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SOPL investigates the use of fiber reinforced composite materials for
concrete reinforcement. Fiber reinforced plastic (FRP) composites offer particular
advantages over conventional steel reinforcement in situations such as bridges, piers,
parking decks, and various structures where magnetic properties are important (magnetic
resonance imaging rooms in hospitals and magnetically levitated train guideways). However,
before FRP reinforcement becomes widely accepted in a traditionally conservative
construction industry governed by codes, there is a need to understand the long-term
performance of FRP's in aggressive environments. This particular research program deals
with the development of test methods for the accelerated characterization of FRP
reinforcement in concrete. SOPL is constantly involved in this research project with other
professionals:
One aspect of the research at SOPL deals with the development of a simple test method for
the accelerated characterization of FRP/concrete bond behavior. To this end, the direct
pullout test shown to the left is used. In this an FRP rod is embedded in a 150 mm cube of
concrete so that one end can be loaded (bottom) and the other end remains free (top). A 60
kip load frame is used to apply loads while loaded and free end slips are measured with an
array of LVDT's connected to a digital data acquisition system. In addition, some rods are
instrumented with an in-house designed strain probe which measures the distribution of
strain within the embeddment length of the rod. This test method enables the measurement
of load-slip relationships as well as the transfer of stress between the rod and concrete.
By measuring internal bond stress development after various accelerated environmental
conditionings, the performance of the rod after many years of natural exposure to the
highly alkaline environment of concrete can be predicted. The effectiveness of the test
method with rods of various shapes has been proven, as has the accuracy of the internal
strain probe.
A close-up view of the strain probes, which consist of thin aluminum tubes instrumented
with strain gages and bonded inside the rod. Tests to verify the acceleration of long term
performance are underway. Verification of the test method by comparison with data obtained
with the RILEM-type split beam is being carried out. It was found that the more realistic
bending condition obtained in the beam specimen provided similar results to the more
simple direct pullout method described above. This means that the results obtained with
the bond test method developed at Penn State can be used for design and prediction in
real-life beam structures.
A second aspect of the research concerns the development of a model to be used for the
prediction of bond behavior in the event it is not possible to run many direct pullout
tests described above. The model used to date consists of a finite element discretization
of the rod and surrounding concrete. A simple axisymmetric model has been used with great
success to date to determine the important FRP material parameters which govern pull-out
behavior. Depending on the geometry of the undulations on the surface of the rod and the
observed failure mode, different parameters were found to be most important. For example,
in some cases, the transverse stiffness and swelling of the rod were dominant, while in
others the shear strength of lugs on the rod were dominant. The finite element model is
useful both as a guide for improved rod design and also as a predictor of bond behavior in
different rod materials and configuration with and without environmental degradation. The
model has been demonstrated to predict very well the measured stress transfer in the
direct pullout tests for carbon and glass FRP rods with and without surface lugs. |
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