Fatigue
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Fatigue Beam at Failure |
Fatigue plays an important role in the performance of highway structures due to
repeated vehicular loading during their design life. Research has been conducted
on the static and shear responses of members strengthened with Fiber Reinforced
Polymer (FRP) materials, but little research had been done on their fatigue performance.
A study was done to investigate the fatigue behavior of reinforced concrete members
strengthened with varying amounts of FRP laminates. Particular emphasis was placed
on the extension of the fatigue life of existing structures.
In this study, six 20 ft. long reinforced concrete tee beams were strengthened in
flexure with two or three layers of externally bonded FRP sheets. The beams were
tested in fatigue at a load ranging from 25% to 50% of the ultimate capacity.
All members were subjected to two-point loading. Application of the loading was
achieved by means of an electro-hydraulic actuator programmed to deliver a sinusoidal
load ranging from 25% to 50% of the ultimate capacity of the specimen. A load cell
resistant to cyclic degradation was used for monitoring the applied load. Deflections
were measured at the supports, load points and at mid-span using linear variable
displacement transducers (LVDT). Strains were monitored at load points and at mid-span
using electrical resistance strain gauge transducers.
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Control Beam Results |
Results For 2-Layer System |
Results For 3-Layer System |
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Fatigue Fracture of Steel |
A control beam was tested to failure to establish the total fatigue life of the
bare specimen. Failure occurred by classic rupture of the steel reinforcement at
approximately 300,000 cycles.
The average fatigue life of beams strengthened with two layers of the MAS2000 Fabric
Wrap System was approximately 1.8 million cycles. This represented a significant
increase over the 300,000 cycles achieved by the control specimen.
Typical results for beams with three layers of MAS2000 Fabric Wrap System demonstrated
a fatigue life exceeding 3 million cycles, a factor of 10 times over the control
specimen.
As mentioned earlier, enhancement of fatigue life of existing structures was an
objective of this study. In order to more accurately represent the conditions existing
in a damaged bridge member, a control specimen was tested under severe fatigue loading
up to about half of the expected fatigue life, after which it was repaired with
two layers of MAS2000 Fiber Wrap System.
Despite the severe cracking exhibited by the specimen, it withstood an additional
two million cycles of fatigue loading. This basic test demonstrated that severely
cracked beams in service could be effectively rehabilitated with FRP laminates.
Furthermore, the rehabilitated control beam had a fatigue life comparable to those
of the specimens, which were strengthened with fabric from the outset. Stiffness
of all the fully wrapped beams was equal to or greater than that of the control
beam.
The results indicate that the use of full wrapping of beams with FRP fabric is an
effective method of rehabilitation and strengthening of concrete structures.
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Fatigue Beam at Failure |
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