cracking and concrete cracking accompanied by longitudinal splitting of the adhesive, in case of cementitious mortar adhesive splitting of the adhesive was dominant influenced by the low tensile strength of the filler material. However, in case of NSM strips the perpendicular component of interactional stress acts towards the thick lateral concrete (exception are reinforcements close to the edge) so splitting failure is less likely to appear
Fig. 5.8 Failure at epoxy concrete interface
[
Fig. 5.9 . Cover splitting failure of NSM round bars a) concrete cracking b)
concrete cracking accompanied by longitudinal splitting of the adhesive c) splitting of the adhesive
5.5.2.3 Edge Splitting
Edge splitting failure can be critical in elements where the reinforcement is close to the edge of the concrete member. It is induced also by the development of interactional stress. Edge splitting failure can
be avoided by keeping a minimum distance from the edge; this should be considered in design Thermal expansion differences between epoxy and concrete can influence edge splitting.
5.5.2.4 FRP Tensile Rupture
Tensile rupture (it has been rarely observed by non prestressed strengthening) should be avoided according to its explosive nature. Structures strengthened with prestressed FRP more frequently fail by fibre tensile rupture because by prestressing the FRP we use a portion of its strain capacity .
6.
CONCLUSIONS
Originally, developed for aerospace and defense applications (aircraft and missile parts) CFRP materials now find wide spread use across a number of industries. Apart from construction field, CFRP materials are used in sporting goods industry, such as golf club shafts, fishing rods, fences, tennis rackets etc. What’s more, CFRP materials are now almost indispensable in the field of medical equipment and general machine construction. As composite materials find increasing use in infrastructure applications were the design lives are typically long, the issue of durability becomes more critical. The most damaging factor faced by CFRP reinforcement is the environment from which steel reinforcement in concrete is shielded automatically. The various external factors such as quality of concrete
surface, temperature conditions, humidity effect, effect of dynamic response with the movement of traffic, impact resistance of composites that affect the process of strengthening of concrete structures using CFRP should be studied in detail before commencement of work.
Carbon Fibre Reinforced Polymers are a real boon in the field of strengthening of structures and concrete repair owing to its good rigidity, high strength, low density, corrosion resistance vibration resistance and low conductivity. The application of carbon FRP laminates is very effective for flexural strengthening of reinforced concrete beams, provided proper anchorage of the laminate is ensured. As the amount of steel reinforcement increases, the additional strength provided by the carbon FRP external reinforcement decreases. Mechanical clamping or wrapping with FRP fabric combined with adhesion is effective in anchoring the FRP laminate and increases the anchorage capacity above that expected for adhesive bond only. If proper anchorage is provided, such as by wrapping or clamping, the effective strain limit (or stress level) currently proposed informally-for FRP reinforcement by ACI 440 is close to being achievable for this particular type of carbon FRP. For lightly (steel) reinforced beams, this design stress level in the FRP can add substantially and economically to the beam strength.
REFERENCES
Debaiky, A. S., Green, M. F. and Hope, B. F., “Carbon Fiber-Reinforced Polymer Wraps for Corrosion Control and Rehabilitation of Reinforced Concrete Columns”, journal of
ACI Materials, March-April 2002, pp.129-137.
2. Anders Carolin.,”Carbon Fibre Reinforced Polymers for strengthening of Structural elements’.(Doctoral Thesis) 2003.
3. J.A.O Barros, S.J.E Dias.,” Near Surface Mounted CFRP Laminates for Shear Strengthening of Concrete Beams”, journal of Cement and Concrete
Composites28 (2006) 276-292, January 2005.
4. Zsombor Kalman Szabo., Gyorgy.L .Balazs., ”Near Surface Mounted FRP Reinforcement for Strengthening of Concrete Structures”,(Research Article),Periodica Polytechnica April 2006,pp33-38
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