STRUCTRAL BEHA VIORE OF REINFORCED CONCRETE BEAMS STRENGTHED FOR SHEAR USING CFRP LAMINATES SUBJECTED TO CYCLIC LOADING

The application of an external strengthening technique such as bonded fibrereinforced
polymer (FRP) laminates seems to be an attractive technique to improve
the structural behaviour of R.C elements under cyclic loading. FRP composite
materials are widely employed because of their high strength to weight ratio,
environmental resistance and ease of application over materials such as steel. In this
research, an analytical model based on non-linear finite element algorithms coded in
FORTRAN language was developed to enable the analysis of R.C beams externally
strengthened for shear using CFRP laminates subjected to cyclic loads.
20-noded isoparametric quadrilateral elements with three degrees of freedom per each
node were used to represent concrete. Material response is assumed to be orthotropic
with tangent stiffness derived from stress-strain relationship for concrete under
general biaxial state of stress. The reinforcement bars were represented in discrete
manner. Three-dimensional space frame elements and space truss elements were used
for this purpose. Material response is assumed to be elastic-perfectly plastic. 20-noded
elements similar to those used to model concrete elements were used to represent
CFRP side plates. Material response is assumed to be elastic-brittle. Discrete cracking
approach was used to represent cracking.
Primary consideration has been given to the representation of shear transfer
mechanisms due to aggregate interlock in cracked concrete and dowel action in
reinforcement. Expressions were derived from an analytical model in conjunction
with experimental data to provide shear stress and stiffness values for special
elements used to model aggregate interlock mechanism. A comparable approach was
used to drive expression for dowel action mechanism. The bond-slip phenomenon
between concrete and reinforcement was accounted for by using non-dimensional spnng elements. Shear stiffness values for such elements are obtained from
expression based on experimental data.
A new experimental methodology that enables to study the interfacial behaviour of
CFRP-to-concrete joints under cyclic shear loading was developed. An experimental
program consisted of testing specified number of push off specimens has been
conducted. Mathematical formulation that govern the behaviour of the interface
element was obtained, which are found in good agreement with the experimental
results. This included the bond-slip behaviour, shear stiffness of interface and its
degradation as number of cycle increased as well as the S-N curve. 3-d interface
element is used to simulate this phenomenon. The element has sixteen nodes, eight
nodes connect to concrete element and the other eight nodes connected to CFRP
sheet. The interface was modelled by three linear springs connecting the joint nodes
with the same coordinates.
A computer program with combined-iterative method was used to solve the non-linear
cyclic problem. A parametric analysis has been carried out to study the effect of
controlling factors such as shear span-depth ratio, CFRP thickness on structural
behaviour of R.C beams strengthened for shear with CFRP laminates subjected to
monotonic or cyclic loading. The results from the analytical model were compared
with corresponding experimental ones in order to confirm the validity of the analytical
algorithm. The comparison between the analytical results and the published results
gave a good agreement which indicates that experimental methodology proved to be
appropriate and valid and that the analytical algorithm is quite efficient tool to study
the structural behaviour of such element under cyclic loading as well as monotonic
loading.
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