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 iii 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.