Discrete crack modelling of plated concrete beams
Strengthening reinforced concrete (RC) beams using externally bonded steel plates or
fibre reinforced polymer (FRP) composites to enhance their structural performance
has been paid great attention in recent years. RC beams strengthened in such way
(plated RC beams) have many failure modes different from those of conventional
concrete beams. A better understanding on the behaviour of the strengthened beams is
essential for safe and economical design of strengthening schemes. This thesis
presents research into discrete crack modelling of plated RC beams using a
specialised computer program developed in this research.
A key factor affecting the behaviour and reliability of strengthened concrete structures
is the bond strength between the steel or FRP plate and the concrete substrate. A
literature review has shown that many different methods have been used to test this
bond strength. An extensive analysis on the stress distribution in various test set-ups
was conducted using the finite element analysis (FEA). Results show that stress
distribution can be significantly different among different set-ups, for similar
materials and geometry. The bond strength and failure modes can be significantly
dependent on the adopted test method. These suggest that it is important to develop a
standard test method so that test results from different sources are comparable.
The research studied a number of issues in using a discrete crack model based FEA
method to model the behaviour of plated RC beams:
Firstly, extensive FEA carried out in this research shows that the accuracy of
predicted stress intensity factors may be significantly improved by adding a rosette
around the crack tip in linear elastic fracture mechanics (LEFM) problems, but the
optimum rosette size is problem dependent. In order to avoid this uncertainty, a new
procedure was devised which resulted in good predictions even for very coarse
Secondly, a mixed-mode discrete crack LEFM based FEA model was developed to
model the behaviour of plated RC beams. Automatic multiple crack propagation
during the whole loading process until the failure of the structure was modelled.
Simulation of the concrete cover separation failure mode has been particularly
success. Numerical results confirmed that the bonding of a plate leads to smaller and
more closely spaced cracks than the un-strengthened beam. For plated beams, the
cracking can have significant effect on the stress distribution in the FRP plates. The
length of the plate has a significant effect on the failure mode.
Finally, 16 numerical strategies were compared for solving problems associated with
sharp snap-back behaviour encountered in modelling discrete crack propagation in
concrete beams using non-linear fracture mechanics. A four-point single notched
shear beam with nonlinear interface elements representing the discrete cracks was
used for this purpose. The results show that the effectiveness and efficiency may vary
considerably from one to another, with the local arc-length based procedures in
conjunction with tangential stiffness strategy and reversible unloading model being
the most robust.