Interfacial stresses and debonding failures in plated beams
Abstract
Extensive research and recent developments in structural engineering has shown that
adhesive bonding of fibre-reinforced polymer (FRP) composite, steel or any other
metallic plate to the tension face of a reinforced concrete (RC), metallic or timber beam
can effectively enhance its strength and other aspects of structural performance. This
technique is now popularly adopted for retro-fitment and rehabilitation of existing
structures. These plated beams often fail prematurely well before attaining the full
flexural capacity by either plate end debonding (PED) or intermediate crack-induced
interfacial debonding (ICD) failure. Concentration of higher interfacial shear and
normal stresses at the plate end due to a geometric discontinuity is believed to be
responsible for PED that initiates at the plate end and propagates inwards. PED includes
concrete cover separation and interfacial debonding initiated at the plate end; and such
failure initiated at a critical diagonal crack. ICD initiates at an intermediate major
flexural or flexural-shear crack in the soffit of the original beam due to high bond stress
and propagates towards one of the plate ends (type-1) or an adjacent crack (type-2).
This thesis presents a study of interfacial stresses and debonding failures in plated
beams. It first presents a simple and novel theoretical solution of interfacial stresses
applicable to any loading considering major deformations like axial and flexural
deformations in the beam and plate within linear elastic range. This solution is then
enhanced with the inclusion of the effect of adherends’ shear deformation by
approximating the displacement field for interfacial shear stress and using
Timoshenko’s beam theory for interfacial normal stress, achieving a better
understanding of the effect of shear deformation which is ill-understood. This resulted
in a first ever solution to include the effect of adherends’ shear deformation under both
interfacial shear and normal stresses. This solution is further advanced by developing a
rigorous and a versatile closed-form solution fully based on Timoshenko’s beam theory
that offered a significant insight.
Interfacial stresses at the plate end cannot be measured directly using available
measurement techniques, and may only be interpreted indirectly from measured plate
strains. The conventional interpretation is based on the assumption that the plate is
under pure tension. A significant drawback of this is that the interfacial normal stresses
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cannot be deduced. A new technique is developed to deduce both interfacial shear and
normal stresses from strain measurements.
The thesis presents three PED strength models for the special case of an RC beam with
the plate terminated in the constant moment region: a theoretical model based on
interfacial fracture mechanics with a reasonable accuracy; a semi-empirical model with
greater accuracy; and an empirical model that is slightly less accurate but simpler to
apply than the semi-empirical model. This is followed by the development of a shear
debonding model to predict the debonding failure in an RC beam with the plate
terminated in high shear and a very low or zero moment region. The two models for
PED failure in pure bending and pure shear zones are then combined to result in an
accurate shear-bending interaction debonding model. An assessment of these models
against a carefully constructed large test database shows that they are more accurate
than existing models and suitable for implementation in design codes or guidelines.
Finally, a structural mechanics formulation for an FRP-to-concrete bonded joint
between two adjacent cracks is developed. It considers axial forces, transverse shear
forces and bending moments in the adherends and uses a linearly softening bond-slip
model. A section analysis with partial interaction and a rotational spring method are
used to relate the applied loading to the interfacial deformation. A closed-form solution
is obtained that may form the basis of a rational ICD design method.